Novel factors as therapeutic targets to treat diabetes. Focus on leptin and ghrelin

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

Alzheimer’s disease and type 2 diabetes mellitus: Pathophysiologic and pharmacotherapeutics links

At present, Alzheimer’s disease (AD) and type 2 diabetes mellitus (T2DM) are two highly prevalent disorders worldwide, especially among elderly individuals. T2DM appears to be associated with cognitive dysfunction, with a higher risk of developing neurocognitive disorders, including AD. These diseases have been observed to share various pathophysiological mechanisms, including alterations in insulin signaling, defects in glucose transporters (GLUTs), and mitochondrial dysfunctions in the brain. Therefore, the aim of this review is to summarize the current knowledge regarding the molecular mechanisms implicated in the association of these pathologies as well as recent therapeutic alternatives. In this context, the hyperphosphorylation of tau and the formation of neurofibrillary tangles have been associated with the dysfunction of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways in the nervous tissues as well as the decrease in the expression of GLUT-1 and GLUT-3 in the different areas of the brain, increase in reactive oxygen species, and production of mitochondrial alterations that occur in T2DM. These findings have contributed to the implementation of overlapping pharmacological interventions based on the use of insulin and antidiabetic drugs, or, more recently, azeliragon, amylin, among others, which have shown possible beneficial effects in diabetic patients diagnosed with AD.

Insulin Signaling Impairment in the Brain as a Risk Factor in Alzheimer's Disease

Type 2 diabetes is a risk factor for developing Alzheimer’s disease (AD). The underlying mechanism that links up the two conditions seems to be the de-sensitization of insulin signaling. In patients with AD, insulin signaling was found to be de-sensitized in the brain, even if they did not have diabetes. Insulin is an important growth factor that regulates cell growth, energy utilization, mitochondrial function and replacement, autophagy, oxidative stress management, synaptic plasticity, and cognitive function. Insulin desensitization, therefore, can enhance the risk of developing neurological disorders in later life. Other risk factors, such as high blood pressure or brain injury, also enhance the likelihood of developing AD. All these risk factors have one thing in common – they induce a chronic inflammation response in the brain. Pro-inflammatory cytokines block growth factor signaling and enhance oxidative stress. The underlying molecular processes for this are described in the review. Treatments to re-sensitize insulin signaling in the brain are also described, such as nasal insulin tests in AD patients, or treatments with re-sensitizing hormones, such as leptin, ghrelin, glucagon-like peptide 1 (GLP-1),and glucose-dependent insulinotropic polypeptide (GIP). The first clinical trials show promising results and are a proof of concept that utilizing such treatments is valid.

GLP-1 receptor agonists show neuroprotective effects in animal models of diabetes

Enzyme-resistant receptor agonists of the incretin hormone glucagon-like peptide-1 (GLP-1) have shown positive therapeutic effects in people with type 2 diabetes mellitus (T2DM). T2DM has detrimental effects on brain function and impairment of cognition and memory formation has been described. One of the underlying mechanisms is most likely insulin de-sensitization in the brain, as insulin improves cognitive impairments and enhances learning. Treatment with GLP-1 receptor agonists improves memory formation and impairment of synaptic plasticity observed in animal models of diabetes-obesity. Furthermore, it has been shown that diabetes impairs growth factor signalling in the brain and reduces energy utilization in the cortex. Inflammation and apoptotic signalling was also increased. Treatment with GLP-1 receptor agonists improved neuronal growth and repair and reduced inflammation and apoptosis as well as oxidative stress. In comparison with the diabetes drug metformin, GLP-1 receptor agonists were able to improve glycemic control and reverse brain impairments, whereas metformin only normalized blood glucose levels. Clinical studies in non-diabetic patients with neurodegenerative disorders showed neuroprotective effects following administration with GLP-1 receptor agonists, demonstrating that neuroprotective effects are independent of blood glucose levels.

High-circulating leptin levels are associated with increased blood pressure in uncontrolled resistant hypertension

Leptin and aldosterone have been associated with the pathophysiological mechanisms of hypertension. However, despite studies showing the association of leptin with intima-media thickness, arterial distensibility and sympathetic nerve activation, the relationship between leptin and blood pressure (BP) in resistant hypertension (RHTN) is unknown. We aimed to assess the correlation of plasma leptin and aldosterone levels with BP in uncontrolled controlled RHTN (UCRHTN) and CRHTN patients. Plasma leptin and aldosterone levels, office BP, ambulatory BP monitoring and heart rate were measured in 41 UCRHTN, 39 CRHTN and 31 well-controlled HTN patients. No differences were observed between the three groups regarding gender, body mass index and age. The UCRHTN group had increased leptin when compared with CRHTN and well-controlled HTN patients (38.2±21.4, 19.6±8.7 and 20.94±13.9 ng ml(-1), respectively; P<0.05). Aldosterone levels values were also statistically different when comparing RHTN, CRHTN and well-controlled HTN patients (9.6±3.8, 8.1±5.0 and 8.0±4.7 ng dl(-1), respectively; P<0.05). As expected, UCRHTN patients had higher heart rate values compared with CRHTN and well-controlled HTN patients (86.2±7.2, 83.5±6.7 and 83.4±8.5, respectively; P<0.05). Plasma leptin positively correlated with systolic (SBP) and diastolic BP (DBP), and aldosterone (r=0.43, 0.35 and 0.47, respectively; all P<0.05) in UCRHTN, but neither in the CRHTN nor in the HTN group. Simple linear regression showed that SBP, DBP and aldosterone may be predicted by leptin (r(2)=0.16, 0.15 and 0.19, respectively; all P<0.05) only in the UCRHTN subgroup. In conclusion, UCRHTN patients have higher circulating leptin levels associated with increased plasma aldosterone and BP levels when compared with CRHTN and HTN subjects.

Role of SST, CORT and ghrelin and its receptors at the endocrine pancreas

Somatostatin (SST), cortistatin (CORT), and its receptors (sst1-5), and ghrelin and its receptors (GHS-R) are two highly interrelated neuropeptide systems with a broad range of overlapping biological actions at central, cardiovascular, and immune levels among others. Besides their potent regulatory role on GH release, its endocrine actions are highlighted by SST/CORT and ghrelin influence on insulin secretion, glucose homeostasis, and insulin resistance. Interestingly, most components of these systems are expressed at the endocrine pancreas and are actively involved in the modulation of pancreatic islet function and, consequently influence glucose homeostasis. In addition, some of them also participate in islet survival and regeneration. Furthermore, under severe metabolic condition as well as in endocrine pathologies, their expression profile is severely deregulated. These findings suggest that SST/CORT and ghrelin systems could play a relevant role in pancreatic function under metabolic and endocrine pathologies. Accordingly, these systems have been therapeutically targeted for the prevention or amelioration of certain metabolic conditions (obesity) as well as for tumor growth inhibition and/or hormonal regulation in endocrine pathologies (neuroendocrine tumors). This review focuses on the interrelationship between SST/CORT and ghrelin systems and their role in severe metabolic conditions and some endocrine disorders.

Ghrelin receptor signaling: a promising therapeutic target for metabolic syndrome and cognitive dysfunction

The neuroendocrine hormone ghrelin is an octanoylated 28-residue peptide that exerts numerous physiological functions. Ghrelin exerts its effects on the body mainly through a highly conserved G protein-coupled receptor known as the growth hormone secretagagogue receptor subtype 1a (GHS-R1a). Ghrelin and GSH-R1a are widely expressed in both peripheral and central tissues/organs, and ghrelin signaling plays a critical role in maintaining energy balance and neuronal health. The multiple orexigenic effects of ghrelin and its receptor have been studied in great detail, and GHS-R1a-mediated ghrelin signaling has long been a promising target for the treatment of metabolic disorders, such as obesity. In addition to its well-characterized metabolic effects, there is also mounting evidence that ghrelin-mediated GHS-R1a signaling exerts neuroprotective effects on the brain. In this review, we will summarize some of the effects of ghrelin-mediated GSH-R1a signaling on peripheral energy balance and cognitive function. We will also discuss the potential pharmacotherapeutic role of GSH-R1a-mediated ghrelin signaling for the treatment of complex neuroendocrine disorders.

Current and potential roles of ghrelin in clinical practice

Ghrelin is a novel GH-releasing peptide, which has been identified as an endogenous ligand for GH-secretagogue receptor. Ghrelin is mainly secreted by the stomach and plays a critical role in a variety of physiological processes including endocrine, metabolic, cardiovascular, immunological, and other actions. Ghrelin stimulates food intake via hypothalamic neurons and causes a positive energy balance and body weight gain by decreasing fat utilization and promoting adiposity. Given the multiple effects of ghrelin, its potential clinical applications have been evaluated in various conditions. Preliminary trials have shown that it may prove valuable in the management of disease-induced cachexia. Ghrelin may improve the wasting syndrome through GH-dependent or GH-independent effects. Moreover, ghrelin may play a role in the management of disorders of gut motility and obesity. Finally, other potential clinical applications of ghrelin include the treatment of patients with diabetes mellitus, infections, rheumatological diseases or GH deficiency and the diagnosis of this hormonal disorder.

Leptin-based therapeutics

Leptin, a pluripotent adipokine, has been discovered as a hormone controlling energy balance in hypothalamic neuroendocrine centers. In addition, recent studies provided ample evidence that leptin can be produced by cells other than adipocytes, and that the hormone can regulate many physiological processes other than energy balance and appetite. In this context, it is not surprising that both leptin excess as well as leptin insufficiency have been implicated in various pathologies. Consequently, despite initially disappointing results with recombinant leptin as the drug for obesity management, new leptin receptor modifiers have been developed and emerged as potential treatment modalities for numerous metabolic, immunological and neoplastic diseases. The major focus of this paper is a systematic review of current experimental leptin-based therapies, including pharmacological advantages and limitations of each prodrug category.