Markus A. Rüegg

Mammalian TOR complex 2 controls the actin cytoskeleton and is rapamycin insensitive

The target of rapamycin (TOR) is a highly conserved protein kinase and a central controller of cell growth. In budding yeast, TOR is found in structurally and functionally distinct protein complexes: TORC1 and TORC2. A mammalian counterpart of TORC1 (mTORC1) has been described, but it is not known whether TORC2 is conserved in mammals. Here, we report that a mammalian counterpart of TORC2 (mTORC2) also exists. mTORC2 contains mTOR, mLST8 and mAVO3, but not raptor. Like yeast TORC2, mTORC2 is rapamycin insensitive and seems to function upstream of Rho GTPases to regulate the actin cytoskeleton. mTORC2 is not upstream of the mTORC1 effector S6K. Thus, two distinct TOR complexes constitute a primordial signalling network conserved in eukaryotic evolution to control the fundamental process of cell growth.

Neuropathology in Mice Expressing Human α-Synuclein

The presynaptic protein α-synuclein is a prime suspect for contributing to Lewy pathology and clinical aspects of diseases, including Parkinsons disease, dementia with Lewy bodies, and a Lewy body variant of Alzheimers disease. α-Synuclein accumulates in Lewy bodies and Lewy neurites, and two missense mutations (A53T and A30P) in the α-synuclein gene are genetically linked to rare familial forms of Parkinsons disease. Under control of mouse Thy1 regulatory sequences, expression of A53T mutant human α-synuclein in the nervous system of transgenic mice generated animals with neuronal α-synucleinopathy, features strikingly similar to those observed in human brains with Lewy pathology, neuronal degeneration, and motor defects, despite a lack of transgene expression in dopaminergic neurons of the substantia nigra pars compacta. Neurons in brainstem and motor neurons appeared particularly vulnerable. Motor neuron pathology included axonal damage and denervation of neuromuscular junctions in several muscles examined, suggesting that α-synuclein interfered with a universal mechanism of synapse maintenance. Thy1 transgene expression of wild-type human α-synuclein resulted in similar pathological changes, thus supporting a central role for mutant and wild-type α-synuclein in familial and idiotypic forms of diseases with neuronal α-synucleinopathy and Lewy pathology. These mouse models provide a means to address fundamental aspects of α-synucleinopathy and test therapeutic strategies.

Skeletal Muscle-Specific Ablation of raptor, but Not of rictor, Causes Metabolic Changes and Results in Muscle Dystrophy

Mammalian target of rapamycin (mTOR) is a central controller of cell growth. mTOR assembles into two distinct multiprotein complexes called mTOR complex 1 (mTORC1) and mTORC2. Here we show that the mTORC1 component raptor is critical for muscle function and prolonged survival. In contrast, muscles lacking the mTORC2 component rictor are indistinguishable from wild-type controls. Raptor-deficient muscles become progressively dystrophic, are impaired in their oxidative capacity, and contain increased glycogen stores, but they express structural components indicative of oxidative muscle fibers. Biochemical analysis indicates that these changes are probably due to loss of activation of direct downstream targets of mTORC1, downregulation of genes involved in mitochondrial biogenesis, including PGC1alpha, and hyperactivation of PKB/Akt. Finally, we show that activation of PKB/Akt does not require mTORC2. Together, these results demonstrate that muscle mTORC1 has an unexpected role in the regulation of the metabolic properties and that its function is essential for life.

Adipose-Specific Knockout of raptor Results in Lean Mice with Enhanced Mitochondrial Respiration

raptor is a specific and essential component of mammalian TOR complex 1 (mTORC1), a key regulator of cell growth and metabolism. To investigate a role of adipose mTORC1 in regulation of adipose and whole-body metabolism, we generated mice with an adipose-specific knockout of raptor (raptor(ad-/-)). Compared to control littermates, raptor(ad-/-) mice had substantially less adipose tissue, were protected against diet-induced obesity and hypercholesterolemia, and exhibited improved insulin sensitivity. Leanness was in spite of reduced physical activity and unaffected caloric intake, lipolysis, and absorption of lipids from the food. White adipose tissue of raptor(ad-/-) mice displayed enhanced expression of genes encoding mitochondrial uncoupling proteins characteristic of brown fat. Leanness of the raptor(ad-/-) mice was attributed to elevated energy expenditure due to mitochondrial uncoupling. These results suggest that adipose mTORC1 is a regulator of adipose metabolism and, thereby, controls whole-body energy homeostasis.

mTORC1 activation in podocytes is a critical step in the development of diabetic nephropathy in mice.

Diabetic nephropathy (DN) is among the most lethal complications that occur in type 1 and type 2 diabetics. Podocyte dysfunction is postulated to be a critical event associated with proteinuria and glomerulosclerosis in glomerular diseases including DN. However, molecular mechanisms of podocyte dysfunction in the development of DN are not well understood. Here we have shown that activity of mTOR complex 1 (mTORC1), a kinase that senses nutrient availability, was enhanced in the podocytes of diabetic animals. Further, podocyte-specific mTORC1 activation induced by ablation of an upstream negative regulator (PcKOTsc1) recapitulated many DN features, including podocyte loss, glomerular basement membrane thickening, mesangial expansion, and proteinuria in nondiabetic young and adult mice. Abnormal mTORC1 activation caused mislocalization of slit diaphragm proteins and induced an epithelial-mesenchymal transition-like phenotypic switch with enhanced ER stress in podocytes. Conversely, reduction of ER stress with a chemical chaperone significantly protected against both the podocyte phenotypic switch and podocyte loss in PcKOTsc1 mice. Finally, genetic reduction of podocyte-specific mTORC1 in diabetic animals suppressed the development of DN. These results indicate that mTORC1 activation in podocytes is a critical event in inducing DN and suggest that reduction of podocyte mTORC1 activity is a potential therapeutic strategy to prevent DN.

Role of mTOR in podocyte function and diabetic nephropathy in humans and mice

Chronic glomerular diseases, associated with renal failure and cardiovascular morbidity, represent a major health issue. However, they remain poorly understood. Here we have reported that tightly controlled mTOR activity was crucial to maintaining glomerular podocyte function, while dysregulation of mTOR facilitated glomerular diseases. Genetic deletion of mTOR complex 1 (mTORC1) in mouse podocytes induced proteinuria and progressive glomerulosclerosis. Furthermore, simultaneous deletion of both mTORC1 and mTORC2 from mouse podocytes aggravated the glomerular lesions, revealing the importance of both mTOR complexes for podocyte homeostasis. In contrast, increased mTOR activity accompanied human diabetic nephropathy, characterized by early glomerular hypertrophy and hyperfiltration. Curtailing mTORC1 signaling in mice by genetically reducing mTORC1 copy number in podocytes prevented glomerulosclerosis and significantly ameliorated the progression of glomerular disease in diabetic nephropathy. These results demonstrate the requirement for tightly balanced mTOR activity in podocyte homeostasis and suggest that mTOR inhibition can protect podocytes and prevent progressive diabetic nephropathy.

Hepatic mTORC2 Activates Glycolysis and Lipogenesis through Akt, Glucokinase, and SREBP1c

Mammalian target of rapamycin complex 2 (mTORC2) phosphorylates and activates AGC kinase family members, including Akt, SGK1, and PKC, in response to insulin/IGF1. The liver is a key organ in insulin-mediated regulation of metabolism. To assess the role of hepatic mTORC2, we generated liver-specific rictor knockout (LiRiKO) mice. Fed LiRiKO mice displayed loss of Akt Ser473 phosphorylation and reduced glucokinase and SREBP1c activity in the liver, leading to constitutive gluconeogenesis, and impaired glycolysis and lipogenesis, suggesting that the mTORC2-deficient liver is unable to sense satiety. These liver-specific defects resulted in systemic hyperglycemia, hyperinsulinemia, and hypolipidemia. Expression of constitutively active Akt2 in mTORC2-deficient hepatocytes restored both glucose flux and lipogenesis, whereas glucokinase overexpression rescued glucose flux but not lipogenesis. Thus, mTORC2 regulates hepatic glucose and lipid metabolism via insulin-induced Akt signaling to control whole-body metabolic homeostasis. These findings have implications for emerging drug therapies that target mTORC2.

New insights into the roles of agrin

The heparan sulphate proteoglycan agrin is expressed as several isoforms in various tissues. Agrin is best known as a crucial organizer of postsynaptic differentiation at the neuromuscular junction, but it has recently also been implicated in the formation of the immunological synapse, the organization of the cytoskeleton and the amelioration of function in diseased muscle. So the activities of agrin might be of broader significance than previously anticipated.

The agrin gene codes for a family of basal lamina proteins that differ in function and distribution.

We isolated two cDNAs that encode isoforms of agrin, the basal lamina protein that mediates the motor neuron-induced aggregation of acetylcholine receptors on muscle fibers at the neuromuscular junction. Both proteins are the result of alternative splicing of the product of the agrin gene, but unlike agrin, they are inactive in standard acetylcholine receptor aggregation assays. They lack one (agrin-related protein 1) or two (agrin-related protein 2) regions in agrin that are required for its activity. Expression studies provide evidence that both proteins are present in the nervous system and muscle and that, in muscle, myofibers and Schwann cells synthesize the agrin-related proteins while the axon terminals of motor neurons are the sole source of agrin.

ALTERNATIVE SPLICING OF AGRIN ALTERS ITS BINDING TO HEPARIN, DYSTROGLYCAN,AND THE PUTATIVE AGRIN RECEPTOR

Agrin is a heparan sulfate proteoglycan that induces aggregation of acetylcholine receptors (AChRs) at the neuromuscular synapse. This aggregating activity is modulated by alternative splicing. Here, we compared binding of agrin isoforms to heparin, alpha-dystroglycan, and cultured myotubes. We find that the alternatively spliced 4 amino acids insert (KSRK) is required for heparin binding. The binding affinity of agrin isoforms to alpha-dystroglycan correlates neither with binding to heparin nor with their AChR-aggregating activities. Moreover, the minimal fragment sufficient to induce AChR aggregation does not bind to alpha-dystroglycan. Nevertheless, this fragment still binds to cultured muscle cells. Its binding is completed only by agrin isoforms that are active in AChR aggregation, and therefore this binding site is likely to represent the receptor that initiates AChR clustering.