Chapter 19 Cell Functions and the ECM

  1. Cell junctions and ECM

    1. Cell cohesion governs the architecture of the body

    2. It is critical for the organization, function, and dynamics of multicellular structures

    3. There are cell-cell junctions and cell-ECM junctions

  2. Connective and epithelial tissues

    1. Epithelial

      1. Cells are connected directly to each other with no matrix between cells 

      2. ECM is at the bottom of the layer of cells and is called the basal lamina

      3. Any stress is tolerated by the cells themselves

    2. Connective

      1. The matrix bears the force of any stress

        1. Skeletal is the strongest tissue organ

      2. The cells are more dispersed, and there is a lot more ECM

      3. Cells do not necessarily connect with each other, but are embedded in the matrix

  3. Cell junctions 

    1. Cell to cell

      1. A junction protein will go through the membrane and bind on the outside of the cell to the junction protein of a neighboring cell

      2. On the inside of the cell, the junction protein will bind to the cytoskeleton

    2. Cadherins

      1. They are classified by their extracellular domain repeats

      2. All animal cells have cadherins, but not all eukaryotes do

      3. All are homophilic binders

        1. They are: classical cadherins (E-cadherins), Fat cadherins, Flamingo, Ret, desmocollin, cadherin 23, protocadherins, T-cadherin

    3. Structure and Function of Cadherins

      1. They bind together at their last EC domains is calcium is present

        1. Calcium being present makes the cadherins stiff so that they can bind

      2. Individual bindings are really weak, but a lot of them together can be strong

        1. This is so the interactions can be broken if needed

    4. Cadherin homophilic binding assists in development

      1. Why

        1. Cells can sort themselves into groups with the same cadherins

          1. They can also sort depending on the concentrations of which cadherins they have

    5. Adherens junctions

      1. The junction proteins are cadherins, and inside, there are adaptor proteins

      2. Adaptor proteins mostly belong to the catenin family

      3. p-120 catenin is bound to beta catenin, which is bound to alpha catenin

      4. The catenins bind to the actin cytoskeleton

    6. Assembly of Adherens junctions

      1. membranes of two cells get close enough for E-cadherins to bind and form a little piece of junction

      2. This triggers a cell signaling pathway and activates Rho family to reorganize actin

        1. It activates the Rho family member Rac

      3. Rac promotes additional protrusions to expand the contact zone

      4. Once they have attached all of the E-cadherins, Rho becomes active

      5. Rho inactivates Rac and stimulates linear bundles of actin

      6. Contractile actin contracts to tighten the junction

    7. Mechanotransduction in adherens junctions

      1. If junctions are exposed to tension that does not break them, they will change and get stronger to resist stress

      2. Tension stretches out alpha catenin, and alpha catenin has additional binding sites for vinculin

      3. Vinculin brings more actin

  4. In epithelia, adherens junctions form the adhesion belt

    1. Adherens junctions are a big patch of cadherins

    2. Across a cell layer, they form a belt structure called the adhesion belt

    3. They are only present about 1/3 of the way from the top of the cell and are present in the same position on all of the neighboring cells

    4. Because they form a belt structure, they can contract in unison and contract the whole cell layer as a unit

      1. An example of this is the formation of the spinal cord during development

  5. Desmosomes provide mechanical strength

    1. intermediate filaments bind adaptors, which bind cadherins (junction proteins), and use the nonclassical cadherins desmoglein and desmocollin, which are homophilic binders

    2. All cadherins are homophilic binders

    3. They look like buttons between cells

  6. Tight junctions

    1. sheets of epithelial cells partition in the human body

    2. All epithelia are polarized (basal vs. apical)

      1. because they have different environments on each side of the layer

    3. All act as selective permeable barriers

    4. Tight junctions seal cells together so that molecules cannot leak freely through the cell sheet

  7. Epithelial cell polarity

    1. Tight junctions make sure nothing can bypass the selective permeability of gut cells

    2. Things only go into the cell if there are transporters for it

  8. Tight junctions prevent the diffusion of molecules and receptor movement to the other side of the cell

    1. added the smallest tracer they could find to both sides of the epithelial cell layer, and none got through to the other side

  9. Tight junctions are branching networks of “sealing strands”

    1. They look like stitching

    2. If one were to break, nothing would get through because there are multiple layers

    3. Junction proteins are claudin and occludin, and they are really short, meaning the cells are very close together

    4. They also keep membrane proteins in the correct place

  10. Scaffold proteins organize junctional protein complexes

    1. Adaptor proteins are zonula occludens (ZOs)

    2. have two major functions: they attach claudin and occluden to the actin cytoskeleton

    3. In addition to being good adaptor proteins, they are also scaffold proteins

      1. They bind claudin and occluden and actin and each other

    4. domains of ZO bind claudin, other ZO, signaling proteins, occludin, and actin

  11. Gap Junctions

    1. Bridge gaps between cells and create direct channels from the cytoplasm of one cell to another

    2. made of connexin proteins in vertebrates, the pore size is 1.4 nm

      1. Only things like H2O molecules, ions, second messengers, and small molecules can fit through

    3. Couples cells, both mechanically and electrically

    4. have open and closed conformations like channels

  12. Structure of a gap junction

    1. called a molecular siv because it looks like a pasta strainer

    2. There is a lot of variability in gap junctions because multiple connexin genes depend on the cell type

      1. Singular proteins that form gap junctions are connexins

      2. 6 connexins together from a pore called a connexon

      3. connexons can be:

        1. homomeric if all connexons are the same

        2. heteromeric if connexins are not the same

      4. 2 connexons combine to form intracellular channels

        1. can be homotypic if made from homomerics

        2. can be heterotypic if made from heteromerics

      5. Homotypic and heterotypic tell us about the halves

        1. could have a homotypic channel made of heteromeric connexons

  13. Plasmodesmata

    1. Plant cell walls are firmly cemented together, so there is no need for adhesion junctions like tight junctions or adherens

    2. The need for communication still remains

    3. The function is similar to gap junctions in animal cells

    4. The plasma membrane of one plant cell is continuous with that of its neighbor cell at each plasmodesmata

      1. Plasmodesmata are very large; whole organelles could fit through

      2. Smooth ER and cytoplasm are shared between neighboring cells

  14. Extracellular matrix

    1. Tissues are made of cells and a complex network of macromolecules called the ECM

    2. Classes of MAcro molecules are similar, but the amount and organization differ

      1. Calcified to form teeth and bone

      2. Transparent cornea

      3. tendons

        1. tendon injury take a long time to heal because they are almost exclusively matrics

      4. play active and complex roles

        1. matrix is an active messenging partner

  15. 3 major classes of Ecm macro molecules

    1. proteoglycans and gags

      1. Made of sugar moleculs and protiens

        1. Gags are sugar molecules

      2. 1 member of the family is just the sugar part

        1. Hyalurona

    2. fibrous proteins

      1. mostly includes collagen family

    3. glycoproteins

      1. organizers of ECM

    4. All tissues have at least one of these

  16. GAGS (glycosaminoglycans)

    1. made of long unbranched polysaccharide chains

      1. repeating disaccharide chains

      2. have - charges ( most have sulfur groups that bear a negative charge)

        1. Positive ions are attracted, and positive ions attract water, making a jelly consistency

    2. help resist compressive forces

    3. all GAGs from hydrated gels to protect tissue from being smashed

  17. Hyaluronan acts as a space filler

    1. gags form hydrated gels by attracting osmotically active cations that suck up water

      1. has huge molecular weight compared to proteins and takes up a lot of space

        1. takes up space because it is big and sugars are stiff, so they can’t compact nicely

  18. Proteoglycans

    1. All gags except hyaluronan are attached ot proteins and are called proteoglycans

    2. Tetrasaccharides are comprised of xylose, galactose, galactose, and glucuronic acid, linking the protein to gag

    3. Proteoglycans have the same function as hyaluronan, used to attach gags to suck up ions and produce hydrated gels

  19. Aggrecan - Example of a proteoglycan

    1. Aggrecan forms aggregates; it sticks together with other Aggrecans and forms an aggrecan aggregate

    2. It is huge and squishy

  20. Fibrous proteins (resist tensile stress)

    1. Collagans

      1. Most abundant protein in mammals

      2. Long, stiff helical proteins that provide structure that provides strength to the ECM

      3. from a triple helix, with lots of proline and glycine

        1. Has to kink backbone so needs proline, has to kink to form helix

        2. Glycine is the only amino acid that fits with proline

    2. Elastin

      1. Gives tissues high flexibility

        1. Skin, lungs, blood vessels, bladder

      2. Network of elastin molecules

        1. Elastin is connected to each other by covalent disulfide bonds

        2. Elastin is intrinsically disordered; it doesn’t have a lot of rigid structure, so it doesn’t need much force to straighten the polypeptide backbone

          1. Overexpansion injury if stretched too far

  21. Glycoproteins - fibronectin

    1. Typically have multiple binding domains

    2. help organize the ECM by binding to multiple elements and receptors on cells

    3. can also respond to tension by binding other fibronectin molecules

  22. Fibronectin

    1. The major organizer fro connective tissue

    2. V-shaped structure with the bottom held together by disulfide bonds

    3. has binding sites for pretty much everything else that exists in the ECM

    4. has 2 beta sheets, 1 has very weak interactions, so tension unfolds it and it recruits more fibronectins when unfolded

  23. Basal lamina

    1. The underpinning of all epithelial sheets

      1. One of the major functions is to pull cells together and keep the epithelial tissue on the connective tissue

    2. structural roles, filtration role, survival, proliferation, differentiation, cell migration, guides synapses

      1. In the kidney glomerulus, the basal lamina and cells help filter blood

      2. The basal lamina surrounds muscle tissue cells

  24. The basal lamina is synthesized by cells on both sides of it

    1. secreted by cells on both sides

  25. Laminin

    1. cross-shaped proteins with binding domains for pretty much every protein in the basement membrane

  26. Laminin is the primary organizer for the basal lamina

    1. cell membrane → cytoskeleton → integrins→ matrix → laimins, grab ll other proteins

  27. Cell to Matrix

    1. Cells make the ECM, organize it, and degrade it, and the matrix exerts powerful effects on the cells

    2. Mediated through adhesion “matrix receptors” (a nickname for integrins), tying the matrix to the cytoskeleton

    3. Integrins play a key role by transmitting mechanical and molecular signals, as well as converting them

  28. Structure of integrins

    1. 24 types of integrins in humans (23 of them interact with actin)

    2. heterodimers (made of alpha and beta subunits)

      1. when they are in the active form, grabbing some type of matrix protein on one side and binding actin through adaptors on the other

      2. Adaptors include: kindlin, talin, and vinculin

    3. They go through the membrane and bind to the cytoskeleton on one side through adaptor proteins and to the matrix on the other side

  29. Integrins switch between active and inactive states in response to intracellular signaling

    1. They have to exist in an inactive form, where they are coiled and not bound to the matrix, and an active form where they are bound to the matrix

      1. For cell crawling, they have to make this transition

  30. Most prominent cell-matrix interactions in epithelia are hemidesmosomes

    1. a ½ desmosome, sticks its fingers down into the matrix and anchors the epithelial sheet to the basal lamina instead of connecting to other hemidesmosomes

  31. Many diseases are caused by defects in integrins

    1. defects in many other junctions we have seen lead to severe blistering diseases

    2. many can lead to death

    3. Blistering can lead to severe dehydration

  32. Focal Adhesion Kinase (FAK) phosphorylates

    1. binds to integrins

    2. binding and letting go allow for cell crawling

  33. Plant cell wall - an elaborate extracellular matrix

    1. The walls of cells are cemented together, forming an ECM that is thicker, stronger, and more rigid than the ECM in animal cells

    2. varies in composition and origin, but has the same general features shared with animals

      1. One component provides tensile strength, and another resists compression

    3. Made almost entirely of polymers that lack nitrogen

      1. Lack of nitrogen means that it lacks proteins because nitrogen is in the protein backbone

  34. Cell walls are largely supported by cellulose and pectin

    1. Pectin resists compressive forces

      1. Pectin is a carbohydrate that sucks up ions and water

    2. Cellulose resists tensile forces

      1. a very long, straight, stiff carbohydrate, and it is placed in alternate directions in layers

  35. The tensile strength of cell walls allows plants to develop turgor pressure

    1. The concentration of a solute externally is less than in the cell walls, which is less than in the cytoplasm

    2. Large internal hydrostatic pressure that pushes outward on the cell wall

    3. This pressure is vital to plants; it drives expansion and provides mechanical rigidity

  36. Turgor pressure also drives the expansion of cells

    1. Cells can either grow vertically or horizontally in response to turgor pressure

      1. which direction they go depends on the direction the cellulose fibers are lying

      2. The cellulose microfibrils are perpendicular to the axis of elongation