Insulin signaling: lessons from the Drosophila tuberous sclerosis complex, a tumor suppressor

Tuberous Sclerosis Complex (TSC) Inactivation Increases Neuronal Network Activity by Enhancing Ca2+ Influx via L-Type Ca2+ Channels

Tuberous sclerosis complex (TSC) is a multisystem developmental disorder characterized by hamartomas in various organs, such as the brain, lungs, and kidneys. Epilepsy, along with autism and intellectual disability, is one of the neurologic impairments associated with TSC that has an intimate relationship with developmental outcomes and quality of life. Sustained activation of the mammalian target of rapamycin (mTOR) via TSC1 or TSC2 mutations is known to be involved in the onset of epilepsy in TSC. However, the mechanism by which mTOR causes seizures remains unknown. In this study, we showed that, human induced pluripotent stem cell-derived TSC2-deficient (TSC2 ^-/-) neurons exhibited elevated neuronal activity with highly synchronized Ca²⁺ spikes. Notably, TSC2 ^-/- neurons presented enhanced Ca²⁺ influx via L-type Ca²⁺ channels (LTCCs), which contributed to the abnormal neurite extension and sustained activation of cAMP response element binding protein (CREB), a critical mediator of synaptic plasticity. Expression of Cav1.3, a subtype of LTCCs, was increased in TSC2 ^-/- neurons, but long-term rapamycin treatment suppressed this increase and reversed the altered neuronal activity and neurite extensions. Thus, we identified Cav1.3 LTCC as a critical downstream component of TSC-mTOR signaling that would trigger enhanced neuronal network activity of TSC2 ^-/- neurons. We suggest that LTCCs could be potential novel targets for the treatment of epilepsy in TSC.SIGNIFICANCE STATEMENT There is a close relationship between elevated mammalian target of rapamycin (mTOR) activity and epilepsy in tuberous sclerosis complex (TSC). However, the underlying mechanism by which mTOR causes epilepsy remains unknown. In this study, using human TSC2 ^-/- neurons, we identified elevated Ca²⁺ influx via L-type Ca²⁺ channels as a critical downstream component of TSC-mTOR signaling and a potential cause of both elevated neuronal activity and neurite extension in TSC2 ^-/- neurons. Our findings demonstrate a previously unrecognized connection between sustained mTOR activation and elevated Ca²⁺ signaling via L-type Ca²⁺ channels in human TSC neurons, which could cause epilepsy in TSC.

Differential metabolic sensitivity of insulin-like-response- and TORC1-dependent overgrowth in Drosophila fat cells

Glycolysis and fatty acid (FA) synthesis directs the production of energy-carrying molecules and building blocks necessary to support cell growth, although the absolute requirement of these metabolic pathways must be deeply investigated. Here, we used Drosophila genetics and focus on the TOR (Target of Rapamycin) signaling network that controls cell growth and homeostasis. In mammals, mTOR (mechanistic-TOR) is present in two distinct complexes, mTORC1 and mTORC2; the former directly responds to amino acids and energy levels, whereas the latter sustains insulin-like-peptide (Ilp) response. The TORC1 and Ilp signaling branches can be independently modulated in most Drosophila tissues. We show that TORC1 and Ilp-dependent overgrowth can operate independently in fat cells and that ubiquitous over-activation of TORC1 or Ilp signaling affects basal metabolism, supporting the use of Drosophila as a powerful model to study the link between growth and metabolism. We show that cell-autonomous restriction of glycolysis or FA synthesis in fat cells retrains overgrowth dependent on Ilp signaling but not TORC1 signaling. Additionally, the mutation of FASN (Fatty acid synthase) results in a drop in TORC1 but not Ilp signaling, whereas, at the cell-autonomous level, this mutation affects none of these signals in fat cells. These findings thus reveal differential metabolic sensitivity of TORC1- and Ilp-dependent growth and suggest that cell-autonomous metabolic defects might elicit local compensatory pathways. Conversely, enzyme knockdown in the whole organism results in animal death. Importantly, our study weakens the use of single inhibitors to fight mTOR-related diseases and strengthens the use of drug combination and selective tissue-targeting.

Lymphangioleiomyomatosis: A Monogenic Model of Malignancy

Lymphangioleiomyomatosis (LAM) is a rare, low-grade, metastasizing neoplasm that arises from an unknown source, spreads via the lymphatics, and targets the lungs. All pulmonary structures become infiltrated with benign-appearing spindle and epithelioid cells (LAM cells) that express smooth-muscle and melanocyte-lineage markers, harbor mTOR-activating mutations in tuberous sclerosis complex (TSC) genes, and recruit abundant stromal cells. Elaboration of lymphangiogenic growth factors and matrix remodeling enzymes by LAM cells enables their access to lymphatic channels and likely drives the cystic lung remodeling that often culminates in respiratory failure. Dysregulated cellular signaling results in a shift from oxidative phosphorylation to glycolysis as the preferred mode of energy generation, to allow for the accumulation of biomass required for cell growth and tolerance of nutrient-poor, anaerobic environments. Symptomatic LAM occurs almost exclusively in females after menarche, highlighting the central but as yet poorly understood role for sex-restricted anatomical structures and/or hormones in disease pathogenesis. LAM is an elegant model of malignancy because biallelic mutations at a single genetic locus confer all features that define cancer upon the LAM cell-metabolic reprogramming and proliferative signals that drive uncontrolled growth and inappropriate migration and invasion, the capacity to exploit the lymphatic circulation as a vehicle for metastasis and access to the lungs, and destruction of remote tissues. The direct benefit of the study of this rare disease has been the rapid identification of an effective FDA-approved therapy, and the collateral benefits have included elucidation of the pivotal roles of mTOR signaling in the regulation of cellular metabolism and the pathogenesis of cancer.

The role of AMPK in cardiomyocyte health and survival

Cellular energy homeostasis is a fundamental process that governs the overall health of the cell and is paramount to cell survival. Central to this is the control of ATP generation and utilization, which is regulated by a complex myriad of enzymatic reactions controlling cellular metabolism. In the cardiomyocyte, ATP generated from substrate catabolism is used for numerous cellular processes including maintaining ionic homeostasis, cell repair, protein synthesis and turnover, organelle turnover, and contractile function. In many instances, cardiovascular disease is associated with impaired cardiac energetics and thus the signalling that regulates pathways involved in cardiomyocyte metabolism may be potential targets for pharmacotherapy designed to help treat cardiovascular disease. An important regulator of cardiomyocyte energy homeostasis is adenosine monophosphate-activated protein kinase (AMPK). AMPK is a serine-threonine kinase that functions primarily as a metabolic sensor to coordinate anabolic and catabolic activities in the cell via the phosphorylation of multiple proteins involved in metabolic pathways. In addition to the direct role that AMPK plays in the regulation of cardiomyocyte metabolism, AMPK can also either directly or indirectly influence other cellular processes such as regulating mitochondrial function, post-translation acetylation, autophagy, mitophagy, endoplasmic reticulum stress, and apoptosis. Thus, AMPK is implicated in the control of a wide variety of cellular processes that can influence cardiomyocyte health and survival. In this review, we will discuss the important role that AMPK plays in regulating cardiac metabolism, as well as the additional cellular processes that may contribute to cardiomyocyte function and survival in the healthy and the diseased heart. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan. F.C. Glatz.

Lipid metabolism in Drosophila: development and disease

Proteins, nucleic acids, and lipids are three major components of the cell. Despite a few basic metabolic pathways, we know very little about lipids, compared with the explosion of knowledge about proteins and nucleic acids. How many different forms of lipids are there? What are the in vivo functions of individual lipid? How does lipid metabolism contribute to normal development and human health? Many of these questions remain unanswered. For over a century, the fruit fly Drosophila melanogaster has been used as a model organism to study basic biological questions. In recent years, increasing evidences proved that Drosophila models are highly valuable for lipid metabolism and energy homeostasis researches. Some recent progresses of lipid metabolic regulation during Drosophila development and in Drosophila models of human diseases will be discussed in this review.

The nuclear receptor DHR3 modulates dS6 kinase-dependent growth in Drosophila

S6 kinases (S6Ks) act to integrate nutrient and insulin signaling pathways and, as such, function as positive effectors in cell growth and organismal development. However, they also have been shown to play a key role in limiting insulin signaling and in mediating the autophagic response. To identify novel regulators of S6K signaling, we have used a Drosophila-based, sensitized, gain-of-function genetic screen. Unexpectedly, one of the strongest enhancers to emerge from this screen was the nuclear receptor (NR), Drosophila hormone receptor 3 (DHR3), a critical constituent in the coordination of Drosophila metamorphosis. Here we demonstrate that DHR3, through dS6K, also acts to regulate cell-autonomous growth. Moreover, we show that the ligand-binding domain (LBD) of DHR3 is essential for mediating this response. Consistent with these findings, we have identified an endogenous DHR3 isoform that lacks the DBD. These results provide the first molecular link between the dS6K pathway, critical in controlling nutrient-dependent growth, and that of DHR3, a major mediator of ecdysone signaling, which, acting together, coordinate metamorphosis.

Molecular basis of the acceleration of the GDP-GTP exchange of human ras homolog enriched in brain by human translationally controlled tumor protein

Ras homolog enriched in brain (Rheb), a small GTPase, positively regulates the mTORC1 pathway. The GDP-GTP exchange of Rheb has been suggested to be facilitated by translationally controlled tumor protein (TCTP). Here we demonstrate that human TCTP (hTCTP) interacts with human Rheb (hRheb) and accelerates its GDP release in vitro and that hTCTP activates the mTORC1 pathway in vivo. To investigate the underlying mechanism, we built structure models of GDP- and GTP-bound hRheb in complexes with hTCTP and performed molecular dynamics simulations of the models, which predict key residues involved in the interactions and region of hRheb undergoing conformational change during the GDP-GTP exchange. These results are verified with site-directed mutagenesis and in vitro biochemical and in vivo cell biological analyses. Furthermore, a crystal structure of the E12V mutant hTCTP, which lacks the guanine nucleotide exchange factor activity, shows that the deficiency appears to be caused by loss of a salt-bridging interaction with Lys-45 of hRheb. These data collectively provide insights into the molecular mechanisms of how hTCTP interacts with hRheb and activates the mTORC1 pathway.

hvps34, an ancient player, enters a growing game: mTOR Complex1/S6K1 signaling

Recent studies have shown that the nutrient input to the mTOR Complex1/S6K1 signaling pathway is mediated by class 3 PI3K or hVps34, the oldest member of the PI3K family. Moreover, studies to date would suggest that during the evolution of multicellular organisms this ancient branch of the pathway was merged with the growth-factor-hormone-controlled class 1 PI3K pathway at the level of mTOR Complex1 to control the development and growth of the organism. However, hVps34 also plays a role in the regulation of macroautophagy - the mechanism by which cells generate nutrients, such as amino acids, through the degradation of intracellular complexes, including mitochondria and ribosomes. These functions of hVps34 initially appear contradictory, since increased mTOR Complex1 activation is triggered by increased amino acid levels, while autophagy is triggered when cells are faced with amino acid deprivation.

Inhibition of protein synthesis by Y box-binding protein 1 blocks oncogenic cell transformation

The multifunctional Y box-binding protein 1 (YB-1) is transcriptionally repressed by the oncogenic phosphoinositide 3-kinase (PI3K) pathway (with P3K as an oncogenic homolog of the catalytic subunit) and, when reexpressed with the retroviral vector RCAS, interferes with P3K- and Akt-induced transformation of chicken embryo fibroblasts. Retrovirally expressed YB-1 binds to the cap of mRNAs and inhibits cap-dependent and cap-independent translation. To determine the requirements for the inhibitory role of YB-1 in P3K-induced transformation, we conducted a mutational analysis, measuring YB-1-induced interference with transformation, subcellular localization, cap binding, mRNA binding, homodimerization, and inhibition of translation. The results show that (i) interference with transformation requires RNA binding and a C-terminal domain that is distinct from the cytoplasmic retention domain, (ii) interference with transformation is tightly correlated with inhibition of translation, and (iii) masking of mRNAs by YB-1 is not sufficient to block transformation or to inhibit translation. We identified a noncanonical nuclear localization signal (NLS) in the C-terminal half of YB-1. A mutant lacking the NLS retains its ability to interfere with transformation, indicating that a nuclear function is not required. These results suggest that YB-1 interferes with P3K-induced transformation by a specific inhibition of translation through its RNA-binding domain and a region in the C-terminal domain. Potential functions of the C-terminal region are discussed.

Disruption of the mouse mTOR gene leads to early postimplantation lethality and prohibits embryonic stem cell development

The mammalian target of rapamycin (mTOR) is a key component of a signaling pathway which integrates inputs from nutrients and growth factors to regulate cell growth. Recent studies demonstrated that mice harboring an ethylnitrosourea-induced mutation in the gene encoding mTOR die at embryonic day 12.5 (E12.5). However, others have shown that the treatment of E4.5 blastocysts with rapamycin blocks trophoblast outgrowth, suggesting that the absence of mTOR should lead to embryonic lethality at an earlier stage. To resolve this discrepancy, we set out to disrupt the mTOR gene and analyze the outcome in both heterozygous and homozygous settings. Heterozygous mTOR (mTOR(+/-)) mice do not display any overt phenotype, although mouse embryonic fibroblasts derived from these mice show a 50% reduction in mTOR protein levels and phosphorylation of S6 kinase 1 T389, a site whose phosphorylation is directly mediated by mTOR. However, S6 phosphorylation, raptor levels, cell size, and cell cycle transit times are not diminished in these cells. In contrast to the situation in mTOR(+/-) mice, embryonic development of homozygous mTOR(-/-) mice appears to be arrested at E5.5; such embryos are severely runted and display an aberrant developmental phenotype. The ability of these embryos to implant corresponds to a limited level of trophoblast outgrowth in vitro, reflecting a maternal mRNA contribution, which has been shown to persist during preimplantation development. Moreover, mTOR(-/-) embryos display a lesion in inner cell mass proliferation, consistent with the inability to establish embryonic stem cells from mTOR(-/-) embryos.

Hairy transcriptional repression targets and cofactor recruitment in Drosophila

Members of the widely conserved Hairy/Enhancer of split family of basic Helix-Loop-Helix repressors are essential for proper Drosophila and vertebrate development and are misregulated in many cancers. While a major step forward in understanding the molecular mechanism(s) surrounding Hairy-mediated repression was made with the identification of Groucho, Drosophila C-terminal binding protein (dCtBP), and Drosophila silent information regulator 2 (dSir2) as Hairy transcriptional cofactors, the identity of Hairy target genes and the rules governing cofactor recruitment are relatively unknown. We have used the chromatin profiling method DamID to perform a global and systematic search for direct transcriptional targets for Drosophila Hairy and the genomic recruitment sites for three of its cofactors: Groucho, dCtBP, and dSir2. Each of the proteins was tethered to Escherichia coli DNA adenine methyltransferase, permitting methylation proximal to in vivo binding sites in both Drosophila Kc cells and early embryos. This approach identified 40 novel genomic targets for Hairy in Kc cells, as well as 155 loci recruiting Groucho, 107 loci recruiting dSir2, and wide genomic binding of dCtBP to 496 loci. We also adapted DamID profiling such that we could use tightly gated collections of embryos (2-6 h) and found 20 Hairy targets related to early embryogenesis. As expected of direct targets, all of the putative Hairy target genes tested show Hairy-dependent expression and have conserved consensus C-box-containing sequences that are directly bound by Hairy in vitro. The distribution of Hairy targets in both the Kc cell and embryo DamID experiments corresponds to Hairy binding sites in vivo on polytene chromosomes. Similarly, the distributions of loci recruiting each of Hairy's cofactors are detected as cofactor binding sites in vivo on polytene chromosomes. We have identified 59 putative transcriptional targets of Hairy. In addition to finding putative targets for Hairy in segmentation, we find groups of targets suggesting roles for Hairy in cell cycle, cell growth, and morphogenesis, processes that must be coordinately regulated with pattern formation. Examining the recruitment of Hairy's three characterized cofactors to their putative target genes revealed that cofactor recruitment is context-dependent. While Groucho is frequently considered to be the primary Hairy cofactor, we find here that it is associated with only a minority of Hairy targets. The majority of Hairy targets are associated with the presence of a combination of dCtBP and dSir2. Thus, the DamID chromatin profiling technique provides a systematic means of identifying transcriptional target genes and of obtaining a global view of cofactor recruitment requirements during development.

Retroviral oncogenes and TOR

Retroviruses have recruited the catalytic subunit of PI 3-kinase and its downstream target, Akt, as oncogenes. These viruses cause tumors in animals and induce oncogenic transformation in cell culture. The oncogenicity of these viruses is specifically inhibited by rapamycin; retroviruses carrying other oncogenes are insensitive to this macrolide antibiotic. Rapamycin is an inhibitor of the TOR (target of rapamycin) kinase whose downstream targets include p70 S6 kinase and the negative regulator of translation initiation 4E-BP. Emerging evidence suggests that the TOR signals transmitted to the translational machinery are essential for oncogenic transformation by the PI 3-kinase pathway.

Cell size regulation by the human TSC tumor suppressor proteins depends on PI3K and FKBP38

TSC1 and TSC2 are responsible for the tumor suppressor gene syndrome tuberous sclerosis (TSC). Mammalian TSC genes have been shown to be involved in cell cycle regulation. Recently, in Drosophila, these data have been confirmed and TSC genes have further been demonstrated to affect cell size control. Here we provide supporting data for the fact that the latter function is conserved in mammals. Human TSC1 and TSC2 trigger mammalian cell size reduction and a dominant-negative TSC2 mutant induces increased size. These effects occur in all cell cycle phases, are dependent on the activity of the phosphoinositide-3-kinase and are abolished by co-overexpression of a dominant-negative Akt mutant. Two independent naturally occurring and disease-causing mutations within the TSC2 gene eliminate tuberin's capacity to affect cell size control, emphasizing the relevance of this function for the development of the disease. The same mutations have earlier been shown not to affect tuberin's antiproliferative capacity. That the consequences of modulated TSC gene expression on cell proliferation and on cell size can be assigned to separable functions is further supported by two findings: A mutation within the TSC1 gene, earlier shown to still harbor anti-proliferative effects, was found to eliminate the cell size regulating functions. An important mammalian cell size regulator, c-Myc, was found to inhibit tuberin's antiproliferative capacity, but to have no effects on tuberin-dependent cell size control. To obtain further mechanistical insights, microarray screens for genes involved in TSC1- or TSC2-mediated cell size effects were performed. Antisense experiments revealed that the so observed regulation of the FK506-binding protein, FKBP38, plays a role in TSC gene-dependent cell size regulation. These data provide new insights into mammalian cell size regulation and allow a better understanding of the function of human TSC genes.

Genes and pathways driving glioblastomas in humans and murine disease models

Human malignant gliomas arise from neural progenitor cells and/or dedifferentiated astrocytes. By now, they are genetically so well characterized that several murine glioma models have emerged that faithfully reiterate the typical histological features of the disease. In experimental animals, only one or two elements of the growth factor/Ras, PI3K/PTEN/PKB, p53/ARF/HDM2, and p16/Rb/cyclinD/CDK4 pathways are targeted. In human gliomas, many additional genes and pathways are targeted due to a most severe mutator phenotype that leads to the accumulation of countless epigenetic and genetic alterations. Changes that convey a growth advantage are selected for, leading to overgrowth of precursor cell populations with increasingly malignant tumor cell clones. While murine models represent a powerful tool for elucidating the role of genetic pathways, mechanisms of response and resistance to new therapeutic agents might be fundamentally different due to the high degree of genomic instability in the human disease. In fact, little is known about the molecular causes of genomic instability involved in gliomas, except for the rare Turcot's syndrome, O(6)-methylguanine-DNA methyltransferase, and the apurinic/apyrimidinic endonuclease Ape-1. Novel approaches that selectively exploit fundamental metabolic differences between tumor and normal cells have to consider these fundamental differences between human disease and presently available, highly sophisticated animal models.

Lethality of Drosophila lacking TSC tumor suppressor function rescued by reducing dS6K signaling

Tuberous sclerosis complex (TSC) is a genetic disorder caused by mutations in one of two tumor suppressor genes, TSC1 and TSC2. Here, we show that absence of Drosophila Tsc1/2 leads to constitutive dS6K activation and inhibition of dPKB, the latter effect being relieved by loss of dS6K. In contrast, the dPTEN tumor suppressor, a negative effector of PI3K, has little effect on dS6K, but negatively regulates dPKB. More importantly, we demonstrate that reducing dS6K signaling rescues early larval lethality associated with loss of dTsc1/2 function, arguing that the S6K pathway is a promising target for the treatment of TSC.

dS6K-regulated cell growth is dPKB/dPI(3)K-independent, but requires dPDK1

Genetic studies in Drosophila melanogaster underscore the importance of the insulin-signalling pathway in controlling cell, organ and animal size. Effectors of this pathway include Chico (the insulin receptor substrate homologue), dPI(3)K, dPKB, dPTEN, and dS6K. Mutations in any of these components have a striking effect on cell size and number, with the exception of dS6K. Mutants in dS6K affect cell size but not cell number, seemingly consistent with arguments that dS6K is a distal effector in the signalling pathway, directly controlled by dTOR, a downstream effector of dPI(3)K and dPKB. Unexpectedly, recent studies showed that dS6K activity is unimpaired in chico-deficient larvae, suggesting that dS6K activation may be mediated through the dPI(3)K docking sites of the Drosophila insulin receptor. Here, we show genetically, pharmacologically and biochemically that dS6K resides on an insulin signalling pathway distinct from that of dPKB, and surprisingly also from that of dPI(3)K. More striking, despite dPKB-dPI(3)K-independence, dS6K activity is dependent on the Drosophila homologue of the phosphoinositide-dependent protein kinase 1, dPDK1, demonstrating that both dPDK1, as well as dTOR, mediated dS6K activation is phosphatidylinositide-3,4,5-trisphosphate (PIP3)-independent.