A Molecular Mechanism Contributing to the Neuronal Specific Defects of Torsina Mutant Mice.

DYT1 dystonia is a movement disorder caused by a glutamic acid deletion mutation (DeltaE) in torsinA, an AAA+ protein that resides in the endoplasmic reticular/nuclear envelope (NE) endomembrane. TorsinA belongs to the torsin protein family including torsinB, torsin2 and torsin3. We previously reported that DeltaE-torsinA accumulates abnormally in the NE and that torsinA null (Tor1a-/-) and DeltaE knock-in ( Tor1aDeltaE/DeltaE) mice both exhibit abnormal NE ultrastructure (blebs), exclusively in neurons despite the widespread expression of torsinA. These findings demonstrated that Tor1a DeltaE is a loss-of-function allele and established a NE-localized function for torsinA uniquely in neurons. This tissue specificity of torsinA loss of function phenotype mimics that of affected tissues in DYT1 dystonia. Therefore, in this thesis work, the mechanism underlying the neural specificity of the torsinA loss-of-function phenotype was explored. To understand the reasons for neural specificity, we investigated the functions of the torsinA interacting partner, LAP1, and torsin family members. To determine whether neurons have a unique requirement for LAP1, a widely expressed NE protein, the phenotype of LAP1 null (Tor1aip1 -/-) mice was characterized. Strikingly, LAP1 null mice develop blebs not only in neurons but also in non-neuronal tissues. These data establish a functional link between LAP1 and torsinA, but fail to explain the neural specificity of torsinA loss of function. Therefore, we explored whether other torsinA family members play a role in this tissue specificity. All torsins co-immunoprecipitated with LAP1 and removing multiple torsins away from their interactors induced blebs. Expression levels of torsinB and torsin3 were strikingly higher in non-neural compared to neural tissue. Furthermore, "substrate trap" forms of torsinA and torsinB, but not torsin2 and torsin3, exhibited LAP1-dependent NE accumulation. Knock-down of torsins in torsinA mutant fibroblasts revealed only torsinB down-regulation can form blebs in these cells. These data establish a functional relationship between torsinA and LAP1 and show that torsinB compensates for the loss of torsinA in non-neuronal tissues. Therefore, the low level of torsinB in neurons contributes to the neural specificity of NE phenotype observed in torsinA mutant mice and may play a role in the neurological specific symptoms of DYT1 dystonia.