The ontogeny of alpha-fetoprotein gene expression in the mouse gastrointestinal tract
A L Tyner1, R Godbout, R S Compton
1Howard Hughes Medical Institute, Princeton University, New Jersey 08544.
Abstract:
The ontogeny of alpha-fetoprotein (AFP) gene expression has been examined in the fetal and adult mouse gastrointestinal tract. AFP mRNA constitutes approximately 0.1% of total mRNA in the fetal gut. The transcripts were localized by in situ hybridization to the epithelial cells lining the villi of the fetal gut. At birth, AFP mRNA declines rapidly to achieve low adult basal levels, which are not affected by different alleles of raf, a gene that determines the adult basal level of AFP mRNA in the liver. The basal level in the adult gut is the consequence of continued AFP transcription in a small number of enteroendocrine cells that are distributed infrequently on the villi. These cells were identified by double antibody staining with antibodies to chromogranin A, an enteroendocrine cell marker and AFP. Previous studies resulted in the generation of a line of transgenic mice containing an internally deleted AFP gene that was greatly overexpressed in the fetal gut. The basis for the inappropriately high level expression of the transgene was shown to be the consequence of very high levels of transcription in the epithelial cells of the villi rather than to expression in inappropriate cell types. The cis-acting DNA sequences required for expression of the AFP gene in the gut were investigated using Caco-2 cells, a human colon adenocarcinoma cell line. These experiments indicated that, with one exception, the regulatory elements required in both the promoter and enhancer regions of the gene coincided with those that are necessary for high level expression in the liver. The one exception was enhancer II, located 5 kbp of DNA upstream of the gene, which exhibited no activity in Caco-2 cells.
Insights
Alpha-fetoprotein (AFP) gene expression in the mouse gut decreases significantly after birth. Adult gut expression originates from enteroendocrine cells, with regulatory elements similar to liver expression, except for enhancer II.
Area of Science:
- Developmental biology
- Molecular genetics
- Gastroenterology
Background:
- Alpha-fetoprotein (AFP) gene expression patterns are crucial for understanding fetal development and adult tissue function.
- Investigating AFP gene regulation in the gastrointestinal tract provides insights into cell-specific gene control.
Purpose of the Study:
- To elucidate the ontogeny of alpha-fetoprotein (AFP) gene expression in the mouse gastrointestinal tract.
- To identify the cell types responsible for adult basal AFP expression in the gut.
- To determine the cis-acting DNA sequences regulating AFP gene expression in the gut.
Main Methods:
- In situ hybridization to localize AFP mRNA in fetal and adult mouse gut.
- Double antibody staining using chromogranin A and AFP antibodies to identify enteroendocrine cells.
- Analysis of transgenic mice with an internally deleted AFP gene.
- Reporter gene assays in Caco-2 cells to study cis-acting regulatory elements.
Main Results:
- AFP mRNA is abundant in fetal gut villi epithelial cells, declining sharply at birth.
- Adult basal AFP expression in the gut is maintained by transcription in a subset of enteroendocrine cells.
- Regulatory elements for gut AFP expression largely overlap with those for liver expression, with enhancer II being an exception.
- Transgenic studies confirmed high fetal gut expression is due to transcriptional levels in villi epithelial cells.
Conclusions:
- The ontogeny of AFP gene expression in the mouse gut involves a transition from widespread epithelial expression in the fetus to restricted expression in adult enteroendocrine cells.
- Specific cis-acting DNA sequences, largely conserved with liver regulatory elements, control AFP expression in the gut.
- Enhancer II represents a gut-specific regulatory element or a lack thereof, highlighting differential gene regulation between tissues.
