- Email: [email protected]
Role for Torsin in Lipid Metabolism
Developmental Cell
Previews Role for Torsin in Lipid Metabolism Aurelio A. Teleman1,* 1German Cancer Research Center (DKFZ), 69120 Heidelberg, Germany *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2016.07.015
DYT1 dystonia is a neurological disease that causes involuntary twisting movements, often caused by dysfunction of the TorsinA gene. In this issue of Developmental Cell, Grillet et al. (2016) use Drosophila to discover that TorsinA regulates lipid metabolism, opening up future directions of research into the causes of this disease.
The advances in genomics capabilities of the past decade are being successfully applied clinically to identify disease-causing genes. Whole-exome sequencing is identifying mutations that cause Mendelian disorders (Yang et al., 2013), whole-genome sequencing of human cancers is uncovering millions of cancer-associated somatic mutations, and genome-wide association studies are linking more and more genes to complex diseases. A major bottleneck now is the functional characterization of the identified genes, which is necessary to translate these findings into therapeutic approaches. Indeed, 50% of human genes are of unknown function, and the function of these genes is often difficult to deduce from cell-culture-based assays, thus requiring in vivo animal model systems to study them. One of the predominant animals used for functional annotation of genes is Drosophila. For instance, work in Drosophila elucidated and pieced together many of the cancerrelevant signaling pathways such as Notch, Wnt, Hedgehog, and TGF/BMP-b signaling (Nu¨sslein-Volhard and Wieschaus, 1980), identified mitochondrial quality control as the function of the familial Parkinson’s genes Pink and Parkin (Imai, 2012), and characterized the molecular function of the TSC2 tumor suppressor (Gao et al., 2002). In this issue of Developmental Cell, Grillet et al. (2016) use Drosophila to study the function of Torsin, a gene whose human homolog TorsinA has been causally linked to DYT1 dystonia. Their studies suggest that Torsin regulates cellular lipid homeostasis. DYT1 dystonia, also known as earlyonset torsion dystonia (EOTD), is a neurological disease that causes involuntary
twisting movements and sustained postures without obvious neurodegeneration (Rose et al., 2015). The most frequent mutation causing DYT1 dystonia is in the TorsinA gene (Ozelius et al., 1997). TorsinA resides in the endoplasmic reticulum (ER) and is a member of the AAA+ (ATPases Associated with a variety of cellular Activities) superfamily of ATPases. These ATPases hydrolyze ATP to exert conformational changes on substrate proteins, to unfold substrates, or to remodel macromolecular complexes. The substrate(s) and function of TorsinA, however, are not clear. TorsinA has been proposed to function in protein quality control in the ER, to regulate protein and vesicle trafficking, to affect the cellular cytoskeleton, and to regulate nuclear envelope dynamics (Rose et al., 2015). Its precise molecular function remains elusive. Grillet et al. (2016) began to address this question by studying Torsin knockout (KO) flies, which were originally generated by the Ito lab (Wakabayashi-Ito et al., 2011). The Ito lab found that loss of Torsin causes lethality, with juveniles displaying locomotor defects. Grillet et al. (2016) unexpectedly found that the lethality could be rescued by reintroducing Torsin expression specifically in the fat body, the organ that combines the functions of mammalian adipose tissue and liver and is the main site of triglyceride (TAG) synthesis and storage. Torsin-KO animals have elevated TAG levels and fused lipid droplets in fat tissue, a phenotype that hints at an imbalance between the synthesis of TAGs, which form the core of lipid droplets, and membrane phospholipids, which form the surface. Indeed, lipidomic analysis revealed that fat bodies of dTorsin-KO animals have 4-fold reduced
levels of phosphatidic acid and 6-fold increased levels of diglycerides, whereas fat bodies of flies overexpressing Torsin have reduced TAG levels and elevated phosphatidylethanolamine levels. Likewise, human U2OS cells overexpressing TorsinA and its cofactor LULL1 have extra stacks of nuclear membrane and elevated levels of the membrane phospholipids phosphatidylcholine and phosphatidylethanolamine, suggesting that the functional role of TorsinA in lipid homeostasis is conserved from flies to humans. One protein that promotes TAG biosynthesis and inhibits membrane lipid biosynthesis is lipin. The activity of lipin, which functions both as a phosphatase and as a transcriptional co-activator, is regulated in part by its subcellular localization. Interestingly, Grillet et al. (2016) found that fat body cells of control animals have predominantly cytosolic lipin, whereas many cells of dTorsin-KO animals have nuclear lipin, suggesting elevated lipin transcriptional activity in the knockouts. Indeed, partial knockdown of lipin in dTorsin-KO flies partially rescued multiple different aspects of the dTorsin-KO phenotype, such as their reduced fat body cell size, their decreased total animal weight, and their low survival to adulthood. In sum, Grillet et al. (2016) describe an unexpected role for Torsin in regulating lipid metabolism. A goal for future research will be to determine whether this role is a conserved aspect of Torsin function. It will be exciting to test in both fly and mammalian models whether this is the key molecular function of Torsin that is relevant for DYT1 dystonia. Many forms of genetic dystonia are not associated with overt neurological lesions or neurodegeneration. Nonetheless,
Developmental Cell 38, August 8, 2016 ª 2016 Elsevier Inc. 223
Developmental Cell
Previews neurodegenerative diseases, as well as brain traumas, can cause dystonia symptoms, as do mutations affecting dopamine synthesis (Breakefield et al., 2008). It therefore appears likely that dystonia results from mild impairments in neuronal function. Given that cerebral-cortex-specific knockdown of TorsinA is sufficient to cause motor deficits (Itin et al., 1994), TorsinA function may be autonomously required in neurons. Thus, one possibility is that altered lipid metabolism in neurons caused by TorsinA loss of function mildly impairs their function. If this is the case, one exciting hypothesis for future research is that rebalancing lipid homeo-
stasis in the neurons of DYT1 patients could ameliorate their symptoms.
Itin, P.H., Pittelkow, M.R., and Kumar, R. (1994). Endocrinology 135, 1793–1798. Nu¨sslein-Volhard, C., and Wieschaus, E. (1980). Nature 287, 795–801.
REFERENCES Breakefield, X.O., Blood, A.J., Li, Y., Hallett, M., Hanson, P.I., and Standaert, D.G. (2008). Nat. Rev. Neurosci. 9, 222–234. Gao, X., Zhang, Y., Arrazola, P., Hino, O., Kobayashi, T., Yeung, R.S., Ru, B., and Pan, D. (2002). Nat. Cell Biol. 4, 699–704. Grillet, M., Dominguez, G., Sicart, A., Po¨ttler, M., Cascalho, A., Billion, K., Hernandez Diaz, S., Swerts, J., Naismith, T.V., Gounko, N.V., et al. (2016). Dev. Cell 38, this issue, 235–247. Imai, Y. (2012). ISRN Cell Biol. 2012, 15.
Ozelius, L.J., Hewett, J.W., Page, C.E., Bressman, S.B., Kramer, P.L., Shalish, C., de Leon, D., Brin, M.F., Raymond, D., Corey, D.P., et al. (1997). Nat. Genet. 17, 40–48. Rose, A.E., Brown, R.S., and Schlieker, C. (2015). Crit. Rev. Biochem. Mol. Biol. 50, 532–549. Wakabayashi-Ito, N., Doherty, O.M., Moriyama, H., Breakefield, X.O., Gusella, J.F., O’Donnell, J.M., and Ito, N. (2011). PLoS One 6, e26183. Yang, Y., Muzny, D.M., Reid, J.G., Bainbridge, M.N., Willis, A., Ward, P.A., Braxton, A., Beuten, J., Xia, F., Niu, Z., et al. (2013). N. Engl. J. Med. 369, 1502–1511.
Of Mice and Snakes: A Tail of Oct4 Natalia A. Shylo1 and Scott D. Weatherbee1,* 1Department of Genetics, Yale University, New Haven, CT 06520, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2016.07.020
The vertebrate axial skeleton comprises regions of specialized vertebrae, which vary in length between lineages. Aires et al. (2016) uncover a key role for Oct4 in determining trunk length in mice. Additionally, a heterochronic shift in Oct4 expression may underlie the extreme elongation of the trunk in snakes. In this issue of Developmental Cell, Aires et al. (2016) tackle an age-old question: How did snakes evolve such long bodies? The authors focus on Gdf11 and Oct4— two known regulators of trunk mesoderm—and, using genetic, genomic, and comparative expression approaches, find that the balance between Gdf11 and Oct4 activities is critical to set the trunktail boundary. During development, vertebrate bodies grow posteriorly, with somites periodically budding off the presomitic mesoderm (PSM), which contains a population of multipotent regenerating cells (DeVeale et al., 2013; Gomez et al., 2008). The somites give rise to vertebrae, whose identities are determined based on their axial positions and which serve specific adaptive functions (Woltering, 2012). Different species make different numbers of somites, and, as a result, different numbers of vertebrae (Woltering, 2012; Gomez et al., 2008). Hence, there is a vast diver-
sity of vertebrate skeletons, with the number of vertebra varying greatly in each region of the body, with different species evolving longer or shorter necks, trunks, or tails. The trunk-to-tail transition occurs at the level of the hindlimb as the PSM begins to break down (Gomez et al., 2008). This transition is particularly important in tetrapods for positioning of the hindlimbs and reproductive system, as well as establishing the posterior end of the organfilled trunk. Axial patterning, individual segment identities, and shifts between body regions are marked and influenced by Hox genes in many model systems (Di-Poı¨ et al., 2010), and major shifts in Hox domains correlate with the extensive elongation of the snake trunk (Cohn and Tickle, 1999). However, despite their important role in determining segment identity, individual Hox genes play only minor roles in determining the length of any individual vertebral domain, such as
224 Developmental Cell 38, August 8, 2016 ª 2016 Elsevier Inc.
the number of segments between forelimbs and hindlimbs in the trunk (Jurberg et al., 2013). This raised the question: What factors control the length of specific body regions within and between species? Strikingly, loss or gain of Gdf11 results in longer or shorter trunks, respectively, suggesting that Gdf11 acts upstream of Hox genes in specifying the trunk-to-tail transition (Figure 1) (Jurberg et al., 2013; McPherron et al., 1999). Gdf11 null mice have an ectopic mass of tissue posterior to the hindlimbs, expressing a set of genes suggestive of an incomplete trunk-to-tail transition (Aires et al., 2016; Jurberg et al., 2013). Aires et al. (2016) found that this mass was also positive for Oct4, a well-known pluripotency factor, at a time when Oct4 is normally restricted to the germline (DeVeale et al., 2013). At earlier stages, Oct4 normally maintains cell viability and proliferation in the posterior of the embryo, and
Previews Role for Torsin in Lipid Metabolism Aurelio A. Teleman1,* 1German Cancer Research Center (DKFZ), 69120 Heidelberg, Germany *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2016.07.015
DYT1 dystonia is a neurological disease that causes involuntary twisting movements, often caused by dysfunction of the TorsinA gene. In this issue of Developmental Cell, Grillet et al. (2016) use Drosophila to discover that TorsinA regulates lipid metabolism, opening up future directions of research into the causes of this disease.
The advances in genomics capabilities of the past decade are being successfully applied clinically to identify disease-causing genes. Whole-exome sequencing is identifying mutations that cause Mendelian disorders (Yang et al., 2013), whole-genome sequencing of human cancers is uncovering millions of cancer-associated somatic mutations, and genome-wide association studies are linking more and more genes to complex diseases. A major bottleneck now is the functional characterization of the identified genes, which is necessary to translate these findings into therapeutic approaches. Indeed, 50% of human genes are of unknown function, and the function of these genes is often difficult to deduce from cell-culture-based assays, thus requiring in vivo animal model systems to study them. One of the predominant animals used for functional annotation of genes is Drosophila. For instance, work in Drosophila elucidated and pieced together many of the cancerrelevant signaling pathways such as Notch, Wnt, Hedgehog, and TGF/BMP-b signaling (Nu¨sslein-Volhard and Wieschaus, 1980), identified mitochondrial quality control as the function of the familial Parkinson’s genes Pink and Parkin (Imai, 2012), and characterized the molecular function of the TSC2 tumor suppressor (Gao et al., 2002). In this issue of Developmental Cell, Grillet et al. (2016) use Drosophila to study the function of Torsin, a gene whose human homolog TorsinA has been causally linked to DYT1 dystonia. Their studies suggest that Torsin regulates cellular lipid homeostasis. DYT1 dystonia, also known as earlyonset torsion dystonia (EOTD), is a neurological disease that causes involuntary
twisting movements and sustained postures without obvious neurodegeneration (Rose et al., 2015). The most frequent mutation causing DYT1 dystonia is in the TorsinA gene (Ozelius et al., 1997). TorsinA resides in the endoplasmic reticulum (ER) and is a member of the AAA+ (ATPases Associated with a variety of cellular Activities) superfamily of ATPases. These ATPases hydrolyze ATP to exert conformational changes on substrate proteins, to unfold substrates, or to remodel macromolecular complexes. The substrate(s) and function of TorsinA, however, are not clear. TorsinA has been proposed to function in protein quality control in the ER, to regulate protein and vesicle trafficking, to affect the cellular cytoskeleton, and to regulate nuclear envelope dynamics (Rose et al., 2015). Its precise molecular function remains elusive. Grillet et al. (2016) began to address this question by studying Torsin knockout (KO) flies, which were originally generated by the Ito lab (Wakabayashi-Ito et al., 2011). The Ito lab found that loss of Torsin causes lethality, with juveniles displaying locomotor defects. Grillet et al. (2016) unexpectedly found that the lethality could be rescued by reintroducing Torsin expression specifically in the fat body, the organ that combines the functions of mammalian adipose tissue and liver and is the main site of triglyceride (TAG) synthesis and storage. Torsin-KO animals have elevated TAG levels and fused lipid droplets in fat tissue, a phenotype that hints at an imbalance between the synthesis of TAGs, which form the core of lipid droplets, and membrane phospholipids, which form the surface. Indeed, lipidomic analysis revealed that fat bodies of dTorsin-KO animals have 4-fold reduced
levels of phosphatidic acid and 6-fold increased levels of diglycerides, whereas fat bodies of flies overexpressing Torsin have reduced TAG levels and elevated phosphatidylethanolamine levels. Likewise, human U2OS cells overexpressing TorsinA and its cofactor LULL1 have extra stacks of nuclear membrane and elevated levels of the membrane phospholipids phosphatidylcholine and phosphatidylethanolamine, suggesting that the functional role of TorsinA in lipid homeostasis is conserved from flies to humans. One protein that promotes TAG biosynthesis and inhibits membrane lipid biosynthesis is lipin. The activity of lipin, which functions both as a phosphatase and as a transcriptional co-activator, is regulated in part by its subcellular localization. Interestingly, Grillet et al. (2016) found that fat body cells of control animals have predominantly cytosolic lipin, whereas many cells of dTorsin-KO animals have nuclear lipin, suggesting elevated lipin transcriptional activity in the knockouts. Indeed, partial knockdown of lipin in dTorsin-KO flies partially rescued multiple different aspects of the dTorsin-KO phenotype, such as their reduced fat body cell size, their decreased total animal weight, and their low survival to adulthood. In sum, Grillet et al. (2016) describe an unexpected role for Torsin in regulating lipid metabolism. A goal for future research will be to determine whether this role is a conserved aspect of Torsin function. It will be exciting to test in both fly and mammalian models whether this is the key molecular function of Torsin that is relevant for DYT1 dystonia. Many forms of genetic dystonia are not associated with overt neurological lesions or neurodegeneration. Nonetheless,
Developmental Cell 38, August 8, 2016 ª 2016 Elsevier Inc. 223
Developmental Cell
Previews neurodegenerative diseases, as well as brain traumas, can cause dystonia symptoms, as do mutations affecting dopamine synthesis (Breakefield et al., 2008). It therefore appears likely that dystonia results from mild impairments in neuronal function. Given that cerebral-cortex-specific knockdown of TorsinA is sufficient to cause motor deficits (Itin et al., 1994), TorsinA function may be autonomously required in neurons. Thus, one possibility is that altered lipid metabolism in neurons caused by TorsinA loss of function mildly impairs their function. If this is the case, one exciting hypothesis for future research is that rebalancing lipid homeo-
stasis in the neurons of DYT1 patients could ameliorate their symptoms.
Itin, P.H., Pittelkow, M.R., and Kumar, R. (1994). Endocrinology 135, 1793–1798. Nu¨sslein-Volhard, C., and Wieschaus, E. (1980). Nature 287, 795–801.
REFERENCES Breakefield, X.O., Blood, A.J., Li, Y., Hallett, M., Hanson, P.I., and Standaert, D.G. (2008). Nat. Rev. Neurosci. 9, 222–234. Gao, X., Zhang, Y., Arrazola, P., Hino, O., Kobayashi, T., Yeung, R.S., Ru, B., and Pan, D. (2002). Nat. Cell Biol. 4, 699–704. Grillet, M., Dominguez, G., Sicart, A., Po¨ttler, M., Cascalho, A., Billion, K., Hernandez Diaz, S., Swerts, J., Naismith, T.V., Gounko, N.V., et al. (2016). Dev. Cell 38, this issue, 235–247. Imai, Y. (2012). ISRN Cell Biol. 2012, 15.
Ozelius, L.J., Hewett, J.W., Page, C.E., Bressman, S.B., Kramer, P.L., Shalish, C., de Leon, D., Brin, M.F., Raymond, D., Corey, D.P., et al. (1997). Nat. Genet. 17, 40–48. Rose, A.E., Brown, R.S., and Schlieker, C. (2015). Crit. Rev. Biochem. Mol. Biol. 50, 532–549. Wakabayashi-Ito, N., Doherty, O.M., Moriyama, H., Breakefield, X.O., Gusella, J.F., O’Donnell, J.M., and Ito, N. (2011). PLoS One 6, e26183. Yang, Y., Muzny, D.M., Reid, J.G., Bainbridge, M.N., Willis, A., Ward, P.A., Braxton, A., Beuten, J., Xia, F., Niu, Z., et al. (2013). N. Engl. J. Med. 369, 1502–1511.
Of Mice and Snakes: A Tail of Oct4 Natalia A. Shylo1 and Scott D. Weatherbee1,* 1Department of Genetics, Yale University, New Haven, CT 06520, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2016.07.020
The vertebrate axial skeleton comprises regions of specialized vertebrae, which vary in length between lineages. Aires et al. (2016) uncover a key role for Oct4 in determining trunk length in mice. Additionally, a heterochronic shift in Oct4 expression may underlie the extreme elongation of the trunk in snakes. In this issue of Developmental Cell, Aires et al. (2016) tackle an age-old question: How did snakes evolve such long bodies? The authors focus on Gdf11 and Oct4— two known regulators of trunk mesoderm—and, using genetic, genomic, and comparative expression approaches, find that the balance between Gdf11 and Oct4 activities is critical to set the trunktail boundary. During development, vertebrate bodies grow posteriorly, with somites periodically budding off the presomitic mesoderm (PSM), which contains a population of multipotent regenerating cells (DeVeale et al., 2013; Gomez et al., 2008). The somites give rise to vertebrae, whose identities are determined based on their axial positions and which serve specific adaptive functions (Woltering, 2012). Different species make different numbers of somites, and, as a result, different numbers of vertebrae (Woltering, 2012; Gomez et al., 2008). Hence, there is a vast diver-
sity of vertebrate skeletons, with the number of vertebra varying greatly in each region of the body, with different species evolving longer or shorter necks, trunks, or tails. The trunk-to-tail transition occurs at the level of the hindlimb as the PSM begins to break down (Gomez et al., 2008). This transition is particularly important in tetrapods for positioning of the hindlimbs and reproductive system, as well as establishing the posterior end of the organfilled trunk. Axial patterning, individual segment identities, and shifts between body regions are marked and influenced by Hox genes in many model systems (Di-Poı¨ et al., 2010), and major shifts in Hox domains correlate with the extensive elongation of the snake trunk (Cohn and Tickle, 1999). However, despite their important role in determining segment identity, individual Hox genes play only minor roles in determining the length of any individual vertebral domain, such as
224 Developmental Cell 38, August 8, 2016 ª 2016 Elsevier Inc.
the number of segments between forelimbs and hindlimbs in the trunk (Jurberg et al., 2013). This raised the question: What factors control the length of specific body regions within and between species? Strikingly, loss or gain of Gdf11 results in longer or shorter trunks, respectively, suggesting that Gdf11 acts upstream of Hox genes in specifying the trunk-to-tail transition (Figure 1) (Jurberg et al., 2013; McPherron et al., 1999). Gdf11 null mice have an ectopic mass of tissue posterior to the hindlimbs, expressing a set of genes suggestive of an incomplete trunk-to-tail transition (Aires et al., 2016; Jurberg et al., 2013). Aires et al. (2016) found that this mass was also positive for Oct4, a well-known pluripotency factor, at a time when Oct4 is normally restricted to the germline (DeVeale et al., 2013). At earlier stages, Oct4 normally maintains cell viability and proliferation in the posterior of the embryo, and