Polycystin-2-dependent control of cardiomyocyte autophagy

Considerable evidence points to critical roles of intracellular Ca²⁺ homeostasis in the modulation and control of autophagic activity. Yet, underlying molecular mechanisms remain unknown. Mutations in the gene (pkd2) encoding polycystin-2 (PC2) are associated with autosomal dominant polycystic kidney disease (ADPKD), the most common inherited nephropathy. PC2 has been associated with impaired Ca²⁺ handling in cardiomyocytes and indirect evidence suggests that this protein may be involved in autophagic control. Here, we investigated the role for PC2 as an essential regulator of Ca²⁺ homeostasis and autophagy.

Activation of autophagic flux triggered by mTOR inhibition either pharmacologically (rapamycin) or by means of nutrient depletion was suppressed in cells depleted of PC2. Moreover, cardiomyocyte-specific PC2 knockout mice (αMhc-cre;Pkd2^F/F mice) manifested impaired autophagic flux in the setting of nutrient deprivation. Stress-induced autophagy was blunted by intracellular Ca²⁺ chelation using BAPTA-AM, whereas removal of extracellular Ca²⁺ had no effect, pointing to a role of intracellular Ca²⁺ homeostasis in stress-induced cardiomyocyte autophagy. To determine the link between stress-induced autophagy and PC2-induced Ca²⁺ mobilization, we over-expressed either wild-type PC2 (WT) or a Ca²⁺-channel deficient PC2 mutant (PC2-D509V). PC2 over-expression increased autophagic flux, whereas PC2-D509V expression did not. Importantly, autophagy induction triggered by PC2 over-expression was attenuated by BAPTA-AM, supporting a model of PC2-dependent control of autophagy through intracellular Ca²⁺. Furthermore, PC2 ablation was associated with impaired Ca²⁺ handling in cardiomyocytes marked by partial depletion of sarcoplasmic reticulum Ca²⁺ stores. Finally, we provide evidence that Ca²⁺-mediated autophagy elicited by PC2 is a mechanism conserved across multiple cell types.

Together, this study unveils PC2 as a novel regulator of autophagy acting through control of intracellular Ca²⁺ homeostasis.