Michie Nakayama

PROP1 coexists with SOX2 and induces PIT1-commitment cells

Prophet of PIT1 (PROP1) is a pituitary-specific factor and responsive gene for the combined pituitary hormone deficiency in Ames dwarf mice and human patients. Our immunohistochemical studies demonstrated that PROP1 is consistently expressed in SOX2-expressing stem/progenitor cells in the rat pituitary from embryonic (E) to postnatal periods. At E13.5, all the cells in Rathkes pouch, the primordium of the pituitary, express PROP1. Afterward, PROP1-positive cells localize along the marginal cell layer, a putative stem cell niche in the pituitary, and stratify in the parenchyma of the anterior pituitary. In the embryonic period, PROP1 coexists transiently with PIT1, which is the anterior pituitary-specific factor and is a target of PROP1, but not any hormones. Thus, the present results imply a regulatory role of PROP1 not only in pituitary organogenesis but also in conversion of PIT1-lineage cells.

HSV type 1 thymidine kinase protein accumulation in round spermatids induces male infertility by spermatogenesis disruption and apoptotic loss of germ cells

HSV type 1 thymidine kinase (HSV1-TK)-introduced transgenic rodents and HSV-infected humans were reported to suffer male infertility. The present study aimed to find novel clues to clarify the cause of HSV1-TK-induced male infertility using an HSV1-tk transgenic rat line. Two truncated HSV1-TK proteins, 37 and 39kDa, were produced and accumulated in the round spermatids, and their transcription initiation site was identified for the first time at the 65 base downstream of the translation start point of the full-length 43kDa HSV1-TK. Spermatozoa from those young transgenic rats showed malformed heads, looped tails, and missing cell membrane in heads and tails. Furthermore, age-dependent germ cell loss was observed. TUNEL assay suggested that this germ cell loss is caused by increased apoptotic germ cell death. These results suggest that the expression of HSV1-TK in testes brings about not only abnormal spermiogenesis but also a loss of germ cells due to apoptosis. These findings could provide a novel clue to elucidate the molecular mechanism underlying male infertility in transgenic animals and HSV-infected patients.

Pituitary homeodomain transcription factors HESX1 and PROP1 form a heterodimer on the inverted TAAT motif

The development and differentiation of the pituitary gland progress through spatial and temporal expressions of many transcription factors. Transcription factor HESX1, which begins to be expressed in the Rathkes pouch at the early stage of pituitary development, acts as a transcription repressor. Another transcription factor, PROP1, which is a pituitary-specific factor and important for the determination of the differentiation of pituitary hormone-producing cells, appears later than HESX1 and is assumed to block the action of HESX1. Both factors are members of the homeodomain family, and the amino acid residue at the 50th position of the homeodomain is glutamine (Gln-50). We recently observed that both factors share the same target sequence through different binding profiles. Hence, using random oligonucleotides and an electrophoretic mobility-shift assay, we have examined the DNA-binding preference of HESX1 by a determination of its binding sequence. HESX1 binds as a monomer to a TAATT motif but not to a TAAT motif. In the presence of PROP1, HESX1 develops to bind to an inverted TAAT motif by forming a heterodimer. Thus, the formation of a heterodimer between HESX1 and PROP1 provides a condition in which, in the early pituitary primordium, HESX1 alters its repressive role to an active one by forming a heterodimer with newly appearing PROP1 so that PROP1 finally replaces HESX1 to advance to the middle stage of pituitary development.