Pitx2 deletion in pituitary gonadotropes is compatible with gonadal development, puberty, and fertility.

This report introduces a gonadotrope-specific cre transgenic mouse capable of ablating floxed genes in mature pituitary gonadotropes. Initial analysis of this transgenic line, Tg(Lhb-cre)1Sac, reveals that expression is limited to the pituitary cells that produce luteinizing hormone beta, beginning appropriately at e17.5. Cre activity is detectable by a reporter gene in nearly every LHbeta-producing cell, but the remaining hormone-producing cell types and other organs exhibit little to no activity. We used the Tg(Lhb-cre)1Sac strain to assess the role Pitx2 in gonadotrope function. The gonadotrope-specific Pitx2 knockout mice exhibit normal expression of LHbeta, sexual maturation, and fertility, suggesting that Pitx2 is not required for gonadotrope maintenance or for regulated production of gonadotropins.

Pituitary-specific Gata2 knockout: effects on gonadotrope and thyrotrope function.

GATA2 is expressed in the pituitary during development and in adult gonadotropes and thyrotropes. It is proposed to be important for gonadotrope and thyrotrope cell fate choice and for TSH production. To test this idea, we produced a pituitary-specific knockout of Gata2, designed so that the DNA-binding zinc-finger region is deleted in the presence of a pituitary-specific recombinase transgene. These mice have reduced secretion of gonadotropins basally and in response to castration challenge, although the mice are fertile. GATA2 deficiency also compromises thyrotrope function. Mutants have fewer thyrotrope cells at birth, male Gata2-deficient mice exhibit growth delay from 3-9 wk of age, and adult mutants produce less TSH in response to severe hypothyroidism after radiothyroidectomy. Therefore, Gata2 appears to be dispensable for gonadotrope and thyrotrope cell fate and maintenance, but important for optimal gonadotrope and thyrotrope function. Gata2-deficient mice exhibit elevated levels of Gata3 transcripts in the pituitary gland, suggesting that GATA3 can compensate for GATA2.

PITX genes are required for cell survival and Lhx3 activation.

The PITX family of transcription factors regulate the development of many organs. Pitx1 mutants have a mild pituitary phenotype, but Pitx2 is necessary for the development of Rathke's pouch, expression of essential transcription factors in gonadotropes, and expansion of the Pit1 lineage. We report that lack of Pitx2 causes the pouch to undergo excessive cell death, resulting in severe pituitary hypoplasia. Transgenic overexpression of PITX2 in the pituitary can increase the gonadotrope population, suggesting that the absolute concentration of PITX2 is important for normal pituitary cell lineage expansion. We show that PITX1 and PITX2 proteins are present in similar expression patterns throughout pituitary development and in the mature pituitary. Both transcription factors are preferentially expressed in adult gonadotropes and thyrotropes, suggesting the possibility of overlap in maintenance of adult pituitary functions within these cell types. Double knockouts of Pitx1 and Pitx2 exhibit severe pituitary hypoplasia and fail to express the transcription factor LHX3. This indicates that these PITX genes are upstream of Lhx3 and have compensatory roles during development. Thus, the combined dosage of these PITX family members is vital for pituitary development, and their persistent coexpression in the adult pituitary suggests a continued role in maintenance of pituitary function.