Signaling Pathways in Development PDF

SIGNALING PATHWAYS IN DEVELOPMENT BIO 130 LEC INTARMED 2030 | Prof. Bordallo/Leonardo | LU2 SEM 1 | SY. 2023-2024 ○ Changes are caused by binding of phosphate TABLE OF CONTENTS groups or sm...

SIGNALING PATHWAYS IN DEVELOPMENT BIO 130 LEC INTARMED 2030 | Prof. Bordallo/Leonardo | LU2 SEM 1 | SY. 2023-2024 ○ Changes are caused by binding of phosphate TABLE OF CONTENTS groups or small molecules (ex. cAMP, Ca2+) I. Overview of Signaling Pathways ○ Cellular responses: Gene transcription or A. Molecules Involved in Signaling activation of biochemical pathways Pathways B. Cell-Cell Communication over Varying Distances a. Induction C. Epithelial-Mesenchymal Interactions a. Epithelial-Mesenchymal Transitions b. Properties of Interactions II. Paracrine Factors in Induction Figure 1: Signal transduction pathway A. Fibroblast Growth Factor (FGF) Family B. Hedgehog (Hh) Family CELL-TO-CELL COMMUNICATION OVER VARYING C. Wnt Family DISTANCES III. Juxtacrine Signaling A. Notch Pathway Juxtacrine Signaling ➔ Communication across two neighboring cells in direct contact I. OVERVIEW OF SIGNALING PATHWAYS Paracrine Signaling ➔ Communication across short distances through the secretion of proteins into the extracellular MOLECULES INVOLVED IN SIGNALING PATHWAYS matrix Signaling Proteins ➔ Proteins secreted from a cell designed to communicate a response to another cell ➔ Also known as ligands Receptors ➔ Proteins within or part of a membrane that bind other membrane-associated proteins or signaling proteins ➔ Receive and interpret signals ➔ Can represent homophilic or heterophilic binding ◆ Homophilic: Receptor of one cell binds to the same kind in another cell ◆ Heterophilic: Binding between different receptor types Cellular responses can involve gene transcription, metabolism, cell migration, cell growth/division, cell death Figure 2: Juxtacrine vs. Paracrine Signaling Activation of signal transduction pathway INDUCTION _______________________________________________________ ○ Binding of signaling proteins changes the Induction conformation of receptor ➔ Process by which a group of tissue influences the ○ Also changes shape of receptor inside the cell, formation or development of another structure allowing it to activate enzymatic reactions from neighboring cells through signals ○ Signals are relayed through a series of Inducer conformational changes and lead to cellular ➔ Tissue that produces a signal that changes cellular response BIO 130 LEC LU 2 SEM 1 | IMED 2030 Page 1 of 6 MUSA, GS; ACAIN, JJ; SAMSON, SDAV; REYES, JCDC SIGNALING PATHWAYS IN DEVELOPMENT BIO 130 LEC INTARMED 2030 | Prof. Bordallo/Leonardo | LU2 SEM 1 | SY. 2023-2024 behavior of another tissue (extracellular matrix, or ECM, composed of laminin ○ Signals are often proteins called paracrine factors and type IV collagen) Responder ○ Mesenchymal cells are unconnected or loosely ➔ Cell or tissue being induced connected migratory cells, can invade tissues and ○ Must have both a receptor protein for the inducer help form organs in new places and competence - the ability to respond to the EPITHELIAL-MESENCHYMAL TRANSITION ______________________________________________________ signal Reciprocal induction Epithelial-Mesenchymal Transition ➔ Structures formed through induction can induce ➔ Series of events through which epithelial cells are other tissues transformed into mesenchymal cells ➔ Next responding tissues can be the original ○ Usually initiated when paracrine factors from inducer neighboring cells activate gene expression in ➔ Ex. Lens cells induce optic vesicle in the next target cells example ◆ Optic vesicle becomes optic cup (wall Target cells are instructed to: differenntiates into pigmented and neural ○ Downregulate cadherins retina) ○ Release attachment to laminin and other basal ➔ Structure does not need to be fully differentiated lamina components to have a function ○ Rearrange actin cytoskeleton ○ Secrete new ECM molecules for mesenchymal cells Ex. Brain and optic vesicles ○ The eye forms through reciprocal paracrine Role of EMT in development interactions ○ Formation of neural crest cells from dorsal region ○ Only the head ectoderm is competent to respond of neural tube to the signals of the optic vesicle ○ Formation of mesoderm in chick embryos ○ Optic vesicles - Paired regions of the brain that ○ Formation of vertebrae precursor cells from bulge out near the surface ecotoderm of the head somites ○ Vesicles secrete paracrine factors and the head ○ Wound healing in adults ectoderm responds to these BMP4 and Fgf8 EMT in cancer metastasis ○ Genes for lens proteins are induced in head ○ Cells in a tumor mass leave and invade other ectoderm tissues and form secondary tumors ○ Prospective lens cells secrete paracrine factors ○ Cells secrete enzymes that degrade the basal that instruct vesicles to form the retina lamina and mesenchymal ECM as they divide Figure 4: Epithelial-mesenchymal transition during the formation of neural crest from dorsal region of neural tube (B) and formation of mesoderm (C) PROPERTIES OF EPITHELIAL-MESENCHYMAL Figure 3: Induction involved in formation of vertebrate eye INTERACTIONS ______________________________________________________ Regional specificity EPITHELIAL-MESENCHYMAL INTERACTIONS ○ Controlled by mesenchyme ○ Specific structures in responder (epithelium) are Cells in embryo can be epithelial or mesenchymal determined by inducer (mesenchyme) ○ Epithelial cells in this transition are polarized and ○ Example: Skin is composed of an epidermis stationary and interact with the basal lamina (epithelial) and dermis (mesenchymal) BIO 130 LEC LU 2 SEM 1 | IMED 2030 Page 2 of 6 MUSA, GS; ACAIN, JJ; SAMSON, SDAV; REYES, JCDC SIGNALING PATHWAYS IN DEVELOPMENT BIO 130 LEC INTARMED 2030 | Prof. Bordallo/Leonardo | LU2 SEM 1 | SY. 2023-2024 Chick epidermis secretes proteins to signal II. PARACRINE FACTORS IN INDUCTION underlying dermis to form condensations Condensed dermal cells secrete factors to signal epidermis to form regionally specific Paracrine factors diffuse over small distances to cutaneous structures (ex. wing vs. thigh induce changes in neighboring cells feathers vs. scales and claws in feet) Work in a range of about 15 cell diameters (40-200µm) Paracrine factors can rearrange cell surface or change ○ Same epithelium can develop varying cutaneous composition of cell membrane structures according to the region where mesenchyme is from FIBROBLAST GROWTH FACTOR (FGF) FAMILY FGFs ➔ Paracrine factors that activate the receptor tyrosine kinase (RTK) pathway RTKs ➔ Have an extracellular domain that binds ligands and an intracellular domain that projects into cytoplasms and is capable of kinase activity FGF receptors (FGFRs) ➔ Subfamily of RTKs ➔ Specifically bind FGFs onto their extracellular domains Figure 5: Regional specificity in chick embryo ○ Stabilized by heparan sulfate proteoglycans (HSPGs) Genetic specificity Ras ○ Controlled by epithelium ➔ G protein involved in the RTK pathway ➔ Responding epithelium can only comply with ➔ Anchored to the membrane by a lipid acting as a instructions of mesenchyme as far as its genome molecular switch permits ○ When GDP is bound to it, pathway is off ○ Mesenchymal tissue instructions can cross species ○ When GTP is bound to it, pathway is on barriers but epithelial response is species-specific RTK pathway 1. 2 FGFs bind to 2 RTKs causing a conformational change that allows dimerization (2 RTKs fuse to form a dimer) 2. Kinase activity of each RTK in dimer is stimulated → Phosphorylation of tyrosine residues (in each RTK’s intracellular domain) 3. GRB2 (adaptor protein) binds to phosphorylated tyrosine residues 4. Sos [guanine exchange factor (GEF)] binds to GRB2, and comes in contact with Ras 5. Sos catalyzes the exchange of GDP with GTP in Ras, activating it 6. GTP-bound (activated) Ras transmits a signal that initiates the mitogen-activated protein (MAP) kinase cascade 7. Raf or MKKK (MAP kinase kinase kinase) in cytostol: first signaling molecule that binds with the activated Ras 8. Raf activates the MEK protein (MAP kinase kinase) 9. Ras is no longer needed and complexes with Figure 6: Example of genetic specificity GTPase-activating protein (GAP) BIO 130 LEC LU 2 SEM 1 | IMED 2030 Page 3 of 6 MUSA, GS; ACAIN, JJ; SAMSON, SDAV; REYES, JCDC SIGNALING PATHWAYS IN DEVELOPMENT BIO 130 LEC INTARMED 2030 | Prof. Bordallo/Leonardo | LU2 SEM 1 | SY. 2023-2024 10. GTP is hydrolyzed back to GDP, Ras returns to ○ Cholesterol also helps in anchoring Hh to its inactive state receptor ○ Note: Mutations in Ras protein that make Pathway when Hh is absent it oncogenic all inhibit the binding of the 1. Patched (Hh receptor) is not activated, inhibiting GAP protein, making Ras remain in its Smoothened active configuration for a longer time 2. Inhibition of smoothened results in transcription 11. Active MEK phosphorylates the ERK protein (MAP factor Gli or Cubitus interruptus (for Drosophilia) kinase) to be held in cytoplasm 12. ERK enters the nucleus and phosphorylates 3. Smoothened is degraded certain transcription factors, activating gene 4. Gli complexes with cleaving proteins tethered to expression and marking an end to the cascade microtubule like Fused, SuFu (Suppressor of Fused), and PKA (protein kinase A) and is cleaved 5. The cleaved Gli enters the nucleus and acts as a repressor (binds to silencer and inhibits transcription) Pathway when Hh is present 1. Hh binds strongly to Patched with the help of co-receptors (Ihog/Cdo, Boi/Boc, Gas1) 2. Patched is endocytosed and degraded → doesn’t inhibit Smoothened 3. Active Smoothened causes inactivation of cleavage proteins → Gli is released from the microtubules 4. Uncleaved Gli enters the nucleus and acts as an activator Figure 7: Summary of RTK Signal Transduction Pathway Examples of FGFs and their specific functions ○ FGF1 or the acidic FGF: For regeneration and tissue homeostasis ○ FGF2 or the basic FGF: For blood vessel formation ○ FGF7 or the keratinocyte growth factor: For skin development ○ FGF8: For segmentation, limb development, and lens induction Figure 8: Summary of Hh Pathways in Vertebrate Cells HEDGEHOG (Hh) FAMILY Developmental events where Hh proteins are Hedgehog (Hh) proteins active ➔ Paracrine factors involved in the regulation of Gli, ○ Limb patterning a transcription factor ○ Neural differentiation and pathfinding The Hh pathways occur in a primary cilium (consisting ○ Retinal and pancreas development of microtubules) present in almost all vertebrate cells ○ Cranial morphogenesis and is involved in signaling instead of motility ○ Feather formation in the chick embryo Role of cholesterol in Hh secretion ○ Hair formation in mammals ○ If there is little to no cholesterol, Hh diffuses too ○ Skin cancer (when misregulated) quickly before being able to bind to its receptor BIO 130 LEC LU 2 SEM 1 | IMED 2030 Page 4 of 6 MUSA, GS; ACAIN, JJ; SAMSON, SDAV; REYES, JCDC SIGNALING PATHWAYS IN DEVELOPMENT BIO 130 LEC INTARMED 2030 | Prof. Bordallo/Leonardo | LU2 SEM 1 | SY. 2023-2024 Examples of Hhs and their specific functions keeping axin and GSK3 bound to the cell ○ Sonic hedgehog (shh): Assures that motor neurons membrane come only from the ventral portion of the neural 4. Uncomplexed β-catenin is not degraded and tube, that chick feathers are in the right places, instead accumulates in the cytoplasm etc. 5. β-catenin enters the nucleus and is free to ○ Desert hedgehog (dhh): Found in Sertoli cells and bind to LEF/TCF, activating transcription promotes spermatogenesis factors for gene expression ○ Indian hedgehog (ihh): Expressed in the gut and cartilage and is important in postnatal blood growth Additional notes ○ Some human cyclopia syndromes are caused by mutations in genes that encode for Hh proteins and enzymes that synthesize cholesterol ○ Cancers and increased mitogenic effects in cells are also often caused by mutations that activate the Hh pathway (ex: basal cell carcinomas, basal cell nevus syndrome) ○ Proposed solutions to decrease mitogenic effects include synthesizing drugs that inhibit Smoothened function Wnt FAMILY Figure 9: Summary of Canonical Wnt Pathway Wnt proteins ➔ Paracrine factors that are cysteine-rich Developmental events where Wnt proteins are glycoproteins active Wnt secretion ○ Polarity of insect and vertebrate limbs ○ Synthesized in the endoplasmic reticulum ○ Proliferation of stem cells ○ Modified by palmitic and palmitoleic acid ○ Regulation of cell fates along axes of various ○ Catalyzed by the enzyme O-acetyltransferase tissues Porcupine ○ Development of mammalian urogenital system ○ Can be secreted through free diffusion, exosomes, ○ Migration of mesenchymal cells and pathfinding or packaged lipoprotein particles axons ○ Associate with glypicans to restrict diffusion, leading to a greater accumulation of Wnt NON-CANONICAL Wnt PATHWAY (β-catenin CANONICAL Wnt PATHWAY (β-catenin dependent) ______________________________________________________ independent) ______________________________________________________ Wnt is absent (β-catenin degraded) Planar cell polarity pathway: Important in the 1. A protein degradation complex (axin, APC formation of germ layers (adenomatous polyposis coli), GSK3 (glycogen ○ Regulates cell division synthase kinase 3)) targets β-catenin ○ Regulates cell shape and migration 2. GSK3 phosphorylates β-catenin, causing its Wnt/Calcium pathway: Regulates the release of ubiquitination (targeted for degradation by calcium ions from the endoplasmic reticulum, allowing proteasomes) it to activate enzymes, transcription factors, 3. LEF/TCF transcription factors repress translation factors, etc. Wnt-responsive genes Wnt is present (β-catenin accumulates) 1. Wnt binds to its two receptors (Frizzled and LRP5/6) making it a multimeric complex 2. This complex binds axin and GSK3 to it 3. Frizzled binds and activates Disheveled, BIO 130 LEC LU 2 SEM 1 | IMED 2030 Page 5 of 6 MUSA, GS; ACAIN, JJ; SAMSON, SDAV; REYES, JCDC SIGNALING PATHWAYS IN DEVELOPMENT BIO 130 LEC INTARMED 2030 | Prof. Bordallo/Leonardo | LU2 SEM 1 | SY. 2023-2024 Developmental events where the Notch pathway III. JUXTACRINE SIGNALING happens ○ Formation of numerous vertebrate organs (ex: Proteins from inducing cell interact with receptor kidney, pancreas, and heart) proteins of adjacent responding cells without diffusion ○ Development of nervous system ○ Lateral inhibition (transcription factors activate genes that are inhibitors; ex: inhibition of NOTCH PATHWAY differentiation of epithelial cells into neurons) Requires direct contact between the signal on the signaling molecule and the receptor on the target cell Named after the receptor, Notch (instead of the signaling molecule) Signaling molecules: Delta, Jagged, or Serrate proteins Pathway when signal is absent 1. Protease (presenilin-1 protease) is inactivated and stays close to Notch 2. In nucleus, the repressor stays on the transcription factor CSL 3. No gene expression occurs Pathway when signal is present 1. When Delta/Jagged/Serrate proteins find Notch, Notch changes conformation 2. Protease is activated and cleaves part of Notch 3. Cleaved Notch enters nucleus and replaces the repressor by binding to the TF (CSL) instead 4. Notch attracts more activators (ex: P300 or the histone-acetyltransferase) 5. Transcription is turned on and gene expression occurs Figure 10: Summary of Notch Pathway BIO 130 LEC LU 2 SEM 1 | IMED 2030 Page 6 of 6 MUSA, GS; ACAIN, JJ; SAMSON, SDAV; REYES, JCDC