The cadherins are anchored (inside or outside) the cell by a complex of proteins called cateninsinsidecadherin-catenin complex forms __________ junctions that help hold epithelial cells together.adherens
Complex binds to the actin cytoskeleton of the cell and integrate epithelial cells into a mechanical unitTwo ways that can prevent the formation of epithelial tissues and cause the cells to disaggregatea) Blocking cadherin functioning (by binding antibodies that disable cadherin)
b) blocking cadherin synthesis (preventing cadherin translation)Three main functions of cadherins1. external domains serve to adhere cells together
2. link to and assemble the actin cytoskeleton, providing the mechanical forces to form cytoskeletal parts
3. can initiate signals that can lead to changes in a cell's gene expressionE-cadherinEarly development: the formation and migration of the epiblast
Later development: restricted to epithelial tissuesP-cadherinplacenta, where it helps the placenta stick to the uterusN-cadherinneural cadherin
- central nervous systemR-cadherinretina formationProtocadherinsFamily of non-classical cadherins. They are produced through alternative splicing, which results in them all having different extracellular domains, but the same intracellular domain. They are used as transient signals in nervous system development.extracellular matrixan insoluble network consisting of macromolecules secreted by cells.
- made up of collagen, proteoglycans,
and a variety of specialized glycoprotein molecules, such as fibronectin and laminin.ProteoglycansA glycoprotein in the extracellular matrix
- critical roles in the ability of matrices to present informative cuesTwo of the most widespread proteoglycansHeparan sulfate and chondroitin sulfateHeparin sulfatecan bind many members of paracrine families and is essential for presenting the paracrine factors in high concentrations to its receptors.Fibronectin(also a glycoprotein)
an intermediary adhesive molecule, linking cells to one another and to other substrates such as
collagen and proteoglycansFibronectin can bind and interact with cell surface molecules such asintegrins___________ forms the "roads" which cells travel on when they are migratingFibronectinLaminin(also a glycoprotein)
plays a role in assembling the extracellular matrix, promoting cell adhesion and growth, changing cell shape, and permitting cell migrationIntegrin familyfamily of receptor proteins is called integrins because they integrate the extracellular and intracellular scaffolds, allowing them to work togetherarginine-glycine-aspartate (RGD)sequence found on the extracellular matrix of many adhesion proteins (integrins, fribronectin)talin and α-actininfound in the intracellular matrix and connect to actin filaments .
Bind to integrinsEpithelialmesenchymal Transition (EMT)an orderly series of events whereby epithelial cells are transformed into mesenchymal cells
- stationary epithelial cell, becomes a migratory mesenchymal cell that can invade tissues and help form organs in new placeswhat initiates an EMTparacrine factors from neighbouring cells alter gene expression to down regulate their expression of cadherins, releasing their attachmentsExamples of EMTs- the formation of neural crest cells from the dorsal most region of the neural tube
- cancer metastasisSteps of EMT1. Epithelial cell receives signal from paracrine factors
2. Cell adhesion is broken and the basal lamina is dissolved
3. cell released from basal lamina (now a mesenchymal cell)Induction: Vertebrate eyethe precise arrangement of tissues in the eye cannot be disturbed without impairing its function. The coordination in the construction of the lens and retina is accomplished by one group of cells communicating an organizing change in the behaviour or developmental trajectory of an
adjacent set of cells. = INDUCTIONTwo components of every induction interactionInducer and responderInducerthe tissue that produces a signal (or signals) that changes the cellular behavior of the other tissue. Often this signal is a secreted protein called a paracrine factor.Responderis the cell or tissue being induced. Cells of the responding tissue must have both a receptor protein for the inducing factor and the ability to respond to the signal.The ability to receive and respond to a specific inductive signal is calledCompetenceVertebrate eye developmentLens cells and optic vessel cells co-construct each other, and the eye forms from reciprocal paracrine interactions.Reciprocal inductionA common sequential feature of induction: One tissue induces another, and that tissue then acts back on the original inducing tissue and induces it, thus the inducer becomes the induced.Instructive interactionA mode of inductive interaction in which a signal from the inducing cell is necessary for initiating new gene expression in the responding cell.Permissive interactionInductive interaction in which the responding tissue has already been specified, and needs only an environment that allows the expression of these traits.Doris Taylor's:
rebuilding of a beating heart- used detergents to remove all the cells from a cadaveric rat heart, which left behind the natural extracellular matrix
- infused this extracellular matrix scaffold with cardiomyocyte progenitor cells
- these cells differentiated and organized into a functionally contracting "recellularized" heart
- Therefore, the environmental conditions of the decellularized extracellular matrix were equipped with instructive guidance for the development of heart muscle.
Example of: permissive interactionWhen membrane proteins on one cell surface interact with receptor proteins on adjacent cell surfaces, the interaction is called aJuxtacrine interaction
Example: CadherinsWhen proteins synthesized by one cell can diffuse over a distance to induce changes in neighboring cells, the interaction is called aParacrine interactionWhen a cell synthesizes a molecule for which it has its own receptor (secretes a paracrine factor which itself can bind); the interaction is a:Autocrine interaction
Ex. placental cytotrophoblast cellsgradients of paracrine factors that regulate gene expressionMorphogens.
- determine the fate of a cell by its concentrationactivin bead experiment- Activin-secreting beads were placed on unspecified cells from an early Xenopus embryo.
- The Activin then diffused from the beads.
- The cells had 3 distinct outcomes depending on the concentration of actin they were exposed to
- the [actin] activated a different set of genes and also inhibit the genes induced at other concentrationsHigh actin concentrationgoosecoid gene expressedlow actin concentrationXbra gene expressedminimal to zero actin concentrationneither gene activated, unspecified cellsResponse resulting from an inducersignal transduction cascadesignal transduction cascadesPathways of response where paracrine factors bind to a receptor that initiates a series of enzymatic reactions within the cell that have often several responses as their end point, such as the regulation of transcription factors and/or the regulation of the cytoskeletonreceptor tyrosine kinase (RTK)A receptor that spans the cell membrane and has an extracellular region, a transmembrane region, and a cytoplasmic region. Ligand (paracrine factor) binding to the extracellular domain causes a conformational change in the receptor's cytoplasmic domains, activating kinase activity that uses ATP to phosphorylate specific tyrosine residues of particular proteins.Four major paracrine families1. The fibroblast growth factor (FGF) family
2. The Hedgehog family
3. The Wnt family
4. The TGF-β superfamilySubfamilies of TGF-β superfamilyTGF-β family, the Activin family, the bone morphogenetic proteins (BMPs), the Nodal proteinsReceptor tyrosine kinase pathway1. Ligand binds to a dormant tyrosine kinase domain
2. ligand induces a conformation change in the receptor structure
3. RTK can be phosphorylated
4. Inactive responding protein comes by, ATP⟶ADP, leaves phosphorylated and ACTIVEFibroblast growth factors (FGFs)A family of paracrine factors that regulate cell proliferation and differentiation.Fgf1 Proteinacidic protein structures, cellular regeneration, apoptosis or necrosisFgf2 ProteinBasic FGF, angiogenesis (blood vessel formation)Fgf7 Proteinimportant for skin development (interacts with collagen). Keratinocyte growth factor; skin developmentFgf8 ProteinSegmentation!! Patterns of dorsal or ventral axis developments. limb development, lens inductionFGFs often work by activating a set of receptor tyrosine kinases called thefibroblast growth factor receptors (FGFRs)RTK pathway + FGF1. FGF binds to FGFRs
2. RTK is phosphorylated
3. intermediate protein GEF is activated
4. GEF activates the Ras G protein by allowing phosphorylation of the GDP-bound Ras
5. GAP desphosphorylates Ras (now inactive)
6. Raf activates MEK, which activates ERK (into the nucleus)
7. Phosphorylated and now Active transcription factor is able to alter gene expressionSimplified version of FGF pathwayLigand
RTK
GEF
Ras
Raf
MEK
ERK
TF
Transcriptionfibroblast growth factor can activate these two pathwaysRTK and JAK/STATJAK-STAT pathway: casein gene activation.The gene for casein is activated during the final phase of mammary gland development, and hormone prolactin secretionJAK-STAT pathwayThe cascade starts when a paracrine factor is bound by the extracellular domain of a receptor that spans the cell membrane, with the cytoplasmic domain of the receptor being linked to JAK (Janus kinase) proteins. The binding of paracrine factor to the receptor activates the JAK kinases and causes them to phosphorylate the STAT (signal transducers and activators of transcription. The phosphorylated STAT is a transcription
factor that can now enter the nucleus and bind to its enhancers.JAK-STAT pathway simpliedLigand
Receptor
JAK
STAT
STAT dimerization
Transcription (along the Casein gene)the JAK-STAT pathway is critically important in regulatinghuman fetal bone growth
Mutations that prematurely activate the STAT pathway have been implicated in some severe forms of dwarfism
- growth plates of the rib and limb bones fail to proliferate
- chondrocytes in the growth plates stop dividing prematurely and the bones fail to growThe original hedgehog gene was found in Drosophila, in which genes are named after their mutant phenotypes:loss-of-function hedgehog mutation causes the fly larva to be covered with pointy denticles on its cuticle (hairlike structures), thus resembling a hedgehog.Desert hedgehog (dhh) protein is found inthe Sertoli cells of the testesIndian hedgehog (ihh) is expressed inthe gut and cartilage
and is important in postnatal bone growthSonic hedgehog (shh)has the greatest number of functions
- responsible for assuring that motor-neurons come only from the ventral portion of the neural tubeHedgehog signaling is capable of regulating these many developmental events because Hedgehog proteins function asmorphogensIn order to secrete the Hedgehog protein, _____ must occurSecretion requires the addition of cholesterol and palmitic acid to the hh proteinIn order for the Hedgehog protein to be secreted as monomers or multimers, packaged as lipoprotein assemblies, or even transported out of the cell within exovesicles, this must occur:cleaving off its carboxyl terminus and associating with both cholesterol and palmitic acid_________ Is critical for Hedgehog to modulate its' extracellular transport AND needed to anchor its' receptor on the cell membranecholesterolPatchedLarge transmembrane receptor which binds HHPatched represses the function of another transmembrane receptor calledSmoothenedIn the absence of Hedgehog binding to PatchedSmoothened is inactive and degradedUpon binding, the Patched protein's shape is altered such that it no longer ___inhibits Smoothened, and Patched enters an endocytic pathway for degradation.In the absence of Hedgehog:
Ci/Gli is cleaved in such a way that a portion of it enters the nucleus and acts as atranscriptional repressorSmoothened releases Ci/Gli from the microtubules (probably by phosphorylation), and the full- length Ci/Gli protein can now enter the nucleus to act as atranscriptional activator of the same genes the cleaved Ci/Gli used to repressHedgehog processing (steps)1. Hh mRNA is translated
2. Autocatalytic cleavage of carboxyl term
3. Modifications (additions of palmitic acid to C-term and cholesterol to N-term
4. Transport to cell membrane
5. Secreted in 1/4 ways4 ways hh can be secreted1. Go through Dispatched and be secreted as a monomer
2. Secreted multimer
3. packaged Lipoprotein assembly (HSPGs)
4. ExovesiclesThe Hedgehog pathway is extremely important invertebrate limb patterning, neural differentiation and pathfinding, retinal and pancreas development, and craniofacial morphogenesis, among many other processescertain chemicals that induce cyclopia do so by interfering with theHedgehog pathwayChemicals found in the corn lily will directly bind to and inhibitSmoothened s' function
- inhibits cholesterol synthesis, which is needed for Hedgehog production and reception.Some human cyclopia syndromes are caused by mutations in genes that encode either Sonic hedgehog or the enzymes that synthesizecholesterolThe Wnts are paracrine factors that make up a large family of cysteine-rich glycoproteins, and are highlyconservedWnt proteins are critical in- establishing the polarity of vertebrate limbs,
- proliferation of stem cells
- in development of the mammalian urogenital system
- guiding the migration of mesenchymal cells and pathfinding axons.Which two families of proteins are synthesized in the ER and are modified by the addition of lipids?Wnt and HedgehogWhat happens if the porcupine gene is lost?Wnt builds up in the ER
- this tells us that the addition of lipids to Wnt is key for transportation to the cell membraneWnt can be secreted by the same mechanisms as hedgehogby free diffusion, by being transported in exovesicles, or by being packaged in lipoprotein particleFunction of Notuma hydrolase that associates with glypican and then cleaves off Wnt's attached lipids in a process of deacylation or delipidationWnt proteins associate with _________________ in the extracellular matrix, which restricts diffusion and leads to a greater accumulation of Wnt closer to the source of productionglypicans (a type of heparan sulfate proteoglycan)When Wnt attaches to the Frizzled receptor on a responding cell, the cell secretes Notum, reducing Wnt signaling because the lipids are essential for Wnt to bind to Frizzled, which creates anegative feedback mechanism for preventing excessive Wnt signaling.canonical Wnt/β-catenin pathwaylipidated Wnt family members interact with a pair of transmembrane receptor proteins: one from the Frizzled family and one large transmembrane protein called LRP5/6canonical Wnt/β-catenin pathway:
In the absence of Wnts- β-catenin is constantly being degraded by a protein
degradation complex containing several proteins
- GSK3 phosphorylates β-catenin so that it will be recognized and degraded by proteosomescanonical Wnt/β-catenin pathway:
Wnts presentthey bring together the Frizzled and LRP5/6 receptors to form a complex.
- enables LRP5/6 to bind both Axin and GSK3, and enables the Frizzled protein to bind Disheveled
- prevents β-catenin from being phosphorylated by GSK3
- β-catenin accumulates and enters the nucleusPlanar cell polarity pathway- certain Wnt proteins signal through Frizzled to activate Disheveled
- leads to the activation of Rho GTPases, such as Rac and RhoA.
- coordinate changes in cytoskeleton organization + cell shape/behaviour change
- works with Janus kinase (JAK) to regulate gene expression.noncanonical (β-catenin independent):
Wnt/Ca2+ pathwayWnt proteins activate Frizzled and Ryk receptors in a way that releases calcium ions from the smooth endoplasmic reticulum (ER) and can result in Ca2+-dependent
gene expression.function of noncanonical Wnt pathwaysIn addition to sending signals to the nucleus, Wnt proteins can also cause changes within the cytoplasm that influence cell function, shape, and behavior.Two types of noncanonical Wnt pathways- Planar cell polarity pathway
- Wnt/Ca2+ pathwayWhich type of Wnt pathway only sends signals to the nucleus?canonical (β-catenin dependent)Function of the PCP pathwayegulate the actin and microtubule cytoskeleton, thus influencing cell shape, and often results in bipolar protrusive behaviors necessary for a cell to migrate
- best known for: instructing cell behaviors along the same spatial plane within a tissue and
hence is called planar polarityWnt/PCP signaling can direct cells todivide in the same plane (rather than forming upper and lower tissue compartments) and to move within that same planeWnt/calcium pathway leads to the release ofcalcium stored within cells, and this
released calcium acts as an important secondary messenger to modulate the function of many downstream targets.each of the three Wnt pathways — β-catenin, PCP, and calcium— all differ yet possess significantcross-interactionsThe TGF-β superfamily includes theTGF-B
BMP
NODAL/ACTIVIN
SMADTGF-β1, 2, 3, and 5 are important in regulating the formation ofthe extracellular matrix between cells and for regulating cell divisionGF-β1 increases the amount ofextracellular matrix that epithelial cells make (both by stimulating collagen and fibronectin synthesis and by inhibiting matrix degradation).What does BMP stand for and what does it do?Bone morphogenic proteins; induce bone formation and regulate cell division, apoptosis (programmed cell death), cell migration, and differentiationBMP1 is not a member of the BMP family at all; rather, it is aprotease.
BMPs are thought to work by diffusion from the cells producing themNodal and Activin proteins are extremely important in specifying the different regions of the mesoderm and for distinguishing theleft-right asymmetry of bilateral organisms
- strongly influenced by a gradient of Nodal from right to left across the
embryoSmads 1 and 5 are activated by theBMP family of TGF-β factorsSmads 2 and 3 are phosphorylated bythe receptors binding Activin, Nodal, and the TGF-β familyThe SMAD pathway resembles what pathway?JAK/STATSmad pathway is activated by...by TGF-β superfamily ligandsSMAD pathway (simplified)TGF-β superfamily ligand
Receptor II
Receptor I
Smad activation
Smad dimerization
New transcriptionHeparan sulfate proteoglycans (HSPGs) in the extracellular matrix often modulatethe stability, reception, diffusion rate, and concentration gradient of FGF, BMP, and Wnt proteinsJuxtacrine interactionsproteins from the inducing cell interact with receptor proteins of adjacent responding cells without diffusing from the producing cellThree of the most widely used families of juxtacrine factors areNotch proteins
Cell adhesion molecules (ex. cadherins)
Ephrin receptors + ligandsEphrins are often seen wherecells are being told where to migrate or where boundaries are formingNotch extends through the cell membrane, and its external surface contacts ______ proteinsDelta, Jagged, or Serrate proteins extending out from an adjacent cell.In both the vertebrate and Drosophila nervous systems, the binding of Delta to Notch tells the receiving cell not to becomeneuralIn the vertebrate eye, the interactions between Notch and its ligands regulate which cells become optic neurons and which becomeglial cellsNotch proteins are involved in the formation of numerous vertebrate organs: _______________ and they are extremely important receptors in the nervous systemkidney, pancreas, and heartCell-to-cell communication can occur between cells in direct contact with one another (________________), or across a distance through cells secreting proteins into the extracellular matrix (________________).juxtacrine signaling
paracrine signalingThe sorting out of one cell type from another results from differences in thecell membraneThe membrane structures responsible for cell sorting are often __________ proteins, cell-cell adhesion molecules that change the surface tension properties of cells adhering to one another.Cadherins:
can cause cells to sort by both quantitative (different amounts of cadherin) and qualitative (different types of cadherin) differences. Cadherins appear to be critical during certain morphological changes.T/F: The extracellular matrix is a source of signals and also serves to modify how such signals may be secreted across cells to influence differentiation and cell migration.TrueComponents of the extracellular matrix (ECM) include:- proteoglycans (such as heparan sulfate and chondroitin sulfate proteoglycan)
- glycoproteins (such as fibronectin and laminin)
- proteins (such as collagen).Cells use these transmembraneadhere to ECM components; on the inside of the cell, integrins are attached to the cytoskeleton; integrins, therefore, integrate the extracellular and intracellular scaffolds, allowing them to work together.Cell migration occurs through changes in theactin cytoskeleton. These changes can be directed by internal instructions (from the nucleus) or by external instructions (such as from the extracellular matrix).Cells can convert from being epithelial to being mesenchymal, this process is calledThe epithelial-mesenchymal transition (EMT)EMT is a series of transformations involved inthe dispersion of neural crest cells and the creation of vertebrae from somitic cells. In adults, EMT is involved in wound healing and cancer metastasis.Inductive interactions involve inducing and responding tissues. The ability to respond to inductive signals depends on thecompetence of the responding cells.Reciprocal induction occurs whenthe two interacting tissues are both inducers and are competent to respond to each other's signals.Cascades of inductive events are responsible fororgan formationParacrine factors are secreted byinducing cellsCompetenceis the ability to bind and to respond to inducers, and it is often the result of a prior induction. Competent cells respond to paracrine factors through signal transduction pathways.Morphogens are secreted signaling molecules that affectgene expression differently at different concentrations.Many paracrine factors fit into four major familieshe fibroblast growth factor (FGF) family, the Hedgehog family, the Wnt family, and the TGF-β superfamily, including Activin, BMPs, Nodal, and Vg1.Signal transduction pathways begin with a paracrine or juxtacrine factor causing aconformational change in its cell membrane receptor. The new shape can result in enzymatic activity in the cytoplasmic domain of the receptor protein. This activity allows the receptor to phosphorylate other cytoplasmic proteins.The cell surface is intimately involved with cell signaling. ____________ and other membrane components can expand or restrict the diffusion of paracrine factors.ProteoglycansNotch-Delta signaling that patterns cell fates throughlateral inhibitionNotch-delta signalling is an example ofJuxtacrine signalling
