Search Results

ABSTRACT

Steroid 15α-hydroxylase (P450_15α) is a female-specific enzyme in the livers of many inbred mice including DBA/2J. Run-on assays using liver nuclei from GH-deficient Little mice indicated that the P450_15α gene is transcriptionally repressed by GH in male mice. BALB/cJ is a variant strain in which the gene is expressed in the males as well as in the females. Genetic crosses between DBA/2J and BALB/cJ indicated that expression of the P450_15α gene in BALB/cJ males is inherited as a recessive trait and is regulated by a single locus. The parental origin of the P450_15α genes, determined using restriction site polymorphism in the exons of the P450_15α genes, divided the F2 males expressing the P450_15α gene into three phenotypes at a ratio of 1:1:2, the individuals expressing the gene from only BALB/cJ or DBA/2J and the individuals expressing the genes from both parents respectively. The results indicate that the repression of the P450_15α gene in male mice is regulated by a trans-acting regulatory locus between the DBA/2J and BALB/cJ pairs. Because hypophysectomy derepressed the P450_15α gene in Fl males and GH repressed the gene in hypophysectomized Fl males, the hormone appears to regulate gene repression through a trans-acting locus, named GH-dependent repression, GDR.

ABSTRACT

Gonadotrophin-releasing hormone (GnRH) is considered to have an important role in the control of reproduction in salmonid fish, although we do not have any direct evidence. To clarify this problem by molecular techniques, we first determined the nucleotide sequence of the mRNA encoding the precursor of salmon-type GnRH (sGnRH) from the masu salmon, Oncorhynchus masou.

The masu salmon sGnRH precursor was composed of a signal peptide, sGnRH and a GnRH-associated peptide (GAP) which was connected to sGnRH by a Gly-Lys-Arg sequence. The amino acid sequence of sGnRH and Gly-Lys-Arg were highly conserved when compared with the corresponding regions of African cichlid sGnRH and mammalian GnRH precursors. However, the GAP region was markedly divergent, with a 66% amino acid similarity to African cichlid GAP and an 8·3–15% similarity to mammalian GAPs. Northern blot analysis indicated the presence of a single mRNA species of about 600 bases in the olfactory bulb and telencephalon and in the diencephalon. The signal was more intense in the former regions.

An in-situ hybridization study further revealed that sGnRH neurones were distributed in the olfactory nerve, the ventral part of the olfactory bulb, the ventral part of the telencephalon, the lateral preoptic area and the preoptic nucleus. The sGnRH neurones were thus longitudinally scattered between the olfactory nerve and the lateral preoptic area in the rostroventral part of brain. The intensity of the hybridization signals and the size of hybridization-positive somata were much greater in the olfactory nerve and the rostral olfactory bulb than in the other regions. Preoptic sGnRH neurones were scarcely detected in immature masu salmon, whereas they were more frequently observed in maturing animals. It is possible that the olfactory and the preoptic sGnRH neurones have different physiological roles in salmonid fish.

ABSTRACT

Salmon gonadotropin-releasing hormone (sGnRH) is considered to have an important role in the control of reproduction in salmonid fish. As a basis for understanding the physiological functioning of sGnRH at the molecular level, we characterized the nucleotide sequences of two types of cDNAs encoding the precursors of sGnRH in sockeye salmon (ss), Oncorhynchus nerka, by a cloning strategy based on reverse transcription-PCR. The two types of cDNAs are referred to as ss-pro-sGnRH-I and -II, and consisted of 435 and 481 bases respectively. Both precursors are predicted to contain a signal peptide, the hormone and a GnRH-associated peptide that is attached to the hormone via a Gly-Lys-Arg sequence. The presence of two types of mRNAs hybridizing with either cDNA was confirmed by Northern blot analysis of brain RNA from sockeye salmon, masu salmon, O. masou, and rainbow trout, O. mykiss. The ss-pro-sGnRH-I cDNA had 97·2% and 82·8% overall identity with sGnRH cDNA from masu salmon and putative sGnRH cDNA deduced from the gene of the Atlantic salmon, Salmo salar respectively, whereas the ss-pro-sGnRH-II cDNA had 80·0% and 91·2% overall identity with the former and the latter respectively. The nucleotide sequences of ss-sGnRH-I and -II cDNAs showed less similarity (79·3%). These results indicated that each salmonid species possesses two differing sGnRH genes. The results of Southern blot analysis using genomic DNA extracted from individuals support this evidence in sockeye salmon, masu salmon and rainbow trout.