Xenopus endo B is a keratin preferentially expressed in the embryonic notochord.

Abstract

Screening of a cDNA library from neurula stage Xenopus laevis for notochord-specific sequences led to the isolation of a cDNA clone, XK endo B {Xenopus keratin endo B), which encodes a nonepidermal type I keratin. In situ and Northern blot hybridizations indicate that expression of XK endo B RNA is concentrated in the notochord, whereas expression in the endoderm is 5-10 times lower. XK endo B mRNA is present in the oocyte and increases from late gastrula. Accumulation peaks by late neurula and is greatly reduced by the tadpole stage; in the adult, a low level of XK endo B RNA is present in the liver. XK endo B shows sequence homology to mouse endo B; genomic Southern blots show that XK endo B is the most similar sequence to mouse endo B in the Xenopus genome, and vice versa, indicating that XK endo B and mouse endo B are homologs. The use of endo B as a marker and the germ layer derivation of the notochord are discussed in light of these results.

XK endo B is preferentially expressed in several induced embryonic tissues during the development of Xenopus laevis

“…Previously, in situ and Northern-blot hybridization indicated that expression of XK endo B was concentrated in the notochord and the endoderm ; while the concentration is highest in the notochord, the large volume of endoderm contains as much XK endo B mRNA as the notochord [31]. Here, we describe the spatial distribution of XK endo B Drotein as determined by immunocytochemistry.…”

“…These antibodies stain endoderm and notochord of stage-28 (tailbud) embryos ( Fig. 2B, C), the two major structures known to express XK endo B RNA [31]; staining in the notochord persists through tadpole stage ( Fig. 2D).…”

“…This gene is the frog homolog of mouse keratin endo B [49] and human keratin 18 [35]. Studies in mammalian systems have emphasized the endodermal expression of this keratin during embryogenesis [14,40,41,551, but it is clear in the frog that the highest concentration of mRNA occurs in the notochord, even though endodermal expression is also important [31]. In the present paper we describe the distribution of XK endo B during development.…”

“…Similar studies have been carried out on the muscle-specific actin gene by other laboratories [15,34,611. In addition, Vgl, later identified as a member of the TGF-P gene superfamily [60], was isolated on the basis of the localization of its RNA in the vegetal pole of the egg [44], thus providing a tool to study the process of RNA localization during oogenesis [33, 631. XK endo B, a type I keratin, was originally identified by its preferential expression in the embryonic notochord, which differentiates early during amphibian mesodermal development [31]. This gene is the frog homolog of mouse keratin endo B [49] and human keratin 18 [35].…”

Keratin 8 and 18 expression in mesenchymal progenitor cells of regenerating limbs is associated with cell proliferation and differentiation

“…The sequenced portion of the NvK18 rod domain, which includes most helix 1B (82 of 101 amino acids) and 11 amino acids of the linker joining helix 1B and helix 2, contains the motif DNARPAADDFR, which is highly conserved in type I keratins. Comparison analysis shows that this region of NvK18 shares 70% and 69% amino acid identity with Xenopus and mouse K18, respectively (Singer et al, 1986;LaFlamme et al, 1988), 47% identity with human keratin 16 (HK16; Rosenberg et al, 1988), and 57% identity with human keratin 17 (HK17; RayChaudhury et al, 1986). The carboxyl tail of NvK18 has high amino acid identity with Xenopus (72%).…”