Chemotherapy-induced peripheral neuropathy: a focus on mitochondria and ER- MITOCHONDRIA INTERACTIONS

Chemotherapy (CT)-induced peripheral neuropathy (CIPN) is a common adverse effect of the treatment with different classes of chemotherapeutic agents, apparently regardless of their specific mechanism of action. At the cellular level, these agents are well known to exert neurotoxic effects over sensory neurons and glial cells supporting them in the peripheral nervous system (PNS), but the precise molecular mechanisms by which they induce damage to these cells are poorly understood. Noteworthy, mitochondria have been reported as common targets of several CT-based regimens, both in terms of their morphology/motility and of their specific functionality, which is of paramount importance for the maintenance of cellular homeostasis. In the context of mitochondrial morphology and functionality, which are deeply interdependent, growing body of evidence highlights the relevance of endoplasmic reticulum (ER)-mitochondria interaction, which ultimately takes place at the level of complex and well-organized morpho-functional units referred to as mitochondria-ER contact sites (MERCS). Therefore, a promising strategy to better unveil CIPN molecular mechanisms could be represented by a focused, in-depth analysis of the possible effects of different classes of antineoplastic drugs at the level of MERCS in both sensory neurons and peripheral glial cells. MERCS are involved in a plethora of intracellular processes, and ER-mitochondria distance has recently emerged as a critical parameter for ensuring their correct realization. Particularly, we focused our attention on calcium (Ca2+) transfer, that at the level of the ER-mitochondria interface is mediated by a complex made of inositol 1,4,5-trisphosphate receptor (IP3R) on the ER side, by 75kDa glucose-regulated protein (Grp75) and by voltage-dependent anion channel 1 (VDAC1) on the outer mitochondrial membrane (OMM). Due to the steric hindrance of this complex, a distance of ~20 nm was proposed as the optimal to promote ER-mitochondria Ca2+ transfer. Accordingly, we decided to investigate both mitochondrial morphology and functionality in MSC80 and F11 cell lines (chosen as models for sensory neurons and peripheral glial cells, respectively) upon treatment with bortezomib (BTZ), a 1st generation proteasome inhibitor approved as first-line therapy for multiple myeloma. Besides overall mitochondrial morphology and distribution, we paid special attention to ER-mitochondria contacts, that we analyzed by exploiting split-GFP contact site sensors (SPLICS). Moving to functionality, we assessed intracellular Ca2+ handling and mitochondrial membrane potential (ΔΨm), taking advantage of both dedicated dyes and genetically-encoded probes. In line with already published results, we observed that BTZ induces profound changes in mitochondrial network, which results collapsed in the perinuclear region; strikingly, BTZ also drastically alters the amount and localization of ER-mitochondria contact sites. In our model of Schwann cells, these alterations are accompanied by a strong BTZ-induced depolarization of the mitochondrial membrane. Of note, this depolarization is partially reverted when the cells are treated with BTZ in combination with isoallopregnenolone and allopregnanolone. These compounds belong to the class of neuroactive steroids, that, intriguingly, have already been reported to exert neuroprotective effects over peripheral nerves in different experimental models of peripheral neuropathy, including docetaxel-induced peripheral neuropathy.