Impaired Development of Somatotropes, Lactotropes and Thyrotropes in Growth-Retarded (grt) Mice

Delayed Postnatal Growth and Anterior Pituitary Development in Growth-Retarded (grt) Female Mice

Growth-retarded (grt) mice display primary congenital hypothyroidism due to the hyporesponsiveness of their thyroid glands to thyroid-stimulating hormone (TSH). We examined somatic growth, anterior pituitary development, and hormonal profiles in female grt mice and normal ones. Although growth in grt females was suppressed 2 weeks after birth, the measured growth parameters and organ weights gradually increased and finally reached close to the normal levels. Grt mice exhibited delayed eye and vaginal openings and remained in a state of persistent diestrus thereafter, plasma estrogen levels being lower than those in normal mice. Grt mice that received normal-donor thyroids showed accelerated growth and their body weights increased up to the sham-normal levels, indicating the importance of early thyroid hormone supplementation. In the anterior pituitary, there were fewer growth hormone (GH) and prolactin (PRL) cells in grt mice than in normal mice as examined at 12 weeks after birth, but the numbers of these cells did not differ from those in normal mice after 24 weeks. Grt mice had more TSH cells than normal mice until 48 weeks. Plasma GH levels in grt mice were lower than those in normal mice at 2 weeks, but did not differ substantially after 5 weeks. Compared with normal mice, grt mice had significantly lower plasma PRL and thyroxine levels, but notably higher TSH levels until 48 weeks. These findings indicate that thyroid hormone deficiency in grt mice causes delayed development and growth, and inappropriate development of GH, PRL and TSH cells, followed by the abnormal secretion of hormones by these pituitary cells.

Gene Expression in Mouse Thyrotrope Adenoma: Transcription Elongation Factor Stimulates Proliferation

Thyrotrope hyperplasia and hypertrophy are common responses to primary hypothyroidism. To understand the genetic regulation of these processes, we studied gene expression changes in the pituitaries of Cga(-/-) mice, which are deficient in the common α-subunit of TSH, LH, and FSH. These mice have thyrotrope hypertrophy and hyperplasia and develop thyrotrope adenoma. We report that cell proliferation is increased, but the expression of most stem cell markers is unchanged. The α-subunit is required for secretion of the glycoprotein hormone β-subunits, and mutants exhibit elevated expression of many genes involved in the unfolded protein response, consistent with dilation and stress of the endoplasmic reticulum. Mutants have elevated expression of transcription factors that are important in thyrotrope function, such as Gata2 and Islet 1, and those that stimulate proliferation, including Nupr1, E2f1, and Etv5. We characterized the expression and function of a novel, overexpressed gene, transcription elongation factor A (SII)-like 5 (Tceal5). Stable expression of Tceal5 in a pituitary progenitor cell line is sufficient to increase cell proliferation. Thus, Tceal5 may act as a proto-oncogene. This study provides a rich resource for comparing pituitary transcriptomes and an analysis of gene expression networks.

Relationship between growth retardation and impaired glucose tolerance in hypothyroidal growth-retarded (grt) mice

Growth-retarded (grt) mice exhibit congenital hypothyroidism and a characteristic growth pause followed by delayed onset of pubertal growth. This pattern of growth has never been reported in any other animal model exhibiting hypothyroidism; therefore, the growth retardation observed in grt mice is unlikely to be explained completely by the low plasma thyroid hormone levels. As growth is closely related to nutrient metabolism, we investigated the relationship between the appearance of growth retardation and glucose utilization, which is the main component of nutrient metabolism, in the peripubertal stage of grt mice. The relative weights of the organs involved in nutrient digestion and absorption were abnormal in grt mice. The intraperitoneal glucose tolerance test (IGTT) showed impaired glucose tolerance in grt mice. Moreover, this symptom appeared in parallel with the progression of growth retardation in grt mice. The impaired blood glucose levels on the IGTT in grt mice were considered to be attributable to decreased plasma insulin levels rather than to impaired insulin sensitivity. The pattern of anti-insulin antibody staining on sections of pancreatic islets from grt mice was almost the same as that in the corresponding sections from normal mice. Insulin treatment accelerated the growth of peripubertal grt mice. These findings suggest that the appearance of growth retardation in grt mice might be partially attributable to a reduction in glucose metabolism and impairment of insulin secretion during the early period of growth.