A balance of Mad and Myc expression dictates larval cell apoptosis and adult stem cell development during Xenopus intestinal metamorphosis

Abstract

The Myc/Mad/Max network has long been shown to be an important factor in regulating cell proliferation, death and differentiation in diverse cell types. In general, Myc–Max heterodimers activate target gene expression to promote cell proliferation, although excess of c-Myc can also induce apoptosis. In contrast, Mad competes against Myc to form Mad–Max heterodimers that bind to the same target genes to repress their expression and promote differentiation. The role of the Myc/Mad/Max network during vertebrate development, especially, the so-called postembryonic development, a period around birth in mammals, is unclear. Using thyroid hormone (T3)-dependent Xenopus metamorphosis as a model, we show here that Mad1 is induced by T3 in the intestine during metamorphosis when larval epithelial cell death and adult epithelial stem cell development take place. More importantly, we demonstrate that Mad1 is expressed in the larval cells undergoing apoptosis, whereas c-Myc is expressed in the proliferating adult stem cells during intestinal metamorphosis, suggesting that Mad1 may have a role in cell death during development. By using transcription activator-like effector nuclease-mediated gene-editing technology, we have generated Mad1 knockout Xenopus animals. This has revealed that Mad1 is not essential for embryogenesis or metamorphosis. On the other hand, consistent with its spatiotemporal expression profile, Mad1 knockout leads to reduced larval epithelial apoptosis but surprisingly also results in increased adult stem cell proliferation. These findings not only reveal a novel role of Mad1 in regulating developmental cell death but also suggest that a balance of Mad and Myc controls cell fate determination during adult organ development.

Thyroid hormone receptor beta is critical for intestinal remodeling during Xenopus tropicalis metamorphosis

“…When apoptosis was analyzed by using TUNEL labeling, we observed the peak level of apoptotic cells after 2 days of T3 treatment in wild type intestine (Fig. 7), as we reported before [68]. In contrast, in TRβ knockout tadpoles, the intestine had reduced number of apoptotic cells after 2 days of T3 treatment compared to the wild type animals (Fig.…”

The balance of two opposing factors Mad and Myc regulates cell fate during tissue remodeling

“…Of particular interest among such candidate stem cell genes is Mad1, which has previously been associated with cell differentiation and anti-apoptotic but not with stem cells [ 11 , 20 – 22 ]. Our recent studies reveal an interesting role for Mad1 during intestinal remodeling [ 58 ].…”

“…Its expression level drops to a much lower level by the end of metamorphosis (stage 66), when intestinal remodeling is completed and the cell-renewal system is established along the trough-crest axis of adult epithelial fold (Fig. 1 A) [ 58 ]. Interestingly, the expression of c-Myc, which is an antagonist of Mad1 and known to be a T3-regulated gene during metamorphosis [ 59 ], has a similar expression pattern during intestinal metamorphosis (Fig.…”

“…Interestingly, the expression of c-Myc, which is an antagonist of Mad1 and known to be a T3-regulated gene during metamorphosis [ 59 ], has a similar expression pattern during intestinal metamorphosis (Fig. 1 A) [ 58 ]. However, the expression of Max, which is the dimerization partner for both Mad1 and c-Myc, changes little during intestinal metamorphosis (Fig.…”

“…(n = 3). See [ 58 ] for more details. B Mad1 but not c-Myc is expressed in the larval cells undergoing apoptosis at the climax of intestinal remodeling.…”