A non-apoptotic role for caspase-9 in muscle differentiation

Abstract

Caspases, a family of cysteine proteases most often investigated for their roles in apoptosis, have also been demonstrated to have functions that are vital for the efficient execution of cell differentiation. One such role that has been described is the requirement of caspase-3 for the differentiation of skeletal myoblasts into myotubes but, as yet, the mechanism leading to caspase-3 activation in this case remains elusive. Here, we demonstrate that caspase-9, an initiator caspase in the mitochondrial death pathway, is responsible for the activation of caspase-3 in differentiating C2C12 cells. Reduction of caspase-9 levels, using an shRNA construct, prevented caspase-3 activation and inhibited myoblast fusion. Myosin-heavychain expression, which accompanies myoblastic differentiation, was not caspase-dependent. Overexpression of Bcl-xL, a protein that inhibits caspase-9 activation, had the same effect on muscle differentiation as knockdown of caspase-9. These data suggest that the mitochondrial pathway is required for differentiation; however, the release of cytochrome c or Smac (Diablo) could not be detected, raising the possibility of a novel mechanism of caspase-9 activation during muscle differentiation.

Old, new and emerging functions of caspases

“…It was observed that depletion of caspase-9 or overexpression of Bcl-xL reduced caspase-3 activation and this had a limiting effect on the differentiation of myoblasts. 88 Osteogenic differentiation of mesenchymal stem cells involves caspase-3 activity, and caspase-3-deficient mice exhibit attenuated differentiation of the bone marrow stromal stem cells and consequently decreased bone mineral density. 89 It is noteworthy that the process of normal osteoblast differentiation requires the transient and potent activation of other caspases, including caspase-8 and -2, without any increase in osteoblast apoptotic cell death.…”

Dronc caspase exerts a non-apoptotic function to restrain phospho-Numb-induced ectopic neuroblast formation in Drosophila

“…In addition, Dronc RNAi is able to further increase ectopic neuroblasts in numb S52F mutant, indicating that Dronc-Numb interaction is normally involved in regulating neuroblast homeostasis. Accumulating evidence suggests that caspases, in addition to their pro-apoptotic functions, also participate in other developmental process without inducing cell death (Geisbrecht and Montell, 2004;Huh et al, 2004a;Huh et al, 2004b;Kuranaga et al, 2006;Kuranaga and Miura, 2007;Maelfait and Beyaert, 2008;Murray et al, 2008). For example, Dronc has been implicated in a non-autonomous role in compensatory proliferation (Huh et al, 2004a;Wells et al, 2006).…”

Parole terms for a killer

“…Several studies have examined the signal cascades that activate and modulate caspase 3 during non-death responses, yet a definitive control mechanism has not been elucidated. [37][38][39] Such inactivation could indirectly end CAD activity by discontinuing proteolysis of ICAD. Intact ICAD (both long and short isoforms) can disassemble the CAD dimer, moreover our observation that ICAD-L is only partial cleaved (with a large pool of intact ICAD-L remaining) suggests that this mechanism maybe operative to prevent extensive DNA fragmentation.…”