The melanocortin receptor in the rat lacrimal gland: a model system for the study of MSH (melanocyte stimulating hormone) as a potential neurotransmitter

Therapeutic Effects of Stimulating the Melanocortin Pathway in Regulating Ocular Inflammation and Cell Death

Alpha-melanocyte-stimulating hormone (α-MSH) and its binding receptors (the melanocortin receptors) play important roles in maintaining ocular tissue integrity and immune homeostasis. Particularly extensive studies have demonstrated the biological functions of α-MSH in both immunoregulation and cyto-protection. This review summarizes the current knowledge of both the physiological and pathological roles of α-MSH and its receptors in the eye. We focus on recent developments in the biology of α-MSH and the relevant clinical implications in treating ocular diseases.

Immunomodulatory Role of Neuropeptides in the Cornea

The transparency of the cornea along with its dense sensory innervation and resident leukocyte populations make it an ideal tissue to study interactions between the nervous and immune systems. The cornea is the most densely innervated tissue of the body and possesses both immune and vascular privilege, in part due to its unique repertoire of resident immune cells. Corneal nerves produce various neuropeptides that have a wide range of functions on immune cells. As research in this area expands, further insights are made into the role of neuropeptides and their immunomodulatory functions in the healthy and diseased cornea. Much remains to be known regarding the details of neuropeptide signaling and how it contributes to pathophysiology, which is likely due to complex interactions among neuropeptides, receptor isoform-specific signaling events, and the inflammatory microenvironment in disease. However, progress in this area has led to an increase in studies that have begun modulating neuropeptide activity for the treatment of corneal diseases with promising results, necessitating the need for a comprehensive review of the literature. This review focuses on the role of neuropeptides in maintaining the homeostasis of the ocular surface, alterations in disease settings, and the possible therapeutic potential of targeting these systems.

The Neuropeptide Alpha-Melanocyte-Stimulating Hormone Is Critical for Corneal Endothelial Cell Protection and Graft Survival after Transplantation

Corneal transplantation is the most common form of tissue transplantation. The success of corneal transplantation mainly relies on the integrity of corneal endothelial cells (CEnCs), which maintain tissue transparency by pumping out excess water from the cornea. After transplantation, the rate of CEnC loss far exceeds that seen with normal aging, which can threaten sight. The underlying mechanisms are poorly understood. Alpha-melanocyte-stimulating hormone (α-MSH) is a neuropeptide that is constitutively found in the aqueous humor with both cytoprotective and immunomodulatory effects. The curent study found high expression of melanocortin 1 receptor (MC1R), the receptor for α-MSH, on CEnCs. The effect of α-MSH/MC1R signaling on endothelial function and allograft survival in vitro and in vivo was investigated using MC1R signaling-deficient mice (Mc1re/e mice with a nonfunctional MC1R). Herein, the results indicate that in addition to its well-known immunomodulatory effect, α-MSH has cytoprotective effects on CEnCs after corneal transplantation, and the loss of MC1R signaling significantly decreases long-term graft survival in vivo. In conclusion, α-MSH/MC1R signaling is critical for CEnC function and graft survival after corneal transplantation.

TFOS DEWS II Sex, Gender, and Hormones Report

One of the most compelling features of dry eye disease (DED) is that it occurs more frequently in women than men. In fact, the female sex is a significant risk factor for the development of DED. This sex-related difference in DED prevalence is attributed in large part to the effects of sex steroids (e.g. androgens, estrogens), hypothalamic-pituitary hormones, glucocorticoids, insulin, insulin-like growth factor 1 and thyroid hormones, as well as to the sex chromosome complement, sex-specific autosomal factors and epigenetics (e.g. microRNAs). In addition to sex, gender also appears to be a risk factor for DED. "Gender" and "sex" are words that are often used interchangeably, but they have distinct meanings. "Gender" refers to a person's self-representation as a man or woman, whereas "sex" distinguishes males and females based on their biological characteristics. Both gender and sex affect DED risk, presentation of the disease, immune responses, pain, care-seeking behaviors, service utilization, and myriad other facets of eye health. Overall, sex, gender and hormones play a major role in the regulation of ocular surface and adnexal tissues, and in the difference in DED prevalence between women and men. The purpose of this Subcommittee report is to review and critique the nature of this role, as well as to recommend areas for future research to advance our understanding of the interrelationships between sex, gender, hormones and DED.

α-Melanocyte-stimulating hormone ameliorates ocular surface dysfunctions and lesions in a scopolamine-induced dry eye model via PKA-CREB and MEK-Erk pathways

Dry eye is a highly prevalent, chronic, and multifactorial disease that compromises quality of life and generates socioeconomic burdens. The pathogenic factors of dry eye disease (DED) include tear secretion abnormalities, tear film instability, and ocular surface inflammation. An effective intervention targeting the pathogenic factors is needed to control this disease. Here we applied α-Melanocyte-stimulating hormone (α-MSH) twice a day to the ocular surface of a scopolamine-induced dry eye rat model. The results showed that α-MSH at different doses ameliorated tear secretion, tear film stability, and corneal integrity, and corrected overexpression of proinflammatory factors, TNF-α, IL-1β, and IFN-γ, in ocular surface of the dry eye rats. Moreover, α-MSH, at 10(-4) μg/μl, maintained corneal morphology, inhibited apoptosis, and restored the number and size of conjunctival goblet cells in the dry eye rats. Mechanistically, α-MSH activated both PKA-CREB and MEK-Erk pathways in the dry eye corneas and conjunctivas; pharmacological blockade of either pathway abolished α-MSH's protective effects, suggesting that both pathways are necessary for α-MSH's protection under dry eye condition. The peliotropic protective functions and explicit signaling mechanism of α-MSH warrant translation of the α-MSH-containing eye drop into a novel and effective intervention to DED.

Tearful relationships? Sex, hormones, the lacrimal gland, and aqueous-deficient dry eye

Sex and the endocrine system exert a significant influence on the physiology and pathophysiology of the lacrimal gland. The purpose of this article is to briefly review the nature and magnitude of these interactions between sex, hormones and lacrimal tissue, and to address how they may relate to the pathogenesis of aqueous-deficient dry eye. Towards this end, this article has a 3-fold approach: first, to summarize the influence of androgens, estrogens, glucocorticoids, mineralocorticoids, retinoic acid, prolactin, alpha-melanocyte stimulating hormone, adrenocorticotropic hormone, luteinizing hormone, follicle-stimulating hormone, growth hormone, thyroid-stimulating hormone, arginine vasopressin, oxytocin, thyroxine, parathyroid hormone, insulin, glucagon, melatonin, human chorionic gonadotropin and cholecystokinin on the structure and function of the lacrimal gland; second, to discuss the mechanism of action of each hormone on lacrimal tissue; and third, to discuss the clinical relevance of the endocrine-lacrimal gland interrelationship, with a particular focus on each hormone's role (i.e. if relevant) in the development of aqueous-tear deficiency.

The MC4 receptor and control of appetite

Mutations in the human melanocortin (MC)4 receptor have been associated with obesity, which underscores the relevance of this receptor as a drug target to treat obesity. Infusion of MC4R agonists decreases food intake, whereas inhibition of MC receptor activity by infusion of an MC receptor antagonist or with the inverse agonist AgRP results in increased food intake. This review addresses the role of the MC system in different aspects of feeding behaviour. MC4R activity affects meal size and meal choice, but not meal frequency, and the type of diet affects the efficacy of MC4R agonists to reduce food intake. The central sites involved in the different aspects of feeding behaviour that are affected by MC4R signalling are being unravelled. The paraventricular nucleus plays an important role in food intake per se, whereas MC signalling in the lateral hypothalamus is associated with the response to a high fat diet. MC4R signalling in the brainstem has been shown to affect meal size. Further genetic, behavioural and brain-region specific studies need to clarify how the MC4R agonists affect feeding behaviour in order to determine which obese individuals would benefit most from treatment with these drugs. Application of MCR agonists in humans has already revealed side effects, such as penile erections, which may complicate introduction of these drugs in the treatment of obesity.

Effect of inflammation on lacrimal gland function

The lacrimal gland is the main contributor to the aqueous layer of the tear film. It secretes proteins, electrolytes and water, which helps to nourish and protect the ocular surface. Lacrimal gland secretion is primarily under neural control, which is achieved through a neural reflex arc. Stimuli to the ocular surface activate afferent sensory nerves in the cornea and conjunctiva. This in turn activates efferent parasympathetic and sympathetic nerves in the lacrimal gland to stimulate secretion. Sex steroid hormones are also important regulators of lacrimal gland functions. A decrease or lack of lacrimal gland secretion is the leading cause of aqueous tear deficient dry eye syndrome (DES). It has been suggested that DES is an inflammatory disorder that affects the ocular surface and the lacrimal gland. In several pathological instances, the lacrimal gland can become a target of the immune system and show signs of inflammation. This can result from autoimmune diseases (Sjögren's syndrome), organ transplantation (graft versus host disease), or simply as a result of aging. The hallmarks of lacrimal gland inflammation are the presence of focal lymphocytic infiltrates and increased production of proinflammatory cytokines. The mechanisms leading to lacrimal gland dysfunction are still poorly understood. Apoptosis, production of autoantibodies, hormonal imbalance, alterations in signaling molecules, neural dysfunction, and increased levels of proinflammatory cytokines have been proposed as possible mediators of lacrimal gland insufficiency in disease states.

Androgen regulation of gene expression in the mouse lacrimal gland

The objective of this study was to determine the nature and extent of androgen influence on gene expression in the lacrimal gland. Lacrimal glands were obtained from orchiectomized mice that had been treated with testosterone or vehicle for 2 weeks, as well as from testicular feminized mice and their Tabby controls. Samples were pooled according to experiment, processed for the isolation of RNA, and analyzed for differentially expressed mRNAs by using primarily CodeLink Bioarrays, GEM 1 and 2 gene chips and quantitative real-time PCR (qPCR) procedures. Gene chip data were analyzed with GeneSifter.Net software. Our results demonstrate that testosterone regulates the expression of over 2000 genes in the lacrimal gland. Gene ontologies most affected by androgen treatment included those related to cell growth, proliferation and metabolism, cell communication and transport, nucleic acid binding, signal transduction and receptor activities. Our findings also indicate that androgen action may be mediated, at least in part, through classical androgen receptors, and may contribute to the sex-related differences in gene expression of lacrimal tissue. Overall, these results support our working hypothesis that androgen action on the lacrimal gland is mediated primarily through a receptor-associated regulation of gene transcription.

New aspects on the melanocortins and their receptors

Knowledge of melanocortins and their receptors has increased tremendously over the last few years. The cloning of five melanocortin receptors, and the discovery of two endogenous antagonists for these receptors, agouti and agouti-related peptide, have sparked intense interest in the field. Here we give a comprehensive review of the pharmacology, physiology and molecular biology of the melanocortins and their receptors. In particular, we review the roles of the melanocortins in the immune system, behaviour, feeding, the cardiovascular system and melanoma. Moreover, evidence is discussed suggesting that while many of the actions of the melanocortins are mediated via melanocortin receptors, some appear to be mediated via mechanisms distinct from melanocortin receptors.

Exocrine gland dysfunction in MC5-R-deficient mice: evidence for coordinated regulation of exocrine gland function by melanocortin peptides

The effects of pituitary-derived melanocortin peptides are primarily attributed to ACTH-mediated adrenocortical glucocorticoid production. Identification of a widely distributed receptor for ACTH/MSH peptides, the melanocortin-5 receptor (MC5-R), suggested non-steroidally mediated systemic effects of these peptides. Targeted disruption of the MC5-R produced mice with a severe defect in water repulsion and thermoregulation due to decreased production of sebaceous lipids. High levels of MC5-R was found in multiple exocrine tissues, including Harderian, preputial, lacrimal, and sebaceous glands, and was also shown to be required for production and stress-regulated synthesis of porphyrins by the Harderian gland and ACTH/MSH-regulated protein secretion by the lacrimal gland. These data show a requirement for the MC5-R in multiple exocrine glands for the production of numerous products, indicative of a coordinated system for regulation of exocrine gland function by melanocortin peptides.

Influence of androgens and pituitary hormones on the structural profile and secretory activity of the lacrimal gland

The present study was designed to examine the influence of androgens on the size, morphological appearance, and fluid and protein secretory capacity of the lacrimal gland in a variety of species. For comparison, we also sought to determine whether other hormones might stimulate the growth and secretory ability of this tissue. Lacrimal glands and tears were obtained from intact, surgically-operated and/or hormone-treated rats, mice, guinea pigs and rabbits and the processed for histological and/or image analysis techniques. Our results demonstrated that: 1) androgens modulate the ultrastructural features of rat lacrimal tissue; 2) androgens may influence the weight, morphological aspects, and secretion of the lacrimal gland, however, these effects are not uniform in mice, rats, guinea pigs and rabbits, but instead are dependent upon the species and/or the animal's endocrine status; and 3) treatment with testosterone, prolactin, growth hormone, alpha-melanocyte stimulating hormone, insulin, or thyroxine plus dexamethasone had no impact on the absolute or relative size of the lacrimal gland, or the tear volume, of hypophysectomized male rats. Overall, these findings indicate that androgens may regulate the structure and secretion of the lacrimal gland. However, the nature and extent of this hormone action is species-dependent and may be modified by alterations in the endocrine environment.

Melanocortins as factors in somatic neuromuscular growth and regrowth

Melanocortins, non-corticotropic fragments of adrenocorticotropic hormone, accelerate growth of the developing neuromuscular system and regrowth of damaged neurons, both in the adult and neonatal rat. Morphological, electrophysiological and behavioral characteristics are all improved by melanocortins, which, however, vary in potency, with alpha-MSH being the most effective. Tissue substrate, dosage, critical time periods and pattern of neuropeptide administration are all important variables. Melanocortins protect central neurons affecting motor behavior during development or following neuronal damage in the adult brain. Possible mechanisms of melanocortin action are discussed.

Heterogeneity of brain melanocortin receptors suggested by differential ligand binding in situ

The existence of multiple brain melanocortin receptor types has been postulated, based on the complex pharmacology of intracerebrally administered melanocortin (melanocyte-stimulating hormone-related) peptides. In this study, this hypothesis was tested by determining whether different brain melanocortin receptor populations can be discriminated on a pharmacologic or neuroanatomic basis. The abilities of various pharmacologically active native melanocortins and structural analogs, as well as other test substances, to compete with biologically active [125I]Nle4,D-Phe7-alpha-MSH ([125I]NDP-MSH) for binding to melanocortin receptors was determined, by in vitro binding and autoradiography in frozen rat brain tissue sections. We have previously shown that native melanocortins including alpha-MSH, gamma-MSH and ACTH1-39 compete with [125I]NDP-MSH for binding to brain tissue sites. In the present studies, each of the melanocortin peptides alpha-MSH, des-acetyl-alpha-MSH, beta-MSH and ACTH1-24 when present at 1 microM virtually eliminated [125I]NDP-MSH binding in each of a series of brain structures, including medial preoptic area, caudate putamen, olfactory tubercle, bed nucleus of the stria terminalis, ventral part of the lateral septal nucleus, hypothalamic periventricular and paraventricular nuclei, dorsal anterior amygdaloid area, substantia innominata and thalamic paraventricular nucleus; as well as in extraorbital lacrimal gland, a peripheral melanocortin target. In contrast, the behaviorally and neurotrophically active melanocortin analogs Met(O2),D-Lys,Phe9-alpha-MSH4-9 (Org2766), ACTH4-9, and the antipyretic peptide alpha-MSH11-13 did not affect [125I]NDP-MSH binding at concentrations up to 100 microM, implying that the receptors or receptor binding sites which mediate the actions of these analogs must comprise additional types, distinct from those which bind [125I]NDP-MSH. Binding of [125I]NDP-MSH was also unaffected by the nonmelanotropic peptides ACTH1-4, ACTH34-39 and vasoactive intestinal polypeptide (VIP) and by the antipyretic drugs acetaminophen and lysine-salicylate. Although some of the brain structures are known to express mRNA encoding a gamma-MSH-preferring melanocortin receptor type known as MC3, the relative order of binding affinities of melanocortins, determined in concentration-response studies, was NDP-MSH > or = ACTH1-24 > or = alpha-MSH > gamma-MSH > ACTH4-10 in all brain structures. This suggests that other melanocortin receptor type(s) in addition to MC3 probably account for most of the [125I]NDP-MSH binding detectable in the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthetic peptides corresponding to the calmodulin-binding domains of skeletal muscle myosin light chain kinase and human erythrocyte Ca2+ pump interact with and permeabilize liposomes and cell membranes

Synthetic calmodulin-binding (CaM-binding) peptides (CBPs) representing CaM-binding domains of Ca2+/CaM-dependent enzymes have been reported to interfere with the activity of the melanocyte-stimulating hormone (MSH) receptor function in melanoma cells [Gerst, J. E., & Salomon, Y. (1988) J. Biol. Chem. 263, 7073-7078]. We postulated that membrane lipids may play an important role in the mode of action of CBPs on cells. We therefore tested the ability of CBPs to interact with membrane bilayers. Using artificial phospholipid vesicles, or M2R melanoma cells and cell membranes derived therefrom, as models, we report here that synthetic peptides representing the CaM-binding domains of skeletal muscle myosin light chain kinase (M5) and the human erythrocyte calcium pump (C28W), as well as other CBPs, interact with lipid bilayers and cell membranes. Significant interactions of CBPs with the lipid bilayer were detected in both model systems. M5 and C28W were found to partition into the lipid bilayer of melanoma cell membranes and soybean lecithin vesicles, and surface partition constants obtained (for the liposome model) were in the range 10(3)-10(4) M-1. In addition, C28W and its N-modified NBD derivative were found to inhibit [125I]iodo-[Nle4,D-Phe7]alpha MSH binding to cultured M2R melanoma cells. These and other CBPs were also found to induce the release of cations and calcein from liposomes, suggesting that the interaction of CBPs with the lipid bilayer increases membrane permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenocorticotropin/alpha-melanocyte-stimulating hormone (ACTH/MSH)-like peptides modulate adenylate cyclase activity in rat brain slices: evidence for an ACTH/MSH receptor-coupled mechanism

The regulation of adenylate cyclase activity by adrenocorticotropin/alpha-melanocyte-stimulating hormone (ACTH/MSH)-like peptides was investigated in rat brain slices using a superfusion method. Adenylate cyclase activity was concentration-dependently increased by ACTH-(1-24), alpha-MSH (EC50 values 16 and 6 nM, respectively), and [Nle4,D-Phe7]alpha-MSH (EC50 value 1.6 nM), in the presence of forskolin (1 microM, optimal concentration). 1-9-Dideoxyforskolin did not augment the response of adenylate cyclase to ACTH-(1-24). Various peptide fragments were tested for their ability to enhance [3H]cyclic AMP production. [Nle4,D-Phe7]alpha-MSH increased [3H]cyclic AMP formation with a maximal effect of 30% and was more potent than ACTH-(1-24), ACTH-(1-16)-NH2, alpha-MSH, ACTH-(1-13)-NH2, [MetO4]alpha-MSH, [MetO2(4),D-Lys8,Phe9]ACTH-(4-9), ACTH-(7-16)-NH2, ACTH-(1-10), and ACTH-(11-24), in order of potency. This structure-activity relationship resembles that found for the previously described peptide-induced display of excessive grooming. ACTH-(1-24) stimulated adenylate cyclase activity in both striatal (maximal effect, approximately 20%) and septal slices (maximal effect, approximately 40%), but not in hippocampal or cortical slices. Lesioning of the dopaminergic projections to the striatum did not result in a diminished effect of [Nle4,D-Phe7]alpha-MSH on [3H]cyclic AMP accumulation, which indicates that the ACTH/MSH receptor-stimulated adenylate cyclase is not located on striatal dopaminergic terminals. ACTH-(1-24) did not affect the dopamine D1 or D2 receptor-mediated modulation of adenylate cyclase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Signaling mechanisms controlled by melanocortins in melanoma, lacrimal, and brain astroglial cells

Melanocortins appear to be involved as regulators in an ever growing number of physiological processes in cells and tissues of diverse functions. While such trends are apparent also in the case of other peptide hormones, it appears that melanocortin receptors can be regarded as unique among G-protein-linked receptors due to their special need for extracellular Ca2+ which may relate to some, yet undetermined selectivity of their actions. The physiological role that Ca2+ may be playing and the diverse signaling mechanisms regulated, as well as the nature of the cell-specific responses elicited in melanocortin-sensitive cells/tissues, have yet to be elucidated. Likewise, it will be of interest to establish the relationship of melanocortins to processes like growth and differentiation of cells, as well as to higher, more complex processes such as those regulated in the CNS.

Mechanism of action of melanocortin peptides. Possible role in astrocyte regulation

Melanocortin peptides exert pleiotropic effect in numerous cell types, controlling processes ranging from adrenal steroidogenesis and melanocyte pigmentation to lacrimation and nerve regeneration. The binding of melanocortins to specific cell surface receptors initiates cellular responses via GTP binding proteins (G-proteins). The affinity of these peptides to the receptor is modulated by extracellular Ca2+ ions, a property unique to melanocortin receptors. In astrocyte cultures derived from the rat brain, melanocortin stimulation elevates cAMP levels that appear to induce morphological changes. However, a transient proliferative response to melanocortins in these cells appears to be cAMP independent. The presence of melanocortin receptors in brain tissue and their unique Ca2+ dependence are discussed in relation to their putative role as regulators of astrocytes.

Melanocortins stimulate proliferation and induce morphological changes in cultured rat astrocytes by distinct transducing mechanisms

Melanocyte stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), and several peptides derived from pro-opiomelanocortin, are present in the dorsolateral hypothalamus and arcuate nucleus of several vertebrate species. These peptides affect central nervous system (CNS) functions including behavior, memory, and foetal brain development. In this study we investigated the effects of ACTH1-24, ACTH1-17, ACTH4-10, alpha-MSH, beta-MSH, and a potent analog (Nle4,D-Phe7)-alpha-MSH (melanocortins) on immunocytochemically defined astroglial cells prepared from primary cultures of 1-2-day-old rat brains. A cyclic adenosine 3',5'-monophosphate (cAMP) response to the melanocortins was only detected in astrocytes and not in other cell types in the culture. The extent of the cAMP response was greatest on day 21, the latest time tested. On the other hand, (methyl3H)-thymidine incorporation in astrocytes was significantly stimulated (1.5-2-fold) by melanocortins only in 7 and not in 14 and 21 day cultures. This mitogenic activity of melanocortins was not mimicked by other agents such as forskolin or isoproterenol which efficiently stimulate cAMP production in astrocytes. ACTH1-17 as a melanocortin representative induced significant morphological changes in 7 and 14 day cultures which included rounding of the cell body and process extension. This response, however, resembled that induced by forskolin and hence appears to be cAMP mediated. These findings suggest that astrocytes in the CNS may serve as a target for melanocortins. These peptides appear to affect differentiation and proliferation of these cells during certain developmental periods. While the morphological effects of melanocortins seem to be cAMP mediated, induction of proliferation of the astrocytes by melanocortins appears to involve an alternative signal transduction pathway.