Jorge Mùnera, Ph.D.: Department of Regenerative Medicine and Cell Biology
Regional specification describes a process through which unique characteristics are subscribed to defined areas of the developing embryo. This process is required for functional specificity throughout the gastrointestinal (GI) tract. Unlike the small intestine, the colon lacks villi and Paneth cells, it has increased tight junction numbers and integrity, and an increased proportion of goblet cells. Perturbations in any of these characteristics occur in colonic diseases including ulcerative colitis (UC) and colorectal cancer (CRC). A global transcriptional change to a proximal small intestine signature is evident in UC . This change in gene expression likely reflects lost expression of a colon specific transcription factor(s).
The earliest known transcription factor to be specifically expressed in the developing colon epithelium is SATB2 (Special AT-rich sequence-binding protein 2). Colonic SATB2 expression is maintained throughout development and the postnatal life of frogs, mice and humans. While Genome Wide Association Studies (GWAS) indicate SATB2 is a susceptibility gene, with reduced expression (of mRNA and protein) in UC and CRC, SATB2 function is unknown. What is known, is that SATB2 binds DNA at matrix associated regions (MARS), sequences in genomic DNA that attach to the nuclear matrix allowing organization of chromatin into structural domains20-22. Through MARS binding, SATB2 induces local chromatin-loop remodeling by recruiting histone deacetylases (HDACs) or histone acetyltransferases (HATs) to promoters and enhancers, suggesting SATB2 is crucial to establish and maintain colon epithelial patterning.
We hypothesize that during human colonic cell formation, bone morphogenetic proteins (BMPs) induce the expression of epithelial SATB2 required for colonic epithelium patterning and potentially other regulators in the mesenchyme such as BCOR, GATA2 and GATA3 that drive the colonic patterning required to establish a genomic environment that supports the transcription of colonic mRNAs and represses the expression of small intestinal mRNAs.
We have generated a model allows us to produce human colonic organoids (HCOs) from human pluripotent stem cells (hPSCs). A 3-day induction of BMP signaling stably patterns mid/hindgut spheroids into SATB2 expressing HCOs containing all of the main epithelial cell types present in the human colon as well as co-developing mesenchymal cells, unlike other organoids systems which are only epithelial. While we have shown that BMP signaling is necessary for expression of SATB2 in model systems and HCOs15, the relationship between the action of BMPs at the cell surface and control of colonic gene expression by SATB2 has not been established. We will test our hypothesis using this human pluripotent stem cell model of colonic differentiation.
