Cell to Cell Junctions and Adhesion

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Cell to Cell Junctions and Adhesion

• Lecture 24 BSCI 420/421 Oct 28/29, 2002
• In our every deliberation, we must consider the
  impact of our decisions on the next seven
  generations.
• - From the Great Law of the Iroquois
  Federation
• A. Cell Adhesion Molecules
• B. Cell-Cell Junctions
• 1. Occluding Jxs
• 2. Anchoring Jxs
• 3. Communicating Jxs

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• Formation of multicell organisms requires
  specific interaction
• between cells to hold the cells together and to
  communicate
• in order to coordinate activities.
• A. 4 types of Cell Adhesion Molecules (CAMs)
• are used to hold animal cells together
• Cadherins
• Ig-like CAMs
• Selectins
• Integrins
• All are single-pass transmembrane proteins
  anchored to the
• cytoskeleton by their cytoplasmic domains.

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• Cadherins Ca2 binding adhering proteins
• When Ca2 is bound to the 5 cadherin-repeat
  domains,
• They can bind to similar domains from an adjacent
  cell.
• Removal of extracell Ca2 can cause dissociation
  of cells.

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Cadherin interactions are homophilic, with
identical cadherens interacting to form tissues
of like cell types. Table 19-3 Classical
cadherins E epithelial embryonic cells P
placental epidermis N Neural cells, lens,
muscles VE Vascular endothelial
cells Non-classical cadherins Desmocollin and
desmoglein desmosomes of epithelia like
skin Protocadherins synapses
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Cadherins are liked to the actin cytoskeleton by
catenins
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2. Ig-like CAMs Are Ca2 independent CAMs 20
different N-CAMs all by alternate splicing of one
gene
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3. Selectins Mediate transient interactions by
heterophilic binding to cell surface
glycoproteins on other cells.
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4. Integrin Binds cells to the extracellular
matrix (fibronectin or laminin) Or to Ig-family
receptors. Binding can be regulated.
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• B. Cell to Cell Junctions are formed when groups
  of molecules
• Interact between two cells.
• 3 broad classes
• Occluding Jxs 2. Anchoring junctions 3.
  Communicating jxs
• seal cells together to prevent
  passage of
• material between cells.
• Main type Tight Junctions
• elect-dense
• tracer to
• Fig 19-3 apical l or
• basolateral r

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TEM of Tight Jxs Freeze-fracture thin
section views
Fig 19-4-pt2
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Recall 2 important fxs of tight jxs In glucose
transport across Int. epithelia 1. Prevent flow
of Glucose between cells 2. Prevent lateral
diffusion of transporters From one mem. Domain
to the other
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2. Anchoring Jxs a. Adherens Jxs
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b. Desmosomes are circular spot junctions that
produce strong adhesions between two cells. Esp.
skin, intestinal epithelia, heart muscle Fig
19-11.
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Diagram of desmosomes Hemidesmosomes look like
½ des, but use integrins inst. of
cadherins
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• 3. Communicating Jxs
• Gap Jxs allow small molecules to flow between
  cells e.g. Ions, sugars, NTs, cAMP
• 2 membranes are separated by a 2 nm gap.
• Six proteins called connexins form a connexon and
  2
• connexons end to end form a 1.5 nm channel
  between 2 cells.
• Fig 19-16

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The size of the gap jxn channel was determined by
injecting fluorescent peptides of different
sizes into Gap jx-coupled salivary gland cells
of Drosophila. 1200 Da mols could pass but 2000
Da mol would not. Injection of Ca2 along w
peptides closed the channel. Why would cells
want to do this?
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Fxs Fig 19-17 Synchronizing beat of heart
muscle cells. Coupling oocyte to Granulosa
cells Mutant Cx37 causes Infertility Oocyte
needs signals from the receptors on The
granulosa cells For ovulation.
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Plasmadesmata serve similarly as comm. jxs in
plants Fig 19-20