ARL6IP1 in endoplasmic reticulum homeostasis and axonal maintenance

ADP-ribosylation factor-like protein 6-interacting protein 1 (ARL6IP1) resides in the endoplasmic reticulum (ER) membrane and contains domains that mediate the ability to bend membranes. Mutations are associated with severe neurological disorders including hereditary spastic paraplegia (HSP) and insensitivity to pain. Although ubiquitously expressed, the protein has a specific role in the survival of long-projecting neurons as described in the clinical picture termed SPG61. In this study, a colocalisation between ARL6IP1 and the ER-phagy receptor FAM134B within the ER was shown and the protein-protein interaction verified by Co-IP. Identified putative LC3-interacting regions (LIRs) in ARL6IP1 were shown to be non-functional. Oligomerisation of ARL6IP1 could be verified and pinpoint to its N-terminal half. An assay points to a cytosolic C-terminus of ARL6IP1. These findings suggest a role of ARL6IP1 ER morphology and compartment regulation through oligomerisation and activity in membrane fragmentation. An Arl6ip1-KO mouse model for was established and analysed in this work. The most striking phenotype found was infantile onset of several characteristic features of the SPG61 clinical picture. This includes a progressive loss of motor functions as shown by gait analysis and grip strength measurements. Nerve conduction velocity studies further revealed a progressive decrease in sensory function. A loss of cerebellar Purkinje neurons and spinal motor neurons was observed in Arl6ip1-KO, verifying neuronal degeneration. Remarkably, the loss of Arl6ip1 in mice results in both motor deficits and sensoneuropathy. Taken together, this study enables the profound characterisation of an animal model mimicking the human neurodegenerative disease phenotype caused by Arl6ip1 loss. Furthermore, it underlines the proteins role in ER morphology and possibly autophagic processes, highlighting the crucial role of this gene product for neuron homeostasis and cellular survival.