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Endocrinology 144(6):2191–2194 Copyright © 2003 by The Endocrine Society doi: 10.1210/en.2003-0127
Minireview: Natriuretic Peptides during Development of the Fetal Heart and Circulation VICKY A. CAMERON
AND
LEIGH J. ELLMERS
Christchurch Cardioendocrine Research Group, Department of Medicine, Christchurch School of Medicine and Health Sciences, Christchurch 8001, New Zealand Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones, secreted by the atria and ventricles, respectively, in the normal adult heart. They participate in the regulation of blood pressure and body fluid homeostasis and modify growth and development of cardiovascular tissues and bone. Levels of ANP are higher in the fetal circulation than in adults, and fetal ventricles express higher levels of ANP and BNP than adult ventricles. The reappearance of ventricular ANP expression in adults is recognized as a marker of the induction of the embryonic gene program in ventricular hypertrophy. The natriuretic peptide system appears to be functional by midgestation, to respond to volume stimuli, and to regulate blood pressure and salt and
T
HE HEART WAS established as an endocrine organ 22 yr ago when de Bold et al. (1) demonstrated that extracts of atrial tissue injected into rats elicited a potent natriuresis in recipient animals. Three years later, this atrial factor was purified, sequenced, and designated human atrial natriuretic polypeptide (2). Subsequently, a family of natriuretic peptides has been identified that regulate blood pressure and body fluid homeostasis through their diuretic, natriuretic, and vasorelaxant effects (3, 4). In addition, these peptides have multiple roles as antiproliferative factors in cardiovascular tissues, neuropeptides in the central nervous system, and paracrine regulators of bone growth. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones, predominantly secreted into the circulation by the atria and ventricles, respectively, in the normal adult heart. The primary acute stimulus for release of ANP and BNP from the heart is increased stretch of the myocardium (3). Although secretion of ANP is influenced by numerous stimuli, including volume-expanded states (hypertension and salt-loading), supine posture, tachycardia, exercise, hypoxia, myocardial ischemia, and vasoconstrictor hormones and drugs, many of these stimuli may in fact be mediated by atrial stretch (4, 5). Both ANP and BNP exert their biological actions by binding to the natriuretic peptide receptor (NPR)-A, resulting in the generation of the second messenger cGMP. A third member of the family, c-type natriuretic peptide (CNP), is expressed primarily in the brain, pituitary, vascular endothelium, kidney (6), and female reproductive tract (7). In contrast to ANP and BNP, very little CNP is produced by cardiac tissue. A second Abbreviations: ANP, Atrial natriuretic peptide; BNP, brain natriuretic peptide; CNP, c-type natriuretic peptide; NPR, natriuretic peptide receptor.
water balance in the developing embryo. In addition, the natriuretic peptides may help regulate the blood supply to the fetus, acting as vasodilators in the placental vasculature. Peaks of ANP and BNP expression during gestation coincide with significant events in cardiac organogenesis, suggesting a role for ANP/BNP in the formation of the heart. In knockout mice lacking the natriuretic peptide receptor (NPR)-A gene (Npr1⫺/⫺), survival is reduced, with hearts enlarged at birth and possible cardiac developmental abnormalities. Surviving adult Npr1⫺/⫺ mice have elevated blood pressure and marked cardiac hypertrophy and fibrosis, indicating that the ANP/ BNP system is an important regulator of myocyte growth during development. (Endocrinology 144: 2191–2194, 2003)
guanylyl cyclase-linked receptor, NPR-B, binds CNP with high affinity (6), and, although CNP also has vasodilatory effects, concentrations of CNP in the circulation are very low. It is thought that CNP acts as a local, paracrine hormone in a number of tissues, including blood vessels, lung, brain, and bone (6). Expression of Natriuretic Peptides in Fetal Heart
In the adult heart, the atrium is the major site of expression of ANP mRNA, with levels in the left ventricle being approximately 0.5–3% those of the atria (5). However, during development, many studies in rodents report higher levels in ventricles than atria both of ANP mRNA expression (8, 9) and of ANP peptide (10). By contrast, in fetal sheep, both ANP mRNA and ANP peptide are greater in atria than ventricles throughout gestation (11, 12). However, in both human and ovine fetal ventricles, ANP mRNA is considerably higher than in adult ventricles and tends to decrease with gestational age (11, 13, 14). Peptide levels of ANP and BNP in fetal ventricles have also been reported to be greater than in the adult ventricle (10, 15, 16). However, this may partly result from a greater storage capacity for ANP in atrial granules than in the ventricle, where secretion is primarily constitutive with a more rapid transit time (17). No CNP expression has been detected in developing hearts of either mouse or human embryos (8, 13). In general, these studies indicate that the relative contribution of ventricular ANP is significantly greater in the embryo than in the adult, and in some species the ventricle is the predominant site of ANP and BNP expression during development. During the molecular processes underlying cardiac hypertrophy, numerous genes normally expressed during development are reactivated, including ventricular ANP (18, 19). Cardiac gene
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expression of both ANP and BNP is increased in animal models of myocardial infarction (20), heart failure (21, 22), and hypertrophy (23), and also in human heart disease (24, 25). Consequently, the appearance of increased ANP expression in adult ventricles has become established as a marker for the induction of the embryonic gene program during the development of ventricular hypertrophy (18). Daily observations of natriuretic peptide expression during embryogenesis in mice demonstrate that ANP and BNP mRNA appear at around 8 –9 d gestation with a large peak in mRNA levels at 12.5 d and smaller peaks at 14.5 and 16.5 d post coitum (8). Each one of these stages coincides with a landmark in the development of the embryonic heart. Day 9 coincides with the start of regular beating of the primitive heart, d 12 marks progressive septal formation to construct the four heart chambers, and at around d 15 the alteration of the heart axis takes place (26). This pattern of expression is consistent with a role for ANP/BNP in the formation of the developing heart. Regulation of Natriuretic Peptide Secretion during Development
During development, levels of ANP in fetal plasma have been shown to be significantly higher than levels in maternal plasma in rats (10) and in sheep (27). These higher circulating ANP levels do not result either from slower removal by the developing kidney (10, 28), or by transfer from maternal circulation (27, 29), and appear to result from a high rate of secretion from the developing heart. The cardiac natriuretic peptides are secreted in response to similar physiological stimuli during development as in the adult animal. Some excellent studies in fetal rats and sheep have demonstrated that ANP secretion is stimulated in response to volume loading (30, 31); hyperosmolality (30); hypoxia (32); and to vasoconstrictors such as angiotensin II and phenylephrine (33), vasopressin (10), and endothelin (34). Cardiac ANP and BNP were also increased in fetal rats with diabetes induced by streptozotocin (35). The high levels of ANP and BNP expression observed in the fetal heart, and their ability to respond to volume stimuli, suggest that these peptide systems are functional by midgestation and may be as important in regulating blood pressure and salt and water balance in the developing embryo as they are in the adult (Fig. 1). Natriuretic Peptides in the Placenta
Expression of BNP and CNP has been demonstrated within the decidua of the mouse placenta (8), and expression of ANP has been demonstrated in the human placenta, particularly by cytotrophoblast cells (36, 37). Furthermore, ANP or BNP administered into the fetal placental circulation have been shown to inhibit the effects of vasoconstrictor agents (38, 39). These studies suggest that the natriuretic peptides act as vasodilators in the fetal-placental vasculature and may help regulate the blood supply to the fetus. Role of Natriuretic Peptides in the Developing Cardiovascular System
Infusion of ANP into the circulation of fetal sheep, albeit at supraphysiological concentrations (increasing plasma lev-
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els from 163 to ⬎2000 pg/ml), decreased arterial blood pressure and elicited diuresis but not natriuresis (30). Consistent with a role in volume regulation in the fetus, specific binding of radiolabeled ANP has been reported in glomeruli of the fetal rat kidney, as well as in ileum, adrenal, lung, liver, skin (15), somites, and brain (40). In addition to volume regulation, ANP modulates cell growth and proliferation, including inhibition of proliferation of vascular smooth muscle cells (41), inhibition of hypertrophy (42), and induction of apoptosis in cultured cardiac myocytes (43). All three natriuretic peptides (ANP, BNP, and CNP) suppress cardiac fibroblast growth (44). This raises the possibility that natriuretic peptides may participate in cardiac organogenesis of the embryo heart and cardiovascular system. Further support for a role of natriuretic peptides in regulating cardiac growth is provided by the phenotype of the NPR-A knockout (Npr1⫺/⫺) mouse (45). These Npr1⫺/⫺ mice lack the receptor that mediates the biological actions of both ANP and BNP and have elevated blood pressure and marked cardiac hypertrophy and fibrosis (45, 46). Cardiac ANP and BNP expression is greatly elevated in Npr1⫺/⫺ mice and correlates with the increased heart weight (47). The cardiac hypertrophy observed in Npr1⫺/⫺ mice is disproportionate to their mildly increased blood pressure and was still evident when the blood pressure of Npr1⫺/⫺ mice was maintained within the normal range by chronic treatment with antihypertensive drugs (46). These findings strongly suggest that the NPR-A pathway directly modulates the hypertrophic response, independent of blood pressure. Significantly, the Npr1⫺/⫺ mice had enlarged hearts at birth, with Npr1⫺/⫺ neonates having heart-weight to body-weight ratios 140% greater than wild-type littermates (46), suggesting that the ANP/BNP system is an important regulator of myocyte growth during development. Survival of Npr1⫺/⫺ mice generated by the Garbers group (48) was reported to be reduced at 3 wk of age, due in part to fetal hydrops. We have established a colony of Npr1⫺/⫺ mice originally generated by the Smithies/Maeda group (45) and backcrossed at least six generations to C57BL/6 mice. Significantly fewer than expected of these Npr1⫺/⫺ mice survive to weaning (17% Npr1⫺/⫺, 58% heterozygous Npr1⫺/⫹, 25% wild-type Npr⫹/⫹ mice; n ⫽ 220; P ⫽ 0.016). Many of the neonates examined have heart abnormalities, with mesocardia or dextrocardia. These abnormalities have not been observed in wild-type littermates, implying that they result from the lack of NPR-A gene function. Knockout mice in which either the ANP (49) or BNP (50) gene is disrupted show no such changes in size or structure in the hearts of neonates. Therefore, it would appear that, if either of the ANP or BNP genes are disrupted, the other can compensate and no effect on cardiac organogenesis occurs. However, if the signaling pathway for both ANP and BNP is disrupted as in the Npr1⫺/⫺ mice, cardiac hypertrophy results, and the hearts become enlarged during development. It should be noted that BNP-overexpressing mice (51) and natriuretic peptide clearance receptor (NPR-C)-knockout mice (52) show skeletal overgrowth, whereas CNP-knockout mice show dwarfism and bone malformation (53). Fetal bone growth is regulated by CNP, via stimulation of chondrocyte
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FIG. 1. Diagram comparing the natriuretic peptide system in embryos (left) and adults (right). The upper shaded boxes show the physiological stimuli known to regulate natriuretic peptide secretion from adult and embryo hearts, and the lower shaded boxes show the major actions ascribed to these peptides.
proliferation and cartilage matrix production (54). Thus, the natriuretic peptides have effects on the development of other tissues besides the heart. Further research will be needed before we understand fully all the roles the natriuretic peptides during embryogenesis.
Address all correspondence and requests for reprints to: Dr. V. A. Cameron, Ph.D., Department of Medicine, Christchurch School of Medicine and Health Sciences, P.O. Box 4345, Christchurch 8001, New Zealand. E-mail:
[email protected]. This work is funded by a Program Grant from the Health Research Council of New Zealand.
Acknowledgments
References
The authors thank Professors Oliver Smithies and Nobuyo Maeda for the generous gift of Npr-1 knockout mice. We also thank Ms. Nicola Scott for performing mouse genotyping assays.
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Received January 27, 2003. Accepted February 21, 2003.
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