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

SFPQ-TFE3 gene fusion reciprocally regulates mTORC1 activity and induces lineage plasticity in a novel mouse model of renal tumorigenesis

The MiT/TFE family gene fusion proteins, such as SFPQ-TFE3 , drive both epithelial (eg, translocation renal cell carcinoma, tRCC) and mesenchymal (eg, perivascular epithelioid cell tumor, PEComa) neoplasms with aggressive behavior. However, no prior mouse models for SFPQ-TFE3 -related tumors exist and the mechanisms of lineage plasticity induced by this fusion remain unclear. Here, we demonstrate that constitutive murine renal expression of human SFPQ-TFE3 using Ksp Cadherin-Cre as a driver disrupts kidney development leading to early neonatal renal failure and death. In contrast, post-natal induction of SFPQ-TFE3 in renal tubular epithelial cells using Pax8 ERT-Cre induces infiltrative epithelioid tumors, which morphologically and transcriptionally resemble human PEComas. As seen in MiT/TFE fusion-driven human tumors, SFPQ-TFE3 expression is accompanied by the strong induction of mTORC1 signaling, which is partially amino acid-sensitive and dependent on increased SFPQ-TFE3 -mediated RRAGC/D transcription. Remarkably, SFPQ-TFE3 expression is sufficient to induce lineage plasticity in renal tubular epithelial cells, with rapid down-regulation of the critical PAX2/PAX8 nephric lineage factors and tubular epithelial markers, and concomitant up-regulation of PEComa differentiation markers in transgenic mice, human cell line models and human tRCC. Pharmacologic or genetic inhibition of mTOR signaling downregulates expression of the SFPQ-TFE3 fusion protein and rescues nephric lineage marker expression and transcriptional activity in vitro. These data provide evidence of a potential epithelial cell-of-origin for TFE3 -driven PEComas and highlight a reciprocal role for SFPQ-TFE3 and mTOR in driving lineage plasticity in the kidney, expanding our understanding of the pathogenesis of MiT/TFE-driven tumors.

RRAGD variants cause cardiac dysfunction in a zebrafish model

The Ras-related GTP-binding protein D (RRAGD) gene plays a crucial role in cellular processes. Recently, RRAGD variants found in patients have been implicated in a novel disorder with kidney tubulopathy and dilated cardiomyopathy. Currently, the consequences of RRAGD variants at the organismal level are unknown. Therefore, this study investigated the impact of RRAGD variants on cardiac function using a zebrafish embryo model. Furthermore, the potential usage of rapamycin, an mTOR inhibitor, as a therapy was assessed in this model. Zebrafish embryos were injected with RRAGD p.S76L and p.P119R cRNA and the resulting heart phenotypes were studied. Our findings reveal that overexpression of RRAGD mutants resulted in decreased ventricular fractional shortening, ejection fraction, and pericardial swelling. In RRAGD S76L-injected embryos, lower survival and heartbeat were observed, whereas survival was unaffected in RRAGD P119R embryos. These observations were reversible following therapy with the mTOR inhibitor rapamycin. Moreover, no effects on electrolyte homeostasis were observed. Together, these findings indicate a crucial role of RRAGD in cardiac function. In the future, the molecular mechanisms by which RRAGD variants result in cardiac dysfunction and if the effects of rapamycin are specific for RRAGD-dependent cardiomyopathy should be studied in clinical studies.NEW & NOTEWORTHY The resultant heart-associated phenotypes in the zebrafish embryos of this study serve as a valuable experimental model for this rare cardiomyopathy. Moreover, the potential therapeutic property of rapamycin in cardiac dysfunctions was highlighted, making this study a pivotal step toward prospective clinical applications.

Ragopathies and the rising influence of RagGTPases on human diseases

RagGTPases (Rags) play an essential role in the regulation of cell metabolism by controlling the activities of both mechanistic target of rapamycin complex 1 (mTORC1) and Transcription factor EB (TFEB). Several diseases, herein named ragopathies, are associated to Rags dysfunction. These diseases may be caused by mutations either in genes encoding the Rags, or in their upstream regulators. The resulting phenotypes may encompass a variety of clinical features such as cataract, kidney tubulopathy, dilated cardiomyopathy and several types of cancer. In this review, we focus on the key clinical, molecular and physio-pathological features of ragopathies, aiming to shed light on their underlying mechanisms.

Heme oxygenase activates calcium release from the endoplasmic reticulum of bovine mammary epithelial cells to promote TFEB entry into the nucleus to reduce the intracellular load of Mycoplasma bovis

Heme oxygenase HO-1 (HMOX) regulates cellular inflammation and apoptosis, but its role in regulation of autophagy in Mycoplasma bovis infection is unknown. The objective was to determine how the HO-1/CO- Protein kinase RNA-like endoplasmic reticulum kinase (PERK)-Ca2+- transcription factor EB (TFEB) signaling axis induces autophagy and regulates clearance of M. bovis by bovine mammary epithelial cells (bMECs). M. bovis inhibited autophagy and lysosomal biogenesis in bMECs and suppressed HO-1 protein and expression of related proteins, namely nuclear factor erythroid 2-related factor 2 (Nrf2) and Kelch-like ECH-associated protein 1 (keap1). Activation of HO-1 and its production of carbon monoxide (CO) were required for induction of autophagy and clearance of intracellular M. bovis. Furthermore, when HO-1 was deficient, CO sustained cellular autophagy. HO-1 activation increased intracellular calcium (Ca2+) and cytosolic localization activity of TFEB via PERK. Knockdown of PERK or chelation of intracellular Ca2+ inhibited HO-1-induced M. bovis autophagy and clearance. M. bovis infection affected nuclear localization of lysosomal TFEB in the MiT/TFE transcription factor subfamily, whereas activation of HO-1 mediated dephosphorylation and intranuclear localization of TFEB, promoting autophagy, lysosomal biogenesis and autophagic clearance of M. bovis. Nuclear translocation of TFEB in HO-1 was critical to induce M. bovis transport and survival of infected bMECs. Furthermore, the HO-1/CO-PERK-Ca2+-TFEB signaling axis induced autophagy and M. bovis clearance, providing a viable approach to treat persistent M. bovis infections.

microRNA-125b-1-3p mediates autophagy via the RRAGD/mTOR/ULK1 signaling pathway and mitigates atherosclerosis progression

Atherosclerosis is characterised by lipid accumulation and formation of foam cells in arterial walls. Dysregulated autophagy is a crucial factor in atherosclerosis development. The significance of microRNA (miR)-125b-1-3p in cardiovascular disease is well-established; however, its precise role in regulating autophagy and impact on atherosclerosis in vascular smooth muscle cells (VSMCs) remain unclear. Here, we observed reduced autophagic activity and decreased miR-125b expression during atherosclerosis progression. miR-125b-1-3p overexpression significantly reduced atherosclerotic plaque development in mice; it also led to decreased lipid uptake and deposition in VSMCs, enhanced autophagy, and suppression of smooth muscle cell phenotypic changes in-vitro. An interaction between miR-125b-1-3p and the RRAGD/mTOR/ULK1 pathway was revealed, elucidating its role in promoting autophagy. Therefore, miR-125b-1-3p plays a pivotal role in enhancing autophagic processes, inhibiting foam cell formation in VSMCs and mitigating atherosclerosis progression, partly through RRAGD/mTOR/ULK1 signaling axis modulation. Thus, miR-125b-1-3p is a promising target for preventive and therapeutic strategies for atherosclerosis.

Tissue-specific expression differences in Ras-related GTP-binding proteins in male rats

The protein kinase Mechanistic Target of Rapamycin (mTOR) in Complex 1 (mTORC1) is regulated in part by the Ras-related GTP-binding proteins (Rag GTPases). Rag GTPases form a heterodimeric complex consisting of either RagA or RagB associated with either RagC or RagD and act to localize mTORC1 to the lysosomal membrane. Until recently, RagA and RagB were thought to be functionally redundant, as were RagC and RagD. However, recent research suggests that the various isoforms differentially activate mTORC1. Here, the mRNA expression and protein abundance of the Rag GTPases was compared across male rat skeletal muscle, heart, liver, kidney, and brain. Whereas mRNA expression of RagA was higher than RagB in nearly all tissues studied, RagB protein abundance was higher than RagA in all tissues besides skeletal muscle. RagC mRNA expression was more abundant or equal to RagD mRNA, and RagD protein was more abundant than RagC protein in all tissues. Moreover, the proportion of RagB in the short isoform was greater than the long in liver, whereas the opposite was true in brain. These results serve to further elucidate Rag GTPase expression and offer potential explanations for the differential responses to amino acids that are observed in different tissues.

The post-translational regulation of transcription factor EB (TFEB) in health and disease

Transcription factor EB (TFEB) is a basic helix-loop-helix leucine zipper transcription factor that acts as a master regulator of lysosomal biogenesis, lysosomal exocytosis, and macro-autophagy. TFEB contributes to a wide range of physiological functions, including mitochondrial biogenesis and innate and adaptive immunity. As such, TFEB is an essential component of cellular adaptation to stressors, ranging from nutrient deprivation to pathogenic invasion. The activity of TFEB depends on its subcellular localisation, turnover, and DNA-binding capacity, all of which are regulated at the post-translational level. Pathological states are characterised by a specific set of stressors, which elicit post-translational modifications that promote gain or loss of TFEB function in the affected tissue. In turn, the resulting increase or decrease in survival of the tissue in which TFEB is more or less active, respectively, may either benefit or harm the organism as a whole. In this way, the post-translational modifications of TFEB account for its otherwise paradoxical protective and deleterious effects on organismal fitness in diseases ranging from neurodegeneration to cancer. In this review, we describe how the intracellular environment characteristic of different diseases alters the post-translational modification profile of TFEB, enabling cellular adaptation to a particular pathological state.