cellular interactions

Transcription

1 cellular interactions chapter 20 tissues cellular interaction in some organisms, cells interact to form defined tissues extracellular matrix allows for cellular interaction extremely important in certain tissues connective tissue epithelial tissue epidermis dermis keratinized cells activelydividing cells epidermis cell to cell contact cell/substrate contact basement membrane extracellular space glycocalyx carbohydrate projections from the plasma membrane basement membrane reticular fiber proteoglycan dermis collagen fiber integrin fibroblast elastic fiber

2 extracellular matrix ECM - present in pretty much all animals evolved in some ancestor of the animals sponges mesohyl basic ECM elements type IV collagen spongin laminins what about plants? special ECM cell wall matrix ECM fibers collagens fibrous glycoproteins most abundant protein in humans very strong structure triple helix arranged in staggered array overlap 25% ECM fibers collagens types I-XIX I-III most common in humans IV - not fibrillar spongin matrix in sponges cuticle of nemata bone cartilage type III - reticular fibers branching network collagen problems collagen-related diseases type I (osteogenesis imperfecta) type II (dwarfism) type IV Alport syndrome kidney disease (glomerular nephritis) blindness (ocular lesions) hearing loss Ehlers-Danlos syndrome hyperflexibility / extensible skin

3 ECM fibers elastic fibers elastin cross-linked into network provide elastic strength important for organs that stretch proteoglycan complexes proteoglycans core protein GAGs (glycosaminoglycans) repeating disaccharide structure cushion cells, help resist crushing elastic fiber fibronectin fibronectin structure glycoproteins found in nearly all animal connective tissue two nearly identical polypeptides fibronectin role in development migration of cells guided by fibronectin provides a substrate for cells to migrate over cells of the neural crest adhere to fibronectin connections can bind to cells and ECM allow cells to attach to ECM

4 basement membrane basement membrane basal lamina flattened layer of ECM ECM secreted by fibroblasts reticular layer lamina reticularis - reticular fibers basement membrane basal lamina 4 main functions structural foundation for epithelium selective membrane facilitate access to proteins serve as guide for developing neurons laminin laminins glycoproteins - triple helix (like collagens) many diverse forms (little homology) form web-like networks resistant to tensile force Molecular and Cell Biology, 6th ed., W.H. Freeman and Co. basal lamina Collagen IV Laminins Nidogens (entactins) Perlecan (proteoglycan)

5 interaction with cells integrins receptors with α and β subunits each with several domains many possible configurations of 26 known subunits contain relatively short transmembrane domain conformations inactive - bent active - unbent / legs apart can adopt many different active configurations αi domain β-propeller β-i domain α subunit β subunit interaction with cells integrins cell-binding sites in extracellular proteins RGD - (arginine-glycine-aspartic acid) recognized integrin binds with RGD sequence two main functions adhesion of cells to substrate transmission of signals from external environment β subunits three main classes: β1, β2, β3 clotting fibrinogen (RGD sequence) binds to integrin clot busters - competitive inhibitors summary collagens - (glycoprotein) - dextral triple helix of 3 sinistral alpha helices - high tensile strength - lots of kinds elastic fibers - cross-linked networks of elastin and fibrillin - allow organs to stretch and recoil proteoglycans - - huge protein-gag complexes - serve as cushioning - form hydrated gels fibronectin - (glycoprotein) - two identical polypeptides - homodimer - bind to integrins - can bind to cells and to ECM fibers - important in migration of cells - anchors cells to proteoglycans or collagen laminins - (glycoprotein) - triple helix of 3 linked polypeptides - high tensile strength - along with collagen IV, comprises basis for basal lamina entactin - (glycoprotein) - connects collagen and laminin networks - web of collagen IV and laminins connected by entactin integrins - - heterodimers - attached to PM - bind to various ECM elements - act as cell receptors cell-ecm junctions hemidesmosomes different class - β4 link to intermediate filaments dense plectin plaque linked to ECM by integrins can be disassembled and reassembled

6 cell-cell junctions junction complex found on lateral aspect of plasma membranes comprise several types of junctions tight junctions complex collections of proteins transmembrane proteins polarity proteins cytoskeletal proteins signaling proteins cell-cell junctions junction complex found on lateral aspect of plasma membranes comprise several types of junctions tight junctions transmembrane proteins claudins occludins junctional adhesion molecule (JAM) function paracellular transport creates membrane polarity: apical, basal membrane domains prevent movement of proteins between domains cell-cell junctions adherens junctions link adjacent cells located just basal to tight junctions transmembrane receptor proteins cadherin dimers bind to identical cadherins on neighboring cells anchor proteins (catenins) link to actin bundles cell-cell junctions septate junctions only in invertebrates similar in function to tight junctions located basal to adherens junctions

7 cell-cell junctions desmosomes cytoplasmic plaques inner dense plaque outer dense plaque connected to intermediate filaments main function: link cells together important for cells exposed to physical stress cell-cell junctions junction complexes (a) tight junctions (b) adherens junctions (c) desmosome (d) hemidesmosome (e) (gap junction) cell-cell communication gap junctions analagous to plasmodesmata vertebrates connexon (2) connexins (6) invertebrates innexon (2) innexins (6) cell membrane cell membrane cytoplasm cytoplasm connexon cell-cell communication membrane nanotubes long tubes between cells allow for exchange hydrophilic channel

8 cell-cell communication plasmodesmata intercellular cytoplasmic channels desmotubule trapped portions of ER passage of materials limited size constraints interconnect protoplasts symplast plasma membrane cell wall plant cell walls comparative cell wall composition for bacteria - mainly peptidoglycan for fungi - chitin Cell wall for plants cellulose cell wall matrix microfibrils 10 μm proteins pectin hemicelluloses Cellulose microfibrils in a plant cell wall 0.5 μm Microfibril Cellulose molecules β Glucose monomer cell wall matrix cell wall matrix microfibrils arranged in radial arrangement restrict cell expansion cell elongation influx of water increases cell volume Nucleus Cellulose microfibrils Elongation Vacuoles composition of cell wall matrix hemicellulose highly branched crosslinked to microfibrils pectin negatively charged galacturonic acids form a gel microfibrils glycoproteins pectin cellulose microfibril hemicellulose 5 µm soluble protein

9 cell wall matrix layers middle lamella (outer layer of pectin) primary cell wall more flexible, laid down first secondary cell wall middle lamella cell wall matrix lignin used to stiffen and strengthen cell walls 2nd most abundant organic polymer hydrophobic polymer p-coumaryl alcohol coniferyl alcohol sinapyl alcohol primary cell wall plasma membrane