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MINIREVIEW
The functionalgenomicsofguanylyl cyclase
⁄
natriuretic
peptide receptor-A:Perspectivesand paradigms
Kailash N. Pandey
Department of Physiology, Tulane University Health Sciences Center School of Medicine, New Orleans, LA, USA
Introduction
The initial work by de Bold et al. [1] established that
atrial extracts contained natriuretic and diuretic
activities, and demonstrated the existence of atrial natri-
uretic factor ⁄ atrial natriuretic peptide (ANP). Members
of a family of endogenous peptide hormones including
atrial natriuretic factor ⁄ ANP, B-type natriuretic peptide
(brain natriuretic peptide) (BNP), C-type natriuretic
peptide (CNP) and urodilatin are considered to play an
integral role in hypertension and cardiovascular
regulation via their ability to mediate excretion of
sodium and water, reduce blood volume, and elicit a
vasorelaxation effect [2–5]. Interestingly, the natriuretic
peptide hormones have been suggested not only to
regulate blood pressure but also to play a role in a
number of additional processes, namely: antimitogenic
effects, inhibition of myocardial hypertrophy, endothe-
lial cell function, cartilage growth, immunity, and
mitochondrial biogenesis [6–9]. ANP and BNP are also
increasingly being utilized to screen and diagnose car-
diac etiologies for shortness of breath and congestive
heart failure (CHF) in emergency situations [10].
One ofthe principal loci involved in the regulatory
action of ANP and BNP is that encoding the receptor
guanylyl cyclase (GC)-A, designated GC-A ⁄ natriuretic
peptide receptor-A (NPRA). Interaction of ANP and
BNP with GC-A ⁄ NPRA produces the intracellular
Keywords
cardiac hypertrophy; functional genomics;
guanylyl cyclase receptor; hypertension;
natriuretic peptides
Correspondence
K. N. Pandey, Department of Physiology,
SL 39, Tulane University Health Sciences
Center, 1430 Tulane Avenue, New Orleans,
LA 70112, USA
Fax: +1 504 9882675
Tel: +1 504 988 1628
E-mail: kpandey@tulane.edu
(Received 2 September 2010, revised 7
December 2010, accepted 2 March 2011)
doi:10.1111/j.1742-4658.2011.08081.x
The cardiac hormones atrial natriuretic peptideand B-type natriuretic pep-
tide (brain natriuretic peptide) activate guanylyl cyclase (GC)-A ⁄ natriuretic
peptide receptor-A (NPRA) and produce the second messenger cGMP.
GC-A ⁄ NPRA is a member ofthe growing family of GC receptors. The
recent biochemical, molecular and genomic studies on GC-A ⁄ NPRA have
provided important insights into the regulation andfunctional activity of
this receptor protein, with a particular emphasis on cardiac and renal pro-
tective roles in hypertension and cardiovascular disease states. The progress
in this field of research has significantly strengthened and advanced our
knowledge about the critical roles of Npr1 (coding for GC-A ⁄ NPRA) in
the control of fluid volume, blood pressure, cardiac remodeling, and other
physiological functions and pathological states. Overall, this review
attempts to provide insights and to delineate the current concepts in the
field offunctionalgenomicsand signaling of GC-A ⁄ NPRA in hypertension
and cardiovascular disease states at the molecular level.
Abbreviations
BNP, B-type natriuretic peptide; CHF, congestive heart failure; CNP, C-type natriuretic peptide; GC, guanylyl cyclase; GCD, guanylyl cyclase
catalytic domain; IP
3,
inositol trisphosphate; KHD, protein kinase-like homology domain; LVH, left ventricular hypertrophy; MAPK, mitogen-
activated protein kinase; NPRA, natriuretic peptide receptor-A; NPRB, natriuretic peptide receptor-B; NPRC, natriuretic peptide receptor-C;
PDE, cGMP-dependent phosphodiesterase; PKG, cGMP-dependent protein kinase; RAA, renin–angiotensin–aldosterone; VSMC, vascular
smooth muscle cell.
1792 FEBS Journal 278 (2011) 1792–1807 ª 2011 The Author Journal compilation ª 2011 FEBS
second messenger cGMP, which plays a central role in
the pathophysiology of hypertension and cardiovascu-
lar disorders [5,11,12]. Gaining insights into the intrica-
cies of ANP–NPRA signaling is of pivotal importance
for understanding both receptor biology andthe disease
state arising from abnormal hormone–receptor interac-
tions. It has been postulated that the binding of ANP
to the extracellular domain ofthe receptor causes a
conformational change, thereby transmitting the signal
to the GC catalytic domain (GCD); however, the exact
mechanism of receptor activation remains unknown.
Recent studies have focused on elucidating, at the
molecular level, the nature and mode of functioning of
GC-A ⁄ NPRA. Both cultured cells in vitro and gene-
targeted mouse models in vivo have been utilized to
gain a better understanding ofthe normal and abnor-
mal control of cellular and physiological processes.
Although there has been much appreciation of the
functional roles of natriuretic peptides and their cog-
nate receptors in renal, cardiovascular, endocrine and
skeletal homeostasis; in-depth research studies are still
needed to fully understand their potential molecular
targets in cardiovascular and other disease states.
Ultimately, it is expected that studies on the natriuretic
peptides and their receptors should yield new therapeu-
tic targets and novel loci for the control and treatment
of hypertension and cardiovascular disorders.
Natriuretic peptide hormone family
ANP is the first described member ofthe natriuretic
peptide hormone family. It is primarily synthesized in
the heart atria, and elicits natriuretic, diuretic and vaso-
relaxant effects, largely directed to the reduction of
fluid volume and blood pressure [2,3,5,7,13,14]. Subse-
quently, BNP and CNP, with biochemical and func-
tional characteristics similar to those of ANP but
derived from separate genes, were identified [15]. BNP
was initially isolated from the brain; however, it is pri-
marily synthesized in the heart, circulates in the
plasma, and displays the most variability in primary
structure. CNP is mainly present in endothelial cells,
and is highly conserved across species. All three
types of natriuretic peptide contain a highly conserved
17-residue disulfide ring, which is essential for the hor-
monal activities, but they show differences from each
other in the N-terminal and C-terminal flanking
sequences (Fig. 1). Although ANP has been considered
to exert its predominant effects in lowering blood
pressure and blood volume, recent evidence indicates
that ANP plays a critical role in preventing cardiac
load and overgrowth ofthe heart in pathological con-
ditions.
Both ANP and BNP are predominantly synthesized
in the heart; ANP levels vary from 50-fold to 100-fold
higher than those of BNP. After processing ofthe 151-
residue preprohormone to the 126-residue prohor-
mone, the secretion of proANP is believed to occur
predominantly in response to atrial distension [14].
Upon secretion, the cleavage of proANP to generate
the active and mature 28-residue ANP molecule is cat-
alyzed by a serine protease, corin [16]. The synthesis
and release of ANP from the heart is enhanced in
response to various agents and settings, such as argi-
nine–vasopressin, endothelin, and vagal stimuli [14,17].
BNP is synthesized as a 134-residue preprohormone,
which yields a 108-residue prohormone. Processing of
the proBNP yields a 75-residue N-terminal BNP and a
32-residue biologically active circulating BNP [18,19].
The atria are the primary sites of synthesis for both
hormones within the heart. Although the ventricles
also produce both ANP and BNP, the concentrations
are 100-fold to 1000-fold less than those in the atria.
The expression of both ANP and BNP increases dra-
matically in both the atria and ventricles in cardiac
hypertrophy [20,21]. It is believed that, in the ventri-
cles, BNP synthesis is regulated by volume overload,
which activates ventricular wall stretch, subsequently
enhancing hormone synthesis at the transcriptional
level [22,23]. Interestingly, higher levels of ventricular
ANP are present in the developing embryo and fetus,
with both mRNA andpeptide levels of ANP declining
rapidly during the prenatal period [24].
CNP is mainly present in the central nervous system
[25], vascular endothelial cells [26], and chondrocytes
[27]. CNP is synthesized as a 103-residue prohormone,
cleaved to a 53-residue peptide by the protease furin,
Fig. 1. Comparison of amino acid sequences ofthe natriuretic pep-
tide hormone family. Comparison of amino acid sequences of
human ANP, BNP and CNP with conserved amino acids, which are
represented by red boxes. The lines between two cysteines in
ANP, BNP and CNP indicate a 17-residue disulfide bridge, which
seems to be essential for the biological activity of these peptide
hormones.
K. N. Pandey Update on functional aspects of GC-A ⁄ NPRA
FEBS Journal 278 (2011) 1792–1807 ª 2011 The Author Journal compilation ª 2011 FEBS 1793
and subsequently processed to yield the biologically
active 22-residue molecule [28]. In addition, a 32-resi-
due peptide termed urodilatin, which is identical to the
C-terminal sequence of proANP, is known to be pres-
ent in urine [29,30]. Urodilatin is not detected in the
circulation, and appears to be a unique intrarenal
natriuretic peptide with unexplored physiological func-
tions [31]. D-type natriuretic peptide is an additional
member ofthe natriuretic peptide hormone family [32].
DNP is present in the venom ofthe green mamba
(Dendroaspis angusticeps) as a 38-residue peptide.
GC
⁄
Natriuretic peptide receptor family
Natriuretic peptides (ANP, BNP, and CNP) bind and
activate specific cognate receptors present on the
plasma membranes of a wide variety of target cells.
Membrane-bound forms of natriuretic peptide recep-
tors have been cloned and sequenced from rat brain
[33,34], human placenta [35], and mouse testis [36].
Molecular cloning and expression of cDNAs have
identified three different forms of natriuretic peptide
receptor, including NPRA, natriuretic peptide recep-
tor-B (NPRB), and natriuretic peptide receptor-
C (NPRC). These constitute the natriuretic peptide
receptor family; however, they show variability in
terms of their ligand specificity and signal transduction
activity. Two of these receptors contain intrinsic GC
activity, and have been designated GC-A ⁄ NPRA and
GC-B ⁄ NPRB; they are also referred to as GC-A and
GC-B, respectively [37–39]. NPRC lacks the GCD,
and has been termed a natriuretic peptide clearance
receptor; it contains a short (37-residue) cytoplasmic
tail, apparently not coupled to GC activation [40].
Both ANP and BNP selectively stimulate NPRA,
whereas CNP primarily activates NPRB, and all three
natriuretic peptides indiscriminately bind to NPRC
[26,39,41]. NPRA is a 135-kDa transmembrane pro-
tein, and ligand binding to the receptor generates the
second messenger cGMP. It has been suggested that
ANP binding to its receptor in vivo requires chloride,
which could exert a chloride-dependent feedback-con-
trol effect on receptor function [42]. The general topo-
logical structure of NPRA is consistent with that seen
in the GC receptor family, containing at least four dis-
tinct regions: an extracellular ligand-binding domain, a
single transmembrane-spanning region, an intracellular
protein kinase-like homology domain (KHD), and a
GCD [36,37]. NPRB has an overall domain structure
similar to that of NPRA, with binding selectivity for
CNP [43]. GC-A ⁄ NPRA is the dominant form of
natriuretic peptide receptor found in peripheral organs,
and mediates most ofthe known actions of ANP and
BNP. By the use of a homology-based cDNA library
screening system, additional members ofthe GC recep-
tor family have also been identified; however, their
specific ligand(s) and ⁄ or activator(s) are not yet known
(Table 1). The other members ofthe GC receptor
family are GC-C [11], GC-D [44], GC-E [45], GC-F
[45], GC-G [46], retinal GC [47], and GC-Y-X1 [48].
Table 1. Ligand specificity, tissue distribution and gene-disrupted phenotypes of particulate GCs ⁄ natriuretic peptide receptors. ROS-GC, rod
outer segment GC.
Receptor Ligand Tissue distribution Gene knockout phenotype in mice
GC-A ⁄ NPRA
(Npr1)
ANP ⁄ BNP
(Nppa ⁄ Nppb)
Adrenal glands, brain, heart, liver,
lung, olfactory glands, ovary,
pituitary gland, placenta, testis,
thymus, vascular beds, and other tissues
High blood pressure,
hypertension, cardiac
hypertrophy and fibrosis,
inflammation, volume
overload, reduced testosterone
levels [21,103–105,108,125,126]
GC-B ⁄ NPRB
(Npr2)
CNP (Nppc) Adrenal glands, brain, cartilage, fibroblast,
heart, lung, ovary, pituitary gland, placenta,
testis, thymus, vascular beds, and other tissues
Dwarfism, decreased adiposity,
female sterility, seizures, vascular
complication [142,143]
GC-C Guanylyn,
uroguanylyn,
enterotoxin
Colon, intestine, kidney Resistance to intestinal
secretion, diarrhea [11]
GC-D Orphan Neuroepithelium, olfactory glands Unknown [44]
GC-E Orphan Pineal gland, retina Unknown [45]
GC-F Orphan Retina Unknown [45]
GC-G Orphan Intestine, kidney, lung, skeletal muscle,
and other tissues
Unknown [46]
ROS-GC Orphan Rod outer segment Unknown [47]
Retinal GC Orphan Retina Unknown [47]
GC-Y-X1 Orphan Sensory neurons Unknown [48]
Update on functional aspects of GC-A ⁄ NPRA K. N. Pandey
1794 FEBS Journal 278 (2011) 1792–1807 ª 2011 The Author Journal compilation ª 2011 FEBS
The intracellular region of NPRA is divided into two
domains: the KHD is the 280-residue region immedi-
ately following the transmembrane domain, and distal
to this is the GCD, which is at the C-terminal portion of
the receptor molecule. More than 80% ofthe conserved
residues that have been found in all protein kinases [49]
are considered to be present in NPRA [5,6]. The GCD
of NPRA has been suggested to consist of a 250-residue
region at the C-terminal end ofthe molecule. Deletion
of the C-terminal region of NPRA results in a protein
that binds to ANP but does not contain GC activity
[38,50,51]. Modeling studies based on the crystal struc-
ture ofthe adenylyl cyclase II C
2
homodimer [52,53]
predicted that the active sites of GCs and adenylyl cyc-
lases are closely related [54,55]. On the basis of these
predictions, the GC catalytic active site of murine
NPRA includes a 31-residue sequence (residues 974–
1004) at the C-terminal end ofthe receptor molecule.
A comprehensive assessment ofthe structure–function
relationship of GC-A ⁄ NPRA has been described in this
series [56]. The transmembrane GC-A ⁄ NPRA contains
a single cyclase catalytic active site per polypeptide mol-
ecule; however, modeling data suggest that two polypep-
tide chains are required to activate the functional
receptor [57]. Thus the transmembrane GC receptors
seem to function as homodimers [58,59]. The dimeriza-
tion region of GC-A ⁄ NPRA has been suggested to be
located between the KHD andthe GCD, and is pre-
dicted to form an amphipathic a-helical structure [58].
NPRB is localized mainly in the brain and vascular
tissues, although it is thought to mediate the actions of
CNP in the vascular beds and in the central nervous
system [43]. The third member ofthe natriuretic peptide
receptor family, NPRC, consists of a large extracellular
domain of 496 residues, a single transmembrane
domain, and a very short 37-residue cytoplasmic
tail that has no homology with any other known recep-
tor protein domain. The extracellular region of NPRC
is 30% identical to those of GC-A ⁄ NPRA and
GC-B ⁄ NPRB. Earlier, it was proposed by default that
NPRC functions as a clearance receptor to clear natri-
uretic peptides from the circulation; however, several
studies have also provided evidence that NPRC plays
roles in the biological actions of natriuretic peptides
[60–62].
Intracellular signal transduction mechanisms of
GC-A
⁄
NPRA
ANP markedly increases cGMP levels in target tissues
in a dose-related manner [63,64]. The production of
cGMP is believed to result from ANP binding to the
extracellular domain of NPRA, which probably allos-
terically regulates an increased specific activity of the
cytoplasmic GCD ofthe receptor molecule
[7,51,65,66]. Because the nonhydrolyzable analogs of
ATP mimic the effect of ANP, it has been suggested
that ATP can allosterically regulate the GC catalytic
activity of NPRA [67–70]. In studies with mutant
NPRA specifically lacking the KHD, it was found that
the mutant receptor was active independently of ANP,
which showed that it had the capacity to be bound
with ligand, and most importantly, that it had basal
GC activity 100-fold greater than that of wild-type
NPRA [70]. Those previous findings suggested that,
under natural conditions, the KHD acts as a negative
regulator ofthe catalytic moiety of NPRA. Initially,
this model was the standard way of explaining the sig-
nal transduction mechanism of GC-coupled natriuretic
peptide receptors [71]. However, the model has not
been supported by the studies of other investigators,
which found that deletion ofthe KHD in NPRA did
not cause an elevation of basal GC activity; neverthe-
less, ATP seems to be obligatory for the transduction
activities of both NPRA and NPRB [65,67,72].
It has been suggested that NPRA exists in the phos-
phorylated form in the basal state, andthe binding of
ANP causes a decrease in phosphate content as well as
a reduction ofthe ANP-dependent GC activity [73].
This apparent mechanism of desensitization of NPRA
is in contrast to what is seen with many other cell sur-
face receptors, which appear to be desensitized by
phosphorylation [74–76]. Some previously reported
observations have also suggested that the GC activity
may, in fact, be regulated by receptor phosphorylation
[77–80]. However, little is known about the molecular
regulatory mechanisms ofthe desensitization and sig-
naling pathways of GC-A ⁄ NPRA, which may involve
more than one process. Internalization and sequestra-
tion of hormone receptors have been suggested to play
important roles in the process of receptor desensitiza-
tion and downregulation [81]. It is possible that NPRA
may undergo homologous desensitization in response
to ANP activation that could be mediated by receptor
internalization, sequestration, and metabolic degrada-
tion, in addition to phosphorylation ⁄ dephosphoryla-
tion mechanisms [82,83].
At the mRNA level, NPRA has been shown to be
regulated by glucocorticoids [84], transforming growth
factor-b [85], chorionic gonadotropin [86], and angio-
tensin II [87,88]. Endogenous transcription factors such
as Ets-1 and p300 have been shown to exert remark-
able stimulating effects on Npr1 transcription and
expression [89,90]. At the protein level, angiotensin II
has been shown to inhibit the GC activity of NPRA
[87,91,92]. Similarly, at the receptor level, NPRA is
K. N. Pandey Update on functional aspects of GC-A ⁄ NPRA
FEBS Journal 278 (2011) 1792–1807 ª 2011 The Author Journal compilation ª 2011 FEBS 1795
downregulated following exposure to its ligand ANP
or 8-bromo-cGMP [51,64,82,93,94].
Ligand-mediated endocytosis of GC-A
⁄
NPRA
After binding to ANP and BNP, GC-A ⁄ NPRA is
internalized and sequestered into intracellular compart-
ments. Therefore, GC-A ⁄ NPRA is a dynamic cellular
macromolecule that traverses different subcellular com-
partments during its lifetime. Evidence indicates that,
after internalization, the ligand–receptor complexes
dissociate inside the cell and a population of GC-A ⁄
NPRA recycles back to the plasma membrane. Sub-
sequently, the dissociated ligands are degraded in the
lysosomes. However a small percentage ofthe ligand
escapes the lysosomal degradative pathway and is
released intact into the culture medium. GC-A ⁄ NPRA
is internalized into subcellular compartments in a
ligand-dependent manner [95–100]. The ligand-depen-
dent endocytosis and sequestration of NPRA involves
a series of sequential sorting steps, through which
ligand–receptor complexes can eventually be degraded.
A proportion of receptor is recycled back to the
plasma membrane, and a small percentage of intact
ligand is released to the cell exterior [51,97,99,100].
The recycling of endocytosed receptor to the plasma
membrane andthe release of intact ligand to the cell
exterior occur simultaneously with processes leading to
degradation ofthe majority of ligand–receptor com-
plexes into lysosomes [51,82]. These findings provided
direct evidence that treatment of cells with unlabeled
ANP accelerates the disappearance of surface recep-
tors, indicating that ANP-dependent downregulation
of GC-A ⁄ NPRA involves internalization ofthe recep-
tor [82]. All three natriuretic peptides (ANP, BNP, and
CNP) are also bind to internalized involving NPRC
and ligand-receptor complexes are internalized. The
metabolic degradation of natriuretic peptides is further
regulated by neprilyisn, as well as by insulin-degrading
enzymes, as discussed in this series [101].
The short GDAY motif in the C-terminal domain of
GC-A ⁄ NPRA serves as a signal for endocytosis and
trafficking [51,82]. Gly920 and Tyr923 are the critical
elements in the GDAY motif. It is thought that
Asp921 provides an acidic environment for efficient
signaling ofthe GDAY motif in the internalization of
GC-A ⁄ NPRA. The mutation of Asp921 to alanine did
not have a major effect on internalization, but signifi-
cantly attenuated the recycling of internalized receptors
to the plasma membrane [82,83]. On the other hand,
mutation of Gly920 and Tyr923 to alanines reduced
the internalization of receptor, but did not have any
discernible effect on receptor recycling. It was sug-
gested that Tyr923 in the GDAY motif modulates the
early internalization of GC-A ⁄ NPRA, whereas Asp921
seems to mediate recycling or later sorting of the
receptor. Increasing evidence indicates that complex
arrays of short signals and recognition peptide
sequences ensure accurate trafficking and distribution
of transmembrane receptors and ⁄ or proteins and their
ligands into intracellular compartments [83,94]. The
short signals usually consist of small, linear amino acid
sequences, which are recognized by adaptor coat pro-
teins along the endocytic and sorting pathways. In
recent years, much has been learned about the function
and mechanisms of endocytic pathways responsible for
the trafficking and molecular sorting of membrane
receptors and their ligands into intracellular compart-
ments; however, the significance and scope of action of
the short motifs in these cellular events of GC-A ⁄
NPRA and GC-NPRB are not well understood.
Interestingly, GC-B ⁄ NPRB is also internalized and
recycled in hippocampal neurons and C6 glioma cell cul-
tures [102]. It was suggested that trafficking of GC-
B ⁄ NPRB occurs ligand-dependently in response to CNP
binding and stimulation ofthe receptor protein. The
internalization and trafficking of GC-B ⁄ NPRB has been
suggested to involve a clathrin-dependent mechanism.
Our recent work indicates that the internalization of
GC-A ⁄ NPRA also involves clathrin-dependent path-
ways [103]. Receptor internalization is severely dimin-
ished by inhibitors of clathrin proteins, such as
chlorpromazine and monodensyl cadaverine. However,
interaction ofthe GDAY motif in GC-A ⁄ NPRA and
GC-B ⁄ NPRB with clathrin adaptor proteins remains to
be established.
Physiological and pathophysiological
functions of GC-A
⁄
NPRA
The interaction of ANP with GC-A ⁄ NPRA reduces
blood volume and lowers blood pressure by enhancing
salt and water release through the kidney and inducing
vasorelaxation of smooth muscle cells. Both ANP and
BNP are implicated in reducing the preload and after-
load ofthe heart in both physiological and pathological
conditions. ANP and BNP acting via GC-A ⁄ NPRA
antagonize cardiac hypertrophic and fibrotic growth,
thus conferring cardioprotective effects in disease states.
ANP has been shown to exert an antimitogenic effect
in response to various growth-promoting agonist
hormones in a number of target cells and tissues. The
binding of ANP and BNP to GC-A ⁄ NPRA produces
increased levels ofthe intracellular second messenger
cGMP, which stimulates three known cGMP effector
molecules, namely: cGMP-dependent protein kinases
Update on functional aspects of GC-A ⁄ NPRA K. N. Pandey
1796 FEBS Journal 278 (2011) 1792–1807 ª 2011 The Author Journal compilation ª 2011 FEBS
(PKGs), cGMP-dependent phosphodiesterases (PDEs),
and cGMP-dependent ion channels. The activation of
these effector molecules elicits a number of physiologi-
cal and pathophysiological roles of GC-A ⁄ NPRA in
several target cells and tissue systems (Fig. 2). Thus,
multiple synergistic actions of ANP and BNP and their
cognate receptor GC-A ⁄ NPRA make them novel
therapeutic targets in renal, cardiac and vascular dis-
eases. The critical physiological and pathophysiological
functions of GC-A ⁄ NPRA are described below.
Protective role of GC-A
⁄
NPRA in blood pressure
regulation
Genetic mouse models with disruption of both Nppa
(coding for proANP) and Npr1 (coding for GC-A ⁄
NPRA) have provided strong support for the central
role ofthe natriuretic peptide hormone–receptor sys-
tem in the regulation of arterial pressure [21,104–109].
Therefore, genetic defects that reduce the activity of
ANP and its receptor system can be considered as can-
didate contributors to essential hypertension [7]. Previ-
ous studies with ANP-deficient (Nppa
) ⁄ )
) mice
demonstrated that a defect in proANP synthesis can
cause hypertension [107]. The blood pressure of homo-
zygous null mutant mice was elevated by 8–23 mmHg
when they were fed with standard-salt or intermediate-
salt diets. Those previous findings indicated that
genetic disruption of ANP production can lead to
hypertension. Transgenic mice overexpressing ANP
developed sustained hypotension with an arterial pres-
sure that was 25–30 mmHg lower than that of their
nontransgenic siblings [110,111]. Interestingly, somatic
delivery ofthe ANP gene in spontaneously hyperten-
sive rats induced a sustained reduction of systemic
blood pressure [112]. Overexpression of ANP in hyper-
tensive mice lowered systolic blood pressure, raising
the possibility of using ANP gene therapy for the
treatment of human hypertension [113]. It has also
been shown that functional alterations ofthe Nppa
promoter are linked to cardiac hypertrophy in proge-
nies of crosses between Wistar Kyoto and Wistar
Kyoto-derived hypertensive rats, and that a single-
nucleotide polymorphism can alter the transcriptional
activity ofthe proANP gene promoter [114].
Genetic studies with Npr1 knockout (Npr1
) ⁄ )
or
zero-copy) mice have indicated that disruption of Npr1
increases blood pressure by 35–40 mmHg as compared
with wild-type (Npr1
+ ⁄ +
or two-copy) animals
[21,104,109]. It has been demonstrated that complete
absence of NPRA causes hypertension in mice and
leads to altered renin and angiotensin II levels
[21,104,109,115–117]. In contrast, increased expression
of NPRA in gene-duplicated mutant mice significantly
reduces blood pressure and increases the levels of
cGMP, in correspondence with the increasing number
of Npr1 copies [106,115,116,118]. Our studies have
examined the quantitative contributions and possible
mechanisms mediating the responses of varying num-
bers of Npr1 copies by determining the renal plasma
flow, glomerular filtration rate, urine flow and sodium
excretion patterns following blood volume expansion
in Npr1-targeted mice in a gene dose-dependent man-
ner [105,116]. Our findings demonstrated that the
ANP–NPRA axis is primarily responsible for mediat-
ing the renal hemodynamic and sodium excretory
responses to intravascular blood volume expansion.
Interestingly, the ANP–NPRA system inhibits aldoste-
rone synthesis and release from adrenal glomerulosa
GTP
y
IP
3
Ligand
Receptor
NPRA NPRB NPRC
ANP BNP CNP
TM
KHD
LBD
cAMP
GCD
DD
IP
3
cGMP
PKG
Phosphorylaon or
dephosphorylaon
Physiological funcons
PDE CNG
GTP
cGMP
Ca
2+
Vasodilataon
Sodium excreon
PKC
MAPK
Anproliferaon
cAMP
TNF
-α
IL-6
Anhypertrophy
Fig. 2. Representation of hormone specificity, ligand-binding
domains, transmembrane-spanning regions, intracellular domains
and signaling systems of GC-A ⁄ NPRA, GC-B ⁄ NPRB, and NPRC.
The arrows indicate the ligand specificity for specific natriuretic
peptide receptors. The extracellular ligand-binding domain (LBD),
transmembrane region (TM), KHD and GCD of GC-A ⁄ NPRA and
GC-B ⁄ NPRB are shown. DD is the dimerization domain of NPRA
and NPRB. The LBD, TM and small intracellular tail of NPRC are
also indicated. Both NPRA and NPRB have been shown to gener-
ate cGMP from the hydrolysis of GTP. An increased level of intra-
cellular cGMP stimulates and activates three known cGMP effector
molecules, namely: PKGs, PDEs, and cGMP-dependent ion-gated
channels (CNGs). The cGMP-dependent signaling may antagonize a
number of pathways, including: intracellular Ca
2+
release, IP
3
for-
mation, activation of protein kinase C (PKC) and MAPKs, and pro-
duction of cytokines such as tumor necrosis factor-a (TNF-a) and
interleukin-6 (IL-6). The resulting cascade can mimic ANP ⁄ NPRA ⁄
cGMP-dependent responses in both physiological and pathophysio-
logical environments. The activation of NPRC may lead to a
decrease in cAMP levels and an increase in IP
3
production.
K. N. Pandey Update on functional aspects of GC-A ⁄ NPRA
FEBS Journal 278 (2011) 1792–1807 ª 2011 The Author Journal compilation ª 2011 FEBS 1797
cells [3,109,115,119], which may account for its renal
natriuretic and diuretic effects. Furthermore, studies
with Npr1-disrupted (zero-copy) mice demonstrated
that, at birth, the absence of NPRA allows higher renin
and angiotensin II levels than in wild-type mice, and
increased renin mRNA expression [109]. However, at
3–16 weeks of age, the circulating renin and angioten-
sin II levels were dramatically decreased in Npr1 homo-
zygous null mutant mice as compared with wild-type
(two-copy) control mice. The decrease in renin activity
in adult Npr1 null mutant mice is probably caused by a
progressive elevation in arterial pressure, leading to
inhibition of renin synthesis and release from the kid-
ney juxtaglomerular cells [116]. On the other hand, the
adrenal renin content and renin mRNA level, as well
as angiotensin II and aldosterone concentrations, were
elevated in adult homozygous null mutant mice as
compared with wild-type mice [109,115]. In light of
these previous findings, it can be suggested that the
ANP–NPRA signaling system may play a key regula-
tory role in the maintenance of both systemic and tis-
sue levels ofthe components ofthe renin–angiotensin–
aldosterone (RAA) system in physiological and patho-
logical conditions. Indeed, ANP–NPRA signaling
appears to oppose almost all actions of angiotensin II
in both physiological and disease states (Table 2).
Although expression of ANP and BNP is markedly
increased in patients with hypertrophic or failing
hearts, it is unclear how the natriuretic peptide system
is activated to play a protective role. The ANP–NPRA
system may act by reducing high blood pressure and
inhibiting the RAA system, or by activating new
molecular targets as a consequence ofthe hypertrophic
changes occurring in the heart [21,105,120,121].
Functional role of GC-A
⁄
NPRA and salt sensitivity
The disruption of Npr1 indicated that the blood pres-
sure of homozygous mutant mice remained elevated
and unchanged in response to either minimal-salt or
high-salt diets [122]. These investigators suggested that
NPRA may exert its major effect at the level of the
vasculature, and probably does so independently of
salt. In contrast, other studies reported that disruption
of Npr1 resulted in chronic elevation of blood pressure
in mice fed with high-salt diets [115,118]. The findings
that adrenal angiotensin II and aldosterone levels are
increased in Npr1-disrupted mice may explain the ele-
vated systemic blood pressure with decreasing Npr1
copy (zero-copy and one-copy) numbers [115]. How-
ever, adrenal angiotensin II and aldosterone levels are
decreased in Npr1 gene-duplicated mice. A low-salt
diet increased adrenal angiotensin II and aldosterone
levels in all Npr1-targeted (gene-disrupted and gene-
duplicated) mice, whereas a high-salt diet reduced
adrenal angiotensin II and aldosterone levels in Npr1-
disrupted mice and wild-type mice, but not in Npr1-
duplicated (three-copy and four-copy) mice. Our
findings suggest that NPRA signaling has a protective
effect against high salt in Npr1-duplicated mice as
compared with Npr1-disrupted (four-copy) mice [115].
Indeed, more studies are needed to clarify the relation-
ship between salt sensitivity and blood pressures in
Npr1-targeted mice.
Protective roles of GC-A
⁄
NPRA in cardiac
dysfunction
It is believed that ANP and BNP concentrations are
markedly increased both in cardiac tissues and in the
plasma of CHF patients [123–125]. Interestingly, in
hypertrophied hearts, ANP and BNP genes are overex-
pressed, suggesting that autocrine and ⁄ or paracrine
effects of natriuretic peptides predominate, and might
serve as an endogenous protective mechanism against
maladaptive pathological cardiac hypertrophy
[21,120,124,126–128]. Evidence suggests that a high
plasma ANP ⁄ BNP level is a prognostic predictor in
humans with heart failure [123,129]. In patients with
severe CHF, concentrations of both ANP and BNP
are higher than control values; however, the increase
in BNP concentration is 10-fold to 50-fold higher than
the increase in ANP concentration [20]. Interestingly,
the half-life of BNP is greater than that of ANP; thus,
the diagnostic evaluations of natriuretic peptides have
favored BNP [125]. The plasma levels of both ANP
and BNP are markedly elevated under the pathophysi-
ological conditions of cardiac dysfunction, including
Table 2. Typical examples of antagonistic actions of ANP–NPRA
on various angiotensin II-stimulated physiological and biochemical
effects in target cells and tissues. CNS, central nervous system;
PKC, protein kinase C.
Parameters Angiotensin II ANP–NPRA
Aldosterone release Stimulation Inhibition
Renin secretion Inhibition Inhibition
Vasopressin release Stimulation Inhibition
Blood vessels Contraction Relaxation
Water intake Stimulation Inhibition
CNS-mediated hypertension Stimulation Inhibition
Gonadotropin release Unknown Stimulation
Testosterone synthesis Unknown Stimulation
Estrodiol synthesis Unknown Stimulation
Intracellular Ca
2+
release Stimulation Inhibition
MAPKs Stimulation Inhibition
PKC Stimulation Inhibition
IP
3
production Stimulation Inhibition
Update on functional aspects of GC-A ⁄ NPRA K. N. Pandey
1798 FEBS Journal 278 (2011) 1792–1807 ª 2011 The Author Journal compilation ª 2011 FEBS
diastolic dysfunction, CHF, pulmonary embolism, and
cardiac hypertrophy [21,124,125,130,131]. It has been
suggested that ventricular expression of ANP and
BNP is more closely associated with local cardiac
hypertrophy and fibrosis than with plasma ANP levels
and systemic blood pressure [21,127]. BNP can be con-
sidered as an important prognostic indicator in CHF
patients; however, N-terminal proBNP is considered to
be a stronger risk bio-indicator for cardiovascular
events [132,133].
The expression of Nppa and Nppb (coding for pro
BNP) is greatly stimulated in hypertrophied hearts,
suggesting that autocrine and ⁄ or paracrine effects of
natriuretic peptides predominate and might serve as an
endogenous protective mechanism against maladaptive
cardiac hypertrophy [21,120,134]. Disruption of Npr1
in mice increases the cardiac mass and incidence of
cardiac hypertrophy to a great extent [21,104,127,135–
137]. Previous studies have demonstrated that Npr1
disruption in mice provokes enhanced expression of
hypertrophic marker genes, proinflammatory cyto-
kines, and matrix metalloproteinases, and enhanced
activation of nuclear factor kappaB, which seem to be
associated with cardiac hypertrophy, fibrosis, and
extracellular matrix remodeling [21,126,127]. Interest-
ingly, the expression of sarcolemmal ⁄ endoplasmic
reticulum Ca
2+
-ATPase-2a progressively decreased in
the hypertrophied hearts of Npr1 homozygous null
mutant mice as compared with wild-type control mice
[21]. It has also been demonstrated that expression of
angiotensin-converting enzyme and angiotensin II
receptor type A is greatly enhanced in Npr1 null
mutant (zero-copy) mice as compared with wild-type
(two-copy) control mice [127]. Moreover, it has also
been suggested that Npr1 antagonizes angiotensin II
receptor-mediated and angiotensin II receptor type
A-mediated cardiac remodeling, and provides an
endogenous protective mechanism in the failing heart
[127,138,139]. The arteries of smooth muscle-specific
and endothelial cell-specific Npr1 knockout mice
exhibited significant arterial hypertension [140]. It has
also been suggested that Npr1 represents a potential
locus for susceptibility to atherosclerosis [141]. The
impact of Npr1 in cardiovascular pathophysiology has
also been described in this series [142]. On the other
hand, Npr2-deleted mice exhibit dysfunctional endo-
chondral ossification and diminished longitudinal
growth in limbs and vertebra, and show normal blood
pressure, as compared with their wild-type counter-
parts [143]. Mutation of Npr2 has been shown to
be associated with Maroteaux-type acromesomedic
dysplasia [144].
Biological actions of GC-A
⁄
NPRA in renal and
vascular cells
ANP–NPRA signaling in the kidneys promotes the
excretion of salt and water, and enhances glomerular
filtration rate and renal plasma flow [3,4,7,116]. Tar-
gets of ANP action in the kidney include the inner
medullary collecting duct, glomerulus, and mesangial
cells [51,145–147]. The increased production of cGMP
at ANP concentrations affecting renal function corre-
lates with the effects of dibutyryl-cGMP, which pre-
vents mesangial cell contraction in response to
angiotensin II [148]. ANP markedly lowers renin secre-
tion and also plasma renin concentrations
[109,149,150]. The role of ANP in mediating the renal
and vascular effects was investigated with selective
NPRA antagonists to eliminate the effect of ANP
[151,152]. ANP–NPRA signaling exerts direct effect on
the kidney, to release sodium and water, by inhibiting
sodium reabsorption. Npr1 knockout mice exhibit an
impaired ability to initiate a natriuretic response to
acute blood volume expansion [105]. In Npr1-dupli-
cated mice, a low dose of ANP decreased the frac-
tional reabsorption of distal sodium, suggesting that
the augmented natriuresis was enhanced by ANP infu-
sions and is mediated by Npr1 dosage [153]. These
findings suggested that ANP–NPRA signaling inhibits
distal sodium reabsorption. ANP–NPRA signaling also
exerts indirect effects on renal sodium and water excre-
tion by inhibiting the RAA system, as previously
described [5,7].
ANP, either in intact aortic segments or in cultured
vascular smooth muscle cells (VSMCs), has always
been shown to increase cGMP levels. The correlative
evidence of ANP-induced cGMP accumulation has
suggested its role as the second messenger of dilatory
responses to ANP in cultured VSMCs [152,154,155].
ANP and cGMP analogs reduced the agonist-depen-
dent increases in cytosolic Ca
2+
levels in VSMCs and
inositol trisphosphate (IP
3
) levels in Leydig cells; thus,
intracellular cGMP has been suggested to mediate the
ANP-induced decrease in cytosolic Ca
2+
and IP
3
levels
[156,157]. ANP has also been found to act as a growth
suppressor in a variety of cell types, including vascula-
ture, kidney and heart cells, and neurons
[51,82,154,155,158]. ANP inhibits mitogen activation
of fibroblasts [159], and induces cardiac myocyte apop-
tosis [160]. However, the mechanisms involved in these
effects of ANP are not yet completely understood.
Clearly, more studies are warranted to elucidate the
molecular mechanisms underlying the antiproliferative
effect of ANP–NPRA signaling in various target cells.
K. N. Pandey Update on functional aspects of GC-A ⁄ NPRA
FEBS Journal 278 (2011) 1792–1807 ª 2011 The Author Journal compilation ª 2011 FEBS 1799
ANP is considered to be a direct smooth muscle
relaxant, and a potent regulator of cell growth and
proliferation. It is expected that the antigrowth
paradigm could potentially operate through the
negative regulation of mitogen-activated protein kinase
(MAPK) activities. ANP may be one ofthe key endog-
enous hormones that interacts negatively with elements
in the MAPK signaling pathway to control cell growth
and proliferation. ANP has been reported to antago-
nize the growth-promoting effects in target cells; how-
ever, the mechanism ofthe antigrowth paradigm of
ANP andthe involvement of specific ANP receptor
subtypes (NPRA and NPRC) in different target cells
are controversial [51,62,161–163].
Association of gene polymorphisms of
Nppa, Nppb and Npr1 in hypertension
and cardiovascular diseases
Recent genetic and clinical studies have indicated an
association of Nppa, Nppb and Npr1 polymorphisms
with hypertension and cardiovascular events in
humans [128,164–166]. An association between an
Nppa promoter polymorphism (–C66UG) and left
ventricular hypertrophy (LVH) has been demon-
strated in Italian hypertensive patients, indicating that
individuals carrying a copy ofthe Nppa variant allele
exhibit a marked decrease in proANP levels associ-
ated with LVH [165]. Interestingly, an association
between a microsatellite marker in the Npr1 promoter
and LVH has also been demonstrated, suggesting that
the ANP–NPRA system contributes to ventricular
remodeling in human essential hypertension [165]. As
the relationship between high blood pressure and car-
diovascular risk is continuous, in the absence of
ANP–NPRA signaling even small increases in blood
pressure have excessive and detrimental effects. Epide-
miological studies have demonstrated that substantial
heritability of blood pressure and cardiovascular risks
can occur, suggesting a role for genetic factors [167].
Intriguingly, a common genetic variant at the
Nppa–Nppb locus was found to be associated with
circulating ANP and BNP concentrations, contribut-
ing to interindividual variations in blood pressure and
hypertension [164]. These authors demonstrated that
a single-nucleotide polymorphism at the Nppa–Nppb
locus was associated with increased plasma ANP and
BNP concentrations, and lower systolic and diastolic
blood pressures.
Rare genetic mutations have been suggested for
monogenic forms of hypertension and blood pressure
in humans [168,169]. However, common variants asso-
ciated with blood pressure regulation were not estab-
lished. A number of pathways, namely the RAA
system andthe adrenergic system, are considered to
regulate blood pressure and hypertension; nevertheless,
the genetic determinants in these pathways contribut-
ing to interindividual differences in blood pressure reg-
ulation have not been elucidated. Therefore, the
findings of those previous studies indicating an associa-
tion of common variants in the Nppa–Nppb locus with
circulating ANP and BNP concentrations are novel
[164]. Interestingly, a ‘four-minus’ haplotype in the
3¢-UTR of Npr1 has been shown to be associated with
an increased level of N-terminal-proBNP in humans
[166]. The ‘four-minus’ haplotype constitutes 4C
repeats at nucleotide position 14 319 and a 4-bp dele-
tion of AGAA at nucleotide position 14 649 of Npr1.
Individuals with genetic defects in Npr1 caused by the
presence ofthe ‘four-minus’ haplotype exhibit signifi-
cantly higher N-terminal proBNP levels. It has been
speculated that the causal mechanism for this effect
could be Npr1 mRNA instability, leading to decreased
translational production of receptor molecules [170].
This could elicit a feedback mechanism, whereby the
diminished function ofthe BNP–NPRA system caused
by the defect in Npr1 provokes compensatory
enhanced expression and release of BNP. Taken
together, these considerations suggest that a positive
association exists between Nppa, Nppb and Npr1 poly-
morphisms and essential hypertension, high blood
pressure and left ventricular mass index in humans.
Further studies are needed for the characterization of
more functionally significant markers of Nppa, Nppb
and Npr1 variants in a larger human population.
Conclusion and future perspectives
The studies outlined in this review provide a unique
perspective for delineating the genetic and molecular
basis of GC-A ⁄ NPRA regulation and function. Recent
studies have utilized molecular approaches to delineate
the physiological functions affected by decreasing or
increasing the number of Npr1 copies as achieved by
gene targeting, such as gene disruption (gene knock-
out) or gene duplication (gene dosage), of Npr1 in
mice. The gene-targeting strategies have produced mice
that contain zero to four copies ofthe Npr1 locus.
Using gene-targeted mouse models, we have been able
to determine the effects of decreasing or increasing the
expression levels of Npr1 in intact mice in vivo. Com-
parative analyses ofthe biochemical and physiological
phenotypes of Npr1-disrupted and Npr1-duplicated
mutant mice will have enormous potential for answer-
ing fundamental questions concerning the biological
importance of ANP–NPRA signaling in disease states
Update on functional aspects of GC-A ⁄ NPRA K. N. Pandey
1800 FEBS Journal 278 (2011) 1792–1807 ª 2011 The Author Journal compilation ª 2011 FEBS
by genetically altering Npr1 copy numbers and product
levels in vivo in intact animals with otherwise identical
genetic backgrounds. The results of these studies have
provided important tools for examination ofthe role
of the ANP–NPRA system in hypertension and cardio-
vascular disease states. Future studies will lead to a
better understanding ofthe genetic basis of Npr1 func-
tion in regulating blood volume and pressure homeo-
stasis, and should reveal new possibilities for
preventing cardiovascular sequelae such as hyperten-
sion, heart attack, and stroke.
Nevertheless, theparadigmsofthe molecular basis
of thefunctional regulation of Npr1 andthe mecha-
nisms of ANP–NPRA action are not yet clearly under-
stood. Currently, natriuretic peptides are considered to
be markers of CHF; however, an understanding of
their therapeutic potential for the treatment of cardio-
vascular diseases such as hypertension, renal insuffi-
ciency, cardiac hypertrophy, CHF and stroke is still
lacking. The results of future investigation should be
of great value in resolving the problems of genetic
complexities related to hypertension and heart failure.
Overall, future studies should be directed at providing
a unique perspective for delineating the genetic and
molecular basis of Npr1 expression, regulation and
function in both normal and disease states. The result-
ing knowledge should yield new therapeutic targets for
treating hypertension and preventing hypertension-
related cardiovascular diseases and other pathological
conditions.
Acknowledgements
My special thanks go to B. B. Aggarwal, Department
of Experimental Therapeutics and Cytokine Research
Laboratory, MD Anderson Cancer Center; and to S.
L. Hamilton, Department of Molecular Physiology
and Biophysics, Baylor College of Medicine, for pro-
viding their facilities during our displacement period
caused by Hurricane Katrina. I thank my wife Kamala
Pandey for her kind help in the preparation of this
manuscript. The research work in the author’s labora-
tory was supported by National Institutes of Health
grants (HL-57531 and HL-62147).
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