Receptor sorting through tubular microdomains of Rab7-positive endosomes in Charcot- Marie-Tooth disease 2B

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Charcot-Marie-Tooth (CMT) disease is an inherited neuropathy which affects the peripheral nervous system, leading to neurodegeneration. Disease onset occurs generally during adolescence or adulthood and is manifested by muscle weakness, loss of fine motor control and sensation in the extremities. In autosomal dominant Charcot-Marie Tooth type 2B (CMT2B), disease is caused by mutations in the late endosomal Rab7-GTPase. In neurons, Rab7- endosomes are important for the trafficking of growth-factors: Following activation by their respective ligand, growth-factor receptors are endocytosed, subsequently sorted into Rab7-positive endosomes and transported retrogradely to the soma where they regulate gene expression of pro-survival genes. Finally, receptors are degraded through the lysosomal pathway. Intriguingly, recent studies have linked CMT2B to impairments in trafficking of Rab7-positive late endosomes, but how mutations in Rab7-GTPase contribute to growth- factor receptors trafficking in CMT2B remains elusive. I hypothesize that endosomes in CMT2B-Rab7 mutants show perturbed sorting abilities due to disturbed formation of tubular microdomains into which receptors are routed during the sorting/trafficking process. Generally, it is considered that sorting occurs on early endosomes where receptors are sorted into the recycling pathways- either directly back to the plasma membrae or through the trans-golgi network (TGN). However, I hypothesize that these tubular microdomains on late endosomes also facilitate receptor sorting, potentially into lysosomes which would be a process uncoupled/in parallel from the process of endosomal maturation. The perturbation in forming such tubular microdomains could therefore affect sorting from Rab7-positive endosomes into lysosomes, defective receptor signalling, a delay in receptor degradation and ultimately downregulation of transcription genes which regulate innervation and/ or survival. All of these effects would explain several of the observed phenotypes in CMT2B, which includes prolonged growth-factor signaling, delay in growth factor sorting into lysosomes and defects in Rab7-endosome trafficking.The main goal of this project is to elucidate the pathogenic mechanisms leading to Charcot-Marie-Tooth type 2B, by focusing on mutations in Rab7-positive late endosomes within the endosomal sorting machinery. I will decipher interactions of Rab7-mutations in CMT2B with the cellular sorting machinery and I will follow subcellular localization of activated growth-factor receptors using high resolution imaging to visualize disturbed trafficking routes. I will address this in mouse DRG neurons as well as iPSC-derived motor- and sensory neurons from CMT2B patients. This proposed work will unravel sorting malfunctions in CMT2B and contribute to the evaluation of new therapeutic approaches by revealing possible target proteins.