4360 Lecture 3 PDF

Lecture 3 ❑Readings: ▪ Loker & Hofkin Chapter 3 (pp 73-106), Chapter 5 (pp 159-180) ▪ Parasite to do list ▪ Pathology resulting from parasitic infections ▪ Introduction to Protozoa ▪ Intestinal physiology 1  The Parasite’s To Do List ❑T...

Lecture 3 ❑Readings: ▪ Loker & Hofkin Chapter 3 (pp 73-106), Chapter 5 (pp 159-180) ▪ Parasite to do list ▪ Pathology resulting from parasitic infections ▪ Introduction to Protozoa ▪ Intestinal physiology 1  The Parasite’s To Do List ❑The Parasite’s To Do List (Chapter 3) 1) Achieve transmission & enter host 2) Migrate to appropriate target tissue or site within the host 3) Maintain its position within the host 4) Finding a mate 5) Successfully reproduce in and release progeny from the host 6) Undergo essential developmental changes within the host 7) Cope with physiological changes within host & evade destruction by host immune system 2  The Parasite’s To Do List 1) Achieve transmission & enter host ▪ Fecal-oral (e.g. Giardia, Entamoeba, Cryptosporidium) ▪ Trophic (e.g. some trematode-infected fishes & snails) ▪ Direct, active penetration (e.g Schistosoma cercaria or hookworm L3 stages into human skin) Cercarial dermatitis Ground itch Lambertucci, J.R. 2010. Mem. Inst. Oswaldo Cruz. 105(4): Loukas et al. 2016. Nature Disease 422-435. Primers. 2: 16088: 3  The Parasite’s To Do List 1) Achieve transmission & enter host ▪ Occurs through intermediate hosts or vectors a) Vector competence- ability of the vector to become infected with a parasite or to transmit a parasite − Biological vectors (essential development and/or reproduction occurs) Cyclodevelopmental transmission Propagative transmission Cyclopropagative transmission b) Vector capacity- extrinsic factors such as regular feeding on host, time spent on host, sufficiently long-life span, abundance & dispersal ability 4  The Parasite’s To Do List 1) Achieve transmission & enter host a) Vector competence − Biological vectors Cyclodevelopmental transmission- undergo essential development but does not increase in number (e.g. some nematodes such as Wuchereria bancrofti ) W. bancrofti microfilariae 5 https://emedicine.medscape.com/article/217776-overview?form=fpf  The Parasite’s To Do List 1) Achieve transmission & enter host a) Vector competence − Biological vectors Propagative transmission- does not undergo developmental progression but multiplies within the vector (e.g. Dengue and West Nile viruses in mosquito) salivary glands Host blood 6  The Parasite’s To Do List 1) Achieve transmission & enter host a) Vector competence − Biological vectors Cyclopropagative transmission- both development & multiplication occurs within the vector 7  The Parasite’s To Do List 1) Achieve transmission & enter host c) Sexual transmission (e.g. Trypanosoma equiperdum causes dourine in horses, Trichomonas vaginalis in humans, Tritrichomonas foetus in the foreskin of bulls) Swelling of genitalia caused by T. equiperidum Suganuma et al. 2016. Parasites & Vectors. 9:481 8  The Parasite’s To Do List 1) Achieve transmission & enter host d) Vertical transmission- from mother to offspring via placenta (e.g. Toxoplasma gondii, Toxocara canis), transovarian (e.g. Babesia bigmina infected gametes in ticks) or via feeding (e.g. breast milk in mammals) Babesia sp Toxoplasma gondii 9 Jalovecka et al. 2019. Tr. Parasitol. 35: 356  The Parasite’s To Do List 1) Achieve transmission & enter host e) Horizontal transmission- contact with infected individuals or via abiotic factors to uninfected individuals that are not in a parent-progeny relationship (e.g. many ectoparasites, aerosols or water, contact with surfaces) Horizontal transmission Cymothoa exigua 10  The Parasite’s To Do List 1) Achieve transmission & enter host f) High reproductive rates (e.g. asexual stages, polyembryony or behavioral adaptations) Gravid proglottid of the tapeworm Echinococcus multilocularis 11  Horizontal transmission Cymothoa exigua Swelling of genitalia caused by T. equiperidum Babesia sp Suganuma et al. 2016. Parasites & Vectors. 9:481 Gravid proglottid of Echinococcus multilocularis 12 Jalovecka et al. 2019. Tr. Parasitol. 35: 356  The Parasite’s To Do List 2) Migrate to appropriate tissue or site within the host ‒ Parasites can preferentially accumulate in some host cells & tissues = tissue tropism ‒ Aberrant migration occurs when infective stages enter an inappropriate (accidental) host Schistosoma mansoni Fasciola hepatica in common bile duct Larvae of the dog hookworm Ancylostoma braziliense causing cutaneous larva migrans  The Parasite’s To Do List 3) Maintain its position within the host ‒ Helminths→ acetabula, hooks, suckers, teeth or other attachment organs ‒ Giardia lamblia → adhesive disc Tapeworm scolices Imprint of Giardia adhesive disc (AD) on villi of the small intestine 14  The Parasite’s To Do List 3) Maintain its position within the host ‒ Plasmodium falciparum → sequestration & rosetting Infected host RBCs 15 Non-infected host RBCs Tr. Parasitol. 33: 309-320 (2017)  The Parasite’s To Do List 4) Finding a mate ‒ Allee effect→ positive correlation between population density and individual fitness ‒ Chemosensory cues → chemoattracts are produced by some intestinal helminths to attract mates (e.g. free sterols in Echinostoma sp) Chai et al 2020. Korean J. Parasitol. 58: 343-371. E. robustum E. cinetorchis E. revolutum E. mekongi 16  The Parasite’s To Do List 5) Successfully reproduce in and release progeny from the host − Propagules move through a portal of exit to the external environment or to the next host − For GI tract parasites → the anus − For sexually transmitted parasites → genital tract − Portal of exit is not always the same location/anatomical structure where the parasite entered the host Hematuria caused by transit and/or lodging of Schistosoma hematobium eggs across the bladder epithelium 17  The Parasite’s To Do List 6) Undergo essential developmental changes within the host − Dramatic developmental changes may occur between hosts → stage-specific changes in gene expression − Leishmania major → 9% of whole genome expression changes as the parasite moves from the sand fly to the human host − T. brucei gambiense → 22% of the genome is differentially expressed between the procyclic form in the tse tse fly and the metacyclic form in mammalian blood − Differentially regulated genes may be involved in nutrient acquisition, calcium signaling, lipid metabolism and response to the host immune system 18  The Parasite’s To Do List 6) Undergo essential developmental changes within the host − Parasite development can be influenced by the external environment, parasite-derived factors, epigenetic mechanisms or co-opting of host-specific signaling molecules 19  The Parasite’s To Do List 6)Undergo essential developmental changes within the host − Some nematodes undergo hypobiosis (developmental arrest) when confronted with unfavourable environmental conditions − Plasmodium maintains premade mRNA molecules inside the P granules of gametocytes → rapid translation inside mosquito gut − T. brucei gambiense slender flagellated forms in mammalian blood produce a stumpy induction factor (SIF) → initiates the transformation of the slender form to the stumpy form (infective to the tse tse fly) − Activation of conserved enzymatic signaling pathways during development (e.g. kinases) − Parasite development can be modulated by vertebrate growth factors, cytokines and other hormones − Epigenetic mechanisms (e.g. histone acetylation/deacetylation, DNA methylation/unmethylation) can alter parasite development 20  The Parasite’s To Do List ❑Epigenetics ▪ The study of phenotypic changes in organisms which are not caused by changes in DNA sequence ▪ Post-translational chemical “tags or marks” such as methyl groups or acetyl groups on histones may be added or removed → effects packing of nucleosomes → accessibility to transcription factors → alters gene expression ▪ Some epigenetic changes (e.g. methylation) may be heritable ▪ Two major ways in which DNA can be modified: 1) Histone modifications (e.g.acetylation/deacetylation, phosphorylation, methylation/demethylation) 2) Direct methylation of DNA sequences 21 21 21 Epigenetic changes and its effect on gene expression Strands of DNA wrapped around histones DNMT= DNA methyltransferase 22  Epigenetic changes and its effect on gene expression Decondensed chromatin (blue lines) 23 Condensed chromatin Modulation of parasite development by vertebrate hormones Host Melatonin Trypanosoma Molec. Biochem. Parasitol. 2012. 181: 1-6 Host melatonin synchronizes Host melatonin can Plasmodium enhance host immune falciparum asexual response to infection replication to natural by activating immune light-dark cycles cells & cytokines 24  The Parasite’s To Do List 7) Cope with physiological changes within the host & evade destruction by the host immune system − Antigenic variation, host immunomodulation/ immunosuppression etc. − To be covered in more detail later 25 Pathology resulting from parasitic infections ❑Pathology resulting from parasitic infections (Chapter 5) ▪ Pathogenicity The ability of an organism to cause disease/harm to the host Genetically determined→ commensals and most opportunistic pathogens lack the inherent ability to cause disease ▪ Virulence ‒ The degree of pathology caused by the organism ‒ Virulence is usually correlated with the ability of pathogen to multiply in the host ‒ Correlated with how much damage the parasite can cause 26 Pathology resulting from parasitic infections ❑Likelihood of disease depends on 3 closely interrelated factors: 1) The status of host defenses 2) Number of parasites present 3) Pathogenicity of those parasites ❑Threshold of disease- the number of parasites required to cause clinical signs & symptoms ‒ Depends on the virulence of the parasite 27  Likelihood of Disease Parasite numbers are the same Rapidly reproducing for the more virulent (red) & less parasite(red) versus slowly virulent (blue) parasite → the reproducing parasite (blue) → more virulent parasite crosses the rapidly reproducing parasite its threshold of disease crosses its threshold of disease 28 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 1) Parasite-induced damage to cells, tissues, and organs 2) Changes in cellular growth patterns 3) Interference with host nutrient acquisition 4) Toxins released by parasites 5) Host immune response to infection 29 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 1) Parasite-induced damage to cells, tissues, and organs ‒ Intestinal or airway obstruction due to Ascaris lumbricoides infection 30 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 1) Parasite-induced damage to cells, tissues, and organs ‒ Mechanical injury to tissues by impingement, penetration through tissue layers or causing cell death Ophthalmic Calcified T. * cysticercosis soilum cysts (Taenia solium) in muscle Invasive Entamoeba Neurocysticercosis histolytica penetrating the (T. solium) Echinococcus. colonic mucosa and granulosus hydatid underlying tissue layers cyst (*) in human liver 31 Parasite-induced changes in cell growth Characteristic size and rate of replication Enlargement of cell size Constraints on cell cycle are disrupted resulting in accelerated proliferation Replacement of cells of one type by cells of a different type 32 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 2) Changes in host cellular growth patterns a. Hypertrophy- increase in cell size which in turn enlarges tissue and organ size ‒ Monocytes & macrophages infected with Leishmania spp. become hypertrophied ‒ Chronic phase Trypanosoma cruzi infections induce cardiac hypertrophy A- uninfected cardiomyocyte B- infected cardiomyocyte Arrows = T.cruzi amastigotes Green= fibronectin 33 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 2) Changes in host cellular growth patterns b. Hyperplasia- increase in cell number (cells remain the same size and retain their functions) ‒ Fasciola hepatica (sheep liver fluke) induce hyperplasia of cells in the bile duct → feeds off newly synthesized cells Hyperplasia Proliferation L of bile duct of the sheep bile duct cells near the caused by F. periphery of hepatica liver (L) tissue J. Pharm. (2015) 5: 48-51. Vet. Pathol (2002). 39: 592-594. 34 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 2) Changes in host cellular growth patterns b. Hyperplasia ‒ The tapeworm Spirometra mansonoides pleurocercoid larvae encysts in rodent muscle & secretes pleurocercoid growth factor (PGF) → accelerated growth in hamster tissues Hamster 9 weeks post-infection with Control (uninfected) D. mansoides 35 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 2) Changes in host cellular growth patterns c. Metaplasia- replacement of delicate cells with more robust cells as a result of chronic irritation or inflammation; usually reversible ‒ Trichinella spiralis (nematode) larva penetrate mammalian skeletal muscle and transform surrounding muscle cells into connective tissue capsule around the developing larva Host skeletal T. spiralis muscle larva (L) surrounded by fibrous L nurse cell (NC) NC 36  Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 2) Changes in host cellular growth patterns c. Metaplasia ‒ Schistosoma hematobium (trematode) infection causes urinary tract epithelial metaplasia → in addition, it increases the likelihood of neoplasia S. hematobium-induced bladder metaplasia. Section of a bladder with keratinized squamous cell carcinoma. Transitional epithelium replaced by hyperkeratotic squamous epithelium (arrowhead) lining the bladder lumen. S. hematobium eggs Tr. Parasitol. 30: 324-332 (2014) 37 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 2) Changes in host cellular growth patterns d. Neoplasia- abnormal proliferation of cells in response to a stimulus, even after the stimulus is removed ‒ The trematodes Opisthorchis viverrini & Clonorchis sinensis can induce carcinomas (e.g. cholangiocarcinoma) Clin. Rev. Microbiol. (2004). 17: 540-552. Intraductal tumour (T) within Tumour (T) of the 38 the common bile duct hepatopancreatic duct Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 2) Changes in host cellular growth patterns d. Neoplasia ‒ Spirocerca lupi (nematode) infection can cause esophageal carcinomas Caudal esophagus of a dog showing multiple parasitic nodules (white arrows) with red Esophageal adult S lupi worms neoplasia (black arrows). induced by Bar = 5.5 cm Spirocerca lupi Veterin. J. 176: 294-309.(2008) 39 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 3) Interference with host nutrient acquisition ‒ Parasites divert host nutrients to themselves ‒ Diphyllobothrium latum tapeworm infection → vitamin B12 deficiency resulting in megaloblastic anemia & neuron demyelinization ‒ Giardia lamblia impairs nutrient absorption→ lactose & fat malabsorption ‒ Giardia, Cryptosporidium & other intestinal protozoa→ impair Na+ absorption & increase luminal Cl- secretion ‒ Plasmodium and hookworm (Ancylostoma or Necator) infections → destruction of erythrocytes →anemia 40 Normal lactose break down and monosaccharide absorption Lactose malabsorption caused by a Giardia infection 41 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 4) Toxins released by parasites ‒ Little evidence for specific toxins produced by protozoa ‒ The digestion of host hemoglobin by Plasmodium sp. releases heme which is toxic to the parasite→ must be detoxified ‒ The process of heme detoxification in Plasmodium produces hemozoin “malaria pigment” which is toxic to the host ‒ Plasmodium falciparum, some parasitic protozoa and other infectious agents produce glycosylphosphatidyl inositol (GPI) which anchors parasite proteins to the cell membrane → induces an inflammatory response 42 from the host immune system The production of hemozoin Structure of hemoglobin Structure of heme Formation of hemozoin (“malaria pigment”) from heme 43 Pathology resulting from parasitic infections ❑Categorizing pathology caused by parasitic infections 5) Host immune response to infection ‒ The pathology of many parasitic infections is caused indirectly as a result a severe inflammatory host immune response = immunopathology ‒ Evolutionary trade off between host protection and parasite destruction (e.g. intestinal helminth infections induce increased host mucus production & enhanced cell division →helps displace the parasite but it impairs secretion of digestive enzymes & absorption of nutrients) ‒ Other examples of immunopathology include cerebral malaria caused by Plasmodium falciparum and chronic tissue granulomas caused by Schistosoma sp ‒ Plasmodium and Trypanosoma cruzi infections can trigger autoimmune responses in the host→ may release/reveal 44 normally sequestered antigens  Cerebral malaria Immunopathology − Inflammatory cytokines & − Plasmodium falciparum vessel occlusion → severe can result in cerebral inflammation & necrosis malaria → reduced − consciousness & other severe neurological symptoms − Parasitized erythrocytes adhere to capillary Infected endothelial cells → RBCs in interferes with blood flow cerebral → hypoxia & anoxia → capillary inflammation & tissue necrosis − Parasitized erythrocytes also attract pro- inflammatory cytokines (e.g. tumour necrosis factor Tissue necrosis alpha and some interleukins) → severe inflammation & cell death 45 Tissue granulomas Schistosomiasis pathology − A structure formed in caused by tissue granulomas host tissue during an inflammatory response Liver granuloma to parasite antigens induced by S. − The host immune mansoni egg system attempts to isolate the parasite antigen and mitigate damage − Gradual build up of host Abdominal ascites (e.g. immune cells around granuloma formation in the parasite antigen the portal vein→ portal (e.g. schistosome egg) hypertension → edema) − Fibroblasts, other immune cells and collagenous material become the dominant cells around the lesion → disrupts organ function 46  Major parasitic groups of animals I. Protozoa- unicellular eukaryotic organisms ❑ Phylum Rhizopoda ❑ Phylum Metamonada ❑ Phylum Apicomplexa ❑ Phylum Kinetoplastida II. Helminths- parasitic worms ❑ Phylum Platyhelminthes- parasitic flatworms ▪ Trematoda- Digeneans ▪ Monogenea ▪ Cestoda ❑ Phylum Nematoda- unsegmented roundworms ▪ Enoplea ▪ Rhabdita III. Arthropods- animals with exoskeleton and jointed appendages ❑ Insects, mites, ticks, lice, fleas 47  Protozoa ❑Protozoan parasites 1) Phylum Rhizopoda ▪ Entamoeba histolytica ▪ Acanthamoeba ▪ Naegleria fowleri 2) Phylum Metamonada ▪ Giardia lamblia (=duodenalis) ▪ Trichomonas vaginalis 3) Phylum Apicomplexa ▪ Plasmodium spp. ▪ Toxoplasma gondii ▪ Cryptosporidium spp. 4) Phylum Kinetoplastida ▪ Leishmania spp. ▪ Trypanosoma spp. 48  Protozoa ❑General Features of Protozoa 1) Feeding & nutrition- most are heterotrophic – Particulate food is acquired by phagocytosis & enclosed in a food vacuole – In some protozoans, the site of phagocytosis is the cytostome (e.g. Balantidium coli) Refractive granule Balantidium coli ingested RBCs nucleus cytostome Enatmoeba histolytica 49  Protozoa ❑General Features of Protozoa 2) Locomotion – Most are motile and move using pseudopodia, cilia or flagella Entamoeba Leishmania donovani Balantidium coli (trophozoite) (promastigote) (trophozoite) kinetoplast cytostome nucleus (mouth) flagellum cilia pseudopod 50  Protozoa ❑General Features of Protozoa 3) Nuclei & mitochondria – Protozoans can have different types of nuclei depending on the arrangement of the nucleolus nucleolus Granular Vesicular Peripheral Complex – Some protozoan cells have degenerate mitochondria- related organelles with reduced functions e.g. (hydrogenosomes & mitosomes) or structures associated with the mitochondrion (e.g. kinetoplast)51  ‒ The origin of mitochondria predates the divergence of Mitochondria eukaryotes & MROs ‒ All known eukaryotes have mitochondria or mitochondria-related organelles (MROs) ‒ MROs are degenerate mitochondria which have reduced function (e.g. hydrogenosomes, mitosomes) ‒ The iron-sulfur cluster (ISC) pathway is ancestral in mitochondria & MROs Burki, F. 2016. Curr. Biol. 26: R408-R412. 52  Yahalomi et al. 2020. A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome. PNAS 117: 5358-5365. H. salmonicola Henneguya salmonicola - myxospores → an animal that lacks aerobic mitochondria Myxobolus squamalis Henneguya salmonicola Mitochondrial nucleosomes (arrows) are present in Myxobolus.squamalis (A) but not Hanneguya salmonicola (B). Electron microscopic image of H. salmonicola showing a mitosome with few cristae (C). → Genome sequencing has revealed that H. salmonicola has lost its mitochondrial genome along with genes involved in aerobic respiration. 53 Kinetoplast ‒ Normally a spherical structure within the mitochondrion which contains its own circular DNA, kinetoplast DNA (kDNA) ‒ The kinetoplast is usually found near the kinetosome (=basal body) but is not involved in motility ‒ Involved in DNA replication & synthesis of mitochondrial proteins ‒ InTrypanosoma, the kinetoplast changes its position during development 54  Protozoa ❑General Features of Protozoa 4) Water balance & excretion – Contractile vacuoles 55  Protozoa ❑General Features of Protozoa 5) Reproduction- asexual & sexual modes ▪ Binary fission (asexual)- a single nucleus undergoes multiple fissions before cell division= schizogony Merozoites infect RBCs Exoerythrocytic schizogony of Plasmodium in the host 56 hepatocyte (liver cell) and the subsequent release of merozoites  Protozoa ❑General Features of Protozoa 5) Reproduction- asexual & sexual modes ▪ Budding (asexual)- when a new organism develops from an outgrowth (bud) → progeny cells are smaller than parental cell, and then grows to adult size (e.g. Plasmodium sp. and internal budding in Toxoplasma) Parent cell Parent cell New progeny cell Parent cell Bud 57  Protozoa ❑General Features of Protozoa 5) Reproduction- asexual & sexual modes ▪ Syngamy (sexual)- fusion of gametes resulting in a zygote ▪ Autogamy or self-fertilization (sexual)- gametes arise from meiosis and fuse to form a zygote in the same individual 58  Protozoa ❑General Features of Protozoa 5) Reproduction- asexual & sexual modes ▪ Conjugation (sexual)- exchange of nuclear material between mating partners that form a temporary union Exchange of micronuclei 59  Protozoa ❑General Features of Protozoa 5) Reproduction- asexual & sexual modes ▪ Cysts- having a resistant, external covering and with greatly reduced or inactive metabolism Toxoplasma Entamoeba Acanthamoeba Giardia cyst oocysts cyst cysts 60  Protozoa ❑Intestinal Protozoa ▪ Many parasitic protozoa inhabit the GI tract as well as the oral cavity and urogenital tract of animals ▪ Many intestinal protozoa are transmitted by the fecal-oral route A protozoan parasite with a direct fecal-oral transmission route 61  Histology of digestive tract wall ❑Layers of the digestive tract wall 1) Mucosa – barrier between lumen & underlying tissue ‒ Mucous membrane- innermost layer; contains exocrine, endocrine and epithelial (enterocyte) cells ‒ Lamina propria- middle layer; connective tissue; contains small blood and lymph vessels, nerve fibers ‒ Muscularis mucosa- outermost layer; sparse layer of smooth muscle; highly folded 2) Submucosa - connective tissue layer → elasticity ‒ Contains larger blood vessels & lymph vessels, immune cells ‒ Stem cells in crypts migrate to villi and mature ‒ Nerve network called submucous (Meissner’s) plexus → controls local activity of the gut 62  Histology of digestive tract wall ❑Layers of the digestive tract wall 3) Muscularis externa – major smooth muscle layer ‒ 2 layers of muscle: circular muscle (inner) and longitudinal muscle (outer) ‒ The myenteric (Auerbach’s) nerve plexus lies between the two muscle layers 4) Serosa ‒ Outer connective tissue covering ‒ Lubricates and prevents friction between digestive organs and surrounding tissues 63 Histology of the digestive tract wall Luminal (apical) side of the digestive tract Villus Mucosa Crypt cells Muscularis mucosa Muscularis externa Basal (serosal) side of the digestive tract 64  Epithelial tissue ❑Epithelium ▪ Cover and line body surfaces ▪ Occurs at surfaces that separate the internal surface of an animal from the environment ▪ e.g. lining of intestinal tract, kidney tubules, fish gills or between different regions of the body ▪ Epithelial cells possess distinct polarity – Apical or luminal (mucosal) – Basal or serosal 65 Epithelial cell structure & body fluid compartments (mucosal) Intracellular fluid (ICF) (serosal) Interstitial fluid (IF) ECF Plasma 66  Epithelial tissue ❑Intestinal mucosal barrier ▪ Intrinsic barrier- tight junctions between cells ▪ Extrinsic barrier- consists of mucus & other epithelial secretions ‒ Mucus is composed of mucins (glycoproteins), salts, and water ‒ Mucus acts as a barrier to diffusion but it is also a barrier to pathogens ‒ If the mucosal barrier is breached, pathogens encounter lymphocytes and other immune cells (e.g. neutrophils, macrophages) 67  Epithelial tissue ❑Epithelial Transport ▪ Substances can be transported across the epithelium by: – Transcellular pathway- can occur via simple diffusion, facilitated diffusion or active transport mechanisms – Paracellular pathway- diffusion between adjacent cells – Transcytosis- transports macromolecules within vesicles through the epithelial cell via endocytosis and exocytosis 68 Epithelial transport Transcellular Paracellular 69  A quick review of transport of substances across the cell membrane The cell membrane is selectively permeable High permeability Low permeability 70  Review of transport of substances across the cell membrane ❑Mechanisms to transport molecules across the plasma membrane 1) Passive transport – Simple (passive) diffusion – Facilitated diffusion 2) Osmosis 3) Active transport – Primary active transport – Secondary active transport 4) Vesicular transport- endocytosis & exocytosis (not covered here) 71 1) Passive transport: Simple (passive) diffusion – All atoms and molecules undergo ceaseless random motions – Internal kinetic energy of random thermal motion drives diffusion – Solutes move from an area of higher concentration to an area of lower concentration – Movement of solutes occurs down a concentration gradient – Passive process (no ATP required) 72  1) Passive transport: Facilitated diffusion- via an ion channels Gated K+ Channel ‒ Aqueous passageway through the membrane ‒ Some are “gated” channels → only open with specific stimuli ‒ Solutes do not bind directly to the inside of channel ‒ Ion selectivity → proteins lining 73 channel & ionic radius 1) Passive transport: Facilitated Diffusion – via a carrier protein Carrier proteins- bind substances on one side of the membrane & transport them to the other side; when a carrier protein binds a substrate, it undergoes a change in shape within the membrane e.g. glucose transporters (GLUTs) Extracellular side Carrier protein Lipid bilayer Intracellular side (cytoplasmic) 74 2) Osmosis and osmotic pressure Solute added to compartment II Fluid in compartment II moves upward since water moves into compartment II, until a new equilibrium is reached − Movement of water can create hydrostatic pressure (pressure exerted by a liquid) across the cell membrane − The amount of external hydrostatic pressure required to oppose the flow of water across a selectively permeable membrane = osmotic pressure 75 2) Osmosis and osmotic pressure Osmotic pressure is determined by the number of solute particles per unit volume of fluid- the mass or chemical nature of the solute does not matter = a colligative property 76 2) Osmosis and osmotic pressure ❑ Osmolarity – Osmotic pressure is determined by the number of solute particles per unit volume of fluid → colligative property – Knowing the osmotic pressure of a solution allows one to predict whether the cell will gain or lose water by osmosis when it interacts with another solution – Some solutes (e.g. Na+, Cl-, K+, Ca2+) can create persistent osmotic pressure gradients across a cell membrane or an epithelial surface (e.g. wall of capillary vessel) – Physiologists use a convenient concentration-based system to express osmotic pressure referred to as osmolarity, which is a measure of the solute concentration in a solution 77 2) Osmosis and osmotic pressure ❑ Osmolarity – Osmolarity is stated as osmolar (Osm) units- a 1 Osm solution behaves osmotically as if it has 6.022 x 1023 (Avogadro’s number) particles dissolved in 1 liter [similar to molarity (M)] – A 1 Osm solution has the same osmotic pressure as a 1 M solution of a non-electrolyte – Most solutions in animal cells are far more dilute than 1 Osm, therefore they are typically expressed as milliosmolar (mOsm)- e.g. osmolarity of mammalian blood is ~ 300 mOsm, osmolarity of seawater is ~ 1000 mOsm, osmolarity of freshwater is ~ 1 mOsm) 78  Capillary Structure & Function ✓ The capillary endothelial cells have spaces between adjacent cells and are highly permeable to water and small molecules ✓ Fluid, small lipid-soluble & water-soluble substances ( osmotic pressure Interstitial fluid Net fluid movement in Net fluid movement Osmotic out pressure Blood pressure < osmotic pressure 81 Intracellular & Extracellular Concentrations of Major Ions in a Mammalian Cell + ++ + −− −− 82  A quick review of transport of substances across the cell membrane Relative volumes of the intracellular fluid, interstitial fluid & plasma The extracellular fluid and intracellular fluid are isotonic with each other but have different chemical compositions 83  A quick review of transport of substances across the cell membrane Homeostasis does not mean equilibrium The body fluid compartments are in a state of chemical disequilibrium 84  A quick review of transport of substances across the cell membrane 4. Active transport- Primary active transport Na+-K+ ATPase (“pump”) Energy (ATP) is used to move solutes across the membrane against an electrochemical gradient 85  A quick review of transport of substances across the cell membrane 4. Active transport- Secondary Active Transport (monosaccharides or Extracellular amino acids) (monosaccharides or amino acids) e.g. Na+-glucose co-transporters Intracellular (SGLTs), Na+- amino acids co- transporters, Na+- K+-2Cl- (NKCC) co- transporter 86  Transport mechanisms in the small intestine: uptake of Na+ and glucose SGLT= sodium- GLUT= glucose glucose transporter Facilitated transporter diffusion via a carrier protein Primary Secondary active active transport transport Facilitated diffusion via a channel protein 87  Thiagarajah and Verkman, 2013. Intestinal Transport Mechanisms Curr. Opin. Pharmacol. 13:888-894. LUMEN Interstitial Interstitial fluid fluid SMCT1= short chain fatty acid transporter eNaC= epithelial sodium channel CFTR= cystic fibrosis transmembrane regulator SGLT1= sodium glucose transporter CaCC= calcium activated chloride channel 88 DRA= bicarbonate-chloride exchanger NKCC= Na+-2Cl- -K+ cotransporter  Protozoa ❑GI tract pathology ▪ Diarrhea- increase in the frequency of stools & fluid in stools; mainly affects small intestine ▪ Dysentery- blood and mucus in stools; mainly large intestine ▪ Enterocolitis- inflammation of the mucosal layer of the small and large intestines ▪ Colitis- inflammation of the colon 89  Protozoa ❑GI tract pathology ▪ Types of diarrhea caused by intestinal pathogens ▪ Typically, more than a single type of diarrhea is involved in any one infection 1) Osmotic 2) Inflammatory 3) Secretory 4) Exudative 5) Motility 90  Protozoa ❑GI tract pathology ▪ Types of diarrhea 1) Osmotic- excessive solutes in lumen are caused by: ‒ Malabsorption ‒ Infections that cause impaired solute reabsorption (e.g. sodium, lactose and fats) ‒ Ingestion of non-absorbable carbohydrates (mannitol, sorbitol) or some divalent ions (e.g. Mg2+) ‒ Damage to villus enterocytes ‒ Blunting of villi and/or atrophy of enterocytes ‒ Crypt cell hyperplasia ‒ Decreased villi : crypt cell ratio ‒ Stem cells in the crypt replicate rapidly to replace damaged cells → immature enterocytes replace mature cells at the villus tips ‒ Hypersecretion of ions (e.g. chloride) 91 − Some cytokines & other molecules can stimulate hypersecretion  Protozoa ❑GI tract pathology ▪ Types of diarrhea 2) Inflammatory ‒ An inflammatory immune response may be caused by release of cytokines (e.g. interleukins, tumour necrosis factors, fibroblast growth factors, etc) ‒ Cytokines can overstimulate ion secretion and/or alter paracellular permeability ‒ Cytokines can increase the production of reactive oxygen species (ROS) → damages macromolecules ‒ Epithelial disruption may result in blood in the lumen as well as leukocytes in the feces ‒ Malabsorption, epithelial damage, inflammation and hypersecretion are not mutually exclusive events → all can contribute to diarrhea 92  Protozoa ❑GI tract pathology ▪ Types of diarrhea 2) Inflammatory − Reactive oxygen species (ROS) are produced as a by-product of normal aerobic metabolism − Oxygen atoms of ROS have a high potential to oxidize lipids, proteins, nucleic acids and carbohydrates → impaired functioning − All aerobic organisms have evolved enzymatic and non- enzymatic (e.g. vitamins C and E, carotenoids, glutathione) antioxidant mechanisms to detoxify ROS − ROS play important roles in human health & are involved in age-related disorders, cancer, neurodegeneration & in the immune system (e.g. host-generated ROS by macrophages is used to kill intracellular parasites) 93  Protozoa ❑GI tract pathology ▪ Types of diarrhea 2) Inflammatory ROS Nitric oxide radical Anti-oxidant enzymes 94  Protozoa ❑GI tract pathology ▪ Types of diarrhea 3) Secretory ‒ Overstimulation of intestinal secretory cells by cytokines or other molecules ‒ Typically associated with toxins produced by bacteria (e.g. Clostridium difficile, pathogenic Escherichia coli strains, Vibrio cholerae) ‒ Intestinal inflammation caused by celiac disease or inflammatory bowel disease may result in excessive secretion of fluids from cells as well ‒ Protozoan-specific toxins that cause secretory diarrhea have not been identified yet 95  Protozoa ❑GI tract pathology ▪ Types of diarrhea 4) Exudative − Disruption of tight junction protein complex and/or death of large numbers of epithelial cells → epithelial cells are sloughed off or lost − Large quantities of blood, pus, and proteinaceous material show up in the feces 5) Motility − Increased smooth muscle contractions → increased intestinal propulsion (peristalsis) of stools 96