Adrenomedullin facilitates reinnervation of phenol-injured perivascular nerves in the rat mesenteric resistance artery

The role of Mas receptor on renal hemodynamic responses to angiotensin II administration in chronic renal sympathectomized male and female rats

Background and purpose

Renal hemodynamics is influenced by renal sympathetic nerves and the renin-angiotensin system. On the other hand, renal sympathetic denervation impacts kidney weight by affecting renal hemodynamics. The current study evaluated the role of the Mas receptor on renal hemodynamic responses under basal conditions and in response to angiotensin II (Ang II) in chronic renal sympathectomy in female and male rats.

Experimental approach

Forty-eight nephrectomized female and male rats were anesthetized and cannulated. Afterward, the effect of chronic renal sympathectomy was investigated on hemodynamic parameters such as renal vascular resistance (RVR), mean arterial pressure (MAP), and renal blood flow (RBF). In addition, the effect of chronic sympathectomy on kidney weight was examined.

Findings/Results

Chronic renal sympathectomy increased RVR and subsequently decreased RBF in both sexes. Renal perfusion pressure also increased after sympathectomy in male and female rats, while MAP did not change, significantly. In response to the Ang II injection, renal sympathectomy caused a greater decrease in RBF in all experimental groups, while it did not affect the MAP response. In addition, chronic sympathectomy increased left kidney weight in right nephrectomized rats.

Conclusion and implications

Chronic renal sympathectomy changed systemic/renal hemodynamics in baseline conditions and only renal hemodynamics in response to Ang II administration. Moreover, chronic sympathectomy increased compensatory hypertrophy in nephrectomized rats. These changes are unaffected by gender difference and Mas receptor blocker.

Adrenomedullin Ameliorates Pulmonary Fibrosis by Regulating TGF-ß-Smads Signaling and Myofibroblast Differentiation

Pulmonary fibrosis is an irreversible, potentially fatal disease. Adrenomedullin (AM) is a multifunctional peptide whose activity is regulated by receptor activity-modifying protein 2 (RAMP2). In the present study, we used the bleomycin (BLM)-induced mouse pulmonary fibrosis model to investigate the pathophysiological significance of the AM-RAMP2 system in the lung. In heterozygous AM knockout mice (AM+/-), hydroxyproline content and Ashcroft scores reflecting the fibrosis severity were significantly higher than in wild-type mice (WT). During the acute phase after BLM administration, FACS analysis showed significant increases in eosinophil, monocyte, and neutrophil infiltration into the lungs of AM+/-. During the chronic phase, fibrosis-related molecules were upregulated in AM+/-. Notably, nearly identical changes were observed in RAMP2+/-. AM administration reduced fibrosis severity. In the lungs of BLM-administered AM+/-, the activation level of Smad3, a receptor-activated Smad, was higher than in WT. In addition, Smad7, an antagonistic Smad, was downregulated and microRNA-21, which targets Smad7, was upregulated compared to WT. Isolated AM+/- lung fibroblasts showed less proliferation and migration capacity than WT fibroblasts. Stimulation with TGF-β increased the numbers of α-SMA-positive myofibroblasts, which were more prominent among AM+/- cells. TGF-β-stimulated AM+/- myofibroblasts were larger and exhibited greater contractility and extracellular matrix production than WT cells. These cells were α-SMA (+), F-actin (+), and Ki-67(-) and appeared to be nonproliferating myofibroblasts (non-p-MyoFbs), which contribute to the severity of fibrosis. Our findings suggest that in addition to suppressing inflammation, the AM-RAMP2 system ameliorates pulmonary fibrosis by suppressing TGF-β-Smad3 signaling, microRNA-21 activity and differentiation into non-p-MyoFbs.

Adrenomedullin-Receptor Activity-Modifying Protein 2 System Ameliorates Subretinal Fibrosis by Suppressing Epithelial-Mesenchymal Transition in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a leading cause of visual impairment. Anti-vascular endothelial growth factor drugs used to treat AMD carry the risk of inducing subretinal fibrosis. We investigated the use of adrenomedullin (AM), a vasoactive peptide, and its receptor activity-modifying protein 2, RAMP2, which regulate vascular homeostasis and suppress fibrosis. The therapeutic potential of the AM-RAMP2 system was evaluated after laser-induced choroidal neovascularization (LI-CNV), a mouse model of AMD. Neovascular formation, subretinal fibrosis, and macrophage invasion were all enhanced in both AM and RAMP2 knockout mice compared with those in wild-type mice. These pathologic changes were suppressed by intravitreal injection of AM. Comprehensive gene expression analysis of the choroid after LI-CNV with or without AM administration revealed that fibrosis-related molecules, including Tgfb, Cxcr4, Ccn2, and Thbs1, were all down-regulated by AM. In retinal pigment epithelial cells, co-administration of transforming growth factor-β and tumor necrosis factor-α induced epithelial-mesenchymal transition, which was also prevented by AM. Finally, transforming growth factor-β and C-X-C chemokine receptor type 4 (CXCR4) inhibitors eliminated the difference in subretinal fibrosis between RAMP2 knockout and wild-type mice. These findings suggest the AM-RAMP2 system suppresses subretinal fibrosis in LI-CNV by suppressing epithelial-mesenchymal transition.

Adrenomedullin: an important participant in neurological diseases

Adrenomedullin, a peptide with multiple physiological functions in nervous system injury and disease, has aroused the interest of researchers. This review summarizes the role of adrenomedullin in neuropathological disorders, including pathological pain, brain injury and nerve regeneration, and their treatment. As a newly characterized pronociceptive mediator, adrenomedullin has been shown to act as an upstream factor in the transmission of noxious information for various types of pathological pain including acute and chronic inflammatory pain, cancer pain, neuropathic pain induced by spinal nerve injury and diabetic neuropathy. Initiation of glia-neuron signaling networks in the peripheral and central nervous system by adrenomedullin is involved in the formation and maintenance of morphine tolerance. Adrenomedullin has been shown to exert a facilitated or neuroprotective effect against brain injury including hemorrhagic or ischemic stroke and traumatic brain injury. Additionally, adrenomedullin can serve as a regulator to promote nerve regeneration in pathological conditions. Therefore, adrenomedullin is an important participant in nervous system diseases.

Nerve growth factor stimulates regeneration of perivascular nerve, and induces the maturation of microvessels around the injured artery

An immature vasa vasorum in the adventitia of arteries has been implicated in induction of the formation of unstable atherosclerotic plaques. Normalization/maturation of the vasa vasorum may be an attractive therapeutic approach for arteriosclerotic diseases. Nerve growth factor (NGF) is a pleotropic molecule with angiogenic activity in addition to neural growth effects. However, whether NGF affects the formation of microvessels in addition to innervation during pathological angiogenesis is unclear. In the present study, we show a new role for NGF in neovessels around injured arterial walls using a novel in vivo angiogenesis assay. The vasa vasorum around arterial walls was induced to grow using wire-mediated mouse femoral arterial injury. When collagen-coated tube (CCT) was placed beside the injured artery for 7-14 days, microvessels grew two-dimensionally in a thin layer on the CCT (CCT-membrane) in accordance with the development of the vasa vasorum. The perivascular nerve was found at not only arterioles but also capillaries in the CCT-membrane. Biodegradable hydrogels containing VEGF and NGF were applied around the injured artery/CCT. VEGF significantly increased the total length and instability of microvessels within the CCT-membrane. In contrast, NGF induced regeneration of the peripheral nerve around the microvessels and induced the maturation and stabilization of microvessels. In an ex vivo nerve-free angiogenesis assay, although NGF potentially stimulated vascular sprouting from aorta tissues, no effects of NGF on vascular maturation were observed. These data demonstrated that NGF had potent angiogenic effects on the microvessels around the injured artery, and especially induced the maturation/stabilization of microvessels in accordance with the regeneration of perivascular nerves.

The mechanism of calcitonin gene-related peptide-containing nerve innervation

Calcitonin gene-related peptide (CGRP) is a major neurotransmitter and CGRP-containing primary sensory neurons play an important role in nociception and potent vasodilation. CGRP-containing nerves in mesenteric arteries are decreased in pathological animal models (hypertension, diabetes, and atherosclerosis). In apolipoprotein E–knockout mice, which have atherosclerosis and peripheral sensory nerve defects, nerve growth factor (NGF)-mediated CGRP nerve facilitation was down-regulated, which may have been caused by the impairment of the Akt–NO–cGMP pathway. In addition, NGF-mediated CGRP neurite outgrowth was decreased in fructose-induced insulin-resistant rats. We recently discovered that renin–angiotensin inhibitors improved CGRP innervation in spontaneously hypertensive rats, indicating that rescuing CGRP nerve innervation might improve pathophysiological conditions. To find a novel reagent that facilitates CGRP nerves, a new model, phenol-injured perivascular nerve model rats, was established. Adrenomedullin, hepatocyte growth factor, and angiotensin II type 2 receptor activation induced CGRP nerve distribution in phenol-injured rats. Furthermore, in insulin-resistant model rats, the down-regulation of CGRP nerves was likely due to the depression of phosphoinositide 3-kinase (PI3K)-dependent Akt activation. Administration of candesartan improves CGRPergic function via the PI3K–Akt pathway in insulin-resistant rats. Thus, clarification of the mechanisms of CGRP nerve defects may constitute future therapeutic targets.

Paracrine control of mesenteric perivascular axo-axonal interaction

Immunohistochemical study of rat mesenteric arteries showed dense innervation of adrenergic nerves, calcitonin gene-related peptide (CGRP)-containing nerves (CGRPergic nerves), nitric oxide-containing nerves (nitrergic nerves). Double-immunostaining revealed that most CGRPergic or nitrergic nerves were in close contact with adrenergic nerves. CGRPergic and transient receptor potential vanilloid-1 (TRPV1)-immunopositive nerves appeared in the same neurone. In rat perfused mesenteric vascular beds without endothelium and with active tone, perfusion of nicotine, or bolus injection of capsaicin and acetylcholine and periarterial nerve stimulation (PNS) lowered pH levels of out flowed perfusate concomitant with vasodilation. Cold-storage denervation of preparations abolished pH lowering induced by nicotine and PNS. Guanethidine inhibited PNS- and nicotine-, but not acetylcholine- and capsaicin-, induced pH lowering. Pharmacological analysis showed that protons were released not only from adrenergic nerves but also from CGRPergic nerves. A study using a fluorescent pH indicator demonstrated that nicotine, acetylcholine and capsaicin applied outside small mesenteric artery lowered perivascular pH levels, which were not observed in Ca(2+) free medium. Exogenously injected hydrochloric acid in denuded preparations induced pH lowering and vasodilation, which was inhibited by denervation, TRPV1 antagonists and capsaicin without affecting pH lowering. These results suggest that excitement of adrenergic nerves releases protons to activate TRPV1 in CGRPergic nerves and thereby induce vasodilation. It is also suggested that CGRPergic nerves release protons with exocytosis to facilitate neurotransmission via a positive feedback mechanism.

Contribution of nerve growth factor to augmented TRPV1 responses of muscle sensory neurons by femoral artery occlusion

In rats, hindlimb muscle ischemia induced by femoral artery occlusion augments the sympathetic nervous response to stimulation of transient receptor potential vanilloid type 1 (TRPV1) by injection of capsaicin into the arterial blood supply of the hindlimb muscles. The enhanced sympathetic response is due to alterations in TRPV1 receptor expression and its responsiveness in sensory neurons. The underlying mechanism by which TRPV1 receptor responses are increased after muscle vascular insufficiency/ischemia is unclear. In this report we tested the hypothesis that muscle ischemia elevates nerve growth factor (NGF) levels in primary afferent neurons, thereby increasing TRPV1 responsiveness. Muscle vascular insufficiency induced by the femoral artery ligation significantly increased NGF in the dorsal root ganglion (DRG) compared with sham controls. Furthermore, when NGF was infused in the hindlimb muscles of healthy rats (72 h using an osmotic minipump), the magnitude of the DRG neuron response to capsaicin was augmented (5.4 +/- 0.54 nA with NGF infusion vs. 3.0 +/- 0.17 nA in control; P < 0.05). With the addition of NGF in the culture dish containing the DRG neurons, the magnitude of the DRG neuron response to capsaicin was greater (6.4 +/- 0.27 nA; P < 0.05 vs. control) than that seen in control (2.9 +/- 0.16 nA). Note that this NGF effect was seen in isolectin B(4)-negative DRG neurons, a group of thin fiber nerves that contain neuropeptides and depend on NGF for survival. These data suggest that NGF affects a selective subpopulation of the afferent neurons in mediating augmented TRPV1 responses after femoral artery occlusion.