cell interactions and extracellular matrix 1 & 2

epithelial and connective tissues (gut)

• epithelial cells (compact sheets)

• basement membrane

• connective tissue (fibroblasts, less compact, more ECM between cells)

• smooth muscle: circular, longitudinal fibers

• connective tissue

• epithelium

tight junctions in transcellular transport

• Na⁺-driven glucose symport located in apical surface of epithelial cells

• passive glucose carrier protein located in basolateral surface

• high glucose concentration in epithelial cells, low in gut lumen and blood

• high Na⁺ concentration in lumen, low in epithelial cells

• glucose transporters cannot diffuse to other surface due to tight junctions between epithelial cells → essential for physiological function

necessities for electron microscopy

• 100,000 V (wavelength electrons = 0.004 nm)

• ultralow pressure vacuum (remove air)

• ultrathin specimen (50-100 nm)

• contrast: stain with osmium tetroxide

tracer molecule analysis of tight junctions

• lanthanum injected into lumen of gut tissue cannot reach basolateral face

• lanthanum injected at basolateral surface cannot diffuse into gut lumen

freeze fracture electron microscopy

1. rapid freezing of cells

2. frozen cells are cleaved along the hydrophobic core of the lipid bilayer → E-face (exoplasmic) and P-face (protoplasmic)

tight junction proteins

• claudins and occludins

• form sealing strands

• N- and C- termini in cytosol

• 4 transmembrane domains

• 2 loops in intercellular space

• anchorage to actin cytoskeleton by ZO proteins

claudins

• 20-27 kDa in humans

• 24 homologous members

occludins

• 65 kDa

• two isoforms from alternative splicing

• phosphorylation serves as signal for localization

average mass of amino acid

100 Da

cytoskeletal fibers involved in cell junctions

• actin filaments - actin

• intermediate filaments - keratin, vimentin, desmin

tight-junction associated hereditary diseases

• neonatal ichthyosis (hard and itchy skin) and sclerosing cholangitis (Cldn-1)

• non-syndromic deafness (Cldn-14, tricellulin)

• familial hypomagnesemia with hypercalciuria and nephrocalcinosis (human) (Cldn-16)

• chronic interstitial nephritis and renal tubular dysplasia (bovine) (Cldn-16)

• hypomagnesemia with hypercalciuria and nephrocalcinosis with visual impairment (Cldn-19)

• familial hypercholamenia (ZO-2)

hypomagnesemia

low magnesium in blood

hypercalciuria

high calcium in urine

actin filament anchoring junctions

• adherens junctions (cell-cell)

• focal adhesions (cell-matrix)

intermediate filament anchoring junctions

• desmosomes (cell-cell)

• hemidesmosomes (cell-matrix)

adherens junctions (adhesion belt)

• anchoring proteins: catenins, vinculin

  • catenins bind cadherin and actin

  • vinculin binds catenins and actin

• transmembrane adhesion protein: cadherin dimers

  • homophilic interaction (same type from each cell)

  • homodimers

  • calcium-dependent

• actin filaments end at junction

folding of epithelial sheet in embryogenesis

1. invagination of sheet caused by organized tightening along adhesion belts in selected regions

   • adhesion belt associated with actin filaments, actin-myosin II contractility

2. tube pinches off from overlying sheet of cells

desmosomes

• anchoring proteins (plaque): plakoglobin, desmoplakin, plakophilin

  • plakoglobin (~80 kDa) binds TM proteins directly

  • desmoplakin (~260/330 kDa) connects plakoglobin and intermediate filaments, coiled-coil rod domain functions as spacer

• transmembrane adhesion proteins: desmoglein, desmocollin

• intermediate filaments are anchored to plaque, but don’t end at junction

desmosome anchoring protein mutations in humans

• cardiomyopathy

• skin diseases

pemphigus vulgaris

• autoantibodies produced against desmoglein

• causes epithelial cells to not be held together as tightly

• cells detach more easily, forming blisters

coiled-coil interactions

• α helix has complete turn every 7 amino acids → hydrophobic side chains line up on one side

• 2 α-helices wrap around each other to minimize exposure of hydrophobic amino acid side chains to aqueous environment

• results in hydrophobic (inside) and ionic (outside) interactions between α-helices, and polar interactions with aqueous environment