TFEB Overexpression, Not mTOR Inhibition, Ameliorates RagCS75Y Cardiomyopathy

Allelic heterogeneity of TTNtv dilated cardiomyopathy can be modeled in adult zebrafish

Allelic heterogeneity (AH) has been noted in truncational TTN-associated (TTNtv-associated) dilated cardiomyopathy (DCM); i.e., mutations affecting A-band-encoding exons are pathogenic, but those affecting Z-disc-encoding exons are likely benign. The lack of an in vivo animal model that recapitulates AH hinders the deciphering of the underlying mechanism. Here, we explored zebrafish as a candidate vertebrate model by phenotyping a collection of zebrafish ttntv alleles. We noted that cardiac function and sarcomere structure were more severely disrupted in ttntv-A than in ttntv-Z homozygous embryos. Consistently, cardiomyopathy-like phenotypes were present in ttntv-A but not ttntv-Z adult heterozygous mutants. The phenotypes observed in ttntv-A alleles were recapitulated in null mutants with the full titin-encoding sequences removed. Defective autophagic flux, largely due to impaired autophagosome-lysosome fusion, was also noted only in ttntv-A but not in ttntv-Z models. Moreover, we found that genetic manipulation of ulk1a restored autophagy flux and rescued cardiac dysfunction in ttntv-A animals. Together, our findings presented adult zebrafish as an in vivo animal model for studying AH in TTNtv DCM, demonstrated TTN loss of function is sufficient to trigger ttntv DCM in zebrafish, and uncovered ulk1a as a potential therapeutic target gene for TTNtv DCM.

Deficiency of diacylglycerol Kinase ζ promotes Beclin1-mediated autophagy via the mTOR/TFEB signaling pathway: Relevance to maladaptive cardiac hypertrophy

The activation Gq protein-coupled receptors (GPCRs) is a crucial factor contributing to maladaptive cardiac hypertrophy, and dysregulation of autophagy is implicated in its prohypertrophic effects. Previous studies have shown that diacylglycerol kinase zeta (DGKζ) can suppress cardiac hypertrophy by inhibiting the diacylglycerol (DAG)-PKC pathway in response to mechanical strain or growth agonists such as endothelin-1 (ET-1). However, the involvement of DGKζ in autophagy regulation remains poorly understood. In this study, we aimed to investigate the role of DGKζ in autophagy regulation during maladaptive cardiac hypertrophy. We found that Beclin1-mediated autophagy was involved in the development of maladaptive cardiac hypertrophy and dysfunction in response to prohypertrophic challenges of transverse aortic constriction (TAC) or ET-1. Deficiency of DGKζ promoted Beclin1-mediated autophagy, aggravated adverse cardiac remodeling, and cardiac dysfunction, which could be ameliorated by genetic deletion of Beclin1 or TFEB. Mechanistically, the deficiency of DGKζ disrupted the activation of AKT/mTOR signaling, the association between mTOR and TFEB, and favored the nuclear translocation of TFEB from the cytoplasm, leading to enhanced activation of Beclin1-mediated autophagy through ULK1/Beclin1 signaling and TFEB-dependent Beclin1 transcription. Taken together, these results suggest that the mechanisms by which DGKζ alleviates pathological cardiac hypertrophy may involve the regulation of Beclin1-mediated autophagy through the mTOR/TFEB signaling pathway.

Raptor mediates the selective inhibitory effect of cardamonin on RRAGC-mutant B cell lymphoma

BACKGROUND

mTORC1 (mechanistic target of rapamycin complex 1) is associated with lymphoma progression. Oncogenic RRAGC (Rag guanosine triphosphatase C) mutations identified in patients with follicular lymphoma facilitate the interaction between Raptor (regulatory protein associated with mTOR) and Rag GTPase. It promotes the activation of mTORC1 and accelerates lymphomagenesis. Cardamonin inhibits mTORC1 by decreasing the protein level of Raptor. In the present study, we investigated the inhibitory effect and possible mechanism of action of cardamonin in RRAGC-mutant lymphoma. This could provide a precise targeted therapy for lymphoma with RRAGC mutations.

METHODS

Cell viability was measured using a cell counting kit-8 (CCK-8) assay. Protein expression and phosphorylation levels were determined using western blotting. The interactions of mTOR and Raptor with RagC were determined by co-immunoprecipitation. Cells overexpressing RagC wild-type (RagCWT) and RagC Thr90Asn (RagCT90N) were generated by lentiviral infection. Raptor knockdown was performed by lentivirus-mediated shRNA transduction. The in vivo anti-tumour effect of cardamonin was assessed in a xenograft model.

RESULTS

Cardamonin disrupted mTOR complex interactions by decreasing Raptor protein levels. RagCT90N overexpression via lentiviral infection increased cell proliferation and mTORC1 activation. The viability and tumour growth rate of RagCT90N-mutant cells were more sensitive to cardamonin treatment than those of normal and RagCWT cells. Cardamonin also exhibited a stronger inhibitory effect on the phosphorylation of mTOR and p70 S6 kinase 1 in RagCT90N-mutant cells. Raptor knockdown abolishes the inhibitory effects of cardamonin on mTOR. An in vivo xenograft model demonstrated that the RagCT90N-mutant showed significantly higher sensitivity to cardamonin treatment.

CONCLUSIONS

Cardamonin exerts selective therapeutic effects on RagCT90N-mutant cells. Cardamonin can serve as a drug for individualised therapy for follicular lymphoma with RRAGC mutations.

De novo missense variants in RRAGC lead to a fatal mTORopathy of early childhood

PURPOSE

Mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) regulates cell growth in response to nutritional status. Central to the mTORC1 function is the Rag-GTPase heterodimer. One component of the Rag heterodimer is RagC (Ras-related GTP-binding protein C), which is encoded by the RRAGC gene.

METHODS

Genetic testing via trio exome sequencing was applied to identify the underlying disease cause in 3 infants with dilated cardiomyopathy, hepatopathy, and brain abnormalities, including pachygyria, polymicrogyria, and septo-optic dysplasia. Studies in patient-derived skin fibroblasts and in a HEK293 cell model were performed to investigate the cellular consequences.

RESULTS

We identified 3 de novo missense variants in RRAGC (NM_022157.4: c.269C>A, p.(Thr90Asn), c.353C>T, p.(Pro118Leu), and c.343T>C, p.(Trp115Arg)), which were previously reported as occurring somatically in follicular lymphoma. Studies of patient-derived fibroblasts carrying the p.(Thr90Asn) variant revealed increased cell size, as well as dysregulation of mTOR-related p70S6K (ribosomal protein S6 kinase 1) and transcription factor EB signaling. Moreover, subcellular localization of mTOR was decoupled from metabolic state. We confirmed the key findings for all RRAGC variants described in this study in a HEK293 cell model.

CONCLUSION

The above results are in line with a constitutive overactivation of the mTORC1 pathway. Our study establishes de novo missense variants in RRAGC as cause of an early-onset mTORopathy with unfavorable prognosis.

METTL14 promotes doxorubicin-induced cardiomyocyte ferroptosis by regulating the KCNQ1OT1-miR-7-5p-TFRC axis

Doxorubicin (DOX) has toxic effects on the heart, causing cardiomyopathy and heart injury, but the underlying mechanisms of these effects require further investigation. This study investigated the role of DOX in promoting ferroptosis to induce myocardial injury. AC16 cardiomyocyte and neonatal rat ventricle cardiomyocytes were used as an in vitro model to study the molecules involved in myocardial injury using gene silencing, ectopic expression, and RNA immunoprecipitation. Messenger RNA and protein level analyses showed that DOX treatment resulted in the upregulation of methyltransferase-like 14 (METTL14), which catalyzes the m6A modification of the long non-coding RNA KCNQ1OT1, a miR-7-5p sponge. The RNA-binding protein IGF2BP1 is associated with KCNQ1OT1 to increase its stability and robustly inhibit miR-7-5p activity. Furthermore, a lack of miR-7-5p expression led to increased levels of transferrin receptor, promoting the uptake of iron and production of lipid reactive oxygen species and demonstrating that DOX-induced ferroptosis occurs in AC16 cells. Additionally, we found that miR-7-5p targets METTL14 in AC16 cells. Meanwhile, the role of METTL14/KCNQ1OT1/miR-7-5p axis in regulating ferroptosis in neonatal rat ventricle cardiomyocytes was also confirmed. Our results indicate that selectively inhibiting ferroptosis mediated by a METTL14/KCNQ1OT1/miR-7-5p positive feedback loop in cardiomyocytes could provide a new therapeutic approach to control DOX-induced cardiac injury.

RagD auto-activating mutations impair MiT/TFE activity in kidney tubulopathy and cardiomyopathy syndrome

Heterozygous mutations in the gene encoding RagD GTPase were shown to cause a novel autosomal dominant condition characterized by kidney tubulopathy and cardiomyopathy. We previously demonstrated that RagD, and its paralogue RagC, mediate a non-canonical mTORC1 signaling pathway that inhibits the activity of TFEB and TFE3, transcription factors of the MiT/TFE family and master regulators of lysosomal biogenesis and autophagy. Here we show that RagD mutations causing kidney tubulopathy and cardiomyopathy are "auto- activating", even in the absence of Folliculin, the GAP responsible for RagC/D activation, and cause constitutive phosphorylation of TFEB and TFE3 by mTORC1, without affecting the phosphorylation of "canonical" mTORC1 substrates, such as S6K. By using HeLa and HK-2 cell lines, human induced pluripotent stem cell-derived cardiomyocytes and patient-derived primary fibroblasts, we show that RRAGD auto-activating mutations lead to inhibition of TFEB and TFE3 nuclear translocation and transcriptional activity, which impairs the response to lysosomal and mitochondrial injury. These data suggest that inhibition of MiT/TFE factors plays a key role in kidney tubulopathy and cardiomyopathy syndrome.

Regulation of mTORC1 by the Rag GTPases

The Rag GTPases are an evolutionarily conserved family that play a crucial role in amino acid sensing by the mammalian target of rapamycin complex 1 (mTORC1). mTORC1 is often referred to as the master regulator of cell growth. mTORC1 hyperactivation is observed in multiple diseases such as cancer, obesity, metabolic disorders, and neurodegeneration. The Rag GTPases sense amino acid levels and form heterodimers, where RagA or RagB binds to RagC or RagD, to recruit mTORC1 to the lysosome where it becomes activated. Here, we review amino acid signaling to mTORC1 through the Rag GTPases.

Transcription factor EB as a key molecular factor in human health and its implication in diseases

Transcription factor EB, as a component of the microphthalmia family of transcription factors, has been demonstrated to be a key controller of autophagy-lysosomal biogenesis. Transcription factor EB is activated by stressors such as nutrition and deprivation of growth factors, hypoxia, lysosomal stress, and mitochondrial injury. To achieve the ultimate functional state, it is controlled in a variety of modes, such as in its rate of transcription, post-transcriptional control, and post-translational alterations. Due to its versatile role in numerous signaling pathways, including the Wnt, calcium, AKT, and mammalian target of rapamycin complex 1 signaling pathways, transcription factor EB-originally identified to be an oncogene-is now well acknowledged as a regulator of a wide range of physiological systems, including autophagy-lysosomal biogenesis, response to stress, metabolism, and energy homeostasis. The well-known and recently identified roles of transcription factor EB suggest that this protein might play a central role in signaling networks in a number of non-communicable illnesses, such as cancer, cardiovascular disorders, drug resistance mechanisms, immunological disease, and tissue growth. The important developments in transcription factor EB research since its first description are described in this review. This review helps to advance transcription factor EB from fundamental research into therapeutic and regenerative applications by shedding light on how important a role it plays in human health and disease at the molecular level.

The Role and Regulatory Mechanism of Transcription Factor EB in Health and Diseases

Transcription factor EB (TFEB) is a member of the microphthalmia-associated transcription factor/transcription factor E (MiTF/TFE) family and critically involved in the maintenance of structural integrity and functional balance of multiple cells. In this review, we described the effects of post-transcriptional modifications, including phosphorylation, acetylation, SUMOylation, and ubiquitination, on the subcellular localization and activation of TFEB. The activated TFEB enters into the nucleus and induces the expressions of targeted genes. We then presented the role of TFEB in the biosynthesis of multiple organelles, completion of lysosome-autophagy pathway, metabolism regulation, immune, and inflammatory responses. This review compiles existing knowledge in the understanding of TFEB regulation and function, covering its essential role in response to cellular stress. We further elaborated the involvement of TFEB dysregulation in the pathophysiological process of various diseases, such as the catabolic hyperactivity in tumors, the accumulation of abnormal aggregates in neurodegenerative diseases, and the aberrant host responses in inflammatory diseases. In this review, multiple drugs have also been introduced, which enable regulating the translocation and activation of TFEB, showing beneficial effects in mitigating various disease models. Therefore, TFEB might serve as a potential therapeutic target for human diseases. The limitation of this review is that the mechanism of TFEB-related human diseases mainly focuses on its association with lysosome and autophagy, which needs deep description of other mechanism in diseases progression after getting more advanced information.