PKB-Mediated Thr649 Phosphorylation of AS160/TBC1D4 Regulates the R-Wave Amplitude in the Heart

AS160 is a lipid-responsive regulator of cardiac Ca2+ homeostasis by controlling lysophosphatidylinositol metabolism and signaling

The obese heart undergoes metabolic remodeling and exhibits impaired calcium (Ca2+) homeostasis, which are two critical assaults leading to cardiac dysfunction. The molecular mechanisms underlying these alterations in obese heart are not well understood. Here, we show that the Rab-GTPase activating protein AS160 is a lipid-responsive regulator of Ca2+ homeostasis through governing lysophosphatidylinositol metabolism and signaling. Palmitic acid/high fat diet inhibits AS160 activity through phosphorylation by NEK6, which consequently activates its downstream target Rab8a. Inactivation of AS160 in cardiomyocytes elevates cytosolic Ca2+ that subsequently impairs cardiac contractility. Mechanistically, Rab8a downstream of AS160 interacts with DDHD1 to increase lysophosphatidylinositol metabolism and signaling that leads to Ca2+ release from sarcoplasmic reticulum. Inactivation of NEK6 prevents inhibition of AS160 by palmitic acid/high fat diet, and alleviates cardiac dysfunction in high fat diet-fed mice. Together, our findings reveal a regulatory mechanism governing metabolic remodeling and Ca2+ homeostasis in obese heart, and have therapeutic implications to combat obesity cardiomyopathy.

In-depth phosphoproteomic profiling of the insulin signaling response in heart tissue and cardiomyocytes unveils canonical and specialized regulation

BACKGROUND

Insulin signaling regulates cardiac substrate utilization and is implicated in physiological adaptations of the heart. Alterations in the signaling response within the heart are believed to contribute to pathological conditions such as type-2 diabetes and heart failure. While extensively investigated in several metabolic organs using phosphoproteomic strategies, the signaling response elicited in cardiac tissue in general, and specifically in the specialized cardiomyocytes, has not yet been investigated to the same extent.

METHODS

Insulin or vehicle was administered to male C57BL6/JRj mice via intravenous injection into the vena cava. Ventricular tissue was extracted and subjected to quantitative phosphoproteomics analysis to evaluate the insulin signaling response. To delineate the cardiomyocyte-specific response and investigate the role of Tbc1d4 in insulin signal transduction, cardiomyocytes from the hearts of cardiac and skeletal muscle-specific Tbc1d4 knockout mice, as well as from wildtype littermates, were studied. The phosphoproteomic studies involved isobaric peptide labeling with Tandem Mass Tags (TMT), enrichment for phosphorylated peptides, fractionation via micro-flow reversed-phase liquid chromatography, and high-resolution mass spectrometry measurements.

RESULTS

We quantified 10,399 phosphorylated peptides from ventricular tissue and 12,739 from isolated cardiomyocytes, localizing to 3,232 and 3,128 unique proteins, respectively. In cardiac tissue, we identified 84 insulin-regulated phosphorylation events, including sites on the Insulin Receptor (InsrY1351, Y1175, Y1179, Y1180) itself as well as the Insulin receptor substrate protein 1 (Irs1S522, S526). Predicted kinases with increased activity in response to insulin stimulation included Rps6kb1, Akt1 and Mtor. Tbc1d4 emerged as a major phosphorylation target in cardiomyocytes. Despite limited impact on the global phosphorylation landscape, Tbc1d4 deficiency in cardiomyocytes attenuated insulin-induced Glut4 translocation and induced protein remodeling. We observed 15 proteins significantly regulated upon knockout of Tbc1d4. While Glut4 exhibited decreased protein abundance consequent to Tbc1d4-deficiency, Txnip levels were notably increased. Stimulation of wildtype cardiomyocytes with insulin led to the regulation of 262 significant phosphorylation events, predicted to be regulated by kinases such as Akt1, Mtor, Akt2, and Insr. In cardiomyocytes, the canonical insulin signaling response is elicited in addition to regulation on specialized cardiomyocyte proteins, such as Kcnj11Y12 and DspS2597. Details of all phosphorylation sites are provided.

CONCLUSION

We present a first global outline of the insulin-induced phosphorylation signaling response in heart tissue and in isolated adult cardiomyocytes, detailing the specific residues with changed phosphorylation abundances. Our study marks an important step towards understanding the role of insulin signaling in cardiac diseases linked to insulin resistance.

Deletion of Tbc1d4/As160 abrogates cardiac glucose uptake and increases myocardial damage after ischemia/reperfusion

BACKGROUND

Type 2 Diabetes mellitus (T2DM) is a major risk factor for cardiovascular disease and associated with poor outcome after myocardial infarction (MI). In T2DM, cardiac metabolic flexibility, i.e. the switch between carbohydrates and lipids as energy source, is disturbed. The RabGTPase-activating protein TBC1D4 represents a crucial regulator of insulin-stimulated glucose uptake in skeletal muscle by controlling glucose transporter GLUT4 translocation. A human loss-of-function mutation in TBC1D4 is associated with impaired glycemic control and elevated T2DM risk. The study's aim was to investigate TBC1D4 function in cardiac substrate metabolism and adaptation to MI.

METHODS

Cardiac glucose metabolism of male Tbc1d4-deficient (D4KO) and wild type (WT) mice was characterized using in vivo [¹⁸F]-FDG PET imaging after glucose injection and ex vivo basal/insulin-stimulated [³H]-2-deoxyglucose uptake in left ventricular (LV) papillary muscle. Mice were subjected to cardiac ischemia/reperfusion (I/R). Heart structure and function were analyzed until 3 weeks post-MI using echocardiography, morphometric and ultrastructural analysis of heart sections, complemented by whole heart transcriptome and protein measurements.

RESULTS

Tbc1d4-knockout abolished insulin-stimulated glucose uptake in ex vivo LV papillary muscle and in vivo cardiac glucose uptake after glucose injection, accompanied by a marked reduction of GLUT4. Basal cardiac glucose uptake and GLUT1 abundance were not changed compared to WT controls. D4KO mice showed mild impairments in glycemia but normal cardiac function. However, after I/R D4KO mice showed progressively increased LV endsystolic volume and substantially increased infarction area compared to WT controls. Cardiac transcriptome analysis revealed upregulation of the unfolded protein response via ATF4/eIF2α in D4KO mice at baseline. Transmission electron microscopy revealed largely increased extracellular matrix (ECM) area, in line with decreased cardiac expression of matrix metalloproteinases of D4KO mice.

CONCLUSIONS

TBC1D4 is essential for insulin-stimulated cardiac glucose uptake and metabolic flexibility. Tbc1d4-deficiency results in elevated cardiac endoplasmic reticulum (ER)-stress response, increased deposition of ECM and aggravated cardiac damage following MI. Hence, impaired TBC1D4 signaling contributes to poor outcome after MI.

Impaired SERCA2a phosphorylation causes diabetic cardiomyopathy through impinging on cardiac contractility and precursor protein processing

Diabetic cardiomyopathy (DCM) is currently a progressive and nonstoppable complication in type 2 diabetic patients. Metabolic insults and insulin resistance are involved in its pathogenesis; however, the underlying mechanisms are still not clearly understood. Here we show that calcium dysregulation can be both a cause and a consequence of cardiac insulin resistance that leads to DCM. A western diet induces the development of DCM through at least three phases in mice, among which an early phase depends on impaired Thr484-phosphorylation of sarcoplasmic/endoplasmic reticulum calcium ATPase 2a (SERCA2a) elicited by insulin resistance. Mutation of SERCA2a-Thr484 to a nonphosphorylatable alanine delays calcium re-uptake into the sarcoplasmic reticulum in the cardiomyocytes and decreases cardiac function at the baseline. Importantly, this mutation blunts the early phase of DCM, but has no effect on disease progression in the following phases. Interestingly, impairment of sarcoplasmic reticulum calcium re-uptake caused by the SERCA2a-Thr484 mutation inhibited processing of insulin receptor precursor through FURIN convertase, resulting in cardiac insulin resistance. Collectively, these data reveal a bidirectional relationship between insulin resistance and impairment of calcium homeostasis, which may underlie the early pathogenesis of DCM. Our findings have therapeutic implications for early intervention of DCM.

No evidence for change in expression of TBC1D1 and TBC1D4 genes in cultured human adipocytes stimulated by myokines and adipokines

TBC1D1 and TBC1D4 proteins play analogous, but not identical role in governing insulin-signalling pathway. Little is known about changes in expression levels of TBC1D1 and TBC1D4 genes in mammals, including humans. Number of factors were studied, but data remain controversial. The aim of this study was to evaluate the effect of selected cytokines, adipokines and myokines with known or putative insulin sensitivity regulation activity (adiponectin, irisin, omentin, interleukin 6, leptin, resistin, and tumour necrosis factor) on TBC1D1 and TBC1D4 expression levels in cultured differentiated human adipocytes. No significant differences were found between the adipocytes treated with different stimuli and this effect was determined not dose dependent. It is reasonable to conclude that relative shortage of data showing no change in TBC1D1 and TBC1D4 from literature results from publication bias; therefore, our finding provides additional insight into the role of both genes.

The effect of diabetes and the common diabetogenic TBC1D4 p.Arg684Ter variant on cardiovascular risk in Inuit in Greenland

Cardiovascular disease (CVD) is a well-known complication of diabetes, but the association has not been studied among Inuit in Greenland. The aim was to examine the association between diabetes and incident CVD among Inuit in Greenland and determine if the common diabetogenic TBC1D4 variant confers increased risk of CVD. We followed an initial study population of 4127 adults in Greenland who had participated in at least one population-based health survey, in national registers. We used Poisson regression to calculate incidence rate ratios (IRR) of cardiovascular endpoints, comparing participants with and without diabetes and comparing homozygous TBC1D4 carriers with heterozygous carriers and non-carriers combined. Close to 10% had diabetes and age range was 18-96 years (45% male). Of the 3924 participants without prior CVD, 362 (~ 9%) had CVD events during a median follow-up of 10 years. Multivariate IRR for the effect of diabetes on CVD was 1.12 (95% CI: 0.80, 1.57) p = 0.50. Using a recessive genetic model, we compared homozygous TBC1D4 carriers with wildtype and heterozygous carriers combined, with a multivariate IRR of 1.20 (95% CI: 0.69, 2.11) p = 0.52. Neither diabetes nor the TBC1D4 variant significantly increased CVD risk among Inuit in Greenland in adjusted models.

Ablating the Rab-GTPase activating protein TBC1D1 predisposes rats to high-fat diet-induced cardiomyopathy

KEY POINTS

Although the role of TBC1D1 within the heart remains unknown, expression of TBC1D1 increases in the left ventricle following an acute infarction, suggesting a biological importance within this tissue. We investigated the mechanistic role of TBC1D1 within the heart, aiming to establish the consequences of attenuating TBC1D1 signalling in the development of diabetic cardiomyopathy, as well as to determine potential sex differences. TBC1D1 ablation increased plasma membrane fatty acid binding protein content and myocardial palmitate oxidation. Following high-fat feeding, TBC1D1 ablation dramatically increased fibrosis and induced end-diastolic dysfunction in both male and female rats in the absence of changes in mitochondrial bioenergetics. Altogether, independent of sex, ablating TBC1D1 predisposes the left ventricle to pathological remodelling following high-fat feeding, and suggests TBC1D1 protects against diabetic cardiomyopathy.

ABSTRACT

TBC1D1, a Rab-GTPase activating protein, is involved in the regulation of glucose handling and substrate metabolism within skeletal muscle, and is essential for maintaining pancreatic β-cell mass and insulin secretion. However, the function of TBC1D1 within the heart is largely unknown. Therefore, we examined the role of TBC1D1 in the left ventricle and the functional consequence of ablating TBC1D1 on the susceptibility to high-fat diet-induced abnormalities. Since mutations within TBC1D1 (R125W) display stronger associations with clinical parameters in women, we further examined possible sex differences in the predisposition to diabetic cardiomyopathy. In control-fed animals, TBC1D1 ablation did not alter insulin-stimulated glucose uptake, or echocardiogram parameters, but increased accumulation of a plasma membrane fatty acid transporter and the capacity for palmitate oxidation. When challenged with an 8 week high-fat diet, TBC1D1 knockout rats displayed a four-fold increase in fibrosis compared to wild-type animals, and this was associated with diastolic dysfunction, suggesting a predisposition to diet-induced cardiomyopathy. Interestingly, high-fat feeding only induced cardiac hypertrophy in male TBC1D1 knockout animals, implicating a possible sex difference. Mitochondrial respiratory capacity and substrate sensitivity to pyruvate and ADP were not altered by diet or TBC1D1 ablation, nor were markers of oxidative stress, or indices of overt heart failure. Altogether, independent of sex, ablation of TBC1D1 not only increased the susceptibility to high-fat diet-induced diastolic dysfunction and left ventricular fibrosis, independent of sex, but also predisposed male animals to the development of cardiac hypertrophy. These data suggest that TBC1D1 may exert cardioprotective effects in the development of diabetic cardiomyopathy.

SPEG Controls Calcium Reuptake Into the Sarcoplasmic Reticulum Through Regulating SERCA2a by Its Second Kinase-Domain

RATIONALE

SPEG (Striated muscle preferentially expressed protein kinase) has 2 kinase-domains and is critical for cardiac development and function. However, it is not clear how these 2 kinase-domains function to maintain cardiac performance.

OBJECTIVE

To determine the molecular functions of the 2 kinase-domains of SPEG.

METHODS AND RESULTS

A proteomics approach identified SERCA2a (sarcoplasmic/endoplasmic reticulum calcium ATPase 2a) as a protein interacting with the second kinase-domain but not the first kinase-domain of SPEG. Furthermore, the second kinase-domain of SPEG could phosphorylate Thr⁴⁸⁴ on SERCA2a, promote its oligomerization and increase calcium reuptake into the sarcoplasmic/endoplasmic reticulum in culture cells and primary neonatal rat cardiomyocytes. Phosphorylation of SERCA2a by SPEG enhanced its calcium-transporting activity without affecting its ATPase activity. Depletion of Speg in neonatal rat cardiomyocytes inhibited SERCA2a-Thr⁴⁸⁴ phosphorylation and sarcoplasmic reticulum calcium reuptake. Moreover, overexpression of SERCA2a^Thr484Ala mutant protein also slowed sarcoplasmic reticulum calcium reuptake in neonatal rat cardiomyocytes. In contrast, domain mapping and phosphorylation analysis revealed that the first kinase-domain of SPEG interacted and phosphorylated its recently identified substrate JPH2 (junctophilin-2). An inducible heart-specific Speg knockout mouse model was generated to further study this SPEG-SERCA2a signal nexus in vivo. Inducible deletion of Speg decreased SERCA2a-Thr⁴⁸⁴ phosphorylation and its oligomerization in the heart. Importantly, inducible deletion of Speg inhibited SERCA2a calcium-transporting activity and impaired calcium reuptake into the sarcoplasmic reticulum in cardiomyocytes, which preceded morphological and functional alterations of the heart and eventually led to heart failure in adult mice.

CONCLUSIONS

Our data demonstrate that the 2 kinase-domains of SPEG may play distinct roles to regulate cardiac function. The second kinase-domain of SPEG is a critical regulator for SERCA2a. Our findings suggest that SPEG may serve as a new target to modulate SERCA2a activation for treatment of heart diseases with impaired calcium homeostasis.

Whole body glucoregulation and tissue-specific glucose uptake in a novel Akt substrate of 160 kDa knockout rat model

Akt substrate of 160 kDa (also called AS160 or TBC1D4) is a Rab GTPase activating protein and key regulator of insulin-stimulated glucose uptake which is expressed by multiple tissues, including skeletal muscle, white adipose tissue (WAT) and the heart. This study introduces a novel rat AS160-knockout (AS160-KO) model that was created using CRISPR/Cas9 technology. We compared male AS160-KO versus wildtype (WT) rats for numerous metabolism-related endpoints. Body mass, body composition, energy expenditure and physical activity did not differ between genotypes. Oral glucose intolerance was detected in AS160-KO versus WT rats (P<0.005). A hyperinsulinemic-euglycemic clamp (HEC) revealed insulin resistance for glucose infusion rate (P<0.05) with unaltered hepatic glucose production in AS160-KO versus WT rats. Genotype-effects on glucose uptake during the HEC: 1) was significantly lower in epitrochlearis (P<0.01) and extensor digitorum longus (P<0.05) of AS160-KO versus WT rats, and tended to be lower for AS160-KO versus WT rats in the soleus (P<0.06) and gastrocnemius (P<0.08); 2) tended to be greater for AS160-KO versus WT rats in white adipose tissue (P = 0.09); and 3) was significantly greater in the heart (P<0.005) of AS160-KO versus WT rats. GLUT4 protein abundance was significantly lower for AS160-KO versus WT rats in each tissue analyzed (P<0.01–0.001) except the gastrocnemius. Ex vivo insulin-stimulated glucose uptake was significantly lower (P<0.001) for AS160-KO versus WT rats in isolated epitrochlearis or soleus. Insulin-stimulated Akt phosphorylation (in vivo or ex vivo) did not differ between genotypes for any tissue tested. Ex vivo AICAR-stimulated glucose uptake by isolated epitrochlearis was significantly lower for AS160-KO versus WT rats (P<0.01) without genotype-induced alteration in AMP-activated protein phosphorylation. This unique AS160-KO rat model, which elucidated striking genotype-related modifications in glucoregulation, will enable future research aimed at understanding AS160’s roles in numerous physiological processes in response to various interventions (e.g., diet and/or exercise).

Disruption of Adipose Rab10-Dependent Insulin Signaling Causes Hepatic Insulin Resistance

Insulin controls glucose uptake into adipose and muscle cells by regulating the amount of GLUT4 in the plasma membrane. The effect of insulin is to promote the translocation of intracellular GLUT4 to the plasma membrane. The small Rab GTPase, Rab10, is required for insulin-stimulated GLUT4 translocation in cultured 3T3-L1 adipocytes. Here we demonstrate that both insulin-stimulated glucose uptake and GLUT4 translocation to the plasma membrane are reduced by about half in adipocytes from adipose-specific Rab10 knockout (KO) mice. These data demonstrate that the full effect of insulin on adipose glucose uptake is the integrated effect of Rab10-dependent and Rab10-independent pathways, establishing a divergence in insulin signal transduction to the regulation of GLUT4 trafficking. In adipose-specific Rab10 KO female mice, the partial inhibition of stimulated glucose uptake in adipocytes induces insulin resistance independent of diet challenge. During euglycemic-hyperinsulinemic clamp, there is no suppression of hepatic glucose production despite normal insulin suppression of plasma free fatty acids. The impact of incomplete disruption of stimulated adipocyte GLUT4 translocation on whole-body glucose homeostasis is driven by a near complete failure of insulin to suppress hepatic glucose production rather than a significant inhibition in muscle glucose uptake. These data underscore the physiological significance of the precise control of insulin-regulated trafficking in adipocytes.

AS160 controls eukaryotic cell cycle and proliferation by regulating the CDK inhibitor p21

AS160 (TBC1D4) has been implicated in multiple biological processes. However, the role and the mechanism of action of AS160 in the regulation of cell proliferation remain unclear. In this study, we demonstrated that AS160 knockdown led to blunted cell proliferation in multiple cell types, including fibroblasts and cancer cells. The results of cell cycle analysis showed that these cells were arrested in the G1 phase. Intriguingly, this inhibition of cell proliferation and the cell cycle arrest caused by AS160 depletion were glucose independent. Moreover, AS160 silencing led to a marked upregulation of the expression of the cyclin-dependent kinase inhibitor p21. Furthermore, whereas AS160 overexpression resulted in p21 downregulation and rescued the arrested cell cycle in AS160-depeleted cells, p21 silencing rescued the inhibited cell cycle and proliferation in the cells. Thus, our results demonstrated that AS160 regulates glucose-independent eukaryotic cell proliferation through p21-dependent control of the cell cycle, and thereby revealed a molecular mechanism of AS160 modulation of cell cycle and proliferation that is of general physiological significance.