[Functions of the septin cytoskeleton and its roles in dopaminergic neurotransmission]

Cytoskeletal polymers are component of cellular infrastructure that are required for fundamental biological processes ranging from cell division to brain functions. Unlike the knowledge available for tubulin and actin, our understanding of unconventional cytoskeletal structures composed of GTP-binding proteins belonging to the septin family is limited, despite their ubiquity and implications in human diseases. Recent studies have revealed that septin plays unique modulatory roles as an accessory component of microtubules and the actin cytoskeleton. Morphological analyses of the mammalian brain and neural cells have revealed that septins preferentially cluster beneath the extra-synaptic membrane domains in dendritic shafts and spine necks, presynaptic terminals of major neurons, and astroglial processes. Live imaging analysis revealed that septin polymers are remarkably stable in these clusters, which may serve as local cytoskeleton and/or scaffold for the organization of specialized cortical domains in neurons and glia. This hypothesis has been supported by the hypo-dopaminergic phenotype of mice that lack the Sept4 subunit and the hyper-dopaminergic phenotype of those with excess Sept4. In these cases, the septin scaffold in the dopamine neurons is considered as a determinant of the quantity of a subset of presynaptic molecules, including tSNAREs (membrane-fusion machinery) and the dopamine transporters. This finding in mouse models is in agreement with the recent findings that qualitative and/or quantitative dysregulation of septins is involved in neurodegenerative disorders such as Parkinson disease and psychological disorders such as schizophrenia and bipolar disorder. Studies on tubulin/actin indicate that a better understanding of the septin family of proteins will improve our insight into neuropathological phenomena in neurodegenerative and psychological disorders, which may help develop diagnostic markers and therapeutic strategies for such diseases.