Saraf-dependent activation of mTORC1 regulates cardiac growth

mTORC1 and SGLT2 Inhibitors-A Therapeutic Perspective for Diabetic Cardiomyopathy

Diabetic cardiomyopathy is a critical diabetes-mediated co-morbidity characterized by cardiac dysfunction and heart failure, without predisposing hypertensive or atherosclerotic conditions. Metabolic insulin resistance, promoting hyperglycemia and hyperlipidemia, is the primary cause of diabetes-related disorders, but ambiguous tissue-specific insulin sensitivity has shed light on the importance of identifying a unified target paradigm for both the glycemic and non-glycemic context of type 2 diabetes (T2D). Several studies have indicated hyperactivation of the mammalian target of rapamycin (mTOR), specifically complex 1 (mTORC1), as a critical mediator of T2D pathophysiology by promoting insulin resistance, hyperlipidemia, inflammation, vasoconstriction, and stress. Moreover, mTORC1 inhibitors like rapamycin and their analogs have shown significant benefits in diabetes and related cardiac dysfunction. Recently, FDA-approved anti-hyperglycemic sodium-glucose co-transporter 2 inhibitors (SGLT2is) have gained therapeutic popularity for T2D and diabetic cardiomyopathy, even acknowledging the absence of SGLT2 channels in the heart. Recent studies have proposed SGLT2-independent drug mechanisms to ascertain their cardioprotective benefits by regulating sodium homeostasis and mimicking energy deprivation. In this review, we systematically discuss the role of mTORC1 as a unified, eminent target to treat T2D-mediated cardiac dysfunction and scrutinize whether SGLT2is can target mTORC1 signaling to benefit patients with diabetic cardiomyopathy. Further studies are warranted to establish the underlying cardioprotective mechanisms of SGLT2is under diabetic conditions, with selective inhibition of cardiac mTORC1 but the concomitant activation of mTORC2 (mTOR complex 2) signaling.

New Insights into the Regulation of mTOR Signaling via Ca(2+)-Binding Proteins

Environmental factors are important regulators of cell growth and proliferation. Mechanistic target of rapamycin (mTOR) is a central kinase that maintains cellular homeostasis in response to a variety of extracellular and intracellular inputs. Dysregulation of mTOR signaling is associated with many diseases, including diabetes and cancer. Calcium ion (Ca²⁺) is important as a second messenger in various biological processes, and its intracellular concentration is tightly regulated. Although the involvement of Ca²⁺ mobilization in mTOR signaling has been reported, the detailed molecular mechanisms by which mTOR signaling is regulated are not fully understood. The link between Ca²⁺ homeostasis and mTOR activation in pathological hypertrophy has heightened the importance in understanding Ca²⁺-regulated mTOR signaling as a key mechanism of mTOR regulation. In this review, we introduce recent findings on the molecular mechanisms of regulation of mTOR signaling by Ca²⁺-binding proteins, particularly calmodulin (CaM).

The roles of transmembrane family proteins in the regulation of store-operated Ca2+ entry

Store-operated Ca2+ entry (SOCE) is a major pathway for calcium signaling, which regulates almost every biological process, involving cell proliferation, differentiation, movement and death. Stromal interaction molecule (STIM) and ORAI calcium release-activated calcium modulator (ORAI) are the two major proteins involved in SOCE. With the deepening of studies, more and more proteins are found to be able to regulate SOCE, among which the transmembrane (TMEM) family proteins are worth paying more attention. In addition, the ORAI proteins belong to the TMEM family themselves. As the name suggests, TMEM family is a type of proteins that spans biological membranes including plasma membrane and membrane of organelles. TMEM proteins are in a large family with more than 300 proteins that have been already identified, while the functional knowledge about the proteins is preliminary. In this review, we mainly summarized the TMEM proteins that are involved in SOCE, to better describe a picture of the interaction between STIM and ORAI proteins during SOCE and its downstream signaling pathways, as well as to provide an idea for the study of the TMEM family proteins.

Bidirectional regulation of calcium release-activated calcium (CRAC) channel by SARAF

Store-operated calcium entry (SOCE) through the Ca²⁺ release–activated Ca²⁺ (CRAC) channel is a central mechanism by which cells generate Ca²⁺ signals and mediate Ca²⁺-dependent gene expression. The molecular basis for CRAC channel regulation by the SOCE-associated regulatory factor (SARAF) remained insufficiently understood. Here we found that following ER Ca²⁺ depletion, SARAF facilitates a conformational change in the ER Ca²⁺ sensor STIM1 that relieves an activation constraint enforced by the STIM1 inactivation domain (ID; aa 475–483) and promotes initial activation of STIM1, its translocation to ER–plasma membrane junctions, and coupling to Orai1 channels. Following intracellular Ca²⁺ rise, cooperation between SARAF and the STIM1 ID controls CRAC channel slow Ca²⁺-dependent inactivation. We further show that in T lymphocytes, SARAF is required for proper T cell receptor evoked transcription. Taking all these data together, we uncover a dual regulatory role for SARAF during both activation and inactivation of CRAC channels and show that SARAF fine-tunes intracellular Ca²⁺ responses and downstream gene expression in cells.

Muscle-specific Cand2 is translationally upregulated by mTORC1 and promotes adverse cardiac remodeling

The mechanistic target of rapamycin (mTOR) promotes pathological remodeling in the heart by activating ribosomal biogenesis and mRNA translation. Inhibition of mTOR in cardiomyocytes is protective; however, a detailed role of mTOR in translational regulation of specific mRNA networks in the diseased heart is unknown. We performed cardiomyocyte genome-wide sequencing to define mTOR-dependent gene expression control at the level of mRNA translation. We identify the muscle-specific protein Cullin-associated NEDD8-dissociated protein 2 (Cand2) as a translationally upregulated gene, dependent on the activity of mTOR. Deletion of Cand2 protects the myocardium against pathological remodeling. Mechanistically, we show that Cand2 links mTOR signaling to pathological cell growth by increasing Grk5 protein expression. Our data suggest that cell-type-specific targeting of mTOR might have therapeutic value against pathological cardiac remodeling.

Regulation of Store-Operated Ca2+ Entry by SARAF

Calcium (Ca²⁺) signaling plays a dichotomous role in cellular biology, controlling cell survival and proliferation on the one hand and cellular toxicity and cell death on the other. Store-operated Ca²⁺ entry (SOCE) by CRAC channels represents a major pathway for Ca²⁺ entry in non-excitable cells. The CRAC channel has two key components, the endoplasmic reticulum Ca²⁺ sensor stromal interaction molecule (STIM) and the plasma-membrane Ca²⁺ channel Orai. Physical coupling between STIM and Orai opens the CRAC channel and the resulting Ca²⁺ flux is regulated by a negative feedback mechanism of slow Ca²⁺ dependent inactivation (SCDI). The identification of the SOCE-associated regulatory factor (SARAF) and investigations of its role in SCDI have led to new functional and molecular insights into how SOCE is controlled. In this review, we provide an overview of the functional and molecular mechanisms underlying SCDI and discuss how the interaction between SARAF, STIM1, and Orai1 shapes Ca²⁺ signaling in cells.

Store-operated Calcium Entry-associated Regulatory Factor Regulates Airway Inflammation and Airway Remodeling in Asthma Mice Models

BACKGROUND

Store-operated calcium entry (SOCE) is involved in the pathogenesis of airway inflammation and remodeling in asthma. Store-operated calcium entry-associated regulatory factor (SARAF) can down-regulate SOCE.

OBJECTIVE

We sought to investigate the role of SARAF in the regulation of airway inflammation and remodeling in asthma mice models, as well as in the functional regulation of human airway smooth muscle cells (hASMCs).

METHODS

Balb/c mice were sensitized and challenged with ovalbumin to establish the asthma mice models. Mice were transfected with lentivirus, which expressed the SARAF gene + GFP or the negative control gene + GFP. Airway resistance was measured with the animal pulmonary function system. Airway inflammation and remodeling were evaluated via histological staining. In vitro cultured hASMCs were transfected with scrambled small interfering RNA(siRNA) or SARAF-specific siRNA respecitvely. The proliferation, migration rate, hypertrophy and SOCE activity of hASMCs were examined with cell counting kit 8, wound healing test, bright field imaging and Ca²⁺ fluorescence imaging, respectively. SARAF expression was measured by quantitative real-time-PCR.

RESULTS

Asthma mice models showed decreased SARAF mRNA expression in the lungs. SARAF overexpression attenuated airway inflammation, resistance and also remodeling. Downregulation of SARAF expression with siRNA promoted the proliferation, migration, hypertrophy and SOCE activity in hASMCs.

CONCLUSIONS

SARAF plays a protective role against airway inflammation and remodeling in asthma mice models by blunting SOCE; SARAF may also be a functional regulating factor of hASMCs.

Amino Acid-Mediated Intracellular Ca2+ Rise Modulates mTORC1 by Regulating the TSC2-Rheb Axis through Ca2+/Calmodulin

Mechanistic target of rapamycin complex 1 (mTORC1) is a master growth regulator by controlling protein synthesis and autophagy in response to environmental cues. Amino acids, especially leucine and arginine, are known to be important activators of mTORC1 and to promote lysosomal translocation of mTORC1, where mTORC1 is thought to make contact with its activator Rheb GTPase. Although amino acids are believed to exclusively regulate lysosomal translocation of mTORC1 by Rag GTPases, how amino acids increase mTORC1 activity besides regulation of mTORC1 subcellular localization remains largely unclear. Here we report that amino acids also converge on regulation of the TSC2-Rheb GTPase axis via Ca²⁺/calmodulin (CaM). We showed that the amino acid-mediated increase of intracellular Ca²⁺ is important for mTORC1 activation and thereby contributes to the promotion of nascent protein synthesis. We found that Ca²⁺/CaM interacted with TSC2 at its GTPase activating protein (GAP) domain and that a CaM inhibitor reduced binding of CaM with TSC2. The inhibitory effect of a CaM inhibitor on mTORC1 activity was prevented by loss of TSC2 or by an active mutant of Rheb GTPase, suggesting that a CaM inhibitor acts through the TSC2-Rheb axis to inhibit mTORC1 activity. Taken together, in response to amino acids, Ca²⁺/CaM-mediated regulation of the TSC2-Rheb axis contributes to proper mTORC1 activation, in addition to the well-known lysosomal translocation of mTORC1 by Rag GTPases.