Glutamine and asparagine activate mTORC1 independently of Rag GTPases.

Nutrient sensing by cells is crucial, and when this sensing mechanism is disturbed, human disease can occur. mTOR complex 1 (mTORC1) senses amino acids to control cell growth, metabolism, and autophagy. Leucine, arginine, and methionine signal to mTORC1 through the well-characterized Rag GTPase signaling pathway. In contrast, glutamine activates mTORC1 through a Rag GTPase-independent mechanism that requires ADP-ribosylation factor 1 (Arf1). Here, using several biochemical and genetic approaches, we show that eight amino acids filter through the Rag GTPase pathway. Like glutamine, asparagine signals to mTORC1 through Arf1 in the absence of the Rag GTPases. Both the Rag-dependent and Rag-independent pathways required the lysosome and lysosomal function for mTORC1 activation. Our results show that mTORC1 is differentially regulated by amino acids through two distinct pathways.

mTOR signaling in stem and progenitor cells.

The mammalian target of rapamycin (mTOR) senses nutrients and growth factors to coordinate cell growth, metabolism and autophagy. Extensive research has mapped the signaling pathways regulated by mTOR that are involved in human diseases, such as cancer, and in diabetes and ageing. Recently, however, new studies have demonstrated important roles for mTOR in promoting the differentiation of adult stem cells, driving the growth and proliferation of stem and progenitor cells, and dictating the differentiation program of multipotent stem cell populations. Here, we review these advances, providing an overview of mTOR signaling and its role in murine and human stem and progenitor cells.

New fixation method for Pauwels type III femoral neck fracture: a finite element analysis of sliding hip screw, L-shaped, and L-shaped with medial plate.

OBJECTIVE: In this study, we aimed to evaluate the biomechanical behavior of three fixations for Pauwels type III fractures (sliding hip screw (SHS), L-shaped, and L-shaped with medial plate), by finite element analysis (FEM). METHODS: Three internal fixators were developed to treat Pauwels type III fracture by finite elements: SHS; L-shaped; and L-shaped with medial plate. Under the same conditions, localized and total vertical fracture displacement, maximum and minimum principal and von Mises stresses were evaluated. RESULTS: The localized and total vertical displacement evaluated for the SHS, L-shaped, and L-shaped with medial plate were 0.15 mm, 0.17 mm, and 0.07 mm (localized), and 4.52 mm, 6.97 mm, and 6.83 mm (total), respectively. The maximum values obtained in the upper region of the femoral neck for the internal fixations were 1.43 MPa, 1.29 MPa, and 1.24 MPa, and the minimum values obtained in the lower region of the femoral neck were - 0.73 MPa, - 1.09 MPa, and - 1.03 MPa, respectively. The maximum Von Mises peak stress values were 6.35 MPa, 10.7 MPa, and 16.2 MPa for the fixation models using the SHS, L-shaped, and L-shaped with medial plate, respectively. CONCLUSION: The present FEM analysis showed that SHS yields better results in terms of total vertical displacements, maximum distribution, and Von Mises peak stresses reduction. On the other hand, the L-shaped construction plus a medial plate decreases localized vertical displacements and maximum principal distribution when compared to the SHS and L-shaped constructions. These results demonstrate that both constructions, SHS and L-shaped plus a medial plate, are biomechanically efficient for the fixation of Pauwels type III femoral neck fractures.

Design and In Vivo Characterization of Immunoconjugates Targeting HIV gp160.

The envelope (Env) glycoprotein of HIV is expressed on the surface of productively infected cells and can be used as a target for cytotoxic immunoconjugates (ICs), in which cell-killing moieties, including toxins, drugs, or radionuclides, are chemically or genetically linked to monoclonal antibodies (MAbs) or other targeting ligands. Such ICs could be used to eliminate persistent reservoirs of HIV infection. We have found that MAbs which bind to the external loop of gp41, e.g., MAb 7B2, make highly effective ICs, particularly when used in combination with soluble CD4. We evaluated the toxicity, immunogenicity, and efficacy of the ICs targeted with 7B2 in mice and in simian-human immunodeficiency virus-infected macaques. In the macaques, we tested immunotoxins (ITs), consisting of protein toxins bound to the targeting agent. ITs were well tolerated and initially efficacious but were ultimately limited by their immunogenicity. In an effort to decrease immunogenicity, we tested different toxic moieties, including recombinant toxins, cytotoxic drugs, and tubulin inhibitors. ICs containing deglycosylated ricin A chain prepared from ricin toxin extracted from castor beans were the most effective in killing HIV-infected cells. Having identified immunogenicity as a major concern, we show that conjugation of IT to polyethylene glycol limits immunogenicity. These studies demonstrate that cytotoxic ICs can target virus-infected cells in vivo but also highlight potential problems to be addressed. IMPORTANCE: It is not yet possible to cure HIV infection. Even after years of fully effective antiviral therapy, a persistent reservoir of virus-infected cells remains. Here we propose that a targeted conjugate consisting of an anti-HIV antibody bound to a toxic moiety could function to kill the HIV-infected cells that constitute this reservoir. We tested this approach in HIV-infected cells grown in the lab and in animal infections. Our studies demonstrated that these immunoconjugates are effective both in vitro and in test animals. In particular, ITs constructed with the deglycosylated A chain prepared from native ricin were the most effective in killing cells, but their utility was blunted because they provoked immune reactions that interfered with the therapeutic effects. We then demonstrated that coating of the ITs with polyethylene glycol minimized the immunogenicity, as has been demonstrated with other protein therapies.

Forward Genetic Screens Identify Mechanisms of Resistance to Small-Molecule Lactate Dehydrogenase Inhibitors.

Altered metabolism is a hallmark of cancer; however, it has been difficult to specifically target metabolism in cancer for therapeutic benefit. Cancers with genetically defined defects in metabolic enzymes constitute a subset of cancers where targeting metabolism is potentially accessible. Hürthle cell carcinoma of the thyroid (HTC) tumors frequently harbor deleterious mitochondrial DNA (mtDNA) mutations in subunits of complex I of the mitochondrial electron transport chain (ETC). Previous work has shown that HTC models with deleterious mtDNA mutations exhibit mitochondrial ETC defects that expose lactate dehydrogenase (LDH) as a therapeutic vulnerability. Here, we performed forward genetic screens to identify mechanisms of resistance to small-molecule LDH inhibitors. We identified two distinct mechanisms of resistance: upregulation of an LDH isoform and a compound-specific resistance mutation. Using these tools, we demonstrate that the anticancer activity of LDH inhibitors in cell line and xenograft models of complex I mutant HTC is through on-target LDH inhibition.

Forward genetic screens identify mechanisms of resistance to small molecule lactate dehydrogenase inhibitors.

Altered metabolism is a hallmark of cancer; however, it has been difficult to specifically target metabolism in cancer for therapeutic benefit. Cancers with genetically defined defects in metabolic enzymes constitute a subset of cancers where targeting metabolism is potentially accessible. Hürthle cell carcinoma of the thyroid (HTC) tumors frequently harbor deleterious mitochondrial DNA (mtDNA) mutations in subunits of complex I of the mitochondrial electron transport chain (ETC). Previous work has shown that HTC models with deleterious mtDNA mutations exhibit mitochondrial ETC defects that expose lactate dehydrogenase (LDH) as a therapeutic vulnerability. Here, we performed forward genetic screens to identify mechanisms of resistance to small molecule LDH inhibitors. We identified two distinct mechanisms of resistance: upregulation of an LDH isoform and a compound-specific resistance mutation. Using these tools, we demonstrate that the anti-cancer activity of LDH inhibitors in cell line and xenograft models of complex I-mutant HTC is through on-target LDH inhibition.

Mitochondrial-Encoded Complex I Impairment Induces a Targetable Dependency on Aerobic Fermentation in Hürthle Cell Carcinoma of the Thyroid.

A metabolic hallmark of cancer identified by Warburg is the increased consumption of glucose and secretion of lactate, even in the presence of oxygen. Although many tumors exhibit increased glycolytic activity, most forms of cancer rely on mitochondrial respiration for tumor growth. We report here that Hürthle cell carcinoma of the thyroid (HTC) models harboring mitochondrial DNA-encoded defects in complex I of the mitochondrial electron transport chain exhibit impaired respiration and alterations in glucose metabolism. CRISPR-Cas9 pooled screening identified glycolytic enzymes as selectively essential in complex I-mutant HTC cells. We demonstrate in cultured cells and a patient-derived xenograft model that small-molecule inhibitors of lactate dehydrogenase selectively induce an ATP crisis and cell death in HTC. This work demonstrates that complex I loss exposes fermentation as a therapeutic target in HTC and has implications for other tumors bearing mutations that irreversibly damage mitochondrial respiration. SIGNIFICANCE: HTC is enriched in somatic mtDNA mutations predicted to affect complex I of the electron transport chain (ETC). We demonstrate that these mutations impair respiration and induce a therapeutically tractable reliance on aerobic fermentation for cell survival. This work provides a rationale for targeting fermentation in cancers harboring irreversible genetically encoded ETC defects. See related article by Gopal et al., p. 1904. This article is highlighted in the In This Issue feature, p. 1749.

Differential requirements for mitochondrial electron transport chain components in the adult murine liver.

Mitochondrial electron transport chain (ETC) dysfunction due to mutations in the nuclear or mitochondrial genome is a common cause of metabolic disease in humans and displays striking tissue specificity depending on the affected gene. The mechanisms underlying tissue-specific phenotypes are not understood. Complex I (cI) is classically considered the entry point for electrons into the ETC, and in vitro experiments indicate that cI is required for basal respiration and maintenance of the NAD+/NADH ratio, an indicator of cellular redox status. This finding has largely not been tested in vivo. Here, we report that mitochondrial complex I is dispensable for homeostasis of the adult mouse liver; animals with hepatocyte-specific loss of cI function display no overt phenotypes or signs of liver damage, and maintain liver function, redox and oxygen status. Further analysis of cI-deficient livers did not reveal significant proteomic or metabolic changes, indicating little to no compensation is required in the setting of complex I loss. In contrast, complex IV (cIV) dysfunction in adult hepatocytes results in decreased liver function, impaired oxygen handling, steatosis, and liver damage, accompanied by significant metabolomic and proteomic perturbations. Our results support a model whereby complex I loss is tolerated in the mouse liver because hepatocytes use alternative electron donors to fuel the mitochondrial ETC.

Mitochondria are specialised structures inside cells that help to convert nutrients into energy. They take electrons from nutrients and use them to power biochemical reactions that supply chemical fuel. Previous studies of cells grown in the laboratory have found that electrons enter this process via a large assembly of proteins in mitochondria called complex I. Understanding the mechanism of energy production is important, as issues with mitochondria can lead to a variety of metabolic diseases. However, it is still unclear how complex I acts in living animals. Lesner et al. addressed this knowledge gap by genetically removing a key protein from complex I in the liver of mice. Surprisingly, the animals did not develop any detectable symptoms and maintained healthy liver function. Mice did not seem to compensate by making energy in a different way, suggesting that complex I is not normally used by the mouse liver for this process. This research suggests that biologists should reconsider the mechanism that mitochondria use to power cells in animals. While the role of Complex I in electron transfer is well established in laboratory-grown cells and some organs, like the brain, it cannot be assumed this applies to the whole body. Understanding energy production in specific organs could help researchers to develop nutrient-based therapies for metabolic diseases.

Scinderin promotes fusion of electron transport chain dysfunctional muscle stem cells with myofibers.

Muscle stem cells (MuSCs) experience age-associated declines in number and function, accompanied by mitochondrial electron transport chain (ETC) dysfunction and increased reactive oxygen species (ROS). The source of these changes, and how MuSCs respond to mitochondrial dysfunction, is unknown. We report here that in response to mitochondrial ROS, murine MuSCs directly fuse with neighboring myofibers; this phenomenon removes ETC-dysfunctional MuSCs from the stem cell compartment. MuSC-myofiber fusion is dependent on the induction of Scinderin, which promotes formation of actin-dependent protrusions required for membrane fusion. During aging, we find that the declining MuSC population accumulates mutations in the mitochondrial genome, but selects against dysfunctional variants. In the absence of clearance by Scinderin, the decline in MuSC numbers during aging is repressed; however, ETC-dysfunctional MuSCs are retained and can regenerate dysfunctional myofibers. We propose a model in which ETC-dysfunctional MuSCs are removed from the stem cell compartment by fusing with differentiated tissue.

Transcriptional changes during hepatic ischemia-reperfusion in the rat.

There are few effective targeted strategies to reduce hepatic ischemia-reperfusion (IR) injury, a contributor to poor outcomes in liver transplantation recipients. It has been proposed that IR injury is driven by the generation of reactive oxygen species (ROS). However, recent studies implicate other mediators of the injury response, including mitochondrial metabolic dysfunction. We examined changes in global gene expression after transient hepatic ischemia and at several early reperfusion times to identify potential targets that could be used to protect against IR injury. Male Wistar rats were subjected to 30 minutes of 70% partial warm ischemia followed by 0, 0.5, 2, or 6 hours of reperfusion. RNA was extracted from the reperfused and non-ischemic lobes at each time point for microarray analysis. Identification of differentially expressed genes and pathway analysis were used to characterize IR-induced changes in the hepatic transcriptome. Changes in the reperfused lobes were specific to the various reperfusion times. We made the unexpected observation that many of these changes were also present in tissue from the paired non-ischemic lobes. However, the earliest reperfusion time, 30 minutes, showed a marked increase in the expression of a set of immediate-early genes (c-Fos, c-Jun, Atf3, Egr1) that was exclusive to the reperfused lobe. We interpreted these results as indicating that this early response represented a tissue autonomous response to reperfusion. In contrast, the changes that occurred in both the reperfused and non-ischemic lobes were interpreted as indicating a non-autonomous response resulting from hemodynamic changes and/or circulating factors. These tissue autonomous and non-autonomous responses may serve as targets to ameliorate IR injury.

Interleukin-33 / Cyclin D1 imbalance in severe liver steatosis predicts susceptibility to ischemia reperfusion injury.

Transplanting donor livers with severe macrosteatosis is associated with increased risk of primary non-function (PNF). The purpose of this study was to identify steatosis-driven biomarkers as a predisposition to severe liver damage and delayed recovery following ischemia reperfusion injury. Wistar rats were fed a methionine- and choline-deficient (MCD) diet for up to three weeks to achieve severe macrosteatosis (>90%). Animals underwent diet withdrawal to control chow and/or underwent ischemia reperfusion and partial hepatectomy injury (I/R-PHx) and reperfused out to 7 days on control chow. For animals with severe macrosteatosis, hepatic levels of IL-33 decreased while Cyclin D1 levels increased in the absence of NF-κB p65 phosphorylation. Animals with high levels of nuclear Cyclin D1 prior to I/R-PHx either did not survive or had persistent macrosteatosis after 7 days on control chow. Survival 7 days after I/R-PHx fell to 57% which correlated with increased Cyclin D1 and decreased liver IL-33 levels. In the absence of I/R-PHx, withdrawing the MCD diet normalized IL-33, Cyclin D1 levels, and I/R-PHx survival back to baseline. In transplanted grafts with macrosteatosis, higher Cyclin D1 mRNA expression was observed. Shifts in Cyclin D1 and IL-33 expression may identify severely macrosteatotic livers with increased failure risk if subjected to I/R injury. Clinical validation of the panel in donor grafts with macrosteatosis revealed increased Cyclin D1 expression corresponding to delayed graft function. This pre-surgical biomarker panel may identify the subset of livers with increased susceptibility to PNF.

Amino acid and small GTPase regulation of mTORC1.

The mammalian target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase that belongs to the phosphatidylinositol 3-kinase-related kinase (PIKK) family. mTOR is the catalytic subunit of mTOR complex 1 (mTORC1), which integrates multiple environmental signals to control cell growth and metabolism. Nutrients, specifically amino acids, are the most potent stimuli for mTORC1 activation. Multiple studies have focused on how leucine and arginine activate mTORC1 through the Rag GTPases, with mechanistic details slowly emerging. Recently, a Rag GTPase-independent glutamine signaling pathway to mTORC1 has been identified, suggesting that mTORC1 is differentially regulated through distinct pathways by specific amino acids. In this review, we summarize our current understanding of how amino acids modulate mTORC1, and the role of other small GTPases in the regulation of mTORC1 activity.