The interphase nuclear envelope (NE) is extensively remodeled during nuclear pore complex (NPC) insertion. How this remodeling occurs and why it requires Torsin ATPases, which also regulate lipid metabolism, remains poorly understood. Here, we show that Drosophila Torsin (dTorsin) affects lipid metabolism via the NEP1R1‐CTDNEP1 phosphatase and the Lipin phosphatidic acid (PA) phosphatase. This includes that Torsins remove NEP1R1‐CTDNEP1 from the NE in fly and mouse cells, leading to subsequent Lipin exclusion from the nucleus. NEP1R1‐CTDNEP1 downregulation also restores nuclear pore membrane fusion in post‐mitotic dTorsinKO fat body cells. However, dTorsin‐associated nuclear pore defects do not correlate with lipidomic abnormalities and are not resolved by silencing of Lipin. Further testing confirmed that membrane fusion continues in cells with hyperactivated Lipin. It also led to the surprising finding that excessive PA metabolism inhibits recruitment of the inner ring complex Nup35 subunit, resulting in elongated channel‐like structures in place of mature nuclear pores. We conclude that the NEP1R1‐CTDNEP1 phosphatase affects interphase NPC biogenesis by lipid‐dependent and lipid‐independent mechanisms, explaining some of the pleiotropic effects of Torsins.

TOR1A/TorsinA mutations cause two incurable diseases: a recessive congenital syndrome that can be lethal, and a dominantly-inherited childhood-onset dystonia (DYT-TOR1A). TorsinA has been linked to phosphatidic acid lipid metabolism in Drosophila melanogaster. Here we evaluate the role of phosphatidic acid phosphatase (PAP) enzymes in TOR1A diseases using induced pluripotent stem cell-derived neurons from patients, and mouse models of recessive Tor1a disease. We find that Lipin PAP enzyme activity is abnormally elevated in human DYT-TOR1A dystonia patient cells and in the brains of four different Tor1a mouse models. Its severity also correlated with the dosage of Tor1a/TOR1A mutation. We assessed the role of excess Lipin activity in the neurological dysfunction of Tor1a disease mouse models by interbreeding these with Lpin1 knock-out mice. Genetic reduction of Lpin1 improved the survival of recessive Tor1a disease-model mice, alongside suppressing neurodegeneration, motor dysfunction, and nuclear membrane pathology. These data establish that TOR1A disease mutations cause abnormal phosphatidic acid metabolism, and suggest that approaches that suppress Lipin PAP enzyme activity could be therapeutically useful for TOR1A diseases.

TOR1A/TorsinA mutations cause poorly explained neurological diseases. A dominantly inherited mutation causes isolated dystonia, while biallelic mutations cause a recessive infant-onset syndrome with cases of lethality. Here we report an unexpected connection between lipid metabolism and these diseases. Lipin phosphatidic acid phosphatase activity was abnormally regulated in TorsinA dystonia patient cells, and in the brains of three different TorsinA disease model mice. Lipin activity was causative to symptoms given that lowering Lipin1 in vivo strongly intervened against lethality in disease mice. Furthermore, Lipin hyperactivity caused cell death in vitro, and Lipin1 deficiency suppressed neurodegeneration in vivo. In addition, it protected the striatal cholinergic interneurons that are implicated in TorsinA movement disorders, and concomitantly suppressed abnormal motor behaviors of TorsinA mice. These data establish the central role of Lipin lipid enzyme hyperactivity in TorsinA disease and show that Lipin inhibition is a therapeutic target for these incurable conditions. One Sentence Summary: Lipin inhibition rescues TorsinA neurological disease TOR1A/TorsinA mutations cause at least two neurological diseases.Dominantly inherited TOR1A/TorsinA dystonia (OMIM #128100) caused by the +/E mutation is characterized by childhood-onset involuntary twisting movements and abnormal postures of unexplained origin (1). Indeed, despite many years of study, no structural or degenerative pathology has been found to explain this isolated dystonia.It is the most common hereditary dystonia, and the genetic insult is a widely used experimental tool to investigate mechanisms leading to dystonia. However, it also remains poorly understood at molecular, cellular and neurobiology system levels.There is also a recessive TorsinA syndrome with a broader set of symptoms. This emerges at birth, and is caused by TorsinA E/E or other biallelic combinations of TorsinA deletions (2-5). Affected infants invariably show arthrogryposis (joint contractures at birth), tremor, and develop speech, cognition and motor deficits.There is no cure for either TorsinA disease. Dominant TorsinA dystonia symptoms often develop into severe, generalized dystonia that is highly debilitating and interferes with all aspects of daily life. It is typically managed with deep brain stimulation. However, this is invasive and often fails to provide full symptom relief. The alternative is botulinum toxin treatment of affected muscle groups if the dystonia remains focal or segmental, but this is also purely symptomatic and only partially effective. There are even fewer options for recessive syndrome patients, and there are examples where infants failed to survive even in high-level health care settings. The surviving children continue to suffer from severe neurological defects, including intellectual impairment, and require continuous support and care (2-5).Poor understanding of disease molecular and cellular pathology is rate-limiting for translating information on...

AbstractTorsin ATPases of the endoplasmic reticulum (ER) and nuclear envelope (NE) lumen inhibit Lipin-mediated phosphatidate (PA) to diacylglycerol (DAG) conversion by an unknown mechanism. This excess PA metabolism is implicated in TOR1A/TorsinA diseases, but it is unclear whether it explains why Torsin concomitantly affects nuclear structure, lipid droplets (LD), organelle and cell growth. Here a fly miniscreen identified that Torsins affect these events via the NEP1R1-CTDNEP1 phosphatase complex. Further, Torsin homo-oligomerization rather than ATPase activity was key to function. NEP1R1-CTDNEP1 activates Lipin by dephosphorylation. We show that Torsin prevents CTDNEP1 from accumulating in the NE and excludes Lipin from the nucleus. Moreover, this repression of nuclear PA metabolism is required for interphase nuclear pore biogenesis. We conclude that Torsin is an upstream regulator of the NEP1R1-CTDNEP1/ Lipin pathway. This connects the ER/NE lumen with PA metabolism, and affects numerous cellular events including it has a previously unrecognized role in nuclear pore biogenesis.HighlightsNuclear envelope PA-DAG-TAG synthesis is independently regulated by Torsin and Torip/LAP1Torsin removes CTDNEP1 from the nuclear envelope and excludes Lipin from the nucleusExcess nuclear envelope NEP1R1-CTDNEP1/ Lipin activity impairs multiple aspects of NPC biogenesisNEP1R1-CTDNEP1/ Lipin inhibition prevents cellular defects associated with TOR1A and TOR1AIP1 / LAP1 disease