Cardiomyocyte protein O-GlcNAcylation is regulated by GFAT1 not GFAT2.

In response to cardiac injury, increased activity of the hexosamine biosynthesis pathway (HBP) is linked with cytoprotective as well as adverse effects depending on the type and duration of injury. Glutamine-fructose amidotransferase (GFAT; gene name gfpt) is the rate-limiting enzyme that controls flux through HBP. Two protein isoforms exist in the heart called GFAT1 and GFAT2. There are conflicting data on the relative importance of GFAT1 and GFAT2 during stress-induced HBP responses in the heart. Using neonatal rat cardiac cell preparations, targeted knockdown of GFPT1 and GFPT2 were performed and HBP activity measured. Immunostaining with specific GFAT1 and GFAT2 antibodies was undertaken in neonatal rat cardiac preparations and murine cardiac tissues to characterise cell-specific expression. Publicly available human heart single cell sequencing data was interrogated to determine cell-type expression. Western blots for GFAT isoform protein expression were performed in human cardiomyocytes derived from induced pluripotent stem cells (iPSCs). GFPT1 but not GFPT2 knockdown resulted in a loss of stress-induced protein O-GlcNAcylation in neonatal cardiac cell preparations indicating reduced HBP activity. In rodent cells and tissue, immunostaining for GFAT1 identified expression in both cardiac myocytes and fibroblasts whereas immunostaining for GFAT2 was only identified in fibroblasts. Further corroboration of findings in human heart cells identified an enrichment of GFPT2 gene expression in cardiac fibroblasts but not ventricular myocytes whereas GFPT1 was expressed in both myocytes and fibroblasts. In human iPSC-derived cardiomyocytes, only GFAT1 protein was expressed with an absence of GFAT2. In conclusion, these results indicate that GFAT1 is the primary cardiomyocyte isoform and GFAT2 is only present in cardiac fibroblasts. Cell-specific isoform expression may have differing effects on cell function and should be considered when studying HBP and GFAT functions in the heart.

The TAB1-p38α complex aggravates myocardial injury and can be targeted by small molecules.

Inhibiting MAPK14 (p38α) diminishes cardiac damage in myocardial ischemia. During myocardial ischemia, p38α interacts with TAB1, a scaffold protein, which promotes p38α autoactivation; active p38α (pp38α) then transphosphorylates TAB1. Previously, we solved the X-ray structure of the p38α-TAB1 (residues 384-412) complex. Here, we further characterize the interaction by solving the structure of the pp38α-TAB1 (residues 1-438) complex in the active state. Based on this information, we created a global knock-in (KI) mouse with substitution of 4 residues on TAB1 that we show are required for docking onto p38α. Whereas ablating p38α or TAB1 resulted in early embryonal lethality, the TAB1-KI mice were viable and had no appreciable alteration in their lymphocyte repertoire or myocardial transcriptional profile; nonetheless, following in vivo regional myocardial ischemia, infarction volume was significantly reduced and the transphosphorylation of TAB1 was disabled. Unexpectedly, the activation of myocardial p38α during ischemia was only mildly attenuated in TAB1-KI hearts. We also identified a group of fragments able to disrupt the interaction between p38α and TAB1. We conclude that the interaction between the 2 proteins can be targeted with small molecules. The data reveal that it is possible to selectively inhibit signaling downstream of p38α to attenuate ischemic injury.

TAB1-Induced Autoactivation of p38α Mitogen-Activated Protein Kinase Is Crucially Dependent on Threonine 185.

p38α mitogen-activated protein kinase is essential to cellular homeostasis. Two principal mechanisms to activate p38α exist. The first relies on dedicated dual-specificity kinases such as mitogen-activated protein kinase kinase (MAP2K) 3 (MKK3) or 6 (MKK6), which activate p38α by phosphorylating Thr180 and Tyr182 within the activation segment. The second is by autophosphorylation of Thr180 and Tyr182 in cis, mediated by p38α binding the scaffold protein TAB1. The second mechanism occurs during myocardial ischemia, where it aggravates myocardial infarction. Based on the crystal structure of the p38α-TAB1 complex we replaced threonine 185 of p38α with glycine (T185G) to prevent an intramolecular hydrogen bond with Asp150 from being formed. This mutation did not interfere with TAB1 binding to p38α. However, it disrupted the consequent long-range effect of this binding event on the distal activation segment, releasing the constraint on Thr180 that oriented its hydroxyl for phosphotransfer. Based on assays performed in vitro and in vivo, the autoactivation of p38α(T185G) was disabled, while its ability to be activated by upstream MAP2Ks and to phosphorylate downstream substrates remained intact. Furthermore, myocardial cells expressing p38α(T185G) were resistant to injury. These findings reveal a mechanism to selectively disable p38α autoactivation and its consequences, which may ultimately circumvent the toxicity associated with strategies that inhibit p38α kinase activity under all circumstances, such as with ATP-competitive inhibitors.

Mechanism and consequence of the autoactivation of p38α mitogen-activated protein kinase promoted by TAB1.

p38α mitogen-activated protein kinase (p38α) is activated by a variety of mechanisms, including autophosphorylation initiated by TGFß-activated kinase 1 binding protein 1 (TAB1) during myocardial ischemia and other stresses. Chemical-genetic approaches and coexpression in mammalian, bacterial and cell-free systems revealed that mouse p38α autophosphorylation occurs in cis by direct interaction with TAB1(371-416). In isolated rat cardiac myocytes and perfused mouse hearts, TAT-TAB1(371-416) rapidly activates p38 and profoundly perturbs function. Crystal structures and characterization in solution revealed a bipartite docking site for TAB1 in the p38α C-terminal kinase lobe. TAB1 binding stabilizes active p38α and induces rearrangements within the activation segment by helical extension of the Thr-Gly-Tyr motif, allowing autophosphorylation in cis. Interference with p38α recognition by TAB1 abolishes its cardiac toxicity. Such intervention could potentially circumvent the drawbacks of clinical pharmacological inhibitors of p38 catalytic activity.

A chemical genetic approach reveals that p38alpha MAPK activation by diphosphorylation aggravates myocardial infarction and is prevented by the direct binding of SB203580.

The use of nonselective pharmacological inhibitors has resulted in controversy regarding the mechanism and consequences of p38 activation during myocardial infarction. Classic p38 inhibitors such as SB203580 rely on a critical "gatekeeper" threonine residue for binding. We addressed these controversies by using mice in which the p38alpha alleles were targeted to cause substitution of the gatekeeper residue and resistance to inhibition. In homozygous drug-resistant compared with wild-type hearts, SB203580 failed to inhibit the activating phosphorylation of p38 or to reduce the infarction caused by myocardial ischemia. However, BIRB796, a p38 inhibitor not reliant on the gatekeeper for binding, similarly reduced p38-activating phosphorylation and infarction in both wild-type and knock-in mice, thereby excluding a nonspecific inhibitor-dependent phenotype resulting from the targeting strategy. Furthermore, the activation during myocardial ischemia involved phosphorylation of both the threonine and tyrosine residues in the activation loop of p38 despite the phosphorylation of the threonine alone being sufficient to create the epitope for dual phosphospecific antibody binding. Finally, SB203580 failed to reduce infarction in heterozygous drug-resistant hearts, suggesting that near complete inhibition of p38alpha kinase activity is necessary to elicit protection. These results indicate that, during myocardial ischemia, p38alpha (i) is the dominant-active p38 isoform, (ii) contributes to infarction, (iii) is responsible for the cardioprotective effect of SB203580, and (iv) is activated by a mechanism consistent with autodiphosphorylation despite this necessitating the phosphorylation of a tyrosine residue by an archetypal serine/threonine kinase.