This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Biology 442 Biology
Developmental Biology Developmental
Lecture 13 Cell-Cell Adhesion and Morphogenesis Morphogenesis The Morphogenetic Movements of Gastrulation Generate the 3 Germ Layers Which Will Interact to Form the Organ Rudiments and Produce a Recognizable Body Organization. Produce How do these interactions take place and what molecules are involved ?
Cell Adhesion is a driving force in Morphogenesis – Cell the organization of cells in to tissues and organs !! the Early studies established “differential” cell adhesion “differential” as a major mechanism for tissue organization. as
1955 Townes and Holtfreter – dissociated neural plate cells 1955 and prospective epidermis cells from newt embryos and mixed them together. them The cells first formed aggregates in which the two cell types were equally mixed. With time cells began to move within the aggregates and neural plate cells sorted to the inside and epidermal cells sorted to the outside – just like in a real embryo. The same pattern of “sorting out” was “sorting observed when other germ layer tissues were disassociated and mixed as single cells. 2 key points from key these experiments: these Cells from different germ layers sort out from each other. Positions occupied by cells in the sorted out embryoids Positions reflects their relative positions in normal embryos. reflects Selective cell affinities help establish and maintain spatial Selective order in the embryo order Cell-Cell adhesive strength determines the sorting behavior of cells cells
Cells that adhere more strongly to each other sort to the more interior position when different cell types are mixed. Different cell types have different kinds and amounts of adhesive proteins on their surfaces. Adhesive strength Isolation and Identification of Cell Adhesion Molecules Isolation
1. Raise antibody to membranes from adhesive cells. 1. Determine whether antibodies can inhibit aggregation of adhesive cells. 1. Identify and purify the protein that the antibodies recognize 1. Determine whether that protein added to FAB can block the ability of the FAB to block cell aggregation. Using the methods just discussed a number of different kinds of adhesion molecules have been identified. Major cell adhesion molecules (CAMs) are integral membrane proteins integral
Mediate homotypic cell-cell adhesion – Cadherins -- in junctons Cadherins -- in – Immunoglobulin (Ig) super family – cell to cell Mediate heterotypic adhesion Mediate – Integrins – cell to substrate adhesion Integrins – Selectins – adhesion to carbohydrates usually on glycoproteins on Ig-superfamily – N-CAMS mediate cell adhesion Ig-superfamily Integral membrane proteins
with 3 domains. Ig domain of one N-CAM binds to the Ig domain of an opposing N-CAM Multiple N-CAMs Ca+2-independent If antibodies to N-CAM are added to cultured neurons, the neurons dissociate PSA = polysialic acid domain – PSA modifies adhesiveness of N-CAM –neg charged – charge repulsion N-CAM molecules can be made either adhesive or repulsive by modifying them with sialic acid residues. Sialic acid is a complex (10-carbon) saccharide that carries a negative charge. Sialic acid can severely reduce the adhesiveness of one cell toward another. If the neighboring cells both express a low-sialic acid form of N-CAM, strong cell adhesion is promoted. If the cells contain the highly sialated forms of N-CAM, cell adhesion will be weakened or inhibited. Cadherins mediate cell-cell adhesion Cadherins Cadherins are integral membrane protein Multiple types of cadherins; each type mediates adhesion of different types of cells (P, E, N, protocadherins) Extracellular domain has Ca+2 binding sites: Adhesion is Ca+2dependent. Degraded in absence of Ca++ Anchored by Catenins that couple them to cytoskeleton – integrate epithelial cells into a mechanical unit. Cadherins are found in 2 types of cell junctions Adherens junctions Adherens
Adherens junctions coordinate cell-cell activity They contain adaptor proteins that connect cadherin to They actin and myosin filaments actin • Actin-myosin
filaments form a “belt” around each cell that allows a group of cells to coordinate cell shape and shape changes. F- actin Cadherin • They are frequently associated with tight Adaptor protein junctions Desmosomes Desmosomes
Important for structural support/mechanical strength of Important tissue (e.g. skin epithelia) tissue Cadherin proteins connect to adaptor proteins that Cadherin connect to intermediate filaments connect Autoimmune disease: patients make antibodies to Autoimmune adaptor protein → skin and mucus membrane blisters skin Intermed filament cadherin Adaptor protein Cadherins join cells together by binding to the same type of cadherin on a neighboring cell. of Cells with E-cadherin stick to other cells with E-cadherin Cells with N-cadherin stick to other cells with N-cadherin If neural cells do not express N-cadherin the neural tube will not separate from the epidermis during neurulation. When tissue layers split from each other they will express different cadherins. Cells that express different amounts of the same cadherin will differ in the strength of their adhesion. If adult epithelial cells lose E-cadherin, they can become malignant. Selectins: Mediate cell movement from one location to another another Lectin domain binds to carbohydrates Leukocytes and Neutrophils (wbc) must move from the blood steam to sites of tissue infection and inflammation and bind to selectins in the process of exiting the blood vessel. Adhesion molecules can play a signaling role. Juxtacrin signaling is mediated through transmembrane proteins proteins Cells are supported and/or surrounded by an extracellular matrix (ECM) extracellular
The matrix consists of a meshwork of fibers and a gel like substance that attracts water. The composition and consistency of the extracellular matrix differs in different cells and tissues. In bones and teeth the matrix is harden by calcification. In bones In tendons the matrix is more fibrous and is maximized for In tendons tensile strength. tensile In the cornea the matrix forms transparent layers. Extracellular matrix between an epithelial cells which secretes a tight laminin based matrix (basal lamina) and a mesenchymal cell that secretes a loose reticular lamina made predominantly of collagen. The extracellular matrix influences cell division, cell shape, cell migration and cell differentiation. The extracellular matrix components bind to cellular receptors – the Integrin Receptors. the Binding of Integrin receptors to extracellullar matrix components can induce changes in gene expression and/or reorganization of the cytoskeleton. The extracellular matrix is composed of a variety of molecules that are secreted by exocytosis from the producing cells into the extracellular space where they selfassemble into the matrix. Components that make up the Extracellular Matrix Components Gel Like Components of ECM include the Gel glycosaminoglycan and proteoglycans. glycosaminoglycan Meshwork of the ECM is composed of fibrous Meshwork glycoproteins. glycoproteins Integrin Receptors mediate cell adhesion to the ECM ECM Glycosaminoglycans: Long unbranched polysaccharide chains Glycosaminoglycans: comoposed of repeating disaccharide units – one sugar is an amino sugar (N-acetylgalactosamine or N-acetylglucosamine) and the other a uronic acid. Most glycosaminoglycans are covalently linked to core proteins which form the side chains of a longer glycosaminoglycan. Most abundant glycosaminoglycans are hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparin, heparan sulfate and keratan sulfate. sulfate Glycosaminoglycans attract water, occupy space and resist compression. They are also involved in cell signaling by binding morphogens. Fibrous Glycoprotein components of the ECM: Fibrous 1) Collagen is the most abundant glycoprotein in the ECM 1) • Collagen prevents tissues/cells from stretching and changing shape • Basic structure: triple helix • Helix is 1 molecule Multiple molecules overlap. Overlaps stabilized by
covalent bonds → fibriles • Fibriles fit
together to form fibers and sheets (think and thin) • Fibers are very strong Collagen fibers have a characteristic repeating pattern There are multiple types of collagen There • Type 1: Connects muscle to Type bone (large fibers) bone • Type II: Major constituent of Type cartilege (small fibers) cartilege • Type IV: Forms the basal Type lamina (basement membrane) lamina Basal lamina is a thick ECM sheet that connects cell receptors to the ECM 2) Fibronectin organizes the ECM: It is a dimer with the 2 chains linked by disulfied bonds a the COOH terminus of the chains. It contains binding sites for cross-linking all of the other components of the ECM and for binding cells. Integrin receptors on cells bind to fibronectin through the RGD tripeptide although other domains contribute to the binding. RGD tripeptide is also found in other ECM components (fibroginin, laminin etc.) 3) Laminins are another family of glycoproteins that play an 3) organizational role in the ECM. organizational Integrins Adhere to Substrate Integrins Integrins contain two subunits (α+β) and bind to ligands with the sequence arg-gly-asp (RGD) fibronectin/laminin (extracellular matrix) and ICAM cell adhesion molecules contain RGD binding site Integrins mediate adhesion of cells to substrate and signal transduction from outside the cell Binding to ligand can mediate a conformational change that initiates signal transduction Many different integrins (over 40 alpha and beta chains, α3β1, α11bβ3, etc.) Integrins can signal 1) cytoskeletal reorganization by coupling to the actin filaments through talin and vinculin and 2) changes in gene expression through the MAP Kinase pathway Activates Map kinase pathway and usually signals cell growth Cell surface proteoglycans are another type of molecule that links cells to the extracellular matrix. Extracellular domain contains different glycosaminoglycans that interact with collagens, fibronectin and other ECM components. Close to the plasma membrane there is a protease cleavage site that, when cleaved releases the extracellular domain. The cytoplasmic domain may link to the cytoskeleton. These molecule, like the integrins can also function in cell signaling Schematic organization of the ECM When we looked at gastrulation we saw several examples of cells requiring interaction with the extracellular matrix for migration. It can serve as a “path” that cells follow. Here is an example of basement membrane directed gene expression in mammary gland tissue development. When placed on plastic (A) mammary gland cells divide and grow but genes for differentiation are not expressed. When they are placed on a basal lamina they start to express differentiation genes in concert with increased integrin-lamina interaction. ...
View Full Document
This note was uploaded on 12/09/2010 for the course BIOL 442 taught by Professor Brewster,r during the Spring '08 term at UMBC.
- Spring '08
- Developmental Biology