Oliferation orapoptosis may be reduced with fasting and recovered with refeeding. In contrast to the muscularis externa, the mucosa is drastically changed by fasting and refeeding, in agreement with previous reports in fish [5], reptiles [11, 12] and other amphibians [13?5]. In particular, the epithelial layer that participates in intestinal specific functions, was down-regulated during fasting and recovered quickly by refeeding. Unlike the mucosa of the mammalian intestines, in which prolonged fasting causes severe atrophy [3] with impairment of mucosal barrier function [4], the structural integrity of the X. laevis intestine mucosa was maintained during fasting, even after 5 months of fasting. This notion was supported byTamaoki et al. Cell Biosci (2016) 6:Page 7 offabp6 H3K9ac 4 2 0 6 H4ac 4 2afabpacdxafxrarplAaa15 10 5 0BabCa a6 4Db abEab ab4 2 0 6 4 2 0 6a5 0 6Fb aGb a bHab a b0 6 4 2bIJb a10 5 0 6aabaabRelative input ( ) H3K4me6 K 4 2 0aLb aMb6 4 2 0Nb a a0 O 6 4 2b a a a2 0a2 0aaPb a aQb a aRbSaTb a abRNAPII55 05aa5 0a5RNAPIIS5P10Ub a aVab a10 5Wb a aXb a10Yb a a55afed fasted DS5565 supplement refedfed fasted refedfed fasted refedfed fasted refedfed fasted refedFig. 4 Epigenetic modifications on fabp1, fabp2, cdx2 and fxr genes in the intestines of fed, fasted and refed X. laevis. Chromatin samples were prepared from the intestines of the frogs that were fed for 22 days (fed), fasted for 22 days (fasted), or fasted for 21 days and PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/28914615 then refed for 1 day (refed). Signals of ChIP on fabp1 (A, F, K, P and U), fabp2 (B, G, L, Q and V), cdx2 (C, H, M, R and W), fxr (D, I, N, S and X) and rpl8 (E, J, O, T and Y) genes were detected by qPCR following immunoprecipitation with antibodies against H3K9ac (A ), H4ac (F ), H3K4me1 (K ), RNAPII (P ) and RNAPIIS5P (U ). Primers used in qPCR are shown in Additional file 6: Table S3. Each value is the mean ?SEM (n = 8). Distinct letters denote significantly different means (p < 0.05). These experiments were repeated at least two times, with similar resultschanges in protein amounts of cell proliferation marker (PCNA) and in transcript amounts of the marker genes for stem cell (lgr5), epithelial cell differentiation (cdx2), cell proliferation (atm, pcna and components of mTOR signaling, mtor, raptor and rictor), whose expression levels correlated well with the morphological changes of the epithelial layer. The marker genes for apoptosis (casp1, casp3, casp7, casp8, casp9, and nos2) [16, 17] were also down-regulated by fasting but remained low at 1 day after refeeding. Under normal conditions, the epithelial cell turnover rate of the X. laevis intestine is approximately 16 days [18], 3? time longer than that in rats, which 4? days [1]. Fasting may further decelerate the usual cell turnover rate by decreasing both cell proliferationand apoptosis, although at present we cannot exclude the possibility that enterocyte hypertrophy contributes to the recovery from the intestinal atrophy as found in the intestines of the snake Python molurus [11] and the frog Cyclorana alboguttata [13]. A similar observation about the decline in both cell proliferation and apoptosis during fasting was also reported in the shark intestine [19]. Reduction in enterocyte number and in villus length with fasting [20], and enterocyte hypertrophy and hyperplasia with refeeding [21] may be common responses in the intestines of lower vertebrates. A histologically interesting finding is that the intestine of the refed frogs had highly branched troug.
