Cell- Cell interactions

Mr. Gunjankumar Mehta,Dept. of Biotechnology,

Shree M. & N. Virani Science College, Rajkot- 360005, Gujarat, India

Email: gjmehta@vsc.edu.in

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Contents• Need of cell- cell interactions• Cell signaling overview & mechanism• A. Cell- cell interaction by direct contact• B. Paracrine signaling• C. Endocrine signaling• D. Synaptic signaling• Summary• References

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Need of Cell- Cell interaction• Cell-to-cell communication is essential for

multicellular organisms.

• The combined effects of multiple signals determine cell response. For example, the dilation of blood vessels is controlled by multiple molecules.

• Cell-cell communication via cell adhesion molecules is critical for assembling cells into tissues, controlling cell shape and cell function (together with cell- matrix interaction).

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Cell- cell interaction

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Cell signaling- overview1. Reception: target cell detects a signaling molecule

coming from outside 2. Transduction: change of the receptor protein

initiating process of cellular response (enzymatic)3. Response: cellular activity: catalysis,

rearrangement of the cytoskeleton, gene activation

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Cell signaling mechanisms

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A. Cell- cell interaction by direct contact• Cells attach themselves to one another with long

lasting bonds called cell junctions.

• 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.

• Four classes of cell junctions are there.

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Cell Junctions- types based on localizationBetween cells

– Tight junctions– Adherence

junctions and desmosomes

– gap junctionsBetween cells and

matrix– Hemidesmosomes

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Cell junctions - types based on function

• Adhering junctions:1. Tight junctions2. Adherence junctions and

desmosomes3. Hemidesmosomes

• Communicating junctions:1. Gap junctions

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Tight junctions (zonula occludens)• Connects plasma

membrane in a sheet.• Belts of proteins that close

extracellular space between cells

• Prevent passage of water and water-soluble substances

• Account for electrical resistance across epithelia.

• Look like honey comb

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Tight junctions- Molecular structure• Claudins (membrane

proteins) zip two membranes together.

• Stabilized by spectrin• Connected to spectrin by

adapter proteins ZO1 and ZO2.

• The “tightness” varies according to the barrier.

• Leaky epithelia where there is need for some traffic.

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Tight junctions- Regulation• Hormones-

Vasopressin• Cytokines• Lack of ATP causes

“leak”• Extravasating

leukocytes open tight junctions.

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Significant example• Tight junction of intestinal epithelium

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Adhesive/ Anchoring junctions• Adherence junctions and

desmosomes• Hold cells tightly together

confering mechanical strength.

• Common in tissue that are subject to severe stress such as skin and cardiac muscle.

• Integral membrane proteins connect a cell’s cytoskeleton to another cell or extracellular matrix.

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Adherence junction- Molecular structure• Belt like junctions

located just below tight junction.

• Simple points of attachment, do not contain channels connecting the interiors of the two attached cells.

• Cell to cell connections are mediated by cadherins binding to other cadherins.

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E- cadherin mediated adherens junctions• Homophilic pairing of E- cadherins• Adapter proteins (plakoglobin and α and βcatenins)

link cadherins to the belt of actin filaments.

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E- cadherin• Ca2+ -dependent cell-cell adhesion

proteins, expressed in almost all cells of vertebrates and invertebrates.

• Transmembrane proteins. • Characteristic structural feature: Tandem repeats of homologous

domains (CAD-domains)Length of about 110 amino acids.β-sheeted structure

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Desmosomes• Button like welds joining

opposing cell membranes• Cadherins bind the

membranes of adjacent cells in a way that gives strength and rigidity to the entire tissue.

• Cadherins attach to intermediate filaments via anchoring proteins.

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Desmosomes- Molecular structure• Two cadherins:• Desmoglein• Desmocollin• Adapter proteins: • Plakoglobin • Desmoplakin• Linked to epidermal

keratins

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Gap junctions• Gap junctions allow cells to exchange

electrical and/or chemical signals via direct door.

• Composed of proteins that form channels that allow small molecules to pass.

• Form electrical synapses- Direct transmission of action potential without transmitter, receptors etc.

• Integrate the metabolism of the cells- Metabolic coupling or metabolic cooperation.

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Gap junction- Molecular structure • A ring of 6 membrane

proteins called connexins- connexons.

• Two connexons on neighboring membranes form a transmembrane channel that interconnects the cytoplasms of two cells.

• Connexons are size filters.

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Regulation of gap junctions• Flip between open and closed states as other

channels do.• Cells may modulate the degree of coupling • Cytoplasmic levels of Ca2+and pH• Phosphorylation• Oleamide – closes gap junctions and induces

sleep

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Significant examples• Cells that use gap junctions• Skin epithelium• Endocrine glands• GI epithelium• Smooth muscle• Cardiac muscle• Osteocytes• Glial cells

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Hemidesmosomes- molecular structure• Similar to desmosomes but totally

different molecular structure.• Cell-matrix adhesions – attach

cells to basal lamina.• Composed of integrins (outside)

that bind to type XVII collagen and laminin-5.

• Cytosolic side consist of a plague composed of adapter proteins (plectin) attaching integrins to keratin filaments.

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Summary- Cell junctions

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Plasmodesmata- plants• In plants, cell walls separate

every cell from all others.• Cytoplasmic connections that

form across the touching plasma membranes.

• Plasmodesmata function much like gap junctions in animal cells- structure differs.

• Lined with plasma membrane and contain a central tubule that connects the ER of the two cells.

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B. Paracrine signaling• Signals with such short-

lived, local effects are called paracrine signals.

• Signal molecules released by cells can diffuse through the extracellular fluid to other cells used or destroyed

• Paracrine signaling plays an important role in early development.

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C. Endocrine signaling• Released signal molecule

remains in the extracellular fluid Circulatory flow

• Travel throughout your body Hormone

• endocrinesignaling (figure 7.3c).

• Both animals and plants use this signaling mechanism extensively.

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D. Synaptic signaling• In animal nervous system,

signal molecules neurotransmitters, do not travel to the distant cells like hormone.

• They are secreted by long fiber like extensions of nerve cells release neurotransmitters from their tips very close to the target cells Chemical synapse.

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References• The Cell- A Molecular Approach- 4th

edition, GEOFFREY M.COOPER, ROBERT E. HAUSMAN.

• Cell and Molecular biology- 6th edition, GERALD KARP

• Cell Biology, Genetics, Molecular Biology, Evolution and Ecology-1st Edition, Verma and Agrawal.

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