cell junctions & the ECM

connective tissue

one of the two major tissue types, ECM directly bears mechanical stress of tension and compression, cells interact with matrix

epithelial tissue

one of the two major tissue types, linings of the cells directly connected, ECM on bottom of layer (separated by basal lamina), mechanical stress transmitted from cell to cell by cytoskeletal filaments anchored to cell-matrix and cell-cell adhesion sites

what are two cell-matrix junctions

actin linked, hemidesmosome

actin linked cell-matrix junction

anchors actin filaments in cell to ECM

hemidesmosome

anchors intermediate filaments in a cell to ECM

tight junctions

seal gaps between epithelial cells

adherens junctions

connect actin filament bundles in one cell with that in the next cell

desmosomes

connect intermediate filaments in one cell to those in the next

gap junctions

allow the passage of small, water soluble molecules from cell to cell

name two cell-cell anchoring junctions

adherens junction and desmosome

what superfamily is cell-cell adhesions

cadherins

what superfamily is cell-matrix adhesions

integrins

what cytoskeletal attachment do hemidesmosomes use

intermediate filaments

adherens adaptor proteins

catenins and vinculin

desmosome adaptor proteins

plakoglobin (gamma catenin), plakophilin, desmoplakin

actin linked cell-matrix junction adaptor proteins

talin, kindlin, vinculin, paxillin, focal adhesion kinase (FAK)

hemidesmosome adaptor proteins

plectin, BP230

cadherins in the cadherin superfamily

classical/E cadherin, fat cadherin, flamingo, ret, desmocollin, cadherin 23, protocadherin, T cadherin

what do cadherins need to bind to each other

calcium

how does cadherin homophilic binding assist in development

cells will self sort by using their different cadherins that bind to each other

what does Rac do

at an adherens junction, recruits more cadherins and catenins

laces the shoe

what does Rho do

inactivates Rac and stimulates inner contractile bundles

tightens shoe laces

what will adherens junctions do in response to stress

strengthen their actin linkages by binding vinculin to alpha catenin and recruiting more actin to the other side of vinculin.

when stress is not present, vinculin not bound because alpha catenin is folded

example of adherens junctions in epithelia

adhesion belt

encircles each of the cells just beneath the apical surface of the epithelium. contraction of this belt allows the force for folding of epithelial-cell sheets into tubes, spheres, and related structures

what do desmosomes do

provide mechanical strength to epithelia by linking intermediate filaments, most common in tissues like the heart and epidermis

epithelia are _____

polarized

tight junction proteins

claudin and occludin, short, homophilic binding proteins

zonula occludens

large scaffold proteins that provide structural support on which the tight junction is built. the ZOs all have multiple protein binding domains linked on a string

gap junction structure

bridges the gaps between cells and creates direct channels of cytoplasm

made of connexins

couple cells electrically and metabolically

open and closed conformations that allow small molecules like ions and second messengers through

connexin

monomer that makes up a connexon

connexon

made of 6 connexins, can be hetero or homomeric

gap junction intercellular channels

connection of connexons, can be homotypic or heterotypic

how are plasmodesmata different from gap junctions

plasmodesmata are continuations of plant cell membranes that cannot be closed

plasmodesmata are huge and can let macromolecules and even mitochondrion through

why do tendons repair slowly?

because they are made of matrix, not cells

what are the three major classes of ECM macromolecules

proteoglycans and GAGs

fibrous proteins

glycoproteins

which GAG is not a proteoglycan

hyaluronan

collagen is a

fibrous protein

glycoproteins are the

organizers of ECM

Glycosaminoglycans

made of unbranched polysaccharide chains

repeating disaccharide of N-acetylglucosamine and glucuronic acid

hyaluronan acts as a _____

space filler that helps resist compressive forces

how do GAGs form hydrated gels

by attracting osmotically active cations that suck up water

a GAG attached to a protein is called a

proteoglycan

what is one proteoglycan that forms huge polymeric complexes

aggrecan

what is the most abundant protein in mammals

collagen

structure of collagen

long, stiff, helical structure that provides strength to the ECM to resist tensile forces

triple helix formed by proline and glycine

what does elastin do

gives tissues flexibility

e.g. skin, bladder, lungs, aorta, blood vessels

network of elastin molecules

made up of single elastins held together by disulfide bonds

intrinsically disordered for stretchiness

glycoprotein fibronectin

typically has multiple binding domains

helps organize the ECM by binding to multiple elements and to receptors on cells

can respond to tension by binding other fibronectin molecules

what is the basal lamina

underpinning of all epithelial sheets

plays role in: structure, filtration, survival, proliferation, differentiation, cell migration, synapses guiding

how is the basal lamina synthesized

by the cells on either side of it

epithelia and connective tissue

laminin

organizes basement membrane

cross shaped coiled coil that binds everything

integrin structure

alpha and beta integrin bind to ECM protein, beta integrin binds talin, vinculin binds talin to actin

integrins switch between active and inactive states in response to _____

intracellular signaling

inactive vs active integrin

inactive: alpha and beta scrunched up together

active: alpha and beta stretched out, together have strong ligand binding on ECM side and beta has strong adaptor protein binding on cell side

what are the most prominent cell-matrix interactions in the epithelia

hemidesmosomes

how do hemidesmosomes work

adaptor proteins BP230 and plectin connect keratin to alpha integrin, beta integrin, and collagen XVII, laminin connects to this structure and binds it to collagen

focal adhesion kinase

phosphorylates integrin focal adhesions which regulates cell adhesion and migration

plant cell wall

forms thicker, stronger, more rigid ECM

cellulose resists tensile forces, pectin acts like protoglycan (hydrated polyschharide

lack nitrogen

tensile strength of cell walls allow plant cells to develop

turgor pressure

turgor pressure _____

drives cell expansion and provides mechanical rigidity