SMAD3 and PINK1 constitute a new positive feedback loop in regulation of mitophagy

Mitophagy, selective degradation of dysfunctional mitochondria by the autophagy-lysosome pathway, is critical for maintaining cellular homeostasis. In recent years, significant progress has been made in understanding how PINK1 (PTEN-induced kinase 1)-mediated phosphorylation and the E3 ubiquitin (Ub) ligase (PRKN/parkin)-mediated ubiquitination form a positive feedforward loop in control of mitophagy. Nevertheless, a fundamental question remains: How is PINK1 transcriptionally modulated under mitochondrial stress to finely support mitophagy? Recently, we unveiled a novel mechanism in control of PINK1 transcription by SMAD3 (SMAD family member 3), an essential component of the TGFB/TGFβ (transforming growth factor beta)-SMAD signaling pathway. Upon mitochondrial depolarization, SMAD3 is activated through PINK1-mediated phosphorylation of SMAD3 at serine 423/425 independent of canonical TGFB signaling. More importantly, the SMAD3-PINK1 regulatory axis appears to functionally provide a pro-survival mechanism against mitochondrial stress. Therefore, PINK1 and SMAD3 constitute a newly discovered positive feedforward loop to regulate mitophagy, highlighting the need for further exploring the crosstalk between TGFB-SMAD signaling and mitophagy.