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Expression of Rat Thyrotropin-Releasing Hormone (TRH) Gene in TRH-Producing Tissues of Transgenic Mice Requires Sequences Located in Exon 1¹

Geriatric Research, Education, and Clinical Center, Veterans Affairs Medical Center; and Departments of Medicine (W.B., M.A.T., P.J.G., B.A.R.) and Neurology (B.A.R.), University of Miami School of Medicine, Miami, Florida 33125

Address all correspondence and requests for reprints to: Wayne Balkan, Geriatric Research, Education, and Clinical Center (11 GRC), Miami Veterans Affairs Medical Center, 1201 Northwest 16th Street, Miami, Florida 33125. E-mail: wbalkan{at}mednet.med.miami.edu

TRH, an amidated tripeptide secreted by certain hypothalamic neurons, is a principal regulator of TSH secretion and thyroid hormone release. TRH is also produced by other neurons in the central nervous system, where it appears to function as a neuromodulator or neurotransmitter, and by certain endocrine cells, where it may act as an autocrine or paracrine factor. The genomic organization of the rat TRH (rTRH) gene is well understood; however, the domains of the rTRH gene that regulate expression are less well characterized. We observed that the region between -47 and +6 of the rTRH gene (relative to the transcription start site at +1) was active in CA-77 cells, a medullary thyroid carcinoma cell line model of TRH production, but was not active in transgenic mice. Inclusion of most of exon 1 (84 out of 103 bp; -47 to +84) increased promoter activity in CA-77 cells and was active in transgenic mice, principally in tissues that normally express the TRH gene. Further lengthening of the 5' end to -243, -547, or -776 retained this expression in TRH-producing tissues in transgenic mice, while further increasing activity in CA-77 cells. These results suggest that cis element(s) located within exon 1 are necessary for the expression of the rTRH gene in vivo.

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