Universität zu Köln

Bagivalu Lakshminarasimha, Amrutha ORCID: 0000-0002-5742-4360 (2023). Endocrine Control of Growth and Reproduction in Zebrafish. PhD thesis, Universität zu Köln.

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Abstract

Energy is the basic currency required for all living organisms. In order to ensure homeostatic supply of energy, various neuro-endocrine systems have evolved in vertebrates, that control food intake, metabolism, and energy expenditure. Even minor discrepancies in the sensitive balance of these systems may result in severe pathologies. Obesity and metabolic disorders are often caused due to disturbances in the homeostasis of energy and is rapidly rising across the globe. Although diet and lifestyle certainly are one of the major factors influencing development of obesity, genetic and epigenetic factors also largely contribute. Monogenic mutations in leptin-melanocortin system have been associated with severe disruptions in the energy homeostasis pathway. In order to mitigate these diseases, better understanding of their basic biology and etiology is crucial. Researches in last decade have relied on diverse animal models to unravel the complex interaction of environment and genetic factors. In this study, we used zebrafish as a model to study the metabolic endpoints caused as a result of longterm obesogenic diet, as well as early lifetime caloric restriction. Our results in publication 1 showed that long-term feeding in zebrafish lead to obesity hallmarks and metabolic phenotypes similar in mammals. While early caloric restriction did not result in exacerbated compensation of growth and other metabolic syndromes in adulthood, we observed phenotypes of catch-up growth in fish that were introduced to obesogenic diets, which caught up with growth and metabolic endpoints comparable to diet-induced obese fish. The Leptin system plays an important role in maintenance of energy homeostasis. In fish, the role of leptin is controversial and is not considered as an adipostat. To understand the functional role of leptin in obesity and in fat regulation, we used three leptin receptor (lepr) and one leptin a (lepa) loss of function (LOF) lines. To validate the LOF alleles, we measured socs3a transcription levels after administration of recombinant mouse leptin and found impaired response in all the LOF alleles. Our results showed that none of the alleles led to obesity or altered growth phenotypes. Given the absence of morbid obesity or drastic body growth phenotypes in lepr mutant strains, we were curious to investigate the role of leptin in regulation of reproduction using leprsa1508 mutant zebrafish line in the publication 3. We found that mutations in the lepr gene caused dysregulation of gonadotropins in the pituitary. Despite an impaired HPG axis, these mutants did not show subfertility. However, they exhibited a delay in ovarian maturation and/or an increased rate of follicular atresia compared to their wild type siblings. In support of this, we found downregulation of candidate genes involved in the process of maturation and upregulation of genes involved in follicular atresia in the LOF line. Next, we elucidated the peripheral or direct role of leptin on the ovarian cells using an in vitro germinal vesicle breakdown (GVBD) assay. In cultured oocytes, leptin promoted GVBD and attenuated the rate of oocyte degradation. Additionally, we found lepr LOF abates the effect of maturation inducing hormone, 17α-20β dihydroxy-4 pregnen-3-one (DHP), leading to reduced GVBD rates in ovarian cultures of mutants in comparison to wild type. In conclusion, we found that leptin has a central as well as peripheral role in the regulation of reproduction in zebrafish. On the long run, these findings will provide insights into the role of leptin in energy mobilisation in fish during reproduction. Alongside, we also studied the central regulation of melanocortin system on body growth and reproduction in zebrafish. Previous research has shown that Agrp neurons are hypophysiotropic and regulate the expression of multiple endocrine axes. Therefore, in our study, we used the Agrp1 LOF model to investigate somatic growth and reproduction in adult zebrafish. We found, despite significant reduction in genes of growth and reproductive axes, adult zebrafish exhibited normal body growth phenotype and normal reproduction. Further we tested for compensation by candidate genes of neuroendocrine axis but found no changes. We conclude that some unknown central and peripheral compensatory mechanisms might allow for normal growth and reproductive function in Agrp1 LOF adult zebrafish, in spite of reduced levels of growth hormone (gh), follicle stimulating hormone (fsh) and luteinising hormone (lh). Together these studies provide a systematic and functional analysis of the regulation of growth and reproduction across neuroendocrine axis in zebrafish.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Bagivalu Lakshminarasimha, Amrutha abagival@uni-koeln.de orcid.org/0000-0002-5742-4360 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-706403
Date:	2023
Place of Publication:	Cologne University Publication Server KUPS
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Biology > Institut für Entwicklungsbiologie
Subjects:	Natural sciences and mathematics
Uncontrolled Keywords:	Keywords Language Zebrafish English Leptin English Reproduction English Oocyte maturation English energy homeostasis English
Date of oral exam:	12 June 2023
Referee:	Name Academic Title Bagivalu Lakshminarasimha, Amrutha UNSPECIFIED
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/70640