Control of sympathetic activity--new insights; new therapeutic targets?

Recently, a large international group of 25 investigators identified a candidate gene for hypertension which codes for phosducin (Pdc) as a novel player in the sympathetic nervous system [1]. In their study, a wide spectrum of findings documented its so-far unrecognized and apparently important role in experimental and human stress-induced hypertension. This conclusion is based on data ranging from detailed analyses of the sympathetic system in genetically manipulated Pdc −/− (knock-out) mice to the demonstration of its relevance in humans: in two separate cohorts, French Canadian and Afro-American individuals, respectively, a correlation was documented between single-nucleotide polymorphisms (SNPs) in the Pdc gene and stress-dependent blood pressure reactions. The target gene identified by the investigators codes for the G-protein regulator phosducin. In mice, targeted Pdc deletion increased blood pressure, increased catecholamine turnover in the peripheral sympathetic system and prolonged action potential firing. This finding suggests that Pdc is a hypotensive principle, explaining why reduced function of Pdc amplifies the hypertensive reaction to stressors. In other words: Pdc puts a brake on sympathetic activity, and conversely Pdc malfunction will lead to hypertension. What is the site of action of Pdc? The Pdc gene and the Pdc protein are expressed in sympathetic ganglia, although to a much lower extent than in the retina and pineal gland, but not in heart, blood vessels and kidney, important players in blood pressure control. To identify the site of action, experimental manipulations sequentially excluded changes in heart function, vascular function or function of the central nervous system as candidates. Early in life, cardiac morphology was not altered in Pdc −/− mice, but left ventricular contractility was increased despite no change of sensitivity to adrenoceptor agonists (norepinephrine, dobutamine)—consistent with increased sympathetic nerve traffic. In older animals (>4 months), however, significant hypertrophy was seen with expression of fetal genes as evidence of target organ injury. Similarly, early in life no structural changes were seen in arterial vessels (intima, media or adventitia), nor was there thickening of media smooth muscle cells. The contractility of the vessels was unchanged. In older animals (>4 months), however, smooth muscle cell hypertrophy was seen in arteries, and endothelial cell dysfunction was noted. Telemetric blood pressure measurements to characterize the circadian rhythm documented nighttime elevation of blood pressure with no change of heart rate. In addition, stress manoeuvres such as exposure to new environments caused an excessive blood pressure increase which was abrogated by the α1-adrenoceptor blocker prazosin pointing to increased sympathetic activity. The norepinephrine concentrations, but not the epinephrine concentrations, were markedly increased, and in target tissues the concentrations of pre- and postreceptor metabolites were found, consistent with increased sympathetic neurotransmitter release. On the way from the brain stem, spinal cord and sympathetic ganglia, detailed experiments excluded brain stem and spinal cord as the culprit, pointing to structures distal to the spinal cord. After having excluded spinal and supraspinal structures as well as target organs, further experiments identified the ganglion as the culprit for the effect of Pdc malfunction. Superior cervical ganglia or stellate ganglia express Pdc. With respect to sympathetic neuron number or neuron morphology, there was no difference between Pdc −/− and controls, yet important function abnormalities could be identified: acetylcholine-induced action potentials persisted longer, and K+ channel currents were significantly greater. These experiments provided the crucial information identifying a functional link between Pdc and stress-induced hypertension. Absence of the Pdc gene product caused ganglion cell-autonomous increased activity of sympathetic neurons. This abnormality was not restricted to ganglions innervating the vasculature: Pdc −/− mice had also significantly larger pupil diameters (mydriasis), pointing to increased sympathetic activity of neurons in the superior cervical ganglia. As a beautiful example of translational research, the investigators then addressed the issue of human relevance of their findings and studied the potential association of human phosducin (PDC) with hypertension. They used a family-based approach in two independent cohorts, French Canadians and Afro-Americans, respectively. They characterized the blood pressure phenotype: awake systolic blood pressure (BP), BP response to standing and BP response to a mathematic test. They sequenced the PDC gene (and the neighboring COX2 gene which proved to be functionally irrelevant to the problem) in 48 French Canadians and 48 Afro-Americans and identified 49 SNPs, of which three were in an untranslated region. In single-point and haplotype-based analyses, significant associations with blood pressure phenotypes were found in the two populations. To take one example of a SNP: in French Canadians homozygous for the G allele, awake systolic BP was higher by 15 mmHg compared to those homozygous for the A allele with intermediate BP in the heterozygotes, suggesting a gene-dosage-dependent effect of PDC. The findings are in excellent agreement with recent reports of genome-wide association studies (GWAS) in Scandinavian type 2 diabetic patients [2] and a cohort of British Caucasians [3]. The search for candidate genes has yielded hundreds of blood-pressure-related phenotypes in genomic regions [4]. The merit of the present study is to go beyond associations and provide solid evidence for causal role of Pdc by combining experimental and population-based evidence. Whether Pdc expression and function of the Pdc protein are changed in individuals with reduced kidney function is unknown, and this will be a topic for future research. Nevertheless, even today one can envisage potentially important implications of the above findings of Beetz et al. [1] for nephrology: In the long run, information on phosducin may help to identify early those renal patients who are at high risk of sympathetic overactivity with implications for blood pressure and (potentially even blood-pressure-independent) effects on progression of renal disease and cardiovascular events. Phosducin may become a potential new target for novel drugs. It may contribute to individualization of treatment, providing arguments for the dedicated use of anti-sympathetic drugs (and in desperate cases potentially even catheter-based renal sympathetic denervation) [5]. Intervention at the level of phosducin may theoretically have advantages over anti-sympathetic medications acting on the receptor level because upstream reduction of sympathetic activity will reduce not only release of catecholamines but also of cotransmitters such as neuropeptide Y (NPY) and ATP, which potentiate noradrenergic pressor responses triggered by sympathetic nerve stimulation [6]. In the past, it was believed that hypertension in and by itself was one important cause of terminal kidney failure. This epidemiological association has recently been identified largely as an epidemiological artifact, at least in Afro-Americans. An association is found between the MYH-9 gene coding for the nonmuscle myosin IIA heavy chain, focal segmental glomerulosclerosis (FSGS) (and other types of renal disease) and renal failure [7,8]. MYH9 is expressed in podocytes and other glomerular cells. Even if it is disputed whether hypertension—with the exception of malignant hypertension—is the sole cause of end-stage renal disease (ESRD), it is undisputed that in individuals with preexisting renal problems high blood pressure accelerates loss of renal function [8–10]. Current management of hypertension in patients with CKD is not satisfactory, and better hypertension control is an important goal in the treatment of patients with CKD as propagated in the message of last year’s world kidney day [11]. In this context, the phosducin gene may well turn out to be an important risk factor for progression of kidney damage, not only because of the association with hypertension [1] but also because of blood-pressure-independent actions of sympathetic overactivity on progression. The association of SNPs in the phosducin gene with type 2 diabetes [2] is of particular interest because in type 2 diabetes higher blood pressure (even within the normotensive range) is a very strong predictor of progression into end-stage kidney disease. Sympathetic overactivity not only drives hypertension but also—in part blood pressure independently [12]—progression of kidney disease. Reducing sympathetic activity, e.g. by the centrally [13] acting sympatholytic agent moxonidine [14], or renal denervation [15] reduced glomerulosclerosis [14] and inflammatory injury, [15] in experimental models. In the very earliest stages of renal dysfunction, e.g. normotensive type 1 diabetic patients with microalbuminuria, moxonidine reduced albuminuria despite no change in ambulatory blood pressure [12], illustrating the relevance of sympathetic activation in human kidney disease. The relation between renal damage and sympathetic overactivity goes two ways [16]: not only does high blood pressure triggered by and/or accompanied by sympathetic overactivity adversely affect renal function in CKD; conversely, impaired renal function is also an important cause of sympathetic overactivity. Stimulatory signals in response to hypoxia and other injuries ascend from the damaged kidney into the central nervous system (CNS) and cause a rise in blood pressure [17]. Recording of post ganglionic sympathetic discharge to blood vessels in skeletal muscle showed increased burst frequency in the sural nerve of haemodialyzed patients [18], in predialysis patients [19] and even hypertensive autosomal dominant polycystic kidney disease (ADPKD) patients despite normal inulin clearance [20]. There is a new player on board, i.e. renalase [21]. The pro-enzyme is synthesized in and secreted from the kidney; conversion to renalase reduces sympathetic activity; it is diminished or absent in CKD [22]. The effectiveness of targeting the sympathetic system has been shown in experimental and clinical studies. Reduction of sympathetic activity by denervation or intervention with sympatholytic drugs even at subantihypertensive doses [14,23] ameliorated progressive kidney damage in renal damage models including administration of imidazoline receptor agonists acting on I1 receptors in the brainstem and the kidney: it reduced glomerulosclerosis and albuminuria in subtotally nephrectomized rats independent of blood pressure [14]. A prospective randomized double-blind study comparing moxonidine vs. nifedipine on top of renin–angiotensin system (RAS) blockade documented less progression despite virtually no difference in blood pressure [24]. Correction of uraemia by transplantation does not normalize sympathetic nerve activity. A beautiful proof for the kidney as a culprit responsible for sympathetic activity was reported by Hausberg et al. [25]: in patients with a functioning kidney graft, microneurography documented sympathetic overactivity which was completely abrogated after bilateral nephrectomy of the recipient’s own anuric kidneys. A further dramatic recent proof for the aetiological role of sympathetic overactivity of the kidney in resistant hypertension is the impressive blood pressure decrease in response to catheter-based renal sympathetic denervation [5]. In renal patients, sympathetic overactivity is important not only because it aggravates progression but also because it contributes to cardiovascular complications [26]. The importance of sympathetic overactivity for the clinical outcome in CKD patients was recently documented by Penne et al. [27]: microneurography showed that a correlation existed between burst frequency (an index of sympathetic overactivity) and cardiovascular events independent of glomerular filtration rate (GFR) and blood pressure. Thus, phosducin may well turn out to be an important risk factor for the cardiovascular complications in chronic kidney disease. Finally, do we really need interventions to reduce sympathetic overactivity in the age of generally available RAS blockade? RAS blockade has to some extent stolen the limelight from alternative interventions including reduction of sympathetic overactivity but in and by itself is very often not sufficient to control blood and target organ damage. Additional interventions on top of RAS blockade have recently become a hot target, and we predict that in this context interventions targeting the sympathetic nervous system will become increasingly attractive. The future will tell whether pharmacological manipulation of phosducin will play a role. Conflict of interest statement. None declared.