Consequences of Caspase Inhibition in Differentiating Myoblasts

Abstract

The apoptotic caspases are proteases that mediate the process of programmed cell death. Canonically, their activation is a point of no return; however, the disruption of myoblast differentiation resulting from caspase inhibition has led some to suggest a non-apoptotic role for caspases in skeletal muscle differentiation. With no identified activation mechanism and no direct observation of cell-autonomous caspase activation in differentiating myoblasts, the validity of this model remains an open question. Here I demonstrate that there likely is no non-apoptotic role for caspases during myoblast differentiation. Rather, caspase inhibition results in the persistence of a population of cells that would have undergone apoptosis had caspases been activate, which I have dubbed UNDEAD cells for upregulators of NF-B dependent on escape from apoptotic death. I found that this population specifically initiates NF-B and type I interferon signaling, resulting in the secretion of inflammatory cytokines. Furthermore, I discovered that conditioned media from caspase-inhibited myoblasts is sufficient to both inhibit the differentiation of and promote the proliferation of naive myoblasts. By preventing the formation of the UNDEAD subpopulation via overexpression of Bcl-xL, I was able to prevent cytokine secretion and nearly abolish the inhibitory effects of caspase inhibition on differentiation. In sum, these results strongly contradict the established model of non-apoptotic caspase function in myoblasts, while simultaneously establishing a novel context in which innate immune signaling can perturb the behavior of local cell populations.