POU transcription factors control expression of CNS stem cell-specific genes

Molecular characterization of the TrkA/NGF receptor minimal enhancer reveals regulation by multiple cis elements to drive embryonic neuron expression

Neural development relies on stringent regulation of key genes that mediate specialized function. TrkA is primarily expressed in neural crest-derived sensory and sympathetic neurons where it transmits critical survival information. We have identified a 457 base pair sequence upstream of the murine first TrkA coding exon that is conserved in human and in chick, and is sufficient for expression in the correct cells with appropriate timing. Mutation analysis of consensus transcription factor binding domains within the minimal enhancer reveals a complex positive regulation that includes sites required for global expression and sites that are specifically required for DRG, trigeminal or sympathetic expression. These results provide a foundation for identification of the transcriptional machinery that specifies neurotrophin receptor expression.

Sox2 regulatory sequences direct expression of a (beta)-geo transgene to telencephalic neural stem cells and precursors of the mouse embryo, revealing regionalization of gene expression in CNS stem cells

Sox2 is one of the earliest known transcription factors expressed in the developing neural tube. Although it is expressed throughout the early neuroepithelium, we show that its later expression must depend on the activity of more than one regionally restricted enhancer element. Thus, by using transgenic assays and by homologous recombination-mediated deletion, we identify a region upstream of Sox2 (−5.7 to −3.3 kb) which can not only drive expression of a (beta)-geo transgene to the developing dorsal telencephalon, but which is required to do so in the context of the endogenous gene. The critical enhancer can be further delimited to an 800 bp fragment of DNA surrounding a nuclease hypersensitive site within this region, as this is sufficient to confer telencephalic expression to a 3.3 kb fragment including the Sox2 promoter, which is otherwise inactive in the CNS. Expression of the 5.7 kb Sox2(beta)-geo transgene localizes to the neural plate and later to the telencephalic ventricular zone. We show, by in vitro clonogenic assays, that transgene-expressing (and thus G418-resistant) ventricular zone cells include cells displaying functional properties of stem cells, i.e. self-renewal and multipotentiality. We further show that the majority of telencephalic stem cells express the transgene, and this expression is largely maintained over two months in culture (more than 40 cell divisions) in the absence of G418 selective pressure. In contrast, stem cells grown in parallel from the spinal cord never express the transgene, and die in G418. Expression of endogenous telencephalic genes was similarly observed in long-term cultures derived from the dorsal telencephalon, but not in spinal cord-derived cultures. Thus, neural stem cells of the midgestation embryo are endowed with region-specific gene expression (at least with respect to some networks of transcription factors, such as that driving telencephalic expression of the Sox2 transgene), which can be inherited through multiple divisions outside the embryonic environment.

Transcription factors involved in pancreatic islet development

The endocrine pancreas is an organ of enormous importance, since its dysfunction causes diabetes, one of the most common human diseases in the world. Regulation of pancreatic endocrine cell determination and differentiation requires a unique set of transcription factors, including basic helix-loop-helix and homeodomain-containing proteins. The physiological role of individual transcription factor has been characterized by gene disruption in the mouse. The results indicate that these genes are not only involved in tissue-specific activation of downstream target genes for islet-specific hormones, but also critical for the proper islet morphogenesis. Future elucidation of the genetic relationship of these genes will lead to a better understanding of the molecular mechanisms controlling endocrine pancreas formation and will contribute to the development of new therapeutic approaches to diabetes.

The POU domain gene, XlPOU 2 is an essential downstream determinant of neural induction

The POU domain gene, XlPOU 2, acts as a transcriptional activator during mid-gastrulation in Xenopus. Overexpression or misexpression of VP16-POU-GR, a fusion protein consisting of the strong activator domain of VP16 and the POU domain of XlPOU 2, results in ectopic expression of the neural-specific genes, nrp-1, en-2, and beta-tubulin. In contrast, overexpressing a dominant-inhibitory form of XlPOU 2 inhibits the chordin-induced neuralization of uncommitted ectoderm, and results in a loss of nrp-1 and en-2 expression in embryos. Furthermore, in uncommitted ectoderm, XlPOU 2 regulates the developmental neural program that includes a number of pre-pattern genes and at least one proneural gene, X-ngnr-1, thus playing a key role during neural determination.

Marrow-mindedness: a perspective on neuropoiesis

There are pluripotent stem cells in the adult brain that might not be very different from those found in bone marrow. Recent and profound advances in the field of neuropoiesis, which often rely on insights from studies of hematopoiesis and in some instances use cross-paradigms with this field, have already revealed that bone marrow has much in common with so-called 'brain marrow'. Proliferative primogenitors and developmentally regulated molecules are hallmarks of both neuropoiesis and hematopoiesis. This article will focus on recent advances in neuropoiesis within a central core of the mature brain that is referred to as brain marrow, discussing its pluripotency and proliferative capacity, in vitro and molecular assays used in its study, and markers of neuropoietic stem/progenitor cells. As hematopoiesis research has led to the discovery of numerous morphogenetic factors, it is anticipated that studies of neuropoiesis should also uncover many new factors and genes that affect the growth and differentiation of neural cells. Recent breakthroughs in the stem-cell field prompt an inclusion of rationale for the persistence of normal stem/progenitor cells even in the aged brain. By analogy with hematopoiesis research, a thorough investigation of brain marrow should provide basic insights into developmental and persistent neurogenesis while anticipating cell-transplant and gene therapies for debilitating neurological diseases.

An octamer-binding site is crucial for the activity of an enhancer active at the embryonic met-/mesencephalic junction

An enhancer sequence found in the Protease Nexin-1 (PN-1) gene was shown to drive lacZ expression specifically at the met-/mesencephalic junction in transgenic mouse embryos. A functional study of this enhancer has been performed to better understand the mechanisms regulating isthmic gene expression. An octamer-binding site for POU domain factors was found to be crucial for the activity of the enhancer in vivo. Comparative expression studies of POU domain factors, electrophoretic mobility shift assays and transient transfection experiments, strongly suggest that Brn-1/-2 regulate the enhancer activity in vivo. In addition, in vitro experiments indicated that FGF-8 was required for the maintenance of the enhancer activity, but not for the synthesis of Bn-1/-2. The data represents the first functional evidence for a role of POU factors in the regulation of met-/mesencephalic gene expression. It also implies that at least two regulatory pathways, namely the FGF-8 signaling and the octamer-binding site pathway, synergistically interact to control the PN-1 enhancer activity in vivo.

Identification of both general and region-specific embryonic CNS enhancer elements in the nestin promoter

In this report, we investigate how nestin expression is controlled in neural progenitor cells of the embryonic CNS. A 374-bp region in the second intron of the human nestin gene is sufficient, and a 120-bp sequence in this region is required, to express the lacZ reporter gene throughout the developing CNS of E9.5-10.5 transgenic mouse embryos. The 120-bp element region contains putative binding sites for nuclear hormone receptors and we show that TRs, RXR, RAR, and COUP-TF bind to these motifs. A separate enhancer, located most probably 5' to the 120-bp sequence in the second intron, controls midbrain expression at E10.5. In conclusion, our data show that the nestin enhancer in the second intron contains elements both for general and for region-specific CNS progenitor cell expression and suggest that nuclear hormone receptors play a role in the regulation of nestin expression in the early CNS.

Multipotent stem/progenitor cells with similar properties arise from two neurogenic regions of adult human brain

Recent in vitro studies have shown that the periventricular subependymal zone (SEZ) of the rodent brain is capable of de novo generation of neurons and glia. There is less information available on neurogenesis in the adult human brain, and no study has shown the clonal generation of neurons and glia from in vitro-generated "neurospheres." Here we describe the isolation of proliferative stem/progenitor cells within neurospheres from two different regions, the SEZ and the hippocampus, from surgical biopsy specimens of adult (24-57 years) human brain. Using light and electron microscopy; immunocytochemistry for a variety of neuronal, glial, and developmental (including extracellular matrix; ECM) markers; and the reverse transcriptase polymerase chain reaction to demonstrate different gene transcripts found in neurospheres, it is shown that the adult human brain harbors a complex population of stem/progenitor cells that can generate neuronal and glial progeny under particular in vitro growth conditions. These methods also show that these neurospheres contain both neurons and glia and demonstrate regional similarities at the mRNA level, indicating common stem/progenitor cell types within two different neurogenic regions of the adult human brain. In addition to the synthesis of developmentally regulated molecules such as the ECM protein tenascin-C, a variety of other genes (e.g., Pax 6) and proteins (e.g. , Bcl-2) involved in cell survival and differentiation are expressed by adult human brain neurospheres.

Plasticity and stem cells in the vertebrate nervous system

How and when do vertebrate neural precursor cells choose their fates? While some studies suggest a series of commitments on the road to fate choice, many recent experiments indicate that precursor fate choices can often be changed. Additionally, the identification of common gene control mechanisms in precursors suggest that these cells share fundamental properties throughout development.