Signalling Role of TRPM2 Channels in Organelle Dynamics in a Cellular Model of Parkinson's Disease

Calcium ions (Ca2+) and reactive oxygen species (ROS) are known to contribute to mitochondrial and lysosomal dysfunction in Parkinson’s diseases (PD), however the precise mechanisms remain unclear. Given that the Transient receptor potential melastatin 2 (TRPM2) channels are regulated by ROS and transport Ca2+ ions, it was hypothesised that coactivation of Ca2+-sensitive NADPH oxidase 2 (NOX2) and the ROS-sensitive TRPM2 channel amplifies Ca2+ and ROS signalling, driving organelle dysfunction and causing neuronal cell death.

To test this hypothesis, I have used the SH-SY5Y cellular model of PD and HEK-293 TRPM2tet cells with tetracycline-inducible TRPM2 expression. SH-SY5Y cells were exposed to PD toxin, MPP+, to induce PD-like phenotype. The effect of MPP+ on the homeostasis of ions and ROS, the integrity of lysosomes and mitochondria as well as cell viability were examined using fluorescent probes. Calcium and zinc ionophores (A23187 and zinc pyrothione, respectively) were also used to investigate the individual roles of Ca2+ and Zn2+ in these processes. The results demonstrate that a positive feedback cycle between TRPM2 and NOX2 amplifies ROS and Ca2+ signals, with their elevation triggering lysosomal membrane permeabilization (LMP). This leads to the translocation of lysosomal Zn2+ to mitochondria, where Zn2+ induces a loss of mitochondrial membrane potential (ΔΨm), mitochondrial ROS (mtROS) production, primarily from complex III, and fragmentation, ultimately leading to apoptotic cell death. In contrast to the widely held notion, Ca2+ does not induce mitochondrial dysfunction directly; rather, Zn2+ signals released from lysosomes play this role. The results also show that the resulting mtROS feeds back to activate TRPM2 channels, restarting the cycle of mtROS production. This amplification of mtROS via positive feedback cycles is essential to impart the pathological effects of TRPM2 downstream signalling.

Finally, an antibody capable of blocking the TRPM2 channel pore mitigates the entire inter-organelle communication and downstream signalling events responsible for neuronal cell death. Taken together, these results describe a signalling cascade, involving ROS-Ca2+ interaction and altered Zn2+ dynamics, promotes impairment of lysosomes and mitochondria leading to apoptotic cell death. This signalling pathway may be relevant to aging-related diseases where organelle damage and cell death play key roles.