Melanocortin receptors mediate alpha-MSH-induced stimulation of neurite outgrowth in neuro 2A cells

Concussive head injury exacerbates neuropathology of sleep deprivation: Superior neuroprotection by co-administration of TiO2-nanowired cerebrolysin, alpha-melanocyte-stimulating hormone, and mesenchymal stem cells

Sleep deprivation (SD) is common in military personnel engaged in combat operations leading to brain dysfunction. Military personnel during acute or chronic SD often prone to traumatic brain injury (TBI) indicating the possibility of further exacerbating brain pathology. Several lines of evidence suggest that in both TBI and SD alpha-melanocyte-stimulating hormone (α-MSH) and brain-derived neurotrophic factor (BDNF) levels decreases in plasma and brain. Thus, a possibility exists that exogenous supplement of α-MSH and/or BDNF induces neuroprotection in SD compounded with TBI. In addition, mesenchymal stem cells (MSCs) are very portent in inducing neuroprotection in TBI. We examined the effects of concussive head injury (CHI) in SD on brain pathology. Furthermore, possible neuroprotective effects of α-MSH, MSCs and neurotrophic factors treatment were explored in a rat model of SD and CHI. Rats subjected to 48h SD with CHI exhibited higher leakage of BBB to Evans blue and radioiodine compared to identical SD or CHI alone. Brain pathology was also exacerbated in SD with CHI group as compared to SD or CHI alone together with a significant reduction in α-MSH and BDNF levels in plasma and brain and enhanced level of tumor necrosis factor-alpha (TNF-α). Exogenous administration of α-MSH (250μg/kg) together with MSCs (1×10⁶) and cerebrolysin (a balanced composition of several neurotrophic factors and active peptide fragments) (5mL/kg) significantly induced neuroprotection in SD with CHI. Interestingly, TiO₂ nanowired delivery of α-MSH (100μg), MSCs, and cerebrolysin (2.5mL/kg) induced enhanced neuroprotection with higher levels of α-MSH and BDNF and decreased the TNF-α in SD with CHI. These observations are the first to show that TiO₂ nanowired administration of α-MSH, MSCs and cerebrolysin induces superior neuroprotection following SD in CHI, not reported earlier. The clinical significance of our findings in light of the current literature is discussed.

Mitogen- and stress-activated protein kinase-1 activation is involved in melanocortin-induced BDNF expression in Neuro2a neuronal cells

Melanocortins are neuropeptides exerting versatile functions in the nervous system. Melanocortin 4 receptor (MC4R) is primarily expressed in the brain and is thought to be a major mediator for melanocortin. Brain-derived neurotrophic factor (BDNF) may be a crucial downstream molecule of MC4R activation, to yield neurite outgrowth, neuroregenerative, anorexigenic and other actions. In this study, we stimulated Neuro2a murine neuronal cells with an α-melanocyte stimulating hormone (α-MSH) analog, [Nle(4), D-Phe(7)]melanocyte-stimulating hormone (NDP-MSH). In Neuro2a cells, NDP-MSH promoted neurite outgrowth. Upon NDP-MSH administration, BDNF expression was greatly enhanced. Furthermore, this effect was effectively reversed by the MC4R antagonist, JKC-363. We found that NDP-MSH treatment activated the ERK cascade and its downstream kinase MSK1 (mitogen- and stress-activated protein kinase-1). Antagonism of the MSK1 cascade by a specific inhibitor or overexpression of a defective MSK1 mutant interrupted the phosphorylation of the transcription factor cAMP-response element binding protein (CREB), blocking BDNF upregulation. In addition, MSK1 activation triggered an epigenetic alteration in histone H3 (Ser10), facilitating the expression of the BDNF gene. Taken together, our results showed that MSK1 kinase positively activates MC4R-induced BDNF expression via modulating the phosphorylation of CREB and histone H3 in Neuro2a neuronal cells.

Activation of NPFF2 receptor stimulates neurite outgrowth in Neuro 2A cells through activation of ERK signaling pathway

Neurite outgrowth is an important process in neural regeneration and plasticity, especially after neural injury, and recent evidence indicates that several G_αi/o protein-coupled receptors play an important role in neurite outgrowth. The neuropeptide (NP)FF system contains two G_αi/o protein-coupled receptors, NPFF₁ and NPFF₂ receptors, which are mainly distributed in the central nervous system. The aim of the present study was to determine whether the NPFF system is involved in neurite outgrowth in Neuro 2A cells. We showed that Neuro 2A cells endogenously expressed NPFF₂ receptor, and the NPFF₂ receptor agonist dNPA inhibited cyclic adenosine monophosphate (cAMP) production stimulated by forskolin in Neuro 2A cells. We also demonstrated that NPFF and dNPA dose-dependently induced neurite outgrowth in Neuro 2A cells, which was completely abolished by the NPFF receptor antagonist RF9. Pretreatment with mitogen-activated protein kinase inhibitors PD98059 and U0126 decreased dNPA-induced neurite outgrowth. In addition, dNPA increased phosphorylation of extracellular signal-regulated kinase (ERK) in Neuro 2A cells, which was completely antagonized by pretreatment with U0126. Our results suggest that activation of NPFF₂ receptor stimulates neurite outgrowth in Neuro 2A cells through activation of the ERK signaling pathway. Moreover, NPFF₂ receptor may be a potential therapeutic target for neural injury and degeneration in the future.

Developments and new vistas in the field of melanocortins

Melanocortins play a fundamental role in several basic functions of the organism (sexual activity, feeding, inflammation and immune responses, pain sensitivity, response to stressful situations, motivation, attention, learning, and memory). Moreover, a large body of animal data, some of which were also confirmed in humans, unequivocally show that melanocortins also have impressive therapeutic effects in several pathological conditions that are the leading cause of mortality and disability worldwide (hemorrhagic, or anyway hypovolemic, shock; septic shock; respiratory arrest; cardiac arrest; ischemia- and ischemia/reperfusion-induced damage of the brain, heart, intestine, and other organs; traumatic injury of brain, spinal cord, and peripheral nerves; neuropathic pain; toxic neuropathies; gouty arthritis; etc.). Recent data obtained in animal models seem to moreover confirm previous hypotheses and preliminary data concerning the neurotrophic activity of melanocortins in neurodegenerative diseases, in particular Alzheimer's disease. Our aim was (i) to critically reconsider the established extrahormonal effects of melanocortins (on sexual activity, feeding, inflammation, tissue hypoperfusion, and traumatic damage of central and peripheral nervous system) at the light of recent findings, (ii) to review the most recent advancements, particularly on the effects of melanocortins in models of neurodegenerative diseases, (iii) to discuss the reasons that support the introduction into clinical practice of melanocortins as life-saving agents in shock conditions and that suggest to verify in clinical setting the impressive results steadily obtained with melanocortins in different animal models of tissue ischemia and ischemia/reperfusion, and finally, (iv) to mention the advisable developments, particularly in terms of selectivity of action and of effects.

Exchange factors directly activated by cAMP mediate melanocortin 4 receptor-induced gene expression

G_s protein-coupled receptors regulate many vital body functions by activation of cAMP response elements (CRE) via cAMP-dependent kinase A (PKA)-mediated phosphorylation of the CRE binding protein (CREB). Melanocortin 4 receptors (MC4R) are prototypical G_s-coupled receptors that orchestrate the hypothalamic control of food-intake and metabolism. Remarkably, the significance of PKA for MC4R-induced CRE-dependent transcription in hypothalamic cells has not been rigorously interrogated yet. In two hypothalamic cell lines, we observed that blocking PKA activity had only weak or no effects on reporter gene expression. In contrast, inhibitors of exchange factors directly activated by cAMP-1/2 (EPAC-1/2) mitigated MC4R-induced CRE reporter activation and mRNA induction of the CREB-dependent genes c-fos and thyrotropin-releasing hormone. Furthermore, we provide first evidence that extracellular-regulated kinases-1/2 (ERK-1/2) activated by EPACs and not PKA are the elusive CREB kinases responsible for MC4R-induced CREB/CRE activation in hypothalamic cells. Overall, these data emphasize the pivotal role of EPACs rather than PKA in hypothalamic gene expression elicited by a prototypical G_s-coupled receptor.

Investigation of the effect of α-melanocyte-stimulating hormone on proliferation and early stages of differentiation of human induced pluripotent stem cells

We have studied the influence of α-melanocyte-stimulating hormone (α-MSH) on proliferation and early stages of differentiation of human induced pluripotent stem cells (iPSc). We have demonstrated that α-MSH receptor genes are expressed in undifferentiated iPSc. The expression levels of MCR1, MCR2, and MCR3 increased at the embryoid body (EB) formation stage. The formation of neural progenitors was accompanied by elevation of MCR2, MCR3, and MCR4 expression. α-MSH had no effect on EB generation and iPSc proliferation at concentrations ranging from 1 nM to 10 μM. At the same time, α-MSH increased the generation of neural rosettes in human iPSc cultures more than twice.

Ionizing radiation induces neuronal differentiation of Neuro-2a cells via PI3-kinase and p53-dependent pathways

PURPOSE

The influence of ionizing radiation (IR) on neuronal differentiation is not well defined. In this study, we investigated the effects of IR on the differentiation of Neuro-2a mouse neuroblastoma cells and the involvement of tumor protein 53 (p53) and mitogen-activated protein kinases (MAPK) during this process.

MATERIALS AND METHODS

The mouse neuroblastoma Neuro-2a cells were exposed to (137)Cs γ-rays at 4, 8 or 16 Gy. After incubation for 72 h with or without inhibitors of p53, phosphatidylinositol-4, 5-bisphosphate 3-kinase (PI3K) and other kinases, the neuronal differentiation of irradiated Neuro-2a cells was examined through analyzing neurite outgrowth and neuronal maker expression and the activation of related signaling proteins by western blotting and immunocytochemistry. Mouse primary neural stem cells (NSC) were exposed to IR at 1 Gy. The change of neuronal marker was examined using immunocytochemistry.

RESULTS

The irradiation of Neuro-2a cells significantly increased the neurite outgrowth and the expression of neuronal markers (neuronal nuclei [NeuN], microtubule-associated protein 2 [Map2], growth associated protein-43 [GAP-43], and Ras-related protein 13 [Rab13]). Immunocytochemistry revealed that neuronal class III beta-tubulin (Tuj-1) positive cells were increased and nestin positive cells were decreased by IR in Neuro-2a cells, which supported the IR-induced neuronal differentiation. However, the IR-induced neuronal differentiation was significantly attenuated when p53 was inhibited by pifithrin-α (PFT-α) or p53-small interfering RNA (siRNA). The PI3K inhibitor, LY294002, also suppressed the IR-induced neurite outgrowth, the activation of p53, the expression of GAP-43 and Rab13, and the increase of Tuj-1 positive cells. The increase of neurite outgrowth and Tuj-1 positive cells by IR and its suppression by LY294002 were also observed in mouse primary NSC.

CONCLUSION

These results suggest that IR is able to trigger the neuronal differentiation of Neuro-2a cells and the activation of p53 via PI3K is an important step for the IR-induced differentiation of Neuro-2a cells.

Astrocytes: new targets of melanocortin 4 receptor actions

Astrocytes exert a wide variety of functions with paramount importance in brain physiology. After injury or infection, astrocytes become reactive and they respond by producing a variety of inflammatory mediators that help maintain brain homeostasis. Loss of astrocyte functions as well as their excessive activation can contribute to disease processes; thus, it is important to modulate reactive astrocyte response. Melanocortins are peptides with well-recognized anti-inflammatory and neuroprotective activity. Although melanocortin efficacy was shown in systemic models of inflammatory disease, mechanisms involved in their effects have not yet been fully elucidated. Central anti-inflammatory effects of melanocortins and their mechanisms are even less well known, and, in particular, the effects of melanocortins in glial cells are poorly understood. Of the five known melanocortin receptors (MCRs), only subtype 4 is present in astrocytes. MC4R has been shown to mediate melanocortin effects on energy homeostasis, reproduction, inflammation, and neuroprotection and, recently, to modulate astrocyte functions. In this review, we will describe MC4R involvement in anti-inflammatory, anorexigenic, and anti-apoptotic effects of melanocortins in the brain. We will highlight MC4R action in astrocytes and discuss their possible mechanisms of action. Melanocortin effects on astrocytes provide a new means of treating inflammation, obesity, and neurodegeneration, making them attractive targets for therapeutic interventions in the CNS.

Plasma α-melanocyte stimulating hormone predicts outcome in ischemic stroke

BACKGROUND AND PURPOSE

α-Melanocyte stimulating hormone (α-MSH) is an endogenously produced neuropeptide derived from the same precursor as adrenocorticotropic hormone. α-MSH has profound immunomodulatory properties and may also be neuroprotective. Nothing is known about α-MSH and changes in its plasma concentrations in patients with acute ischemic stroke.

METHODS

In this prospective observational study, plasma concentrations of α-MSH, adrenocorticotropic hormone, cortisol, and interleukin 6 were assessed longitudinally over the course of 1 year after stroke onset in 111 patients. Logistic regression was used to the effect of initial plasma α-MSH, adrenocorticotropic hormone, cortisol, and interleukin 6 on long-term outcome.

RESULTS

There was an early decrease in plasma α-MSH in patients with severe stroke (National Institutes of Health Stroke Scale≥17) that normalized over the course of the year; these same patients evidenced elevations in plasma cortisol and interleukin 6. Higher initial plasma α-MSH, but not adrenocorticotropic hormone, cortisol, or interleukin 6, was independently predictive of good long-term outcome.

CONCLUSIONS

This research is the first to study endogenous changes in plasma α-MSH after stroke. The independent effect of early plasma α-MSH on stroke outcome, as well as a growing body of experimental data demonstrating improved stroke outcome with exogenous α-MSH administration, suggests a potential therapeutic role for α-MSH in the treatment of stroke.

The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology

The melanocortin-4 receptor (MC4R) was cloned in 1993 by degenerate PCR; however, its function was unknown. Subsequent studies suggest that the MC4R might be involved in regulating energy homeostasis. This hypothesis was confirmed in 1997 by a series of seminal studies in mice. In 1998, human genetic studies demonstrated that mutations in the MC4R gene can cause monogenic obesity. We now know that mutations in the MC4R are the most common monogenic form of obesity, with more than 150 distinct mutations reported thus far. This review will summarize the studies on the MC4R, from its cloning and tissue distribution to its physiological roles in regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain inflammation, and anxiety. I will then review the studies on the pharmacology of the receptor, including ligand binding and receptor activation, signaling pathways, as well as its regulation. Finally, the pathophysiology of the MC4R in obesity pathogenesis will be reviewed. Functional studies of the mutant MC4Rs and the therapeutic implications, including small molecules in correcting binding and signaling defect, and their potential as pharmacological chaperones in rescuing intracellularly retained mutants, will be highlighted.

Melanocortin receptor 4 is induced in nerve‐injured motor and sensory neurons of mouse

We previously identified melanocortin receptor 4 (MC4R) in a search for genes associated with hypoglossal nerve regeneration. As melanocortins promote nerve regeneration after axonal injury, we investigated whether MC4R functions as a key receptor for peripheral nerve regeneration. In situ hybridization revealed that MC4R mRNA is induced in mouse hypoglossal motor neurons after axonal injury, whereas mRNAs for MC1R, MC2R, MC3R, and MC5R are not expressed either before or after nerve injury. This result was confirmed by RT-PCR. The level of MC4R mRNA expression increased significantly from day 3 after axotomy, reached a peak on day 5, and decreased to the control level on day 14. Similar induction of MC4R was observed in axotomized mouse dorsal root ganglia (DRGs). MC4R mRNA expression was induced exclusively among the MCR family in the L4-6 DRG after sciatic nerve injury. We further examined whether alpha-melanocortin stimulating hormone (alpha-MSH) promotes neurite elongation via MC4R. In mouse DRG neuron culture, alpha-MSH significantly promoted neurite outgrowth at a concentration of 10(-8) mol/L. This neurite-elongation effect was entirely inhibited by the addition of a selective MC4R blocker, JKC-363. Therefore, it is concluded that alpha-MSH could stimulate neurite elongation via MC4R in DRG neurons. The present results suggest that induction of MC4R is crucial for motor and sensory neurons to regenerate after axonal injury.

Constitutive traffic of melanocortin-4 receptor in Neuro2A cells and immortalized hypothalamic neurons

Melanocortin-4 receptor (MC4R) is a G protein-coupled receptor (GPCR) that binds alpha-melanocyte-stimulating hormone (alpha-MSH) and has a central role in the regulation of appetite and energy expenditure. Most GPCRs are endocytosed following binding to the agonist and receptor desensitization. Other GPCRs are internalized and recycled back to the plasma membrane constitutively, in the absence of the agonist. In unstimulated neuroblastoma cells and immortalized hypothalamic neurons, epitopetagged MC4R was localized both at the plasma membrane and in an intracellular compartment. These two pools of receptors were in dynamic equilibrium, with MC4R being rapidly internalized and exocytosed. In the absence of alpha-MSH, a fraction of cell surface MC4R localized together with transferrin receptor and to clathrin-coated pits. Constitutive MC4R internalization was impaired by expression of a dominant negative dynamin mutant. Thus, MC4R is internalized together with transferrin receptor by clathrin-dependent endocytosis. Cell exposure toalpha-MSH reduced the amount of MC4R at the plasma membrane by blocking recycling of a fraction of internalized receptor, rather than by increasing its rate of endocytosis. The data indicate that, in neuronal cells, MC4R recycles constitutively and that alpha-MSH modulates MC4R residency at the plasma membrane by acting at an intracellular sorting step.

FAS inhibitor cerulenin reduces food intake and melanocortin receptor gene expression without modulating the other (an)orexigenic neuropeptides in chickens

Cerulenin, a natural fatty acid synthase (FAS) inhibitor, and its synthetic analog C75 are hypothesized to alter the metabolism of neurons in the hypothalamus that regulate ingestive behavior to cause a profound decrease of food intake and an increase in metabolic rate, leading to body weight loss. The bulk of data exclusively originates from mammals (rodents); however, such effects are currently lacking in nonmammalian species. We have, therefore, addressed this issue in broiler chickens because this species is selected for high growth rate and high food intake and is prone to obesity. First, we demonstrate that FAS messenger and protein are expressed in the hypothalamus of chickens. FAS immunoreactivity was detected in a number of brain regions, including the nucleus paraventricularis magnocellularis and the nucleus infundibuli hypothalami, the avian equivalent of the mammalian arcuate nucleus, suggesting that FAS may be involved in the regulation of food intake. Second, we show that hypothalamic FAS gene expression was significantly (P < 0.05) decreased by overnight fasting similar to that in liver, indicating that hypothalamic FAS gene is regulated by energy status in chickens. Finally, to investigate the physiological consequences of in vivo inhibition of fatty acid synthesis on food intake, we administered cerulenin by intravenous injections (15 mg/kg) to 2-wk-old broiler chickens. Cerulenin administration significantly reduced food intake by 23 to 34% (P < 0.05 to P < 0.0001) and downregulated FAS and melanocortin receptors 1, 4, and 5 gene expression (P < 0.05). However, the known orexigenic (neuropeptide Y, agouti gene-related peptide, orexin, and orexin receptor) and anorexigenic (pro-opiomelanocortin and corticotropin-releasing hormone) neuropeptide mRNA levels remained unchanged after cerulenin treatment. These results suggest that the catabolic effect of cerulenin in chickens may be mediated through the melanocortin system rather than the other neuropeptides known to be involved in food intake regulation.

Interactions between delta opioid receptors and alpha-adrenoceptors

1. Several studies have reported functional interactions between different subtypes of opioid and alpha2A-adrenoceptors in the induction of spinal cord analgesia. The mechanisms underlying this phenomenon are not well characterized. We propose that direct receptor-receptor associations could account for some of the observed functional interactions. In the present study, we examined the presence of delta opioid receptors and alpha2A-adrenoceptors in interacting complexes and the functional implications of such interactions on receptor activity. 2. Using the proximity based bioluminescence resonance energy transfer (BRET) assay, we found that the delta opioid receptors and alpha2A-adrenoceptors are in close enough proximity (< 100 A) in live cells that can foster physical interactions. 3. Using coimmunoprecipitation of differentially epitope-tagged receptors, we found that delta opiate receptors exist in interacting complexes with alpha2A-adrenoceptors in heterologous cells. 4. Finally, using receptor activity mediated neurite outgrowth in Neuro 2A cells as a physiological readout, we found that interactions between delta opiate receptors and alpha2A-adrenoceptors have functional consequences. The expression of alpha2A-adrenoceptors is sufficient to promote delta opiate receptor-mediated neurite outgrowth, suggesting that the presence of inactive alpha2A-adrenoceptors can enhance delta opiate receptor-mediated signalling. 5. Taken together, these findings suggest that modulation of receptor function as a result of physical associations between delta opiate receptors and alpha2A-adrenoceptors may account for the observed synergy between opiate and adrenergic agonists in spinal analgesia.

Proopiomelanocortin and melanocortin receptors in the adult rat retino-tectal system and their regulation after optic nerve transection

The aim of this study was to characterise the expression of the melanocortin system in the normal and injured rat visual system. Using real-time polymerase chain reaction and immunohistochemistry, we detected melanocortin MC(3), MC(4) and MC(5) receptors and proopiomelanocortin in adult retina and superior colliculus. Melanocortin MC(4) receptor mRNA was the most abundant receptor. Melanocortin MC(3), MC(4) and MC(5) receptors were localised to the ganglion cell and inner nuclear layers and the melanocortin MC(3) and MC(4) receptors were localised to retinal ganglion cells. Transection of the optic nerve leads to ganglion cell death and both melanocortin receptor and proopiomelanocortin expression decreased in superior colliculus after transection whereas the expression was unchanged or even increased in the retina. alpha-Melanocyte-stimulating hormone elicited neurite outgrowth from embryonic retinal explants. Together, these data implicate a role for the melanocortin system in the adult rat retina and that melanocortins can stimulate neurite growth from retinal neurons.

Accelerating sensory recovery after sciatic nerve crush: non-selective versus melanocortin MC4 receptor-selective peptides

Melanocortin receptor ligands accelerate functional recovery after peripheral nerve crush. It is not known which mechanism is involved or via which melanocortin receptor this effect occurs, albeit indirect evidence favours the melanocortin MC4 receptor. To test whether the melanocortin MC4 receptor is involved in the effects of melanocortins on functional recovery, we used melanocortin compounds that distinguish the melanocortin MC4 receptor from the melanocortin MC1, MC3 and MC5 receptors on basis of selectivity and agonist/antagonist profile. Activation and binding studies indicated that the previously described peptides JK1 (Ac-Nle-Gly-Lys-D-Phe-Arg-Trp-Gly-NH2) and [D-Tyr4]melanotan-II ([D-Tyr4]MTII. Ac-Nle-c[Asp-His-D-Tyr-Arg-Trp-Lys]NH2) are selective for the rat melanocortin MC4 receptor as compared to the rat melanocortin MC3 and MC5 receptors, but are also potent on the melanocortin MC1 receptor. Both peptides did not accelerate sensory recovery in rats with a sciatic nerve crush, whereas the non-selective melanocortin agonist melanotan-II (MTII, Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]NH2) was effective. The melanocortin MC3/MC4 receptor antagonist SHU9119 (Ac-Nle-c[Asp-His-D-Nal(2)-Arg-Trp-Lys]NH2) also enhanced sensory recovery. This effect was probably not due to interaction with the melanocortin MC4 receptor, since JK46 (Ac-Gly-Lys-His-D-Nal(2)-Arg-Trp-Gly-NH2), a selective melanocortin MC4 receptor antagonist, was ineffective. Taken together, these data suggest that melanocortins do not accelerate sensory recovery via interaction with the melanocortin MC4 receptor. From the known melanocortin receptors, only the involvement of the melanocortin MC5 receptor in acceleration of recovery could not be excluded.

Targeting Melanocortin Receptors as a Novel Strategy to Control Inflammation

Adrenocorticotropic hormone and alpha-, beta-, and gamma-melanocyte-stimulating hormones, collectively called melanocortin peptides, exert multiple effects upon the host. These effects range from modulation of fever and inflammation to control of food intake, autonomic functions, and exocrine secretions. Recognition and cloning of five melanocortin receptors (MCRs) has greatly improved understanding of peptide-target cell interactions. Preclinical investigations indicate that activation of certain MCR subtypes, primarily MC1R and MC3R, could be a novel strategy to control inflammatory disorders. As a consequence of reduced translocation of the nuclear factor kappaB to the nucleus, MCR activation causes a collective reduction of the major molecules involved in the inflammatory process. Therefore, anti-inflammatory influences are broad and are not restricted to a specific mediator. Short half-life and lack of selectivity could be an obstacle to the use of the natural melanocortins. However, design and synthesis of new MCR ligands with selective chemical properties are already in progress. This review examines how marshaling MCR could control inflammation.

Evidence for the interaction of protein kinase C and melanocortin 3-receptor signaling pathways

The melanocortin-3 receptor, MC3-R, is abundant in the brain and is activated by gamma-2-melanocyte stimulating hormone (gamma-2-MSH). We have previously reported the translocation of protein kinase C (PKC) in spontaneous hypertensive rat (SHR) brain synaptosomes treated with gamma-2-MSH. In this study, the expression of PKA and the related PKB in SHR brain synaptosomes was analyzed. PKA was detected in total synaptosomal fractions but not in particulate fractions, whereas PKB was not detected in either fraction. We next tested the hypothesis that the PKC pathway is involved in MC3-R signaling in a neuronal, CAD, cell line. Mobilization of intracellular Ca2+ was analyzed by dual fluorescence imaging of Fura-2AM loaded MC3-R transfected cells. An increase in intracellular Ca2+ was observed upon treatment with gamma-2-MSH. A MC3-R-green fluorescent protein (GFP) fusion protein was expressed and shown to localize mainly to the plasma membrane in the soma and to neurites in differentiated CAD cells. Treatment with gamma-2-MSH led to a punctate appearance and co-immunoprecipitation of the receptor fusion protein with protein kinase C-gamma (PKC-gamma). Differentiation of some neuronal cells has been shown to be associated with changes in the expression levels of protein kinase C isoenzymes. Induction of CAD cell differentiation was associated with down-regulation of the atypical PKC-zeta and protein kinase B (PKB/Akt1), that was less pronounced in MC3-R transfected cells. However, the levels of classical PKC isozymes, PKC-alpha, PKC-gamma, and PKC-beta were unchanged. These studies therefore indicate a role for PKC isozymes in gamma-2-MSH/MC3-R receptor signaling and in neuronal cell differentiation.

The role of melanocortins and their receptors in inflammatory processes, nerve regeneration and nociception

The melanocortins are a family of bioactive peptides derived from proopiomelanocortin. Those peptides, included among hormones and comprising ACTH, alpha-MSH, beta-MSH and gamma-MSH, are best known mainly for their physiological effects, such as the control of skin pigmentation by alpha-MSH, and ACTH effects on pigmentation and steroidogenesis. Melanocortins are released in various sites in the central nervous system and in peripheral tissues, and participate in the regulation of multiple physiological functions. They are involved in grooming behavior, food intake and thermoregulation processes, and can also modulate the response of the immune system in inflammatory states. Research of the past decade provided evidence that melanocortins could elicit their diverse biological effects by binding to a distinct family of G protein-coupled receptors with seven transmembrane domains. To date, five melanocortin receptor genes have been cloned and characterized. Those receptors differ in their tissue distribution and in their ability to recognize various melanocortins. These advances have opened up new horizons for exploring the significance of melanocortins, their ligands and their receptors for a variety of important physiological functions. We reviewed the origin of MSH peptides, the function and distribution of melanocortin receptors and their endogenous and exogenous ligands and the role of melanocortins and their receptors in inflammatory processes, nerve regeneration and nociception. Moreover, we analyzed their interaction with opioid peptides and finally, we discussed the postulated role of the melanocortin system in pain transmission at the spinal cord level.

Putative targets of CNS melanocortin receptor activity

Chronic antagonism of hypothalamic melanocortin receptors, primarily melanocortin-4 receptor (MC4R), is the molecular basis for "agouti obesity syndrome," whereas suppression of MC4R gene activity due to genetic mutations induces obesity in both rodents and humans. However, little is known about the neurocircuitry of MC4R-mediated control of energy balance, the regulation of MC4R gene expression, or how suppression of MC4R activity leads to differential expression of potential downstream central nervous system (CNS) targets or effectors of melanocortin signaling. This paper focuses on strategies for mapping CNS melanocortin circuits using transgenic mouse models for conditional expression of MC4R and MC3R as well as progress in characterizing the murine MC4R promoter. Additionally, preliminary studies that focus on putative targets of melanocortinergic signaling will include a discussion of CD81, a gene identified using the polymerase chain reaction-based method of suppression subtractive hybridization. CD81, first described as TAPA-1 (target of antiproliferative antibody), is a member of the tetraspanin family of cell surface proteins believed to function in cell-cell adhesion, signal transduction, and possibly neuronal plasticity. Elevated expression of CD81 mRNA in hypothalamic regions of obese yellow mice suggests that loss of MC4R activity may lead to altered neuronal function via modulation of the cell surface protein CD81.

The potent melanocortin receptor agonist melanotan-II promotes peripheral nerve regeneration and has neuroprotective properties in the rat

The neurotrophic and neuroprotective potential of the alpha-melanocyte-stimulating hormone (alpha-MSH) analog cyclo-[Ac-Nle(4),Asp(5),D-Phe(7),Lys(10)]alpha-MSH-(4-10) amide (melanotan-II), a potent melanocortin receptor agonist, was investigated. The sciatic nerve crush model was used as a paradigm to investigate the neurotrophic properties of melanotan-II. Melanotan-II significantly enhanced the recovery of sensory function following a crush lesion of the sciatic nerve in the rat at a dose of 20 microg kg(-1) per 48 h, s.c., but not at a dose of 2 or 50 microg kg(-1). In addition, we observed that melanotan-II also possesses neuroprotective properties, as it partially protected the nerve from a toxic neuropathy induced by cisplatin. Thus, the present data for the first time demonstrate the effectiveness of the potent alpha-MSH analog melanotan-II in nerve regeneration and neuroprotection.

Generation of expression constructs that secrete bioactive alphaMSH and their use in the treatment of experimental autoimmune encephalomyelitis

alpha Melanocyte-stimulating hormone (alphaMSH) is a 13 amino acid peptide with potent anti-inflammatory effects. We created two DNA expression constructs (miniPOMC and pACTH1-17) that encode bioactive versions of the alphaMSH peptide, and tested these constructs for therapeutic effects in experimental autoimmune encephalomyelitis (EAE). Each construct contained the sequences for alphaMSH, as well as the sequences that are involved in the secretion and processing of the POMC gene with the assumption that these sequences would promote processing and release of the encoded alphaMSH peptide. The differences between the two constructs lie at the C-terminal end where amino acids necessary for amidation of alphaMSH were included in only the pACTH1-17 construct. These two constructs were tested in vitro in bioassays, and in vivo in a mouse model of EAE. The results show that although bioactive peptides are secreted from cells transfected with either construct, there appears to be a significant therapeutic effect only with the pACTH1-17 construct which contains the extra C-terminal amino acids. The data suggest that it is possible to engineer DNA expression vectors encoding small secreted peptides such as alphaMSH, and that similar type constructs may be useful as therapeutics for the treatment of inflammatory diseases.

2,3-Diaryl-5-anilino[1,2,4]thiadiazoles as melanocortin MC4 receptor agonists and their effects on feeding behavior in rats

The melanocortin-4 receptor (MC4) modulates physiological functions such as feeding behavior, nerve regeneration, and drug addiction. Using a high throughput screen based on (125)I-NDP-MSH binding to the human MC4 receptor, we discovered 2,3-diaryl-5-anilino[1,2,4]thiadiazoles 3 as potent and selective MC4 receptor agonists. Through SAR development on the three attached aryl rings, we improved the binding affinity from 174 nM to 4.4 nM IC(50). When delivered intraperitoneally, compounds 3a, 3b, and 3c induced significant inhibition of food intake in a fasting-induced feeding model in rats. When delivered orally, these compounds lost activity, mainly due to rapid metabolism to inactive imidoylthiourea reduction products.

Modeling the pathways of energy balance using the N1E-115 murine neuroblastoma cell line

A good in vitro model within which to investigate molecular interactions between feeding relevant neuropeptide systems has been lacking. Consequently, we began using reverse transcriptase-polymerase chain reaction (RT-PCR) to screen various neuronal cell lines for the presence of feeding relevant neuropeptides and receptors. N1E-115 murine neuroblastoma cells have emerged as an attractive candidate for further analysis because they contain mRNA for a variety of key systems implicated in the regulation of energy homeostasis.

Mouse melanocortin-4 receptor gene 5'-flanking region imparts cell specific expression in vitro

Weight homeostasis is exquisitely sensitive to changes in the abundance of melanocortin-4 receptor (MC4-R). To begin to understand the factors that regulate MC4-R gene expression, we determined there are no introns in the gene, there are multiple starts of transcription, and a cluster of 3' ends. A series of MC4-R-luciferase gene reporter chimerics was developed and transfected into cell lines expressing (UMR106; GT1-7; HEK293) and not expressing (Neuro 2A) endogenous MC4-R mRNA. The longest construct, which includes approximately 3.3 kb 5'-flanking, 425 bp 5'-untranslated (UTR) and 1852 bp 3'-flanking, significantly increased luciferase reporter gene expression 24-, 13-, and 3-fold compared to pGL3-basic when expressed in HEK293, UMR106, and GT1-7 cells, respectively. Deletion analysis of mMC4-R 5'-flanking cDNA identified full mMC4-R promoter activity within 178 bp upstream of the major start of transcription. The mMC4-R gene structure and reporter chimerics provide a fundamental framework for the identification of specific factors regulating MC4-R gene expression.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Topiramate promotes neurite outgrowth and recovery of function after nerve injury

Topiramate is a structurally novel neurotherapeutic agent with a unique combination of pharmacological properties and currently is available in most world markets for treating several seizure disorders. Because its pharmacological profile was suggestive of possible activity as a neuroprotectant, topiramate was evaluated and found to be active in several animal models of stroke or neuropathic pain. This prompted an evaluation of topiramate as a possible neurotrophic agent. In this study, topiramate enhanced the recovery of facial nerve function after injury when administered orally at therapeutically relevant doses, and significantly increased neurite outgrowth in cell cultures derived from fetal rat cortical and hippocampal tissues.

Melanocortins and the treatment of nervous system disease. Potential relevance to the skin?

For several decades melanocortins have been implicated in the modulation of brain function. More recently, this idea has been supported by the identification and cloning of melanocortin (MC) receptors in the nervous system. MCs stimulate axonal growth in fetal neural tissue or in neural cell lines in culture. This feature was utilized in screening their neurotrophic or neuroprotection potential in animal studies of nervous system disease (peripheral nerve and spinal cord trauma, toxic and metabolic neuropathies, EAN, EAE, etc.). Some of these effects may be mediated by MC4 receptor activation, although as yet unknown receptors may also be involved (for instance, protection by Org 2766). To what extent MC-nervous system effects are related to known effects of MCs in skin- and neuro-immune systems, remains to be discovered. Nevertheless, it is of interest to note that activation of brain MC4 receptors profoundly affects care behavior for the body surface (skin and fur). The excessive grooming response in rodents exhibits a remarkable functional correlation with MSH activity in a brain-skin axis.

Mechanisms of antiinflammatory action of alpha-MSH peptides. In vivo and in vitro evidence

alpha-Melanocyte stimulating hormone (alpha-MSH) modulates all forms of inflammation by acting on peripheral inflammatory cells, glial inflammatory cells, and on CNS receptors that activate descending antiinflammatory neural pathways. The multiple actions of this ancient peptide suggest that there is no singular biochemical mechanism through which it exerts its antiinflammatory activity. However, research on IL-10 deficient and Agouti protein hypersecreting mice provide new insights into the actions of the peptide in living animals. Studies of cultured human astrocytes, whole murine brain, and human monocyte/macrophages indicate that a primary effect of the peptide is modulation of activation of the nuclear transcription factor kappa B. The latter influence may underlie the established reduction of gene expression and production of proinflammatory peptides and inducible nitric oxide by alpha-MSH peptides.

Alpha-melanocyte stimulating hormone promotes regrowth of injured axons in the adult rat spinal cord

Peptides related to melanotropin (alphaMSH) and corticotropin (ACTH), collectively termed melanocortins, are known to improve the postlesion repair of injured peripheral nerves. In addition, melanocortins exert trophic effects on the outgrowth of neurites from central nervous system neurons in vitro. Here we report, for the first time, the stimulation by alpha-MSH of spinal neurite outgrowth in vivo after injury. In the in vivo model, spinal cord trauma was produced at lower thoracic spinal levels of adult rats. Under a surgical microscope a laminectomy was performed exposing the dorsum of the spinal cord. Then the dura was cut longitudinally and the dorsal columns were identified. Iridectomy scissors were used to transect the dorsal half of the spinal cord bilaterally, thereby completely lesioning the main corticospinal tract component. Then the lesion gap was immediately filled with a solid collagen matrix. Ingrowth of fibers was quantified using an advanced image analyser using a video image of sections transmitted by a camera. In the control situation virtually no ingrowth of sprouting injured fibers into the collagen implant in the lesion gap was seen. However, when the collagen matrix contained 10(-8) M alpha-MSH, a profound and significant stimulation of fiber ingrowth into the implant was observed (alpha-MSH, 21.5 +/- 2.9%; control, 1.4 +/- 0.6% p < 0.01). A small percentage of these ingrowing fibers was CGRP-immunoreactive (17.0 +/- 4%), whereas no serotonergic ingrowth was observed. Furthermore, we found that local application of alpha-MSH directs a substantial amount of lesioned anterogradely labelled corticospinal tract axons to regrow into the collagen implant (alpha-MSH, 15.2 +/- 5.2%; control, 0.5 +/- 0.3%, p < 0.01). The observed fiber ingrowth is not accompanied by an invasion of astroglial or reactive microglial cells into the implant. In conclusion, inclusion of alpha-MSH in the collagen implant stimulates the regrowth of injured axons in the adult rat spinal cord.

Expression of melanocortin receptors and pro-opiomelanocortin in the rat spinal cord in relation to neurotrophic effects of melanocortins

Although neurotrophic effects of alpha-melanocyte-stimulating hormone (alpha-MSH) are well established, the mechanism underlying these effects is unknown. To identify candidate components of the signaling system that may mediate these effects, in the present study rat spinal cord, dorsal root ganglia, sciatic nerve and soleus muscle were analysed for the expression of the neural MC3, MC4 and MC5 receptors and for the expression of the melanocortin precursor pro-opiomelanocortin (POMC). In rat lumbar spinal cord, the MC4 receptor was the only MC receptor subtype for which mRNA was detectable using RNAse protection assays. In situ binding studies using 125I-NDP-MSH, a synthetic alpha-MSH analogue, demonstrated MC receptor protein in the rat spinal cord, predominantly localised in substantia gelatinosa and area X, surrounding the central canal. Furthermore, POMC mRNA was demonstrated in rat spinal cord and dorsal root ganglia. These findings suggest a functional melanocortin system in the rat spinal cord, that might be involved in peripheral nerve repair. Regulation of POMC or MC receptor transcripts does not appear to be involved in the response to peripheral nerve crush in rats, since no change in mRNA expression patterns was detected after sciatic nerve crush, using quantitative RNAse protection assays. Nevertheless, subtle changes in melanocortin receptor binding did occur postsurgically in several regions of the spinal cord in both sham-operated and sciatic nerve-lesioned rats. The robust expression of MC receptor protein in spinal cord regions that are generally associated with nociception suggests a potentially broader involvement of endogenous melanocortins in spinal pathways which mediate the responses to peripheral injury, in addition to any direct melanocortin effects on sprouting and neurite outgrowth.

Interaction of Agouti protein with the melanocortin 1 receptor in vitro and in vivo

Agouti protein and Agouti-related protein (Agrp) are paracrine-signaling molecules that normally regulate pigmentation and body weight, respectively. These proteins antagonize the effects of alpha-melanocyte-stimulating hormone (alpha-MSH) and other melanocortins, and several alternatives have been proposed to explain their biochemical mechanisms of action. We have used a sensitive bioassay based on Xenopus melanophores to characterize pharmacologic properties of recombinant Agouti protein, and have directly measured its cell-surface binding to mammalian cells by use of an epitope-tagged form (HA-Agouti) that retains biologic activity. In melanophores, Agouti protein has no effect in the absence of alpha-MSH, but its action cannot be explained solely by inhibition of alpha-MSH binding. In 293T cells, expression of the Mc1r confers a specific, high-affinity binding site for HA-Agouti. Binding is inhibited by alpha-MSH, or by Agrp, which indicates that alpha-MSH and Agouti protein bind in a mutually exclusive way to the Mc1r, and that the similarity between Agouti protein and Agrp includes their binding sites. The effects of Agouti and the Mc1r in vivo have been examined in a sensitized background provided by the chinchilla (Tyrc-ch) mutation, which uncovers a phenotypic difference between overexpression of Agouti in lethal yellow (Ay/a) mice and loss of Mc1r function in recessive yellow (Mc1re/Mc1re) mice. Double and triple mutant studies indicate that a functional Mc1r is required for the pigmentary effects of Agouti, and suggest that Agouti protein can act as an agonist of the Mc1r in a way that differs from alpha-MSH stimulation. These results resolve questions regarding the biochemical mechanism of Agouti protein action, and provide evidence of a novel signaling mechanism whereby alpha-MSH and Agouti protein or Agrp function as independent ligands that inhibit each other's binding and transduce opposite signals through a single receptor.

Melanocortin and MCH precursor-derived NEI effects on striatum-midbrain co-cultures

The possibility of developmental effects of POMC-derived melanocortins and analogs on neurons of fetal rat brain regions exhibiting marked developmental melanocortin receptor expression, was studied in serum-free co-cultures of gestational day 18 striatal and mesencephalic cells, and compared with NEI and NGE. These two peptide fragments of the melanin concentrating hormone precursor, occurring in brain areas devoid of POMC terminals, cross-react with alpha-MSH antibodies; NEI elicits grooming similar to alpha-MSH. Neurofilament protein (NF), growth-associated protein (GAP-43) and synaptophysin of the synaptosomal fraction were determined by ELISA as markers for neuritogenesis, growth cones, and nerve terminal differentiation. Cell survival was analyzed by MTT assay, proportions of major cell types by immunocytochemistry. alpha-Melanocyte-stimulating hormone (alpha-MSH, effective concentration 250-2500 nM), the analog Nle4-, D-Phe7-alpha-MSH (NDP, 3.1-750 nM), and NEI (250 nM) increased NF in 3 day cultures by 11%, 17%, and 22%, respectively, whereas ACTH(1-24) and ACTH(1-39) (25 2500 nM) were ineffective. In 11 day cultures, alpha-MSH (250-750 nM), but not NDP, ACTH(1-24) or ACTH(1-39), increased synaptosomal synaptophysin by 11%. GAP-43 and cell survival remained unaffected. These data indicate that selected melanocortins as well as NEI can influence differentiation of neural processes in brain neurons.

Beneficial effects of the melanocortin alpha-melanocyte-stimulating hormone on clinical and neurophysiological recovery after experimental spinal cord injury

OBJECTIVE

Melanocortins, peptides related to melanocyte-stimulating hormone (MSH) and corticotropin (ACTH), exhibit neurotrophic and neuroprotective activity in several established models of peripheral and central nervous system damage. The beneficial effects of melanocortins on functional recovery after experimental brain damage and central demyelinating diseases have prompted us to investigate alpha MSH treatment in a weight drop model of traumatic spinal cord injury in rats.

METHODS

In two independent randomized blinded experiments, treatment with either alpha MSH (75 micrograms/kg of body weight administered subcutaneously every 48 h for 3 weeks after trauma) or single high-dose (30 mg/kg, 30 min after injury) methylprednisolone was compared with saline treatment in rats subjected to a moderately severe 20-gcm weight drop injury. Spinal cord function was monitored using behavioral, electrophysiological, and histological parameters.

RESULTS

In both experiments, alpha MSH significantly improved recovery, as illustrated by Tarlov scores, thoracolumbar height, and amplitude of rubrospinal motor evoked potentials. The magnitude of the alpha MSH effect on motor performance was comparable with the one observed after treatment with methylprednisolone.

CONCLUSION

The reproducible neurological and electrophysiological improvement in spinal cord function of animals treated with alpha MSH suggests a new lead in the treatment of traumatic spinal cord injury.