Fatal gastrointestinal obstruction and hypertension in mice lacking nitric oxide-sensitive guanylyl cyclase

Abstract

The signaling molecule nitric oxide (NO), first described as endothelium-derived relaxing factor (EDRF), acts as physiological activator of NO-sensitive guanylyl cyclase (NO-GC) in the cardiovascular, gastrointestinal, and nervous systems. Besides NO-GC, other NO targets have been proposed; however, their particular contribution still remains unclear. Here, we generated mice deficient for the ␤1 subunit of NO-GC, which resulted in complete loss of the enzyme. GC-KO mice have a life span of 3-4 weeks but then die because of intestinal dysmotility; however, they can be rescued by feeding them a fiber-free diet. Apparently, NO-GC is absolutely vital for the maintenance of normal peristalsis of the gut. GC-KO mice show a pronounced increase in blood pressure, underlining the importance of NO in the regulation of smooth muscle tone in vivo. The lack of an NO effect on aortic relaxation and platelet aggregation confirms NO-GC as the only NO target regulating these two functions, excluding cGMP-independent mechanisms. Our knockout model completely disrupts the NO/cGMP signaling cascade and provides evidence for the unique role of NO-GC as NO receptor.cardiovascular ͉ knockout mice ͉ cGMP ͉ platelet aggregation ͉ smooth muscle relaxation T he nitric oxide (NO)/cGMP signaling cascade regulates a plethora of physiological functions in the cardiovascular, neuronal, and gastrointestinal systems (1-3). In the vascular system, NO, first recognized as endothelium-derived relaxing factor (EDRF; ref. 4), has been shown to mediate smooth muscle relaxation and inhibition of platelet aggregation. NO is synthesized by the family of NO synthases which exist in endothelial, neuronal, and inducible forms. The prominent receptor known to date is the enzyme NO-sensitive guanylyl cyclase (NO-GC). Stimulation of NO-GC by NO results in the production of the second messenger, cGMP, which exerts its effects via cGMPdependent kinases, channels, or phosphodiesterases (5-8). Besides these cGMP-mediated effects, NO is thought to mediate a variety of effects via cGMP-independent mechanisms in the cardiovascular system (for a review, see ref. 9).To gain further insight into the NO/cGMP signaling cascade, mice deficient in NO synthases (NOS) have been generated (10)(11)(12)(13)(14). Although these mouse lines have tremendously helped to understand NO/cGMP signaling, it is still not known which of NO's effects are mediated via NO-GC and thus cGMP, or alternatively, via pathways not involving cGMP.To address this point and to further investigate the physiological role of the enzyme and of the NO/cGMP signaling cascade in vivo, we generated an NO-GC-deficient mouse line. NO-GC is a heterodimer made up of two subunits, ␣ and ␤. Two isoforms are known to exist (␣ 1 ␤ 1 and ␣ 2 ␤ 1 ; ref. 15) in which the ␤ 1 subunit acts as the dimerizing partner for either ␣ subunit. ␣ subunits in the absence of the ␤ 1 subunit do not form dimers and are not catalytically active. Thus, deletion of the ␤ 1 subunit should completely eliminate NO-GC and yield a mouse line ...

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