The development of adult intestinal stem cells: Insights from studies on thyroid hormone-dependent anuran metamorphosis

Abstract

Vertebrates organ development often takes place in two phases: initial formation and subsequent maturation into the adult form. This is exemplified by the intestine. In mouse, the intestine at birth has villus, where most differentiated epithelial cells are located, but lacks any crypts, where adult intestinal stem cells reside. The crypt is formed during the first 3 weeks after birth when plasma thyroid hormone (T3) levels are high. Similarly, in anurans, the intestine undergoes drastic remodeling into the adult form during metamorphosis in a process completely dependent on T3. Studies on Xenopus metamorphosis have revealed important clues on the formation of the adult intestine during metamorphosis. Here we will review our current understanding on how T3 induces the degeneration of larval epithelium and de novo formation of adult intestinal stem cells. We will also discuss the mechanistic conservations in intestinal development between anurans and mammals.

Keywords: Intestine; Metamorphosis; Postembryonic development; Stem cell; Thyroid hormone receptor; Xenopus laevis; Xenopus tropicalis.

Fig. 1

Intestinal maturation takes place during postembryonic development in vertebrates. Both mouse intestinal maturation (upper panel) and frog intestinal remodeling during Xenopus metamorphosis (lower panel) involve the formation of adult epithelial stem cells. Some of the pre-existing epithelial cells develop into adult stem cells during the period when plasma thyroid hormone (T3) levels are high. In mouse, intervillus, sonic hedgehog (hh)-expressing epithelial cells develop into stem cells that express both hh and protein arginine methyltransferase 1 (PRMT1) in the newly formed intestinal crypts within the first 3 weeks after birth. The Xenopus adult intestinal stem cells are formed during metamorphosis through dedifferentiation of some larval epithelial cells to also express high levels of PRMT1 and Shh. The adult stem cell markers LGR5 and Msi1 are expressed in the developing adult intestinal stem cells at the climax of metamorphosis, although they have not been analyzed in the maturing neonatal mouse intestine. Modified after Ishizuya-Oka, A., & Shi, Y.B. (2011). Evolutionary insights into postembryonic development of adult intestinal stem cells. Cell & Bioscience 1, 37.

Fig. 2

Proliferating adult epithelial stem cells are formed de novo at the climax of Xenopus metamorphosis. Edu (5-ethynyl-2′-deoxyuridine, labeling newly synthesized DNA) were injected into tadpoles at premetamorphic stage 54 (A), metamorphic climax (B, stage 62), or the end of metamorphosis (C, stage 66) to label proliferating cells. One hour later, the animals were sacrificed for preparation of intestinal cross-sections, which were then double-stained for EdU and differentiated intestinal epithelial cell marker IFABP (intestinal fatty acid binding protein) by immunohistochemistry. The epithelium-mesenchyme boundary is marked by dotted lines. Note that at the climax, high levels of EdU-labeled, IFABP-negative proliferating cells were present as clusters between the connective tissue and differentiated cells expressing IFABP (B). After metamorphosis, the proliferating cells were in the troughs where IFABP was low (C), mimicking that in the adult mammalian crypt. Modified after Okada, M., Wen, L., Miller, T.C., Su, D., & Shi, Y.B. (2015). Molecular and cytological analyses reveal distinct transformations of intestinal epithelial cells during Xenopus metamorphosis. Cell & Bioscience 5, 74.

Fig. 3

TRs are not required for the initiation of metamorphosis but essential for animal survival and the completion of metamorphosis. TRα and TRβ double knockout (TR-DKO) Xenopus tropicalis tadpoles can develop to the climax stage of 61. Compared to wild type (WT) tadpoles at stage 61, the tadpoles lacking any TR have larger gills, suggesting inhibition of gill resorption due to the TR knockout. Interestingly, these TR-DKO tadpoles are stalled at stage 61 for 2 weeks or so and then die while the WT tadpoles at stage 61 need only about 1 week to complete the remaining metamorphic changes to become tailless froglets. See Shibata, Y., Wen, L., Okada, M., & Shi, Y.B. (2020). Organ-specific requirements for thyroid hormone receptor ensure temporal coordination of tissue-specific transformations and completion of xenopus metamorphosis. Thyroid 30, 300–313 for more details.

Fig. 4

Distinct expression patterns for the oncogene c-Myc and its antagonist Mad1 in the intestinal epithelium during metamorphosis. in situ hybridization analysis of Mad1 and c-Myc mRNAs was carried out on intestinal cross-sections of Xenopus laevis premetamorphic tadpoles (stage 54), metamorphosing tadpoles (stages 61/62 or climax), and post metamorphic froglets (stage 66). Panels a and b are higher magnification photos of the boxed region at stage 61/62 for Mad1 and c-Myc, respectively. Note that both Mad1 and c-Myc have little expression during premetamorphosis but are strongly activated at the climax (stages 61/62) with Mad expressed in the dying larval epithelial cells (arrowheads) while c-Myc expressed in the epithelial region close to the connective tissue (arrows). The dotted lines mark the approximate boundary between the epithelium (Ep) and the connective tissue (C). Scale bar, 50μm. Lu, lumen; Mu, muscle; Ty, typhlosole. Modified after Okada, M., Miller, T.C., Wen, L., & Shi, Y.B. (2017). A balance of Mad and Myc expression dictates larval cell apoptosis and adult stem cell development during Xenopus intestinal metamorphosis. Cell Death & Disease 8, e2787.