Mog1 to tbx5‐cryab/hspb2: A novel signalling network potentiates heart failure?

Today, heart failure remains a serious global health challenge. It affects more than 6.2 million people in the United States according to the 2019 data and this number is predicted to reach over 8 million by 2030. The ontology of heart failure involves a pathological process termed adverse remodelling, which is defined by cardiomyocyte hypertrophy and fibrosis. In the current issue of Acta Physiologica, Wang's group reported a novel signalling cascade from Mog1 to tbx5-cryab/hspb2 whose disruption contributes to cardiac hypertrophy and heart failure. Wang's group explored the in vivo phenotype of mog1 knockout (KO) using two lines of homozygous mog1 KO zebrafish.1 MOG1 (human Ran guanine nucleotide release factor RANGNRF) is a small protein that binds to Ran and regulates the cytoplasm-nucleus transportation of RNA and proteins. Recent genetic studies have identified several mutant alleles of MOG1 including p. E83D and p. E61X, associated with loss of function in the sodium current. Despite its known association in arrhythmia syndrome and heart failure, MOG1’s in vivo physiological function had not been discovered. In the mog1 KO zebrafish embryos, pericardial oedema together with enhanced expression of nppa were observed. Adult mog1 KO zebrafish exhibited significantly reduced ejection fraction (EF) and thickened ventricular wall, suggesting the onset of cardiac hypertrophy and heart failure. Upon isoproterenol (ISO) treatment, more severe phenotypes of cardiac hypertrophy and heart failure were observed in mog1 KO zebrafish at both the embryonic and adult stage. Thus, the paper for the first time demonstrated MOG1's role in in vivo physiological functions in cardiac morphogenesis, cardiac hypertrophy and heart failure. Another highlight of the study is the establishment of a novel signalling cascade from Mog1 to tbx5-cryab/hspb2. Previous cell culture studies conducted by Wang's group have demonstrated that MOG1 interacts with the voltage-gated cardiac sodium channel Nav1.5, and further promotes Nav1.5 intracellular trafficking from endoplasmic reticulum to plasma membranes.2 Mutation of the SCN5A gene that encodes Nav1.5 is reported to cause hereditary cardiac arrhythmias including Brugada syndrome (BrS).3 However, cell surface scn5a expression and sodium current densities were unchanged in the mog1 KO zebrafish heart, contrary to the association between MOG1 dosage and cell surface expression of Nav1.5 that had been noted in various cultured mammalian cells. This conflict might be explained by a genetic compensation response in gene KO zebrafish. In the current paper, a novel Mog1 hierarchy involving tbx5-cryab-hspb2 is reported. Identified by RNA-seq and validated by RT-PCR, the Mog1 dependant expressions of tbx5, cryab and hspb2 placed these genes downstream of Mog1. Although it is unknown by what mechanism Mog1 affects tbx5b expression, co-transfection of Mog1 and Tbx5 significantly enhanced luciferase activity when binding to the cryab or hspb2 promoter regions. This indicated that cryab and hspb2 are downstream tagerts of Tbx5. Thus, a TBX5 consensus binding motif was suggested. However, to be proven, confirmation assays such as a luciferase reporter assay with site-specific mutations are further needed. Importantly, overexpression of cryab and hspb2 partially rescued the pericardial oedema caused by Mog1 deletion. Taken together, these results established tbx5-cryab-hspb2 to be downstream of Mog1. Both CRYAB and HSPB2 are highly expressed in the human heart. Although their molecular functions remain unclear, CRYAB has been suggested to be involved in Calcineurin/NFAT signaling, and cytochrome c realease, which then affect the Ca2+/Na+ exchange and cardiomyocyte survival. Various cardiac phenotypes—including cardiac hypertrophy, heart failure, abnormal cardiac morphogenesis and development—suggest that Mog1 is required in various molecular pathways for normal cardiac development and physiological function. In human, haploinsufficiency of TBX5 causes Holt-Oram syndrome, manifested by congenital heart defects, conduction-system abnormalities and upper-limb deformities. In a Tbx5 KO zebrafish model, Deborah et al reported slow heart rate, which contributes to progressive stretching into a non-functional heart.4 A harmonious activity of Nkx2-5 and Tbx5 is required for the development of cardiac electrophysiological system.5, 6 Tbx5 mutant mice fail to develop a mature conduction system which results in atrioventricular node maturation failure, patterning defects of the ventricular bundle branches, and electrocardiographic abnormalities.5 In addition to reduced ejection fraction (EF) and thickened ventricular wall, the mog1 KO zebrafish had slow heart rate, along with elongated RR interval, QRS duration and QTc. Considering the important role of Tbx5 in the development of cardiac conduction system, some electrocardiographic abnormalities might be due to a congenital conduction system defect caused by tbx5 disruption during development, which further promotes the development and progression of heart failture later in life. In addition, mog1 KO zebrafish also display pericardial oedema and severe defects in cardiac looping, both associated with high mortality rates. The expression of multiple important transcription factors required for heart development, such as tbx5, nkx2.5, gata4 and hand2 was also significantly downregulated in mog1 KO zebrafish. Despite surgical correction, patients with cardiac abnormalities carry a life-long risk for cardiovascular complications, and thus are at high risk for heart failure with an incidence up to 70% in selected populations. Various cardiac abnormalities that develop during embryonic stages potentiate and promote the progression of heart failure in adulthood. Thus, rather than direct prevention for heart failure, the novel signalling network from Mog1 to tbx5-cryab/hspb2 may contribute to the heart development, which likely includes the maturation of cardiac electrophysiology. It is not yet clarified how tbx5 downstream genes, cryab and hspb2, are involved in cardiac development during embryogenesis nor in heart failure development during adulthood. Although overexpression of cryabb and hspb2 partially rescued the pericardial oedema in mog1 KO zebrafish, its efficacy in rescuing electrophysiological abnormalities and preventing isoproterenol (ISO)-induced heart failure remains to be investigated. Therefore, the functional signalling from tbx5 to cryabb and hspb2 contributing to heart development and rhythm maintenance still has much to uncover. Nonetheless, the study revealed a novel molecular mechanism involving tbx5 in which mog1 KO causes cardiac hypertrophy and heart failure. As pointed out by the authors, one limitation of the study is that they do not address the molecular mechanism by which mog1 regulates tbx5 expression. Mog1 is a known coordinator of the Ran-GTP cycle by binding to small GTPase Ran. The classic function of Ran is to regulate the cycle of nuclear import and export, which connects the nuclear transcription to the cytoplasmic translation of proteins. The predominant form of Ran in the nucleus is bound to GTP, while in the cytoplasm it is bound to GDP. Ran-GTP binding to importins promotes cargo release, while binding to exportins stabilizes the exportin-cargo interaction. In mitosis, the Ran-GTP gradient around chromosomes restricts the activity of several spindle assembly factors, which then regulate the spindle assembly and cell division. MOG1 controls the Ran-GTP cycle by stimulating the release of GTP from Ran-GTP and remaining bound to nucleotide-free Ran. Therefore, deleting Mog1 results in defective nuclear-protein import. It is possible that Mog1 affects TBX5 level by regulating its transportation from the nucleus to the cytoplasm. To be noted, a recent study by Oliete-Calvo et al suggests that Mog1 affects RNA transcription via epigenetic regulation. MOG1 is required for the H2B ubiquitination, which further determines the downstream histone H3 methylation, transcription and mRNA export.7 The mog1 KO study by Wang's group observed mRNA level changes of multiple transcription factors including tbx5, nkx2.5, gata4 and hand2, suggesting that the transcriptional regulation under mog1 may be at the whole genome level rather than at specific genomic regions. Future studies are needed to answer this important question: what molecular mechanism does mog1 alter to regulate the expressions of tbx5 and other transcription factors? While questions remain, the new finding of the Wang group offers a novel working model for understanding cardiomyocyte hypertrophy and heart failure that Mog1 to tbx5-cryab/hspb2 signalling is both developmentally required and physiologically relevant. The present study provides an important starting point to understand both, although translation of these findings to humans will need critical work performed on additional models. The author declares no conflict of interests.