Apoptosis and non-inflammatory phagocytosis can be induced by mitochondrial damage without caspases

A central issue regarding vertebrate apoptosis is whether caspase activity is essential, particularly for its crucial biological outcome: non-inflammatory clearance of the dying cell. Caspase-9 is required for the proteolytic cascade unleashed by the mitochondrial outer membrane permeabilization (MOMP) regulated by the Bcl-2 protein family. However, despite the severely blunted apoptosis in cells from Casp9^-/- mice, some organs with copious apoptosis, such as the thymus, appear unaffected. To address this paradox, we investigated how caspase-9 loss affects apoptosis and clearance of mouse fibroblasts and thymocytes. Although Casp9^-/- cells were initially refractory to apoptotic insults, they eventually succumbed to slower caspase-independent cell death. Furthermore, in γ-irradiated mice, the dying Casp9^-/- thymocytes were efficiently cleared, without apparent inflammation. Notably, MOMP proceeded normally, and the impaired mitochondrial function, revealed by diminished mitochondrial membrane potential (Δ Ψ_m), committed cells to die, as judged by loss of clonogenicity. Upon the eventual full collapse of Δ ψ_m, presumably reflecting failure of respiration, intact dying Casp9^-/- cells unexpectedly exposed the prototypic eat-me signal phosphatidylserine, which allowed their recognition and engulfment by phagocytes without overt inflammation. Hence, caspase-9-induced proteolysis accelerates apoptosis, but impaired mitochondrial integrity apparently triggers a default caspase-independent program of cell death and non-inflammatory clearance. Thus, caspases appear dispensable for some essential biological functions of apoptosis.