Transcript Slide 1

CELL JUNCTIONS
Yasir Waheed
• Cells are small, deformable , filled with aqueous
medium and surrounded by fragile plasma membrane;
yet they combine in different manners to form strong
and massive structures like a horse or a tree.
• The building technologies of animals and plants are
different, and each type of organism is formed of many
types of tissues, in which the cells are assembled and
bound together in different ways. In both animals and
plants, however, an essential part is played in most
tissues by the extracellular matrix.
• In animals, the cells of most tissues are bound directly
to one another by cell-cell junctions
Figure 19-1. A cross-sectional view of part of the wall of the intestine. This long,
tubelike organ is constructed from epithelial tissue (red), connective tissue (green),
and muscle tissue (yellow). Each tissue is an organized assembly of cells held
together by cellcell adhesions, extracellular matrix, or both.
Cell Junctions
• Specialized cell junctions occur at points of cell-cell and cellmatrix contact in all tissues, and they are particularly plentiful
in epithelia.
• Cell junctions can be classified into three functional groups:
• 1. Occluding junctions seal cells together in an epithelium in a
way that prevents even small molecules from leaking from
one side of the sheet to the other.
• 2. Anchoring junctions mechanically attach cells (and their
cytoskeletons) to their neighbors or to the extracellular
matrix.
• 3. Communicating junctions mediate the passage of chemical
or electrical signals from one interacting cell to its partner.
Tight Junctions
• Tight junctions have this barrier role in vertebrates
e.g. the mammalian small intestine.
• The epithelial cells lining the small intestine form a
barrier that keeps the gut contents in the gut cavity /
lumen. At the same time, the cells transport selected
nutrients across the epithelium from the lumen into
the extracellular fluid, or gut. These nutrients diffuse
into small blood vessels to provide nourishment to
the organism.
Figure 19-2. The role of tight junctions in
transcellular transport. Transport proteins are
confined to different regions of the plasma
membrane in epithelial cells of the small
intestine. This segregation permits a vectorial
transfer of nutrients across the epithelium
from the gut lumen to the blood. In the
example
shown,
glucose
is
actively
transported into the cell by Na+-driven glucose
symports at the apical surface, and it diffuses
out of the cell by facilitated diffusion mediated
by glucose carriers in the basolateral
membrane. Tight junctions are thought to
confine the transport proteins to their
appropriate membrane domains by acting as
diffusion barriers within the lipid bilayer of the
plasma membrane; these junctions also block
the backflow of glucose from the basal side of
the epithelium into the gut lumen.
Figure 19-3. The role of tight junctions in allowing epithelia to serve as barriers to
solute diffusion. (A) The drawing shows how a small extracellular tracer molecule
added on one side of an epithelium cannot traverse the tight junctions that seal
adjacent cells together. (B) Electron micrographs of cells in an epithelium in which a
small, extracellular, electron-dense tracer molecule has been added to either the
apical side (on the left) or the basolateral side (on the right). In both cases, the tracer
is stopped by the tight junction.
Figure 19-5. A current model of a tight junction. (A) This drawing shows how the
sealing strands hold adjacent plasma membranes together. The strands are
composed of transmembrane proteins that make contact across the intercellular
space and create a seal. (B) This drawing shows the transmembrane claudin and
occludin proteins in a tight junction.
Figure 19-6. A septate junction. A conventional electron micrograph of a
septate junction between two epithelial cells in a mollusk. The
interacting plasma membranes, seen in cross section, are connected by
parallel rows of junctional proteins. The rows, which have a regular
periodicity, are seen as dense bars, or septa.
Anchoring junctions
• Anchoring junctions mechanically attach cells
(and their cytoskeletons) to their neighbors or
to the extracellular matrix.
• Anchoring junctions are widely distributed in
animal tissues and are most abundant in
tissues that are subjected to severe
mechanical stress, such as heart, muscle, and
epidermis.
• Anchoring Junctions are composed of two main
classes of proteins.
• Intracellular anchor proteins are present on the
cytoplasmic face of the plasma membrane and
connect the junctional complex to either actin
filaments or intermediate filaments.
• Transmembrane adhesion proteins have a
cytoplasmic tail that binds to one or more
intracellular anchor proteins and an extracellular
domain that interacts with either the extracellular
matrix or the extracellular domains of specific
transmembrane adhesion proteins on another cell.
Figure 19-7. Anchoring junctions in an epithelium. This
drawing illustrates, in a very general way, how anchoring
junctions join cytoskeletal filaments from cell to cell and from
cells to the extracellular matrix.
Figure 19-8. The construction of an anchoring junction from two classes of
proteins. This drawing shows how intracellular anchor proteins and
transmembrane adhesion proteins form anchoring junctions.
• Anchoring junctions occur in two functionally
different forms:
• 1. Adherens junctions and desmosomes hold
cells together and are formed by
transmembrane adhesion proteins that belong
to the cadherin family.
• 2. Focal adhesions and hemidesmosomes
bind cells to the extracellular matrix and are
formed by transmembrane adhesion proteins
of the integrin family.
Figure 19-9. Adherens junctions. (A) Adherens junctions, in the form of adhesion belts,
between epithelial cells in the small intestine. The beltlike junction encircles each of the
interacting cells. Its most obvious feature is a contractile bundle of actin filaments
running along the cytoplasmic surface of the junctional plasma membrane. (B) Some of
the molecules that form an adherens junction. The actin filaments are joined from cell
to cell by transmembrane adhesion proteins called cadherins. The cadherins form
homodimers in the plasma membrane of each interacting cell. The extracellular domain
of one cadherin dimer binds to the extracellular domain of an identical cadherin dimer
on the adjacent cell. The intracellular tails of the cadherins bind to anchor proteins that
tie them to actin filaments. These anchor proteins include a-catenin, b-catenin, gcatenin (also called plakoglobin), a-actinin, and vinculin.
(C) The structural components of a desmosome. On the cytoplasmic surface of each
interacting plasma membrane is a dense plaque composed of a mixture of
intracellular anchor proteins. A bundle of keratin intermediate filaments is attached
to the surface of each plaque. Transmembrane adhesion proteins of the cadherin
family bind to the plaques and interact through their extracellular domains to hold
the adjacent membranes together.
(D) Some of the molecular components of a desmosome. Desmoglein and
desmocollin are members of the cadherin family of adhesion proteins. Their
cytoplasmic tails bind plakoglobin (g-catenin), which in turn binds to
desmoplakin. Desmoplakin also binds to the sides of intermediate filaments,
thereby tying the desmosome to these filaments.
(B) Some of the proteins that form focal adhesions. The transmembrane adhesion
protein is an integrin heterodimer, composed of an alpha and a beta subunit. Its
extracellular domains bind to components of the extracellular matrix, while the
cytoplasmic tail of the beta subunit binds indirectly to actin filaments via several
intracellular anchor proteins.
Figure 19-13. Desmosomes and
hemidesmosomes. The distribution
of
desmosomes
and
hemidesmosomes in epithelial cells
of the small intestine. The keratin
intermediate filament networks of
adjacent cells are indirectly
connected to one another through
desmosomes and to the basal
lamina through hemidesmosomes.
Gap Junctions
Most cells in animal tissues are in communication with
their neighbors via gap junctions.
Each gap junction appears in conventional electron
micrographs as a patch where the membranes of
two adjacent cells are separated by a uniform
narrow gap of about 2 - 4 nm.
The gap is spanned by channel-forming proteins
(connexins). The channels they form (connexons)
allow inorganic ions and other small water-soluble
molecules to pass directly from the cytoplasm of
one cell to the cytoplasm of the other, thereby
coupling the cells both electrically and metabolically.
Figure 19-14. Determining the size of a gap-junction channel. When
fluorescent molecules of various sizes are injected into one of two cells
coupled by gap junctions, molecules with a mass of less than about 1000
daltons can pass into the other cell, but larger molecules cannot.
Figure 19-15. Gap junctions. (A) A three-dimensional drawing showing the
interacting plasma membranes of two adjacent cells connected by gap junctions.
The apposed lipid bilayers (red) are penetrated by protein assemblies called
connexons (green), each of which is formed by six connexin subunits. Two connexons
join across the intercellular gap to form a continuous aqueous channel connecting
the two cells. (B) The organization of connexins into connexons and connexons into
intercellular channels. The connexons can be homomeric or heteromeric, and the
intercellular channels can be homotypic or heterotypic.
Figure 19-17. Gap junction coupling in the
ovarian follicle. The oocyte is surrounded
by a thick layer of extracellular matrix
called the zona pellucida (discussed in
Chapter 20). The surrounding granulosa
cells are coupled to each other by gap
junctions formed by connexin 43 (Cx43). In
addition, the granulosa cells extend
processes through the zona pellucida and
make gap junctions with the oocyte. These
gap junctions contain a different connexin
(Cx37). Mutations in the gene encoding
Cx37 cause infertility by disrupting
the development of both the granulosa
cells and the oocyte.
Figure 19-19. A summary of the various cell junctions found in a vertebrate
epithelial cell. The drawing is based on epithelial cells of the small intestine.
Figure 19-20. Plasmodesmata. (A) The cytoplasmic channels of plasmodesmata
pierce the plant cell wall and connect all cells in a plant together. (B) Each
plasmodesma is lined with plasma membrane that is common to two connected
cells. It usually also contains a fine tubular structure, the desmotubule, derived
from smooth endoplasmic reticulum.
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