Caspase 3 cleavage of Pax7 inhibits self-renewal of satellite cells

Compensatory growth and regeneration of skeletal muscle is dependent on the resident stem cell population, satellite cells (SCs). Self-renewal and maintenance of the SC niche is coordinated by the paired-box transcription factor Pax7, and yet continued expression of this protein inhibits the myoblast differentiation program. As such, the reduction or removal of Pax7 may denote a key prerequisite for SCs to abandon self-renewal and acquire differentiation competence. Here, we identify caspase 3 cleavage inactivation of Pax7 as a crucial step for terminating the self-renewal process. Inhibition of caspase 3 results in elevated Pax7 protein and SC self-renewal, whereas caspase activation leads to Pax7 cleavage and initiation of the myogenic differentiation program. Moreover, in vivo inhibition of caspase 3 activity leads to a profound disruption in skeletal muscle regeneration with an accumulation of SCs within the niche. We have also noted that casein kinase 2 (CK2)-directed phosphorylation of Pax7 attenuates caspase-directed cleavage. Together, these results demonstrate that SC fate is dependent on opposing posttranslational modifications of the Pax7 protein

Doxorubicin-induced chronic dilated cardiomyopathy—the apoptosis hypothesis revisited

The chemotherapeutic agent doxorubicin (DOX) has significantly increased survival rates of pediatric and adult cancer patients. However, 10% of pediatric cancer survivors will 10–20 years later develop severe dilated cardiomyopathy (DCM), whereby the exact molecular mechanisms of disease progression after this long latency time remain puzzling. We here revisit the hypothesis that elevated apoptosis signaling or its increased likelihood after DOX exposure can lead to an impairment of cardiac function and cause a cardiac dilation. Based on recent literature evidence, we first argue why a dilated phenotype can occur when little apoptosis is detected. We then review findings suggesting that mature cardiomyocytes are protected against DOX-induced apoptosis downstream, but not upstream of mitochondrial outer membrane permeabilisation (MOMP). This lack of MOMP induction is proposed to alter the metabolic phenotype, induce hypertrophic remodeling, and lead to functional cardiac impairment even in the absence of cardiomyocyte apoptosis. We discuss findings that DOX exposure can lead to increased sensitivity to further cardiomyocyte apoptosis, which may cause a gradual loss in cardiomyocytes over time and a compensatory hypertrophic remodeling after treatment, potentially explaining the long lag time in disease onset. We finally note similarities between DOX-exposed cardiomyocytes and apoptosis-primed cancer cells and propose computational system biology as a tool to predict patient individual DOX doses. In conclusion, combining recent findings in rodent hearts and cardiomyocytes exposed to DOX with insights from apoptosis signal transduction allowed us to obtain a molecularly deeper insight in this delayed and still enigmatic pathology of DC