A leptina é um componente importante do complexo sistema fisiológico que regula o armazenamento, o equilíbrio e o uso de energia pelo organismo. É descrito na literatura que a concentração plasmática de leptina e/ou a quantidade de mRNA de leptina nos adipócitos, correlacionam-se positivamente com o gênero, a ingesta calórica, o IMC, e a massa de tecido adiposo. A leptina pode aumentar o gasto energético por meio da estimulação nervosa simpática. A ação tecidual da leptina se da através de sua ligação com seu receptor. O receptor de leptina apresenta heterogeneidade na população humana, com naturais ocorrências de polimorfismos que codificam para o aminoácido das posições 109, 223 e 656 entre outros. Acredita-se que mutações no gene do receptor podem modificar sua função e afetar os níveis séricos de leptina na população em geral, sugerindo que este polimorfismo possa contribuir para a correlação da obesidade com a doença cardiovascular associado à atividade nervosa simpática. Neste estudo, avaliamos a relação entre as variantes alélicas do gene do receptor de leptina e mudanças na composição corporal, nas variáveis metabólicas, neurovasculares e hemodinâmicas de mulheres obesas antes e após um programa de treinamento físico e dieta hipocalórica. Como achado principal do nosso estudo, observamos que as pacientes obesas com polimorfismo Gln223Arg/Arg223Arg apresentaram maior massa gorda em comparação ao grupo de mulheres obesas Gln223Gln no período inicial do estudo. Não foram encontradas diferenças nas demais variáveis antropométricas, metabólicas, neurovasculares e hemodinâmicas do códon 223. As variáveis antropométricas, metabólicas, neurovasculares e hemodinâmicas permaneceram semelhantes entre as formas genéticas dos códons 109, 656. Como esperado, a mudança no hábito de vida impactou em algumas das variáveis antropométricas, metabólicas, neurovasculares e hemodinâmicas, porém as variantes genéticas de cada códon não alteraram essas variáveis. A partir dos dados apresentados, podemos dizer que o polimorfismo Gln223Arg/Arg223Arg influencia no aumento de massa gorda em mulheres obesas. Entretanto, após quatro meses de intervenção nos hábitos de vida dessas pacientes, a associação entre o polimorfismo citado e o aumento da massa gorda não se manteve, o que pode ser atribuído à melhora da resistência a ação da leptina, que sabidamente é favorecida por perda de peso, como ocorreu na nossa população / Leptin is an important component of a complex physiological system that contributes to the regulation of the organic energy balance. Leptin plasmatic concentration or mRNA quantities are positive related to gender, caloric intake, BMI and fat mass. Energy expenditure can be altered by leptin action through the nervous sympathetic stimulus. Leptin acts via its specific receptor. These receptor presents heterogeneity in human population, with natural occurrences of polymorphisms that codify to different proteins. Mutations in gene receptor can modify the leptin level and consequently its action. This suggests that the polymorphism of leptin receptor could contribute to the relation of obesity and cardiovascular disease, and the sympathetic nervous system is involved in this process. In this study we evaluate the relation between allelic variants of leptin receptor gene with changes in corporal composition, metabolic, neurovascular and hemodynamic variables in obese women, before and after a life style change program. The main result of this investigation is that the women that carrier a polymorphism in codon 223, with mutation of glicina to arginina presented higher fat mass than the other polymorphisms. There are no differences in the other antropometric, metabolic, neurovascular and hemodynamic variables in this codon. All the studied variables were similar between the genetic forms 109 and 656. As expected, the life style changes positive impact in some of the anthropometric, metabolic, neurovascular and hemodynamic variables, but the allelic variants did not alter these variables. In conclusion, the data presented permit us to say that the Gln223Arg/Arg223Arg polymorphism influences the fat mass gain in obese women. However, after four months of exercise training and diet, this difference in fat mass gain did not keep, wich can be atributted to the improvement in the leptin resistence and the weigh loss, which occurred in our population

A leptina é um componente importante do complexo sistema fisiológico que regula o armazenamento, o equilíbrio e o uso de energia pelo organismo. É descrito na literatura que a concentração plasmática de leptina e/ou a quantidade de mRNA de leptina nos adipócitos, correlacionam-se positivamente com o gênero, a ingesta calórica, o IMC, e a massa de tecido adiposo. A leptina pode aumentar o gasto energético por meio da estimulação nervosa simpática. A ação tecidual da leptina se da através de sua ligação com seu receptor. O receptor de leptina apresenta heterogeneidade na população humana, com naturais ocorrências de polimorfismos que codificam para o aminoácido das posições 109, 223 e 656 entre outros. Acredita-se que mutações no gene do receptor podem modificar sua função e afetar os níveis séricos de leptina na população em geral, sugerindo que este polimorfismo possa contribuir para a correlação da obesidade com a doença cardiovascular associado à atividade nervosa simpática. Neste estudo, avaliamos a relação entre as variantes alélicas do gene do receptor de leptina e mudanças na composição corporal, nas variáveis metabólicas, neurovasculares e hemodinâmicas de mulheres obesas antes e após um programa de treinamento físico e dieta hipocalórica. Como achado principal do nosso estudo, observamos que as pacientes obesas com polimorfismo Gln223Arg/Arg223Arg apresentaram maior massa gorda em comparação ao grupo de mulheres obesas Gln223Gln no período inicial do estudo. Não foram encontradas diferenças nas demais variáveis antropométricas, metabólicas, neurovasculares e hemodinâmicas do códon 223. As variáveis antropométricas, metabólicas, neurovasculares e hemodinâmicas permaneceram semelhantes entre as formas genéticas dos códons 109, 656. Como esperado, a mudança no hábito de vida impactou em algumas das variáveis antropométricas, metabólicas, neurovasculares e hemodinâmicas, porém as variantes genéticas de cada códon não alteraram essas variáveis. A partir dos dados apresentados, podemos dizer que o polimorfismo Gln223Arg/Arg223Arg influencia no aumento de massa gorda em mulheres obesas. Entretanto, após quatro meses de intervenção nos hábitos de vida dessas pacientes, a associação entre o polimorfismo citado e o aumento da massa gorda não se manteve, o que pode ser atribuído à melhora da resistência a ação da leptina, que sabidamente é favorecida por perda de peso, como ocorreu na nossa população / Leptin is an important component of a complex physiological system that contributes to the regulation of the organic energy balance. Leptin plasmatic concentration or mRNA quantities are positive related to gender, caloric intake, BMI and fat mass. Energy expenditure can be altered by leptin action through the nervous sympathetic stimulus. Leptin acts via its specific receptor. These receptor presents heterogeneity in human population, with natural occurrences of polymorphisms that codify to different proteins. Mutations in gene receptor can modify the leptin level and consequently its action. This suggests that the polymorphism of leptin receptor could contribute to the relation of obesity and cardiovascular disease, and the sympathetic nervous system is involved in this process. In this study we evaluate the relation between allelic variants of leptin receptor gene with changes in corporal composition, metabolic, neurovascular and hemodynamic variables in obese women, before and after a life style change program. The main result of this investigation is that the women that carrier a polymorphism in codon 223, with mutation of glicina to arginina presented higher fat mass than the other polymorphisms. There are no differences in the other antropometric, metabolic, neurovascular and hemodynamic variables in this codon. All the studied variables were similar between the genetic forms 109 and 656. As expected, the life style changes positive impact in some of the anthropometric, metabolic, neurovascular and hemodynamic variables, but the allelic variants did not alter these variables. In conclusion, the data presented permit us to say that the Gln223Arg/Arg223Arg polymorphism influences the fat mass gain in obese women. However, after four months of exercise training and diet, this difference in fat mass gain did not keep, wich can be atributted to the improvement in the leptin resistence and the weigh loss, which occurred in our population

Abstract Leptin is a hormone secreted by adipose tissue. It is involved in the regulation of appetite and energy expenditure. Leptin binds to its receptor (LEPR) that is expressed in the central nervous system as well as in other tissues including adipocytes and endothelial cells. Plasma leptin level reflects the amount of adipose tissue and previously, it has been shown to be associated with the risk for coronary artery disease. Two LEPR polymorphisms, Lys109Arg and Gln223Arg, have been extensively studied and they have been associated with several risk factors of atherosclerosis. Earlier studies have also shown that the risk for developing atherosclerosis and various other diseases might already be determined during the fetal period or immediately after birth. It seems that intrauterine undernourishment might cause changes on epigenetic level and result in alterations in gene expression. It has been suggested that the impaired fetal growth could affect plasma leptin level and leptin messenger RNA expression from adipose tissue. Long-term fructose consumption has also been shown to result in leptin resistance. Recently, leptin has been observed to be associated with autophagy. Autophagy has been demonstrated to act in several interesting processes such as fat storage in adipocytes and liver. Autophagy and the leptin system might also regulate each another. The aim of this thesis was to investigate the association of LEPR polymorphisms with thickness of the wall of carotid artery as well as with fatal and nonfatal cardiovascular disease events. In addition, we aimed to clarify the effects of fetal undernourishment and fructose consumption on the leptin system and autophagy. We were also interested in studying the role of the leptin system and autophagy in elevated triglycerides and liver fat accumulation seen as a result of high-fructose diet. In our studies, we observed that LEPR polymorphisms, Lys109Arg and Gln223Arg, are associated with intima-media thickness of carotid artery. Moreover, 19-year follow-up study showed that 109Arg homozygotes display lower incidence of cardiovascular events and lower total mortality. In our animal experiments, we were able to detect that fructose diet affects both LEPR isoform and autophagy gene expression. It seems that these changes might partly explain the mechanism behind the rise in blood triglyceride levels and liver fat accumulation caused by fructose diet. In conclusion, the results of this study clarify the role of leptin receptor in cardiovascular diseases. In addition, they offer new information especially about the effects of fructose diet on the leptin system, the dysfunction of which might predispose to the development of diseases. / Tiivistelmä Leptiini on rasvakudoksen tuottama hormoni. Se osallistuu ruokahalun ja energiankulutuksen säätelyyn. Leptiini sitoutuu reseptoriinsa (LEPR), joita on sekä keskushermostossa että muissakin kudoksissa, myös adiposyyteissä ja endoteelisoluissa. Plasman leptiinitaso heijastaa rasvakudoksen määrää ja sen on aiemmin osoitettu olevan yhteydessä sepelvaltimotaudin riskiin. Erityisesti kahta LEPR:n polymorfiaa, Lys109Arg ja Gln223Arg, on tutkittu aiemmin ja niiden on osoitettu olevan yhteydessä useisiin ateroskleroosin riskitekijöihin. Aiemmat tutkimukset ovat myös osoittaneet, että ateroskleroosiin ja useisiin muihin sairauksiin sairastumisen riski voi osittain määräytyä jo sikiöaikana tai varhain syntymänjälkeisen kehityksen aikana. Vaikuttaa siltä, että sikiöaikainen aliravitsemus voi aikaansaada muutoksia epigeneettisellä tasolla ja aiheuttaa näin muutoksia geeniekspressiossa. On ehdotettu, että sikiön heikentynyt kasvu vaikuttaisi plasman leptiinitasoon ja rasvakudoksen leptiinin lähetti-RNA:n ilmentymiseen. Pitkäaikaisen fruktoosinkulutuksen on myös osoitettu aiheuttavan leptiiniresistenssiä. Hiljattain leptiinin on havaittu olevan yhteydessä myös autofagiaan. Autofagian on osoitettu vaikuttavan useisiin kiinnostaviin prosesseihin, kuten rasvan varastoitumiseen adiposyytteihin sekä maksaan. Autofagia ja leptiinijärjestelmä mahdollisesti myös säätelevät toisiaan. Tämän väitöskirjan tavoitteena oli tutkia LEPR-polymorfioiden yhteyttä kaulavaltimon seinämän paksuuteen sekä sydän- ja verisuonitautitapahtumiin ja kuolleisuuteen. Pyrimme lisäksi selvittämään sikiöaikaisen aliravitsemuksen ja fruktoosin käytön vaikutusta leptiinijärjestelmään sekä autofagiaan ja olimme kiinnostuneita tutkimaan näiden osuutta fruktoosin kulutuksen seurauksena nähtävien metabolisten muutosten, kuten kohonneiden triglyseridien sekä maksan rasvoittumisen, synnyssä. Tutkimuksessamme havaittiin yhteys LEPR polymorfioiden Lys109Arg ja Gln223Arg sekä kaulavaltimon paksuuden välillä. Lisäksi 19-vuoden seurantatutkimus osoitti 109Arg-homotsygotian liittyvän pienentyneeseen sydän- ja verisuonitapahtumien ilmaantuvuuteen sekä matalampaan kokonaiskuolleisuuteen. Eläinmallissamme havaitsimme sekä LEPR-muotojen että autofagiageenien ilmentymisen muuttuneen fruktoosidieetin vaikutuksesta. Vaikuttaa siltä, että nämä muutokset voisivat osaltaan selittää esimerkiksi fruktoosiruokavalion aiheuttaman veren triglyseriditasojen nousun sekä maksan rasvoittumisen rotilla. Tutkimuksen tulokset selventävät leptiinireseptorin roolia sydän- ja verisuonitautien taustalla. Lisäksi ne tarjoavat uutta tietoa erityisesti fruktoosinkulutuksen vaikutuksesta leptiinijärjestelmään, jonka häiriöt altistavat sairauksien kehittymiselle.

Indiana University-Purdue University Indianapolis (IUPUI) / The entire hematopoietic system rests upon a group of very rare cells called hematopoietic stem cells (HSCs). Due to this extraordinarily crucial role, after birth HSCs are localized to the deep bone marrow niche, a hypoxic environment inside the bone where HSCs are under well-orchestrated regulation by both cellular and humoral factors. Among the cellular components regulating hematopoiesis are Leptin Receptor (LEPR)-expressing mesenchymal/stromal cells and adipocytes; both have been demonstrated to have significant influence on the maintenance of HSCs under homeostasis and in stress-related conditions. It has been reported in early work by others that HSCs and hematopoietic progenitor cells (HPCs) express LEPR. However, whether LEPR+ HSCs/HPCs are functionally different from other HSCs/HPCs was unknown. In this study, I demonstrated for the first time that murine LEPR+ Lineage-Sca-1+cKit+ (LSK, a heterogenous population consisting of HSCs/HPCs) cells even though constituting a small portion of total LSK cells are significantly enriched for both phenotypic and functional self-renewing long-term (LT) HSCs as shown in primary and secondary transplants in lethally irradiated recipients. LEPR+LSK cells are also more enriched for colony-forming progenitor cells assessed by colony-forming unit (CFU) assays. In addition, LEPR+ HSCs (defined as LSKCD150+CD48-) exhibited robust repopulating potential as compared to LEPR-HSCs in long-term competitive transplantation assays. To elucidate the molecular pathways that may govern functional properties of LEPR+HSCs, bulk RNA-seq on freshly sorted cells was done. Gene set enrichment analyses (GSEA) revealed Interferon Type I and Interferon γ (IFNγ) Pathways were significantly enriched in LEPR+HSCs while mitochondrial membrane protein gene set was significantly enriched in LEPR-HSCs. Interestingly, proinflammatory signaling including IFNγ pathway has been suggested to be critical for the emergence of embryonic HSCs from the hemogenic endothelium. Altogether, our work demonstrated that LEPR+HSCs represent a small subset of highly engrafting adult BM HSCs. These results may have potential therapeutic implications in the field of hematopoietic transplantation as LEPR is highly conserved between mice and humans.

Truncal adiposity impairs ventilation in obese adults by altering normal ventilatory mechanics. Leptin, an inflammatory adipocytokine, is elevated in obesity and has been shown to alter ventilatory responses to exercise. Leptin’s bioavailability appears to be regulated by its soluble receptor (LRe), which is reduced in obesity. Roux-en-Y gastric bypass surgery (RYGBS) is a weight loss intervention that reduces total fat mass and improves several obesity related co-morbidities including pulmonary dysfunction. The purpose of this study was to first evaluate the differences between ventilatory responses to carbon dioxide (VE/VCO2 slope) during progressive treadmill walking in morbidly obese and normal weight females. Second, we will analyze the relationships between the VE/VCO2 slope, truncal adiposity, serum leptin, and LRe. Lastly, we want to evaluate the changes in the ventilatory responses to exercise (VE/VCO2 slope), truncal adiposity, serum leptin, and LRe 3 months following Roux-en Y gastric bypass surgery. Thirteen obese (OB 37.7 ±11.4 years, 42.0 ± 4.8 kg/m2) and 12 normal weight females (NW 36.1 ±8.0 years, 22.8 ± 1.2 kg/m2) participated in this study. Blood samples for measure of fasting serum leptin and soluble leptin receptor were obtained prior to exercise. Cardiopulmonary variables were measured throughout exercise. Regional adiposity was determined through dual energy x-ray absorptiometry. Truncal adiposity was significantly greater in the obese group than the normal weight group. Serum leptin was greater in the obese group while LRe was lower than the normal weight group. The VE/VCO2 slopes were lower in obese group when compared to the normal weight group. There were no significant group differences in maximal ventilation, tidal volume or respiratory rate. Stepwise regression determined that truncal adiposity accounted for 31.5% of variance in VE/VCO2 slope (R= 0.561, R2 =0.315, p = 0.004). At 3 months post-surgery we observed significant reductions in the obese group in total percentages of fat, truncal adiposity, serum leptin. The soluble leptin receptor was not changed at any measured time point following RYGBS. There were no changes in 3 months post-surgery VE/VCO2 slopes in the obese group. Truncal adiposity, serum leptin and LRe were associated with reduced ventilatory responses to weight bearing exercise (VE/VCO2 slope) in obese females when compared to normal weight females. There were no differences between obese and normal weight females in maximal minute ventilation, tidal volume or respiratory rate. This result suggests that differences in VE/VCO2 slopes may not be entirely from maximal pulmonary capacity. Rather, the differences in VE/VCO2 slope may be attributed to truncal adiposity and its positive relationship with leptin. Elevated leptin in the obese group may indicate a state of central leptin resistance which has been shown to reduce the ventilatory responses to exercise. At 3 months post RYGBS significant reductions in total percent fat, serum leptin, truncal adiposity and BMI were observed. However, despite improvement in fat mass and serum leptin there were no changes in the VE/VCO2 slope and LRe at 3 months post RYGBS. Therefore, it is possible that the improvements in body composition and leptin following RYGBS were not sufficient to increase ventilation responses to weight bearing exercise in obese females.

Humanos e camundongos deficientes tanto na produção da leptina quanto na expressão do seu receptor (LepR) são obesos, hiperfágicos e inférteis; o tratamento com reposição de leptina em indivíduos deficientes na produção desse hormônio garante que haja redução no peso corporal, seja retomada ciclicidade estral e a fertilidade. Ainda, o tratamento com leptina em fêmeas prépúberes normais acelera o início da puberdade, sugerindo que esse hormônio tem papel crucial no tempo de início da puberdade. Ao relacionarmos leptina e puberdade, uma informação interessante é que, neurônios que secretam hormônio liberador de gonadotrofinas (GnRH), localizados na área pré-óptica medial (MPO) não coexpressam o LepR. Além disso, embora poucos neurônios que expressam o RNAm que codifica o gene Kiss1 do núcleo arqueado sejam responsivos à leptina, a sinalização da leptina nesses neurônios só ocorre após a puberdade e, a inativação do LepR nesses neurônios não afeta a reprodução. Por esses motivos, acredita-se que a leptina atue de forma indireta no controle da reprodução. Além dos núcleos MPO e ARH, o núcleo pré-mamilar ventral (PMv), que apresenta uma parcela de neurônios que expressam o LepR, também está envolvido na regulação neuroendócrina da reprodução. Estudos anteriores demonstraram que alterações nos níveis circulantes de estrógeno alteram características biofísicas e morfológicas de neurônios que expressam o gene Kiss1. Embora a leptina seja fundamental na maturação sexual não sabemos se os neurônios que expressam o LepR são suscetíveis a essas alterações durante a maturação sexual. Portanto, o objetivo deste trabalho foi investigar se neurônios LepR presentes nos núcleos MPO, ARH e PMv sofrem alterações morfológicas durante a maturação sexual e mediante ausência de hormônios gonadais. Foram utilizados camundongos (fêmeas) LepR-Cre tdTomato, divididas em 4 grupos experimentais: pré-púberes, púberes, adultas e ovarectomizadas (OVX). A maturação sexual foi avaliada, para determinação do dia da abertura vaginal e primeiro estro. Nas idades específicas os encéfalos foram processados e coletados para posterior quantificação e aferição da área de superfície de neurônios LepR. Realizamos, também, imunoistoquímica de fluorescência para pSTAT3 para quantificar o percentual de neurônios responsivos à leptina. Os resultados obtidos demostraram que ocorreu um aumento no número de neurônios LepR no núcleo ARH com o avanço da idade. Porém, não houveram diferenças significativas quanto ao número de neurônios nos núcleos MPO e PMv. Também não observamos alterações na área de superfície nos núcleos analisados. Em relação a ativação de neurônios LepR via marcação de pSTAT3, o núcleo MPO apresentou uma diminuição no percentual de neurônios ativos quando comparamos os grupos pré-púberes com os grupos adulta e OVX. O oposto aconteceu no núcleo ARH onde obtivemos um aumento desse percentual com o avanço da idade. Em ambos os núcleos (MPO e ARH) o percentual de neurônios LepR ativados no grupo OVX foi estatisticamente semelhante ao grupo adulta, diferindo somente do grupo pré-púbere. O núcleo PMv não apresentou diferenças estatísticas entre os grupos analisados. Nossos dados sugerem que estrógeno e leptina podem interagir de formas distintas em diferentes regiões hipotalâmicas. / Leptin or leptin receptor (LepR) deficient humans and mice are obese, hyperphagic and infertile; leptin replacement in leptin deficient subjects is capable of reducing body mass and restores estrous cyclicity and fertility. In addition, leptin accelerates the onset of puberty in normal female mice, suggesting that this hormone plays a critical role in the timing of puberty. Interestingly, the gonadotropin releasing hormone neurons, located at the medial pre-optic área (MPO) does not coexpress the LepR. Although few kisspeptin neurons in the arcuate nucleus (ARH; 15%) are responsive to leptin, leptin signaling in kisspeptin neurons arises only after pubertal development and leptin receptor inactivation in kisspeptin cells did not affect reproduction. Therefore, the effects of leptin on reproduction is believed to occur indirectly. In addition to ARH and MPO, the ventral premammillary nucleus (PMv) is also involved in the neuroendocrine regulation of reproduction and a fraction of PMv neurons coexpress LepR. Curiously, previous studies have shown that changing levels of estrogen alters the biophysical properties and morphology of hypothalamic neurons, such as expressing the ones expressing the Kiss1 gene. Although leptin has an important role on the puberty onset, it is not known whether LepR-expressing neurons are also susceptible to changes in their morphology during sexual maturation. Therefore, our goal was to investigate whether LepR-expressing neurons in the MPO, ARH and PMv nuclei are susceptible to cell morphology modifications throughout the sexual maturation and after manipulation of estrogen levels. LepR-Cre tdTomato mice (females) were divided into 4 experimental groups: prepubertal, pubertal, adult and ovarectomized (OVX). The sexual maturation was evaluated to determine the day of vaginal opening and first estrus. At specific ages the brains were processed and collected for further quantification and measurement of the surface area of LepR neurons. We also performed fluorescence immunohistochemistry for pSTAT3 to quantify the percentage of neurons responsive to leptin. The results showed that there is an increase in the number of LepR neurons in the ARH nucleus with advancing age. However, there were no significant differences in the number of neurons in the MPO and PMv nuclei. We also did not observe changes in the surface area in the nuclei analyzed. In relation to the activation of LepR neurons via pSTAT3 labeling, the MPO nucleus showed a decrease in the percentage of active neurons when comparing the prepubertal groups with the adult and OVX groups. The opposite happened in the ARH nucleus where we had an increase of this percentage with the advancement of the age. In both nuclei (MPO and ARH) the percentage of LepR neurons activated the OVX group was statistically similar to the adult group, differing only from the pre-pubertal group. The PMv nucleus did not present statistical differences between the analyzed groups. Our data suggest that estrogen and leptin may interact differently in different hypothalamic regions.

<p>Thymus atrophy is highly inducible by stress and prolonged thymus atrophy can contribute to T cell deficiency or inhibit immune recovery after acute peripheral T cell depletion. Little is known regarding the mechanisms driving thymic involution or thymic reconstitution after acute stress. Leptin deficiency in mice results in chronic thymic atrophy, suppressed cell-mediated immunity, and decreased numbers of total lymphocytes, suggesting a role for leptin in regulating thymopoiesis and overall immune homeostasis. Exogenous leptin administration during stress has been shown to protect against thymic damage, yet the mechanisms governing these thymostimulatory effects are currently undefined. Studies herein define the impact of endotoxin-induced thymic damage in the stromal and lymphoid compartment of the thymus and systemic glucocorticoid and cytokine responses in the animal. We report here the novel finding that leptin receptor expression is restricted to medullary thymic epithelial cells in the normal thymus. Using a model of endotoxin-induced acute thymic involution and recovery, we have demonstrated a role for the metabolic hormone leptin in protection of medullary thymic epithelial cells from acute endotoxin-induced damage. We also demonstrated that systemic leptin treatment decreased endotoxin-induced apoptosis of double positive thymocytes and promoted proliferation of double negative thymocytes in vivo through a leptin receptor isoform b-specific mechanism. Leptin treatment increased thymic expression of IL-7, an important soluble thymocyte growth factor produced by medullary thymic epithelial cells. We also found leptin to inhibit systemic glucocorticoid and pro-inflammatory cytokine responses. Using leptin-deficient and leptin receptor-deficient mice in our stress model, we found that endotoxin-induced thymic atrophy was exacerbated in the absence of leptin, despite an inability to mount a proper pro-inflammatory cytokine response. Together, these data support a model in which leptin can function to protect the thymus gland from stress-induced acute damage in part by reduction of systemic corticosteroid and pro-inflammatory cytokine responses, and intrathymically through a mechanism orchestrated by medullary thymic epithelial cells and their soluble mediators (e.g. IL-7). Taken together, these studies suggest a physiological role for leptin signaling in the thymus for maintaining healthy thymic epithelium and promoting thymopoiesis, which is revealed when thymus homeostasis is perturbed by stress.</p> / Dissertation

The work presented in this thesis deals with the analysis of protein crystal structures with an emphasis on the stereochemical aspects of the folded conformation of proteins. The various analyses described have been performed on a data-set of 250 high resolution and non-homologous protein structures derived from the Protein Data Bank. The overall objective of the work has been to analyse conformational features of the non-secondary structural regions in proteins and identify structural motifs present therein. The results can be useful in the three-dimensional modelling of proteins, altering the stability of proteins, design of peptide mimics and in understanding the structural rules that guide protein folding. The contents of this thesis can be broadly classified into three parts, (a) Conformational preferences of amino acid residues to occur in the partially allowed regions of the Ramachandran map, (b) conformational features of structural motifs formed by side-chain/main-chain hydrogen bonds by polar residues and (c) analysis and characteristic features of isolated β-strands. Chapter 1 of the thesis gives an introduction, briefly discussing the conformation of polypeptide chains, structural features of globular proteins and applications of protein structural analysis etc. Chapter 2 describes the occurrence of left-handed α-helical conformation in protein structures. A data-set of 250 high resolution (< 2.0A) non-homologous protein crystal structures derived from the Protein Data Bank (PDB) has been analysed for occurrences of left-handed α-helical (αL) conformations. A total of 2,573 αL residues were identified from the data-set. About 59% of the observed examples of at conformations were found to be glycyl residues and about 41% non-glycyl. Continuous long stretches of αL residues are seldom found in protein structures. They are most commonly found as singlets represented by 78% of the observed αL examples. The doublets, triplets and quadruplets account for a very minor fraction of the observed examples. There is only a single example of a stretch of four contiguous αL residues, from the protein thermolysin, which forms a single turn of a left-handed α-helix. A majority of the αL residues are nevertheless part of well-defined substructures in proteins. They play singular roles as part of β-turns and helix termination sites in maintaining the characteristic main-chain hydrogen bonds needed for the stability of these structures. They are also found to be effective in the termination of β-strands. The stereo-chemistry and sequence environment around such structures are discussed. The analysis of the side-chain torsion angles of αL residues indicate that the g+ rotamer is highly unfavourable due to stereo-chemical violations posed by the atoms of the side-chain with those of the backbone. The αL residues are highly conserved by residue type as well as conformation among related proteins indicating their vital importance in protein structures Chapter 3 provides an explanation for the unusual preference of glycyl residues to occur in the bridge regions of the Ramachandran map. The Ramachandran steric map and energy diagrams for the glycyl residue are fully symmetric. Though a plot of the (Φ,Ψ) angles of glycyl residues derived from a data-set of 250 non-homologous and high-resolution protein structures is also largely symmetric, there is a clear aberration in the symmetry. While there is a cluster of points corresponding to the right-handed a-helical region, the "equivalent" cluster is shifted to centre around the (Φ,Ψ)values of (90°, 0°) instead of being centred at the left-handed a-helical region of (60°, 40°). An analysis of glycyl conformations in small peptide structures and in "coil" proteins, which are largely devoid of helical and sheet regions, shows that glycyl residues prefer to adopt conformations around (±90°, 0°) instead of right and left handed a-helical regions. Using theoretical calculations, such conformations are shown to have highest solvent accessibility in a system of two-linked peptide units with glycyl residue at the central Cα atom. This is found to be consistent with the observations from 250 non-homologous protein structures where glycyl residues with conformations close to (±90°, 0°) are seen to have high solvent accessibility. Analysis of a sub-set of non-homologous structures with very high resolution (1.5A or better) shows that water molecules are indeed present at distances suitable for hydrogen bond interaction with glycyl residues possessing conformations close to (±90°, 0°). It is concluded that water molecules play a key role in determining and stabilising these conformations of glycyl residues and explains the aberration in the symmetry of glycyl conformations in proteins. Chapter 4 discusses an analysis of backbone mimicry performed by polar side-chains in protein structures. Backbonemimicry bythe formation of closed loop C7, C10, C13 (mimics of γ-, β- and α-turns) conformations through side-chain main-chain hydrogen bonds by polar groups is found to be a frequent observation in protein structures. A data-set of 250 non-homologous and high-resolution protein structures was used to analyse these conformations for their characteristic features. Seven out of the nine polar residues (Ser, Thr, Asn, Asp, Gin, Glu and His) have hydrogen bonding groups in their side-chains which can participate in such mimicry and as many as 15% of all these polar residues engage in such conformations. The distributions of dihedral angles of these mimics indicate that only certain combinations of the involved dihedral angles aids the formation of these mimics. The observed examples have been categorised into various classes based on these combinations resulting in well-defined motifs. Asn and Asp residues show a very high capability to perform such backbone secondary structural mimicry. The most highly mimicked backbone structure is of the Cio conformation by the Asx residues. The mimics formed by His, Ser, Thr and Glx residues are also discussed. The role of such conformations in initiating the formation of regular secondary structures during the course of protein folding seems significant. Chapter 5 presents a description of deterministic features of side-chain main-chain hydrogen bonds as observed in protein structures. A total of 19,835 polar residues from the data set of 250 non-homologous and highly resolved protein crystal structures were used to identify side-chain main-chain (SC-MC) hydrogen bonds. The ratio of the total number of polar residues to the number of SC-MC hydrogen bonds is close to 2:1, indicating the ubiquitous nature of such hydrogen bonds. Close to 56% of the SC-MC hydrogen bonds are local involving side-chain acceptor/donor (‘i’) and a main-chain donor/acceptor within the window i-5 to i+5. These short-range hydrogen bonds form well defined conformational motifs characterised by specific combinations of backbone and side-chain torsion angles. Some of the salient features of such hydrogen bonds are as follows, (a) The Ser/Thr residues show the greatest preference in forming intra-helical hydrogen bonds between the atoms Oyi and Oi-4 Such hydrogen bonds form motifs of the form αRαRαRαR(g") and are most commonly observed at the middle of α-helices. (b) These residues also show great preference to form hydrogen bonds between OYi and Oi-3, which are closely related to the previous type and though intra-helical, these hydrogen bonds are more often found at the C-termini of helices than at the middle. The motif represented by αRαRαRaR(g+) is most preferred in these cases, (c) The Ser, Thr and Glu (between the side-chain and main-chain of the same residue), (d) The side-chain acceptor atoms of Asn/Asp and Ser/Thr residues show high preference to form hydrogen bonds with acceptors two residues ahead in the chain, which are characterised by the motifs β(tt’)αR and β(t)αR, respectively. These hydrogen bonded segments referred to as Asx turns, are known to provide stability to type I and type I’ β-turns. (e) Ser/Thr residues often form a combination of SC-MC hydrogen bonds, with the side-chain donor hydrogen bonded to the carbonyl oxygen of its own peptide backbone and the side-chain acceptor hydrogen bonded to an amide hydrogen three residues ahead in the sequence. Such motifs are quite often seen at the beginning of a-helices, which are characterised by the β (g+)αRαR motif. A remarkable majority of all these hydrogen bonds are buried from the protein surface, away from the surrounding solvent. This strongly indicates the possibility of side-chains playing the role of the backbone, in the protein interiors, to satisfy the potential hydrogen bonding sites and maintaining the network of hydrogen bonds which is crucial to the structure of the protein. Chapter 6 provides a detailed characterisation of isolated β-strands. Reason for the formation of β-strands in proteins is often associated with the formation of β -sheets. However β-strands, not part of β-sheets, commonly occur in proteins. This raises questions about the structural role and stability of such isolated β-strands. Using a data set consisting of 250 proteins, 518 isolated β-strands have been identified from 187 proteins. The two important features that distinguish isolated β-strands from p-strands occurring in β-sheets are (i) the high preponderance of prolyl residues to occur in isolated β-strands and (ii) their high solvent exposure. It is shown that the high propensity for proline residues to occur in isolated β-strands is not due to the occurrence of polyproline type segments in the data-set. The propensities of other amino acids to occur in isolated β-strands follows the same trend as those for β-sheet forming β-strands. Isolated β-strands are characterised often by their main-chain amide and carbonyl groups involved in hydrogen bonding with polar side-chains or water. They are often flanked by irregular loop structures indicating that they are part of long of loops. Analysis of the conservation of such strands among families of homologous protein structures indicates that a sizeable fraction of them are highly conserved. It is suggested that though the formation of isolated β-strands are driven by the intrinsic preferences of amino acid residues, they have many characteristics like loop segments but with repetitive (Φ,Ψ) values falling within the β-region of the Ramachandran map. In addition of the material described in the six chapters above, the thesis also contains the details of work carried out on an aspect slightly different from the main theme of the thesis. This pertains to the comparative analysis of the members of a family of cytokine receptors to derive information to model new members of the family. The three dimensional modelling of the leptin receptor has been used as a case study and the details are included as an appendix. Appendix describes the 3-dimensional model of the satiety factor receptor (the leptin receptor) modelled using principles of homology modelling. Recessive mutations in the mouse obese (ob) and diabetes (db) genes result in obesity and diabetes in a syndrome resembling human obesity. Data from parabiosis (cross circulation) experiments suggested that the ob gene coded, and was responsible for the generation of a circulating factor called leptin which regulated energy balance and the db gene encoded the receptor for this factor. While the structure of the leptin has been determined that of its cognate receptor is as yet unknown. The leptin receptor shows low but clear sequence similarity to the members of the interleukin type 6 family of receptors. The structures of the members of this family are characterised by two p-sandwich like domains connected by a short 4-residue helical linker. The 3-dimensional models for the N- and C-terminal domains of the leptin receptor was generated using the corresponding structures of the signal transducing component of gpl30, the erythropoetin receptor and the prolactin receptor. Further using the evidence that the leptin binds to its receptor with a stoichiometry of 1:1, the relative orientation of the two domains was modelled based on the structure of the human growth hormone receptor, which also binds its ligand with similar stoichiometry. The complex of leptin with its receptor was also modelled based on the structure of human growth hormone/receptor complex. The final energy minimised model of the complex elucidates the mode of interaction between the leptin and its receptor.

Leptin is a peripheral adiposity signal and a key hormone in energy balance regulation in mammals, acting as a link between nutritional status and the endocrine reproductive axis. If this is also the role of leptin in fish is not fully understood. This thesis investigates how different components of the leptin system are affected by sexual maturation and seasonal changes in energy balance in male Atlantic salmon (Salmo salar L.) parr under fully fed and feed-restricted conditions. Moreover, the role of sex steroids as being one of the possible mechanisms by which sexual maturation interacts with leptin is explored. The salmon leptin-a genes, lepa1 and lepa2, were expressed mainly in liver and the leptin receptor (lepr) in brain and ubiquitously in peripheral tissues. Seasonal characterization of the lepa genes and lepr during the growth and reproductive season in one-year old males showed that hepatic lepa1 and lepa2 mRNA levels and plasma leptin levels were down-regulated concomitantly with an increase in weight and body fat. Feed restriction up-regulated hepatic leptin, and pituitary lepr expression as well as plasma leptin levels. Correlation between leptin levels and body lipid stores were either lacking or negative. These findings show that leptin and lepr are sensitive to changes in energy balance, but that leptin might not reflect adiposity in juvenile salmon. Hepatic lepa1 and lepa2, and testicular lepr expression increased during mid- to late spermatogenesis in early maturing males. This up-regulation was preceded by rapid gonadal growth and elevated pituitary follicle-stimulating hormone gene expression levels, whereas peak leptin levels coincided with peak pituitary luteinizing hormone expression and the presence of running milt in the testes. The sex steroids testosterone (T), 11-ketotestosterone and 17-β estradiol stimulated lepa1 and lepa2 gene expression in Atlantic salmon hepatocytes in vitro differentially depending on developmental stage. T was also able to stimulate hepatic lepa1 and pituitary lepa1 and lepr gene expression in immature male salmon in vivo. These results suggest that leptin plays a role in male fish reproduction during later stages of the maturational process and that the elevation of leptin expression during spermatogenesis could be caused by androgen stimulation.

Πολλές μελέτες έχουν συνδέσει τη λεπτίνη με την παθοφυσιολογία των νεοπλασιών. Εμείς ερευνήσαμε την άμεση επίδραση της λεπτίνης σε κακοήθη αιματοποιητικό ιστό που περιελάμβανε κύτταρα οξείας μυελογενούς λευχαιμίας (AML) και λευχαιμικές κυτταρικές σειρές. Τα PBMC, τα Τ κύτταρα, τα Β κύτταρα και τα μονοκύτταρα από υγιείς δότες χρησιμοποιήθηκαν ως δείγματα αναφοράς. Προσδιορίσαμε τα πρότυπα έκφρασης των OB-R ισομορφών στα AML κύτταρα και σε λευχαιμικές κυτταρικές σειρές σε σύγκριση με τα κύτταρα αναφοράς με RT-PCR. Η ανασυνδυασμένη λεπτίνη αύξησε την έκφραση του OB-R και την ενδογενή λεπτίνη στους AML βλάστες και σε συγκεκριμένες κυτταρικές σειρές, μα όχι στα κύτταρα αναφοράς. Η CBA ανάλυση προ- και αντι-φλεγμονωδών κυτταροκινών έδειξε ότι η ανανσυνδυασμένη λεπτίνη αυξάνει την έκφραση της IL-6 από τα ΑΜL κύτταρα, πολλών κυτταροκινών από τις λευχαιμικές κυτταρικές σειρές που εξετάστηκαν, και έκκριση της IL-10 από τα PBMC κύτταρα. Ανάλυση western αποκάλυψε ότι η επίδραση της ανασυνδυασμένης λεπτίνης ήταν ανεξάρτητη από τα JAK-2/phospho-JAK-2 πρωτεϊνικά επίπεδα. Προτείνουμε πως το σύστημα λεπτίνης/ υποδοχέα επιδρά άμεσα και ισχυρά στην υποστήριξη ανάπτυξης αιματοποιητικών ανωμαλιών. / Several studies have implicated leptin in the pathophysiology of neoplasias. We investigated the direct effect of leptin on malignant hematopoietic tissue that included primary acute myeloid leukemia (AML) cells και leukemic cell lines. PBMC, T-cells, B-cells and monocytes from healthy subjects served as controls. We defined the patterns of OB-R isoform expression in AML cells and leukemic cell lines in comparison to control cells by RT-PCR. rLeptin upregulated the expression of OB-R and endogenous leptin in AML blasts and certain cell lines but not in control cells. Cytometric Bead Array analysis of pro- and anti-inflammatory cytokines showed that rleptin upregulates IL-6 secretion by AML cells, various cytokines by the leukemic cell lines tested and IL-10 secretion by control PBMC, contributed by monocytes. Western immunoblotting revealed that the effect of rleptin was independent of JAK-2/phospho-JAK-2 protein levels. We propose that the leptin/OB-R system is strongly and directly involved in supporting the growth of hematopoietic malignancies.