<i>Islet1</i> and Its Co-Factor <i>Ldb1</i> Are Expressed in Quiescent Cells of Mouse Intestinal Epithelium

<div><p><i>Islet1</i> belongs to Lim homeobox (<i>Lhx</i>) gene family which encodes transcription factors that have been conserved in evolution. They form complexes with other transcriptional regulators, among them obligatory co-factors encoded by <i>Ldb</i> genes. <i>Isl1 (Islet1)</i>, <i>Lhx</i> and <i>Ldb1</i> genes play a crucial role in organ patterning, cell fate determination and cell differentiation in both embryonic and adult tissues. In this study we analyzed expression pattern of <i>Isl1</i> and its co-factor <i>Ldb1</i> in small intestine. We also studied the biological role of <i>Ldb1</i> in gut endoderm. Quantitative PCR analysis revealed a relatively high level of expression of <i>Lhx1, Isl1, Isl2, Lmx1a, Ldb1</i> and <i>Ldb2</i> mRNAs in the gut tissue as compared to the level of less abundant detectable <i>Lmx1b</i> mRNA. Immunohistochemical studies demonstrated a unique pattern of Ldb1 and Islet1 proteins in the crypt compartment. Ldb1 is produced at a low level in majority of crypt cells; but, its abundant expression was demonstrated for some single cells. Islet1 is also expressed in single cells of the crypt. Double staining experiments with Ldb1 and Isl1 antibodies showed that both genes are co-expressed in certain cells of the crypt. Further analysis revealed the Ldb1-expressing cells in the gut are both of endodermal and mesodermal origin. Proliferation studies using antibodies to phospho-histone H3 and Ki-67 antigens, as well as long-term BrdU labeling, showed that cells prominently expressing Ldb1/Islet1 are quiescent but do not belong to any known terminally differentiated cell lineages. They may represent a group of stem-like cells in the crypt. Further experiments by cell lineage tracing should be performed to better characterize this cell population. Functional studies of mice with <i>Ldb1</i> gene ablated in gut endoderm revealed no specific role of <i>Ldb1</i> in that tissue.</p>