Laminin and integrin

How do mutations in laminin and integrin cause disease?

Basement membranes function to anchor cells via laminin and integrin. Defects in the attachment of cells to ECM lead to disease. The epidermis shows how defects can occur at multiple points in this linkage.

What is the structure of laminin?

Laminin is a high MW glycoprotein (around 800 kDa) with 3 chains in a cruciform structure. It is the next most abundant protein in the basement membrane after collagen IV. At the top of the crucifix are 3 globular domains separated by spacers with EGF repeats. 3 chains join together to form a coiled-coil (forms the 2 arms and bottom of the cross) - the alpha, beta and gamma chains are made up of alpha-helices. At the bottom is the alpha chain which forms the C-terminal globular domain - where cells attach.

The alpha globular domain consists of 5 LG domains that interact with cell surface receptors such as integrins and dystroglycan heparin.

How does laminin polymerise and connect to collagen IV?

Laminin 1 spontaneously forms a network in vitro (similar to col IV). Their N-terminal globular domains (3 top points of the cross) promote polymerisation into a network - join to each other at the N termini.

The laminin network interacts with the collagen IV network via accessory molecules - collagen provides structure, laminin provides cell attachment sites.

How many laminin isoforms are there?

There are 11 different laminin genes, but these only combine to form 15 different heterotrimeric combinations - 5 alpha chains, 3 beta chains and 3 gamma chains. Different laminin isoforms show tissue specific expression; for example, genetic deletion in mice indicates that some laminins are essential throughout the animal (a1b1g1 is the laminin found in the embryonic BM). 

Mutations in specific lamina (alpha1, beta1, gamma1) can cause early embryonic death. Some others cause early death after birth.

What is Pierson Syndrome and how is it caused?

Loss in laminin B2 isoform results in Pierson syndromes. This causes congenital nephrotic syndrome progressing to end stage renal disease, eye abnormalities, and severe muscular hypotonia. Laminin 11 (a5b2g1) is expressed in the glomerular BM, eye, and synaptic BM, explaining the disease phenotype.

• In mice, they can phenocopy for human diseases. It shows that for Pierson syndrome, the defect in beta 2 results in a different isoform replacing laminin 11 which cannot provide the correct function - poor filtration barrier in GBM. Collagen IV is not affected.

How do laminin mutations cause skin conditions? Give an example.

Specific laminin isoforms in the epidermis link to the underlying collagen and BM in the dermis. Mutations in this cause skin conditions. For example, Epidermolysis bullosa (EB) are a related group of conditions where skin blisters follow mechanical trauma. Mutations affect the mechanical strength of the dermal/epidermal junction. Position of the break depends upon the genetic defect.

How does a mutation in laminin V cause epidermolysis bullosa?

Laminin 5 (a3b3g2) is a skin-specific BM. It links cell surface adhesion proteins (integrins) in structures called hemidesmosomes to the underlying collagen IV. Collagen VII are anchoring fibres that link the BM to the collagen I network in the dermis. Collagen VII is also specific to basement membranes.

• Laminin 5 causes junctional EB where the epidermis lifts from BM. Complete loss of any Laminin 5 chain leads to Herlitz type JEB - lethal within the first few months after birth . Other mutations with perturbed laminin 5 function lead to milder forms of condition (non-Herlitz JEB) - missense, splice site mutations leading to reduced Lm5 expression or variant with partial function. These are all autosomal recessive conditions of any of the laminin isoforms.

• Use of genetic models in mice prove that the loss of laminin 5 causes JEB since laminin 5 deletion in mice phenocopies Herlitz JEB.

What are integrins?

In the cell-ECM junctions in the skin, there are structural links between the cytoskeleton and matrix. This provides physical strength to tissues.

Integrins are cell/ECM adhesion receptors on most cells. They are heterodimers of alpha and beta subunits, with a large extracellular domain, single transmembrane spanning domain (each chain goes through membrane with a single alpha helix) and a short cytoplasmic domain.

How do integrins vary?

There are many alpha and beta subunits, with known alpha-beta heterodimers. These have distinct but overlapping specificity for different ECM proteins.

Their cytoplasmic domain interacts with the cytoskeleton. Most integrins associate with actin, but in hemidesmosomes, they link to IFs made up of keratin.

Is there tissue specific distribution of integrins?

There is tissue specific distribution of integrin isoforms - some are necessary for life.

Why are B4 integrins necessary for health?

Hemidesmosome specific B4 integrins are required for the integrity of skin. Deletion of these leads to the same phenotype as laminin 5 deletions (JEB). The loss of B4 integrin results in the loss of hemidesmosomes, but not the cell-cell adhesions that link to IFs. They attach to each other, but not the skin which is why it lifts.

What is junctional epidermolysis bullosa with pyloric atresia?

Junctional epidermolysis bullosa with pyloric atresia (PA-JEB) is a rare autosomal recessive condition associated with loss of a6b4 integrin. It causes neonatal mucocutaneous blistering and gastric outlet obstruction through loss of function in gastrointestinal, genitourinary and respiratory epithelium. The disease is fatal.

The loss of B4 leads to the loss of A6 integrin. However, the formation of the BM appears to still occur as seen by the presence of laminin 5 - deletion of A6 integrin has the same effect as the loss of B4.

What is epidermolysis bullosa simplex?

EB simplex has a milder phenotype than other forms of EB. It is usually restricted to blisters on regions subject to mechanical stress. It can heal without significant scarring. When compared with JEB which has 40% mortality in the first year. It is mostly associated with mutations in keratins 5 and 14 meaning the skin is still attached, but the cell integrity is weaker.

How is epidermolysis bullosa simplex phenocopied in mice?

The partial deletion of a B4 cytoplasmic tail in mice results in EB simplex - the cells are weakened internally due to the weakened connection, but the BM is still intact. The cells rip apart leaving part still attached to the BM via integrin.

Patients can also have mutations in any of the cytoplasmic proteins that link the b4 integrin cytoplasmic domain with intermediate filaments. These show similar phenotype as b4 integrin cytoplasmic domain deletions.