Opa1 participates in inner mitochondrial membrane fusion and cristae morphogenesis. Here, we show that muscle-specific Opa1 ablation causes reduced muscle fiber size, dysfunctional mitochondria, enhanced Fgf21, and muscle inflammation characterized by NF-κB activation, and enhanced expression of pro-inflammatory genes. Chronic sodium salicylate treatment ameliorated muscle alterations and reduced the muscle expression of Fgf21. Muscle inflammation was an early event during the progression of the disease and occurred before macrophage infiltration, indicating that it is a primary response to Opa1 deficiency. Moreover, Opa1 repression in muscle cells also resulted in NF-κB activation and inflammation in the absence of necrosis and/or apoptosis, thereby revealing that the activation is a cell-autonomous process and independent of cell death. The effects of Opa1 deficiency on the expression NF-κB target genes and inflammation were absent upon mitochondrial DNA depletion. Under Opa1 deficiency, blockage or repression of TLR9 prevented NF-κB activation and inflammation. Taken together, our results reveal that Opa1 deficiency in muscle causes initial mitochondrial alterations that lead to TLR9 activation, and inflammation, which contributes to enhanced Fgf21 expression and to growth impairment.

The metzincin clan encompasses several families of zinc-dependent metalloproteases with proven function both in physiology and pathology. They act either as broad spectrum protein degraders or as sheddases, operating through limited proteolysis. Among the structurally uncharacterized metzincin families are the pappalysins, of which the most thoroughly studied member is human pregnancy-associated plasma protein A (PAPP-A), a heavily glycosylated 170-kDa multidomain protein specifically cleaving insulin-like growth factor (IGF)-binding proteins (IGFBPs). Proulilysin is a 38-kDa archaeal protein that shares sequence similarity with PAPP-A but encompasses only the pro-domain and the catalytic domain. It undergoes calcium-mediated autolytic activation, and the mature protein adopts a three-dimensional structure with two subdomains separated by an active site cleft containing the catalytic zinc ion. This structure is reminiscent of human members of the adamalysin/ADAMs (a disintegrin and a metalloprotease) family of metzincins. A bound dipeptide yields information on the substrate specificity of ulilysin, which specifically hydrolyzes IGFBP-2 to -6, insulin, and extracellular matrix proteins but not IGFBP-1 or IGF-II. Accordingly, ulilysin has higher proteolytic efficiency and a broader substrate specificity than human PAPP-A. The structure of ulilysin represents a prototype for the catalytic domain of pappalysins. Insulin-like growth factors (IGF)-I and -II4 regulate human somatic growth and development. Their activity is also correlated with several diseases, including atherosclerosis, cardiovascular disease, diabetes, and cancer (1). Most circulating IGF molecules are sequestered in complexes with soluble IGF-binding proteins (IGFBP-1 to -6) (2). IGFBPs antagonize binding of IGFs to their receptors because of their much higher affinity, and they are thus carriers, mediators, and reservoirs of IGFs (3). IGFs are released from these complexes with IGFBP through proteolytic inactivation mediated by IGFBP proteases, which include serine proteinases, cysteine proteinases, and metalloproteases (MPs). Among these, human PAPP-A specifically inactivates IGFBPs (4 -6). This MP was originally identified as an antigen in human pregnancy plasma (7). It is ubiquitously expressed and plays central roles in ovarian follicular development, myogenesis, human embryo implantation, and wound healing (8). Mature PAPP-A is a glycosylated multidomain protein of 1,547 residues that specifically hydrolyzes human IGFBP-4 in an IGF-dependent manner. Only human IGFBP-5 and bovine and porcine IGFBP-2 have been identified as further substrates (6). In addition to its proteolytic domain, three Lin12-Notch repeats (LNR-1, -2, and -3), modules that regulate ligand-induced proteolytic cleavage of the Notch receptor, and five complement control protein modules (CCP1-5) have been identified (9). Human PAPP-A is the founding member of the pappalysin family of MPs, which further comprehends the paralogue PAPP-A2 (10) and has been included in the metzinc...