Random chromosome segregation in mouse intestinal epithelial stem cells.

The mammalian intestinal epithelium is endowed with a high cell turnover sustained by a few stem cells located in the bottoms of millions of crypts. Until recently, it was generally assumed that the extreme sensitivity to DNA damaging agents leading to cell death and the asymmetric mode of chromosome segregation of intestinal epithelial stem cells prevented the illicit survival of mutated stem cells and guarded against mistakes leading to aneuploidy and neoplastic transformation. Recent evidence points instead to a pool of mutipotent self-renewing stem cells capable of repairing DNA by homologous recombination significantly more efficiently than other crypt cells. Furthermore, the equilibrium between cell division and differentiation is achieved at the level of the cell population obeying to a random mode of chromosome segregation and a predominantly symmetric mode of cell division. This review summarizes the experimental findings on the mode of cell division adopted by intestinal epithelial stem cells.

The intestinal epithelium tuft cells: specification and function.

The intestinal epithelium, composed of at least seven differentiated cell types, represents an extraordinary model to understand the details of multi-lineage differentiation, a question that is highly relevant in developmental biology as well as for clinical applications. This review focuses on intestinal epithelial tuft cells that have been acknowledged as a separate entity for more than 60 years but whose function remains a mystery. We discuss what is currently known about the molecular basis of tuft cell fate and differentiation and why elucidating tuft cell function has been so difficult. Finally, we summarize the current hypotheses on their potential involvement in diseases of the gastro-intestinal tract.

Expression of S100B during embryonic development of the mouse cerebellum.

BACKGROUND: In the cerebellum of newborn S100B-EGFP mice, we had previously noted the presence of a large population of S100B-expressing cells, which we assumed to be immature Bergmann glial cells. In the present study, we have drawn on this observation to establish the precise spatio-temporal pattern of S100B gene expression in the embryonic cerebellum. RESULTS: From E12.5 until E17.5, S100B was expressed in the primary radial glial scaffold involved in Purkinje progenitor exit from the ventricular zone and in the Sox9+ glial progenitors derived from it. During the same period coinciding with the primary phase of granule neuron precursor genesis, transient EGFP expression tagged the Pax6+ forerunners of granule precursors born in the cerebellar rhombic lip. CONCLUSION: This study provides the first characterization of S100B-expressing cell types of the embryonic mouse cerebellum in a high-resolution map. The transient activation of the S100B gene distinguishes granule neuron precursors from all other types of precursors so far identified in the rhombic lip, whereas its activation in radial glial precursors is a feature of Bergmann cell gliogenesis.

Visualization of S100B-positive neurons and glia in the central nervous system of EGFP transgenic mice.

S100B, the EF-hand Ca(++)-binding protein with gliotrophic and neurotrophic properties implicated in the pathogenesis of Alzheimer's disease, is coined as a glial marker, despite its documented presence in rodent brain neurons. We have generated a transgenic mouse whose EGFP reporter, controlled by the -1,669/+3,106 sequence of the murine S100B gene, allows the direct microscopic observation of most S100B-expressing cells in the central nervous system (CNS). From embryonic day 13 onward, EGFP expression was targeted to selected neuroepithelial, glial, and neuronal cells, indicating that cell-specific expression of S100B is regulated at the transcriptional level during development. In adult mice, the highest level of EGFP expression was found in ependymocytes; astrocytes; and spinal, medullar, pontine, and deep cerebellar S100B neurons. Our results, thus, agree with earlier reports suggesting that S100B is not a CNS glial-specific marker. In addition, we detected EGFP and S100B in forebrain neurons previously thought not to express S100B in the mouse, including neurons of primary motor and somatosensory neocortical areas, the ventral pallidum and prerubral field. Another interesting finding was the selected EGFP targeting to neonatal S100B oligodendrocytes and adult NG2 progenitors as opposed to mature S100B oligodendrocytes. This finding suggests that, except for oligodendrocytes at the last stage of myelin maturation, the -1,669/+3,106 sequence of the S100B gene is a useful reagent for driving expression of transgenes in most S100B-expressing cells of mouse brain.

Intestinal epithelial stem cells do not protect their genome by asymmetric chromosome segregation.

The idea that stem cells of adult tissues with high turnover are protected from DNA replication-induced mutations by maintaining the same 'immortal' template DNA strands together through successive divisions has been tested in several tissues. In the epithelium of the small intestine, the provided evidence was based on the assumption that stem cells are located above Paneth cells. The results of genetic lineage-tracing experiments point instead to crypt base columnar cells intercalated between Paneth cells as bona fide stem cells. Here we show that these cells segregate most, if not all, of their chromosomes randomly, both in the intact and in the regenerating epithelium. Therefore, the 'immortal' template DNA strand hypothesis does not apply to intestinal epithelial stem cells, which must rely on other strategies to avoid accumulating mutations.

S100B expression defines a state in which GFAP-expressing cells lose their neural stem cell potential and acquire a more mature developmental stage.

During the postnatal development, astrocytic cells in the neocortex progressively lose their neural stem cell (NSC) potential, whereas this peculiar attribute is preserved in the adult subventricular zone (SVZ). To understand this fundamental difference, many reports suggest that adult subventricular GFAP-expressing cells might be maintained in immature developmental stage. Here, we show that S100B, a marker of glial cells, is absent from GFAP-expressing cells of the SVZ and that its onset of expression characterizes a terminal maturation stage of cortical astrocytic cells. Nevertheless, when cultured in vitro, SVZ astrocytic cells developed as S100B expressing cells, as do cortical astrocytic cells, suggesting that SVZ microenvironment represses S100B expression. Using transgenic s100b-EGFP cells, we then demonstrated that S100B expression coincides with the loss of neurosphere forming abilities of GFAP expressing cells. By doing grafting experiments with cells derived from beta-actin-GFP mice, we next found that S100B expression in astrocytic cells is repressed in the SVZ, but not in the striatal parenchyma. Furthermore, we showed that treatment with epidermal growth factor represses S100B expression in GFAP-expressing cells in vitro as well as in vivo. Altogether, our results indicate that the S100B expression defines a late developmental stage after which GFAP-expressing cells lose their NSC potential and suggest that S100B expression is repressed by adult SVZ microenvironment.