Expression and functional characteristics of calpain 3 isoforms generated through tissue-specific transcriptional and posttranscriptional events

Calpain 3 is a nonlysosomal cysteine protease whose biological functions re main unknown. We previously demonstrated that this protease is altered in l imb girdle muscular dystrophy type 2A patients. Preliminary observations su ggested that its gene is subjected to alternative splicing. In this paper, we characterize transcriptional and posttranscriptional events leading to a lterations involving the NS, IS1, and IS2 regions and/or the calcium bindin g domains of the mouse calpain 3 gene (capn3), These events can be divided into three groups: (i) splicing of exons that preserve the translation fram e, (ii) inclusion of two distinct intronic sequences between exons 16 and 1 7 that disrupt the frame and would lead, if translated, to a truncated prot ein lacking domain IV, and (iii) use of an alternative first exon specific to lens tissue. In addition, expression of these isoforms seems to be regul ated. Investigation of the proteolytic activities and titin binding abiliti es of the translation products of some of these isoforms clearly indicated that removal of these different protein segments affects differentially the biochemical properties examined. In particular, removal of exon 6 impaired the autolytic but not fodrinolytic activity and loss of exon 16 led to an increased titin binding and a loss of fodrinolytic activity, These results are likely to impact our understanding of the pathophysiology of calpainopa thies and the development of therapeutic strategies.