BIOCHEMISTRY OF LEPTIN Shalini Jain and Hariom Yadav Animal Biochemistry Division Natioanal Dairy Research Institute Karnal (Haryana), India

How leptin is discovered Three theories evolved :  Thermo regulation influenced the VMH  Glucostatic theory – plasma glucose regulated over all energy stores.  Lipostatic theory – product of fat metabolism that circulated in the blood and interacted with the VMH  Factor was discovered in 1994 by Dr. Jeffrey Friedman's team.  Factor was termed ‘Leptin’.

Leptin (OB) 16 KDa protein encoded by ob gene. Expressed & secreted by – adipocytes, placenta, gastric epithelium. Directly proportional to the total amount of fat in the body. mice are homozygous for single gene mutation. ob/ob –protein hormone leptin db/db –receptor for leptin. High degree of homology

Leptin structure  146 a.a residue non glycosylated polypeptide  Member of helical cytokine family Primary structure of leptin

Contd. folding pattern compatible with the helical cytokines Mutant form of leptin E- 100, sustitution at 100 position Glu – Trp. 4 antiparallel helices each about 5-6 turn long Two long loops connecting helices A-B and C-D, shorter loop connecting helices B-C.

Small helical segment E

Two cysteine residues at the 96 and 146 position Disulphide bridge and kink in the D-helix are a essential for proper folding and receptor binding.

Contd. Act as binding protein. A tail with 34 a.a residues – OB-R a form. A tail of 32 & 40 a.a –OB-R c form and OB-R d form. OB-R e form terminate before transmembrane segment and this represent a soluble receptor In db/db mice gene under go abnormal splicing- truncated receptor. OB bind with OB-R with high affinity(K d = pM).

Signalling Pathway of Leptin Action

Physiological effects of Leptin Regulation of food intake,energy expenditure and body weight. Thermogenesis. Reproductive function. Supressed bone formation. Directly act on the cells of liver and muscles. Related to inflammatory response. Contribute to early hematopoiesis.

How leptin works ?

Role of leptin in regulation of food intake and body weight Decrease hunger and food consumption - inhibition of neuropeptide Y synthesis. Food intake linked to its ability to regulate the neuroendocrine system.

Neuropeptide Y 36 a.a residue produce in the arcuate nucleus of the hypothalamus. Rich in tyrosine residues. Appetite stimulating hypothalamic peptide

Contd. Found in many organ, high level of NPY are found in brainstem and hypothalamus. Stimulates leptin production in adipose tissue by increasing food intake and insulin secretion. Action through the parasympathetic nervous system.

Role of leptin in thermogenesis

Role of leptin in reproduction Fertility influenced by stored body fat Leptin signals the onset of puberty. Regulates hypothalamic- pituitary – ovarian function.

Contd.

Role of leptin in lipid metabolism Inhibits intracellular lipid concentration Activate 5 –AMP-activated protein kinase (AMPK) Inhibits acetyl coenzyme-A carboxylase (ACC) Increase in fatty acid oxidation and reducing the fat tissue in muscles and liver. Increase insulin sensitivity.

Inflammatory response Long form of leptin receptor is expressed by T- lymphocytes, bone marrow, spleen. Leptin released in response to inflammatory cytokines attenuating its response and hence. modulating inflammatory response. Stimulates the expression of POMC -processed to  - MSH. Against the auto aggressive effects of the immune system.

Regulation of leptin expression Two transcription factors PPAR  and C/EBP  control the adipocyte differentiation C/EBP  promotes the leptin expression. PPAR  decrease leptin expression Regulated by environmental and hormonal factors

Inducers and suppressers of leptin expression InducersEffectSpecies Feeding+Rodent + man Glucocorticoids+Rodent + man Insulin+Rodent C or +Man Cytokines+Rodent Obesity+Rodent + man Fasting-Rodent + man Pertussis toxin-Rodent  -receptor antagonists -Rodent Thiazolidinediones-Rodent cAMP-Rodent

Feedback loop Food intake trigger the output of glucocorticoids and insulin Favour fat accumulation & increase leptin Leptin travels to hypothalamus Regulate body mass & control body energy intake, energy expenditure NPY also regulate body fat mass

Obesity Main focus of leptin research. Dramatic effects on obesity in mice. In human a body mass index over 27.3 for man & 27.8 for women. Hypothalamic insensitivity to leptin – fundamental mechanism of obesity.

Leptin resistance Mutation of the gene for leptin receptors in the brain Post receptor abnormalities in leptin signal transduction Impaired leptin transport across blood- brain barrier

What about humans: Human fat cells also manufacture leptin protein (167 a.a ) Mutation in gene for leptin or its receptor are rarely found in obese people High blood conc. leptin indicates leptin resistance Extreme obesity in 5 members of two families that are homozygous for mutation in their leptin gene –like ob/ob mice.

Extreme obesity among three members of a family homozygous for mutation in leptin receptor gene-like db/db mice. Only moderate obesity in people heterozygous for leptin gene. Recombinant human leptin available. The 16 september 1999 issue the New England Journal of Medicine reports- 9 year old girl homozygous for frame shift mutation leptin gene. Contd.

Factor in obesity -  -3 adrenoreceptor Defect contribute leptin resistance/leptin expression A paradox exists-comparison between mouse & human research cannot be made

Leptin research results Weight lost with mutated ob gene Not effective without genetic defect ob gene More obese less sensitive high level leptin

Future prospects More focus on leptin receptor & involvement in leptin resistance Relation to reproduction Mechanisms involved in regulation of leptin

Conclusion Much more research needs to be done to fully realize the potential of leptin in the body When the medical community does learn more about leptin’s control & regulation,it will surely have a profound impact on the treatment of obesity, infertility