Spastin and microtubules: Functions in health and disease.

SPG4, the gene encoding for spastin, a member of the ATPases associated with various cellular activities (AAA) family, is mutated in around 40% of cases of autosomal dominant hereditary spastic paraplegia (AD-HSP). This group of neurodegenerative diseases is characterized by a progressive spasticity and lower limb weakness with degeneration of terminal axons in cortico-spinal tracts and dorsal columns. Spastin has two main domains, a microtubule interacting and endosomal trafficking (MIT) domain at the N-terminus and the C-terminus AAA domain. Early studies suggested that spastin interacts with microtubules similarly to katanin, a member of the same subgroup of AAA. Recent evidence confirmed that spastin possesses microtubule-severing activity but can also bundle microtubules in vitro. Understanding the physiologic and pathologic involvement of these activities and their regulation is critical in the study of HSP.