I kid(ney) you not…natriuretic peptides which promote natriuresis but not hypotension

Originally identified as components of secretory granules in atrial tissue^{1, 2}, natriuretic factors (now termed natriuretic peptides, or NPs) A and B function as myocardial-generated hormones with multiple biological effects on diverse target tissues³. NP family members activate membrane bound receptors termed NP receptors (NPRs). Upon activation by NP binding, NPRs function as guanylate cyclases to catalyze intracellular formation of cyclic GMP (cGMP)³. cGMP in turn modulates effector proteins including ion channels, cGMP-regulated phosphodiesterases, and the cGMP-dependent protein kinase (PKG)⁴.

NP activation of NPRs regulates a host of tissue-specific physiological processes including arteriolar relaxation, promotion of urinary sodium excretion (natriuresis), and direct inhibition of myocardial hypertrophy and fibrosis. The related peptide urodilatin (URO), a product of the gene for ANP, undergoes differential processing in renal tissue and contains additional amino acids, which may render it more potent in inducing renal natriuresis and resisting enzymatic degradation by endopeptidases^{3, 5}.

Given the important roles of NPs and cGMP in promoting generally beneficial physiological processes, interest has grown in administering these peptides in disease states. However, administration of synthetic BNP (nesiritide) and other NPs has been limited by excessive hypotension presumably arising from cGMP-mediated vasodilation⁶. In fact, hypotension has proven an important general limitation of a number of other cGMP augmenting therapies, raising interest in identifying mechanisms to preserve cGMP effects in target organs but avoid excessive vasodilation.

In a study in this issue of Circulation Research⁷, Chen and colleagues take an innovative approach to circumvent these limitations and to probe renal-protective effects of NPs. Using what they have termed a designer peptide approach, the group has generated a peptide, CRRL269, based on the structure of BNP and of urodilatin⁸. They previously demonstrated CRRL269 to possess increased cGMP generating effects in renal epithelial cells compared with either BNP or URO alone. Further, the combination peptide produced improvement in glomerular filtration rate (GFR, the measure of renal filtering function) yet did not significantly lower blood pressure in healthy animals.

In the present study⁷ Chen et al tested the effects of this novel NP in a large animal aortic cross clamping model of acute renal injury. This procedure temporarily interrupts blood flow to the kidneys resulting in reduced renal function during and after the clamping process. In elegant in vivo experiments, they observed that, compared with control treated animals, the designer NP increased plasma and urinary cGMP levels, and augmented renal tissue cGMP after infusion. Only the NP, but not control treatment, acutely improved GFR to baseline. CRRL269 treatment stimulated significantly increased urine volume and urine sodium excretion, consistent with the natriuretic properties of NPs. Remarkably, the animals treated with the designer NP experienced no reductions in blood pressure. Only right atrial pressure decreased in the treated animals, likely reflecting the diuretic effect of the peptide.

The investigators then observed that CRRL269 treatment correlated with reduced evidence of cellular apoptosis in the renal cortex. Using an ischemia/reoxygenation protocol in human cultured proximal tubular epithelial cells, the authors detected similar reductions of apoptosis and apoptotic gene expression with CRRL269, which occurred to a greater extent than with ANP or BNP treatment. Critically, the authors demonstrated that inhibition of PKG kinase activity in the cell culture model blocked the anti-apoptotic effects of CRRL269, indicating that CRRL269 effects require PKG kinase activity and thus most likely occur through augmentation of cGMP synthesis.

By identifying that the NP effect on renal function and urine output does not require vascular effects, these experiments provide important conceptual insights. Specifically, they support the emerging paradigm that rational modulation of NPs can exploit the beneficial effects of cGMP in target tissues, but avoid excess vasodilation and hypotension which has limited these therapies previously. Prior work does support important roles for NPs, cGMP, and PKG in renal tissue⁹, but the physiologic and cellular findings of the current study raise an important question: what are the critical substrates of PKG in renal tissue mediating the NP anti-apoptotic and natriuretic effects, and do they differ from PKG substrates in vascular tissue which mediate vasorelaxation? Understanding this could identify additional downstream candidate therapeutic targets for renal disease.

Another question relates to the degree to which prevention of renal cellular apoptosis truly mediates the CRRL269 effects on urine output and GFR. Additional translational questions include whether CRRL269 might improve renal function in other models of kidney damage, and, of course, in humans with different types of renal dysfunction. Finally, the authors carefully studied the effects of their designer NP on acute renal physiology, but the effects on longer term kidney function remain unclear. Future investigations will hopefully answer these clinically relevant questions.

Although this study focused on renal function in a preclinical surgical model, the findings have important cardiovascular implications. Renal dysfunction and volume overload remain major consequences of heart failure. However, conventional strategies to reduce volume overload with diuretics often prove inadequate or ultimately contribute to further renal dysfunction, one form of a spectrum of conditions known as cardiorenal syndrome¹⁰. Alternative strategies using synthetic NPs like nesiritide to improve cardiac output and thus renal perfusion in heart failure frequently induce limiting hypotension,⁶ and have not improved outcomes in this condition. Thus it will be informative to evaluate the effects of CRRL269, and other innovative NPs, as potentially hemodynamically neutral treatments of renal dysfunction and volume overload in the setting of heart failure.

Finally, the authors demonstrated that CRRL269, augments cGMP not only in renal tissue, but also in the plasma and urine. Other NP-modulating drugs, like neprilysin inhbitors, also appear to increase extracellular cGMP¹¹. Why does cGMP, which functions as an intracellular second messenger, become regulated in the extracellular space, and what might be the clinical and biological significance of this?

In the meantime, the current work represents an important step in parsing out the biological effects of NPs and downstream cGMP-PKG signaling. And, this work supports the greater concept that understanding the basic biology underlying physiological processes can lead directly to novel therapeutic strategies for patients with cardiovascular (and cardiorenal) disease.