A functional role for Septin-2 in the maintenance of the axon initial segment and in human cognitive development

The establishment of neuronal polarity, whereby somatodendritic and axonal cellular compartments are defined, is a critical determinant for the development of neuronal networks and patterning during neurogenesis. The axon initial segment (AIS), a key structure in the establishment of this polarity, is formed through interactions between the microtubule and actin cytoskeleton, Ankyrin G, TRIM46 and multiple transmembrane and perimembranous proteins. Here, we implicate a component of the septin cytoskeleton, Septin-2, in the maintenance and function of the AIS through the study of mutations found in five unrelated human individuals and one mother-daughter duo with a majority presenting with cognitive impairment. Septins form octameric rods that assemble into higher-order filamentous scaffolds driven by Septin-2 homodimerization. Mutant Septin-2 is predicted to impart a dominant negative blockade on septin octamers forming these structures by precluding Septin-2 homodimerization. Expression of mutant Septin-2 constructs in neurons leads to the disappearance of canonical hallmarks of the AIS. This includes loss of Ankyrin G in the AIS, aberrant localization of MAP2 within the distal axon, axonal shortening and electrophysiological hypoexcitability. We further show that Septin-2 binds to a neuron-specific domain of Ankyrin G, an interaction that is largely ablated by these mutations. These data establish a role for Septin-2 in the maintenance and function of the AIS and implicate cytoskeletal structures composed of septin oligomers in the establishment of higher cognitive functions in humans.