Impact of PARL-mediated mitochondrial protease activity on calcium regulation

The presenilin-associated rhomboid-like protein (PARL) is a mitochondrial inner membrane serine protease that is a key regulator of several cellular processes, including apoptosis, metabolism, inflammation and stress responses. While recent studies suggest that PARL may play a role in mitochondrial calcium homeostasis, the underlying mechanisms remain poorly understood. In this study, we investigated the effects of PARL modulation on mitochondrial and cytosolic calcium dynamics, as well as mitochondrial membrane potential. Our results show that altering PARL protein levels, through both overexpression and silencing, significantly affects mitochondrial calcium uptake, without influencing cytosolic calcium transients or mitochondrial membrane potential. Despite the observed changes in mitochondrial calcium dynamics, PARL does not interact with the mitochondrial calcium uniporter complex (mtCU) regulators MICU1 and MICU2, which are critical for regulating mitochondrial calcium influx. However, we observed alterations in the protein levels of MICU1 and MICU2, either in their monomeric or dimeric forms, suggesting that PARL may influence these mtCU components indirectly. Interestingly, the pore-forming subunit MCU, and the structural subunit EMRE, essential for the assembly of the mtCU, were unaffected by PARL modulation. These findings suggest that the role of PARL in modulating mitochondrial calcium homeostasis may involve indirect mechanisms, potentially involving other regulatory pathways. Overall, our study provides novel insights into the functional role of PARL in mitochondrial calcium regulation, offering potential avenues for further investigation into its broader cellular functions.