a search for the biological factor responsible for energy balance and signal transduction to the hypothalamus was initiated by studies on animal models . in 1994 , a molecular defect in the obese gene ( ob ) , a gene responsible for the obesity phenotype in ob / ob mice was identified using a positive cloning method . ob gene has found on chromosome 6 in mice and on chromosome 7q31.3 in humans and has been shown to encode 4.5 kb - long mrna . the protein encoded by the ob gene has been isolated and named leptin - from the greek " leptos " meaning thin . it has been demonstrated that leptin biological activity strongly depends on its proper interactions with ob - r receptors .
ob - r receptor , encoded by db gene has been identified for the first time in a murine vascular plexus using a cloning technique .
it is a member of class i cytokine receptors family that , apart from , ob - r consist of gp130 subunit of il-2r , il-3 , il-4 , il-6 , il-7 , lif ( leukemia inhibitory factor ) , g - csf ( granulocytecolony stimulating factor ) , growth hormone , prolactin , and erythropoietin receptors .
ob - r expressed in six isoforms , is the product of alternative rna splicing of db gene ( diabetes gen ) . according to its structural differences
, the receptor 's isoforms were divided into three classes : long , short and secretory .
all ob - r isoforms have a similar , extracellular ligand- binding domain located in n - terminus of the protein .
the domain is constituted by 816 amino acids and contains four cysteine residues , wsxws motif ( trp - ser - x - trp - ser ) and a different number of fibronectin iii domains .
five isoforms , including the long isoform ob - rb and short ones - ob - ra , ob - rc , ob - rd , and ob - rf , have a transmembrane domain consisting of 34 amino acids .
the intracellular domain of the long isoform consists of 303 amino acids at carboxyl terminus , while intracellular domains of short isoforms are shorter and consists of 32 - 40 amino acids .
apart from identical extracellular and transmembrane domains , ob - r isoforms ( short and long ) have the same sequence of the first intracellular 29 amino acids .
the length of the further intracellular amino acid sequences depends on the alternative mrna splicing .
ob - rb isoform contain additional box 2 and socs ( suppressor of cytokine signaling ) motifs .
a sixth , secreted ob - re isoform , does not contain intracellular and cytoplasmic domains and is secreted to the bloodstream as a soluble receptor ( figure 1 ) [ 6,8 - 10 ] .
cr - cytokine receptor domain , f - iii - type iii fibronectin domains , box 1 , 2 , 3 - constant , intracellular motifs [ according to 7 , 8 ] . a long , fully active isoform of ob - rb is expressed mainly in the hypothalamus , where it takes part in energy homeostasis and in the regulation of secretory organs ' activity . ob - rb is also present in all types of immune cells , involved in innate and adaptive immunity ( figure 2 ) [ 7,9,11 - 14 ] .
ob - rb , long isoform of leptin receptor : jak - tyrosine kinases , stat - signal transducer and activator of transcription , socs proteins - cytokine signal transduction inhibitors , p - phosphate residues , cr - cytokine receptor domain , f - iii - type iii fibronectin domains [ according to 7 , 17 ] .
lack of a full - length ob - rb receptor is responsible for the development of the early obesity phenotype in db / db mice and in obese rats . in db
/ db mice , a short ob - ra isoform with limited activity is synthesized .
the mice strain is also characterized by cold intolerance and elevated concentration of glucosteroid hormones .
moreover , db / db mice phenotype includes a significantly elevated leptin concentration , with no ability to respond to leptin signal .
short leptin isoforms that contain box 1 motif are able to bind jak kinases ( janus kinases ) and to activate some signal transduction cascades .
however , the effect of short isoform activation differs from that of long isoform activation .
a short isoform , ob - ra , is the most common ob - r isoform that can be found in many various cells and tissues , including kidney , lungs , liver , spleen , and macrophages .
a soluble isoform of the receptor , ob - re , is probably a result of alternative transcript splicing of db gene or a consequence of transmembrane ob - r receptor destruction .
circulating ob - re is able to bind serum leptin and to inhibit signal transduction pathways . on the other hand ,
the receptor can regulate serum leptin concentration and serve as a carrier protein delivering the hormone to its membrane receptors able to transduct the signal into the cell . in normal conditions ,
only 5 - 25% of all ob - r isoforms are present on the cell surface , whereas the majority of receptors are localized within the cell .
after ligand binding , the receptors are internalized into early endosomes via clathrin - coated vesicles .
a decrease in ob - rb expression is much higher than changes in ob - ra expression , and short isoform ob - ra is much faster recycled to the cell membrane .
relatively weak signal transduction through long ob - rb isoform observed in obese , hyperleptinemic patients is related to delayed receptor expression on the cell surface , which may explain leptin resistance in these patients .
the major role in leptin signal transduction through membrane receptors is mediated through jak / stat ( signal transduction and activation of transcription ) pathway . among all ob - r isoforms ,
only the fulllength isoform ob - rb is able to fully transduce an activation signal into the cell .
ob - rb is considered a fully active receptor , because it contains 3 intracellular motifs necessary to activate the jak / stat pathway .
the described motifs are the following : box 1 and box 2 that bind jak tyrosine kinase , and box 3 motif that binds stat transcription factor . additionally , in intracellular domain , ob - rb contains four tyrosine residues ( tyr974 , tyr985 , tyr1077 and tyr1138 ) that activate intracellular signal transduction pathways ( figure 2 ) .
after ligand binding to the ob - rb receptor , a cytoplasmic tyrosine kinase jak2 binds to box-1 and box-2 motifs .
the activated kinase is autophosphorylated and activates specific tyrosine residues ( tyr 985 and tyr1138 ) in the receptor molecule . next , after tyr1138 phosphorylation , stat proteins bind to box-3 motif .
stat proteins ( stat1 , stat3 , stat5 , stat6 ) , including the key signal transduction molecule stat3 , are phosphorylated on tyrosine residues by jak2 kinases .
the process leads to the dissociation of proteins from the receptor , homo- or heterodimerization and translocation of the proteins to the nucleus . in the nucleus ,
the described proteins serve as transcription factors activating the expression of a suppressor of cytokine signaling 3 ( socs-3 ) or protein tyrosine phosphatase 1b ( figure 3 ) [ 17,24 - 26 ] .
after leptin binding , the ob - rb receptor changes its conformation that leads to jak translocation .
jak proteins become activated and start to exert their kinase activity and phosphorylate tyrosine residues of other jak proteins and of ob - rb receptor .
tyr1138 phosphorylation allows stat3 protein binding and next , stat proteins become the substrate for jak proteins related to ob - r receptor .
stat3 protein phosphorylation leads to dissociation of the protein from the receptor , dimerization and translocation to the nucleus .
stat proteins are activated by leptin and depending on the target tissue ; the type of activated protein may differ between various types of target cells .
leptin induces socs3 expression by binding to the socs3 sh2 domain with tyr985 or through the inhibition of socs3 dependent jak2 phosphorylation activity .
in addition , ptp1b ( protein tyrosine phosphatase 1b ) is another negative leptin signal transduction modulator that regulates signal transduction through dephosphorylation of jak2 .
ptpb1 overexpression decreases jak2 phosphorylation and inhibits leptin - induced socs3 and c - fos ( proto - oncogene ) transcription .
mapk kinases ( mitogen - activated protein kinase ) , irs1 ( insulin receptor substrate-1 ) , and phosphatidylinositol kinase ( pi3k ) are important pathways responsible for ob - rb receptor activation by leptin in various cells , e.g. , t - cells .
the mapk kinase signal transduction pathway is mediated by erk ( estrogen receptor ) and p38 kinases .
it has been demonstrated in osteoblasts that leptin , after activation of ob - rb , induces apoptosis through the activation of mapk , erk1/2-dependent activation of cytoplasmic phospholipase a , and subsequent cytochrome c release and activation of caspases 3 and 9 .
osmotic stress , heat shock , and cytokines are able to activate another mapk family protein , namely p38 . in mononuclear cells , after binding to ob - rb , leptin increases the level of p38mapk phosphorylation . in lps
( lipopolysaccharide)-stimulated kupffer cells , leptin significantly increases tnf secretion through the activation of p38 and jnk / mapk ( c - jun n - terminal kinase ) , whereas in smooth muscle cells it is able to induce hypertrophy through the activation of p38 mapk .
pi3k is believed to constitute an important common element for signal transduction pathways activated by insulin and leptin and its receptor . in the central nervous system ,
adipose tissue , liver , and pancreas , leptin induces a similar pathway to that activated by insulin , including pi3kdependent activation of pde3b ( phosphodiesterase 3b ) and camp ( cyclic adenosine monophosphate ) reduction .
it seems that pi3k / pde3b / camp pathway cooperates with the jak2/stat cascade and is an important element of leptin signal transduction pathways in the hypothalamus .
lepr gene ( leptin receptor gene ) mutations are extremely rare in human and animals . a single nucleotide substitution ( g to a ) in exon 6 leads to a deficiency in intracellular and transmembrane domain of the receptor .
the mutation caused by premature stop codon insertion at the 3 ' terminus of leprb mrna was found in db / db mice , whereas in zucker rats , the mutation was caused by amino acid substitution ( gln to pro ) at position 269 of the extracellular domain of the receptor .
the described mutation resulted in a severe reduction of ob - r expression on the cell surface and limitation in the leptin - receptor binding .
obese koletsky rats had a point mutation at 763 position resulting in premature stop codon introduction in the intracellular domain of the receptor that led to a total deficiency in all ob - r isoforms on the cell surface . both zucker and koletsky rats are characterized by morbid obesity , hyperphagia , hyperlipidemia , and numerous hormonal disorders . in humans ,
a rare mutation caused by a single substitution of g to a in exon 16 was identified .
it leads to an abnormal expression of transmembrane and intracellular domains of receptors . as in case of laboratory animals , humans with lepr gene defects
it has been demonstrated that the polymorphisms may lead to the impairment of signal transduction from the receptor into the cell through premature ob - ra , instead of ob - rb isoform production , or through a decrease in ob - r expression on the cell surface and limitation in leptin - receptor interactions .
lepr gene defects influence the development of hyperphagia , obesity , pubertal development disorders , neuroendocrine system regulation impairment , and diabetes caused by -cell apoptosis in the pancreas .
the most commonly seen polymorphism is gln223arg , encoding the extracellular domain of the receptor responsible for leptin binding .
glutamine to arginine change may be responsible for an impaired signal transducing capacity of the leptin receptor . a connection between gln223arg polymorphism and breast cancer development and progression has been suggested .
genetic interactions between leptin and gln223arg polymorphisms of lepr gene can increase the risk of the development of non - hodgkin lymphoma in obese patients .
it also is suggested that the discussed polymorphisms are related to hemodynamic and metabolic disorders found in obese patients .
overweight and obesity commonly observed after the management of childhood acute lymphoblastic leukemia also may have to do with the gln223arg polymorphism of lepr gene .
 leptin or obesity receptor ( ob - r ) is a member of class i cytokine receptor family . 
 ob - r , expressed in six isoforms , is the product of alternative rna splicing of db gene . according to its structural differences , 
 the receptor 's isoforms are divided into three classes : long , short , and secretory isoforms . 
 a long , fully active isoform of ob - rb is expressed mainly in the hypothalamus , where it takes part in energy homeostasis and in the regulation of secretory organs ' activity . 
 ob - rb is also present on all types of immune cells , involved in innate and adaptive immunity . 
 short leptin isoforms ( ob - ra , ob - rc , ob - rd , and ob - re ) that contain box 1 motif are able to bind jak kinases ( janus kinases ) as well as to activate some other signal transduction cascades . 
 a soluble isoform ( ob - re ) can regulate serum leptin concentration and serve as a carrier protein delivering the hormone to its membrane receptors and is able to transduce the signal into the cell . 
 jak / stat pathway plays the major role in leptin signal transduction through membrane receptors . among all ob - r isoforms , only full - length isoform ( ob - rb ) is able to fully transduce activation signal into the cell . 
Introduction
The Structure of Leptin Receptors
Signal Transduction Pathways
Leptin Receptor Gene Polymorphism
Conflicts of interest