# ER Morphology in the Pathogenesis of Hereditary
## 4 Microtubule (MT) Dynamics
### Spastin (Spg4)  
About 40% of pure AD-HSP are caused by mutations in the _SPG4/SPAST_ gene, encoding Spastin [15]. Spastin, a member of the AAA (ATPase associated with various cellular activities) family, is a MT-severing protein that plays a major role in cytoskeleton regulation [107; 108; 109; 110]. In addition to the ATPase domain, it also contains a MT Binding Domain (MTBD) necessary for MT binding, and a MT Interacting and Transport (MIT) domain [107]. To exert its severing activity, Spastin is believed to assemble into hexamers, dock on MTs and break them up [111; 112]. Recently, it has been discovered that severases can also promote MT growth likely through a mechanism in which the MT fragments created by severing act as seeds for the growth of new MTs [113; 114]. Moreover, Spastinsevering activity is regulated by MT post-translational modification [115; 116; 117]. Considering these features altogether, it is clear that fine control of Spastin function is necessary for regulating MT number, mobility and distribution in the cell [108; 109; 110; 118]. Over 300 mutations and deletions have been described in the SPG4 gene. The wide spectrum of mutations includes non-sense, frameshift, splice site, missense mutations, as well as extended deletions. Loss-of-function (haploinsufficiency or dominant-negative) has been proposed as the pathological mechanism [119; 120; 121; 110], supported by the fact that mutant Spastin proteins have not been detected in cells and tissues derived from human patients [121; 122], as well as by experiments in a _Drosophila_ SPG4 model [123]. However, two independently generated Spastin knockout mouse models failed to recapitulate major HSP hallmarks [124; 125], questioning the validity of the haploinsufficiency model. On this basis, the contribution of an additional gain of function mechanism has been proposed for specific missense mutations [126]. The _SPAST_ gene produces a 530 amino acid (60 kDa) ubiquitous isoform called M87 and a much less abundant 616 amino acid (68 kDa) isoform called M1 whose expression is restricted to the spinal cord [127]. The MIT, MTBD and AAA domains are present in both M1 and M87 Spastin isoforms, whereas M1 Spastin features a specific 86-aminoacid \(N\)-terminal domain containing a hydrophobic region suggested to form a hairpin that can partially insert into the ER membrane. Spastin has been reported to interact with ER resident proteins ATL1, RTN1 and REEP1 via its hydrophobic hairpin [128; 129; 130; 89; 131]. Despite suggestions that these interactions may be important for ER-MT interplay [132; 132; 89; 107], their actual role in the building and maintenance of the tubular ER network is still unclear. The precise role of Spastin at the ER remains unknown, since no requirement for endogenous Spastin in ER shaping has yet been demonstrated. Available evidence shows that ER morphology is affected when Spastin is downregulated or when pathogenic variants are expressed [133; 134; 131]. Experiments in _Drosophila_ showed that the expression of the pathological variant spastin[8467R] (known to induce MT hyper-stabilization via a dominant-negative effect) shifts the ER sheets/tubules balance toward the formation of sheets. However, administration the MT-destabilizing drug vinblastine, which has been shown to restore MT organization [123], rescues also ER morphology, suggesting that ER perturbation occurs as a consequence of the impairment of MT dynamics [134]. Thus, currently available evidence does not support a direct role for Spastin in regulating ER network shape. Likewise, it has been suggested that Spastin regulates ER-endosome tethering, lysosome size, and LD size [135; 136; 137; 138], but there is no conclusive proof that these effects are associated with activities of the protein beyond its MT-destabilizing role.