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We disrupted the Mcl-1 locus in murine ES cells to determine the developmental roles of this Bcl-2 family member. Deletion of Mcl-1 resulted in peri-implantation embryonic lethality. Mcl-1(-/-) embryos do not implant in utero, but could be recovered at E3.5-4.0. Null blastocysts failed to hatch or attach in vitro, indicating a trophectoderm defect, although the inner cell mass could grow in culture. Of note, Mcl-1(-/-) blastocysts showed no evidence of increased apoptosis, but exhibited a delay in maturation beyond the precompaction stage. This model indicates that Mcl-1 is essential for preimplantation development and implantation, and suggests that it has a function beyond regulating apoptosis.

Background/aims: Enteroendocrine cell subpopulations are uniquely distributed along the crypt-villus and cephalocaudal axes of the small intestine. These regional differences in enteroendocrine cell expression, which are maintained in spite of rapid turnover of the epithelium, serve as descriptive markers of physiological differences along the length of the bowel. This study aimed to determine the influence of luminal contents on the maintenance of regional differentiation patterns of enteroendocrine and enterocytic phenotypes., Methods: Sections of jejunum and ileum were surgically transposed in rats, leaving the innervation and blood supply to the transposed segments intact. The animals were killed 1, 4, and 8 weeks after surgery. Enteroendocrine cell subpopulations and enterocytic cell markers were studied immunohistochemically., Results: No change in regional expression patterns was seen in response to the altered luminal environment by any of the enterocytic markers and four of the five enteroendocrine cell subpopulation markers. Eight weeks after surgery, the number of gastrin-expressing enteroendocrine cells increased in ileal segments transplanted proximally., Conclusions: Although luminal signals can affect intestinal stem cells to alter their proliferation rates, the luminal environment has only limited effects on the regional-specific expression of enteroendocrine or enterocytic products.

SV-40 T antigen (TAg), human K-rasVal12, and a dominant negative mutant of human p53 (p53Ala143) have been expressed singly and in all possible combinations in postmitotic enterocytes distributed throughout the duodenal-colonic axis of 1-12-mo-old FVB/N transgenic mice to assess the susceptibility of this lineage to gene products implicated in the pathogenesis of human gut neoplasia. SV-40 TAg produces re-entry into the cell cycle. Transgenic pedigrees that produce K-rasVal12 alone, p53Ala143 alone, or K-rasVal12 and p53Ala143 have no detectable phenotypic abnormalities. However, K-rasVal12 cooperates with SV-40 TAg to generate marked proliferative and dysplastic changes in the intestinal epithelium. These abnormalities do not progress to form adenomas or adenocarcinomas over a 9-12-mo period despite sustained expression of the transgenes. Addition of p53Ala143 to enterocytes that synthesize SV-40 TAg and K-rasVal12 does not produce any further changes in proliferation or differentiation. Mice that carry one, two, or three of these transgenes were crossed to animals that carry Min, a fully penetrant, dominant mutation of the Apc gene associated with the development of multiple small intestinal and colonic adenomas. A modest (2-5-fold) increase in tumor number was noted in animals which express SV-40 TAg alone, SV-40 TAg and K-rasVal12, or SV-40 TAg, K-rasVal12 and p53Ala143. However, the histopathologic features of the adenomas were not altered and the gut epithelium located between tumors appeared similar to the epithelium of their single transgenic, bi-transgenic, or tri-transgenic parents without Min. These results suggest that (a) the failure of the dysplastic gut epithelium of SV-40 TAg X K-rasVal12 mice to undergo further progression to adenomas or adenocarcinomas is due to the remarkable protective effect of a continuously and rapidly renewing epithelium, (b) initiation of tumorigenesis in Min mice typically occurs in crypts rather than in villus-associated epithelial cell populations, and (c) transgenic mouse models of neoplasia involving members of the enterocytic lineage may require that gene products implicated in tumorigenesis be directed to crypt stem cells or their immediate descendants. Nonetheless, directing K-rasVal12 production to proliferating and nonproliferating cells in the lower and upper half of small intestinal and colonic crypts does not result in any detectable abnormalities.

Axial pattern formation is sustained in the mammalian gut epithelium despite rapid and continuous renewal of its four principal cell lineages. The mouse and rat liver fatty acid-binding protein (L-FABP) genes (Fabpl) represent an excellent model for understanding the mechanisms that determine differentiation-dependent, cell lineage-specific, and distinct regional patterns of expression along the crypt-to-villus and duodenal-to-ileal axes of the gut, as well as within the liver acinus. We have used transgenic mice to map cis-acting elements in rat Fabpl that control these patterns of gene expression. Seven transgenes were analyzed, representing sequential deletions of the 5'-nontranscribed domain of Fabpl linked to the human growth hormone (hGH) gene beginning at its nucleotide +3 (L-FABP/hGH+3). Several pedigrees of mice containing each one of the L-FABP/hGH+3 transgenes were examined at the end of their 8th and 20th weeks of postnatal life using immunocytochemical and RNA hybridization analyses. A remarkably compact sequence spanning nucleotides -132 to +21 of Fabpl is sufficient to establish and maintain a distribution of reporter mRNA and protein in villus-associated enterocytes located along the duodenal-to-ileal axis of the gut that resembles the pattern of expression of the endogenous Fabpl gene. L-FABP-132 to +21/hGH+3 is also expressed in surface and pit mucous cells of gastric units and in enterocytes located in the colonic homologs of small intestinal villi, the surface epithelial cuffs. This pattern of transgene expression in the stomach and colon recapitulates that of the intact endogenous donor rat Fabpl but not that of mouse Fabpl, which is silent in these proximal and distal segments of the gastrointestinal tract. Analysis of mice containing L-FABP-4000 to +21/hGH+3, L-FABP-1600 to +21/hGH+3, L-FABP-596 to +21/hGH+3, L-FABP-246 to +21/hGH+3, and L-FABP-186 to +21/hGH+3 indicate that Fabpl's cephalocaudal gradient is influenced by cis-acting suppressors of cecal and colonic expression located between nucleotides -4000 and -1600 and by cis-acting activators of cecal and colonic expression located between nucleotides -597 and -351. L-FABP-132 to +21/hGH+3 is precociously activated in proliferating and nonproliferating epithelial cells located in intestinal crypts. The suppressor(s) of L-FABP accumulation in crypt epithelial cell populations are not represented between nucleotides -4000 and +21, indicating that different cis-acting sequences regulate regional and differentiation-dependent patterns of Fabpl expression.(ABSTRACT TRUNCATED AT 400 WORDS)

The mouse intestinal epithelium is able to establish and maintain complex lineage-specific, spatial, and temporal patterns of gene expression despite its rapid and continuous renewal. A multipotent stem cell located near the base of each intestinal crypt gives rise to progeny which undergo amplification and allocation to either enterocytic, Paneth cell, goblet cell, or enteroendocrine cell lineages. Differentiation of these four lineages occurs during their geographically ordered migration along the crypt-villus axis. Gut stem cells appear to have a "positional address" which is manifested by differences in the differentiation programs of their lineal descendants along the duodenal-colonic (cephalocaudal) axis. We have used the intestinal fatty acid binding protein gene (Fabpi) as a model to identify cis-acting elements which regulate cell- and region-specific patterns of gene expression in the gut. Nucleotides -1178 to +28 of rat Fabpi direct a pattern of expression of a reporter (human growth hormone [hGH]) which mimics that of mouse Fabpi (a) steady-state levels of hGH mRNA are highest in the distal jejunum of adult transgenic mice and fall progressively toward both the duodenum and the mid-colon; and (b) hGH is confined to the enterocytic lineage and first appears as postmitotic, differentiating cells exit the crypt and migrate to the base of small intestinal villi or their colonic homologs, the surface epithelial cuffs. Nucleotides -103 to +28, which are highly conserved in rat, mouse and human Fabpi, are able to correctly initiate transgene expression in late fetal life, restrict hGH to the enterocytic lineage, and establish an appropriate cephalocaudal gradient of reporter expression. This cephalocaudal gradient is also influenced by cis-acting elements located between nucleotides -1178 and -278, and -277 and -185 that enhance and suppress (respectively) expression in the ileum and colon and by element(s) located upstream of nucleotide -277 that are needed to sustain high levels of hGH production after weaning. Nucleotides -277 to -185 contain part of a domain conserved between the three orthologous Fabpi genes (nucleotides -240 to -159), a 24-bp element (nucleotides -212 to -188) that binds nuclear factors present in colonic but not small intestinal epithelial cells, and a portion of a CCAAT/enhancer binding protein footprint (C/EBP alpha, nucleotides -188 to -167). Removal of nucleotides -277 to -185 (yielding I-FABP-184 to +28/hGH+3) results in inappropriate expression of hGH in proliferating and nonproliferating epithelial cells located in the mid and upper portions of duodenal, jejunal, ileal, and colonic crypts without affecting the "shape" of the cephalocaudal gradient of transgene expression.(ABSTRACT TRUNCATED AT 400 WORDS)

Little is known about lineage relationships and differentiation programs of various epithelial cells present in mouse gastric units. We have previously used rat liver fatty acid binding protein/human growth hormone (L-FABP/hGH) transgenes to define epithelial cell lineages relationships in the small intestine of fetal and adult mice and to examine regulation of their terminal differentiation programs along the crypt-to-villus and duodenal-to-ileal axes. We have now used these transgenes to explore similar issues in the stomach. Immunocytochemical studies of fetal and adult transgenic L-FABP/hGH animals and their normal littermates revealed that the intact endogenous mouse L-FABP gene (Fabpl) is not expressed in gastric epithelium. Nucleotides-596 to +21 of the rat L-FABP gene direct "inappropriate" expression of hGH in the gastric epithelium as early as fetal day 15. From 1 to 13 mo, L-FABP-596 to +21/hGH expression occurs only in surface mucous cells of zymogenic and mucous gastric units; the reporter is not detectable in the enteroendocrine, parietal and chief cell populations of zymogenic glands. Electron microscopic immunocytochemistry revealed that hGH is directed to apical secretory granules in surface and pit mucous cells expressing the transgene. hGH levels vary widely among surface mucous cells both within single pits and between gastric units in a given animal. The heterogeneity noted in reporter expression suggests that there are marked differences in the regulatory environments of individual cells of a single type within a given gastric unit. This raises the possibility that cell differentiation programs in the stomach may not be as tightly coupled to cellular translocation as in the small intestine. Finally, the lack of expression of L-FABP-596 to +21/hGH in gastrin- and serotonin-immunoreactive cells of the stomach contrasts with its efficient expression in comparable cell types located in the duodenum; providing a model system for examining differential regulation of gene expression in terminally differentiated cell types represented in both gastric and intestinal epithelium.

The four principal cellular constituents of the mouse intestinal epithelium are all derived from a multipotent stem cell functionally anchored near the base of its crypts. Differentiation of enterocytes, enteroendocrine, and goblet cells occurs during an orderly upward migration from monoclonal crypts supplied by a single active stem cell to adjacent polyclonal small intestinal villi or to their colonic homologs, the surface epithelial cuffs. Paneth cells differentiate as they descend to the base of crypts. This epithelium undergoes rapid and perpetual renewal yet is able to maintain cephalocaudal (duodenal-to-colonic) differences in the differentiation programs of its four cell types from the time of its initial cytodifferentiation in late fetal life (embryonic (E) days 16-17). Rat liver fatty acid-binding protein/human growth hormone transgenes (Fabpl/hGH) have been used as novel phenotypic markers to describe the biological properties of gut stem cells and the differentiation programs of their enterocytic and enteroendocrine lineages. To determine whether the multipotent stem cell is able to retain a "positional" address in the absence of luminal signals, we prepared isografts from the proximal small intestine or distal small intestine and colon of E15-E16 Fabpl/hGH transgenic mice and their normal littermates and implanted them into the subcutaneous tissues of young, adult male CBY/B6 nude mice. Immunocytochemical and histochemical studies indicate that appropriate position-specific differences in the differentiation programs of each of the four principal cell lineages are present along the cephalocaudal and crypt-to-villus (or crypt-to-epithelial cuff) axes of isografts harvested 4-6 weeks after implantation. This suggests that the gut stem cell can be characterized not only by its multipotency and enormous capacity for self-renewal but also by its ability to be programmed (? imprinted) with positional information. Transgene expression is reduced in a number of enteroendocrine subpopulations in small intestinal and colonic isografts compared to the intact gut. Moreover, the decision to express the Fabpl/hGH transgene appears to be coordinated between adjacent crypts as evidenced by (i) the presence of multicrypt patches of wholly reporter (hGH)-positive or reporter-negative cells in the intact colon and in colonic isografts and (ii) by the presence of coherent bands of reporter-positive or -negative cells that emanate from adjacent monophenotypic crypts and extend to the apical extrusion zone of distal small intestinal villi.(ABSTRACT TRUNCATED AT 400 WORDS)

The mouse intestinal epithelium represents a unique mammalian system for examining the relationship between cell division, commitment, and differentiation. Proliferation and differentiation are rapid, perpetual, and spatially well-organized processes that occur along the crypt-to-villus axis and involve clearly defined cell lineages derived from a common multipotent stem cell located near the base of each crypt. Nucleotides -1178 to +28 of the rat intestinal fatty acid binding protein gene were used to establish three pedigrees of transgenic mice that expressed SV-40 large T antigen (TAg) in epithelial cells situated in the uppermost portion of small intestinal crypts and in already committed, differentiating enterocytes as they exited these crypts and migrated up the villus. T antigen production was associated with increases in crypt cell proliferation but had no apparent effect on commitment to differentiate along enterocytic, enteroendocrine, or Paneth cell lineages. Single- and multilabel-immunocytochemical studies plus RNA blot hybridization analyses suggested that the differentiation programs of these lineages were similar in transgenic mice and their normal littermates. This included enterocytes which, based on the pattern of [3H]thymidine and 5-bromo-2'-deoxyuridine labeling and proliferating nuclear antigen expression, had reentered the cell cycle during their migration up the villus. The state of cellular differentiation and/or TAg production appeared to affect the nature of the cell cycle; analysis of the ratio of S-phase to M-phase cells (collected by metaphase arrest with vincristine) and of the intensities of labeling of nuclei by [3H]thymidine indicated that the duration of S phase was longer in differentiating, villus-associated enterocytes than in the less well-differentiated crypt epithelial cell population and that there may be a block at the G2/M boundary. Sustained increases in crypt and villus epithelial cell proliferation over a 9-mo period were not associated with the development of gut neoplasms--suggesting that tumorigenesis in the intestine may require that the initiated cell have many of the properties of the gut stem cell including functional anchorage.

Min is a fully penetrant dominant mutation that leads to the development of multiple intestinal adenomas throughout the duodenal-to-colonic axis. Min/+ C57BL6/J mice have an average life-span of 120 d. Multi-label immunocytochemical studies of these lesions demonstrate patches of differentiated enterocytes, and scattered enteroendocrine, goblet and Paneth cells. Expression of endogenous marker genes within these differentiated cells can be directly correlated with the position occupied by the adenoma along the duodenal-to-colonic axis and mirrors the regional differentiation of the normal gut epithelium. The presence of multiple lineages in adenomas together with their retention of spatial information suggests that tumorigenesis in Min/+ mice may be initiated in a multipotent stem cell normally located at the base of intestinal crypts. To study the time-dependent properties of these tumors, genetic conditions were sought in which Min/+ animals could survive for up to 300 d. Min is fully penetrant in hybrids with either AKR/J or MA/MyJ. However, the hybrids demonstrate a reduction in the number of intestinal adenomas. Preliminary backcross analysis is consistent with a single major modifier locus unlinked to Min in both the AKR/J and MA/MyJ strains. The increased lifespan of the hybrid animals is also associated with the development of invasive tumors. New tumors do not arise continuously over the lifespan of these animals; instead all adenomas appear to be established by 100 d of age or sooner. These studies indicate that the Min/+ mouse is a powerful model system for analyzing the mechanisms that establish and maintain a balance between proliferation and differentiation in the continuously renewing gut epithelium and for an assessment of the multi-step hypothesis of intestinal neoplasia.

Transgenes, composed of elements of the 5' nontranscribed region of the liver fatty acid-binding protein (L-FABP) gene linked to various reporters, have previously been used to explore the cellular, regional, and temporal differentiation of the mouse intestinal epithelium. In this report, we have analyzed a pedigree of L-FABP/human growth hormone (hGH) transgenic mice that display a stable, heritable, mosaic pattern of reporter expression: wholly hGH-positive or hGH-negative populations of differentiating enterocytes arise from hGH-positive or hGH-negative crypts, respectively, and migrate as vertical coherent bands up the villus producing striped (polyclonal) villi. The ability of enteroendocrine cells within a given villus stripe to support hGH expression coincides with the enterocytic reporter phenotype, suggesting that these two terminally differentiated cells arise from a common multipotent stem cell. hGH-negative crypts are nonrandomly distributed around each villus and their frequency increases along the duodenal-to-ileal axis. Statistical analysis of the observed villus striping pattern suggests that transgene expression is not independently determined in individual crypts but rather in multicrypt "patches." The intact endogenous mouse L-FABP gene (Fabpl) exhibits a similar striped villus pattern of expression in a portion of the distal small intestine. These studies indicate that Fabpl and L-FABP/hGH transgenes represent sensitive markers for exploring the biological properties of gut stem cells and how positional information is encoded in this rapidly and continuously renewing epithelium.

Transgenes consisting of segments of the rat liver fatty acid-binding protein (L-FABP) gene's 5' non-transcribed domain linked to the human growth hormone (hGH) gene (minus its regulatory elements) have provided useful tools for analyzing the mechanisms that regulate cellular and spatial differentiation of the continuously renewing gut epithelium. We have removed the jejunum from normal and transgenic fetal mice before or coincident with, cytodifferentiation of its epithelium. These segments were implanted into the subcutaneous tissues of young adult CBY/B6 nude mouse hosts to determine whether the bipolar, migration-dependent differentiation pathways of gut epithelial cells can be established and maintained in the absence of its normal luminal environment. Immunocytochemical analysis of isografts harvested 4-6 wk after implantation revealed that activation of the intact endogenous mouse L-FABP gene (fabpl) in differentiating enterocytes is perfectly recapitulated as these cells are translocated along the crypt-to-villus axis. Similarly, Paneth and goblet cells appear to appropriately differentiate as they migrate to the crypt base and villus tip, respectively. The enteroendocrine cell subpopulations present in intact 4-6-wk-old jejunum are represented in these isografts. Their precise spatial distribution along the crypt-to-villus axis mimics that seen in the intact gut. A number of complex interrelationships between enteroendocrine subpopulations are also recapitulated. In both "intact" and isografted jejunum, nucleotides -596 to +21 of the rat L-FABP gene were sufficient to direct efficient expression of the hGH reporter to enterocytes although precocious expression of the transgene occurred in cells located in the upper crypt, before their translocation to the villus base. Inappropriate expression of hGH occurred in a high percentage (greater than 80%) of secretin, gastrin, cholecystokinin, and gastric inhibitory peptide producing enteroendocrine cells present in the intact jejunum of 4-6-wk-old L-FABP-596 to +21/hGH transgenics. Addition of nucleotides -597 to -4,000 reduced the percentage of cells co-expressing this reporter four- to eightfold in several of the subpopulations. Jejunal isografts from each transgenic pedigree studied contained a lower percentage of hGH positive enteroendocrine cells than in the comparably aged intact jejunum.(ABSTRACT TRUNCATED AT 400 WORDS)

The intestinal epithelium is characterized by the rapid and continuous renewal of its four principal cell types and by its ability to establish and maintain remarkably complex spatial differentiation along its crypt-to-villus and duodenal-to-colonic axes. We have previously used transgenic mice containing liver fatty acid binding protein/human growth hormone (L-FABP/hGH) fusion genes to analyze the molecular mechanisms responsible for encoding positional information in this epithelium. Because these studies could not distinguish whether cis-acting sequences in the L-FABP promoter or hGH structural gene were responsible for the observed cellular and regional patterns of transgene transcription in the gut, a second model fusion gene has now been constructed. It consists of nucleotides -596 to +21 of rat L-FABP linked to a cDNA encoding a chimeric protein, human decay-accelerating factor (DAF, minus the site of attachment of its COOH-terminal glycophospholipid anchor), coupled to the transmembrane (TM) and cytoplasmic domains of human HLA-B44. RNA blot hybridization and immunocytochemical analyses revealed that the cell-specific and region-specific expressions of DAF-TM and hGH in adult mice appear identical along both axes of the gut, indicating that cis-acting elements contained within the 5' nontranscribed region of the L-FABP gene rather than in the reporter are largely responsible for these observed patterns of transgene expression. Unlike pre-hGH, a prototypical secreted protein, DAF-TM is a membrane protein. The ability to direct its expression along the length of both axes of the gut provides an opportunity to analyze in vivo the sorting pathways of membrane-associated proteins in normal epithelial cells as a function of their location and differentiation. Light microscopic studies indicate that DAF-TM is targeted to the basolateral and apical surfaces of villus-associated enterocytes.

The intestinal epithelium establishes and maintains a precise spatial organization despite its continuous and rapid renewal. We have used transgenic mice containing liver fatty acid-binding protein/human growth hormone (L-FABP/hGH) fusion genes to begin to define the molecular mechanisms which are responsible for appropriate regional and cell-specific expression of genes in the gut. Multilabel immunocytochemical methods were employed to characterize the patterns of expression of two transgenes in the enteroendocrine and enterocytic populations of late gestation fetal mice at the time of initial cytodifferentiation of the gastrointestinal epithelium (fetal days 16-19). Surveys of the enteroendocrine cell population using a panel of antibodies directed against 11 neuroendocrine products revealed that these cells are scarce prior to fetal day 17, show a progressive increase in number through day 19, and while the relative proportion of subpopulations (defined by their principal peptide product) are somewhat different than in adults, their geographic distribution along the duodenal to colonic and intervillus(crypt) to villus axes are very similar to that encountered in adult (2-5 month old) mice. Immunoreactive L-FABP is first detectable at fetal day 17 and at this time of first appearance shows an adult pattern of regional enterocytic expression: i.e. it is present in cells overlying nascent villi but not those in the intervillus zone, it is highest in proximal small bowel, declines distally, and is absent from colonocytes. Colocalization studies indicate that L-FABP is not present in enteroendocrine cells during fetal life. Mapping studies indicate that nucleotides -596 to +21 of the rat L-FABP gene are sufficient to reproduce an appropriate temporal, cellular, and regional pattern of reporter (hGH) expression in fetal transgenic mice (with the exception that a subset(s) of enteroendocrine cells, typically containing immunoreactive gastric inhibitory peptide, support transgene but not L-FABP expression). This is in marked contrast to adult transgenic mice where inappropriate hGH accumulation occurs in crypt-associated epithelial cells, in colonocytes, and in many enteroendocrine populations. These studies indicate the importance of considering developmental stage when interpreting the results of any mapping study of cis-acting elements that regulate cell-specific and regional expression of genes in the perpetually renewing intestinal epithelium. Moreover, they also raise the possibility of using transgenes to define fundamental temporal changes in the gut's epithelial cell populations.

We have used liver fatty acid-binding protein/human growth hormone (L-FABP/hGH) fusion genes to explore the temporal and spatial differentiation of intestinal epithelial cells in 1- to 12-month-old transgenic mice. The intact, endogenous L-FABP gene (Fabpl) was not expressed in the colon at any time. Young adult transgenic mice containing nucleotides -596 to +21 of the rat L-FABP gene linked to the hGH gene (minus its 5' nontranscribed domain) demonstrated inappropriate expression of hGH in enterocytes and many enteroendocrine cells of most proximal and mid-colonic crypts (glands). Rare patches of hGH-negative crypts were present. With increasing age, a wave of "extinction" of L-FABP (-596 to +21)/hGH expression occurred, first in the distal colon and then in successively more proximal regions, leaving by 10 months of age only rare hGH-positive multicrypt patches. At no time during this progressive silencing of transgene expression were crypts observed that contained a mixture of hGH-positive and -negative cells at a particular cell stratum. Young (5-7 weeks) mice containing a L-FABP (-4000 to +21)/hGH transgene also demonstrated inappropriate expression of the transgene in most proximal colonic crypts. However, the additional 3.3 kilobases of upstream sequence resulted in much more rapid extinction of reporter expression, leaving by 5 months of age only scattered single crypts with detectable levels of hGH. This age-related extinction of L-FABP/hGH expression did not involve enterocytes and enteroendocrine cells in the (proximal) small intestine. These results indicate that cis-acting elements outside of nucleotides -4000 to +21 are necessary to fully modulate suppression of colonic L-FABP expression. They also define fundamental changes in colonic epithelial cell populations during adult life. Our data suggest that (i) a single stem cell gives rise to all cells that populate a given colonic crypt, (ii) stem cells represented in several adjacent crypts may be derived from a common progenitor, and (iii) such a progenitor cell may repopulate colonic crypts with stem cells during adult life. Since each colonic crypt contains the amplified descendants of its stem cell, transgenes may be powerful tools for characterizing the spatial and biological features of gut stem cells and their progenitors during life.

Human liver fatty acid binding protein is a 127 residue cytoplasmic protein synthesized in liver and in the intestinal epithelium. Previous studies of normal and transgenic mice indicated that the liver fatty acid-binding protein gene is a sensitive marker of enterocytic differentiation. This study shows the use of immunohistochemical methods to examine liver fatty acid-binding protein gene expression in normal human colonic epithelium, colonic villoglandular adenomas, nonmucinous and mucinous adenocarcinomas, and several types of noncolonic epithelial neoplasms. Cells containing liver fatty acid-binding protein were found in normal colonic epithelium, in two thirds of colorectal villoglandular adenomas and nonmucinous adenocarcinomas, and in one third of mucinous adenocarcinomas but not in noncolonic, nonhepatic carcinomas. All liver fatty acid-binding protein-positive colonic adenomas and adenocarcinomas contained patches of immunoreactive cells distributed among histologically identical patches of cells without liver fatty acid-binding protein immunoreactivity. This "mosaicism" was also found in metastases from liver fatty acid-binding protein-positive colonic adenocarcinomas. Immunostaining of these liver fatty acid-binding protein-positive tissues for carcinoembryonic antigen did not show a mosaic cellular pattern in its expression. These data suggest that within a given neoplasm, differences exist in the differentiation programs of monoclonally-derived, malignant colonic epithelial cells and that liver fatty acid-binding protein is a useful marker for operationally defining these subpopulations. Liver fatty acid-binding protein is also a potentially useful diagnostic marker for colorectal and hepatic carcinomas.

The mammalian intestinal epithelium undergoes continuous and rapid renewal of its four principal terminally differentiated cell types. These cells arise from multipotent stem cells located at or near the base of the crypts of Lieberkühn. The differentiation process is precisely organized along two spatial dimensions (axes)--from the crypt to the villus tip and from the duodenum to the colon. The enteroendocrine cell population provides a sensitive marker of the intestine's topologic differentiation. At least 15 different regionally distributed subsets have been described based on their principal neuroendocrine products. We have used immunocytochemical methods to characterize the spatial relationships of the serotonin-, secretin-, and substance P-containing enteroendocrine cell subsets in normal adult C57BL/6J x LT/Sv mice as well as in transgenic littermates that contain rat liver fatty acid-binding protein-human growth hormone fusion genes. Our results reveal precise spatial interrelationships between these populations and suggest a differentiation pathway that may involve the sequential expression of substance P, serotonin, and secretin.

The gastrointestinal tract is lined with a monolayer of cells that undergo perpetual and rapid renewal. Four principal, terminally differentiated cell types populate the monolayer, enterocytes, goblet cells, Paneth cells, and enteroendocrine cells. This epithelium exhibits complex patterns of regional differentiation, both from crypt-to-villus and from duodenum-to-colon. The "liver" fatty acid binding protein (L-FABP) gene represents a useful model for analyzing the molecular basis for intestinal epithelial differentiation since it exhibits cell-specific, region-specific, as well as developmental stage specific expression. We have previously linked portions of the 5' nontranscribed domain of the rat L-FABP gene to the human growth hormone (hGH) gene and analyzed expression of the fusion gene in adult transgenic mice. High levels of hGH expression were noted in enterocytes as well as cells that histologically resembled enteroendocrine cells. In the present study, we have used immunocytochemical techniques to map the distribution of enteroendocrine cells in the normal adult mouse gut and to characterize those that synthesize L-FABP. In addition, L-FABP/hGH fusion genes were used to identify subsets of enteroendocrine cells based on their ability to support hGH synthesis in several different pedigrees of transgenic mice. The results reveal remarkable differences in transgene expression between, and within, enteroendocrine cell populations previously classified only on the basis of their neuroendocrine products. In some cases, these differences are related to the position occupied by cells along the duodenal-to-colonic and crypt-to-villus axes of the gut. Thus, transgenes appear to be sensitive tools for examining the cellular and regional differentiation of this class of intestinal epithelial cells.