Mitochondrial gene expression in the human gastrointestinal tract

Divide and conquer: two stem cell populations in squamous epithelia, reserves and the active duty forces

Stem cells are of great interest to the scientific community due to their potential role in regenerative and rejuvenative medicine. However, their role in the aging process and carcinogenesis remains unclear. Because DNA replication in stem cells may contribute to the background mutation rate and thereby to cancer, reducing proliferation and establishing a relatively quiescent stem cell compartment has been hypothesized to limit DNA replication-associated mutagenesis. On the other hand, as the main function of stem cells is to provide daughter cells to build and maintain tissues, the idea of a quiescent stem cell compartment appears counterintuitive. Intriguing observations in mice have led to the idea of separated stem cell compartments that consist of cells with different proliferative activity. Some epithelia of short-lived rodents appear to lack quiescent stem cells. Comparing stem cells of different species and different organs (comparative stem cell biology) may allow us to elucidate the evolutionary pressures such as the balance between cancer and longevity that govern stem cell biology (evolutionary stem cell biology). The oral mucosa and its stem cells are an exciting model system to explore the characteristics of quiescent stem cells that have eluded biologists for decades.

Localization of adenylate kinase 4 in mouse tissues

Adenylate kinase (AK) is a key enzyme in the high-energy phosphoryl transfer reaction in living cells. Of its isoforms, AK4 has a similar sequence and subcellular localization to that of AK3 in the mitochondrial matrix. However, unlike AK3, AK4 lacks the guanosine triphosphate: adenosine monophosphate phosphotransferase activity. To elucidate the physiological role of AK4, we explored the protein localization of AK4 in various mouse tissues by immunohistochemical analysis. AK4 protein was detected in the kidney, liver, brain, heart, stomach, intestine, and gonads but not in the lung and spleen. Interestingly, cell-type specific expression was evident in the brain, gastrointestinal tract, and gonads. In the cerebellum, AK4 was detected in granular cells but not in Purkinje cell bodies. In the gastrointestinal tract, AK4 was highly expressed in epithelia. In the ovary, AK4 was detected in oocytes and corpora lutea. In the testis, AK4 was detected in spermatocytes but not in spermatogonia. Our findings demonstrate that AK4 localizes uniquely in a cell-type and tissue-specific manner in mouse tissues.

Inflammatory bowel disease-associated gene expression in intestinal epithelial cells by differential cDNA screening and mRNA display

Intestinal epithelial cells are actively involved in the pathogenesis of inflammatory bowel disease resulting in an altered functional phenotype. The modulation of epithelial gene expression may occur as a consequence of proliferative, metabolic, immune, inflammatory, or genetic abnormalities. Differential screening of epithelial-cell-derived cDNA libraries (from control, ulcerative colitis, and Crohn's disease epithelial cells) and differential display of mRNA were used for investigation of disease-associated gene expression and modulation. Intestinal epithelial gene expression was successfully analyzed by both approaches. Using differential screening with clones encoding mitochondrial genes, quantitative overexpression was observed in both ulcerative colitis and Crohn's disease, while a unique expression of small RNA was noticed in Crohn's disease cells using Alu-homologous clones. Differential display demonstrated that several genes were differentially displayed among control, ulcerative colitis, and Crohn's disease epithelial cells. This was confirmed by immunohistochemical staining of pleckstrin, desmoglein 2 and voltage-dependent anion channel in control and inflammatory bowel disease mucosal samples. In summary, several inflammatory bowel disease-related associations were found. Since both differential screening and display have advantages and limitations, the combination of both techniques can generate complementary information, facilitate search for novel genes, and potentially identify genes uniquely associated with inflammatory bowel disease.

Expression and human chromosomal localization to 17q25 of the growth-regulated gene encoding the mitochondrial ribosomal protein MRPL12

Mitochondrial activity requires the expression of nuclear genes, whose products are part of multiproteic complexes leading to ATP production and delivery. We recently characterized a growth-activated mRNA encoding the human mitochondrial ribosomal MRPL12 protein, which is thought to act as a translational regulator of mitochondrial mRNAs. We show here that MRPL12 mRNA expression is enhanced in growth-stimulated cells as a result of transcriptional activation, a feature lost in transformed cell lines. MRPL12 mRNA is highly expressed in the colon, in which a reduction in mitochondrial activity was shown to be associated with tumor formation. The human MRPL12 protein is encoded by a unique gene located on chromosome 17 (q25-qter). As no predisposition to colon cancer linked to this chromosomal region was hitherto reported, the MRPL12 gene might be involved in the process of differentiation of colonic epithelial cells.

The synthesis of mRNA in isolated mitochondria can be maintained for several hours and is inhibited by high levels of ATP

The dependence for the maintenance of the synthesis and maturation of mitochondrial RNA on the supply of nucleo-cytoplasmic factors has been investigated by a novel in organello RNA synthesis system. We found that mitochondrial DNA transcription can be maintained for several hours in isolated mitochondria. Analysis of the individual mitochondrial RNA species revealed that: the processing of the rRNA precursors and the stability of the mature rRNAs, but not the transcription itself, is severely impaired after short periods of incubation, indicating that these processes are strongly dependent on the mitochondrial interaction with the nucleo-cytoplasmic compartment; the events that lead to the synthesis, processing and turnover of the mitochondrial mRNAs do not require the continuous supply of nucleo-cytoplasmic factors, that are accumulated in excess by mitochondria. Furthermore, we present evidence indicating an inhibition of high ATP levels on the mitochondrial RNA polymerase activity, both in organello and in vitro. Consequently, it is proposed that mitochondrial mRNA synthesis can be regulated in response to changes in intramitochondrial ATP levels. This regulation of mitochondrial mRNA synthesis together with their very rapid turnover described here and elsewhere [Gelfand, R. & Attardi, G. (1981) Mol. Cell Biol. 1, 497-511], could represent a mechanism that would allow each individual mitochondrion to adjust its optimal levels of mRNA, and hence its translation capacity, in response to local energetic demands.

Patterns of gene expression along the crypt-villus axis in mouse jejunal epithelium

BACKGROUND

There is considerable interest in gene expression along the crypt-villus axis of the small intestinal epithelium, particularly in the identification of genes expressed in intestinal crypts.

METHODS

In an attempt to identify crypt-expressed genes, single-stranded cDNA made from normal mouse jejunal epithelium was used in subtractive hybridization against single-stranded cDNA from epithelium from which crypt cells were depleted by 2,000 rads of gamma irradiation. Partial DNA sequence and in situ hybridization of 72 resulting clones were determined.

RESULTS

The sequence of 45 clones matched previously published genes. Gene expression patterns fell into three categories: expression throughout the crypt-villus axis, expression restricted to the villus, and expression restricted to the crypt. Clones in the first two categories could be further divided into three subgroups: those with uniform expression, those with an increasing gradient of expression, and those with a decreasing gradient of expression along the crypt-villus axis. Twenty two clones showed a stronger expression in crypt and lower villus cells, four of these were differentially localized to the crypt. Two of the crypt localized clones were uniformly expressed throughout the crypt, expression of one was stronger in the lower crypt, and expression of the remaining clone was enhanced Paneth cells. We report the full-length cDNA sequence of the Paneth-cell-enhanced clone.

CONCLUSIONS

The screen isolated crypt-expressed genes that may prove useful tools in the study of crypt biology. In a companion report, we characterize one of the crypt clones.