General informations
Structure
Leptin receptors belong to the family of class 1 cytokine receptors.
The leptin receptor protein is a molecule that exists in different forms : short or long. It consists of an extracellular chain of 816 amino acids, a transmembrane domain of 23 amino acids and a cytoplasmic domain [1] [2]. This cytoplasmic domain is made up of 34 amino acids for the short form of the leptin receptor and 303 amino acids for the long form.
The extracellular part is itself subdivided into 5 functional domains :
- distal membrane homology of the first cytokine receptor (CRH1),
- immunoglobulin-like domain (IGD),
- second homology of the cytokine receptor (CRH2),
- two membrane proximal fibronectine type-III (FNIII) domains.
Location
Leptin receptors are found in different regions depending on the short or long form of the protein.
The long form (Ob-Rl) is mainly present in the hypothalamus, notably in the arcuate nucleus, dorsomedial and ventromedial nuclei, and in the paraventricular nucleus (PVN). It is also found in lesser amounts in the islets of Langerhans in mouses, as well as in the liver, spleen, heart, white fat, lymph node, jejunum and hematopoietic stem cells.
Short-form leptin receptors are located primarily in the lungs, kidneys and islets of Langerhans. In ovaries, only this short form of the protein can be found. The short form Ob-Ra is also present in smaller quantities in the brain at the hypothalamus.
Important functions of the protein
The leptin receptor helps regulate the release of leptin in the body. Leptin playing a large number of roles, its receptor allows the regulation and proper functioning of all these tasks. Most of these functions have been little developed or are being researched or simply have not been found.
However, we know that leptin is necessary for the proper development of the human body, as well as its good health. Its main function is related to the feeling of satiety. This therefore makes it possible not to overeat. It also plays an important role in the use of energy provided by nutrition. The main role of the receptor is therefore linked to its release in the body and therefore to its upstream production, since the level of leptin depends on the body mass of the person. The bigger a person, the more leptin they produce. It therefore has a strong link with obesity.
The second important role of the leptin receptor is the development of the body for reproduction and then during pregnancy. Regarding reproduction, studies have shown in mice that the absence of leptin, and therefore the proper functioning of its receptor, had an impact on their fertility. For pregnancy, there are two phenomena, one in the mother and the other in the fetus. In the mother there is a significant increase in leptin production throughout pregnancy and then a return to normal just before childbirth. This production takes place in the placenta, the leptin receptor which is located on the wall. of this, therefore allows the mother to have access to this leptin. The fetus recovers very little leptin from the placenta, despite this presence of the receptor. It uses more of the "normal" route, where leptin passes through the receptor located in the hypothalamus and releases leptin based on the individual's body fat.
Interactions of the protein
Activation
The activation of the leptin receptor [3]is done through the CRH2, IGD and FN III domains [4].
The CRH2 domain is the main leptin binding site on the receptor. This domain is required for the activation of the receptor. It is composed of a region of four consecutive hydrophobic residues.
The IGD domain has no affinity for leptin but is nevertheless required for receptor activation. In the absence of this domain, the result is a receptor with a wild-type affinity for leptin, but completely devoid of biological activity.
In the FN III domains, there are two conserved cysteines that are crucial for the activation of the receptor.
In contrast, the receptor functionality is hardly affected when the CRH1 domain is deleted.
Signaling pathways
The leptin receptor has cytokine receptor characteristics. The transduction of the signal by this type of receptor involves the formation of multimeric complexes. This leads to the recruitment of tyrosine kinases, in particular those of the JAK family, which will phosphorylate STAT-like transcription factors at the cytoplasmic tail of the receptor. Recruitment and activation of secondary signalling molecules enable the leptin receptor signalling via the MAPK,AMPK, PI3K and mTOR pathways.
Regulation
The best known regulation of the leptin receptor is negative regulation by OB-RGRP. The gene encoding the leptin receptor (OB-R) also encodes a second transcript called OB-R gene related protein (OB-RGRP). Stopping the expression of the OB-RGRP gene by interfering RNAs increases the sensitivity of the cells' receptors to leptin [5]. This prevents the development of obesity despite a diet that may be high in fat.
Disease
The receptor allows the release of leptin in the body. An abnormality of this receptor can cause a large number of illnesses, because the level of leptin is too much or too little.
Obesity is partly due to the too weak uptake of leptin by the receptor, the individual does not feel the feeling of satiety. And the production of leptin continues.
