Anti-obesity phenotypic screening looking to increase OBR cell surface expression

Leptin, Obesity, and Leptin Resistance: Where Are We 25 Years Later?

Leptin, a hormone that is capable of effectively reducing food intake and body weight, was initially considered for use in the treatment of obesity. However, obese subjects have since been found to have high levels of circulating leptin and to be insensitive to the exogenous administration of leptin. The inability of leptin to exert its anorexigenic effects in obese individuals, and therefore, the lack of clinical utility of leptin in obesity, is defined as leptin resistance. This phenomenon has not yet been adequately characterized. Elucidation of the molecular mechanisms underlying leptin resistance is of vital importance for the application of leptin as an effective treatment for obesity. Leptin must cross the blood–brain barrier (BBB) to reach the hypothalamus and exert its anorexigenic functions. The mechanisms involved in leptin transportation across the blood–brain barrier continue to be unclear, thereby preventing the clinical application of leptin in the treatment of obesity. In recent years, new strategies have been developed to recover the response to leptin in obesity. We have summarized these strategies in this review.

Endospanin 1 Determines the Balance of Leptin-Regulated Hypothalamic Functions

Endospanin 1 (Endo1), a protein encoded in humans by the same gene than the leptin receptor (ObR), and increased by diet-induced obesity, is an important regulator of ObR trafficking and cell surface exposure, determining leptin signaling strength. Defective intracellular trafficking of the leptin receptor to the neuronal plasma membrane has been proposed as a mechanism underlying the development of leptin resistance observed in human obesity. More recently, Endo1 has emerged as a mediator of "selective leptin resistance." The underlying mechanisms of the latter are not completely understood, but the possibility of differential activation of leptin signaling pathways was suggested among others. In this respect, the expression level of Endo1 is crucial for the appropriate balance between different leptin signaling pathways and leptin functions in the hypothalamus and is likely participating in selective leptin resistance for the control of energy and glucose homeostasis.

[Traffic and signalisation of the leptin receptor]

Receptors are the master regulators conveying the information provided by the hormone from the extracellular environment to the intracellular milieu. As a result, the level of receptors at the cell surface can determine the signaling strength. Regulation of receptor trafficking to the cell surface or receptor retention processes in intracellular compartments are key mechanisms for leptin receptor (ObR) activity. An alteration of these mechanisms leads to the development of obesity. However, the canonical mechanism of plasma membrane receptors activation is challenged by the discovery that intracellular receptors also have their own signaling activity inside specific intracellular compartments. These intracellular receptors can trigger signaling that regulates a particular function, different from, or in continuity with, surface receptor signaling. We will address both these aspects by focusing particularly on the case of the leptin receptor (ObR), i.e., i) the regulation of its level of exposure to the cell surface and its impact on the development of obesity, and ii) the discovery of its location and signaling in some intracellular compartments.

Endospanin 1 silencing in the hypothalamic arcuate nucleus contributes to sustained weight loss of high fat diet obese mice

Leptin targets specific receptors (OB-R) expressed in the hypothalamus to regulate energy balance. Leptin decreases food intake in normal weight individuals, but this effect is blunted in obese subjects who are characterized by a state of leptin resistance. The prevention of leptin resistance is one of the major goals of obesity research. Recently, we identified endospanin 1 as a negative regulator of OB-R, which by interacting with OB-R retains the receptor inside the cell. We show here that in obese mice endospanin 1 is upregulated in the hypothalamic arcuate nucleus (ARC), the major brain structure involved in body weight regulation, suggesting that endospanin 1 is implicated in obesity development and/or the installation of leptin resistance. In contrast, silencing of endospanin 1 with lentiviral vectors in the ARC of obese mice fully restores leptin responsiveness when combined with a switch to ad libitum fed chow diet. The recovery of central leptin sensitivity is accompanied by sustained body weight loss and amelioration of blood lipid parameters and steatosis. Collectively, our results define endospanin 1 as a novel therapeutic target against obesity.

New pharmacological perspectives for the leptin receptor in the treatment of obesity

After its discovery in 1994, leptin became the great hope as an anti-obesity treatment based on its ability to reduce food intake and increase energy expenditure. However, treating obese people with exogenous leptin was unsuccessful in most cases since most of them present already high circulating leptin levels to which they do not respond anymore defining the so-called state of "leptin resistance." Indeed, leptin therapy is unsuccessful to lower body weight in commonly obese people but effective in people with rare single gene mutations of the leptin gene. Consequently, treatment of obese people with leptin was given less attention and the focus of obesity research shifted toward the prevention and reversal of the state of leptin resistance. Many of these new promising approaches aim to restore or sensitize the impaired function of the leptin receptor by pharmacological means. The current review will focus on the different emerging therapeutic strategies in obesity research that are related to leptin and its receptor.