ITGA7

Structure

ITGA7 encodes the protein alpha-7 integrin. Alpha-7 integrin is 128.9 kDa in molecular weight and 1181 amino acids in length. Integrins are heterodimeric integral membrane proteins composed of an alpha chain and a beta chain. Alpha-7 integrin undergoes post-translational cleavage within the extracellular domain to yield disulfide-linked light and heavy chains that join with beta 1 to form an integrin that binds to the extracellular matrix protein laminin-1. The primary binding partners of alpha-7 integrin are laminin-1 (alpha1-beta1-gamma1), laminin-2 (alpha2-beta1-gamma1) and laminin-4 (alpha2-beta2-gamma1). Alpha-7/beta-1 is the major integrin complex expressed in differentiated muscle cells.

Splice variants of alpha-7 integrin that differ in both the extracellular and cytoplasmic domains exist in the mouse and are developmentally regulated in mouse and rat muscle tissue. The X1/X2 alternative splicing region lies in the extracellular domain and alters the ligand binding site; specifically, the conserved homology repeat domains 3 and 4. The first identified human transcript contains extracellular and cytoplasmic domains corresponding to the mouse X2 and B variants, respectively. A unique extracellular splice variant was also identified in human. The differentially spliced variants detected in rodents have also been detected in humans. Major cytoplasmic, developmentally regulated variants, alpha-7A and alpha-7B, as well as extracellular variants, X1 and X2 were identified in humans. Moreover, the D variant, but not the C variant was detected in humans.

Alpha-7 integrin is highly expressed in striated muscle, namely skeletal and cardiac muscle, and functions as the major laminin-binding integrin. It was later shown that alpha-7 integrin is also highly expressed in smooth muscle. The two major splice variants of alpha-7 integrin appear to have developmentally regulated expression; alpha-7A integrin is expressed solely in skeletal muscle, however alpha-7B integrin is expressed more loosely in striated muscle as well as the vasculature.

Function

The function of alpha-7 integrin, as is the case for most integrins is to mediate cell membrane interactions with extracellular matrix.

The alpha-7/beta-1 integrin complex clearly plays a role in the development of striated muscle and smooth muscle. Alpha-7/beta-1 integrin promotes the adhesion and motility of myoblasts, and is likely important in the recruitment of myogenic precursors during muscle differentiation. It was shown however that beta-1D integrin appears at embryonic day 11 and alpha-7 integrin does not appear until embryonic day 17; thus, beta-1D associates with alternate alpha subunits (alpha-5, alpha-6A) prior to alpha-7. In human skeletal muscle, alpha-7 integrin is also developmentally regulated, being first detected at age 2.

In adult striated muscle cells, alpha-7 integrin (complexed to beta-1 integrin) is localized to Z-discs and costamere structures, bound to the four and one half LIM domain proteins, FHL1 and FHL2. It has been demonstrated that alpha-7 integrin can be mono-ADP-ribosylated on the cell surface in skeletal muscle cells; however, the functional significance of this modification has not been investigated.

Insights into the function of alpha-7 integrin have come from studies employing mouse transgenesis. A mouse expressing a null allele of the ITGA7 gene are viable, suggesting that alpha-7 integrin is not essential for normal myogenesis; however, these mice develop a phenotype that resembles muscular dystrophy. In soleus muscle, there was a significant disruption of myotendinous junctions, variation in the size of fibers, centrally located nuclei, necrosis, phagocytosis, and elevated serum levels of creatine kinase. It has also been proposed that alpha-7 integrin and gamma-sarcoglycan have overlapping functions in skeletal muscle. In support of this, a double knockout of gamma-sarcoglycan and alpha-7 integrin produced a phenotype that was far worse than either knockout alone. Mice died within 1 month of birth and had severe muscle degeneration, suggesting that the roles of these proteins may overlap to maintain the stability of the sarcolemma. Moreover, the double knockout of dystrophin and alpha-7 integrin produced a Duchenne muscular dystrophy-like phenotype, and demonstrated that alterations in alpha-7 integrin affect the pathological changes observed in dystrophin deficiencies. In support of this notion, AAV overexpression of ITGA7 in skeletal muscle of Duchenne muscular dystrophy (DMD) mice showed a significant protective effect against adverse functional parameters associated with DMD, combined with a reversal of these negative features, suggesting that alpha-7 integrin may be a potential therapeutic candidate to treat Duchenne muscular dystrophy.

Studies employing mutant alpha-7 integrin constructs have shown that the cytoplasmic tail of alpha-7B integrin is essential for regulation of lamellipodia formation and regulation of cell mobility regulation via laminin-1/E8 and p130(CAS)/Crk complex formation.

Clinical Significance

Mutations in ITGA7 have been found in patients with unclassified congenital myopathy. Additionally, in patients with severe congenital fiber type disproportion and left ventricular non-compaction cardiomyopathy, a missense mutation, Glu882Lys, was identified in ITGA7 along with a missense mutation in MYH7B, both novel disease genes having a synergistic effect on disease severity.

Alpha-7B integrin expression has been shown to be significantly decreased at sarcolemmal membranes in patients with laminin alpha2 chain-deficient congenital muscular dystrophy. Additionally, in Duchenne muscular dystrophy and Becker muscular dystrophy, the expression of alpha-7B integrin was enhanced.

Interactions

ITGA7 has been shown to interact with: