Calpain 3, the "gatekeeper" of proper sarcomere assembly, turnover and maintenance

Abstract

Calpain 3 is a member of the calpain family of calcium-dependent intracellular proteases. Thirteen years ago it was discovered that mutations in calpain 3 (CAPN3) result in an autosomal recessive and progressive form of limb girdle muscular dystrophy called limb girdle muscular dystrophy type 2A. While calpain 3 mRNA is expressed at high levels in muscle and appears to have some role in developmental processes, muscles of patients and mice lacking calpain 3 still form apparently normal muscle during prenatal development; thus, a functional calpain 3 protease is not mandatory for muscle to form in vivo but it is a pre-requisite for muscle to remain healthy. Despite intensive research in this field, the physiological substrates of the calpain 3 protein (hereafter referred to as CAPN3) and its alternatively spliced isoforms remain elusive. The existence of these multiple isoforms complicates the search for the physiological functions of CAPN3 and its pathophysiological role. In this review, we summarize the genetic and biochemical evidence that point to loss of function of the full-length isoform of CAPN3, also known as p94, as the pathogenic isoform. We also argue that its natural substrates must reside in its proximity within the sarcomere where it is stored in an inactive state anchored to titin. We further propose that CAPN3 has many attributes that make it ideally suited as a sensor of sarcomeric integrity and function, involved in its repair and maintenance. Loss of these CAPN3-mediated activities can explain the "progressive" development of muscular dystrophy.

Figure 1

A. Distribution of evolutionary non-conserved amino acid residues among mammalian mature CAPN3's (blue) and reported pathogenic LGMD2A missense mutations (red) using a scanning window and bins of respectively 10 and 20 amino acids. B. Schematic representation of CAPN3 and its diverse protein domains (I-IV) with the three calpain3-specific domains, NS, IS1 and IS2.

Figure 2. Calpain 3 may function in conjunction with E3 ubiquitin ligases to mediate sarcomere remodeling

Shown is a cartoon illustrating calpain 3's role in sarcomere remodeling. Calpain 3 (red) is anchored to the sarcomere at the N2 line and M line, through its association with titin (not shown). The contractile proteins are highly organized and entwined and cannot be degraded in the proteasome without the initiating step of proteolytic dissociation of this complex (for simplicity, only actin and myosin are shown). Data suggest that calpain 3 is one protein that performs the initial proteolytic cleavage that allows E3 ubiquitin ligases to ubiquitinate the peptides and target them for degradation in the proteasome (see [29]).