ACTH receptors in the adrenal: specific binding of ACTH-125I and its relation to adenyl cyclase

[The insulin receptor discovery is 50 years old - A review of achieved progress]

The isolation of insulin from the pancreas and its purification to a degree permitting its safe administration to type 1 diabetic patients were accomplished 100 years ago at the University of Toronto by Banting, Best, Collip and McLeod and constitute undeniably one of the major medical therapeutic revolutions, recognized by the attribution of the 1923 Nobel Prize in Physiology or Medicine to Banting and McLeod. The clinical spin off was immediate as well as the internationalization of insulin's commercial production. The outcomes regarding basic research were much slower, in particular regarding the molecular mechanisms of insulin action on its target cells. It took almost a half-century before the determination of the tri-dimensional structure of insulin in 1969 and the characterization of its cell receptor in 1970-1971. The demonstration that the insulin receptor is in fact an enzyme named tyrosine kinase came in the years 1982-1985, and the crystal structure of the intracellular kinase domain 10 years later. The crystal structure of the first intracellular kinase substrate (IRS-1) in 1991 paved the way for the elucidation of the intracellular signalling pathways but it took 15 more years to obtain the complete crystal structure of the extracellular receptor domain (without insulin) in 2006. Since then, the determination of the structure of the whole insulin-receptor complex in both the inactive and activated states has made considerable progress, not least due to recent improvement in the resolution power of cryo-electron microscopy. I will here review the steps in the development of the concept of hormone receptor, and of our knowledge of the structure and molecular mechanism of activation of the insulin receptor.

Insulin: A pacesetter for the shape of modern biomedical science and the Nobel Prize

Background

The 100th anniversary of the discovery of insulin in Toronto in 1921 is an important moment in medical and scientific history. The demonstration that an extract of dog pancreas reproducibly lowered blood glucose, initially in diabetic dogs and then in humans with type 1 diabetes, was a medical breakthrough that changed the course of what was until then a largely fatal disease. The discovery of the “activity”, soon named “insulin”, was widely celebrated, garnering a Nobel Prize for Banting and McLeod in 1923. Over the ensuing 100 years, research on insulin has advanced on many fronts, producing insights that have transformed our understanding of diabetes and our approach to its treatment.

Scope of Review

This paper will review research on insulin that had another consequence of far broader scientific significance, by serving as a pacesetter and catalyst to bioscience research across many fields. Some of this was directly insulin-related and was also recognized by the Nobel Prize. Equally important, however, was research stimulated by the discovery of insulin that has profoundly influenced biomedical research, sometimes also recognized by the Nobel Prize and sometimes without this recognition.

Major Conclusions

By reviewing some of the most notable examples of both insulin-related and insulin-stimulated research, it becomes apparent that insulin had an enormous and frequently under-appreciated impact on the course of modern bioscience.

Advances in covalent kinase inhibitors

This comprehensive review details recent advances, challenges and innovations in covalent kinase inhibition within a 10 year period (2007–2018).

Function of cAMP scaffolds in obstructive lung disease: Focus on epithelial-to-mesenchymal transition and oxidative stress

Over the past decades, research has defined cAMP as one of the central cellular nodes in sensing and integrating multiple pathways and as a pivotal role player in lung pathophysiology. Obstructive lung disorders, such as chronic obstructive pulmonary disease (COPD), are characterized by a persistent and progressive airflow limitation and by oxidative stress from endogenous and exogenous insults. The extent of airflow obstruction depends on the relative deposition of different constituents of the extracellular matrix, a process related to epithelial-to-mesenchymal transition, and which subsequently results in airway fibrosis. Oxidative stress from endogenous and also from exogenous sources causes a profound worsening of COPD. Here we describe how cAMP scaffolds and their different signalosomes in different subcellular compartments may contribute to COPD. Future research will require translational studies to alleviate disease symptoms by pharmacologically targeting the cAMP scaffolds. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Stability and Turnover of the ACTH Receptor Complex

Glucocorticoid production in mammals is principally regulated by the action of the pituitary hormone adrenocorticotropin (ACTH) acting on its cognate membrane receptor on the zona fasciculata cells of the adrenal cortex. The receptor for ACTH consists of two essential components, a small seven transmembrane domain G protein-coupled receptor of the melanocortin receptor subgroup known as the melanocortin 2 receptor (MC2R) and a small single transmembrane domain protein that adopts a antiparallel homodimeric form and which is known as the melanocortin 2 receptor accessory protein (MRAP). MRAP is essential for the trafficking of the MC2R to the cell surface as well as being required for receptor responsiveness to ACTH at physiological concentrations-probably by facilitating ACTH binding, but possibly also by supporting G protein interaction with the MC2R. A number of studies have shown that ACTH stimulates the expression of functional receptor at the cell surface and the transcription of both MC2R and MRAP mRNA. However, the time course of these transcriptional effects differs such that MRAP is expressed relatively rapidly whereas MC2R transcription responds much more slowly. Furthermore, recent data suggests that MRAP protein is turned over with a short half-life whereas MC2R has a significantly longer half-life. These findings imply that these two ACTH receptor proteins have distinct trajectories and that it is likely that MRAP-independent MC2R is present at the cell surface. In such a situation newly transcribed and translated MRAP could enable the rapid recruitment of functional receptor at the plasma membrane without the need for new MC2R translation. This may be advantageous in circumstances of significant stress in that the potentially complex and perhaps inefficient process of de novo MC2R translation, folding, post-translational modification and trafficking can be avoided.

The Hypothalamic-Pituitary-Adrenal Axis: A Brief History

The hypothalamic-pituitary-adrenal (HPA) axis is central to homeostasis, stress responses, energy metabolism, and neuropsychiatric function. The history of this complex system involves discovery of the relevant glands (adrenal, pituitary, hypothalamus), hormones (cortisol, corticotropin, corticotropin-releasing hormone), and the receptors for these hormones. The adrenal and pituitary were identified by classical anatomists, but most of this history has taken place rather recently, and has involved complex chemistry, biochemistry, genetics, and clinical investigation. The integration of the HPA axis with modern neurology and psychiatry has cemented the role of endocrinology in contemporary studies of behavior.

A Serendipitous Scientist

Growing up in a middle-class Jewish home in the Bronx, I had only one professional goal: to become a physician. However, as with most of my Vietnam-era MD colleagues, I found my residency training interrupted by the Doctor Draft in 1968. Some of us who were academically inclined fulfilled this obligation by serving in the US Public Health Service as commissioned officers stationed at the National Institutes of Health. This experience would eventually change the entire trajectory of my career. Here I describe how, over a period of years, I transitioned from the life of a physician to that of a physician-scientist; my 50 years of work on cellular receptors; and some miscellaneous thoughts on subjects as varied as Nobel prizes, scientific lineages, mentoring, publishing, and funding.

Hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes: sex differences in regulation of stress responsivity

Gonadal hormones play a key role in the establishment, activation, and regulation of the hypothalamic-pituitary-adrenal (HPA) axis. By influencing the response and sensitivity to releasing factors, neurotransmitters, and hormones, gonadal steroids help orchestrate the gain of the HPA axis to fine-tune the levels of stress hormones in the general circulation. From early life to adulthood, gonadal steroids can differentially affect the HPA axis, resulting in sex differences in the responsivity of this axis. The HPA axis influences many physiological functions making an organism's response to changes in the environment appropriate for its reproductive status. Although the acute HPA response to stressors is a beneficial response, constant activation of this circuitry by chronic or traumatic stressful episodes may lead to a dysregulation of the HPA axis and cause pathology. Compared to males, female mice and rats show a more robust HPA axis response, as a result of circulating estradiol levels which elevate stress hormone levels during non-threatening situations, and during and after stressors. Fluctuating levels of gonadal steroids in females across the estrous cycle are a major factor contributing to sex differences in the robustness of HPA activity in females compared to males. Moreover, gonadal steroids may also contribute to epigenetic and organizational influences on the HPA axis even before puberty. Correspondingly, crosstalk between the hypothalamic-pituitary-gonadal (HPG) and HPA axes could lead to abnormalities of stress responses. In humans, a dysregulated stress response is one of the most common symptoms seen across many neuropsychiatric disorders, and as a result, such interactions may exacerbate peripheral pathologies. In this review, we discuss the HPA and HPG axes and review how gonadal steroids interact with the HPA axis to regulate the stress circuitry during all stages in life.

ACTH Receptor (MC2R) Specificity: What Do We Know About Underlying Molecular Mechanisms?

Coincidentally, the release of this Research Topic in Frontiers in Endocrinology takes place 25 years after the discovery of the adrenocorticotropic hormone receptor (ACTHR) by Mountjoy and colleagues. In subsequent years, following the discovery of other types of mammalian melanocortin receptors (MCRs), ACTHR also became known as melanocortin type 2 receptor (MC2R). At present, five types of MCRs have been reported, all of which share significant sequence similarity at the amino acid level, and all of which specifically bind melanocortins (MCs)-a group of biologically active peptides generated by proteolysis of the proopiomelanocortin precursor. All MCs share an identical -H-F-R-W- pharmacophore sequence. α-Melanocyte-stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH) are the most extensively studied MCs and are derived from the same region. Essentially, α-MSH is formed from the first 13 amino acid residues of ACTH. ACTHR is unique among MCRs because it binds one sole ligand-ACTH, which makes it a very attractive research object for molecular pharmacologists. However, much research has failed, and functional studies of this receptor are lagging behind other MCRs. The reason for these difficulties has already been outlined by Mountjoy and colleagues in their publication on ACTHR coding sequence discovery where the Cloudman S91 melanoma cell line was used for receptor expression because it was a "more sensitive assay system." Subsequent work showed that ACTHR could be successfully expressed only in endogenous MCR-expressing cell lines, since in other cell lines it is retained within the endoplasmic reticulum. The resolution of this methodological problem came in 2005 with the discovery of melanocortin receptor accessory protein, which is required for the formation of functionally active ACTHR. The decade that followed this discovery was filled with exciting research that provided insight into the molecular mechanisms underlying the action of ACTHR. The purpose of this review is to summarize the advances in this fascinating research field.

Regulation of zonation and homeostasis in the adrenal cortex

The adult adrenal cortex is organized into concentric zones, each specialized to produce distinct steroid hormones. Cellular composition of the cortex is highly dynamic and subject to diverse signaling controls. Cortical homeostasis and regeneration rely on centripetal migration of steroidogenic cells from the outer to the inner cortex, which is accompanied by direct conversion of zona glomerulosa (zG) into zona fasciculata (zF) cells. Given the important impact of tissue structure and growth on steroidogenic function, it is essential to understand the mechanisms governing adrenal zonation and homeostasis. Towards this end, we review the distinctions between each zone by highlighting their morphological and ultra-structural features, discuss key signaling pathways influencing zonal identity, and evaluate current evidence for long-term self-renewing stem cells in the adult cortex. Finally, we review data supporting zG-to-zF transdifferentiation/direct conversion as a major mechanism of adult cortical renewal.

Trends in Modern Drug Discovery

Drugs discovered by the pharmaceutical industry over the past 100 years have dramatically changed the practice of medicine and impacted on many aspects of our culture. For many years, drug discovery was a target- and mechanism-agnostic approach that was based on ethnobotanical knowledge often fueled by serendipity. With the advent of modern molecular biology methods and based on knowledge of the human genome, drug discovery has now largely changed into a hypothesis-driven target-based approach, a development which was paralleled by significant environmental changes in the pharmaceutical industry. Laboratories became increasingly computerized and automated, and geographically dispersed research sites are now more and more clustered into large centers to capture technological and biological synergies. Today, academia, the regulatory agencies, and the pharmaceutical industry all contribute to drug discovery, and, in order to translate the basic science into new medical treatments for unmet medical needs, pharmaceutical companies have to have a critical mass of excellent scientists working in many therapeutic fields, disciplines, and technologies. The imperative for the pharmaceutical industry to discover breakthrough medicines is matched by the increasing numbers of first-in-class drugs approved in recent years and reflects the impact of modern drug discovery approaches, technologies, and genomics.

TLR4 at the Crossroads of Nutrients, Gut Microbiota, and Metabolic Inflammation

Obesity is accompanied by the activation of low-grade inflammatory activity in metabolically relevant tissues. Studies have shown that obesity-associated insulin resistance results from the inflammatory targeting and inhibition of key proteins of the insulin-signaling pathway. At least three apparently distinct mechanisms-endoplasmic reticulum stress, toll-like receptor (TLR) 4 activation, and changes in gut microbiota-have been identified as triggers of obesity-associated metabolic inflammation; thus, they are expected to represent potential targets for the treatment of obesity and its comorbidities. Here, we review the data that place TLR4 in the center of the events that connect the consumption of dietary fats with metabolic inflammation and insulin resistance. Changes in the gut microbiota can lead to reduced integrity of the intestinal barrier, leading to increased leakage of lipopolysaccharides and fatty acids, which can act upon TLR4 to activate systemic inflammation. Fatty acids can also trigger endoplasmic reticulum stress, which can be further stimulated by cross talk with active TLR4. Thus, the current data support a connection among the three main triggers of metabolic inflammation, and TLR4 emerges as a link among all of these mechanisms.

Diabetes insipidus: celebrating a century of vasopressin therapy

Diabetes mellitus, widely known to the ancients for polyuria and glycosuria, budded off diabetes insipidus (DI) about 200 years ago, based on the glucose-free polyuria that characterized a subset of patients. In the late 19th century, clinicians identified the posterior pituitary as the site of pathology, and pharmacologists found multiple bioactivities there. Early in the 20th century, the amelioration of the polyuria with extracts of the posterior pituitary inaugurated a new era in therapy and advanced the hypothesis that DI was due to a hormone deficiency. Decades later, a subset of patients with polyuria unresponsive to therapy were recognized, leading to the distinction between central DI and nephrogenic DI, an early example of a hormone-resistant condition. Recognition that the posterior pituitary had 2 hormones was followed by du Vigneaud's Nobel Prize winning isolation, sequencing, and chemical synthesis of oxytocin and vasopressin. The pure hormones accelerated the development of bioassays and immunoassays that confirmed the hormone deficiency in vasopressin-sensitive DI and abundant levels of hormone in patients with the nephrogenic disorder. With both forms of the disease, acquired and inborn defects were recognized. Emerging concepts of receptors and of genetic analysis led to the recognition of patients with mutations in the genes for 1) arginine vasopressin (AVP), 2) the AVP receptor 2 (AVPR2), and 3) the aquaporin 2 water channel (AQP2). We recount here the multiple skeins of clinical and laboratory research that intersected frequently over the centuries since the first recognition of DI.

Insulin's discovery: new insights on its ninetieth birthday

2012 marks the 90th year since the purification of insulin and the miraculous rescue from death of youngsters with type 1 diabetes. In this review, we highlight several previously unappreciated or unknown events surrounding the discovery. (i) We remind readers of the essential contributions of each of the four discoverers--Banting, Macleod, Collip, and Best. (ii) Banting and Best (each with his own inner circle) worked not only to accrue credit for himself but also to minimize credit to the other discoverers. (iii) Banting at the time of the insulin research was very likely suffering from post-traumatic stress disorder (PTSD) that originated during his heroic service as a surgeon in World War I on the Western Front in 1918, including an infected shrapnel wound that threatened amputation of his arm. His war record along with the newly discovered evidence of a suicide threat goes along with his paranoia, combativeness, alcohol excess, and depression, symptoms we associate with PTSD. (iv) Banting's eureka idea, ligation of the pancreatic duct to preserve the islets, while it energized the early research, was unnecessary and was bypassed early. (v) Post discovery, Macleod uncovered many features of insulin action that he summarized in his 1925 Nobel Lecture. Macleod closed by raising the question--what is the mechanism of insulin action in the body?--a challenge that attracted many talented investigators but remained unanswered until the latter third of the 20th century.

The C-terminal domains of melanocortin-2 receptor (MC2R) accessory proteins (MRAP1) influence their localization and ACTH-induced cAMP production

ACTH binding to the human melanocortin-2 receptor (MC2R) requires the presence of the MC2R accessory protein1 isoforms, MRAPα or MRAPβ. This study evaluated the role of the isoform-specific C-terminal domains of MRAP with regard to their cellular localization, topology, interaction with MRAP2 and cAMP production. When stably expressed in HEK293/FRT cells or in B16-G4F mouse melanoma cells (an MSH receptor-deficient cell clone), MRAPα and MRAPdCT (truncated MRAP1, N-terminal only) localized mainly around the nuclear envelope and within dense intracellular endosomes, while MRAPβ exhibited a strong localization at the plasma membrane, and partially with rapid recycling endosomes. MRAPβ and MRAPdCT both exhibited dual-topology (N(cyto)/C(exo) and N(exo)/C(cyto)) at the plasma membrane whereas MRAPα exhibited only N(cyto)/C(exo) topology at the plasma membrane while adopting dual-topology in intracellular compartments. Both MRAPα and MRAP2 colocalized in intracellular compartments, as opposed to weak colocalization between MRAPβ and MRAP2. MRAP2 and MC2R enhanced the expression of MRAP1 isoforms and vice versa. Moreover, in both HEK293/FRT and B16-G4F cells, ACTH failed to activate MC2R unless MRAP1 was present. MRAP1 expression enhanced MC2R cell-surface expression as well as concentration-dependent cAMP accumulation. In the presence of human or zebrafish MC2R, MRAPβ induced the highest cAMP accumulation while MRAPdCT induced the lowest. Together, the present findings indicate that the C-terminal domains of MRAP dictate their intracellular localization in addition to regulating ACTH-induced cAMP production. These preferential localizations suggest that MRAPα is involved in MC2R targeting to the plasma membrane, while MRAPβ may enhance ACTH-MC2R coupling to cAMP production.

Mechanisms of melanocortin-2 receptor (MC2R) internalization and recycling in human embryonic kidney (hek) cells: identification of Key Ser/Thr (S/T) amino acids

ACTH is the most important stimulus of the adrenal cortex. The precise molecular mechanisms underlying the ACTH response are not yet clarified. The functional ACTH receptor includes melanocortin-2 receptor (MC2R) and MC2R accessory proteins (MRAP). In human embryonic kidney 293/Flp recombinase target cells expressing MC2R, MRAP1 isoforms, and MRAP2, we found that ACTH induced a concentration-dependent and arrestin-, clathrin-, and dynamin-dependent MC2R/MRAP1 internalization, followed by intracellular colocalization with Rab (Ras-like small guanosine triphosphate enzyme)4-, Rab5-, and Rab11-positive recycling endosomes. Preincubation of cells with monensin and brefeldin A revealed that 28% of the internalized receptors were recycled back to the plasma membrane and participated in total accumulation of cAMP. Moreover, certain intracellular Ser and Thr (S/T) residues of MC2R were found to play important roles not only in plasma membrane targeting and function but also in promoting receptor internalization. The S/T residues T131, S140, T204, and S280 were involved in MRAP1-independent cell-surface MC2R expression. Other mutants (S140A, S208A, and S202D) had lower cell-surface expressions in absence of MRAPβ. In addition, T143A and T147D drastically impaired cell-surface expression and function, whereas T131A, T131D, and S280D abrogated MC2R internalization. Thus, the modification of MC2R intracellular S/T residues may positively or negatively regulate its plasma membrane expression and the capacity of ACTH to induce cAMP accumulation. Mutations of T131, T143, T147, and S280 into either A or D had major repercussions on cell-surface expression, cAMP accumulation, and/or internalization parameters, pointing mostly to the second intracellular loop as being crucial for MC2R expression and functional regulation.

Localisation of the melanocortin-2-receptor and its accessory proteins in the developing and adult adrenal gland

The melanocortin-2-receptor (MC2R)/MC2R accessory protein (MRAP) complex is critical to the production of glucocorticoids from the adrenal cortex. Inactivating mutations in either MC2R or MRAP result in the clinical condition familial glucocorticoid deficiency. The localisation of MC2R together with MRAP within the adrenal gland has not previously been reported. Furthermore, MRAP2, a paralogue of MRAP, has been shown in vitro to have a similar function to MRAP, facilitating MC2R trafficking and responsiveness to ACTH. Despite similar MC2R accessory functions, in vivo, patients with inactivating mutations of MRAP fail to be rescued by a functioning MRAP2 gene, suggesting differences in adrenal expression, localisation and/or function between the two MRAPs. In this study on the rat adrenal gland, we demonstrate that while MRAP and MC2R are highly expressed in the zona fasciculata, MRAP2 is expressed throughout the adrenal cortex in low quantities. In the developing adrenal gland, both MRAP and MRAP2 are equally well expressed. The MC2R/MRAP2 complex requires much higher concentrations of ACTH to activate compared with the MC2R/MRAP complex. Interestingly, expression of MC2R and MRAP in the undifferentiated zone would support the notion that ACTH may play an important role in adrenal cell differentiation and maintenance.

Melanocortin 1 receptor agonists reduce proteinuria

Membranous nephropathy is one of the most common causes of nephrotic syndrome in adults. Recent reports suggest that treatment with adrenocorticotropic hormone (ACTH) reduces proteinuria, but the mechanism of action is unknown. Here, we identified gene expression of the melanocortin receptor MC1R in podocytes, glomerular endothelial cells, mesangial cells, and tubular epithelial cells. Podocytes expressed most MC1R protein, which colocalized with synaptopodin but not with an endothelial-specific lectin. We treated rats with passive Heymann nephritis (PHN) with MS05, a specific MC1R agonist, which significantly reduced proteinuria compared with untreated PHN rats (P < 0.01). Furthermore, treatment with MC1R agonists improved podocyte morphology and reduced oxidative stress. In summary, podocytes express MC1R, and MC1R agonism reduces proteinuria, improves glomerular morphology, and reduces oxidative stress in nephrotic rats with PHN. These data may explain the proteinuria-reducing effects of ACTH observed in patients with membranous nephropathy, and MC1R agonists may provide a new therapeutic option for these patients.

Mechanisms of disease: the adrenocorticotropin receptor and disease

The action of the peptide hormone adrenocorticotropin (ACTH) to stimulate glucocorticoid production by the adrenal gland is an essential physiologic process, yet is dependent on a single unique genetic component--the ACTH receptor or melanocortin 2 receptor. Genetic defects that cause abnormalities in this receptor or in a protein required for its expression at the cell surface result in a potentially fatal disease (familial glucocorticoid deficiency). Overexpression of this receptor or inability to desensitize it is found in adrenal adenomas or hyperplasia associated with glucocorticoid overproduction (Cushing syndrome). These disorders are uncommon, but there are considerable data to show that the hypothalamo-pituitary-adrenal axis is overactive, or in some circumstances underactive, in more common situations including depressive illness and septic shock. The origin of these latter disturbances is undoubtedly complex and multifactorial, but there is good evidence that a component of this phenomenon is an altered responsiveness of the ACTH receptor to ACTH. Understanding the basis of ACTH responsiveness might, therefore, contribute to the understanding of disorders such as these and perhaps enable the hypothalamo-pituitary-adrenal axis to be manipulated beneficially in these circumstances.

Aggressive behavior linked to corticotropin-reactive autoantibodies

BACKGROUND

Altered stress response is characteristic for subjects with abnormal aggressive and antisocial behavior, but the underlying biological mechanisms are unclear. We hypothesized that autoantibodies (autoAbs) directed against several stress-related neurohormones may exist in aggressive subjects.

METHODS

Using enzyme-linked immunosorbent assay, we studied whether autoAbs directed against corticotropin (ACTH), alpha-melanocyte-stimulating hormone (alpha-MSH), oxytocin, and vasopressin are present in serum of male subjects with conduct disorder and prisoners with history of violence. Healthy blood donors served as control subjects.

RESULTS

Both conduct disorder and prisoners groups displayed strongly increased levels of ACTH-reactive immunoglobulin G (IgG) and immunoglobulin M (IgM) autoAbs compared with control subjects. Levels of oxytocin-reactive IgM autoAbs were slightly increased in both groups of aggressive subjects, whereas levels of vasopressin-reactive IgG and IgM autoAbs were lower only in conduct disorder. No differences in the levels of alpha-MSH-reactive autoAbs were found between aggressive and control subjects.

CONCLUSIONS

High levels of ACTH-reactive autoAbs as well as altered levels of oxytocin- and vasopressin-reactive autoAbs found in aggressive subjects may interfere with the neuroendocrine mechanisms of stress and motivated behavior. Our data suggest a new biological mechanism of human aggressive behavior that involves autoAbs directed against several stress-related neurohormones.

The discovery of signal transduction by G proteins: a personal account and an overview of the initial findings and contributions that led to our present understanding

The realization that there existed a G-protein coupled signal transduction mechanism developed gradually and was initially the result of an ill fated quest for uncovering the mechanism of action of insulin, followed by a refocused research in many laboratories, including mine, on how GTP acted to increase hormonal stimulation of adenylyl cyclase. Independent research into how light-activated rhodopsin triggers a response in photoreceptor cells of the retina and the attendant biochemical studies joined midway and, without the left hand knowing well what the right hand was doing, preceded classical G protein research in identifying the molecular players responsible for signal transduction by G proteins.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

Expression of the human melanocortin-2 receptor in different eukaryotic cells

The human melanocortin-2 receptor (hMC2R) is mainly present in the adrenal cortex and has been difficult to express in heterologous cells. The hMC2R fused to the EGFP at its C-terminus has been stably transfected in the murine M3 melanoma and HEK293 cells. In the M3 cells, the hMC2R-EGFP was well-addressed to the cell membrane and functional whereas in the HEK293 cells, the hMC2R-EGFP was retained intracellularly. These results suggest that some specific factors, missing in cells, which do not express any melanocortin receptor, are involved in the correct addressing of the hMC2R to the cell membrane.

Oligomerization of G protein-coupled receptors: past, present, and future

G protein-coupled receptor (GPCR)-mediated signal transduction has been studied for more than a century. Despite the intense focus on this class of proteins, a molecular understanding of what constitutes the functional form of the receptor is still uncertain. GPCRs have traditionally been conceptualized as monomeric proteins, and this view has changed little over the years until relatively recently. Recent biochemical and biophysical studies have challenged this traditional concept, and point instead to a mechanistic view of signal transduction wherein the receptor functions as an oligomer. Cooperative interactions within such an oligomeric array may be critical for the propagation of an external signal across the cell membrane and to the G protein, and may therefore underlie the mechanistic basis of signaling.

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.

Desensitization of the Y1 cell adrenocorticotropin receptor: evidence for a restricted heterologous mechanism implying a role for receptor-effector complexes

Receptor desensitization provides a potential mechanism for the regulation of adrenocortical adrenocorticotropin (ACTH) responsiveness. Using the mouse adrenocortical Y1 cell line we demonstrate that ACTH effectively desensitizes the cAMP response of its own receptor, the melanocortin 2 receptor (MC2R), in these cells with a maximal effect between 30 and 60 min. Neither forskolin nor isoproterenol (in Y1 cells stably transfected with the beta(2)-adrenergic receptor) desensitize this ACTH response. ACTH desensitizes its receptor at concentrations at which only a fraction of receptors are occupied, implying that this mechanism acts on agonist-unoccupied receptors. Y1 cells express G protein-coupled receptor kinase (GRK) 2 and 5, but stable expression of a dominant negative GRK2 (K220W) only marginally reduces the desensitization by ACTH. The protein kinase A (PKA) inhibitor, H89, extinguishes almost the entire desensitization response over the initial 30-min period at all concentrations of ACTH. A mutant MC2R in which the single consensus PKA phosphorylation site has been mutated (S208A) when expressed in MC2R-negative Y6 cells is also unable to desensitize. These data imply a heterologous, PKA-dependent, mode of desensitization, which is restricted to agonist-occupied and -unoccupied MC2R, possibly as a consequence of receptor/effector complexes that functionally compartmentalize this receptor.

A quantitative approach to signal transduction

The high affinity receptor for IgE (FcepsilonRI), is one of a family of immunoreceptors whose antigen-induced clustering leads to a variety of cellular responses. The signaling pathways are enormously complex but by focusing on only the most initial steps, it is now possible to sketch plausible molecular models that relate the interaction of multivalent antigens with the receptor-bound IgE to the earliest cellular events. In this paper, we describe how we have combined quantitative experimentation and mathematical modeling to probe this system further. We also discuss some of the formidable challenges that remain before we can claim reasonably complete understanding of even these early events.

The 1994 Upjohn Award Lecture. Molecular and genetic approaches to the study of signal transduction in the adrenal cortex

This review highlights contributions from my laboratory in which the sites and mechanisms of action of the adrenocorticotropic hormone (ACTH) in the adrenal cortex have been explored. Early studies showing that ACTH stimulates adrenal steroidogenesis by interacting with specific receptors at the cell surface are summarized. Next, the development of a strategy of genetic analysis to define the signalling events that follow ACTH interaction with its receptor is described. This strategy involved the isolation and characterization of mutant adrenal cell lines harboring specific defects in the ACTH-responsive steroidogenic pathway. I describe the isolation and characterization of several of these mutants and demonstrate how these mutants have helped to establish obligatory roles for adenylyl cyclase, cyclic AMP (cAMP), and cAMP-dependent protein kinase in the steroidogenic actions of ACTH. Finally, some of our studies on the regulated expression of the steroidogenic cytochrome P450 enzymes in Y1 adrenal cells are reviewed. These latter studies have led to the discovery of a novel promoter element and transcription factor (designated steroidogenic factor 1) that participates in the coordinate expression of these cytochrome P450 enzymes and that is required for their regulated expression by ACTH and cAMP.

Sensitive and specific radioreceptor assay for calcitonin gene-related Peptide

Abstract Calcitonin gene-related peptide is a 37-amino-acid neuropeptide which has a direct action on the heart and is an extremely potent vasodilator. Immunoreactive calcitonin gene-related peptide is present in plasma, cerebrospinal fluid and in other tissues. We have previously established the specific binding characteristics of this neuropeptide to the cerebellar membranes. This paper describes a sensitive, rapid and specific radioreceptor assay for the measurement of receptor-active calcitonin gene-related peptide in tissues and in plasma extracts using cerebellar membrane preparations as the binding agent. This assay should prove a valuable tool in studies of the physiology of this vasoactive neuropeptide and of its involvement in disease.

Immunocytochemical localization of ACTH-like immunoreactivity in rat adrenal glomerulosa and fasciculata cells

The role of ACTH in the synthesis of the adrenocortical hormones has been largely described. In order to investigate the localization of this peptide at the subcellular level of the adrenal glomerulosa and fasciculata cells, an immunocytological method was used. Rat adrenals were fixed and frozen. Ultrathin sections obtained by cryoultramicrotomy, were incubated with anti-beta (1-24) ACTH or anti-alpha (17-39) ACTH sera. The antigen-antibody reaction was detected by PAP complexes (revealed by 4-chloro-1-naphthol) or with protein A-colloidal gold or IgG-colloidal gold. The results obtained were the same whatever the antisera of the technique employed. All the cells of the adrenal zona glomerulosa and zona fasciculata were labelled. ACTH-like immunoreactivity in zona glomerulosa and zona fasciculata cells was observed at the plasma membrane level, in cytoplasmic matrix, mitochondria and nucleus (in the euchromatin close to the heterochromatin aggregations and, occasionally, associated with the nucleolus). No immunoreactivity was observed when non-immune serum or anti-ACTH serum preincubated with ACTH were used, nor there was any modification of the immunocytochemical reaction when anti-ACTH serum incubated with heterologous antigens was employed. These data, demonstrate the presence of endogenous ACTH in both adrenal glomerulosa and fasciculata cells, and suggest that the peptide is internalized after binding to the plasma membrane.

Fusion-mediated microinjection of active amine and diamine oxidases into cultured cells: effect on protein and DNA synthesis in chick embryo fibroblasts and in glioma cells

Serum amine oxidase and/or porcine kidney diamine oxidase were trapped within reconstituted Sendai virus envelopes, and retained their activity. The trapped enzymes that were detected by radioimmunoblots were microinjected into cultured cells by fusion. When diamine oxidase was microinjected into cultured fibroblasts of chick or rat embryos, a temporary arrest in protein and DNA synthesis was observed. The inhibitory effect was more significant when both serum amine oxidase and kidney diamine oxidase were microinjected into those cultured cells. Fibroblasts of either chick or rat embryos transformed by Rous sarcoma virus were more susceptible to the injected enzymes than the normal cultures, showing a complete arrest in protein and DNA synthesis within 4 hours. Similar results were obtained by microinjecting diamine oxidase into cultured glioma cells. The injected enzyme catalyzed the oxidation of intracellular polyamines. The resulting oxidation product (hydrogen peroxide and aminoaldehydes) apparently caused the arrest in the synthesis of macromolecules.