Conference Agenda

Chemotherapy-induced peripheral neurotoxicity (CIPN) is one of the most frequent and dose-limiting long-term side effects that affects patients receiving several widely used anticancer drugs as taxanes, platinum derivatives, vinka-alkaloids and proteasome inhibitors. In particular, CIPN represents a disabling side effect since it consists in a predominantly sensory neuropathy that may be accompanied by motor and autonomic deficits. CIPN occurs because neurotoxic chemotherapy drugs reach peripheral nerves and dorsal root ganglia (DRG), sited outside the blood brain barrier and vascularized by fenestrated capillaries. For these reasons, the peripheral DRG neurons have, for a long time, been considered the only reasonable target of investigations for CIPN, that led to the discovery of several pathogenetic mechanisms of CIPN. Nevertheless, it is important to consider that in DRG, sensory neurons are tightly surrounded by the most abundant satellite glial cells (SGCs) with which they form a functional unit, ensuring the neuronal homeostasis. Therefore, it is plausible that changes in this non-neuronal and replicating counterpart could occur during CIPN onset and development, considering their potential vulnerability to the action of anti-replicating anticancer drugs. Recent data suggest that SGCs of the DRG are involved in neurotoxicity and pain development. Glial proliferation/activation, altered SGC-SGC and/or SGC-neuron interactions as well as changes in perineuronal neuroactive signalling molecules were reported in animal models of pain and following some chemotherapy treatments. This work will report how SGCs may act as principal actors in CIPN, paving the way for the identification of new druggable targets for the treatment and prevention of CIPN.

Charcot-Marie-Tooth (CMT) neuropathies represent a broad group of disorders generally characterized by progressive muscular atrophy and weakness, with an age at onset usually comprised between the first and the second decade of life.

CMT4B is a severe autosomal recessive demyelinating neuropathy with childhood onset characterized by the presence of redundant loops of myelin in the nerve termed myelin outfoldings. This morphology may be the consequence of an excessive longitudinal growth of the myelinated internode during postnatal nerve development.

CMT4B comprises three distinct genetic and clinical subtypes named CMT4B1, B2, and B3. While CMT4B1 and B2 present with a classical motor and sensory neuropathy phenotype, CMT4B3-associated phenotypes range from a pure demyelinating poly-neuropathy to an axonal neuropathy plus complex central nervous system (CNS) phenotypes. CMT4B neuropathies are caused by loss-of-function mutations in the myotubularin-related 2 (MTMR2, CMT4B1), MTMR13 (also known as SBF2, SET binding factor 2, CMT4B2), and MTMR5 (also known as SBF1, CMT4B3) genes. MTMR2, MTMR5, and MTMR13 belong to a broad family of protein tyrosine phosphatase/dual specificity-like phosphatases (PTP/DSP), which consists of 14 members in mammals. Six MTMRs are catalytically inactive, whereas eight are catalytically active enzymes, which dephosphorylate 3-phosphoinositides (PIs). Interestingly, MTMR5 and MTMR13 are catalytically inactive proteins, whereas MTMR2 is a catalytically active enzyme, which in vitro is predicted to dephosphorylate the 3-phosphoinositides PtdIns3P and PtdIns(3,5)P₂ in the 3-position of the inositol ring. Heterodimers of MTMR2 with either MTMR5 or MTMR13 are thought to possess higher enzymatic activity and a different sub-cellular localization as compared to MTMR2 homodimers. How loss of MTMR2/R5/R13 results in severe neuropathy or complex CNS phenotype remains to be clarified.

In this talk, we will revise recent advances on CMT4B-associated MTMRs and discuss potential therapeutic strategies for these disorders.

Charcot-Marie-Tooth type 2B (CMT2B) is an inherited peripheral neuropathy caused by missense mutations in the RAB7A gene.

RAB7A, a ubiquitous small GTPase, controls transport to late endosomes and lysosomes, and, given its pivotal role on endocytosis, impacts on many other cellular pathways. In particular, RAB7A controls neuronal specific processes such as neurotrophin trafficking and signaling, neurite outgrowth and neuronal migration, and it interacts with peripherin, an intermediate filament protein expressed mainly in peripheral neurons, regulating its assembly.

Previous studies demonstrated that CMT2B-causing RAB7A mutant proteins display altered GTP cycle impacting on neurite outgrowth, peripherin and vimentin interaction and assembly, autophagy, lysosomal functionality, lipid metabolism, lipid droplet breakdown but also mitochondrial contact sites and axonal growth.

Notably, RAB7A has a key role in mitophagy and in organelle contact sites, in particular between mitochondria, endosomes and lysosomes. In addition, recently, it has been discovered that late endosomes bearing RAB7A are mRNA translation sites sustaining mitochondria in axons. These data strongly indicate a key role of RAB7A in the regulation of different aspects of mitochondria dynamics and functions, particularly in neuronal cells. Considering that dysfunctions of mitochondrial turnover, morphology and functions have been identified as the cause of numerous diseases including those of the peripheral nervous system, this could be relevant also for CMT2B.

Thus, as mitochondria-lysosome cross-talk is fundamental for cellular metabolism and viability and considering that RAB7A has been found to regulate mitochondrial dynamics and mitochondria-lysosome contact sites, we started to investigate the relationship between RAB7A and mitochondria in CMT2B, looking at mitochondrial morphology, activity and biogenesis. We found that mitochondria are fragmented in patient cells and are dysfunctional. Furthermore, we found important defects in mitochondrial biogenesis. Altogether, the data obtained suggest that alterations of lysosomes and mitochondria and of their cross-talk could be responsible, at least in part, for neurodegeneration.