Adult-type myogenesis of the frog Xenopus laevis specifically suppressed by notochord cells but promoted by spinal cord cells in vitro

Abstract

Larval-to-adult myogenic conversion occurs in the dorsal muscle but not in the tail muscle during Xenopus laevis metamorphosis. To know the mechanism for tail-specific suppression of adult myogenesis, response character was compared between adult myogenic cells (Ad-cells) and larval tail myogenic cells (La-cells) to a Sonic hedgehog (Shh) inhibitor, notochord (Nc) cells, and spinal cord (SC) cells in vitro. Cyclopamine, an Shh inhibitor, suppressed the differentiation of cultured Ad (but not La) cells, suggesting the significance of Shh signaling in promoting adult myogenesis. To test the possibility that Shh-producing axial elements (notochord and spinal cord) regulate adult myogenesis, Ad-cells or La-cells were co-cultured with Nc or SC cells. The results showed that differentiation of Ad-cells were strongly inhibited by Nc cells but promoted by SC cells. If Ad-cells were "separately" co-cultured with Nc cells without direct cell-cell interactions, adult differentiation was not inhibited but rather promoted, suggesting that Nc cells have two roles, one is a short-range suppression and another is a long-range promotion for adult myogenesis. Immunohistochemical analysis showed both notochord and spinal cord express the N-terminal Shh fragment throughout metamorphosis. The "spinal cord-promotion" and long-range effect by Nc cells on adult myogenesis is thus involved in Shh signaling, while the signaling concerning the short-range "Nc suppression" will be determined by future studies. Interestingly, these effects, "Nc suppression" and "SC promotion" were not observed for La-cells. Situation where the spinal cord/notochord cross-sectional ratio is quite larger in tadpole trunk than in the tail seems to contribute to trunk-specific promotion and tail-specific suppression of adult myogenesis during Xenopus metamorphosis.

Cell Interaction During Larval-To-Adult Muscle Remodeling in the Frog, Xenopus laevis

“…On the other hand, from the fact that tail muscle does not convert to adult type even though they contain AMS cells, it is conceivable that during metamorphosis the growth and differentiation of AMS cells is specifically activated in the trunk DM portions but suppressed in the tail DM portions. As to this spatial control, Yamane et al (2011) suggested the possibility that the trunk-specific adult myogenesis is regulated by two cell-interactive mechanisms: a promotion by spinal cord (SP) cells and a suppression by notochord (Nc) cells (see section 6).…”

“…Then, is there any regulation from cells (or tissues) other than myoblasts (or muscles) for the trunkspecific adult myogenesis? From this point of view, Yamane et al (2011) reported the importance of interaction between myogenic cells and non-myogenic cells (i.e. notochord and spinal cord cells) for the trunk specific adult muscle differentiation during X. laevis metamorphosis.…”

“…Because, both two tissues (notochord and spinal cord) are known to regulate early axial myogenesis in vitro (Munsterberg and Lassar, 1995;Stern and Hauschka, 1995) and in vivo (Blagden et al, 1997). At first, Yamane et al (2011) examined the difference in cross-sectional areas between trunk and tail regions using histological sections of Xenopus tadpoles and noticed that the cross-sectional ratio of spinal cord to notochord area (SC/Nc ratio) is nearly 1:1 in the trunk but about 1:15 in the tail (Fig.11). This big difference is due to the situation that notochord area in the tail region is about 1.5 times larger than that in the trunk region and, on the contrary, spinal cord area in the tail is extremely smaller (around 1/10) than that in the trunk part.…”

“…During this period, most of preexisted larval body organs, i.e., 'larval-specific organs' such as tail and gills degenerated (Nishikawa and Hayashi, 1995) and new 'adult-specific organs' such as fore-and hindlimbs formed (Brown et al, 2005). This cell replacement is thought to be essential for amphibian metamorphosis and deeply involved in various fundamental biological processes such as cell growth, programmed cell death, differentiation, morphogenesis and cell-cell and cell-environment interactions (Nishikawa, 1997;Shibota et.al., 2000;Shimizu-Nishikawa et al, 2002;Yamane et al, 2011). How are these remodelingevents regulated by metamorphic hormones, triiodothyronine (T3) and thyroxine (T4)?…”

“…4) Regulatory mechanism for muscle cell fates, death or adult differentiation, by interaction between two types of myoblasts (larval and adult types) (Shimizu-Nishikawa et al, 2002). 5) Regulatory mechanism of adult type myogenesis by interaction between myoblasts and notochord (notochord-suppression) and spinal cord cells (spinal cord-promotion) (Yamane et al, 2011). The rationale for studying a cell-to-cell interaction during larval-to-adult muscle conversion in the amphibian is that such an analysis will provide important insights into the processes occurring during the differentiation (and/or adult organogenesis) of other stem cells such as mammalian embryonic and tissue (adult) stem cells.…”