A Sideroflexin/Connexin adaptor ties kinesin-3 to mitochondria in neurons

Various neurological diseases are linked to changes in mitochondrial trafficking in axons. Thus, it is crucial to understand how dynamics of mitochondria are regulated on the molecular level. From a candidate screen, we identified SFXN-1.2 to regulate both morphologies as well as dynamics of mitochondria. SFXN-1.2 is an ortholog of human Sideroflexin 1/3 associated with Alzheimer's disease and Parkinson's diseases. We demonstrate that SFXN-1.2 binds to kinesin-3 KIF1A(UNC-104) via CX32 (Connexin 32 or GJB1), a protein known to be linked to Charcot-Marie-Tooth disease. While SFXN-1.2 and CX32 affect the dynamics of mitochondria, they also affect the motility of the molecular motor UNC-104 alone. From yeast two-hybrid, co-immunoprecipitation and bimolecular fluorescent complementation assays, we narrowed down critical interaction schemes of the UNC-104/CX32/SFXN-1.2 complex providing a complex molecular model.Interestingly, though UNC-104 binds to synaptic vesicles via its PH domain, we found no role of this domain in linking the motor to mitochondria. While UNC-104 trafficking is diminished in neurons of sfxn-1.2 mutants, motility of conventional kinesin-1 UNC-116 remains unaffected. Our data suggest that UNC-104 and UNC-116 act in parallel pathways likely co-transporting mitochondria. We also show that sfxn-1.2 -induced changes in mitochondrial morphologies are independent of the fusion/fission machinery and that neither mitochondrial bioenergetics nor mitophagy are affected by sfxn-1.2 mutations. Strikingly, mutations in SFXN-1.2 lead to motor- and sensory neuron defects in C. elegans negatively affecting the animal's touch responses as well as restricting body movements - pathological phenotypes commonly seen in neurological diseases.