Paracrine and endocrine pathways of natriuretic peptides assessed by ligand-receptor mapping in the Japanese eel brain

The natriuretic peptide (NP) family consists of cardiac NPs (ANP, BNP, and VNP) and brain NPs (CNPs) in teleosts. In addition to CNP1-4, a paralogue of CNP4 (named CNP4b) was recently discovered in basal teleosts including Japanese eel. Mammals have lost most Cnps during the evolution, but teleost cnps were conserved and diversified, suggesting that CNPs are important hormones for maintaining brain functions in teleost. The present study evaluated the potency of each Japanese eel CNP to their NP receptors (NPR-A, NPR-B, NPR-C, and NPR-D) overexpressed in CHO cells. A comprehensive brain map of cnps- and nprs-expressing neurons in Japanese eel was constructed by integrating the localization results obtained by in situ hybridization. The result showed that CHO cells expressing NPR-A and NPR-B induced strong cGMP productions after stimulation by cardiac and brain NPs, respectively. Regarding brain distribution of cnps, cnp1 is engaged in the ventral telencephalic area and periventricular area including the parvocellular preoptic nucleus (Pp), anterior/posterior tuberal nuclei, and periventricular gray zone of the optic tectum. cnp3 is found in the habenular nucleus and prolactin cells in the pituitary. cnp4 is expressed in the ventral telencephalic area, while cnp4b is expressed in the motoneurons in the medullary area. Such CNP isoform-specific localizations suggest that function of each CNP has diverged in the eel brain. Furthermore, the Pp lacking the blood-brain barrier expressed both npra and nprb, suggesting that endocrine and paracrine NPs interplay for regulating the Pp functions in Japanese eels.

Vascular endothelial cell-specific overexpression of CNP did not improve liver fibrosis in HFFCD-induced NASH, but did improve renal lesions

Mice with endothelial-cell-specific overexpression of C-type natriuretic peptide (E-CNP Tg mice) were shown to be protected against hepatic fibrosis and inflammation induced by high fat diet (HFD) feeding, with improved insulin sensitivity and attenuated weight gain. A recently developed high-fat, high-fructose, high-cholesterol diet (HFFCD) is considered to be a superior model to HFD, owing to the resemblance to human non-alcoholic steatohepatitis (NASH). In this study, we therefore aimed to reveal whether these previous findings with E-CNP Tg mice on HFD can be observed in a newly developed NASH model.　Patients with NASH have been suggested to be at higher risk of developing chronic kidney disease, so we also assessed the kidney histology of these mice. After 8 months of HFFCD feeding, the livers of E-CNP Tg mice and controls showed progressive fibrosis, which resembled the features of human NASH. However, no significant differences were observed in NAFLD activity scores between E-CNP Tg mice and controls, although there was a tendency for improvement in E-CNP Tg mice. The reduced levels of GCB, a receptor for CNP, may have weakened the action of CNP in the current model. In the kidneys, HFFCD showed glomerular hypertrophy and tubular atrophy in the cortical region, which were suppressed in E-CNP Tg mice. The present study did not prove the therapeutic effect of CNP on NASH in the HFFCD model, but provided evidence of its potential beneficial effects on NASH-associated renal damage.

cGMP and mitochondrial K+ channels-Compartmentalized but closely connected in cardioprotection

The 3',5'-cGMP pathway triggers cytoprotective responses and improves cardiomyocyte survival during myocardial ischaemia and reperfusion (I/R) injury. These beneficial effects were attributed to NO-sensitive GC induced cGMP production leading to activation of cGMP-dependent protein kinase I (cGKI). cGKI in turn phosphorylates many substrates, which eventually facilitate opening of mitochondrial ATP-sensitive potassium channels (mitoK_ATP ) and Ca²⁺ -activated potassium channels of the BK type (mitoBK). Accordingly, agents activating mitoK_ATP or mitoBK provide protection against I/R-induced damages. Here, we provide an up-to-date summary of the infarct-limiting actions exhibited by the GC/cGMP axis and discuss how mitoK_ATP and mitoBK, which are present at the inner mitochondrial membrane, confer mito- and cytoprotective effects on cardiomyocytes exposed to I/R injury. In view of this, we believe that the functional connection between the cGMP cascade and mitoK⁺ channels should be exploited further as adjunct to reperfusion therapy in myocardial infarction.

Mechanism of CNP-mediated DG-PKC and IP4 signaling pathway in diabetic rats with gastric motility disorder

In the precedent research conducted by the same team, it concluded that the activities in C-type natriuretic peptide (CNP)/cyclic guanosine monophosphate (cGMP)/cyclic adenosine monophosphate (cAMP)/β-type phospholipase C (PLCβ) pathways of rat antral smooth muscle were changed due to diabetes, which was the key pathogenetic mechanism for diabetic gastric dysmotility. As the follow-on step, this study was designed to probe into the downstream signaling pathway of CNP/PLCβ. The results showed that level of α-type protein kinase C (PKCα),cell membrane to cytoplasm ratio of PKCα, cell membrane to cytoplasmic ratio of βI-type protein kinase C (PKCβI) and level of Phosphor-PKCα (P-PKCα) were significantly reduced in diabetes rat antral smooth muscle samples. The content of tetraphosphate inositol (IP4) in gastric antral smooth muscle of diabetic rats reduced, and the content of diacyl-glycerol (DG) was unchanged. CNP significantly decreased the content of IP4 and DG, this effect was more obvious in diabetic rats. Subsequent to the addition of protein kinase A (PKA) blocker N-[2- (p-Bromocin-namylamino)ethyl]-5 -isoquinolinesulfonamide dihydrochloride (H-89) before CNP treatment, the inhibitory effect of CNP was reduced; subsequent to the addition of protein kinase G (PKG) blocker KT5823 before CNP treatment, the inhibitory effect of CNP was also reduced. With the addition of the combination of H-89 and KT5823 before CNP treatment, the inhibition by CNP could be offset. These results were concluded that CNP inhibited the activity of PKC family in rat smooth muscle and reduced the levels of IP4 and DG through the PKG/PKA-PLCβ pathways, causing inhibited muscular contractions, which may be a key pathogenetic factor for diabetic gastroparesis.

C-type natriuretic peptide restores impaired skeletal growth in a murine model of glucocorticoid-induced growth retardation

Glucocorticoids are widely used for treating autoimmune conditions or inflammatory disorders. Long-term use of glucocorticoids causes impaired skeletal growth, a serious side effect when they are used in children. We have previously demonstrated that C-type natriuretic peptide (CNP) is a potent stimulator of endochondral bone growth. In this study, we investigated the effect of CNP on impaired bone growth caused by glucocorticoids by using a transgenic mouse model with an increased circulating CNP level. Daily administration of a high dose of dexamethasone (DEX) to 4-week-old male wild-type mice for 4weeks significantly shortened their naso-anal length, which was restored completely in DEX-treated CNP transgenic mice. Impaired growth of the long bones and vertebrae by DEX was restored to a large extent in the CNP transgenic background, with recovery in the narrowed growth plate by increased cell volume, whereas the decreased proliferation and increased apoptosis of the growth plate chondrocytes were unaffected. Trabecular bone volume was not changed by DEX treatment, but decreased significantly in a CNP transgenic background. In young male rats, the administration of high doses of DEX greatly decreased N-terminal proCNP concentrations, a marker of CNP production. In organ culture experiments using fetal wild-type murine tibias, longitudinal growth of tibial explants was inhibited by DEX but reversed by CNP. These findings now warrant further study of the therapeutic potency of CNP in glucocorticoid-induced bone growth impairment.