Impact of the chromatin remodeller SMARCAD1 on murine intestinal intraepithelial lymphocyte and white adipose tissue biology.

Chromatin remodelling factors use the energy of ATP hydrolysis to drive the movement of and/or affect molecular changes to the nucleosome. One such factor, SMARCAD1 (SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily A containing DEAD/H box 1), has been previously shown to restore heterochromatin at the replication fork in vitro. This project aimed to assess the impact of SMARCAD1 on mammalian biology, utilising an animal model in which the catalytic ATPase domain of murine SMARCAD1 had been deleted using Cre/lox technology.

Preliminary results had implicated SMARCAD1 in adaptive-immunity and white adipose tissue biology, and SMARCAD1 expression in these tissues/cells was confirmed by tissue-panel western blot. This project therefore aimed to build on these results to understand better the impact of SMARCAD1 on adaptive immune development and white adipose tissue biology. In addition, fewer than expected viable Smarcad1-/- homozygous offspring were produced during Smarcad1+/- x +/- matings, which both confirmed the observation from a previous knockout model of Smarcad1, and limited the number of knockout animals available for this study.

Investigation of systemic B- and T-cells in the bone marrow, thymus and spleen had previously suggested there was no significant defect in adaptive immune development in Smarcad1-/- mice, however a tissue-specific and age-related loss of intra-epithelial (IEL) T-lymphocytes was found in the small intestine by flow cytometry. Analysis by qPCR of duodenal RNA suggested that differentiation rather than inflammation may underpin any loss-of-IEL phenotype, although further examination of cell-proliferation and crypt/villus anatomy by EdU incorporation and immunofluorescence revealed no overt cell-anatomical or proliferative difference in the knockout mice. The requirement for large numbers of aged mice made further investigation of the intestinal IEL phenotype logistically prohibitive.

The reduction of epididymal white adipose tissue (eWAT) size had also been observed in male Smarcad1-/- mice, and serum from these mice showed elevated triglyceride (TG) and free fatty acids (FFA) levels. Transcriptomic analysis by RNA-seq of whole-WAT revealed an elevation in macrophage-related markers in knockout mice, which was confirmed by flow cytometry. As a number of reports have implicated SMARCAD1 in stem cell biology, putative adipose stem cells were isolated from +/+ and -/- mice by FACS and used for adipogenic differentiation assays ex-vivo In parallel, mouse embryonic fibroblasts from +/- and -/- mice were also assayed for adipogenic differentiation. While no significant differences in adipogenesis were observed, Smarcad1-/- mice challenged with a (60%) high fat diet did show increased weight gain over +/+ mice, and measurements of adipocyte size and cell cycle/cell proliferation analysis suggested hyperplasia rather than defects in adipogenesis may drive any WAT-related pathology in these mice.