Cell adhesion molecules and matrix proteins

Presented by : Navneet Kaur M.Pharm (P’cology)2nd Sem

EXTRACELLULAR MATRIX Three groups of macromolecules,

which are often physically associate, constitute the ECM:

1. fibrous structural proteins such as collagens and elastins

2. a diverse group of adhesive glycoproteins

3. proteoglycans and hyaluronic acid

These macromolecules are present in intercellular junctions and cell surfaces and may assemble into 2 general organizations: interstitial matrix and basement membrane(BM)

Collagen : provide extracellular framework for all multicellular organisms

Composed of a triple helix of 3 polypeptide α chains having a gly-x-y repeating sequence. Currently 27 different types of collagens encoded by 41 genes dispersed on at least 14 chromosomes are known.

-Type I, Type II, Type III,Type V,Type XI: interstitial or fibrillar collagens.

- Type IV: nonfibrillar and main component of BM together with laminin

Fibrillar collagen is synthesised from procollagen, a precursor molecule derived from preprocollagen,which is transcribed from collagen genes.

Elastin,fibrillin,and elastic fibres: Tissues such as blood vessels,skin,uterus,lungs require elasticity for their function.

-Ability of tissues to recoil is provided by elastic

fibres.These fibres can stretch to several times their length and then return to their original size after release of tension. Elastin fibres consist of a central core made of elastin, surrounded by a peripheral network of microfibrils.The peripheral microfibrillar network that surrounds the core consists largely of fibrillin ,a 350 kD secreted glycoprotein which associates either with itself or with other components of ECM.

Cell adhesion proteins: It basically include fibronectin and laminin.

Fibronectin is a large protein that binds to many molecules,such as collagen,fibrin, proteoglycans and cell surface receptors.

Laminin is the most abundant glycoprotein in the basement membrane and has binding domains for both ECM and cell surface receptors.

Proteoglycans and hyaluronic acid(HA): Proteoglycans consists of a core protein linked to

one or more polysaccharides called as glycosaminoglycans (GAGs). Some of the most common are heparan sulfate, chondroitin sulfate and dermatan sulfate. They have diverse role in regulating connective tissue structure and permeability. By binding to other proteins and fibroblast growth, they act as modulators of cell growth and differentiation.

HA is a polysaccharide of GAG family found in ECM of many tissues. It binds a large amount of water forming a viscous hydrated gel that gives connective tissue the ability to resist compression forces. It also provides lubrication to many type of connective tissue,notably for the cartilage in joints.

CELL ADHESION MOLECULES are proteins located on cell surface involved with the binding with other cells or ECM in process called cell adhesion.

Cell adhesion molecules

CAMs

cadherins

integrins

selectins

IgSF CAMs: Immunoglobin superfamily CAMs are either homophilic or heterophilic and bind integrins or different IgSF CAMs. Some molecules of this family are:-NCAMs (neural CAMs)-ICAM(intercellular CAMs)-VCAMs(vascular CAMs)-PECAMs(platelet-endothelial CAMs)

CADHERINS:Cadherins are Ca++Dependent cell-cell adhesion molecules. They are rapidly degraded by proteases in the absence of Ca++. There are 3 major types: E-cadherins(epithethial),P -cadherins(placental), N-cadherins(neural)

.

Cadherins can have homophilic binding or heterophilic binding.

INTEGRINS: are receptors that mediate attachment between a cell and the tissues surrounding it,which may be other cells or the ECM. Integrins shows heterophilic binding

Integrins are a family of transmembrane glycoproteins that are composed of 2 chains, α and β.There are 40 different types of α chains and 8 types of β chains that can combine to form a large number of different integrin molecules. The α chain has binding sites for Ca++ and Mg++ which are needed for integrins to adhere. The cytoplasmic tail of integrins is connected to a linker protein that connects to the cytoskeleton. A bridge from ECM to cytoskeleton.

Integrins pass information about the chemical composition and mechanical status of ECM into cell

Involved in cell signaling and regulation of cell cycle,shape and motility .

Intracellular domains interact with variety of signaling and structural proteins: Talin and vinculin (adapter proteins) to form

focal adhesions Link to actin to form stress fibers Paxillin (adapter proteins) to activate FAK (focal adhesion kinase)

Functions of integrins:1. Attachment of cell to ECM2. Signal transduction from ECM to cell

have relation to- -cell growth -cell division -cell survival -cellular differentiation -apoptosis -cell migration during embryogenesis

, thrombosis, haemostasis,wound healing etc.

SELECTINS: are single chain transmembrane glycoproteins,they have lectin like carbohydrate binding

domain.

• endothelial• Transfer leukocytes to

particular tissuesE-selectin

• leukocytes• Responsible for

inflammatory reactionL-selectin

• platelet• In blood clotting and

platelet activationP-selectin

The best characterised ligand for three selectins is P-selectin glycoprotein ligand-1 (PSGL-1), which is a , mucin type glycoprotein expressed on all WBCs. The name selectin comes from the words “selected” and “lectins” which are a type of carbohydrate recognizing proteins.

Another ligand for E-selectin is sialylated Lewis X Ag (sLe(x)).

PHYSIOLOGICAL ROLES:

•Leukocyte-endothelial interactions: The following steps are included : 1. leukocyte trapping or rolling 2. leukocyte activation and adhesion 3. diapedesis 4. Termination

Blood coagulation: vascular injury produces endothelial denudation or retraction, platelets rapidly adhere to exposed sub endothelium. The platelet adherent to damaged vessel wall may directly recruit leukocytes by initiating selectin- integrin dependent leukocyte adhesion to surface bound platelets. Moreover accumulated leukocytes bound to adherent platelets may promote fibrin deposition, thereby contributing to thrombus formation.

Morphho genesis and CAMs:The ability of cells to detach from a formerly cohesive structure and to migrate and contribute to formation of new tissues is a key event during morphogenesis. - early development - implantation of embryo into uterine epithelium involves both E- and P- cadherin. -Gastrulation and embryogenesis

THERAPEUTIC UTILITY:1. cancer: selectins can be used as

nanodevices to treat cancer. Researchers are trying to create a device capable of killing cancer cells circulating in the blood.

2. osteoporosis: osteoporosis is a disease that occurs when bone creating cells called osteoblasts become to scarce. Osteoblast developed from stem cells can be able to treat osteoporosis by adding stem cells to a patient’s bone marrow. E-selectins are constitutively expressed in the bone marrow, and researchers have shown that tagging stem cells with a certain glycoprotein causes those cells to migrate to bone marrow.

3.Ischemic heart disease: GPIIb/IIIa antagonists (like abciximab,tirofiban,eptifibatide)are effective antithrombotic agents. As compared to alone thrombolytic agent, combination of GPIIb/IIIa antagonists and a thrombolytic agent produces more rapid and extensive clot lysis,reduces the risk of re-occlusion and dimnishes infarct size.

Trials of these antagonists along with other thrombolytic agents and anticoagulants are currently ongoing in MI and unstable angina. Other conditions which can be benefited are stroke, thrombotic thrombocytopenic purpura and microvascular surgery etc.

4.Inflammatory diseases: Overzealous accumulation of leukocytes in tissues contributes to a wide variety of diseases.

Atherosclerosis Asthma and COPD Chronic IBD Rheumatoid arthritis Multiple sclerosis Juvenile diabetes etc therapeutic strategies are thus directed to

reduce or prevent leukocyte-endothelial cell interactions and communication, in order to limit the progression of inflammatory diseases.

5.Bacterial diseases: advantage of adherent state to bacterias:

required for colonization and for subsequent development of disease

provides significantly greater resistance against clearance by normal cleansing mechanisms,killing by normal immune factors,bacteriolytic enzymes and antibodies

better ability to acquire nutrients,further enhancing ability to survive and infect the host.

Thus the target is to reduce contact between host tissues and pathogens,either by prevention or reversal of adhesion(anti-adhesive agents) of infectious agents.

Glycoprotein IIb/IIIa antagonists Main adhesion molecule involved in platelet

aggregation is the membrane protein, GPIIb/IIIa complex. GPIIb/IIIa is an integrin receptor present at high density on platelets.  It exists as an inactive form in resting platelets. Platelet activation, associated with the release of agonists (epinephrine, serotonin, thromboxane A2) from the platelet-dense granules, leads to the recruitment of the internal pool of glycoprotein IIb/IIIa receptors, and this increases the number of surface receptors by 50%. It induces conformational changes of GPIIb/IIIa, which becomes competent to bind soluble plasma fibrinogen.  The receptor-bound fibrinogen acts as a bridge between two GPIIb/IIIa molecules on adjacent platelets. This is the final common pathway of platelet aggregation.

MOA: GP IIb/IIIa is unable to bind fibrinogen unless the platelet is first activated by an agonist, which then induces a conformational change in GP IIb/IIIa receptor, exposing the binding domain and rendering the molecule a competent fibrinogen binder. GP IIb/IIIa antagonists block the final step in platelet aggregation triggered by all platelet activators.INTRAVENOUS ANTAGONISTS: Abiciximab, Eptifibatide, Tirofiban and Lamifiban

ORAL ANTAGONISTS: Xemilofiban, orofiban, sibrafiban, lefradafiban.

Abciximab(ReoPro): Abciximab is a monoclonal antibody that targets the GP IIb/IIIa receptor on the surface of platelets.

Abciximab is indicated as an adjunct to aspirin and heparin for prevention of ischemic complications in patients undergoing PCI. It is

also indicated for the short-term (1 month) reduction of risk of MI in patients who have unstable angina that is not responding to full conventional therapy and who are to undergo PCI. Side effects are bleeding and thrombocytopenia

Eptifibatide (Integrilin): Eptifibatide is a synthetic cyclic heptapeptide and is oneof the small-molecule GP IIb/IIIa inhibitors

It reversibly inhibits platelet aggregation by preventing the binding of fibrinogen and other adhesive ligands to the GP IIb/IIIa receptor. Eptifibatide is indicated for the prevention of early MI in patients presenting with unstable angina or non-Q-wave MI (NSTEMI) who have had chest pain within the last 24 hours and who have ECG changes and/or elevated cardiac enzymes.

Tirofiban (Aggrastat): Tirofiban is a non-peptidal antagonist of the GPIIb/IIIa receptor. . It prevents fibrinogen from binding to the GP IIb/IIIa receptor, thus blocking platelet aggregation. Tirofiban is indicated for the prevention of early MI in patients presenting with unstable angina or NSTEMI who have had chest pain within the last 12 hours and who have ECG changes and/or elevated cardiac enzymes.

Anti integrin therapy: Integrins are the foremost family of cell adhesion

molecules that regulate immune cell trafficking in health and diseases. Different integrin antagonists can be used in therapeutics:

1. Natalizumab(tysabri): Integrin alpha4 mediates organ-specific migration of immune cells to the inflamed brain, thereby playing the critical role in the pathogenesis of multiple sclerosis. Anti-alpha4 integrin therapy (natalizumab) aiming to block infiltration of autoreactive lymphocytes to the inflamed brain has been validated in several clinical trials for the treatment of multiple sclerosis. Other potential uses of this type of therapeutic mAb include inflammatory and/or autoimmune conditions with a high impact in general population, including asthma etc

2. Efalizumab(raptiva): for the therapy of moderate to severe forms of psoriasis. The other therapeutic uses of anti-LFA-1 mAb include the prevention of bone marrow graft failure, and the treatment of kidney allograft rejection, asthma 3. Abciximab( ReoPro): is a humanised version of a function-blocking mAb (7E3) that recognises the β3 subunit and blocks the platelet fibrinogen receptor αIIbβ3.It is used in the treatment of unstable angina and as an adjuvant to percutaneous coronary interventions. 4.Erlizumab(rhuMab):is a  recombinant humanized monoclonal antibody that was an immunosuppressive drug. Erlizumab was to treat heart attack, stroke, and traumatic shock.

 

The drug works by blocking a growth factor in blood vessels. Specifically, erlizumab targets CD18 and an LFA-1 integrin. Erlizumab was meant to stop lymphocyte movement into inflamed tissue, thereby reducing tissue damage.

5.Enlimomab: is an anti-ICAM monoclonal antibody, inhibits leukocyte adhesion to the vascular endothelium, thereby decreasing leukocyte extravasation and inflammatory tissue injury. It is beneficial in reducing both disease activity in refractory rheumatoid arthritis and the incidence of acute rejection after kidney and liver allograft transplantations.