The THO Ribonucleoprotein Complex Is Required for Stem Cell Homeostasis in the Adult Mouse Small Intestine

Abstract

RNA processing and transport are mediated by cotranscriptionally assembled ribonucleoprotein (RNP) complexes. RNPs have been postulated to help specify coordinated gene expression, but the requirements for specific RNP complexes in mammalian development and tissue homeostasis have not been extensively evaluated. THO is an evolutionarily conserved RNP complex that links transcription with nuclear export. THO is not essential for Saccharomyces cerevisiae viability, but it is essential for early mouse embryonic development. Embryonic lethality has limited the characterization of THO requirements in adult tissues. To overcome this limitation, a mouse model has been generated that allows widespread inducible deletion of Thoc1, which encodes an essential protein subunit of THO. Widespread Thoc1 deletion disrupts homeostasis within the small intestine but does not have detectable effects in other epithelial tissues such as the related mucosa of the large intestine. Thoc1 loss compromises the proliferation and lineage-generating capacity of small intestinal stem cells, disrupting the supply of differentiated cells in this rapidly renewing tissue. These findings demonstrate that the effects of THO deficiency in the adult mouse are tissue and cell type dependent.

The role of TREX in gene expression and disease

“…Depletion of THO components at this stage results in embryonic lethality as the blastocyst cannot be effectively formed due to increased apoptosis of the ICM [56,57,167]. However, depletion of these components in differentiated adult cells does not result in immediate cell death [168] and THOC1 expression is decreased in nonproliferating cells [52], suggesting a greater dependency on TREX in undifferentiated cells early on in embryogenesis. Furthermore, the balance between differentiation and self-renewal within stem cells may be regulated by TREX through its function in mRNA export.…”

“…The small intestine is remarkably sensitive to the loss of TREX, presumably due to the presence of stem-cell niches in the intestinal crypt. THOC1 expression is higher in intestinal crypts harboring stem-cell niches [168], and THOC1 depletion results in degeneration of the small-intestinal epithelium and increased apoptosis of cells in the niche [168,172]. Interestingly, the stem-cell niches in the large intestine are not as affected by THOC1 depletion [168], demonstrating the tissue-specific effects of TREX loss as well as the increased requirement for the complex in differentiating tissues.…”

An approach for controlling the timing and order of engineered mutations in mice

“…Mice were treated with tamoxifen essentially as described (Pitzonka et al, 2013); tamoxifen (Sigma-Aldrich, St. Louis, Missouri) at 20 mg/mL was prepared and 0.1 mL administered (70–75 mg/kg body weight) to 9–12-week-old mice once per day for 5 days via intraperitoneal injection. Weight was monitored during and after treatment.…”

“…To initiate Cre recombination, the Meox2 tm1(cre)Sor allele (Tallquist & Soriano, 2000) or the PB-Cre4 transgene (Wu et al, 2001) were bred into Gt(ROSA)26Sor tm1dwg containing mice. Mice were treated with tamoxifen essentially as described (Pitzonka et al, 2013); tamoxifen (Sigma-Aldrich, St. Louis, Missouri) at 20 mg/mL was prepared and 0.1 mL administered (70-75 mg/kg body weight) to 9-12-week-old mice once per day for 5 days via intraperitoneal injection. Weight was monitored during and after treatment.…”

Evaluating Effects of Hypomorphic Thoc1 Alleles on Embryonic Development in Rb1 Null Mice

“…Loss of Thoc1 causes early embryonic death (17); thus, its function is nonredundant. In adult mice, induced Thoc1 deletion causes defects in a limited number of rapidly dividing cells like intestinal stem cells and myeloid progenitor cells (19,20). Mice homozygous for a hypomorphic Thoc1 allele expressing reduced Thoc1 are viable (18,21).…”