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8/4/2019 Cellular Junctions
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Cellular JunctionsGroup 1:
Tom Nicolas Ong, Kerstin Ponsaran, Paolo Robert Bueno, Donna Rose Santos, and Pia Monique Hipolito
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Cell junctions Cells in the body frequently need to
stick together or communicate withone another.
Plasma membrane areas specialized to
provide contact between cells
Dense clusters of cell adhesion
molecules on the outside
linked to cytoskeleton on the
inside through adapter proteins
Mediates information between cell-
to-cell activities
These junctions functions as: abinder, an anchor, and a communicator
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Cell junctions Cell junctionscan be classified
into three functional groups:
Occluding junctions seal cellstogether in an epithelium in a way
that prevents evensmallmoleculesfrom leaking from oneside of the sheet to the other.
Anchoring junctions mechanicallyattach cells (and theircytoskeletons)to their neighbors or totheextracellular matrix.
Communicating junctions mediate thepassage of chemical or electrical
signals from one interacting cell to
its partner.
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Cell junctions:
AnimalTissue
In many animal tissues(e.g., connective tissue), eachcell is separated from the next by
an extracellular coatingormatrix.
However, in some tissues(e.g., epithelia), the plasmamembranes of adjacent cells are
pressed together. Four kinds of
junctions occur in vertebrates:
Tight junctions Adherens junctions Gap junctions Desmosomes
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Cell junctions:TightJunctions also referred to as a zonula
occludens / occluding junctions
Belts of proteins that closeextracellular space between cells
seal epithelial layers to preventmaterials leaking across theepithelium between the cells
instead materials must pass through:
receptor-mediated endocytosis at theapical surface
exocytosisat the basolateral surface act as a barrier so that materials
cannot pass between two interactingcells
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Cell junctions:TightJunctions
they also divide theepithelial cell membrane
into apical (luminal) and
basolateral membranes
Molecular structure of tightjunctions Claudins (membrane
proteins) zip two
membranes together
Stabilized by spectrin Connected to spectrin by
adapter proteins ZO1 and
ZO2
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Cell junctions:TightJunctions
Regulation of tight junctions The tightness varies according to
the barrier needs
Leaky epithelia where there is need for
some traffic
Hormones
Vasopressin
Lack of ATP causes leak
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Cell junctions:AdherensJunctions
provide strong mechanical attachments
between adjacent cells
Confer mechanical strength
reinforce tight junctions
are composed of membrane proteinsattached to cytoskeleton microfilaments
Common in tissue that are subject to
severe stress such as skin and cardiac
muscle
Adherens junctions also bind epithelial
cells together, where they may form a
band around the circumference of each
cell, called zonula adherens
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Cell junctions:AdherensJunctions not only tether cells together
or to the extracellular matrixbut also transduce signalsinto and out of the cell,
influencing a variety of
cellular behaviors:
proliferation,
migration,
differentiation.
They may also bind cells
together at discrete spots
(focal adhesions). They may also bind cells to the
extracellular matrix (ECM) at
spots called adhesion plaques
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Cell junctions:AdherensJunctions Adherens junctions are built
from:
cadherins transmembraneproteins (shown in red) whose
extracellular segments bind
to each other and
whose intracellular
segments bind to
catenins (yellow). Cateninsare connected to actinfilaments
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Cell junctions:AdherensJunctions
Molecular structure of adherensjunctions:
Belt like junctions located just
below tight junction
Simple points of attachment, do
not contain channels connectingthe interiors of the two attached
cells
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Cell junctions:Desmosomes are similar in some
respects to focal adhesions
of the adherens type and
also contain cadherins, but
they link in to the
intermediate filaments ofthe cytoskeleton
Mechanical links that bind
cells together
Hemidesmosomes modifieddesmosomes (cell-matrix
adhesions) anchor cells to the
underlying basement membrane/
basal lamina
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Cell junctions:Desmosomes
their association with
the keratin -based cytoskeleton(intermediate filaments)
proteins run across the membraneinto the intracellular space, to
connect the two cells together
'transmembrane linker' proteins
are called desmoglein anddesmocollin, which are typesofcadherin
Integrin - transmembraneproteins of hemidesmosomes
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Cell junctions:GapJunctions are the most widespread of all
cell junctions in animal tissues
couple cells electrically andmetabolically, enabling cells tocommunicate with each otherdirectly
an open and close in response to
changes in calcium levels, andpH.
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Cell junctions:GapJunctions
Form electrical synapses Direct transmission of action
potential without
transmitter, receptors etc.
Integrate the metabolism of the
cells Metabolic coupling or
metabolic cooperation transfer of molecules that
the neighboring cell can not
synthesize (secondmessengers, precursors of
DNA and RNA)
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Cell junctions:GapJunctions
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Cell junctions:GapJunctions Cells that use gap junctions:
Skin epithelium
Endocrine glands
GI epithelium
Smooth muscle
Cardiac muscle
Osteocytes
Glial cells
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Cell junctions:GapJunctions
Molecular structure of gapjunction
Connexin - basic building blockof each gap junction
Connexons- a channel formed by sixblocks of connexins
Two connexons on neighboring
membranes form a transmembrane
channel that interconnects the
cytoplasms of two cells
Connexons are size filters
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Cell junctions:GapJunctions
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Cell junctions:GapJunctions
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Cell junctions:GapJunctions Cells may modulate the degree
of coupling Cytoplasmic levels of
Ca2+and pH
Phosphorylation
Oleamide closes gapjunctions and induces sleep Structure of Oleamide
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Cell junctions:
PlantTissue
In many plant tissues, it turnsout that the plasma membrane of
each cell is continuous with that
of the adjacent cells.
The membranes contact each other
through openings in the cell wall
called Plasmodesmata.
middle lamella is a glue thatkeeps plant cells together
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Cell junctions:Plasmodesmata
allow molecules to be shared
between adjacent plant cells
provide an easy route for the movement
of ions, small molecules like sugarsand amino acids, and even
macromolecules like RNA and proteins,
between cells
The larger molecules pass throughwith the aid ofactin filaments.
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Cell junctions:Plasmodesmata
Molecular structure of gap junction
are sheathed by a plasma membrane
that is simply an extension of the
plasma membrane of the adjoining
cells
the endoplasmic reticulum ofadjacent plant cells are connected
through the plasmodesmata via a
tubule called a desmotubule
form a continuous cytoplasmicconnection between cells calledthesymplast
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Cellular Junctions
THE END!
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