Metamorphosis‐dependent transcriptional regulation of xak‐c, a novel Xenopus type I keratin gene

Abstract

Anuran larvae transform their epidermis to the adult counterpart during metamorphosis. The major event of this process is the proliferation of larval epidermal basal cells and their differentiation into adult ones. The present study isolated novel type I keratin cDNA dubbed xak-c (Xenopus adult keratin-c) that was exclusively expressed in adult epidermal basal cells. The gene started its expression in the larval epidermis at the onset of metamorphosis. Thyroid hormone (TH) induced the precocious expression of the gene in the epidermis of premetamorphic tadpoles. To study the transcriptional regulation of this gene in relation to epidermal metamorphosis, a 2.8 kb 5 -flanking region of xak-c was cloned and its promoter activity was investigated. Gene constructs were made so as to contain the xak-c promoter region and gene of EGFP or luciferase as a reporter gene and were transfected into various types of cells, which revealed that the 5 -flanking region had an epidermal cellspecific transcriptional activity in both anurans and mammals. Larval skin tissues of Xenopus were transfected with the constructs and cultured in the presence and absence of TH, which showed that the promoter region is responsive to TH, although the region did not contain the consensus TH response element-like sequence. In sharp contrast, the promoter region did not respond to TH in the adult skin, clearly indicating that the cloned region contains specific sequences that respond to metamorphosis-dependent transcription factor(s).

The keratin-related Ouroboros proteins function as immune antigens mediating tail regression in Xenopus metamorphosis

“…Specific primers were used for ouro1 (sense: 5Ј-TTT-GAT-AAC-ACG-CCC-AAA-CTG-G-3Ј, antisense 5Ј-CAT-CTT-CAC-TGC-CAA-GAG-GTC-3Ј) and ouro2 (sense: 5Ј-GGC-ATT-TTC-TTT-GGG-GCG-TTC-TTT-GAC-TGC-3Ј, antisense: 5Ј-GCT-CTC-AGT-TTG-TTT-AAT-GCA-GTG-GTG-AGG-3Ј). PCR was performed for 22-32 cycles as described for xlk, xak (22), and rpl8 (23). The amplified products were run on 2% agarose gels and visualized by ethidium bromide.…”

Tadpole skin dies autonomously in response to thyroid hormone at metamorphosis

“…As thyroid hormone (TH) concentration rises in the tadpole during metamorphic climax the outer two cell layers of the epidermis are thought to die by apoptosis (3)(4)(5)(6). The basal cells of the tadpole epidermis are progenitors for the adult frog epidermis (7)(8)(9). At metamorphosis the subepithelial fibroblasts secrete proteolytic enzymes such as collagenase-3 and invade the collagen lamella, the dermis thickens, and adult skin glands form (2,10).…”

Characterization of a novel type I keratin gene and generation of transgenic lines with fluorescent reporter genes driven by its promoter/enhancer in Xenopus laevis

“…In the amphibian Xenopus laevis, various keratin genes have been identified and characterized according to their unique expression changes during ontogenesis (Miyatani et al, 1986;Mathisen and Miller, 1987;Watanabe et al, 2001Watanabe et al, , 2002. Keratin gene switching from larval-type to adult counterparts occurs in the tadpole body epidermis, which is remodeled by thyroid hormone (TH) during spontaneous metamorphosis (Suzuki et al, 2009;Mukhi et al, 2010).…”

“…Several keratin genes are strongly induced in the regenerating tail, while another keratin gene is inactivated in the limb blastema (Tazaki et al, 2005;Tazawa et al, 2006). Although there have been some reports regarding the transcriptional regulation of X. laevis epidermal keratin genes (Jonas et al, 1989;Warshawsky and Miller, 1995;Watanabe et al, 2002;Suzuki et al, 2009), the mechanisms of keratin gene regulation in amphibians remain largely unknown.…”