AUTHOR=Crespo Diego , Skaftnesmo Kai Ove , Kjærner-Semb Erik , Yilmaz Ozlem , Norberg Birgitta , Olausson Sara , Vogelsang Petra , Bogerd Jan , Kleppe Lene , Edvardsen Rolf B. , Andersson Eva , Wargelius Anna , Hansen Tom J. , Fjelldal Per Gunnar , Schulz Rüdiger W. 

TITLE=Pituitary Gonadotropin Gene Expression During Induced Onset of Postsmolt Maturation in Male Atlantic Salmon: In Vivo and Tissue Culture Studies

JOURNAL=Frontiers in Endocrinology

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2022.826920

DOI=10.3389/fendo.2022.826920

ISSN=1664-2392

ABSTRACT=<p>Precocious male maturation causes reduced welfare and increased production costs in Atlantic salmon (<italic>Salmo salar</italic>) aquaculture. The pituitary produces and releases follicle-stimulating hormone (Fsh), the gonadotropin triggering puberty in male salmonids. However, little is known about how Fsh production is regulated in Atlantic salmon. We examined, <italic>in vivo</italic> and <italic>ex vivo</italic>, transcriptional changes of gonadotropin-related genes accompanying the initial steps of testis maturation, in pituitaries of males exposed to photoperiod and temperature conditions promoting maturation (constant light and 16°C). Pituitary <italic>fshb</italic>, <italic>lhb</italic> and <italic>gnrhr2bba</italic> transcripts increased <italic>in vivo</italic> in maturing males (gonado-somatic index &gt; 0.1%). RNA sequencing (RNAseq) analysis using pituitaries from genetically similar males carrying the same genetic predisposition to mature, but differing by responding or not responding to stimulatory environmental conditions, revealed 144 differentially expressed genes, ~2/3rds being up-regulated in responders, including <italic>fshb</italic> and other pituitary hormones, steroid-related and other puberty-associated transcripts. Functional enrichment analyses confirmed gene involvement in hormone/steroid production and gonad development. In <italic>ex vivo</italic> studies, whole pituitaries were exposed to a selection of hormones and growth factors. Gonadotropin-releasing hormone (Gnrh), 17β-estradiol (E<sub>2</sub>) and 11-ketotestosterone (11-KT) up-regulated <italic>gnrhr2bba</italic> and <italic>lhb</italic>, while <italic>fshb</italic> was up-regulated by Gnrh but down-regulated by 11-KT in pituitaries from immature males. Also pituitaries from maturing males responded to Gnrh and sex steroids by increased <italic>gnrhr2bba</italic> and <italic>lhb</italic> transcript levels, but <italic>fshb</italic> expression remained unchanged. Growth factors (inhibin A, activin A and insulin-like growth factor 1) did not change <italic>gnrhr2bba</italic>, <italic>lhb</italic> or <italic>fshb</italic> transcript levels in pituitaries either from immature or maturing males. Additional pituitary <italic>ex vivo</italic> studies on candidates identified by RNAseq showed that these transcripts were preferentially regulated by Gnrh and sex steroids, but not by growth factors, and that Gnrh/sex steroids were less effective when incubating pituitaries from maturing males. Our results suggest that a yet to be characterized mechanism up-regulating <italic>fshb</italic> expression in the salmon pituitary is activated in response to stimulatory environmental conditions prior to morphological signs of testis maturation, and that the transcriptional program associated with this mechanism becomes unresponsive or less responsive to most stimulators <italic>ex vivo</italic> once males had entered pubertal developmental <italic>in vivo</italic>.</p>