Induction of Purkinje fiber differentiation by coronary arterialization

Abstract

A synchronized heart beat is controlled by pacemaking impulses conducted through Purkinje fibers. In chicks, these impulse-conducting cells are recruited during embryogenesis from myocytes in direct association with developing coronary arteries. In culture, the vascular cytokine endothelin converts embryonic myocytes to Purkinje cells, implying that selection of conduction phenotype may be mediated by an instructive cue from arteries. To investigate this hypothesis, coronary arterial development in the chicken embryo was either inhibited by neural crest ablation or activated by ectopic expression of fibroblast growth factor (FGF). Ablation of cardiac neural crest resulted in Ϸ70% reductions (P < 0.01) in the density of intramural coronary arteries and associated Purkinje fibers. Activation of coronary arterial branching was induced by retrovirus-mediated overexpression of FGF. At sites of FGF-induced hypervascularization, ectopic Purkinje fibers differentiated adjacent to newly induced coronary arteries. Our data indicate the necessity and sufficiency of developing arterial bed for converting a juxtaposed myocyte into a Purkinje fiber cell and provide evidence for an inductive function for arteriogenesis in heart development distinct from its role in establishing coronary blood circulation.heart development ͉ cardiac conduction system ͉ coronary artery ͉ retrovirus ͉ neural crest T he contractive rhythm of the vertebrate heart depends on specialized tissue components involved in the generation and conductive spread of electrical excitation (1-3). The dominant pacemaking site in the heart is the sinuatrial node. From this focus of automaticity, activation is spread from cell to cell through atrial myocardium, eventually focusing into the atrioventricular node, where impulse propagation is delayed briefly before ventricular activation. After exit from the node, the propagating action potential accelerates along the atrioventricular bundle and its branched limbs, finally spreading into working ventricular muscle via a peripheral network of Purkinje fibers. Current understanding of the mechanisms regulating development of the cardiac pacemaking and conduction system is rudimentary (4-7). Fundamental issues, including whether common or different processes govern development of its disparate elements, remain unsettled. Our work in avian hearts has provided some progress on these questions, particularly in relation to the origin and differentiation of Purkinje fibers constituting the peripheral conduction system (4,6,8). By using replication-defective retroviruses, it was demonstrated that periarterial Purkinje fibers share common cellular progenitors with working myocytes (8). Of particular significance, this recruitment occurred in a site-specific pattern, with only those embryonic myocytes in direct proximity to coronary arterial vessels undergoing further differentiation into Purkinje fibers. Furthermore, we have recently shown (9) that embryonic myocytes in culture can be induced to convert to a Purkinje cell phen...

Elevated expression of Nkx‐2.5 in developing myocardial conduction cells

“…Although apparently elevated expression might represent local maintenance of a higher steady-state level of Nkx-2.5 mRNA from earlier stages of development, this cannot be the case for these newly recruited cells, especially the periarterial Purkinje fibres. These arise in the chick embryo from a previously lowexpressing population following in-growth of coronary arterial progenitors (Gittenberger-de Groot et al, 1998;Hyer et al, 1999). We propose our results support the hypothesis of a role for Nkx-2.5 in the regulation of selection and/or maintenance of specialized fate within the cardiomyogenic lineage.…”

Epicardium‐derived cells are important for correct development of the Purkinje fibers in the avian heart

“…The recruitment of cardiomyocytes into the Purkinje fiber system has been demonstrated to be related to hemodynamic influences derived from the coronary vasculature and endocardial lining of the ventricular lumen Hyer et al, 1999), with mature ET-1 and the ECE1 metalloprotease as key inductive factors (Takebayashi-Suzuki et al, 2000;Reckova et al, 2003;Hall et al, 2004). Earlier work demonstrated dramatic effects on the development of cardiac architecture and coronary vasculature when epicardial differentiation was disturbed (Gittenberger-de Groot et al, 2000;Tevosian et al, 2000;Lie-Venema et al, 2003;Eralp et al, 2005).…”

“…Retroviral tagging experiments showed that the Purkinje fibers of the peripheral conduction system differentiate from cardiomyocytes, in close spatiotemporal relation to the developing coronary vasculature Hyer et al, 1999), whereas the subendocardial Purkinje fibers develop in the proximity of endocardial cells. These unique differentiation sites suggested an inductive role both for periarterial and subendocardial EPDCs and for a paracrine signal from the endocardium and arterial beds in the recruitment of cardiomyocytes into the Purkinje fiber network (Gittenberger-de Groot et al, 2003b;Gourdie et al, 2003).…”

Iroquois homeobox gene 3 establishes fast conduction in the cardiac His–Purkinje network

“…At the cellular level, efficient impulse propagation through this network is dependent upon the interplay between cell morphology, membrane excitability, and electrical coupling of adjacent cells via gap junctions, the latter of which is the major determinant for rapid and directional conduction (3). Although regulation of early VCS specification and morphogenesis is becoming well understood (4)(5)(6)(7)(8)(9), less is known about how cells of the VCS gain their specialized conduction properties as they mature.…”