C-type natriuretic peptide is a Schwann cell-derived factor for development and function of sensory neurones

Anatomical Evidence for Parasympathetic Innervation of the Renal Vasculature and Pelvis

BACKGROUND

The kidneys critically contribute to body homeostasis under the control of the autonomic nerves, which enter the kidney along the renal vasculature. Although the renal sympathetic and sensory nerves have long been confirmed, no significant anatomic evidence exists for renal parasympathetic innervation.

METHODS

We identified cholinergic nerve varicosities associated with the renal vasculature and pelvis using various anatomic research methods, including a genetically modified mouse model and immunostaining. Single-cell RNA sequencing (scRNA-Seq) was used to analyze the expression of AChRs in the renal artery and its segmental branches. To assess the origins of parasympathetic projecting nerves of the kidney, we performed retrograde tracing using recombinant adeno-associated virus (AAV) and pseudorabies virus (PRV), followed by imaging of whole brains, spinal cords, and ganglia.

RESULTS

We found that cholinergic axons supply the main renal artery, segmental renal artery, and renal pelvis. On the renal artery, the newly discovered cholinergic nerve fibers are separated not only from the sympathetic nerves but also from the sensory nerves. We also found cholinergic ganglion cells within the renal nerve plexus. Moreover, the scRNA-Seq analysis suggested that acetylcholine receptors (AChRs) are expressed in the renal artery and its segmental branches. In addition, retrograde tracing suggested vagus afferents conduct the renal sensory pathway to the nucleus of the solitary tract (NTS), and vagus efferents project to the kidney.

CONCLUSIONS

Cholinergic nerves supply renal vasculature and renal pelvis, and a vagal brain-kidney axis is involved in renal innervation.

Atrial Natriuretic Peptide Promotes Neurite Outgrowth and Survival of Cochlear Spiral Ganglion Neurons in vitro Through NPR-A/cGMP/PKG Signaling

Sensorineural hearing loss (SNHL) is a dominant public health issue affecting millions of people around the globe, which is correlated with the irreversible deterioration of the hair cells and spiral ganglion neurons (SGNs) within the cochlea. Strategies using bioactive molecules that regulate neurite regeneration and neuronal survival to reestablish connections between auditory epithelium or implanted electrodes and SGN neurites would become attractive therapeutic candidates for SNHL. As an intracellular second messenger, cyclic guanosine-3’,5’-monophosphate (cGMP) can be synthesized through activation of particulate guanylate cyclase-coupled natriuretic peptide receptors (NPRs) by natriuretic peptides, which in turn modulates multiple aspects of neuronal functions including neuronal development and neuronal survival. As a cardiac-derived hormone, atrial natriuretic peptide (ANP), and its specific receptors (NPR-A and NPR-C) are broadly expressed in the nervous system where they might be involved in the maintenance of diverse neural functions. Despite former literatures and our reports indicating the existence of ANP and its receptors within the inner ear, particularly in the spiral ganglion, their potential regulatory mechanisms underlying functional properties of auditory neurons are still incompletely understood. Our recently published investigation revealed that ANP could promote the neurite outgrowth of SGNs by activating NPR-A/cGMP/PKG cascade in a dose-dependent manner. In the present research, the influence of ANP and its receptor-mediated downstream signaling pathways on neurite outgrowth, neurite attraction, and neuronal survival of SGNs in vitro was evaluated by employing cultures of organotypic explant and dissociated neuron from postnatal rats. Our data indicated that ANP could support and attract neurite outgrowth of SGNs and possess a high capacity to improve neuronal survival of SGNs against glutamate-induced excitotoxicity by triggering the NPR-A/cGMP/PKG pathway. The neuroregenerative and neuroprotective effects of ANP/NPRA/cGMP/PKG-dependent signaling on SGNs would represent an attractive therapeutic candidate for hearing impairment.

Circulating products of C-type natriuretic peptide and links with organ function in health and disease

Paracrine actions of CNP and rapid degradation at source severely limit study of CNP's many roles in vivo. However provided sensitive and validated assays are used, there is increasing evidence that low concentrations of bioactive CNP in plasma, and the readily detectable concentrations of the bio-inactive processed product of proCNP (aminoterminal proCNP), can be used to advance understanding of the hormone's role in pathophysiology. Provided renal function is normal, concordant changes in both CNP and NTproCNP reflect change in tissue production of proCNP whereas change in CNP alone results from altered rates of bioactive CNP degradation and are reflected in the ratio of NTproCNP to CNP. As already shown in juveniles, where plasma concentration of CNP products are higher and are associated with concurrent endochondral bone growth, measurements of plasma CNP products in mature adults have potential to clarify organ response to stress and injury. Excepting the role of CNP in fetal-maternal welfare, this review examines evidence linking plasma CNP products with function of a wide range of tissues in adults, including the impact of extraneous factors such as nutrients, hormone therapy and exercise.

Atrial Natriuretic Peptide Improves Neurite Outgrowth from Spiral Ganglion Neurons In Vitro through a cGMP-Dependent Manner

The spiral ganglion neurons (SGNs) are the primary afferent neurons in the spiral ganglion (SG), while their degeneration or loss would cause sensorineural hearing loss. As a cardiac-derived hormone, atrial natriuretic peptide (ANP) plays a critical role in cardiovascular homeostasis through binding to its functional receptors (NPR-A and NPR-C). ANP and its receptors are widely expressed in the mammalian nervous system where they could be implicated in the regulation of multiple neural functions. Although previous studies have provided direct evidence for the presence of ANP and its functional receptors in the inner ear, their presence within the cochlear SG and their regulatory roles during auditory neurotransmission and development remain largely unknown. Based on our previous findings, we investigated the expression patterns of ANP and its receptors in the cochlear SG and dissociated SGNs and determined the influence of ANP on neurite outgrowth in vitro by using organotypic SG explants and dissociated SGN cultures from postnatal rats. We have demonstrated that ANP and its receptors are expressed in neurons within the cochlear SG of postnatal rat, while ANP may promote neurite outgrowth of SGNs via the NPR-A/cGMP/PKG pathway in a dose-dependent manner. These results indicate that ANP would play a role in normal neuritogenesis of SGN during cochlear development and represents a potential therapeutic candidate to enhance regeneration and regrowth of SGN neurites.

Natriuretic peptide system expression in murine and human submandibular salivary glands: a study of the spatial localisation of ANB, BNP, CNP and their receptors

The natriuretic peptide (NP) system comprises of three ligands, the Atrial Natriuretic Peptide (ANP), Brain Natriuretic peptide (BNP) and C-type Natriuretic peptide (CNP), and three natriuretic peptide receptors, NPRA, NPRB and NPRC. Here we present a comprehensive study of the natriuretic peptide system in healthy murine and human submandibular salivary glands (SMGs). We show CNP is the dominant NP in mouse and human SMG and is expressed together with NP receptors in ducts, autonomic nerves and the microvasculature of the gland, suggesting CNP autocrine signalling may take place in some of these glandular structures. These data suggest the NP system may control salivary gland function during homeostasis through the regulation of electrolyte re-absorption, neural stimulation and/or blood vessel wall contraction/relaxation. We also show abnormal expression of NPRA in the stroma of a subset of human SMGs resected from patients diagnosed with oral squamous cell carcinoma (OSCC) of non-salivary gland origin. This finding warrants further research to investigate a possible correlation between early OSCC invasion and NPRA overexpression.

Transcriptome profiles of corticosterone-induced cytotoxicity reveals the involvement of neurite growth-related genes in depression

Corticosterone (CORT), the main HPA-axis glucocorticoid hormone in rodents, is involved in the regulation of animal stress responses. However, the neural mechanisms underlying the effects of corticosteroids on depression are yet to be elucidated. We found that fluoxetine reversed neurite growth inhibition induced by CORT in PC12 cells, a widely used model system for neurobiological and neurotoxicological studies. Transcriptome profiling showed that 1,609 genes were up-regulated, whereas 1,764 genes were down-regulated significantly in the CORT group in comparison with the Control group. Of them, the expression of 589 DEGs was reversed after fluoxetine treatment, and genes related to cell morphogenesis, neurite growth, and immune function were involved in the neuroprotective effect of fluoxetine against CORT. Furthermore, expression of neurite growth-related genes, such as such as Calpain 2 (Capn2), vesicle-associated membrane protein 7 (Vamp7) and C-type natriuretic peptide (Cnp), altered in a brain region- or treatment-specific manner in the animal models of depression. Therefore, the interaction between stress, glucocorticoids, and neurite growth inhibition may be a candidate pathophysiology underlying major depressive disorder (MDD), and the identification of Capn2, Vamp7 and Cnp might provide insight into treatment of MDD.

Two isoforms of cyclic GMP-dependent kinase-I exhibit distinct expression patterns in the adult mouse dorsal root ganglion

cGMP-dependent kinase-I (cGKI) is known to regulate spinal pain processing. This enzyme consists of two isoforms (cGKIα and cGKIβ) that show distinct substrate specificity and tissue distribution. It has long been believed that the α isoform is exclusively expressed in the adult dorsal root ganglion. The aim of the present study was to reexamine the expression of cGKI isoforms in the adult mouse dorsal root ganglion using isoform-specific cGKI antibodies whose specificities had been validated in the previous studies. Immunoblot and immunohistochemical analyses revealed the presence of both isoforms in the dorsal root ganglion. Moreover, cGKIα was found to be mainly expressed within the cytoplasm of small- to medium-sized peptidergic and nonpeptidegic C-fibers, whereas cGKIβ was located within the nuclei of a wide range of dorsal root ganglion neurons. In addition, glutamine synthetase-positive satellite glial cells expressed both isoforms to varying degrees. Finally, using an experimental model for neuropathic pain produced by L5 spinal nerve transection, we found that cGKIα expression was downregulated in the injured, but not in the uninjured, dorsal root ganglion. In contrast, cGKIβ expression was upregulated in both the injured and uninjured dorsal root ganglions. Also, injury-induced cGKIβ upregulation was found to occur in small-to-medium-diameter dorsal root ganglion neurons. These data thus demonstrate the existence of two differently distributed cGKI isoforms in the dorsal root ganglion, and may provide insight into the cellular and molecular mechanisms of pain.

Brain-specific natriuretic peptide receptor-B deletion attenuates high-fat diet-induced visceral and hepatic lipid deposition in mice

C-type natriuretic peptide (CNP) and its receptor, natriuretic peptide receptor-B (NPR-B), are abundantly distributed in the hypothalamus. To explore the role of central CNP/NPR-B signaling in energy regulation, we generated mice with brain-specific NPR-B deletion (BND mice) by crossing Nestin-Cre transgenic mice and mice with a loxP-flanked NPR-B locus. Brain-specific NPR-B deletion prevented body weight gain induced by a high-fat diet (HFD), and the mesenteric fat and liver weights were significantly decreased in BND mice fed an HFD. The decreased liver weight in BND mice was attributed to decreased lipid accumulation in the liver, which was confirmed by histologic findings and lipid content. Gene expression analysis revealed a significant decrease in the mRNA expression levels of CD36, Fsp27, and Mogat1 in the liver of BND mice, and uncoupling protein 2 mRNA expression was significantly lower in the mesenteric fat of BND mice fed an HFD than in that of control mice. This difference was not observed in the epididymal or subcutaneous fat. Although previous studies reported that CNP/NPR-B signaling inhibits SNS activity in rodents, SNS is unlikely to be the underlying mechanism of the metabolic phenotype observed in BND mice. Taken together, CNP/NPR-B signaling in the brain could be a central factor that regulates visceral lipid accumulation and hepatic steatosis under HFD conditions. Further analyses of the precise mechanisms will enhance our understanding of the contribution of the CNP/NPR-B system to energy regulation.

Nitric oxide-mediated pain processing in the spinal cord

A large body of evidence indicates that nitric oxide (NO) plays an important role in the processing of persistent inflammatory and neuropathic pain in the spinal cord. Several animal studies revealed that inhibition or knockout of NO synthesis ameliorates persistent pain. However, spinal delivery of NO donors caused dual pronociceptive and antinociceptive effects, pointing to multiple downstream signaling mechanisms of NO. This review summarizes the localization and function of NO-dependent signaling mechanisms in the spinal cord, taking account of the recent progress made in this field.

Oxidant-induced activation of cGMP-dependent protein kinase Iα mediates neuropathic pain after peripheral nerve injury

AIMS

Emerging lines of evidence indicate that oxidants such as hydrogen peroxide exert specific signaling functions during the processing of chronic pain. However, the mechanisms by which oxidants regulate pain processing in vivo remain poorly understood. Here, we investigated whether cyclic guanosine monophosphate (cGMP)-dependent protein kinase Iα (cGKIα), which can be activated by oxidants independently of cGMP, serves as a primary redox target during pain processing.

RESULTS

After peripheral nerve injury, oxidant-induced cGKIα activation is increased in dorsal root ganglia of mice. Knock-in (KI) mice in which cGKIα cannot transduce oxidant signals demonstrated reduced neuropathic pain behaviors after peripheral nerve injury, and reduced pain behaviors after intrathecal delivery of oxidants. In contrast, acute nociceptive, inflammatory, and cGMP-induced pain behaviors were not impaired in these mice.

INNOVATION

Studying cGKIα KI mice, we provide the first evidence that oxidants activate cGKIα in sensory neurons after peripheral nerve injury in vivo.

CONCLUSION

Our results suggest that oxidant-induced activation of cGKIα specifically contributes to neuropathic pain processing, and that prevention of cGKIα redox activation could be a potential novel strategy to manage neuropathic pain.

Corin in natriuretic peptide processing and hypertension

Corin is a serine protease originally isolated from the heart. Functional studies show that corin is the long-sought enzyme responsible for activating cardiac natriuretic peptides. In mice, lack of corin prevents natriuretic peptide processing, causing salt-sensitive hypertension. In humans, corin variants and mutations that reduce corin activity have been identified in patients with hypertension and heart failure. Decreased plasma levels of corin antigen and activity have been reported in patients with heart failure and coronary artery disease. Low levels of urinary corin also have been found in patients with chronic kidney disease. Most recent studies show that corin also acts in the uterus to promote spiral artery remodeling and prevent pregnancy-induced hypertension. Here, we review the role of corin in natriuretic peptide processing and cardiovascular diseases such as hypertension, heart disease, pre-eclampsia, and chronic kidney disease.

Improved genetically-encoded, FlincG-type fluorescent biosensors for neural cGMP imaging

Genetically-encoded biosensors are powerful tools for understanding cellular signal transduction mechanisms. In aiming to investigate cGMP signaling in neurones using the EGFP-based fluorescent biosensor, FlincG (fluorescent indicator for cGMP), we encountered weak or non-existent fluorescence after attempted transfection with plasmid DNA, even in HEK293T cells. Adenoviral infection of HEK293T cells with FlincG, however, had previously proved successful. Both constructs were found to harbor a mutation in the EGFP domain and had a tail of 17 amino acids at the C-terminus that differed from the published sequence. These discrepancies were systematically examined, together with mutations found beneficial for the related GCaMP family of Ca²⁺ biosensors, in a HEK293T cell line stably expressing both nitric oxide (NO)-activated guanylyl cyclase and phosphodiesterase-5. Restoring the mutated amino acid improved basal fluorescence whereas additional restoration of the correct C-terminal tail resulted in poor cGMP sensing as assessed by superfusion of either 8-bromo-cGMP or NO. Ultimately, two improved FlincGs were identified: one (FlincG2) had the divergent tail and gave moderate basal fluorescence and cGMP response amplitude and the other (FlincG3) had the correct tail, a GCaMP-like mutation in the EGFP region and an N-terminal tag, and was superior in both respects. All variants tested were strongly influenced by pH over the physiological range, in common with other EGFP-based biosensors. Purified FlincG3 protein exhibited a lower cGMP affinity (0.89 μM) than reported for the original FlincG (0.17 μM) but retained rapid kinetics and a 230-fold selectivity over cAMP. Successful expression of FlincG2 or FlincG3 in differentiated N1E-115 neuroblastoma cells and in primary cultures of hippocampal and dorsal root ganglion cells commends them for real-time imaging of cGMP dynamics in neural (and other) cells, and in their subcellular specializations.

The C-type natriuretic peptide induces thermal hyperalgesia through a noncanonical Gβγ-dependent modulation of TRPV1 channel

Natriuretic peptides (NPs) control natriuresis and normalize changes in blood pressure. Recent studies suggest that NPs are also involved in the regulation of pain sensitivity, although the underlying mechanisms remain essentially unknown. Many biological effects of NPs are mediated by guanylate cyclase (GC)-coupled NP receptors, NPR-A and NPR-B, whereas the third NP receptor, NPR-C, lacks the GC kinase domain and acts as the NP clearance receptor. In addition, NPR-C can couple to specific Gα(i)-Gβγ-mediated intracellular signaling cascades in numerous cell types. We found that NPR-C is coexpressed in transient receptor potential vanilloid-1 (TRPV1)-expressing mouse dorsal root ganglia (DRG) neurons. NPR-C can be coimmunoprecipitated with Gα(i), and C-type natriuretic peptide (CNP) treatment induced translocation of protein kinase Cε (PKCε) to the plasma membrane of these neurons, which was inhibited by pertussis toxin pretreatment. Application of CNP potentiated capsaicin- and proton-activated TRPV1 currents in cultured mouse DRG neurons and increased their firing frequency, an effect that was absent in DRG neurons from TRPV1(-/-) mice. CNP-induced sensitization of TRPV1 activity was attenuated by pretreatment of DRG neurons with the specific inhibitors of Gβγ, phospholipase C-β (PLCβ), or PKC, but not of protein kinase A, and was abolished by mutations at two PKC phosphorylation sites in TRPV1. Furthermore, CNP injection into mouse hindpaw led to the development of thermal hyperalgesia that was attenuated by administration of specific inhibitors of Gβγ or TRPV1 and was also absent in TRPV1(-/-) mice. Thus, our work identifies the Gβγ-PLCβ-PKC-dependent potentiation of TRPV1 as a novel signaling cascade recruited by CNP in mouse DRG neurons that can lead to enhanced nociceptor excitability and thermal hypersensitivity.

cGMP signaling and branching of sensory axons in the spinal cord

Axonal branching is essential for neurons to establish contacts to different targets. It therefore provides the physical basis for the integration and distribution of information within the nervous system. During embryonic and early postnatal development, several axonal branching modes may be distinguished that might be regulated by activities of the growth cone or by the axon shaft. The various forms of axonal branching are dependent on intrinsic components and are regulated by extrinsic factors that activate specific signaling systems. This article focuses on components implicated in cyclic guanosine monophosphate signaling that regulate axon bifurcation – a specific form of branching – within the spinal cord in animal models. This cascade is composed of the ligand CNP, the guanylyl cyclase Npr2 and the cyclic guanosine monophosphate-dependent kinase I. In the absence of one of these components, axons of dorsal root ganglion neurons do not form T-shaped branches when entering the spinal cord, while collateral (interstitial) branching, another branching mode of the same type of the neuron, is not affected. It will be important to analyze human patients with mutations in the corresponding genes to get insights into the pathophysiological effects of impaired sensory axon branching in the spinal cord.

CNGA3: a target of spinal nitric oxide/cGMP signaling and modulator of inflammatory pain hypersensitivity

A large body of evidence indicates that nitric oxide (NO) and cGMP contribute to central sensitization of pain pathways during inflammatory pain. Here, we investigated the distribution of cyclic nucleotide-gated (CNG) channels in the spinal cord, and identified the CNG channel subunit CNGA3 as a putative cGMP target in nociceptive processing. In situ hybridization revealed that CNGA3 is localized to inhibitory neurons of the dorsal horn of the spinal cord, whereas its distribution in dorsal root ganglia is restricted to non-neuronal cells. CNGA3 expression is upregulated in the superficial dorsal horn of the mouse spinal cord and in dorsal root ganglia following hindpaw inflammation evoked by zymosan. Mice lacking CNGA3 (CNGA3(-/-) mice) exhibited an increased nociceptive behavior in models of inflammatory pain, whereas their behavior in models of acute or neuropathic pain was normal. Moreover, CNGA3(-/-) mice developed an exaggerated pain hypersensitivity induced by intrathecal administration of cGMP analogs or NO donors. Our results provide evidence that CNGA3 contributes in an inhibitory manner to the central sensitization of pain pathways during inflammatory pain as a target of NO/cGMP signaling.

Regulation of C-type natriuretic peptide expression

C-type natriuretic peptide (CNP) is a member of the small family of natriuretic peptides that also includes atrial natriuretic peptide (ANP) and brain, or B-type natriuretic peptide (BNP). Unlike them, it performs its major functions in an autocrine or paracrine manner. Those functions, mediated through binding to the membrane guanylyl cyclase natriuretic peptide receptor B (NPR-B), or by signaling through the non-enzyme natriuretic peptide receptor C (NPR-C), include the regulation of endochondral ossification, reproduction, nervous system development, and the maintenance of cardiovascular health. To date, the regulation of CNP gene expression has not received the attention that has been paid to regulation of the ANP and BNP genes. CNP expression in vitro is regulated by TGF-β and receptor tyrosine kinase growth factors in a cell/tissue-specific and sometimes species-specific manner. Expression of CNP in vivo is altered in diseased organs and tissues, including atherosclerotic vessels, and the myocardium of failing hearts. Analysis of the human CNP gene has led to the identification of a number of regulatory sites in the proximal promoter, including a GC-rich region approximately 50 base pairs downstream of the Tata box, and shown to be a binding site for several putative regulatory proteins, including transforming growth factor clone 22 domain 1 (TSC22D1) and a serine threonine kinase (STK16). The purpose of this review is to summarize the current literature on the regulation of CNP expression, emphasizing in particular the putative regulatory elements in the CNP gene and the potential DNA-binding proteins that associate with them.

Natriuretic peptide system: an overview of studies using genetically engineered animal models

The mammalian natriuretic peptide system, consisting of at least three ligands and three receptors, plays critical roles in health and disease. Examination of genetically engineered animal models has suggested the significance of the natriuretic peptide system in cardiovascular, renal and skeletal homeostasis. The present review focuses on the in vivo roles of the natriuretic peptide system as demonstrated in transgenic and knockout animal models.

Central and peripheral forms of C-type natriuretic peptide (CNP): evidence for differential regulation in plasma and cerebrospinal fluid

Aminoterminal proCNP (NTproCNP), a stable product of CNP gene expression and readily measured in human plasma, provides a new approach to studies of CNP which is rapidly degraded at source. CNP is detectable in human CSF but the presence and proportions of NTproCNP in CSF are unknown. Since CNP is widely expressed throughout the CNS, we hypothesized that the ratio of NTproCNP to CNP in CSF is greatly increased when compared to plasma and that CSF CNP peptides may contribute to their concentrations in the systemic circulation. Concurrent plasma and CSF concentrations of CNP forms were measured in 51 subjects undergoing spinal anesthesia for arranged orthopedic procedures. Elevated concentrations of NTproCNP (1045 ± 359 pmol/L), characterized by HPLC-RIA, were found in CSF and greatly exceeded those of CNP (7.9 ± 3.2 pmol/L). The ratio of NTproCNP to CNP in CSF (145 ± 55) was much higher than in plasma (31 ± 27). A significant inverse relation was found between plasma and CSF CNP concentrations (r = -0.29, p < 0.05). cGMP and neprilysin were unrelated to CNP levels in CSF. We conclude that CNP is differentially regulated across the brain in normal health. Despite markedly elevated levels of NTproCNP in CSF, it is unlikely that these contribute to systemic levels in healthy adults. Identifying NTproCNP as the dominant CNP form in CSF opens up the possibility of its use in future studies exploring CNP regulation within the CNS and possible applications in the diagnosis and monitoring of subjects with central neural disorders.

SNO-ing at the nociceptive synapse?

Nitric oxide is generally considered a pronociceptive retrograde transmitter that, by activation of soluble guanylyl cyclase-mediated cGMP production and activation of cGMP-dependent protein kinase, drives nociceptive hypersensitivity. The duality of its functions, however, is increasingly recognized. This review summarizes nitric-oxide-mediated direct S-nitrosylation of target proteins that may modify nociceptive signaling, including glutamate receptors and G-protein-coupled receptors, transient receptor potential channels, voltage-gated channels, proinflammatory enzymes, transcription factors, and redoxins. S-Nitrosylation events require close proximity of nitric oxide production and target proteins and a permissive redox state in the vicinity. Despite the diversity of potential targets and effects, three major schemes arise that may affect nociceptive signaling: 1) S-Nitrosylation-mediated changes of ion channel gating properties, 2) modulation of membrane fusion and fission, and thereby receptor and channel membrane insertion, and 3) modulation of ubiquitination, and thereby protein degradation or transcriptional activity. In addition, S-Nitrosylation may alter the production of nitric oxide itself.

C-type natriuretic peptide as a new regulator of food intake and energy expenditure

The physiological implication of C-type natriuretic peptide (CNP) including energy metabolism has not been elucidated, because of markedly short stature in CNP-null mice. In the present study we analyzed food intake and energy expenditure of CNP-null mice with chondrocyte-targeted CNP expression (CNP-Tg/Nppc(-/-) mice), in which marked skeletal dysplasia was rescued, to investigate the significance of CNP under minimal influences of skeletal phenotypes. In CNP-Tg/Nppc(-/-) mice, body weight and body fat ratio were reduced by 24% and 32%, respectively, at 20 wk of age, and decreases of blood glucose levels during insulin tolerance tests were 2-fold exaggerated at 17 wk of age, as compared with CNP-Tg/Nppc(+/+) mice. Urinary noradrenalin excretion of CNP-Tg/Nppc(-/-) mice was greater than that of CNP-Tg/Nppc(+/+) mice by 28%. In CNP-Tg/Nppc(-/-) mice, rectal temperature at 1600 h was higher by 1.1 C, and uncoupling protein-1 mRNA expression in the brown adipose tissue was 2-fold increased, which was canceled by propranolol administration, as compared with CNP-Tg/Nppc(+/+) mice. Oxygen consumption was significantly increased in CNP-Tg/Nppc(-/-) mice compared with that in CNP-Tg/Nppc(+/+) mice. Food intake of CNP-Tg/Nppc(-/-) mice upon ad libitum feeding and refeeding after 48 h starvation were reduced by 21% and 61%, respectively, as compared with CNP-Tg/Nppc(+/+) mice. This study unveiled a new aspect of CNP as a molecule regulating food intake and energy expenditure. Further analyses on precise mechanisms of CNP actions would lead to the better understanding of the significance of the CNP/guanylyl cyclase-B system in food intake and energy expenditure.

Gastrointestinal tract disorder in natriuretic peptide receptor B gene mutant mice

Natriuretic peptide receptor B (NPR-B), which has high affinity for C-type natriuretic peptide (CNP) and synthesizes intracellular cGMP, may be involved in gastrointestinal tract (GIT) regulation. A mutant allele of the NPR-B-encoding gene (Npr2) is responsible for the phenotype of the short-limb dwarfism (SLW) mouse. Homozygosity for this autosomal-recessive gene (slw/slw) leads to dwarfism and death before weaning because of milk retention in the stomach and intestinal distention. To elucidate the relationship between CNP/NPR-B signaling and GIT function, we investigated the association between Npr2 mutation and the GIT phenotype in slw/slw mice. The pylorus and large intestine of the mutants did not respond to CNP stimulation; further, they showed pyloric lumen narrowing with randomly aligned circular muscle cells. Comparison of the cGMP and neuronal marker distribution in GIT tissues confirmed cGMP expression in neuronal tissues. An Auerbach's plexus and submucosal tissues of the mutants didn't express cGMP and expressed Ca(2+). In contrast, those of normal mice (controls) expressed both cGMP and Ca(2+). Sequencing revealed that the causative Npr2 mutation was a 7-base deletion in exon 8, resulting in a frameshift and premature termination codon appearance. Therefore, the GIT phenotype of slw/slw mice is because of a CNP/NPR-B-signaling defect caused by an Npr2 mutation. These results facilitate better understanding of the role of CNP/NPR-B signaling in GIT motility.

Expression of guanylyl cyclase (GC)-A and GC-B during brain development: evidence for a role of GC-B in perinatal neurogenesis

Atrial (ANP) and C-type (CNP) natriuretic peptide generate physiological effects via selective activation of two closely related membrane receptors with guanylyl cyclase (GC) activity, known as GC-A and GC-B. As yet, however, the discrete roles for ANP/GC-A vs. CNP/GC-B signaling in many mammalian tissues are still poorly understood. We here used receptor affinity labeling and GC assays to characterize comparatively GC-A/GC-B expression and functional activity during rat brain development. The study revealed that GC-B predominates in the developing and GC-A in the adult brain, with regional differences each between cerebral cortex, cerebellum, and brain stem. Whereas GC-A levels nearly continuously increase between embryonal d 18 and adult, GC-B expression in brain is highest and widely distributed around postnatal d 1. The striking perinatal GC-B peak coincides with elevated expression of nestin, a marker protein for neural stem/progenitor cells. Immunohistochemical investigations revealed a cell body-restricted subcellular localization of GC-B and perinatal abundance of GC-B-expressing cells in regions high in nestin-expressing cells. However, and supported by examination of nestin-GFP transgenic mice, GC-B and nestin are not coexpressed in the same cells. Rather, GC-B(+) cells are distinguished by expression of NeuN, an early marker of differentiating neurons. These findings suggest that GC-B(+) cells represent neuronal fate-specific progeny of nestin(+) progenitors and raise the attention to specific and pronounced activities of CNP/GC-B signaling during perinatal brain maturation. The absence of this activity may cause the neurological disorders observed in GC-B-deficient mice.

Regulation of axonal development by natriuretic peptide hormones

Natriuretic peptides (NPs) are a family of cardiac- and vascular-derived hormones that are well known for regulating blood pressure, but their expression in the brain poses an intriguing yet unanswered question concerning their roles in the nervous system. Here, we report several unique activities of these hormones in regulating axonal development of dorsal root ganglion (DRG) neurons in the spinal cord. First, the C-type NP (CNP) is expressed in a restricted area of the dorsal spinal cord and provides a cue that is necessary for bifurcation of central sensory afferents. Second, in the culture of embryonic DRG neurons, CNP stimulates branch formation, induces axon outgrowth, and attracts growth cones. Furthermore, these activities are mediated by cyclic guanosine-3',5'-monophosphate (cGMP) signaling and can be elicited by other members of NP hormones. Thus, NPs represent a new class of extracellular factors that regulate key axonal processes during development. Because their receptors are present in many regions of the embryonic and adult brain, we propose that these hormones have wide influence on the development and function of the nervous system.

No NO, no pain? The role of nitric oxide and cGMP in spinal pain processing

A large body of evidence indicates that nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) essentially contribute to the processing of nociceptive signals in the spinal cord. Many animal studies have unanimously shown that inhibition of NO or cGMP synthesis can considerably reduce both inflammatory and neuropathic pain. However, experiments with NO donors and cGMP analogs also caused conflicting results because dual pronociceptive and antinociceptive effects of these molecules have been observed. Here, we summarize the most recent advances in the understanding of NO- and cGMP-dependent signaling pathways in the spinal cord and further unravel the role of NO and cGMP in pain processing.