Lesson 2: Cell membrane Differentiations and Interactions and Communication

Where are cells joined together and what is the composition

In tissues

-formed by Extracellular matrix and cells

Microvilli

-On apical surface of epithelial tissues

-Increased absorption

-physical barrier

-maintain their shape thank to cytoskeleton

Lateral interdigitations

-increase cell adhesion between cells

-with Ca2+ and Cytoskeleton

Cell junctions

Contact points between the plasma membranes of cells or between

cells and extracellular matrix.

Types of cell junctions

1. Occluding or Tight junctions

2. Anchoring Junctions to the cytoskeleton:

• Actin filament attachment:

2A-Adherens junction (cell-cell)

2B-Focal adhesions (cell-extracellular matrix)

• Intermediate filaments attachment:

2C-Desmosomes (cell-cell)

2D-Hemidesmosomes (cell-extracellular matrix)

3. Gap or communicating junctions (Channel-forming junctions)

Functions of cell junctions

Interaction between cell and its environment

Membrane adhesion molecules

Membrane adhesion molecules Classification

• Ca2+-dependent:

• CADHERINS

• SELECTINS

• INTEGRINS

• Non- Ca2+ dependent:

• IMMUNOGLOBULINS

Cadherins

-adhesion between cells

-Embryogenesis: hold cells together of embryo

-Homophilic binding: same type (E and E cadherin)

CadherinS types

-Adherence junctions

• indirectly bind the actin cytoskeleton of neighboring cells.

Desmosomes

-bind intermediate filaments of neighboring cells.

Cadherin classification

Classical cadherins:

-E-Cadherin (Epitehial cells)

-N-Cadherin (nerve)

-P-cadherin (placenta, epidermis)

Non classical cadherin:

-with adhesion function: desmosomal

-without adhesion function: T-cadherin (signaling)

Integrins

• Adhesion proteins that connect components of the extracellular

matrix to the cell cytoskeleton.

-transmembrane glycoprotein

-are on the cell surface and switch from inactive to active when they bind to ligands

Exception: in blood cells, also cell-cell junction.

Structure

• Consist of 2 subunits: heterodimer, 2 transmembrane glycoproteins

• domains on extracellular side tat bind ions (Ca2+ or Mg2+)

• Heterophobia binding: Integrins from one cell binding to different protein demo another cell or matrix

Integrins types

They are found in:

- Hemidesmosomes:

• They attach intermediate filaments to the matrix.

- Focal junctions:

• Bind the actin cytoskeleton to the matrix.

• On the extracellular side, integrins are connected to:

• the complex network of matrix filaments (collagen fibers, ....)

• the adhesion glycoproteins of the matrix (laminin, nidogen, ...)

Integrin functions

Functions:

• Adhesion

• Movement (fibroblast or macrophage)

• Form/Shape

• Growth

• Differentiation

• Signal transmission

Integrins Signalling from the outside to the inside

Important in platelets and white blood cells (leukocytes):

-these cells circulate in blood with integrins exposed but inactive.

• When they encounter the appropriate stimulus, the integrins are activated.

-integrin just changes its conformation

Selectins

-transmembrane proteins capable of recognising carbohydrates.

Types of selectin

• L-selectin in leukocytes (white blood cells).

• P-selectin on platelets and endothelial cells activated locally by

inflammation

• E-Selectin on endothelial cells

Selectin functions

Ca2+ dependent Leukocyte trafficking

Immunoglobulin

-most of the Ca2+-independent Intercellular adhesion

-weaker than Cadherins

-homophilic binding

Immunoglobulin functions

-Adhesion

-Signal transmission.

Cell communication

transmission of chemical extracellular signals between cells

-producing a response in the receiving cell

Signaling cell

Cell that produces the signal molecule

Types of signaling

-secrete signal molecules (by exocytosis or diffusion across the membrane)

-contact-dependent signalling: exposing signal molecules on their cell surface (influences cells that are in close contact)

1. Type of signaling cells

Endocrine cells

-most common type of communication

-release ligands(hormones) into the bloodstream

-distributed throughout the body (long distances)

2. Type of signaling cells

Paracrine cells

-Inflammation and healing

-act over short distances → local communication

-signal molecule short half life → eliminated by enzymes or taken up

3. type of signaling cells

Autocrine / contact-dependent cells

-Embryonic developement

-secret hormone or chemical messenger → autocrine agent

-binds autocrine receptors to specific cell

4. type of signaling cells

Neuronal cells

-synaptic transmission

-Neurotransmitter released by neuron and activate receptors of postsynaptic cell

-long distances

-trigger membrane potential changes of postsynaptic cell

Receptor / target cell

Protein that recognises the ligand

Types of receptor / target cells

Surface receptor

-transmembrane proteins activated by ligands - transmit signal into cell

-3 types of surface receptors

Intracellular receptors

-intracellular proteins that bind small, hydrophobic ligands

-2 types of intracellular receptors

Surface receptors

-Ion channel-gated receptors (Ligand opens channel, changing permeability of the membrane to some ions)

-Enzyme-linked receptors (receptors acts like enzymes or binds to enzyme and activates it.

-G protein-linked receptors (Ligand-receptor activates a G protein, which activates an enzyme or ion channel)

Intracellular receptors

Ligand-activated nuclear receptors (ligand binds and conformation changes which activates gene transcription

Cytoplasmic enzyme-linked receptors (cytoplasmic enzyme that activates signalling pathways, activate or inhibit responses)

Complexity

-each cell recognises only few

-same signal an generate different responses