The Calcineurin Variant CnAβ1 Controls Mouse Embryonic Stem Cell Differentiation by Directing mTORC2 Membrane Localization and Activation

A cellular atlas of calcineurin signaling

Intracellular Ca2+ signals are temporally controlled and spatially restricted. Signaling occurs adjacent to sites of Ca2+ entry and/or release, where Ca2+-dependent effectors and their substrates co-localize to form signaling microdomains. Here we review signaling by calcineurin, the Ca2+/calmodulin regulated protein phosphatase and target of immunosuppressant drugs, Cyclosporin A and FK506. Although well known for its activation of the adaptive immune response via NFAT dephosphorylation, systematic mapping of human calcineurin substrates and regulators reveals unexpected roles for this versatile phosphatase throughout the cell. We discuss calcineurin function, with an emphasis on where signaling occurs and mechanisms that target calcineurin and its substrates to signaling microdomains, especially binding of cognate short linear peptide motifs (SLiMs). Calcineurin is ubiquitously expressed and regulates events at the plasma membrane, other intracellular membranes, mitochondria, the nuclear pore complex and centrosomes/cilia. Based on our expanding knowledge of localized CN actions, we describe a cellular atlas of Ca2+/calcineurin signaling.

Polypyrimidine tract binding protein 1 regulates the activation of mouse CD8 T cells

The RNA-binding protein polypyrimidine tract binding protein 1 (PTBP1) has been found to have roles in CD4 T-cell activation, but its function in CD8 T cells remains untested. We show it is dispensable for the development of naïve mouse CD8 T cells, but is necessary for the optimal expansion and production of effector molecules by antigen-specific CD8 T cells in vivo. PTBP1 has an essential role in regulating the early events following activation of the naïve CD8 T cell leading to IL-2 and TNF production. It is also required to protect activated CD8 T cells from apoptosis. PTBP1 controls alternative splicing of over 400 genes in naïve CD8 T cells in addition to regulating the abundance of ∼200 mRNAs. PTBP1 is required for the nuclear accumulation of c-Fos, NFATc2, and NFATc3, but not NFATc1. This selective effect on NFAT proteins correlates with PTBP1-promoted expression of the shorter Aβ1 isoform and exon 13 skipped Aβ2 isoform of the catalytic A-subunit of calcineurin phosphatase. These findings reveal a crucial role for PTBP1 in regulating CD8 T-cell activation.

ROCK ‘n TOR: An Outlook on Keratinocyte Stem Cell Expansion in Regenerative Medicine via Protein Kinase Inhibition

Keratinocyte stem cells play a fundamental role in homeostasis and repair of stratified epithelial tissues. Transplantation of cultured keratinocytes autografts provides a landmark example of successful cellular therapies by restoring durable integrity in stratified epithelia lost to devastating tissue conditions. Despite the overall success of such procedures, failures still occur in case of paucity of cultured stem cells in therapeutic grafts. Strategies aiming at a further amplification of stem cells during keratinocyte ex vivo expansion may thus extend the applicability of these treatments to subjects in which endogenous stem cells pools are depauperated by aging, trauma, or disease. Pharmacological targeting of stem cell signaling pathways is recently emerging as a powerful strategy for improving stem cell maintenance and/or amplification. Recent experimental data indicate that pharmacological inhibition of two prominent keratinocyte signaling pathways governed by apical mTOR and ROCK protein kinases favor stem cell maintenance and/or amplification ex vivo and may improve the effectiveness of stem cell-based therapeutic procedures. In this review, we highlight the pathophysiological roles of mTOR and ROCK in keratinocyte biology and evaluate existing pre-clinical data on the effects of their inhibition in epithelial stem cell expansion for transplantation purposes.

Downregulating MFN2 promotes the differentiation of induced pluripotent stem cells into mesenchymal stem cells via the PI3K/Akt/GSK-3β/Wnt signaling pathway

Understanding the mechanism of the differentiation of induced pluripotent stem cells (iPSCs) into mesenchymal stem cells (MSCs) and promoting the production efficiency of iPSC-derived MSCs (iPSC-MSCs) are critical to periodontal tissue engineering. However, the gene networks that control this differentiation process from iPSCs into MSCs are poorly understood. We demonstrated that MFN2 knockdown showed a positive effect on the triploblastic and MSC differentiation from iPSCs. Activation of the PI3K/Akt signaling pathway by MFN2 knockdown activated the Wnt/β-catenin signaling pathway by inhibiting GSK-3β and reducing β-catenin degradation. Inhibitor of the PI3K/Akt signaling pathway normalized the enhanced efficiency of differentiation into MSCs of MFN2-KD iPSCs and Wnt activator treated control iPSCs. MFN2-OE iPSCs displayed an opposite phenotype. In conclusion, downregulating MFN2 promotes the differentiation of iPSCs into MSCs by activating the PI3K/Akt/GSK-3β/Wnt signaling pathway. Our results reveal a crucial function and mechanism for MFN2 in regulating MSC differentiation from iPSCs, which will provide new ideas for periodontal tissue engineering and periodontal regenerative treatment by using iPSC-MSCs.

Calcineurin in the heart: New horizons for an old friend

Calcineurin, also known as PP2B or PPP3, is a member of the PPP family of protein phosphatases that also includes PP1 and PP2A. Together these three phosphatases carryout the majority of dephosphorylation events in the heart. Calcineurin is distinct in that it is activated by the binding of calcium/calmodulin (Ca2+/CaM) and therefore acts as a node for integrating Ca2+ signals with changes in phosphorylation, two fundamental intracellular signaling cascades. In the heart, calcineurin is primarily thought of in the context of pathological cardiac remodeling, acting through the Nuclear Factor of Activated T-cell (NFAT) family of transcription factors. However, calcineurin activity is also essential for normal heart development and homeostasis in the adult heart. Furthermore, it is clear that NFAT-driven changes in transcription are not the only relevant processes initiated by calcineurin in the setting of pathological remodeling. There is a growing appreciation for the diversity of calcineurin substrates that can impact cardiac function as well as the diversity of mechanisms for targeting calcineurin to specific sub-cellular domains in cardiomyocytes and other cardiac cell types. Here, we will review the basics of calcineurin structure, regulation, and function in the context of cardiac biology. Particular attention will be given to: the development of improved tools to identify and validate new calcineurin substrates; recent studies identifying new calcineurin isoforms with unique properties and targeting mechanisms; and the role of calcineurin in cardiac development and regeneration.

Palmitoylation targets the calcineurin phosphatase to the phosphatidylinositol 4-kinase complex at the plasma membrane

Calcineurin, the conserved protein phosphatase and target of immunosuppressants, is a critical mediator of Ca2+ signaling. Here, to discover calcineurin-regulated processes we examined an understudied isoform, CNAβ1. We show that unlike canonical cytosolic calcineurin, CNAβ1 localizes to the plasma membrane and Golgi due to palmitoylation of its divergent C-terminal tail, which is reversed by the ABHD17A depalmitoylase. Palmitoylation targets CNAβ1 to a distinct set of membrane-associated interactors including the phosphatidylinositol 4-kinase (PI4KA) complex containing EFR3B, PI4KA, TTC7B and FAM126A. Hydrogen-deuterium exchange reveals multiple calcineurin-PI4KA complex contacts, including a calcineurin-binding peptide motif in the disordered tail of FAM126A, which we establish as a calcineurin substrate. Calcineurin inhibitors decrease PI4P production during Gq-coupled GPCR signaling, suggesting that calcineurin dephosphorylates and promotes PI4KA complex activity. In sum, this work discovers a calcineurin-regulated signaling pathway which highlights the PI4KA complex as a regulatory target and reveals that dynamic palmitoylation confers unique localization, substrate specificity and regulation to CNAβ1.

Regulation of mRNA translation in stem cells; links to brain disorders

Translational control of gene expression is emerging as a cardinal step in the regulation of protein abundance. Especially for embryonic (ESC) and neuronal stem cells (NSC), regulation of mRNA translation is involved in the maintenance of pluripotency but also differentiation. For neuronal stem cells this regulation is linked to the various neuronal subtypes that arise in the developing brain and is linked to numerous brain disorders. Herein, we review translational control mechanisms in ESCs and NSCs during development and differentiation, and briefly discuss their link to brain disorders.

The SRSF4-GAS5-Glucocorticoid Receptor Axis Regulates Ventricular Hypertrophy

Supplemental Digital Content is available in the text.

RBPs (RNA-binding proteins) play critical roles in human biology and disease. Aberrant RBP expression affects various steps in RNA processing, altering the function of the target RNAs. The RBP SRSF4 (serine/arginine-rich splicing factor 4) has been linked to neuropathies and cancer. However, its role in the heart is completely unknown.

Pseudogene ACTBP2 increases blood-brain barrier permeability by promoting KHDRBS2 transcription through recruitment of KMT2D/WDR5 in Aβ1-42 microenvironment

The blood–brain barrier (BBB) has a vital role in maintaining the homeostasis of the central nervous system (CNS). Changes in the structure and function of BBB can accelerate Alzheimer’s disease (AD) development. β-Amyloid (Aβ) deposition is the major pathological event of AD. We elucidated the function and possible molecular mechanisms of the effect of pseudogene ACTBP2 on the permeability of BBB in Aβ_1–42 microenvironment. BBB model treated with Aβ_1–42 for 48 h were used to simulate Aβ-mediated BBB dysfunction in AD. We proved that pseudogene ACTBP2, RNA-binding protein KHDRBS2, and transcription factor HEY2 are highly expressed in ECs that were obtained in a BBB model in vitro in Aβ_1–42 microenvironment. In Aβ_1–42-incubated ECs, ACTBP2 recruits methyltransferases KMT2D and WDR5, binds to KHDRBS2 promoter, and promotes KHDRBS2 transcription. The interaction of KHDRBS2 with the 3′UTR of HEY2 mRNA increases the stability of HEY2 and promotes its expression. HEY2 increases BBB permeability in Aβ_1–42 microenvironment by transcriptionally inhibiting the expression of ZO-1, occludin, and claudin-5. We confirmed that knocking down of Khdrbs2 or Hey2 increased the expression levels of ZO-1, occludin, and claudin-5 in APP/PS1 mice brain microvessels. ACTBP2/KHDRBS2/HEY2 axis has a crucial role in the regulation of BBB permeability in Aβ_1–42 microenvironment, which may provide a novel target for the therapy of AD.

ERG1 plays an essential role in rat cardiomyocyte fate decision by mediating AKT signaling

ERG1, a potassium ion channel, is essential for cardiac action potential repolarization phase. However, the role of ERG1 for normal development of the heart is poorly understood. Using the rat embryonic stem cells (rESCs) model, we show that ERG1 is crucial in cardiomyocyte lineage commitment via interactions with Integrin β1. In the mesoderm phase of rESCs, the interaction of ERG1 with Integrin β1 can activate the AKT pathway by recruiting and phosphorylating PI3K p85 and focal adhesion kinase (FAK) to further phosphorylate AKT. Activation of AKT pathway promotes cardiomyocyte differentiation through two different mechanisms, (a) through phosphorylation of GSK3β to upregulate the expression levels of β-catenin and Gata4; (b) through promotion of nuclear translocation of nuclear factor-κB by phosphorylating IKKβ to inhibit cell apoptosis, which occurs due to increased Bcl2 expression. Our study provides solid evidence for a novel role of ERG1 on differentiation of rESCs into cardiomyocytes.

Inhibition of lncRNA MAAT Controls Multiple Types of Muscle Atrophy by cis- and trans-Regulatory Actions

Muscle atrophy is associated with negative outcomes in a variety of diseases. Identification of a common therapeutic target would address a significant unmet clinical need. Here, we identify a long non-coding RNA (lncRNA) (muscle-atrophy-associated transcript, lncMAAT) as a common regulator of skeletal muscle atrophy. lncMAAT is downregulated in multiple types of muscle-atrophy models both in vivo (denervation, Angiotensin II [AngII], fasting, immobilization, and aging-induced muscle atrophy) and in vitro (AngII, H₂O₂, and tumor necrosis factor alpha [TNF-α]-induced muscle atrophy). Gain- and loss-of-function analysis both in vitro and in vivo reveals that downregulation of lncMAAT is sufficient to induce muscle atrophy, while overexpression of lncMAAT can ameliorate multiple types of muscle atrophy. Mechanistically, lncMAAT negatively regulates the transcription of miR-29b through SOX6 by a trans-regulatory module and increases the expression of the neighboring gene Mbnl1 by a cis-regulatory module. Therefore, overexpression of lncMAAT may represent a promising therapy for muscle atrophy induced by different stimuli.

MBNL1 reverses the proliferation defect of skeletal muscle satellite cells in myotonic dystrophy type 1 by inhibiting autophagy via the mTOR pathway

Skeletal muscle atrophy is one of the clinical symptoms of myotonic dystrophy type 1 (DM1). A decline in skeletal muscle regeneration is an important contributor to muscle atrophy. Skeletal muscle satellite cells (SSCs) drive skeletal muscle regeneration. Increased autophagy can reduce the proliferative capacity of SSCs, which plays an important role in the early regeneration of damaged skeletal muscle in DM1. Discovering new ways to restore SSC proliferation may aid in the identification of new therapeutic targets for the treatment of skeletal muscle atrophy in DM1. In the pathogenesis of DM1, muscleblind-like 1 (MBNL1) protein is generally considered to form nuclear RNA foci and disturb the RNA-splicing function. However, the role of MBNL1 in SSC proliferation in DM1 has not been reported. In this study, we obtained SSCs differentiated from normal DM1-04-induced pluripotent stem cells (iPSCs), DM1-03 iPSCs, and DM1-13-3 iPSCs edited by transcription activator-like (TAL) effector nucleases (TALENs) targeting CTG repeats, and primary SSCs to study the pathogenesis of DM1. DM1 SSC lines and primary SSCs showed decreased MBNL1 expression and elevated autophagy levels. However, DM1 SSCs edited by TALENs showed increased cytoplasmic distribution of MBNL1, reduced levels of autophagy, increased levels of phosphorylated mammalian target of rapamycin (mTOR), and improved proliferation rates. In addition, we confirmed that after MBNL1 overexpression, the proliferative capability of DM1 SSCs and the level of phosphorylated mTOR were enhanced, while the autophagy levels were decreased. Our data also demonstrated that the proliferative capability of DM1 SSCs was enhanced after autophagy was inhibited by overexpressing mTOR. Finally, treatment with rapamycin (an mTOR inhibitor) was shown to abolish the increased proliferation capability of DM1 SSCs due to MBNL1 overexpression. Taken together, these data suggest that MBNL1 reverses the proliferation defect of SSCs in DM1 by inhibiting autophagy via the mTOR pathway.

Calcineurin Aβ-Specific Anchoring Confers Isoform-Specific Compartmentation and Function in Pathological Cardiac Myocyte Hypertrophy

BACKGROUND

The Ca²⁺/calmodulin-dependent phosphatase calcineurin is a key regulator of cardiac myocyte hypertrophy in disease. An unexplained paradox is how the β isoform of the calcineurin catalytic A-subunit (CaNAβ) is required for induction of pathological myocyte hypertrophy, despite calcineurin Aα expression in the same cells. It is unclear how the pleiotropic second messenger Ca²⁺ drives excitation-contraction coupling while not stimulating hypertrophy by calcineurin in the normal heart. Elucidation of the mechanisms conferring this selectivity in calcineurin signaling should reveal new strategies for targeting the phosphatase in disease.

METHODS

Primary adult rat ventricular myocytes were studied for morphology and intracellular signaling. New Förster resonance energy transfer reporters were used to assay Ca²⁺ and calcineurin activity in living cells. Conditional gene deletion and adeno-associated virus-mediated gene delivery in the mouse were used to study calcineurin signaling after transverse aortic constriction in vivo.

RESULTS

CIP4 (Cdc42-interacting protein 4)/TRIP10 (thyroid hormone receptor interactor 10) was identified as a new polyproline domain-dependent scaffold for CaNAβ2 by yeast 2-hybrid screen. Cardiac myocyte-specific CIP4 gene deletion in mice attenuated pressure overload-induced pathological cardiac remodeling and heart failure. Blockade of CaNAβ polyproline-dependent anchoring using a competing peptide inhibited concentric hypertrophy in cultured myocytes; disruption of anchoring in vivo using an adeno-associated virus gene therapy vector inhibited cardiac hypertrophy and improved systolic function after pressure overload. Live cell Förster resonance energy transfer biosensor imaging of cultured myocytes revealed that Ca²⁺ levels and calcineurin activity associated with the CIP4 compartment were increased by neurohormonal stimulation, but minimally by pacing. Conversely, Ca²⁺ levels and calcineurin activity detected by nonlocalized Förster resonance energy transfer sensors were induced by pacing and minimally by neurohormonal stimulation, providing functional evidence for differential intracellular compartmentation of Ca²⁺ and calcineurin signal transduction.

CONCLUSIONS

These results support a structural model for Ca²⁺ and CaNAβ compartmentation in cells based on an isoform-specific mechanism for calcineurin protein-protein interaction and localization. This mechanism provides an explanation for the specific role of CaNAβ in hypertrophy and its selective activation under conditions of pathologic stress. Disruption of CaNAβ polyproline-dependent anchoring constitutes a rational strategy for therapeutic targeting of CaNAβ-specific signaling responsible for pathological cardiac remodeling in cardiovascular disease deserving of further preclinical investigation.

Identifying New Substrates and Functions for an Old Enzyme: Calcineurin

Biological processes are dynamically regulated by signaling networks composed of protein kinases and phosphatases. Calcineurin, or PP3, is a conserved phosphoserine/phosphothreonine-specific protein phosphatase and member of the PPP family of phosphatases. Calcineurin is unique, however, in its activation by Ca2+ and calmodulin. This ubiquitously expressed phosphatase controls Ca2+-dependent processes in all human tissues, but is best known for driving the adaptive immune response by dephosphorylating the nuclear factor of the activated T-cells (NFAT) family of transcription factors. Therefore, calcineurin inhibitors, FK506 (tacrolimus), and cyclosporin A serve as immunosuppressants. We describe some of the adverse effects associated with calcineurin inhibitors that result from inhibition of calcineurin in nonimmune tissues, illustrating the many functions of this enzyme that have yet to be elucidated. In fact, calcineurin has essential roles beyond the immune system, from yeast to humans, but since its discovery more than 30 years ago, only a small number of direct calcineurin substrates have been shown (∼75 proteins). This is because of limitations in current methods for identification of phosphatase substrates. Here we discuss recent insights into mechanisms of calcineurin activation and substrate recognition that have been critical in the development of novel approaches for identifying its targets systematically. Rather than comprehensively reviewing known functions of calcineurin, we highlight new approaches to substrate identification for this critical regulator that may reveal molecular mechanisms underlying toxicities caused by calcineurin inhibitor-based immunosuppression.

Growth Factor-Dependent and -Independent Activation of mTORC2

The target of rapamycin complex 2 (TORC2) was discovered in 2002 in budding yeast. Its mammalian counterpart, mTORC2, was first described in 2004. Soon thereafter it was demonstrated that mTORC2 directly phosphorylates Akt on Ser473, ending a long search for the elusive 'second' insulin-responsive Akt kinase. In this review we discuss key evidence pertaining to the subcellular localization of mTORC2, highlighting a spatial heterogeneity that relates to mTORC2 activation. We summarize current models for how growth factors (GFs), such as insulin, trigger mTORC2 activation, and we provide a comprehensive discussion focusing on a new exciting frontier, the molecular mechanisms underpinning GF-independent activation of mTORC2.

Low doses of BPA induced abnormal mitochondrial fission and hypertrophy in human embryonic stem cell-derived cardiomyocytes via the calcineurin-DRP1 signaling pathway: A comparison between XX and XY cardiomyocytes

Humans are inevitably exposed to bisphenol A (BPA) via multiple exposure ways. Thus, attention should be raised to the possible adverse effects related to low doses of BPA. Epidemiological studies have outlined BPA exposure and the increased risk of cardiovascular diseases (such as cardiac hypertrophy), which has been confirmed to be sex-specific in rodent animals and present in few in vitro studies, although the molecular mechanism is still unclear. However, whether BPA at low doses equivalent to human internal exposure level could induce cardiac hypertrophy via the calcineurin-DRP1 signaling pathway by disrupting calcium homeostasis is unknown. To address this, human embryonic stem cell (H1, XY karyotype and H9, XX karyotype)-derived cardiomyocytes (CM) were purified and applied to study the low-dose effects of BPA on cardiomyocyte hypertrophy. In our study, when H1- and H9-CM were exposed to noncytotoxic BPA (8 ng/ml), markedly elevated hypertrophic-related mRNA expression levels (such as NPPA and NPPB), enhanced cellular area and reduced ATP supplementation, demonstrated the hypertrophic cardiomyocyte phenotype in vitro. The excessive fission produced by BPA was promoted by CnAβ-mediated dephosphorylation of DRP1. At the molecular level, the increase in cytosolic Ca²⁺ levels by low doses of BPA could discriminate between H1- and H9-CM, which may suggest a potential sex-specific hypertrophic risk in cardiomyocytes in terms of abnormal mitochondrial fission and ATP production by impairing CnAβ-DRP1 signaling. In CnAβ-knockdown cardiomyocytes, these changes were highly presented in XX-karyotyped cells, rather than in XY-karyotyped cells.

Severe Cardiac Dysfunction and Death Caused by Arrhythmogenic Right Ventricular Cardiomyopathy Type 5 Are Improved by Inhibition of Glycogen Synthase Kinase-3β

BACKGROUND

Arrhythmogenic cardiomyopathy/arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited cardiac disease characterized by fibrofatty replacement of the myocardium, resulting in heart failure and sudden cardiac death. The most aggressive arrhythmogenic cardiomyopathy/ARVC subtype is ARVC type 5 (ARVC5), caused by a p.S358L mutation in TMEM43 (transmembrane protein 43). The function and localization of TMEM43 are unknown, as is the mechanism by which the p.S358L mutation causes the disease. Here, we report the characterization of the first transgenic mouse model of ARVC5.

METHODS

We generated transgenic mice overexpressing TMEM43 in either its wild-type or p.S358L mutant (TMEM43-S358L) form in postnatal cardiomyocytes under the control of the α-myosin heavy chain promoter.

RESULTS

We found that mice expressing TMEM43-S358L recapitulate the human disease and die at a young age. Mutant TMEM43 causes cardiomyocyte death and severe fibrofatty replacement. We also demonstrate that TMEM43 localizes at the nuclear membrane and interacts with emerin and β-actin. TMEM43-S358L shows partial delocalization to the cytoplasm, reduced interaction with emerin and β-actin, and activation of glycogen synthase kinase-3β (GSK3β). Furthermore, we show that targeting cardiac fibrosis has no beneficial effect, whereas overexpression of the calcineurin splice variant calcineurin Aβ1 results in GSK3β inhibition and improved cardiac function and survival. Similarly, treatment of TMEM43 mutant mice with a GSK3β inhibitor improves cardiac function. Finally, human induced pluripotent stem cells bearing the p.S358L mutation also showed contractile dysfunction that was partially restored after GSK3β inhibition.

CONCLUSIONS

Our data provide evidence that TMEM43-S358L leads to sustained cardiomyocyte death and fibrofatty replacement. Overexpression of calcineurin Aβ1 in TMEM43 mutant mice or chemical GSK3β inhibition improves cardiac function and increases mice life span. Our results pave the way toward new therapeutic approaches for ARVC5.

Inflammation-induced mammalian target of rapamycin signaling is essential for retina regeneration

Upon retina injury, Müller glia in the zebrafish retina respond by generating multipotent progenitors to repair the retina. However, the complete mechanisms underlying retina regeneration remain elusive. Here we report inflammation-induced mammalian target of rapamycin (mTOR) signaling in the Müller glia is essential for retina regeneration in adult zebrafish. We show after a stab injury, mTOR is rapidly activated in Müller glia and later Müller glia-derived progenitor cells (MGPCs). Importantly, mTOR is required for Müller glia dedifferentiation, as well as the proliferation of Müller glia and MGPCs. Interestingly, transient mTOR inhibition by rapamycin only reversibly suppresses MGPC proliferation, while its longer suppression by knocking down Raptor significantly inhibits the regeneration of retinal neurons. We further show mTOR promotes retina regeneration by regulating the mRNA expression of key reprogramming factors ascl1a and lin-28a, cell cycle-related genes and critical cytokines. Surprisingly, we identify microglia/macrophage-mediated inflammation as an important upstream regulator of mTOR in the Müller glia and it promotes retina regeneration through mTOR. Our study not only demonstrates the important functions of mTOR but also reveals an interesting link between inflammation and the mTOR signaling during retina regeneration.

New Insights Into the Role of mTOR Signaling in the Cardiovascular System

The mTOR (mechanistic target of rapamycin) is a master regulator of several crucial cellular processes, including protein synthesis, cellular growth, proliferation, autophagy, lysosomal function, and cell metabolism. mTOR interacts with specific adaptor proteins to form 2 multiprotein complexes, called mTORC1 (mTOR complex 1) and mTORC2 (mTOR complex 2). In the cardiovascular system, the mTOR pathway regulates both physiological and pathological processes in the heart. It is needed for embryonic cardiovascular development and for maintaining cardiac homeostasis in postnatal life. Studies involving mTOR loss-of-function models revealed that mTORC1 activation is indispensable for the development of adaptive cardiac hypertrophy in response to mechanical overload. mTORC2 is also required for normal cardiac physiology and ensures cardiomyocyte survival in response to pressure overload. However, partial genetic or pharmacological inhibition of mTORC1 reduces cardiac remodeling and heart failure in response to pressure overload and chronic myocardial infarction. In addition, mTORC1 blockade reduces cardiac derangements induced by genetic and metabolic disorders and has been reported to extend life span in mice. These studies suggest that pharmacological targeting of mTOR may represent a therapeutic strategy to confer cardioprotection, although clinical evidence in support of this notion is still scarce. This review summarizes and discusses the new evidence on the pathophysiological role of mTOR signaling in the cardiovascular system.

Role of mTOR Signaling in Female Reproduction

Mammalian target of rapamycin (mTOR) is a conserved serine/threonine kinase of the phosphatidylinositol kinase-related kinase family that regulates cell growth, metabolism, and autophagy. Extensive research has linked mTOR to several human diseases including cancer, neurodegenerative disorders, and aging. In this review, recent publications regarding the mechanisms underlying the role of mTOR in female reproduction under physiological and pathological conditions are summarized. Moreover, we assess whether strategies to improve or suppress mTOR expression could have therapeutic potential for reproductive diseases like premature ovarian failure, polycystic ovarian syndrome, and endometriosis.

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.

The unique C terminus of the calcineurin isoform CNAβ1 confers non-canonical regulation of enzyme activity by Ca2+ and calmodulin

Calcineurin, the conserved Ca2+/calmodulin-regulated phosphatase and target of immunosuppressants, plays important roles in the circulatory, nervous, and immune systems. Calcineurin activity strictly depends on Ca2+ and Ca2+-bound calmodulin (Ca2+/CaM) to relieve autoinhibition of the catalytic subunit (CNA) by its C terminus. The C terminus contains two regulatory domains, the autoinhibitory domain (AID) and calmodulin-binding domain (CBD), which block the catalytic center and a conserved substrate-binding groove, respectively. However, this mechanism cannot apply to CNAβ1, an atypical CNA isoform generated by alternative 3'-end processing, whose divergent C terminus shares the CBD common to all isoforms, but lacks the AID. We present the first biochemical characterization of CNAβ1, which is ubiquitously expressed and conserved in vertebrates. We identify a distinct C-terminal autoinhibitory four-residue sequence in CNAβ1, 462LAVP465, which competitively inhibits substrate dephosphorylation. In vitro and cell-based assays revealed that the CNAβ1-containing holoenzyme, CNβ1, is autoinhibited at a single site by either of two inhibitory regions, CBD and LAVP, which block substrate access to the substrate-binding groove. We found that the autoinhibitory segment (AIS), located within the CBD, is progressively removed by Ca2+ and Ca2+/CaM, whereas LAVP remains engaged. This regulatory strategy conferred higher basal and Ca2+-dependent activity to CNβ1, decreasing its dependence on CaM, but also limited maximal enzyme activity through persistence of LAVP-mediated autoinhibiton during Ca2+/CaM stimulation. These regulatory properties may underlie observed differences between the biological activities of CNβ1 and canonical CNβ2. Our insights lay the groundwork for further studies of CNβ1, whose physiological substrates are currently unknown.

Extensive alternative splicing transitions during postnatal skeletal muscle development are required for calcium handling functions

Postnatal development of skeletal muscle is a highly dynamic period of tissue remodeling. Here, we used RNA-seq to identify transcriptome changes from late embryonic to adult mouse muscle and demonstrate that alternative splicing developmental transitions impact muscle physiology. The first 2 weeks after birth are particularly dynamic for differential gene expression and alternative splicing transitions, and calcium-handling functions are significantly enriched among genes that undergo alternative splicing. We focused on the postnatal splicing transitions of the three calcineurin A genes, calcium-dependent phosphatases that regulate multiple aspects of muscle biology. Redirected splicing of calcineurin A to the fetal isoforms in adult muscle and in differentiated C2C12 slows the timing of muscle relaxation, promotes nuclear localization of calcineurin target Nfatc3, and/or affects expression of Nfatc transcription targets. The results demonstrate a previously unknown specificity of calcineurin isoforms as well as the broader impact of alternative splicing during muscle postnatal development.

Calcineurin signaling in the heart: The importance of time and place

The calcium-activated protein phosphatase, calcineurin, lies at the intersection of protein phosphorylation and calcium signaling cascades, where it provides an essential nodal point for coordination between these two fundamental modes of intracellular communication. In excitatory cells, such as neurons and cardiomyocytes, that experience rapid and frequent changes in cytoplasmic calcium, calcineurin protein levels are exceptionally high, suggesting that these cells require high levels of calcineurin activity. Yet, it is widely recognized that excessive activation of calcineurin in the heart contributes to pathological hypertrophic remodeling and the progression to failure. How does a calcium activated enzyme function in the calcium-rich environment of the continuously contracting heart without pathological consequences? This review will discuss the wide range of calcineurin substrates relevant to cardiovascular health and the mechanisms calcineurin uses to find and act on appropriate substrates in the appropriate location while potentially avoiding others. Fundamental differences in calcineurin signaling in neonatal verses adult cardiomyocytes will be addressed as well as the importance of maintaining heterogeneity in calcineurin activity across the myocardium. Finally, we will discuss how circadian oscillations in calcineurin activity may facilitate integration with other essential but conflicting processes, allowing a healthy heart to reap the benefits of calcineurin signaling while avoiding the detrimental consequences of sustained calcineurin activity that can culminate in heart failure.