Cellular Junctions

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 Cells that use gap junctions:

Skin epithelium

Endocrine glands

GI epithelium

Smooth muscle

Cardiac muscle

Osteocytes

Glial cells


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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 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

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Cellular Junctions

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