Cell Membrane, Part 3 - Cell Junctions

tight communities of cells

• those that hold cells together

• consist of multi protein complexes that provide contact btwn neighboring cells or btwn a cell and ECM

• bond cells together • reduce stress placed upon cells • build up paracellular barrier of epithelia • control paracellular transport • enable communication btwn neighboring cells

• epithelial tissues 2. connective tissues

• ex: lining of gut or epidermal covering of skin • cells tightly bound together into sheets called epithelia • direct attachments btwn cells are made by cell-cell junctions • cytoskeletal filaments are anchored onto these junctions and also linked to basal lamina through cell-matrix junctions

• ex: bone or tendon • formed from an ECM produced by cells that are distributed sparsely in matrix (fibroblasts) • matrix bears most of the mechanical stress to which tissue is subjected, not cells • direct attachments btwn cells are rare • cells have important attachments to matrix via cell-matrix junctions

• cardiac muscle • smooth muscle • nervous tissue

• tight junctions • cell-cell anchoring junctions • adherens junction • desmosomes • gap junctions (communicating junctions) • channel-forming junctions

• cell-matrix anchoring junctions • actin linked cell-matrix junctions • hemidesmosomes

• seal gap btwn epithelial cells • seal the intercellular space from luminal environment • seal is not absolute • impermeable to macromolecules • permeability to ions and other small molecule varies • serve as selective permeability barriers for paracellular transport • mostly at apical location • found in epithelial cells • ex: lining of intestinal mucosa, bladder, etc

• depends on transmembrane adhesion proteins called CADHERINS (calcium-dependent adhesion) that span membrane, with one end linking to cytoskeleton and other end linking to structures outside it • anchoring junctions form strong, membrane-spanning structures that are tethered inside cell to the tension-bearing filaments of the cytoskeleton • widely distributed in tissues and most abundant in tissues that are subjected to severe mechanical stress such as heart, muscle and epidermis

• adherens junctions (desmosomes): hold cells together and are formed by transmembrane adhesion proteins that belong to CADHERIN family • focal adhesions and hemidesmosomes bind cells to the eCM and are formed by transmembrane proteins of the INTEGRIN family

• aka zonula adherens, intermediate junction, “belt desmosome” or adhesion belt • connect one actin filament bundle in one cell with that in the next cell • provide anchorage site for cytoskeleton • help modeling shape of multicellular structures • guides organization of developing tissues • ex: construction of vertebrate nervous system • important in tissue remodeling • located more basal than tight junctions • found in epithelial cells, cardiac muscle, fibroblasts

• aka macula adherens • connect intermediate filaments in one cell to those in the next cell • provide anchorage site for cytoskeleton • structurally similar to adherens junctions but contain specialized proteins that link with intermediate filaments instead of actin • main function: provide mechanical strength • located in tissues that suffer a lot of stress/stretch like skin, intestines, stomach, bladder

• allow passage of small water-soluble molecules from cell to cell • without having to pass through plasma membrane • ex: ions, glucose • gap is narrow • very important in tissues containing electrically excitable cells • action potentials can spread rapidly from cell to cell • ex: cardiac contraction, peristaltic movements of intestines • located in connective tissue, epithelial tissue, cardiac muscle, neurons…

• depend on interns (transmembrane proteins (receptors)) like cadherins that bridge cell-cell and cell-matrix interactions • activated after binding ECM ligands or intracellular activator proteins, and produce intracellular signals that can influence almost an aspects of cell behavior (proliferation, survival, polarity and migration) • sense and respond to mechanical forces acting across junction

• aka focal adhesion • anchor actin filaments in cell to ECM • can be small and transient or large and durable • ex: focal adhesions of fibroblasts to a culture dish; myotendinous junctions attaching muscle cells to their tendons

• anchor intermediate filaments in a cell to ECM • similar in form to desmosomes • instead of desmogleins, hemidesmosomes use integrins • found in epithelial cells connecting basal face of cell to basal lamina