Myotube growth is associated with cancer-like metabolic reprogramming and is limited by phosphoglycerate dehydrogenase

Funding Information: Brendan M. Gabriel was supported by fellowships from the Novo Nordisk Foundation ( NNF19OC0055072 ) & the Wenner-Gren Foundation , an Albert Renold Travel Fellowship from the European Foundation for the Study of Diabetes , and an Eric Reid Fund for Methodology from the Biochemical Society . Abdalla D. Mohamed was funded initially by Sarcoma UK (grant number SUK09.2015 ), then supported by funding from Postdoctoral Fellowship Program ( Helmholtz Zentrum München, Germany ), and currently by Cancer Research UK . Publisher Copyright: © 2023 The AuthorsPeer reviewedPublisher PD

Patterning and Development of the Conduction System of the Heart. Origins of the Conduction System in Development

This chapter focuses on the cellular origin of the conduction system components and the molecular genetic mechanisms that may control their phenotype and position within the developing heart. It discusses the connection between heart cell precursor pools, which in a temporal pattern form the heart, and the genesis of the conduction system components. The distinct components of the cardiac conduction system of the heart are essentially myocardial. They are innervated by cardiac ganglia largely derived from neural crest. In addition, a large fraction of cells in the mature conduction system is noncardiac, and insulating layers of fibrous tissue are found around conduction system components, such as the SAN and AV bundle. These noncardiac cell types are derived from the epicardium (fibroblasts), endocardium, neural crest (neural innervations), and other sources, although their origins have not been defined in detail. Although these nerves and fibrous tissues are important, or even a prerequisite, for conduction system formation and function, the cardiomyocytes are essential for impulse generation and propagation. Furthermore, in the embryo the functional myocardial conduction system is not yet innervated and interstitial fibroblast and fibrous tissues in association with the conduction system are sparse or absent. Studies of the development of the conduction system components strongly suggest that they originate from myocardial precursors, which in turn are derived from mesoderm and pericardial wall mesenchyme

Morphogenesis of the Vertebrate Heart

The adult four-chambered heart of higher vertebrates functions as a sophisticated pump, driving a pulmonary and a systemic circulation that have been separated during evolution. During cardiac development, assemblage of all the different components that make this structure functional is achieved by complex morphogenetic processes that we are just beginning to appreciate. Starting as two cardiac progenitor fields residing in the mesodermal layer of the embryonic disc, the heart begins to shape when these heart fields fuse and fold to form the heart tube. Subsequently, myocardial components are added to both poles of this tube and formation of the functional components of the adult heart, such as the chambers and the cardiac conduction system, is initiated. The molecular and cellular processes that control these morphogenetic processes are emerging and involve multiple gene programs controlled by conserved transcriptional regulators, such as T-box factors, Nkx2-5, and GATA4. In this chapter, we will highlight the morphological changes that the developing heart undergoes before the mature four-chambered heart emerges. Furthermore, we will take a closer look at recent progress that has been made in deciphering the molecular pathways underlying these processes

TGF-β Regulates Collagen Type I Expression in Myoblasts and Myotubes via Transient Ctgf and Fgf-2 Expression

Transforming Growth Factor β (TGF-β) is involved in fibrosis as well as the regulation of muscle mass, and contributes to the progressive pathology of muscle wasting disorders. However, little is known regarding the time-dependent signalling of TGF-β in myoblasts and myotubes, as well as how TGF-β affects collagen type I expression and the phenotypes of these cells. Here, we assessed effects of TGF-β on gene expression in C2C12 myoblasts and myotubes after 1, 3, 9, 24 and 48 h treatment. In myoblasts, various myogenic genes were repressed after 9, 24 and 48 h, while in myotubes only a reduction in Myh3 expression was observed. In both myoblasts and myotubes, TGF-β acutely induced the expression of a subset of genes involved in fibrosis, such as Ctgf and Fgf-2, which was subsequently followed by increased expression of Col1a1. Knockdown of Ctgf and Fgf-2 resulted in a lower Col1a1 expression level. Furthermore, the effects of TGF-β on myogenic and fibrotic gene expression were more pronounced than those of myostatin, and knockdown of TGF-β type I receptor Tgfbr1, but not receptor Acvr1b, resulted in a reduction in Ctgf and Col1a1 expression. These results indicate that, during muscle regeneration, TGF-β induces fibrosis via Tgfbr1 by stimulating the autocrine signalling of Ctgf and Fgf-2

Reduced growth rate of aged muscle stem cells is associated with impaired mechanosensitivity

Aging-associated muscle wasting and impaired regeneration are caused by deficiencies in muscle stem cell (MuSC) number and function. We postulated that aged MuSCs are intrinsically impaired in their responsiveness to omnipresent mechanical cues through alterations in MuSC morphology, mechanical properties, and number of integrins, culminating in impaired proliferative capacity. Here we show that aged MuSCs exhibited significantly lower growth rate and reduced integrin-a7 expression as well as lower number of phospho-paxillin clusters than young MuSCs. Moreover, aged MuSCs were less firmly attached to matrigel-coated glass substrates compared to young MuSCs, as 43% of the cells detached in response to pulsating fluid shear stress (1 Pa). YAP nuclear localization was 59% higher than in young MuSCs, yet YAP target genes Cyr61 and Ctgf were substantially downregulated. When subjected to pulsating fluid shear stress, aged MuSCs exhibited reduced upregulation of proliferation-related genes. Together these results indicate that aged MuSCs exhibit impaired mechanosensitivity and growth potential, accompanied by altered morphology and mechanical properties as well as reduced integrin-a7 expression. Aging-associated impaired muscle regenerative capacity and muscle wasting is likely due to aging-induced intrinsic MuSC alterations and dysfunctional mechanosensitivity

Tbx3 controls the sinoatrial node gene program and imposes pacemaker function on the atria

The sinoatrial node initiates the heartbeat and controls the rate and rhythm of contraction, thus serving as the pacemaker of the heart. Despite the crucial role of the sinoatrial node in heart function, the mechanisms that underlie its specification and formation are not known. Tbx3, a transcriptional repressor required for development of vertebrates, is expressed in the developing conduction system. Here we show that Tbx3 expression delineates the sinoatrial node region, which runs a gene expression program that is distinct from that of the bordering atrial cells. We found lineage segregation of Tbx3-negative atrial and Tbx3-positive sinoatrial node precursor cells as soon as cardiac cells turn on the atrial gene expression program. Tbx3 deficiency resulted in expansion of expression of the atrial gene program into the sinoatrial node domain, and partial loss of sinoatrial node-specific gene expression. Ectopic expression of Tbx3 in mice revealed that Tbx3 represses the atrial phenotype and imposes the pacemaker phenotype on the atria. The mice displayed arrhythmias and developed functional ectopic pacemakers. These data identify a Tbx3-dependent pathway for the specification and formation of the sinoatrial node, and show that Tbx3 regulates the pacemaker gene expression program and phenotype

T-box transcription factor TBX3 reprogrammes mature cardiac myocytes into pacemaker-like cells.

Item does not contain fulltextAIM: Treatment of disorders of the sinus node or the atrioventricular node requires insights into the molecular mechanisms of development and homoeostasis of these pacemaker tissues. In the developing heart, transcription factor TBX3 is required for pacemaker and conduction system development. Here, we explore the role of TBX3 in the adult heart and investigate whether TBX3 is able to reprogramme terminally differentiated working cardiomyocytes into pacemaker cells. METHODS AND RESULTS: TBX3 expression was ectopically induced in cardiomyocytes of adult transgenic mice using tamoxifen. Expression analysis revealed an efficient switch from the working myocardial expression profile to that of the pacemaker myocardium. This included suppression of genes encoding gap junction subunits (Cx40, Cx43), the cardiac Na(+) channel (Na(V)1.5; I(Na)), and inwardly rectifying K(+) ion channels (K(ir) genes; I(K1)). Concordantly, we observed conduction slowing in these hearts and reductions in I(Na) and I(K1) in cardiomyocytes isolated from these hearts. The reduction in I(K1) resulted in a more depolarized maximum diastolic potential, thus enabling spontaneous diastolic depolarization. Neither ectopic pacemaker activity nor pacemaker current I(f) was observed. Lentiviral expression of TBX3 in ventricular cardiomyocytes resulted in conduction slowing and development of heterogeneous phenotypes, including depolarized and spontaneously active cardiomyocytes. CONCLUSIONS: TBX3 reprogrammes terminally differentiated working cardiomyocytes and induces important pacemaker properties. The ability of TBX3 to reduce intercellular coupling to overcome current-to-load mismatch and the ability to reduce I(K1) density to enable diastolic depolarization are promising TBX3 characteristics that may facilitate biological pacemaker formation strategies