Natriuretic Peptide Receptor Type B (NPRB)
The natriuretic peptides (NP) system is a family of peptides named A, B, and C-type natriuretic peptides (ANP, BNP, CNP) and urodilatin (URO), synthetized by the heart (ANP and BNP), kidney (URO), and vascular smooth muscle (CNP) but also by other cells widely dispersed in different organs and tissues (Koller and Goeddel 1992). The NP family plays an essential role on the regulation of blood pressure, the intravascular volume, and electrolyte homeostasis in mammals. Based on the fact that their main actions are exerted on regulation of the cardiovascular system, the NP are included within the big family of vasoactive peptides, together with angiotensin II, endothelins, bradikinin, and vasopressin among others (Padney 2015).
Binding of NP to either natriuretic peptide receptor type A (NPRA) or type B (NPRB) leads to activation of particulate guanylate cyclase (pGC) catalytic domain generating the second messenger cyclic GMP (cGMP) signaling cascade, which mediates most biological actions of these peptides. The phosphorylation state of one domain of these receptors via cGMP-dependent protein kinases I and II (cGKI and cGKII) determines their sensitivity to ligand (Garbers et al. 2006). The binding affinity of the NPs for NPRB is the following: CNP > ANP > BNP (Cantú et al. 2015).
NPRB is encoded by Npr2 gene (OMIM 108961) which is located in chromosome 9 (p12–21). Human NPRB gene is approximately 16.5 kilobases (kb), which contain 22 exons, 21 introns, and the junction between introns and exons follows the GT-AG patron. The gene encodes for a peptide of 1061 amino acids (Rehemudula et al. 1999).
NPRB Structure, Activation, and General Actions
NPRB is a 135 kDa transmembrane protein that binds selectively to CNP (Pandey and Singh 1990) (Fig. 1). NPRB is a membrane guanylyl cyclase homodimeric receptor characterized by a general structure consistent to the guanylyl cyclase receptor family: an extracellular ligand-binding domain, highly glycosylated on asparagine residues, and three intramolecular disulfide bonds (Garbers 1992). In basal conditions, it is highly phosphorylated while the loss of phosphate residues leads to its inactivation (Potter 2011). Its transmembrane domain comprises four highly conserved regions ordered as follows (from membrane to cytoplasm): the juxtamembrane domain (JMD), the protein kinase-like homology domain (KHD) of 280 amino acid, the dimerization domain (DD), and the pGC catalytic domain. Ligand binding stimulates pGC activity that converts GMP to cGMP, which in turn activates several target molecules, such as cGKI and cGKII, cyclic nucleotide-regulated ion channels, and cGMP-regulated phosphodiesterases (PDE) like PDE5. Not only extracellular ligand-binding is required for NPRB activation but also the phosphorylation of up to six residues within the KHD. NPRB has a high degree of similarity with NPRA: approximately 43% in the ligand-binding domain, 72% in the KHD, and 91% in the pGC domain (Garbers 1992; Khurana and Pandey 1993; Koller and Goeddel 1992; Pandey and Singh 1990; Schlueter et al. 2014). NPRB is internalized in a ligand-dependent manner once CNP binds to it. It is degraded by lysosomes and the recycled molecules go back to the plasmatic membrane to form new receptors (Padney 2015). The expression of NPRB takes place in brain, bones, chondrocytes, lungs, and ovary tissue, as well as in vascular smooth muscle cells and fibroblasts (Potter 2011) (Fig. 2). It has also been found in cardiac myocyte caveoles (Padney 2015). NPRB plays an important role in long bone growth by regulating endochondral ossification. In the brain, the binding of CNP to NPRB was associated with central regulation of food intake and energy metabolism (Schlueter et al. 2014).
Npr2 Gene and Growth Disorders
CNP and its receptor NPRB are recognized as important regulators of longitudinal growth. Animal models allowed investigators to found the relation between CNP or NPRB genes (Nppc or Npr2, respectively) and the fundamental role of their products CNP/NPRB in endochondral ossification (Vasques et al. 2014).
It has been shown that height variability in healthy individuals is related to polymorphisms in two genes related to the CNP pathway. Bartels et al. demonstrated that a severe skeletal dysplasia characterized by dwarfism and short limbs called acromesomelic dysplasia type Maroteux (AMDM) is caused by biallelic mutations that generate the loss of function in Npr2 gene. Furthermore, homozygous mutations produce a severe short stature and body disproportion (Vasques et al. 2014). It has also been found heterozygous mutations in Npr2 gene that seem to be associated with mild and variable growth impairment without a distinct skeletal phenotype (Wang 2015). On the other hand, heterozygous gain-of-function mutations in Npr2 were pointed out as a cause of tall stature, and a similar phenotype had been observed in an individual with overexpression of the CNP caused by a balanced translocation (Vasques et al. 2014).
Considering all the data available until today, CNP should be studied as a promising therapy for achondroplasia (ACH) and growth impairment diseases. In fact, it is an ongoing investigation using a once-daily subcutaneous administration of a CNP analog called BMN-111, which has an extended half-life due to neutral endopeptidase resistance. Preclinical studies in mouse models of ACH using this new molecule showed improvement of dwarfism. In humans, a phase 2 multicenter and multinational trial has been initiated, with estimated study completion in 2017 (ClinicalTrials.gov Identifier: NCT02055157).
NPRB in Cardiovascular Diseases and Hypertension
CNP bound to NPRB promotes antiproliferative, proapoptotic, antifibrotic, and antihypertrophic effects on cardiomyocytes. It was observed that NPRB expression occurred at the site of vascular injury, and the administration of CNP after an arterial damage inhibited intimal proliferation, and therefore neointimal restenosis. The interaction between CNP and NPRB has been related to the progression of atherosclerotic lesions and was also found to reduce cardiac ischemia-reperfusion injury. All these beneficial effects could be associated with the inhibition of cardiac fibroblasts collagen synthesis by CNP, positioned this natriuretic peptide as a potent antihypertrophic agent through its interaction with NPRB (Garbers et al. 2006; Cantú et al. 2015).
It was described that CNP/NPRB induces vasorelaxation in large conduction vessels and small resistant arteries, and also inhibits smooth muscle proliferation, leading to a reduction in blood pressure. Studies conducted in spontaneously hypertensive rats with normal CNP blood levels demonstrated that alterations of NPRB were related to hypertension and vasorelaxation attenuation (Rahmutula et al. 2001).
Systematic screening of polymorphism of Npr2 was conducted by Rahmutula et al. looking for an association of these changes to cardiovascular diseases. They identified a 9 bp insertion/deletion (I/D) in intron 18 that was not related to essential hypertension, contrary to an 11-repeat allele of the GT repeat polymorphism in intron 2. Also, and related to this polymorphism, they found a C to T transition at nucleotide (nt) 2077 in exon 11 but it was not associated with myocardial infarction (Rahmutula et al. 2001).
Npr2 Gene and Infertility
Several studies showed the relation between CNP and the granulosa cells of the ovarian cortex. In that tissue, CNP acting through its receptor NPRB inhibits the oocyte maturation. To confirm this observation, investigators demonstrated the administration of CNP to infertile female mice derived in ovarian growth. Furthermore, after this treatment and with the external supplement of gonadotropins, ovulation was successfully induced (Vasques et al. 2014).
In a particular natural mutation of Npr2 occurring in female mice, it was observed no progression of the oocyte to the two-cell embryo stage due to premature meiotic resumption. Also, female mice lacking of NPRB showed infertility associated with the absence of estrus cycle and uterine atrophy with thickness reduction of endometrium and myometrium (Vasques et al. 2014).
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