TBC1D7 Is a Third Subunit of the TSC1-TSC2 Complex Upstream of mTORC1

The tuberous sclerosis complex (TSC) tumor suppressors form the TSC1-TSC2 complex, which limits?cell growth in response to poor growth conditions. Through its GTPase-activating protein (GAP) activity toward Rheb, this complex inhibits the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1), a key promoter of cell growth. Here, we identify and biochemically characterize TBC1D7 as a stably associated and ubiquitous third core subunit?of the?TSC1-TSC2 complex. We demonstrate that the TSC1-TSC2-TBC1D7 (TSC-TBC) complex is?the functional complex that senses specific cellular?growth conditions and possesses Rheb-GAP activity. Sequencing analyses of samples from TSC patients suggest that TBC1D7 is unlikely to represent TSC3. TBC1D7 knockdown decreases the association of TSC1 and TSC2 leading to decreased Rheb-GAP activity, without effects on the localization of TSC2 to the lysosome. Like the other TSC-TBC components, TBC1D7 knockdown results in increased mTORC1 signaling, delayed induction of autophagy, and enhanced cell growth under poor growth conditions.     

Multiple amino acid sensing inputs to mTORC1

The evolutionarily conserved target of rapamycin complex 1 (TORC1) is a master regulator of cell growth and metabolism. In mammals, growth factors and cellular energy stimulate mTORC1 activity through inhibition of the TSC complex (TSC1-TSC2-TBC1D7), a negative regulator of mTORC1. Amino acids signal to mTORC1 independently of the TSC complex. Here, we review recently identified regulators that link amino acid sufficiency to mTORC1 activity and how mutations affecting these regulators cause human disease.     

Identification of TBC7 having TBC domain as a novel binding protein to TSC1-TSC2 complex

TBC7, a TBC (Tre-2/Bub2/Cdc16) 1 domain protein, was identified as a novel binding protein to the TSC1-TSC2 tumor suppressor complex by peptide mass fingerprinting analysis of the proteins immunoprecipitated with FLAG-epitope tagged TSC1 and TSC2 from the transfected mammalian cells. The in vivo and in vitro association of TBC7 and the TSC1-TSC2 complex was confirmed by the co-immunoprecipitation and pull-down analysis, respectively, and TBC7 was revealed to bind to the C-terminal half region of TSC1, which is distinct from the binding site with TSC2. The immunofluorescence microscopy and subcellular fractionation showed that TBC7 co-localizes with the tumor suppressor complex in the endomembrane. Overexpression of TBC7 enhanced ubiquitination of TSC1 and increased phosphorylation of S6 protein by S6 kinase, that is located in the mTOR-signaling pathway. These results indicate TBC7 could take a part in the negative regulation of the tumor suppressor complex through facilitating the downregulation of TSC1.     

Phosphorylation and binding partner analysis of the TSC1-TSC2 complex

Tuberous sclerosis complex (TSC) is an autosomal dominant benign tumour syndrome caused by mutations to either the TSC1 or TSC2 tumour suppressor gene. The TSC1 and TSC2 gene products, TSC1 and TSC2, form a protein complex that integrates inputs from multiple signalling cascades to inactivate the small GTPase rheb, and thereby inhibit mTOR-dependent cell growth. We have used matrix-assisted laser desorption/ionisation time-of-flight and Fourier transform mass spectrometry to identify TSC1 and TSC2 phosphorylation sites and candidate TSC1 and TSC2 interacting proteins. We identified three sites of TSC2 phosphorylation and a novel site of TSC1 phosphorylation, and investigated the roles of these sites in regulating the activity of the TSC1-TSC2 complex. In addition, we identified three TSC1-TSC2 interacting proteins, including DOCK7 a putative rhebGEF.     

Identification of FIP200 interaction with the TSC1-TSC2 complex and its role in regulation of cell size control

FIP200 (focal adhesion kinase [FAK] family interacting protein of 200 kD) is a newly identified protein that binds to the kinase domain of FAK and inhibits its kinase activity and associated cellular functions. Here, we identify an interaction between FIP200 and the TSC1-TSC2 complex through FIP200 binding to TSC1. We found that association of FIP200 with the TSC1-TSC2 complex correlated with its ability to increase cell size and up-regulate S6 kinase phosphorylation but was not involved in the regulation of cell cycle progression. Conversely, knockdown of endogenous FIP200 by RNA interference reduced S6 kinase phosphorylation and cell size, which required TSC1 but was independent of FAK. Furthermore, overexpression of FIP200 reduced TSC1-TSC2 complex formation, although knockdown of endogenous FIP200 by RNA interference did not affect TSC1-TSC2 complex formation. Lastly, we showed that FIP200 is important in nutrient stimulation-induced, but not energy- or serum-induced, S6 kinase activation. Together, these results suggest a cellular function of FIP200 in the regulation of cell size by interaction with the TSC1-TSC2 complex.     

The TSC1-TSC2 complex is required for proper activation of mTOR complex 2

The mammalian target of rapamycin (mTOR) is a protein kinase that forms two functionally distinct complexes important for nutrient and growth factor signaling. Both complexes phosphorylate a hydrophobic motif on downstream protein kinases, which contributes to the activation of these kinases. mTOR complex 1 (mTORC1) phosphorylates S6K1, while mTORC2 phosphorylates Akt. The TSC1-TSC2 complex is a critical negative regulator of mTORC1. However, how mTORC2 is regulated and whether the TSC1-TSC2 complex is involved are unknown. We find that mTORC2 isolated from a variety of cells lacking a functional TSC1-TSC2 complex is impaired in its kinase activity toward Akt. Importantly, the defect in mTORC2 activity in these cells can be separated from effects on mTORC1 signaling and known feedback mechanisms affecting insulin receptor substrate-1 and phosphatidylinositol 3-kinase. Our data also suggest that the TSC1-TSC2 complex positively regulates mTORC2 in a manner independent of its GTPase-activating protein activity toward Rheb. Finally, we find that the TSC1-TSC2 complex can physically associate with mTORC2 but not mTORC1. These data demonstrate that the TSC1-TSC2 complex inhibits mTORC1 and activates mTORC2, which through different mechanisms promotes Akt activation.     

Analysis of TSC1 truncations defines regions involved in TSC1 stability,aggregation and interaction

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterised by the development of hamartomas in a variety of organs and tissues. The disease is caused by mutations in either the TSC1 gene on chromosome 9q34, or the TSC2 gene on chromosome 16p13.3. The TSC1 and TSC2 gene products, TSC1 and TSC2, interact to form a protein complex that inhibits signal transduction to the downstream effectors of the target of rapamycin complex 1 (TORC1). Here we investigate TSC1 structure and function by analysing a series of truncated TSC1 proteins. We identify specific regions of the protein that are important for TSC1 stability, localisation, interactions and function.     

Ubiquitination and Degradation of the Hominoid-Specific Oncoprotein TBC1D3 Is Mediated by CUL7 E3 Ligase

Expression of the hominoid-specific TBC1D3 oncoprotein enhances growth factor receptor signaling and subsequently promotes cellular proliferation and survival. Here we report that TBC1D3 is degraded in response to growth factor signaling, suggesting that TBC1D3 expression is regulated by a growth factor-driven negative feedback loop. To gain a better understanding of how TBC1D3 is regulated, we studied the effects of growth factor receptor signaling on TBC1D3 post-translational processing and turnover. Using a yeast two-hybrid screen, we identified CUL7, the scaffolding subunit of the CUL7 E3 ligase complex, as a TBC1D3-interacting protein. We show that CUL7 E3 ligase ubiquitinates TBC1D3 in response to serum stimulation. Moreover, TBC1D3 recruits F-box 8 (Fbw8), the substrate recognition domain of CUL7 E3 ligase, in pull-down experiments and in an in vitro assay. Importantly, alkaline phosphatase treatment of TBC1D3 suppresses its ability to recruit Fbw8, indicating that TBC1D3 phosphorylation is critical for its ubiquitination and degradation. We conclude that serum- and growth factor-stimulated TBC1D3 ubiquitination and degradation are regulated by its interaction with CUL7-Fbw8.     

The TSC1-TSC2 complex: a molecular switchboard controlling cell growth

TSC1 and TSC2 are the tumour-suppressor genes mutated in the tumour syndrome TSC (tuberous sclerosis complex). Their gene products form a complex that has become the focus of many signal transduction researchers. The TSC1-TSC2 (hamartin-tuberin) complex, through its GAP (GTPase-activating protein) activity towards the small G-protein Rheb (Ras homologue enriched in brain), is a critical negative regulator of mTORC1 (mammalian target of rapamycin complex 1). As mTORC1 activity controls anabolic processes to promote cell growth, it is exquisitely sensitive to alterations in cell growth conditions. Through numerous phosphorylation events, the TSC1-TSC2 complex has emerged as the sensor and integrator of these growth conditions, relaying signals from diverse cellular pathways to properly modulate mTORC1 activity. In the present review we focus on the molecular details of TSC1-TSC2 complex regulation and function as it relates to the control of Rheb and mTORC1.     

The tuberin-hamartin complex negatively regulates beta-catenin signaling activity

Tuberous sclerosis complex (TSC) is characterized by the formation of hamartomas in multiple organs resulting from mutations in the TSC1 or TSC2 gene. Their protein products, hamartin and tuberin, respectively, form a functional complex that affects cell growth, differentiation, and proliferation. Several lines of evidence, including renal tumors derived from TSC2+/- animals, suggest that the loss or inhibition of tuberin is associated with up-regulation of cyclin D1. As cyclin D1 can be regulated through the canonical Wnt/beta-catenin signaling pathway, we hypothesize that the cell proliferative effects of hamartin and tuberin are partly mediated through beta-catenin. In this study, total beta-catenin protein levels were found to be elevated in the TSC2-related renal tumors. Ectopic expression of hamartin and wild-type tuberin, but not mutant tuberin, reduced beta-catenin steady-state levels and its half-life. The TSC1-TSC2 complex also inhibited Wnt-1 stimulated Tcf/LEF luciferase reporter activity. This inhibition was eliminated by constitutively active beta-catenin but not by Disheveled, suggesting that hamartin and tuberin function at the level of the beta-catenin degradation complex. Indeed, hamartin and tuberin co-immunoprecipitated with glycogen synthase kinase 3 beta and Axin, components of this complex in a Wnt-1-dependent manner. Our data suggest that hamartin and tuberin negatively regulate beta-catenin stability and activity by participating in the beta-catenin degradation complex.     

Tuberin phosphorylation regulates its interaction with hamartin. Two proteins involved in tuberous sclerosis

Hamartin and tuberin are products of the tumor suppressor genes, TSC1 and TSC2, respectively. When mutated, a characteristic spectrum of tumor-like growths develop resulting in the syndrome of tuberous sclerosis complex. The phenotypes associated with TSC1 and TSC2 mutations are largely indistinguishable suggesting a common biochemical pathway. Indeed, hamartin and tuberin have been shown to interact stably in vitro and in vivo. Factors that regulate their interaction are likely critical to the understanding of disease pathogenesis. In this study, we showed that tuberin is phosphorylated at serine and tyrosine residues in response to serum and other factors, and it undergoes serial phosphorylation that can be detected by differences in electrophoretic mobilities. A disease-related TSC2 mutation (Y1571H) nearly abolished tuberin phosphorylation when stimulated with pervanadate. Expression of this mutant tuberin caused a marked reduction in TSC1-TSC2 interaction compared with wild-type protein and significantly curtailed the growth inhibitory effects of tuberin when overexpressed in COS1 cells, consistent with a loss of function mutation. Examination of a second pathologic mutation, P1675L, revealed a similar relationship between limited phosphorylation and reduced interaction with hamartin. Our data show for the first time that 1) tuberin is phosphorylated at tyrosine and serine residues, 2) TSC1-TSC2 interaction is regulated by tuberin phosphorylation, and 3) defective phosphorylation of tuberin is associated with loss of its tumor suppressor activity. These findings suggest that phosphorylation may be a key regulatory mechanism controlling TSC1-TSC2 function.     

Tuberous sclerosis complex: from Drosophila to human disease

Tuberous sclerosis complex (TSC) is a human syndrome characterized by a widespread development of benign tumors. This disease is caused by mutations in the TSC1 or TSC2 tumor suppressor genes; the molecular mechanisms underlying the activity of these have long been elusive. Recent studies of Drosophila and mammalian cells demonstrate that the TSC1-TSC2 complex functions as GTPase activating protein against Rheb - a Ras-like small GTPase, which in turn regulates TOR signaling in nutrient-stimulated cell growth. These findings provide a new paradigm for how proteins involved in nutrient sensing could function as tumor suppressors and suggest novel therapeutic targets against TSC. Here, we review these exciting developments with an emphasis on Drosophila studies and discuss how Drosophila can be a powerful model system for an understanding of the molecular mechanisms of the activity of human disease genes.     

14-3-3beta binds to and negatively regulates the tuberous sclerosis complex 2 (TSC2) tumor suppressor gene product,tuberin

TSC2, or tuberin, is the product of the tuberous sclerosis tumor suppressor gene TSC2 and acts downstream of the phosphatidylinositol 3-kinase-Akt signaling pathway to negatively regulate cellular growth. One mechanism underlying its function is to assemble into a heterodimer with the TSC1 gene product TSC1, or hamartin, resulting in a reduction in phosphorylation, and hence activation, of the ribosomal subunit S6 kinase (S6K). We identified a novel interaction between TSC2 and 14-3-3beta. We found that 14-3-3beta does not interfere with TSC1-TSC2 binding and can form a ternary complex with these two proteins. Association between 14-3-3beta and TSC2 requires phosphorylation of TSC2 at a unique residue that is not a known Akt phosphorylation site. The overexpression of 14-3-3beta compromises the ability of the TSC1-TSC2 complex to reduce S6K phosphorylation. The antagonistic activity of 14-3-3beta toward TSC is dependent on the 14-3-3beta-TSC2 interaction, since a mutant of TSC2 that is not recognized by 14-3-3beta is refractory to 14-3-3beta. We suggest that 14-3-3 proteins interact with the TSC1-TSC2 complex and negatively regulate the function of the TSC proteins.     

Primary cells derived from Tuberous Sclerosis Complex patients show autophagy alteration in the haploinsufficiency state

Tuberous sclerosis complex (TSC) is an autosomal dominant cancer predisposition disorder caused by heterozygous mutations in TSC1 or TSC2 genes and characterized by mTORC1 hyperactivation. TSC-associated tumors develop after loss of heterozygosity mutations and their treatment involves the use of mTORC1 inhibitors. We aimed to evaluate cellular processes regulated by mTORC1 in TSC cells with different mutations before tumor development. Flow cytometry analyses were performed to evaluate cell viability, cell cycle and autophagy in non-tumor primary TSC cells with different heterozygous mutations and in control cells without TSC mutations, before and after treatment with rapamycin (mTORC1 inhibitor). We did not observe differences in cell viability and cell cycle between the cell groups. However, autophagy was reduced in mutated cells. After rapamycin treatment, mutated cells showed a significant increase in the autophagy process (p=0.039). We did not observe differences between cells with distinct TSC mutations. Our main finding is the alteration of autophagy in non-tumor TSC cells. Previous studies in literature found autophagy alterations in tumor TSC cells or knock-out animal models. We showed that autophagy could be an important mechanism that leads to TSC tumor formation in the haploinsufficiency state. This result could guide future studies in this field.     

Muscle atrophy in transgenic mice expressing a human TSC1 transgene

Muscle mass is regulated by a wide range of hormonal and nutritional signals, such as insulin and IGF. Tuberous sclerosis complex (TSC) is an inherited hamartoma disease with tumor growth in numerous organs. TSC is caused by mutation in either TSC1 or TSC2 tumor suppressor genes that negatively regulate insulin-induced S6K activation and cell growth. Here we report that expression of human TSC1 (hTSC1) in mouse skeletal muscle leads to reduction of muscle mass. Expression of hTSC1 stabilizes endogenous TSC2 and leads to inhibition of the mTOR signaling. The hTSC1-mTSC2 hetero-complex and its downstream components remain sensitive to insulin stimulation and nutrition signals. This study suggests that an increase in the steady state level of resident TSC1-TSC2 complex is sufficient to reduce muscle mass and cause atrophy.     

Recessive TBC1D24 Mutations Are Frequent in Moroccan Non-Syndromic Hearing Loss Pedigrees

Mutations in the TBC1D24 gene are responsible for four neurological presentations: infantile epileptic encephalopathy, infantile myoclonic epilepsy, DOORS (deafness, onychodystrophy, osteodystrophy, mental retardation and seizures) and NSHL (non-syndromic hearing loss). For the latter, two recessive (DFNB86) and one dominant (DFNA65) mutations have so far been identified in consanguineous Pakistani and European/Chinese families, respectively. Here we report the results of a genetic study performed on a large Moroccan cohort of deaf patients that identified three families with compound heterozygote mutations in TBC1D24. Four novel mutations were identified, among which, one c.641G>A (p.Arg214His) was present in the three families, and has a frequency of 2% in control Moroccan population with normal hearing, suggesting that it acts as an hypomorphic variant leading to restricted deafness when combined with another recessive severe mutation. Altogether, our results show that mutations in TBC1D24 gene are a frequent cause (>2%) of NSHL in Morocco, and that due to its possible compound heterozygote recessive transmission, this gene should be further considered and screened in other deaf cohorts.     

Mutations in TBC1D24, a Gene Associated With Epilepsy,Also Cause Nonsyndromic Deafness DFNB86

Inherited deafness is clinically and genetically heterogeneous. We recently mapped DFNB86, a locus associated with nonsyndromic deafness, to chromosome 16p. In this study, whole-exome sequencing was performed with genomic DNA from affected individuals from three large consanguineous families in which markers linked to DFNB86 segregate with profound deafness. Analyses of these data revealed homozygous mutation c.208G>T (p.Asp70Tyr) or c.878G>C (p.Arg293Pro) in TBC1D24 as the underlying cause of deafness in the three families. Sanger sequence analysis of TBC1D24 in an additional large family in which deafness segregates with DFNB86 identified the c.208G>T (p.Asp70Tyr) substitution. These mutations affect TBC1D24 amino acid residues that are conserved in orthologs ranging from fruit fly to human. Neither variant was observed in databases of single-nucleotide variants or in 634 chromosomes from ethnically matched control subjects. TBC1D24 in the mouse inner ear was immunolocalized predominantly to spiral ganglion neurons, indicating that DFNB86 deafness might be an auditory neuropathy spectrum disorder. Previously, six recessive mutations in TBC1D24 were reported to cause seizures (hearing loss was not reported) ranging in severity from epilepsy with otherwise normal development to epileptic encephalopathy resulting in childhood death. Two of our four families in which deafness segregates with mutant alleles of TBC1D24 were available for neurological examination. Cosegregation of epilepsy and deafness was not observed in these two families. Although the causal relationship between genotype and phenotype is not presently understood, our findings, combined with published data, indicate that recessive alleles of TBC1D24 can cause either epilepsy or nonsyndromic deafness.     

The TSC-mTOR Pathway Mediates Translational Activation of TOP mRNAs by Insulin Largely in a Raptor- or Rictor-Independent Manner

The stimulatory effect of insulin on protein synthesis is due to its ability to activate various translation factors. We now show that insulin can increase protein synthesis capacity also by translational activation of TOP mRNAs encoding various components of the translation machinery. This translational activation involves the tuberous sclerosis complex (TSC), as the knockout of TSC1 or TSC2 rescues TOP mRNAs from translational repression in mitotically arrested cells. Similar results were obtained upon overexpression of Rheb, an immediate TSC1-TSC2 target. The role of mTOR, a downstream effector of Rheb, in translational control of TOP mRNAs has been extensively studied, albeit with conflicting results. Even though rapamycin fully blocks mTOR complex 1 (mTORC1) kinase activity, the response of TOP mRNAs to this drug varies from complete resistance to high sensitivity. Here we show that mTOR knockdown blunts the translation efficiency of TOP mRNAs in insulin-treated cells, thus unequivocally establishing a role for mTOR in this mode of regulation. However, knockout of the raptor or rictor gene has only a slight effect on the translation efficiency of these mRNAs, implying that mTOR exerts its effect on TOP mRNAs through a novel pathway with a minor, if any, contribution of the canonical mTOR complexes mTORC1 and mTORC2. This conclusion is further supported by the observation that raptor knockout renders the translation of TOP mRNAs rapamycin hypersensitive.     

Growth signaling: TSC takes its place

The protein products of the tumor suppressor genes tuberous sclerosis complex 1 and 2 form a protein complex, TSC1-TSC2, that inhibits growth. Several new studies suggest that TSC1-TSC2 does this by inhibiting TOR and S6 kinase, and that PI 3-kinase-Akt signaling relieves this inhibition.