Thyroid hormone-dependent and independent processes of red blood cell transition from larval to adult type during metamorphosis in Xenopus laevis.

Amphibian metamorphosis results in drastic whole-body remodeling. Thyroid hormone (TH) drives most of these metamorphic changes. A prominent event during this remodeling is the red blood cell (RBC) transition from larval to adult forms, which exclusively contain larval and adult hemoglobin, respectively. However, the role of TH in RBC transition remains unclear. Here we reconfirmed that RBC transition of Xenopus laevis is completed much later than morphological metamorphosis. Further, larval and adult RBCs/erythroblasts proliferated both in the erythropoietic liver and in circulation during metamorphic climax. RBC transition was also confirmed in Rana ornativentris, but in contrast to X. laevis, adult RBC-specific proliferation was observed from the early climax stages. We also revealed in either species that RBC transition occurs in the liver prior to circulating RBCs. Moreover, anemia induction using phenylhydrazine during the prometamorphosis of X. laevis caused precocious RBC transition even when TH synthesis was blocked, resulting in metamorphosis-arrested larvae in which most of RBCs were of adult type. These results indicate that a decline in larval RBCs facilitates RBC transition during metamorphosis in a TH-independent manner. Further, combined administration of phenylhydrazine and TH induced precocious appearance of adult RBCs in early prometamorphic X. laevis tadpoles, whereas individual treatment with phenylhydrazine or TH did not cause precocious RBC transition; this suggests that TH is required to initiate RBC transition by promoting the differentiation of adult erythroblasts during early prometamorphosis in X. laevis. These results show that TH-dependent and independent processes are present in RBC transition in X. laevis.

The gastrin-releasing peptide/bombesin system revisited by a reverse-evolutionary study considering Xenopus.

Bombesin is a putative antibacterial peptide isolated from the skin of the frog, Bombina bombina. Two related (bombesin-like) peptides, gastrin-releasing peptide (GRP) and neuromedin B (NMB) have been found in mammals. The history of GRP/bombesin discovery has caused little attention to be paid to the evolutionary relationship of GRP/bombesin and their receptors in vertebrates. We have classified the peptides and their receptors from the phylogenetic viewpoint using a newly established genetic database and bioinformatics. Here we show, by using a clawed frog (Xenopus tropicalis), that GRP is not a mammalian counterpart of bombesin and also that, whereas the GRP system is widely conserved among vertebrates, the NMB/bombesin system has diversified in certain lineages, in particular in frog species. To understand the derivation of GRP system in the ancestor of mammals, we have focused on the GRP system in Xenopus. Gene expression analyses combined with immunohistochemistry and Western blotting experiments demonstrated that GRP peptides and their receptors are distributed in the brain and stomach of Xenopus. We conclude that GRP peptides and their receptors have evolved from ancestral (GRP-like peptide) homologues to play multiple roles in both the gut and the brain as one of the 'gut-brain peptide' systems.

Identification of the sexually dimorphic gastrin-releasing peptide system in the lumbosacral spinal cord that controls male reproductive function in the mouse and Asian house musk shrew (Suncus murinus).

Several regions of the brain and spinal cord control male reproductive function. We previously demonstrated that the gastrin-releasing peptide (GRP) system, located in the lumbosacral spinal cord of rats, controls spinal centers to promote penile reflexes during male copulatory behavior. However, little information exists on the male-specific spinal GRP system in animals other than rats. The objective of this study was to examine the functional generality of the spinal GRP system in mammals using the Asian house musk shrew (Suncus murinus; suncus named as the laboratory strain), a specialized placental mammal model. Mice are also used for a representative model of small laboratory animals. We first isolated complementary DNA encoding GRP in suncus. Phylogenetic analysis revealed that suncus preproGRP was clustered to an independent branch. Reverse transcription-PCR showed that GRP and its receptor mRNAs were both expressed in the lumbar spinal cord of suncus and mice. Immunohistochemistry for GRP demonstrated that the sexually dimorphic GRP system and male-specific expression/distribution patterns of GRP in the lumbosacral spinal cord in suncus are similar to those of mice. In suncus, we further found that most GRP-expressing neurons in males also express androgen receptors, suggesting that this male-dominant system in suncus is also androgen-dependent. Taken together, these results indicate that the sexually dimorphic spinal GRP system exists not only in mice but also in suncus, suggesting that this system is a conserved property in mammals. J. Comp. Neurol. 525:1586-1598, 2017. © 2016 Wiley Periodicals, Inc.