Barrier Immunology in the Gut PDF

Barrier Immunology in the Gut Part 2 Text: Kuby’s Immunology, 8th ed. BMS 150 Week 12 Chapter 13  IgA and Intestinal Immunity IgA is secreted from plasma cells from 3 major sources: ILFs and Peyer’s patches – most of the IgA secreted in the lumen is from these...

Barrier Immunology in the Gut Part 2 Text: Kuby’s Immunology, 8th ed. BMS 150 Week 12 Chapter 13  IgA and Intestinal Immunity IgA is secreted from plasma cells from 3 major sources: ILFs and Peyer’s patches – most of the IgA secreted in the lumen is from these two sources Plasma cells in the mesenteric lymph nodes located around the abdominal aorta ▪ After the B cell is activated, it travels back to the mucosa to secrete IgA  IgA and Intestinal Immunity IgA class switching: ▪ T-dependent – follicular T cells (Tfh) induce IgA class switching in B-cells via TGF- beta, and CD40L/iCOS interactions – RA plays a role but is not secreted by follicular T cells This is a long process, and can involve somatic hypermutation – takes at least 7 days It will result in the production of ONE or A FEW specific antibodies  IgA and Intestinal Immunity IgA class switching: ▪ T-independent – BAFF and APRIL are secreted by mucosal dendritic cells and enterocytes Low-affinity antibodies that are produced very quickly MANY different types of antibodies produced Much more likely to be produced in a tolerogenic environment  ILC3 and Th17 in the Gut Initially Th17 cells and ILC3 cells were thought to be most clinically relevant in situations where they were pro- inflammatory However, they play a major role in tolerance and normal development of the intestinal immune system ▪ There are quite a few ILC3 cells and Th17 cells in a healthy gut… not so many ILC2/ILC1 or Th1/Th2 cells Human ILC3 cells seem to respond: ▪ Directly to microbes via TLRs (this does not happen in mice) ▪ To RA and IL-23 released from innate immune cells and enterocytes  ILC3 and Th17 in the Gut When activated, ILC3 cells: ▪ Secrete IL-22 & IL-17, which leads to increased production of AMPs by enterocytes and Paneth cells ▪ Secrete factors that induce the full development of Peyer’s patches and ILFs and IgA production These factors are still being elucidated ▪ Amplify the Th17 response in the gut This can be tolerogenic or pathogenic, depending on the presence of pro- or anti- inflammatory cytokines  ILC3 and Th17 in the Gut Interestingly, Th17 cells can be induced to become either: ▪ Tfh cells → assist antibody production in follicles and lymph nodes ▪ Treg cells → anti-inflammatory cytokine production, downregulation of APCs So, although Th17 and ILC3 are strongly implicated in autoimmunity and inflammatory disease, they are also crucial for tolerance Comparison – Large and Small Intestines Small intestine: ▪ Much smaller and less diverse microbial community ▪ Many Paneth cells, fewer goblet cells ▪ More prevalent M cells, Peyer’s patches present, fewer ILFs Large intestine: ▪ Huge microbial community (1,000 – 1,000,000 times more than the small intestine) ▪ Lots of goblet cells, very thick layer of mucous ▪ Lots of ILFs, no Paneth cells, fewer M cells The full impact of these differences is still being researched ▪ Different microbes invade the small and large intestines ▪ Excess large intestinal bacteria in the small intestine is known as small intestinal bacterial overgrowth ▪ Different autoimmune disorders affect the large vs. the small intestine Immune Tolerance and the Microbiome Commensals tend to: Stimulate the development and accumulation of Tregs ▪ Firmicutes, Actinobacteria, Bacteroidetes ▪ Can be through secretion of short-chain fatty acids (SCFA) SCFAs influence dendritic cells → induction of regulatory T-cells Aid the development of MALT ▪ Can be through TLR signaling Human ILC3s can detect commensals via TLR signaling → IL-17, IL-22 and secretion of MALT-developing signals Firmicutes (segmented and filamentous bacteria) enhance IgA production and Th17 development Immune Tolerance and the Microbiome  Failure of Tolerance – Celiac Disease Gluten “messes with” our intestinal immune in a complex and multi-step way: 1. A degradation product known as alpha-gliadin is resistant to proteolytic degradation by pancreatic enzymes 2. Gliadin binds to a chemokine receptor – CXCR3 → production and release of zonulin extracellularly ▪ Zonulin binds to its receptor → disassembly of ZO proteins → disassembly of tight junctions 3. Gliadin ALSO causes production of IL-15 by enterocytes ▪ This causes intra-epithelial lymphocytes to express NK cell “activating receptors” that bind to stress proteins on the enterocyte Failure of Tolerance – Celiac Disease Gluten “messes with” our intestinal immune in a complex and multi-step way: 4. Gliadin – and other pro-inflammatory molecules, likely – leaks through the damaged tight junctions 5. APCs phagocytose gliadin and some individuals will express HLA-2 molecules that present gliadin in a way that activates Th cells (usually Th1 or Th17) Step 5 is the “kiss of death” – this type of HLA molecule (either HLA-DQ2 or HLA-DQ8) seems to be the necessary factor to perpetuate inflammation and destruction of villi HLA-DQ2 or DQ8 expression is strongly linked to development of celiac disease ▪ Though many with this HLA-type do not develop celiac disease – still being studied  Pathogenetic model – celiac disease A combination of enterocyte destruction by intra-epithelial lymphocytes (NKG2D recognizes stress proteins), loss of tight junction integrity, and ongoing inflammation driven by recognition of gliadin as an antigen leads to: ▪ Development of self-antibodies – in particular tissue- transglutaminase antibodies ▪ Destruction of villi, crypt hyperplasia ▪ Migration of immune cells into the crypts and lamina propria Robbins Pathologic Basis of Disease, fig. 17.26 Celiac disease – Clinical Presentation Adults and children often present differently: Adult presentation (becoming more commonly detected) ▪ Anemia, chronic diarrhea ▪ Bloating, fatigue ▪ Deficiencies in B12 and iron Pediatric presentation: ▪ Irritability, anorexia, chronic diarrhea, weight loss muscle wasting (malabsorption) ▪ Some present with abdominal pain, nausea, vomiting, bloating, or constipation These children appear less nutrient deprived  Celiac disease – Clinical Presentation Extra-intestinal manifestations are very common In all ages: ▪ include arthritis or joint pain, aphthous stomatitis, iron deficiency anemia ▪ Dermatitis herpetiformis – up to 10% of patients Itchy, erythematous, blistering macular- vesicular lesion Often on torso, can present in a variety of areas In children: ▪ seizure disorders ▪ pubertal delay and short stature  Celiac disease – Diagnosis, Prognosis Diagnosis: ▪ Anti-tissue transglutaminase antibodies have a >95% specificity and sensitivity Most labs detect IgA tTG – if IgA deficiency, then need to look for IgG ▪ Duodenal biopsy is the gold diagnostic standard Prognosis: ▪ Very good prognosis if gluten can be avoided ▪ Continual exposure to gluten will result in intestinal and extra-intestinal manifestations In a minority, can result in the development of a B-cell lymphoma  Barrier Immunology in the Gut Text: Kuby’s Immunology, 8th ed. BMS 150 Week 12 Chapter 13 T-helper polarization – a quick review IL-12 iCOS, CD40 interactions T-helper polarization – a quick review IL-12 iCOS, CD40 interactions Antibody production: Th1 & Th2 review Different cytokines secreted by the T-helper cells will induce class switching ▪ TH1 cells secrete IFN-y which stimulates class switching to IgG subtypes ▪ TGF-beta and Retinoic Acid seem to stimulate class switching to IgA IL-21 ▪ TH2 cells secrete IL-4 & IL- IL-4 5, which stimulates class switching to IgE Also secretion of large amounts to IgM Kuby Immunology (6th ed) Figure 11-19, page 271 Antibody classes: IgE review Secreted as a monomer in small quantities Functions: ▪ Binds to cells with an Fc receptor for IgE triggering degranulation of granulocytes Eosinophils, basophils, mast cells TOP: Adapted from: https://upload.wikimedia.org/wikipedia/commons/1/14/2221_Five_Classes_of_Antibodies_new.jpg BOTTOM: Kuby Immunology (6th ed) Figure 4-16, page 94 Antibody classes: IgA Predominantly found as a dimer secreted into the GI and respiratory tract mucous ▪ Also tears, saliva, breastmilk Functions: ▪ Neutralizing and aggregating pathogens ▪ Important function in developing tolerance within the mucosal immune system The antibody produced in the highest quantity in our body – up to 5 grams/day ▪ Mostly secreted across mucosa Adapted from: https://upload.wikimedia.org/wikipedia/commons/1/14/2221_Five_Classes_of_Antibodies_new.jpg Innate Lymphoid Cells - Review Major types of innate lymphoid cells (ILCs): ▪ NK cells – already discussed as a cytotoxic monitor of and responder to abnormal-looking or stressed cells ▪ “Resident” ILCs – these cells live in barrier tissues Type 1 ILCs (ILC1) → secrete cytokines such as IFN- and TNF- → “pushes” the barrier into a “Type 1” response and favours the development of Th1 cells Type 2 ILCs (ILC2) → secrete cytokines such as IL-4, IL-5, IL-9, IL-13 → “pushes” the barrier into a “Type 2” response and favours the development of Th2 cells Type 3 ILCs (ILC3) → secrete IL-17, IFN- → effective against extracellular bacteria ▪ also contribute to lymphoid tissue development at the barrier and developing gut tolerance Tight Junctions - Review Key proteins: Claudins – trans-membrane proteins that can act as channels for small molecules (paracellular) Occludin – trans-membrane protein, function not clear Junctional adhesion molecules (JAM) ▪ Trans-membrane protein that may mediate permeability to larger molecules ZO-proteins ▪ Important in tight junction formation, interact with the cytoskeleton https://commons.wikimedia.org/wiki/File:Life_cycle_and_protein_associations_of_connexins.jpg An Immunology Model of the Gut  Immune Structures in the Gut Peyer’s patches: large collections of lymphoid nodules in the ileum ▪ A few cm in length, can be palpated, most individuals have about 100 ▪ MALT that is very thick and well- developed, extends right to the submucosa ▪ Luminal surface lined by M (microfold) cells Isolated lymphoid follicles (ILFs) ▪ Found throughout the gut, MALT nodules without capsules ▪ Much smaller than Peyer’s patches ▪ Can still have M (microfold) cells at the A Peyer’s Patch luminal surface Gut Cells with Immune Functions Enterocytes: Many PRRs – these PRRs tend to be intracellular or located at the basolateral surface ▪ Studies suggest that these PRRs are “meant” to detect bacteria that have invaded the enterocyte (intracellular) or have penetrated the epithelial tight junction barrier (basolateral surface) ▪ Some PRRs are expressed at the luminal surface – more later Translocation of IgA ▪ enterocytes “grab” secreted IgA from plasma cells in the lamina propria → assemble it with the secretory component and the J-chain → and then exocytose it into the luminal layer of mucus ▪ Polymeric IgA receptor = the receptor that binds to secreted IgA at the basolateral surface of the enterocyte Gut Cells with Immune Functions IgA secretion by enterocytes IgA is secreted bound to the secretory component and the J (“joining”) chain Studies suggests that glycans on the secretory component may also have a role in binding bacteria (as well as the Fab fragment)  Gut Cells with Immune Functions Goblet Cells: All throughout the intestine, but the highest population in the colon Secrete mucous, which presents a barrier to bacterial invasion (commensal or pathogenic) Secrete anti-microbial peptides (AMPs) that prevent bacteria from getting “too close” to the epithelial lining Can transport antigen from the lumen to APCs in the lamina propria Paneth Cells (located in the crypts): Secrete large quantities of AMPs  Gut Cells with Immune Functions Microfold cells Very specialized cells present over the surface of Peyer patches and isolated lymphoid follicles (ILFs) Smooth apical surface that captures antigen Large basolateral “pocket” that intimately contacts APCs and lymphocytes Less mucous is found over sites with M-cells basement membrane is sieve-like (see next slide) and allows movement of lymphocytes/dendritic cells past it A close-up view of M-cells in the ileum Before we begin: The immunology of the barrier tissues – ESPECIALLY the GI mucosal barrier – is extremely complex and is changing as studies accumulate and study methods improve ▪ It seems like every few years “non-traditional” immunology mechanisms are proven to be physiologically and clinically relevant ▪ An example – the secretory portion of the IgA molecule is very likely able to bind to microbes, not just the Fab segments We will discuss immunology and disease models that are unlikely to change much in the future because they have been fairly well-established… but they might change The Challenges of the Gut The GI tract MUST ▪ maintain commensal bacteria and be exposed to potentially antigenic macronutrients in the diet ▪ fight off pathogenic microbes The GI tract MUST NOT (most of the time) ▪ develop an inflammatory response to potentially antigenic macronutrients or healthy commensals Although these challenges are present in all mucosal tissue, they are especially pronounced in the gut ▪ The GI tract has the best-developed barrier immune system and has developed unique “uses” for antibodies and AMPs that are unique from the systemic, internal immune system Immune Tolerance in the Gut – an Overview Immune Tolerance in the Gut Maintaining distance Mucous in the lumen impairs bacterial mobility and makes it difficult for bacteria to penetrate the epithelial barrier ▪ The mucins are glycoproteins that impair bacterial mobility ▪ This layer is very thick in the large intestine → lots and lots of goblet cells Enterocytes and Paneth cells secrete large amounts of AMPs ▪ Defensins, phospholipases, and lysozyme all can degrade bacterial cell walls or create pores in them ▪ REG3 – secreted mostly by Paneth cells, mainly toxic against gram (+)-ve bacteria but also seems to have some activity against gram (-)-ves Unique to the GI tract Immune Tolerance in the Gut Maintaining distance Secreted IgA can perform a number of functions ▪ “unshuffled” IgA – the antibody sequence has not undergone affinity maturation – is often broadly specific for a wide range of microbes Recognize a number of microbial molecular patterns and do not tend to bind specifically or with high affinity Generally inhibits microbe penetration into the mucosa Likely itself somewhat tolerogenic – reduces the likelihood of inflammation in the mucosa ▪ “shuffled”, high-affinity IgA – the antibody sequence has undergone affinity maturation due to Th-B cell interactions Can fight off pathogens that have been recognized to be pathogenic – higher affinity, more “deadly” antibody May be more likely to be related to overall inflammation in the gut (more later)  Immune Tolerance in the Gut Different methods of antigen presentation Each of the methods illustrated below may be tolerogenic or cause the development of inflammation, depending on the activation state of APC and the presence of messengers ▪ Alarmins, other cytokines ▪ DAMPs Note that IgA bound to antigen can be endocytosed and later presented by an APC If a microbe penetrates the epithelial barrier, then a pro- inflammatory response is more likely Immune Tolerance in the Gut What is a “happy”, non-inflammatory gut environment? ▪ Results from tolerance in the GI tract, because we can’t avoid the presence of “non-self” molecules in the gut ▪ “Tolerogenic” features and molecules: Very low or zero levels of molecules that together promote either Type 1 or Type 2 inflammation Low levels of signals that are associated with Th17 and ILC3 activation “Normal” levels of anti-inflammatory cytokines and regulatory Th cells Immune Tolerance in the Gut Signals that are associated with maintenance of tolerance in the gut: ▪ IL-10, retinoic acid (RA), TGF-beta Produced by many immune and non-immune cells Tend to enhance Treg and IgA production as well as inhibiting inflammation ▪ APRIL, BAFF These are pro-B cell messengers released by epithelial cells and resident APCs They enhance T-independent B cell production of IgA ▪ Low levels of Th17-type cytokines IL-23, IL-17, IL-22 Enhance production of anti-microbial proteins Physiology 5.04 Pathology 5.0X Stomach physiology and pathology Dr. Hurnik BMS 150 Week 11 Outline PHL5.04: Stomach anatomy and physiology Stomach anatomy Regions, sphincters, curvatures Arterial and vascular supply Innervation Stomach histology Mucosa Gastric glands, cell types Submucosa, muscularis externa, serosa Stomach physiology Motility Secretion PAT 5.02 - Stomach pathology Acute and chronic gastritis Peptic ulcer disease Learning outcomes Coming soon… Stomach - intro Stomach is a J-shaped enlargement of GI tract directly inferior to diaphragm in abdomen Most distensible part of GI tract: can accommodate a large quantity of food Functions: § Serves as reservoir for food before release into SI § Mixes saliva, food and gastric juice to form chyme Stomach: Anatomy 4 main regions § Cardia § Fundus Moore et al et. al., Moore’s Clinically Oriented § Body Anatomy (7th ed). Fig 2.37, p. 233 § Pyloric 2 sphincters: § lower esophageal § Pyloric 2 main curvatures: § Lesser § Greater Anatomy and Physiology (Betts et al). Figure 23.15 Stomach: Anatomy 4 main regions § Cardia § Fundus § Body § Pylorus 2 sphincters: § lower esophageal § Pyloric 2 main curvatures: § Lesser § Greater Anatomy and Physiology (Betts et al). Figure 23.15 Stomach: Anatomy 4 main regions § Cardia § Fundus § Body § Pylorus 2 sphincters: § lower esophageal § Pyloric 2 main curvatures: § Lesser § Greater Anatomy and Physiology (Betts et al). Figure 23.15 Stomach – Vasculature: arteries Main arterial supply comes from celiac trunk of aorta 4 main arteries supply stomach: § à Hepatic artery à Right gastric à Right gastro-omental § à Celiac trunk à Left gastric § à Splenic artery à Left gastro- omental Moore et al et. al., Moore’s Clinically Oriented Anatomy (7th ed). Fig 2.40, p. 236  Stomach – Vasculature: Veins Veins run parallel with arteries Drain into hepatic portal vein or superior mesenteric vein § Left gastric vein à Hepatic Portal Vein § Right gastric à Hepatic Portal Vein § Left gastro-omental vein à Superior Mesenteric Vein § Right gastro-omental à Superior Mesenteric Vein Moore et al et. al., Moore’s Clinically Oriented Anatomy (7th ed). Fig 2.41, p. 237 Stomach - Innervation Parasympathetic Supply: § Anterior and posterior vagal trunks from vagus nerve Sympathetic Supply: § From T5-T9 segments of sympathetic trunk § Passes to celiac plexus via greater splanchnic nerve Moore et al et. al., Moore’s Clinically Oriented Anatomy (7th ed). Fig 2.35, p. 231 Stomach - Innervation Parasympathetic Supply: § Anterior and posterior vagal trunks from vagus nerve Sympathetic Supply: § From T5-T9 segments of sympathetic trunk § Passes to celiac plexus via greater splanchnic nerve Moore et al et. al., Moore’s Clinically Oriented Anatomy (7th ed). Fig 2.35, p. 231  Stomach - Histology Mucosa § Epithelium § Lamina propria Loose CT, smooth muscle, lymphoid cells § Muscularis mucosae Arranged in 3 layers: inner circular, outer longitudinal Anatomy and Physiology (Betts et al). Figure 23.16 and outermost circular Stomach - Histology Epithelium and lamina propria are arranged into glands § Glands have three regions: Neck Pit Base Neck (Isthmus) Base § Different cells types are found in different regions of the glands § Different regions of the stomach have different glands Anatomy and Physiology (Betts et al). Figure 23.16 Stomach - Histology Cells types: § Surface epithelium and gastric pits: Surface mucus cells: § Simple columnar epithelium lining surface of stomach and gastric pits Lots of mucin granules in apical surface Mucin is a large glycoprotein Short microvilli Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.1 Stomach - Histology Cells types: § Neck/Ismuth Mucus neck cells Simple columnar epithelium found within neck of gastric glands usually interspersed between parietal cells Shorter and contain less mucin granules in apical surface Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.1 Stomach - Histology Cells types: § Neck & base Parietal cells (Oxyntic) § Found mainly in upper half of gastric gland § Rounded/ pyramidal shape § Tubuloversicular structures in apical region Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.1 Re-arrange to form lumen canaliculi when active § Function: produce HCl and IF Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.3 Stomach - Histology Cells types: § Base Chief cells (aka zymogenic) § Found in lower regions of gastric glands § Abundant RER for synthesizing proteins § Contain granules containing pepsinogen § Function Pepsinogen secretion Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.1 Stomach - Histology Cells types: § Glands Enteroendocrine cells § Found deep within gastric pits § Types Enterochromaffin-like cells Secrete histamine G-cells Secreted gastrin D cells Secrete somatostatin Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.1 Stomach - Histology Submucosa § Dense, irregular collagenous CT § Rich vascular and lymphatic network draining lamina propria Anatomy and Physiology (Betts et al). Figure 23.16 Anatomy and Physiology (Betts et al). Figure 23.15 Stomach - Histology Muscularis externa § 3 layers Inner oblique Middle circular Outermost longitudinal Serosa Anatomy and Physiology (Betts et al). Figure 23.16 Stomach Physiology - Motility There are 4 different stages: § 1. Food entry into stomach § 2. Storage in Fundus § 3. Mixing (aka churning) § 4. Emptying into small intestine Stomach Physiology – Motility - 1 1. Food entry into stomach - § Lower esophageal sphincter Function § Controls movement of food into the stomach § Also prevent reflux of gastric contents into the esophagus Resting tone is maintained via intrinsic myogenic properties of sphincter muscles & cholinergic regulation § Relaxation is required to facilitate entry of food into the stomach Stomach Physiology – Motility - 1 1. Food entry into stomach § To allow food to enter the stomach a wave of relaxation moves along esophagus, lower esophageal sphincter, and into stomach and small intestine Initiated by a vasovagal reflex called receptive relaxation § Triggers by swallowing and esophageal distension § Then, a wave of peristalsis from esophagus approaches stomach and pushes food into the stomach Stomach Physiology – Motility - 2 2. Storage in fundus § Food can be stored in the fundus until it is ready to be processed. § Presence of food stretches the stomach walls Reduces the tone in muscular wall of body of the stomach (aka active dilation) § This is known as gastric accommodation § Stomach wall bulges progressively outward to accommodate larger and larger quantities of food FYI - Completely relaxed stomach can hold ~0.8-1.5L Stomach Physiology – Motility - 3 3. Mixing (aka churning) § Presence of food triggers mixing waves Initiated by gastric pacemakers § Waves start in mid- to upper portion and move toward pyloric antrum This is called propulsion § Magnitude of contraction increases on approach to pyloric antrum Contractions in pyloric antrum “grinds” the food bolus § Aka Grinding § Contents are forced into pylorus under high pressure Pylorus opening is very small (few milliliters) so antral contents are pushed back upstream toward body of stomach This is called retropulsion Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.15 Stomach Physiology – Motility - 3 3. Mixing (aka churning) § Only liquid can leave the stomach through the pyloric sphincter If particles are > 2mm in size, mixing continues § Eventually anything remaining in the stomach will eventually be emptied into the duodenum by the migrating motor complex Occurs during the inter-digestive period ~2 hours after eating Stomach Physiology - Motility 4. Gastric emptying § Movement of liquid chyme from the stomach into the small intestine § The rate of gastric emptying is governed signals from the stomach and duodenum This ensures pH inside the duodenum therefore does not become too acidic § Why is this important? Ensure travel time is slow enough for nutrient absorption Physiology – Secretions: Gastric Acid Gastric acid § Released from _______ cells § pH of 1-2 § Composed of: Hydrochloric acid Large amounts of KCl & small amounts of NaCl Small amounts of NaCl § Functions Digestion of _______ § How does it contribute? Bacteriostatic Needed for conversion of pepsinogen to pepsin Physiology – Secretions: Gastric Acid Gastric acid – secretion mechanism § Water dissociates into H+ and OH- in cell cytoplasm § CO2 combines with OH- to form bicarbonate ions Enzyme? § H+ is pumped into lumen of canaliculus H+ / K+ ATPase § Cl- transported passively from cytoplasm of parietal cell into lumen of canaliculus Guyton and Hall Textbook of Medical Physiology (Hall). 13th ed. Figure 65-5, page 822 Physiology – Secretions: Gastric Acid Gastric acid – secretion mechanism § H+/K+ ATPase is blocked by the class of drugs called “proton pump inhibitors” Guyton and Hall Textbook of Medical Physiology (Hall). 13th ed. Figure 65-5, page 822 Physiology – Secretions: Gastric Acid Parietal cells can be stimulated by several sources: § Acetylcholine acting on Muscarinic receptors Parasympathetic stimulation § Gastrin acting on CCK2 receptors § Histamine acting on H2 receptors Medical Physiology (Boron and What cell secretes gastrin? Boulpaepl). 3rd ed. Figure 42.5 Physiology – Secretions: Histamine Histamine § Histamine is stored and released from enterochromaffin-like cells (ECL) of the stomach Type of enteroendocrine cell § Function: Histamine acts on _______ receptors on ________ cells Stimulates release of gastric acid Stimulates vasodilation Physiology – Secretions: Gastric Acid Parietal cells can be stimulated by several sources: § Another commonly used class of drug in GERD are “H2 receptor antagonists”, they work by blocking the H2 receptor Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.5 Physiology – Secretions: Gastrin Gastrin § Secreted by _______ cells Type of enteroendocrine cell § Secreted in response to: Stomach distension Vagal stimulation Presence of partially digestion proteins (peptides and amino acids) § Function: Acts on ECL cells to stimulate release of histamine § What does histamine do? Directly stimulates parietal cells by binding to _____ receptor Physiology – Secretions: Gastric Acid Parietal cells can be inhibited by: § Somatostatin § Prostaglandins FYI - The drug misoprostol used in GERD acts on the prostaglandin receptor § Prostaglandin analogue Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.6 Physiology – Secretions: somatostatin Somatostatin aka growth hormone inhibiting hormone (GHIH) § Released from D cells in the stomach and intestine (also delta cells in the pancreas) § Function Acts on parietal cells to reduce secretion of gastric acid Also reduce release of gastrin, secretin, and histamine Suppresses released of pancreatic hormones § Secreted in response to: Luminal H+ § What kind of feedback is this? Stomach Physiology – Secretions: Gastric acid Gastric acid secretion is substantially higher after meals and low between meals Phases of gastric acid secretion after meals can be divided into three phases: § Cephalic § Gastric § Intestinal Secretions: Gastric acid – putting it all together Phases of gastric acid secretion after meals can be divided into three phases: § Cephalic Triggered by smell, sight, taste, thought and swallowing food Primarily mediated by the vagus nerve § Vagus nerve releases Ach Ach acts directly on ______ cells to release H+ Ach acts on ______ cells to release histamine Ach acts on D cells, inhibiting release of ______ § Vagus nerve releases GRP to induce _____ release from G cells Accounts for 30% of total acid secretion Secretions: Gastric acid – putting it all together Phases of gastric acid secretion after meals can be divided into three phases: § Gastric Food enters the stomach, distending the gastric mucosa and activating: § Vagovagal reflex & local ENS reflex Initiates same 4 mechanisms of gastric acid secretion seen in the cephalic phase Partially digested proteins stimulate G cells to produce ______. § FYI – carbohydrates nor lipids participate in gastric acid secretion, but components of wine, beer, and coffee can promote gastric acid secretion by stimulating G cells Negative feedback § Low luminal pH stimulates D cells to Medical Physiology (Boron and secrete ________, which inhibits gastrin Boulpaepl). 3rd ed. Figure 42.10 production Accounts of 50-60% of gastric acid secretion Secretions: Gastric acid – putting it all together Phases of gastric acid secretion after meals can be divided into three phases: § Intestinal Presence of amino acids and partially digested peptides in proximal intestine § Stimulates G cells in duodenum to secrete gastrin Accounts for 5-10% of total gastric acid secretion Physiology - Secretions: Intrinsic Factor Intrinsic Factor § Glycoprotein § Also secreted by parietal cells § Function Required for the absorption of vitamin B12 in the ileum § Details covered in B vitamin biochemistry in year 2 Physiology - Secretions: Intrinsic Factor Pepsinogen § Secreted from Chief cells via exocytosis Pepsinogen is spontaneously cleaved to active pepsin in the presence of HCl Pepsin function § _______ digestion How does it contribute? § Function of pepsin is dependent on low pH (optimal 1.8- 3.5) § Secretion is stimulated by: Ach release from vagus nerve or ENS § Ach bind to M receptors on chief cells § *Most important stimulus Presence of acid in the duodenum triggers secretin from S cells § Secretin also stimulates chief cells to release more pepsinogen Protecting the gastric mucosa How is the stomach able to withstand the low pH and high pepsin levels? § Gastric diffusion barrier – maintained by: Mucus gel layer on surface epithelium Bicarbonate microclimate adjacent to surface epithelial Tight junctions in gastric glands Protecting the gastric mucosa How is the stomach able to withstand the low pH and high pepsin levels? § Gastric diffusion barrier – maintained by: Mucus gel layer on surface epithelium § Gastric mucin is secreted by which cell types? § Mucus combines with phospholipids, electrolytes, and water to form a gel layer Protects against: acid, pepsin, bile acid, & ethanol Also lubricates gastric mucosa to minimize abrasions from food § Mucin secretion is induced by: Vagal stimulation Chemical irritation Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.13 Protecting the gastric mucosa How is the stomach able to withstand the low pH and high pepsin levels? § Gastric diffusion barrier – maintained by: Bicarbonate microclimate § Surface epithelial cells secrete HCO3-, which remains trapped into the mucus gel layer § HCO3- can neutralize most acid that diffuses through the mucosal layer and inactivate any pepsin that penetrates the mucus § HCO3- secretion is induced by: Vagal stimulation PGE2 Intraluminal pH Medical Physiology (Boron and Boulpaepl). 3rd ed. Figure 42.13 Gastric pathologies We will discuss two main pathologies: § Gastritis Acute & chronic § Peptic ulcer disease § Gastric neoplasms will be covered later in the term with other GI neoplasms Gastric pathologies - definitions Gastritis – inflammation of stomach mucosa Gastric erosion § Damage is limited to the gastric mucosa (ie. does not penetrate beyond the lamina propria) Peptic Ulcer § Damage extends beyond lamina propria Gastric Atrophy § Loss of gastric glandular cells Acute Gastritis - Intro Gastric mucosal inflammation caused by an imbalance between protective factors and secretion of acid and pepsin § Ranges in severity - can be asymptomatic and discovered incidentally or can cause catastrophic blood loss, anemia, or peritonitis Etiology: § NSAID toxicity, alcohol, bile, shock/sepsis, intracranial lesions, H. pylori H. pylori more closely linked to chronic gastritis Acute Gastritis – Etiology & pathogenesis NSAIDs § Inhibit prostaglandins Review - What was the effect of prostaglandins on gastric acid secretion and the gastric diffusion barrier? Effect is most significant for nonselective inhibitors (aspirin, ibuprofen, and naproxen) it can also occur with selective COX-1 inhibitors (celecoxib) Alcohol consumption § Causes direct cellular damage Intracranial lesions § Thought to stimulate the parasympathetic nervous system Acute gastritis - pathophysiology Notes Kumar et. al., Robbins and Cotran Pathologic Basis of Disease 9th ed. Fig 17.12, p. 765 Acute gastritis - pathology Initially mild inflammation § Lamina propria shows moderate edema and slight vascular congestion § Surface epithelium intact with scattered neutrophils Progression to active inflammation § Lots of neutrophils found above the basement membrane, in direct contact with epithelial cells In severe cases, mucosal damage progressed to erosions and bleeding with pronounced neutrophilic infiltrate & fibrin- containing purulent exudate can be found in stomach lumen § Erosion = loss of superficial epithelium (damage does not penetrate beyond ________) § Bleeding may occur § Erosion + bleeding = acute erosive hemorrhagic gastritis Acute gastritis – Clinical features Clinical features § May be asymptomatic but often includes: Dyspepsia Nausea, vomiting, loss of appetite, belching, and bloating Acute abdominal pain Complications § Perforation leading to peritonitis – medical emergency § Bleeding § Chronic gastritis Chronic gastritis One of the most common GI disorders Etiology: § Most common: H. pylori Gram negative motile curved rod that lives in the mucous layer Most commonly infected site is stomach antrum Retrieved from: § Can progress to gastric body or fundus https://upload.wikimedia.org/wikipe dia/commons/d/d6/EMpylori.jpg Common § Prevalence of Canadian population is 20-30% § Other causes: pernicious anemia, Crohn’s disease, radiation toxicity, amyloidosis Chronic gastritis Types: § Non-atrophic Inflammation without loss of gastric glandular cells Caused by H.pylori § Atrophic Loss of gastric glandular cells Replaced by intestinal epithelium, pyloric-type glands, fibrous tissue Change of one differentiated cell type to another is called _____? § Associated with development of gastric carcinoma § Caused by H. pylori and autoimmunity § FYI - Other (uncommon) Eosinophilic Lymphocytic Granulomatous Chronic H. pylori gastritis – pathogenesis Pathogenic mechanisms are poorly understood: § Virulence factors allow for survival of bacteria in the stomach Flagella allow them to be motile within viscous mucus Produce ammonia via an enzyme called urease, § Also toxic to epithelial cells Produce adhesins that allow bacterial to adhere to surface epithelial cells Release toxins that may be linked to Kumar et. al., Robbins and Cotran development of malignancy Pathologic Basis of Disease 9th ed. Fig 17.13, p. 768 § Elicit a robust inflammatory response – intraepithelial neutrophils, plasma cells, lymphocytes § Bacteria seem to cause reduced mucous and bicarbonate secretion Chronic H. pylori gastritis – pathogenesis H. pylori infections most often occur predominantly in the stomach antrum § Increase local gastric production and increased acid secretion is seen § This leads to a greater risk of PUD Moore et al et. al., Moore’s Clinically Oriented Anatomy (7th ed). Fig 2.37, p. 233 Long-standing H. pylori can progress to involve gastric body and fundus § Associated with gastric atrophy Chronic H. pylori gastritis – pathology Pathology: § Erythematous antral mucosa with large amounts of plasma cells and increased levels of lymphocytes, macrophages, and neutrophils in the lamina propria § Neutrophils can infiltrate the basement membrane and accumulate in lumen of gastric glands to form pit accesses. § With progression, gastric epithelium can become atrophic Atrophic gastric is associated with what? § Increases the risk of gastric adenocarcinoma § Overgrowth of MALT associated with H. pylori may also be associated with gastric lymphoma Chronic H. pylori gastritis – Clinical features & diagnosis Clinical features § May be asymptomatic but can cause: Epigastric pain Nausea, vomiting, anorexia, early satiety Weight loss Diagnosis includes: § Presence of antibodies to H. pylori in serum § Fecal bacteria detection § Urea breath test Chronic H. pylori gastritis – Treatment and complications Complications: § PUD § Gastric adenocarcinoma § MALT lymphoma Treatment: § Triple therapy: two antibiotics + PPI § Eradication of bacteria tends to cure the disease Comparison of acute & chronic gastritis Acute gastritis Chronic gastritis Inflammation of gastric mucosa with Erythematous mucosa with plasma cells, presence of lots of neutrophils among lymphocytes, macrophages, and epithelial cells, glands, and within the neutrophils in lamina propria. Neutrophils stomach lumen in advanced cases. can infiltrate the basement membrane to (minimal lymphocytes and plasma cells) form pit abbesses. Can progress to erosion and bleeding Can progress to atrophy of gastric epithelium and promote overgrowth of MALT Symptoms: Symptoms: Complications: Complications: Peptic ulcer disease Major complication of chronic gastritis Two main types: § Duodenal 4x more common Lower likelihood of perforation and malignancy § Gastric 4% are malignant, higher likelihood of perforation Tend to occur in the antrum, along the lesser curvature Etiology § H. pylori infection, NSAIDs, or cigarette smoking Most common- induce by H. pylori infection Peptic ulcer disease - pathogenesis Pathogenesis § Ultimately occurs due to imbalance between defense mechanisms and damaging factors causing chronic gastritis. Typically develops as a result of chronic gastritis § Duodenal ulcers & antral gastric ulcers Not well understood, linked to H. pylori despite H. pylori not colonizing duodenum Colonization may be linked to: § Decreased bicarbonate secretion in the duodenum § Increased gastric acid secretion in the stomach § Gastric ulcers in fundus or body Caused by mucosal atrophy Slightly higher acid secretion and greatly decreased production of mucin and other protective factors Peptic ulcer disease - Pathology Pathology: § Round to oval shaped, sharply punched-out defect § Mucosal margin usually level with surrounding mucosa but may overhand the base Base is usually smooth & clean with granulation tissue below and fibrous/collagenous scar Retrieved from: § Larger vessels within scarred area https://commons.wikimedia.org/wiki/File:Ga can thickened and thrombosed stric_Ulcer_Antrum.jpg Peptic ulcer disease – Clinical features Clinical Features: § Vague, though sometimes intense, pain with a variable relationship with meals More intense pain can be associated with perforation, bleeding, and peritonitis Duodenal ulcers § Are relieved by food, worsened 2-3 hours after eating § Duodenal ulcers commonly awaken a patient at night Gastric ulcers § Poorly relieved by eating – pain re-occurs shortly after a meal § Weight loss is common § Can also present with iron-deficiency anemia, bleeding, nausea/vomiting, bloating, belching Peptic ulcer disease - treatment and prognosis Treatment: § Withdraw offending agent § Eradicate H. pylori infection Complications § Perforation leading to peritonitis – medical emergency § Bleeding § Gastric adenocarcinoma & MALT lymphoma References Moore et al. Moore’s Clinically Oriented Anatomy (7th ed). Kumar et. al., Robbins and Cotran. Pathologic Basis of Disease (9th ed). Boron and Boulpeap. Medical Physiology (3rd ed). Guyton and Hall. Textbook of Medical Physiology (13th ed). Betts et al. Anatomy and Physiology. OpenStax Images: § https://commons.wikimedia.org/wiki/File:Gastric_Ulcer_Antrum.jp g § https://upload.wikimedia.org/wikipedia/commons/d/d6/EMpylori.j pg Small intestinal Anatomy & Physiology  Small Intestine: Anatomy Divided into 3 regions: Duodenum Shortest and widest portion of the small intestine starts at pyloric sphincter, about 25 cm long C-shaped, mostly retroperitoneal Closely associated with the head of the pancreas Jejunum About 1 meter and extends to ileum Ileum Longest: about 2 meters Joins the large intestine at ileocecal sphincter  Duodenal Anatomy Note the relationships with: The major ducts and vessels associated with the liver and gall bladder The stomach The pancreas The vast majority of the duodenum is retroperitoneal Intra-peritoneal structures removed in this dissection Jejunal and Ileal Anatomy The jejunum has larger plicae circulares The ileum has many large lymphoid nodules (Peyer patches) Arterial supply to the small intestine First 2/3 of the duodenum from the hepatic artery off the celiac trunk Hepatic art. → gastroduodenal art. → superior pancreaticoduodenal art. Pancreaticoduodenal artery supplies… the pancreas and the duodenum The inferior pancreaticoduodenal artery is supplied by the superior mesenteric artery (SMA) The pancreas and duodenum receive arterial blood from both the SMA and hepatic artery  Arterial supply to the small intestine The rest of the small intestine (last part of duodenum → ileum) as well as the first part of the large intestine (up to the transverse/descending colon) receives arterial blood from the superior mesenteric artery Jejunal and ileal arteries Basic venous drainage of the intestines Superior mesenteric vein – receives venous blood from small intestine and portions of the large intestine, stomach, and pancreas Splenic vein receives blood from: Stomach, spleen, pancreas Distal large intestine via the inferior mesenteric vein The splenic vein and superior mesenteric vein join to form the hepatic portal vein, which drains almost all sub-diaphragmatic fore-, mid-, and hindgut structures The hepatic portal vein drains into the liver – more about liver vasculature on liver physiology day Overview of abdominal visceral venous drainage  GI: Small Intestine Histology Mucosa Epithelium Simple columnar, villi Lamina propria Loose CT, blood vessels, lymph vessels, nerves, smooth muscle Duodenum Crypts of Lieberkuhn Jejunum Crypts of Lieberkuhn Ileum Crypts of Lieberkuhn, Peyer’s patches Muscularis Inner circular, outer longitudinal mucosae Submucosa Plica circulares – more notable in the jejunum and ileum Duodenum Brunner’s glands Jejunum No glands Ileum No glands Muscularis externa Inner circular, outer longitudinal Serosa / adventitia Serosa and adventitia  Small Intestinal Histology Plica circulares Folds of mucosa and submucosa Permanent ridges about 10 mm “tall” (projecting into the lumen) Enhance absorption by increasing surface area Plicae circulares cause chyme to Encourages mixing → “spiral” and “slosh” through the intestine  GI Small Intestine Histology Villi Finger-like projections of mucosa that are 0.5 – 1 mm long Vastly increase the SA of epithelium Each villus: Covered by epithelium with core of lamina propria In connective tissue: arteriole, venule, capillary network, lacteal  Microvilli Projections of apical membrane of absorptive cell 1 um long cylindrical, contains bundle of 20-30 actin filaments Too small to be seen individually (smaller than a cell) = brush border Greatly increase surface area  GI Small Intestine: Histology Cell types: Surface absorptive cell / enterocytes Simple columnar epithelium with microvilli Life span is short: a few days Goblet cell: Scattered among the absorptive cells Specialized for secretion of mucus Facilitates passage of material through bowel Paneth cells: Typical serous-secretory appearance, with basophilic basal cytoplasm and apical secretory vesicles basal portion of the intestinal crypts, below the stem cells, release lysozyme, phospholipase A2, and defensins regulate the microenvironment of the intestinal crypts and innate immune responses (more during e-learning)  GI Small Intestine: Histology Cell types that secrete hormones: Enteroendocrine cells I cells: Cholecystokinin S cells: Secretin D cells: Somatostatin K cells: GIP L cells: Peptide YY, incretins Mo cells: Motilin Mucosal cells: Secrete incretins, similar to L-cells Incretins will be discussed in more depth next week  Important Enteroendocrine Cells Cell Location Hormone (Stimulus) Main Hormonal Functions Stomach, Somatostatin Generally “turns down” the release of hormones D duodenum, (many different stimuli cause from nearby cells pancreas release) ECL – stomach ECL – histamine (stimulated by ECL – stimulates acid production EC – stomach, vagus) EC – increased motility EC, ECL small and large EC – serotonin, substance P intestines (mechanical, neural, endocrine) Gastrin (amino acids in the Increases secretion of stomach acid G* Stomach stomach, vagal stimulation, gastrin-releasing peptide) Small Intestine CCK (fats and proteins in the Pancreatic enzyme secretion, gallbladder I* (especially duodenum) contraction, satiety duodenum) Inhibits gastric acid secretion Glucagon-like peptide (amino GLP - Insulin secretion, satiety acids & carbs) Inhibits gastric acid secretion L Small intestine Peptide YY (distal small Peptide YY – inhibits gastric secretion & intestine) motility – slows gastric emptying Motilin (fasting) Migrating motor complex Mo* Small intestine Secretin (acid in small intestine, Bicarbonate and water secretion from pancreas S* Small intestine especially duodenum) Inhibits gastric acid secretion and gastric emptying Small Intestine: Regulation and Motility Mixing / Segmentation Contractions Stretch from chyme against intestinal wall elicits concentric contractions (local reflex) Spaced 1 to 5 cm = causes segmentation and “chain of sausage” appearance Contraction band does not progress Chyme from one contracting Mixes chyme with segment forced into relaxed areas digestive enzymes creates motion that chops and mixes luminal content → Circulates chyme for optimal exposure  Small Intestine: Regulation and Motility Propulsive Movements (Peristalsis) Chyme propelled through small intestine by peristaltic waves (weak) Occur in any part of small intestine and move towards anus Velocity: 0.5 to 2.0cm/sec Peristaltic rush a powerful wave of contractile activity that travels long distances down the small intestine caused by intense irritation or unusual distension  GI Small Intestine: Movements Control of peristalsis Nervous Peristaltic reflex: “law of the gut” mediated by ENS “law of the gut” – distention in the alimentary canal causes distal parts of the canal to relax and proximal parts to contract (circular muscle) Major mechanic of peristalsis Chyme entering duodenum Gastroenteric reflex initiated by distension of the stomach and conducted via the myenteric plexus Hormonal Enhanced by: Gastrin, CCK, serotonin Inhibited by: Secretin, Peptide YY, epinephrine Control of gastric emptying Requirements for chyme entering the duodenum: Food particles must be very small (< 2 mm) Small volumes of low pH fluid Large volumes of acidic fluid will damage the duodenum and denature pancreatic enzymes Gradual release Time is needed for pancreatic enzymes to perform chemical digestion Large volumes “released right away” will limit the absorptive time at the microvilli Therefore the duodenum is the major organ that regulates the rate of gastric emptying Other segments of the small intestine also contribute in a similar fashion Small intestinal control of gastric emptying Nervous control: Both sympathetic nervous system reflexes (at the level of the spinal cord) and enteric nervous system reflexes (submucosal and myenteric plexuses) will inhibit gastric emptying in response to: Increased or decreased osmolarity in the duodenum Decreased pH in the duodenum Distention or any chemical irritation of the duodenum Breakdown products of proteins and fats (mostly proteins) in the duodenum Other areas of the small intestine may participate, but the duodenum seems to be the major nervous regulator Small intestinal control of gastric emptying Hormonal Control: Duodenal hormones: CCK – secreted by I cells throughout the small intestine, but mostly in the duodenum Substances that elicit CCK release: Fats > peptides > carbohydrates (carbohydrates quite ineffective) CCK is likely the major paracrine regulator or gastric emptying – increased CCK release → slowed gastric emptying Secretin – secreted by S cells throughout the duodenum and jejunum Substances that elicit secretin release: low pH > fats, capsaicin, bile acids Mild impact on gastric emptying – slows gastric emptying Small intestinal control of gastric emptying Hormonal Control: Distal small intestine Peptide YY - major hormone that performs the role of the “ileal brake”, secreted by L-cells in the ileum Substances that elicit peptide YY release: Fats > carbohydrates, amino acids If undigested foods are reaching the distal small intestine, then peptide YY “slows everything down” – not just gastric motility, but motility of the proximal intestines as well Other hormones were hypothesized to limit gastric emptying, but were later shown to be unimportant in physiologic conditions Glucagon-like peptide, gastrin-inhibitory peptide – many Type of meal and gastric emptying Explain the variability in the rate of gastric emptying based on its hormonal and neural regulation discussed in the last 3 slides  GI Small Intestine: Movements Emptying at the Ileocecal Valve Protrudes into lumen of cecum Forcefully closed when excess pressure builds up in cecum Wall of ileum has thickened circular muscle called ileocecal sphincter which remains mildy constricted GI Small Intestine Secretions Intestinal Digestive juices by enterocytes Secrete large quantities of water and electrolytes Rate of about 1800mL/day Very similar to extracellular fluid, pH 7.5 to 8.0 A little more alkaline, so a bit more HCO3- Rapidly reabsorbed by villi Supplies a watery vehicle for absorption of substances from the chyme as it comes in contact with villi  GI Small Intestine Secretions Brunner’s Glands Found proximal to the sphincter of Oddi Secrete alkaline mucous in response to: Tactile stimuli or irritating stimuli of the overlying mucosa Vagal stimulation GI hormones: especially secretin Functions Protect duodenal wall from digestion by gastric juice Inhibited by sympathetic stimulation Pancreatic and Hepatic Involvement in Digestion – the Basics 1. Chyme arrives in the duodenum through the pyloric spincter 2. Nutrients and low pH stimulate enteroendocrine cells as well as receptors in the mucosa 3. The enteric nervous system and enteroendocrine cells respond by: CCK secretion → gall bladder contraction, pancreatic enzyme secretion, relaxation of the sphincter of Oddi Secretin release → bicarbonate-rich secretions from the pancreas and the small intestine enterocytes 4. Bile and pancreatic enzymes chemically digest macromolecules in the chyme Bile – emulsification of lipids Pancreatic enzymes – hydrolysis of fats, proteins, carbohydrates into smaller molecules Ducts of the biliary apparatus Bile is secreted by the liver by the right and left hepatic ducts → Join to form the common hepatic duct → The common bile duct is formed at the junction of the cystic duct and common hepatic duct → The sphincter of Oddi regulates release of bile into the duodenum from the ampulla of Vater Sphincter of Oddi = major duodenal papilla Ampulla of Vater = hepato-pancreatic ampulla Ducts of the biliary apparatus and pancreas If the sphincter of Oddi is closed, then bile is stored in the gall bladder The main pancreatic duct meets the common bile duct at the ampulla of Vater The sphincter of Oddi regulates both biliary and pancreatic secretions into the duodenum Sphincter of Oddi = major duodenal papilla Ampulla of Vater = hepato-pancreatic ampulla  Digestive enzymes of the pancreas Protein digestion Enzyme Name Inactive Form Activator Basic Function (FYI*) Trypsin Trypsinogen Enterokinase Cleaves peptide bonds next to arginine or lysine* Chymotrypsin Chymotrypsinogen Trypsin Cleaves peptide bonds next to a wide range of a.a.s Elastase Proelastase Trypsin Cleaves elastin Carboxypeptidase Procarboxypeptidase Trypsin Cleaves the carboxy-terminal ends of peptides Carbohydrate digestion Enzyme Name Inactive Form Activator Basic Function Pancreatic amylase N/A N/A Digests starch Lipid digestion Enzyme name Inactive Form Activator Basic Function Pancreatic N/A Co-lipase activates Cleaves triglycerides to FAs and 2-monoacyl lipase/co-lipase lipase at the micelle glycerol Phospholipase A2 Pro-phospholipase Trypsin Cleaves phospholipids Chemical Digestion – Review Chemical digestion = breaking down macromolecules into smaller molecules to increase absorption Enzymatic digestion – enzymes break macronutrients down into smaller and smaller particles through the process of hydrolysis All pancreatic, gastric, and brush-border enzymes use hydrolysis as a means of breaking a macromolecule into smaller molecules Digestion of Carbohydrates in the Mouth & Stomach Food is first mixed with saliva which contains the enzyme ptyalin (α-amylase or salivary amylase) secreted mainly by the parotid glands Ptyalin hydrolyzes starch into the disaccharide maltose and other small polymers of glucose (3-9 glucose molecules) Activity of ptyalin is blocked by the acid of gastric secretions (pH < 4)  Digestion of Carbohydrates in the Small Intestine Digestion by pancreatic amylase almost identical in its function with salivary amylase ptyalin, but several times as powerful Pancreatic amylase is the most important enzyme for digestion of starches Starches are almost totally converted into maltose and other very small glucose polymers (mostly disaccharides) before they have passed beyond the duodenum or upper jejunum  Hydrolysis of Disaccharides into Monosaccharides The brush border formed by the microvilli contain 3 major enzymes that digest the major disaccharide sugars in our diet Lactase Maltase (Isomaltase) Sucrase Carbohydrate Absorption - Review Monosaccharides are then able to be absorbed via facilitated diffusion or via co-transport via the sodium gradient SGLT-1 GLUT5 GLUT2 When monosaccharides build up in the cytosol of the enterocyte, they are then transported to the capillary network across the basolateral surface Digestion of Proteins Mouth Mechanical only, no enzymatic digestion Stomach HCL Denatures protein, not involved in hydrolysis Denaturation improves the “exposure” of peptide bonds to digestive enzymes Pepsin Most active between pH 1.5 to 3.5 Only begins the process of digestion (10 to 20%) Ability to digest collagen FYI - Most efficient in cleaving peptide bonds between hydrophobic and aromatic amino acids However, there are multiple different types of pepsins with somewhat different activities Digestion of Proteins by Pancreatic Secretions Most protein digestion occurs in duodenum and upper jejunum from proteolytic enzymes of pancreatic secretion Protein that is digested by pepsin are still mostly in the form of large polypeptides Upon entering the small intestine, they are attacked by enzymes: trypsin, chymotrypsin, carboxypolypeptidase, proelastase  Targets peptide bonds next Trypsin to lysine or arginine Split protein molecules into small polypeptides Chymotrypsin Targets peptide bonds next to phenylalanine, tyrosine, tryptophan Also methionine, asparagine, histidine Carboxypeptidase Cleaves individual amino acids from carboxyl ends of polypeptides A Aromatic neutral or aliphatic FYI neutral amino acids Don’t need to B know the Basic amino acids: lysine, peptide bonds arginine, ornithine cleaved Protein digestion in the duodenum The brush Trypsinogen is activated by enterokinase in the border lumen of the duodenum Enterokinase = a brush border enzyme After trypsinogen is activated to trypsin, it activates a wide variety of other inactive enzymes (zymogens) Chymotrypsin, pro-elastase, procarboxypeptidases Pro-phospholipaseA2 Trypsin ALSO digests pancreatic hormones after the nutritional substrate (food) has “run out” Carboxypeptidase digests proteins at their carboxy-terminal ends The rest are known as “endopeptidases” – they are not limited to the ends of polypeptides Protein Digestion - Brush Border Peptidases The brush border Membrane-bound peptidases aid in digestion of large peptides to di- and tri-peptides FYI – brush-border peptidases: Aminopeptidases Attack N-terminal end of oligopeptides Dipeptidases Hydrolyze N terminal side of dipeptides Tripeptidases Cleave tripeptide into amino acids and dipeptide Absorption of Amino Acids & Peptides 33% of protein absorption as free amino acids, 67% of protein absorption as peptides Multiple ATP dependent systems with overlapping specificity, many are Na+ co-transporters Amino acids Active transport or secondary transport with Na+ Dipeptides and tripeptides Some are Na+ dependent Some are via secondary active transport with H+ Within cytoplasm of the enterocyte most peptides are hydrolyzed to free amino acids Fat digestion - Intro Digestion of fat presents a challenge: Not very water soluble, so tends to accumulate in large droplets Reduces surface area Reduced SA, poor water solubility impairs the process of chemical digestion as well as absorption Answer: emulsify the fat droplets into very small droplets that have an amphipathic molecule on the outside of the droplet Structure = micelle Improved interaction of water, much larger SA:volume ratio Fat emulsification and bile Bile contains a variety of amphipathic molecules that help the formation of micelles Lecithins (i.e. phosphatidylcholine) Bile salts Cholesterol Mixing movements in the duodenum and bile secretion help sequester dietary lipids into micelles “outside” of the micelle – hydrophilic portions of bile components Inside of the micelle Hydrophobic portions of bile components Dietary lipids Fat digestion Once micelles have been formed in the duodenum and the jejunum, pancreatic lipase can act on the triglycerides in “interior” of the micelle Vast majority of dietary lipids are triglycerides Less cholesterol and phospholipids Pancreatic lipase is not very effective alone Needs colipase to help it insert into the micelle and activate it Pancreatic lipase breaks triglycerides into 2 free fatty acids and 2- monoacyl glycerol These lipids can then diffuse from the micelle into the enterocyte, across the brush border 1 Pancreatic 2 + Lipase 3 Triglyceride 2-Monoglyceride Free Fatty Acid  Digestion of Fats Phospholipase works similarly Phospholipids broken down into fatty acids and other components After lipids have been chemically digested and absorbed into the enterocyte, they’re reassembled into triglycerides and other lipids and inserted into chylomicrons Chylomicrons are built within the enterocyte and secreted into the lymphatic vessels at the base of the villi transported to a variety of cells via the thoracic duct → venous blood Chylomicrons are necessary for transporting triglycerides (longer fatty acids), cholesterol esters into the circulation Chylomicron = a lipoprotein Spherical structure with a phospholipid bilayer and proteins on the outside, absorbed triglycerides, cholesterol on the inside Proteins on the outside can have enzymatic and receptor functions that help the chylomicron be “delivered” to and used by cells throughout the body Assimilation of Lipids lecithin Emulsified lipase-colipase 2-MG FOOD FFA bile salts fat micelles (enterocyte) bile salts apoprotein + TG 2-MG 2-MG TG FFA FFA (micelles) chylomicrons lymph vessel A chylomicron  Absorption at the villus Carbohydrates and amino acids are transported across the basement membrane of the enterocyte, and diffuse into the capillary loops within the villus Small free fatty acids can diffuse into the blood and be carried by serum proteins (i.e. albumin) Larger lipids need to be carried by chylomicrons Chylomicrons are secreted via exocytosis into the lamina propria Enter the openings of the lacteals (lymphatic capillaries within the villi) Malabsorption – an intro When absorption at the small intestine fails, nutrients remain in the lumen of the intestine. This can result in: Diarrhea much of the contents of chyme are osmotically active → they draw water into the lumen of the intestine and/or prevent the overall absorption of fluid Bacteria can also metabolize undigested/unabsorbed nutrients → gas, abdominal distention, abdominal pain Micronutrient deficiency (aka vitamin, mineral deficiencies) Macronutrient deficiencies Inadequate caloric and protein intake for basic metabolic requirements Malabsorption – an intro Types of malabsorption can be pathophysiologically organized into four major problems: Disturbances in intraluminal digestion Enzymes/bile that are secreted into the lumen by the stomach, pancreas or gall bladder/liver are inadequate for the near- complete breakdown of proteins, carbohydrates, or fats Disturbances in terminal digestion The brush border enzymes cannot break down small peptides or disaccharides Disturbances in transepithelial transport nutrient, fluid, and/or electrolyte transport are disordered Disturbances in lymphatic transport – lipid transport is impaired  Malabsorption – common illnesses Disease Intraluminal Terminal Trans- Basic pathogenesis Digestion Digestion Epithelial Transport Celiac disease No Yes Yes Damage to the villi and microvilli results in loss of surface area and overall absorptive enterocyte function Chronic Yes No No Lack of major digestive enzymes from the pancreas pancreatitis leads to major impairment of absorption, diarrhea, and nutrient deficiencies Disaccharidase No Yes No Lack of disaccharide results in unabsorbed sugar in the deficiencies lumen → bacterial gas (lactose production and osmotic intolerance) diarrhea Gastroenteritis No Yes Yes Damage to the brush border or dysregulation of electrolyte (bacterial, viral, transport results in impaired parasitic) ability to absorb nutrients