Expression of the cholinergic signal-transduction pathway components during embryonic rat heart development

Control of the heart rate of rat embryos during the organogenic period

The aim of this study was to gain insight into whether the first trimester embryo could control its own heart rate (HR) in response to hypoxia. The gestational day 13 rat embryo is a good model for the human embryo at 5–6 weeks gestation, as the heart is comparable in development and, like the human embryo, has no functional autonomic nerve supply at this stage. Utilizing a whole-embryo culture technique, we examined the effects of different pharmacological agents on HR under normoxic (95% oxygen) and hypoxic (20% oxygen) conditions. Oxygen concentrations ≤60% caused a concentration-dependent decrease in HR from normal levels of ~210 bpm. An adenosine agonist, AMP-activated protein kinase (AMPK) activator and K_ATP channel opener all caused bradycardia in normoxic conditions; however, putative antagonists for these systems failed to prevent or ameliorate hypoxia-induced bradycardia. This suggests that the activation of one or more of these systems is not the primary cause of the observed hypoxia-induced bradycardia. Inhibition of oxidative phosphorylation also decreased HR in normoxic conditions, highlighting the importance of ATP levels. The β-blocker metoprolol caused a concentration-dependent reduction in HR supporting reports that β₁-adrenergic receptors are present in the early rat embryonic heart. The cAMP inducer colforsin induced a positive chronotropic effect in both normoxic and hypoxic conditions. Overall, the embryonic HR at this stage of development is responsive to the level of oxygenation, probably as a consequence of its influence on ATP production.

Mouse Model of Human Congenital Heart Disease: Progressive Atrioventricular Block Induced by a Heterozygous Nkx2-5 Homeodomain Missense Mutation

BACKGROUND

Heterozygous human NKX2-5 homeodomain (DNA-binding domain) missense mutations are highly penetrant for varied congenital heart defects, including progressive atrioventricular (AV) block requiring pacemaker implantation. We recently replicated this genetic defect in a murine knockin model, in which we demonstrated highly penetrant, pleiotropic cardiac anomalies. In this study, we examined postnatal AV conduction in the knockin mice.

METHODS AND RESULTS

A murine knockin model (Arg52Gly, Nkx2-5(+/R52G)) in a 129/Sv background was analyzed by histopathology, surface, and telemetry ECG, and in vivo electrophysiology studies, comparing with control Nkx2-5(+/+) mice at diverse postnatal stages, ranging from postnatal day 1 (P1) to 17 months. PR prolongation (first degree AV block) was present at 4 weeks, 7 months, and 17 months of age, but not at P1 in the mutant mice. Advanced AV block was also occasionally demonstrated in the mutant mice. Electrophysiology studies showed that AV nodal function and right ventricular effective refractory period were impaired in the mutant mice, whereas sinus nodal function was not affected. AV nodal size was significantly smaller in the mutant mice than their controls at 4 weeks of age, corresponding to the presence of PR prolongation, but not P1, suggesting, at least in part, that the conduction abnormalities are the result of a morphologically atrophic AV node.

CONCLUSIONS

The highly penetrant and progressive AV block phenotype seen in human heterozygous missense mutations in NKX2-5 homeodomain was replicated in mice by knocking in a comparable missense mutation.

Caffeine acts via A1 adenosine receptors to disrupt embryonic cardiac function

Background

Evidence suggests that adenosine acts via cardiac A1 adenosine receptors (A1ARs) to protect embryos against hypoxia. During embryogenesis, A1ARs are the dominant regulator of heart rate, and A1AR activation reduces heart rate. Adenosine action is inhibited by caffeine, which is widely consumed during pregnancy. In this study, we tested the hypothesis that caffeine influences developing embryos by altering cardiac function.

Methodology/Principal Findings

Effects of caffeine and adenosine receptor-selective antagonists on heart rate were studied in vitro using whole murine embryos at E9.5 and isolated hearts at E12.5. Embryos were examined in room air (21% O₂) or hypoxic (2% O₂) conditions. Hypoxia decreased heart rates of E9.5 embryos by 15.8% and in E12.5 isolated hearts by 27.1%. In room air, caffeine (200 µM) had no effect on E9.5 heart rates; however, caffeine increased heart rates at E12.5 by 37.7%. Caffeine abolished hypoxia-mediated bradycardia at E9.5 and blunted hypoxia-mediated bradycardia at E12.5. Real-time PCR analysis of RNA from isolated E9.5 and E12.5 hearts showed that A1AR and A2aAR genes were expressed at both ages. Treatment with adenosine receptor-selective antagonists revealed that SCH-58261 (A2aAR-specific antagonist) had no affects on heart function, whereas DPCPX (A1AR-specific antagonist) had effects similar to caffeine treatment at E9.5 and E12.5. At E12.5, embryonic hearts lacking A1AR expression (A1AR−/−) had elevated heart rates compared to A1AR+/− littermates, A1AR−/− heart rates failed to decrease to levels comparable to those of controls. Caffeine did not significantly affect heart rates of A1AR−/− embryos.

Conclusions/Significance

These data show that caffeine alters embryonic cardiac function and disrupts the normal cardiac response to hypoxia through blockade of A1AR action. Our results raise concern for caffeine exposure during embryogenesis, particularly in pregnancies with increased risk of embryonic hypoxia.

Ablation of Nkx2-5 at mid-embryonic stage results in premature lethality and cardiac malformation

AIMS

Human congenital heart disease linked to mutations in the homeobox transcription factor, NKX2-5, is characterized by cardiac anomalies, including atrial and ventricular septal defects as well as conduction and occasional defects in contractility. In the mouse, homozygous germline deletion of Nkx2-5 gene results in death around E10.5. It is, however, not established whether Nkx2-5 is necessary for cardiac development beyond this embryonic stage. Because human NKX2-5 mutations are related to septum secundum type atrial septal defects (ASD), we hypothesized that Nkx2-5 deficiency during the processes of septum secundum formation may cause cardiac anomalies; thus, we analysed mice with tamoxifen-inducible Nkx2-5 ablation beginning at E12.5 when the septum secundum starts to develop.

METHODS AND RESULTS

Using tamoxifen-inducible Nkx2-5 gene-targeted mice, this study demonstrates that Nkx2-5 ablation beginning at E12.5 results in embryonic death by E17.5. Analysis of mutant embryos at E16.5 shows arrhythmias, contraction defects, and cardiac malformations, including ASD. Quantitative measurements using serial section histology and three-dimensional reconstruction demonstrate growth retardation of the septum secundum and enlarged foramen ovale in Nkx2-5-ablated embryos. Functional cardiac defects may be attributed to abnormal expression of transcripts critical for conduction and contraction, including cardiac voltage-gated Na(+) channel pore-forming α-subunit (Na(v)1.5-α), gap junction protein connexin40, cardiac myosin light chain kinase, and sarcolipin within 4 days after tamoxifen injection.

CONCLUSION

Nkx2-5 is necessary for survival after the mid-embryonic stage for cardiac function and formation by regulating the expression of its downstream target genes.

Developmental changes in the expression of nicotinic acetylcholine receptor alpha-subunits in the rat heart

Neuronal nicotinic acetylcholine receptors (nAChR) are ligand-gated ion channels that consist of various subunits. During ontogeny, muscular and neuronal nAChR undergo changes in the distribution and subunit composition in skeletal muscle and brain, respectively. Here, we have investigated the occurrence of the ligand-binding alpha-subunits of neuronal nAChR by means of reverse transcription/polymerase chain reaction and immunohistochemistry in the rat heart during prenatal and postnatal development and after capsaicin-induced sensory denervation. mRNAs coding for the alpha4, alpha5, alpha7 and alpha10 subunits were detected throughout all developmental stages. Messenger coding for the alpha2 subunit was first detectable at developmental stage E20; alpha3 subunit mRNA was expressed throughout all prenatal developmental stages, whereas it was restricted postnatally to the atria. mRNA for alpha6 was observed at E14-P8 but was absent thereafter. At no developmental stage could an unequivocal signal for alpha9 nAChR subunit mRNA be obtained. The expression pattern was unchanged by capsaicin treatment. Immunohistochemistry demonstrated alpha7 subunits on cardiac neurons, fibroblasts and cardiomyocytes and alpha2/4 subunits on cardiomyocytes with a postnatal redistribution to intercalated discs, as shown by cryo-immunoelectron microscopy. Our results indicate an additional non-neuronal expression of nAChR subunits in the rat heart that, as in skeletal muscle, precedes functional innervation and then undergoes changes in its distribution on the surface of cells.

Acetylcholinesterase function is dispensable for sensory neurite growth but is critical for neuromuscular synapse stability

The enzyme acetylcholinesterase (AChE) terminates synaptic transmission at cholinergic synapses by hydrolyzing the neurotransmitter acetylcholine. In addition, AChE is thought to play several 'non-classical' roles that do not require catalytic function. Most prominent among these is facilitation of neurite growth. Here, we report that the zebrafish zieharmonika (zim) locus encodes AChE. We show that one mutant zim allele is caused by a pre-mature stop codon, resulting in a truncated protein that lacks both the catalytic site and the carboxy-terminal neuritogenic domain. To explore the 'non-classical' role of AChE, we examined embryos mutant for this allele. In contrast to previous results using a catalytic-inactive allele, our analysis demonstrates that AChE is dispensable for muscle fiber development and Rohon-Beard sensory neuron growth and survival. Moreover, we show that in the absence of AChE, acetylcholine receptor clusters at neuromuscular junctions initially assemble, but that these clusters are not maintained. Taken together, our results demonstrate that AChE is dispensable for its proposed non-classical roles in muscle fiber formation and sensory neuron development, but is crucial for regulating the stability of neuromuscular synapses.

Functional M3 muscarinic acetylcholine receptors in mammalian hearts

In contrast to most peripheral tissues where multiple subtypes of muscarinic acetylcholine receptor (mAChR) coexist, with each of them playing its part in the orchestra of parasympathetic innervation, the myocardium has been traditionally considered to possess a single mAChR subtype. Although there is much evidence to support the notion that one receptor subtype (M2) orchestrates myocardial muscarinic transduction, there is emerging evidence that M1 and M3 receptors are also expressed and are of potential physiological, pathophysiological and pharmacological relevance. Clarifying this issue has a profound impact on our thinking about the cholinergic control of the heart function and disease and approaches to new drug development for the treatment of heart disease associated with parasympathetic dysfunction. This review article presents evidence for the presence of the M3 receptor subtype in the heart, and analyzes the controversial data from published pharmacological, functional and molecular studies. The potential roles of the M3 receptors, in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias, are discussed. On the basis of these considerations, we have made some proposals concerning the future of myocardial M3 receptor research.

Ontogeny of humoral heart rate regulation in the embryonic mouse

Catecholamines, acetylcholine, and adenosine are known to influence cardiac function, yet the effects of these agents on mammalian embryonic myocardium are largely unknown. To address this issue, we compared the chronotrophic effects of adenosinergic, adrenergic, and muscarinic agents on cultured murine embryos from postcoital day (PC) 8.0, when the fusing heart tubes first begin to beat, to PC 14, when cardiogenesis is essentially complete. At PC 8.0 and older, A(1)-adenosine receptor (A(1)AR) activation significantly decreased heart rates. Adrenergic stimulation caused modest increases in heart rates (145-155% of baseline) beginning at PC 9.0. Muscarinic activation decreased heart rates only after PC 13. When receptor gene expression was examined, A(1)ARs and beta(1)ARs were expressed in isolated hearts as early as PC 9.0, and beta(2)ARs and m(2)-muscarinic receptor genes were expressed at PC 11.0. These results identify the adenosinergic system as the earliest and most potent regulator of embryonic cardiac function and show that prenatal responsiveness to catecholamines and acetylcholine develops at later embryonic stages.

Characterization of acetylcholinesterase and butyrylcholinesterase forms in normal and dystrophic Lama2dy mouse heart

In searching for possible differences in acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) forms of dystrophic heart, the properties of ChE species in normal (NH) and dystrophic Lama2dy mouse heart (DH) were investigated. BuChE predominated over AChE. Loosely- and tightly-bound ChEs were released with saline (extract S1) and saline-Triton X-100 buffers (S2). About 50% of AChE, and 25% of BuChE, in NH or DH was measured in S1, and the rest in S2. Asymmetric AChE forms A12 (15%) and A8 (11%), globular hydrophilic G(H)4 (8%), amphiphilic G(A)4 (15%), and G(A)2+G(A)1 (51%) AChE species, and BuChE forms G(H)4 (13%), G(A)4 (3%), and G(A)2+G(A)1 (84%) were identified in NH and DH. Most of the asymmetric and G(A)4 AChE species were bound to Triticum vulgaris (WGA) or Ricinus communis (RCA) agglutinins. About half of G(H)4 and G(A)2+G(A)1 AChE were bound to WGA, and less (10%) to RCA. Variable amounts of G(H)4+G(A)4 (60%), and G(A)2+G(A)1 (75%) BuChE bound to WGA, and 50 and 10% to RCA. The lack of structural differences between ChE species in NH and DH indicates that, in contrast to the ChE forms in mouse skeletal muscle, the biosynthesis of ChE components in heart is not disturbed by dystrophy.