The biological function of NPRA is
The biological function of NPRA is demonstrated primarily through the ANP/BNP-dependent GC catalytic activity of the receptor and the production of cGMP, which is regulated by several factors, including hormones, growth factors, physiological milieu, and the ligand itself [26,34,, , , , , , ]. Several of our studies have established that the gene targeting (gene-disruption and gene-duplication) of Npr1 (encoding NPRA) in mouse models shows the hallmark significance of NPRA signaling in lowering arterial pressure and protecting against renal, cardiac, and vascular diseases [, , , , , ]. More recent evidences suggest that reduced levels of ANP seem to be associated with insulin resistance, obesity, lifestyle-associated metabolic syndrome, and essential hypertension [10,, , ]. In addition, ANP/NPRA system has been suggested to play role in vascular remodeling of uterine spiral artery . Previous studies have suggested that endothelial action of ANP enhances the myocardial inflammatory infiltration in early phase after acute infarction . Further, ANP has been shown to exhibit anti-inflammatory properties and tumor-associated immune response [54,55]. Moreover, the synthetic novel designer natriuretic peptides have provided the renal enhancing actions of these hormones over the naturally occurring molecules [, , , ].
NPRA is designated as the cognate receptor for both ANP and BNP; nevertheless, ANP binds to the receptor with a higher affinity than BNP . However, it is not yet clear whether ANP and BNP complete each other for the receptor binding. The ligand binding of both NPRA and NPRB has demonstrated that bound ligand-receptor complexes of NPRA are promptly internalized into cells, redistributed into intracellular compartments, and ultimately degraded in lysosomes [35,40,, , , ]. Ligand-receptor complexes of ANP-NPRA are distributed, possibly through the endosomes, to lysosomal compartments, where they are largely metabolized; however, a population of ligand-receptor complexes escapes the lysosomal compartments, allowing these receptors to recycle back to the plasma membrane [26,34,42,62]. Moreover, internalization of ANP, BNP, and CNP also occurs through NPRC [65,66]. Virtually, the ligand-mediated receptor Ki16425 is a vital mechanism for physiological responses. Clathrin-mediated endocytosis is a well-established mechanism for numerous membrane-bound hormone receptors containing glucagon, insulin, platelet-derived growth factor (PDGF), and epidermal growth factor (EGF), which are also internalized by ligand-mediated endocytosis [35,67,68]. Our recent studies have demonstrated that NPRA is dynamin-dependently endocytosed in clathrin-coated vesicles . Here, we provide a comprehensive summary of past and present findings on the internalization, intracellular trafficking, and concurrent signaling mechanisms of NPRA focused on cell-based studies as well as in intact live cells and animals in vivo to establish the actions of NPRA in the dynamic physiological context.
Historical background Several studies have clearly established that ANP-BNP/NPRA ligand-receptor complexes are internalized in various cell types upon stimulation by ligand. However, the question of whether NPRA is internalized remained open for a long time. For example, earlier it has been stated that neither NPRA nor NPRB ligand-receptor complexes are internalized and that only NPRC receptor is internalized to downregulate the cGMP signaling process . Although remarkable progress has been made toward determining the structure-function relationship of NPRA and NPRB, the issues of internalization and the trafficking itinerary of these GC receptors remained controversial until recently. In the past, debate focused on whether ANP/NPRA complexes were internalized at all or whether cells used some other mechanisms to cell-specifically release ANP from its receptor. Indeed, it was reported by default that among the three natriuretic peptide receptors, only NPRC was internalized with bound ligand . It was suggested that endogenous NPRA was not internalized in cultured renal medullary interstitial cells and that rapid dissociation of ligand-receptor complexes occurred after ANP binding to NPRA at 370C. However, it was difficult to interpret the results of such findings because the dissociation of ligand was carried out in a medium containing high concentrations of unlabeled ANP to preclude the rebinding of dissociated hormone to receptors . One study using this ligand-binding protocol indicated that intact 125I-ANP was released into the culture medium of 293 T cells expressing recombinant NPRA . However, in a later study, the authors reconciled the absence of ANP/NPRA internalization in 293 T cells, stating that it might have resulted from the slow rate of cell-specific ANP degradation in these cells .