Lineage analysis using retrovirus vectors

Transcriptional regulation of MGE progenitor proliferation by PRDM16 controls cortical GABAergic interneuron production

The mammalian cortex is populated by neurons derived from neural progenitors located throughout the embryonic telencephalon. Excitatory neurons are derived from the dorsal telencephalon, whereas inhibitory interneurons are generated in its ventral portion. The transcriptional regulator PRDM16 is expressed by radial glia, neural progenitors present in both regions; however, its mechanisms of action are still not fully understood. It is unclear whether PRDM16 plays a similar role in neurogenesis in both dorsal and ventral progenitor lineages and, if so, whether it regulates common or unique networks of genes. Here, we show that Prdm16 expression in mouse medial ganglionic eminence (MGE) progenitors is required for maintaining their proliferative capacity and for the production of proper numbers of forebrain GABAergic interneurons. PRDM16 binds to cis-regulatory elements and represses the expression of region-specific neuronal differentiation genes, thereby controlling the timing of neuronal maturation. PRDM16 regulates convergent developmental gene expression programs in the cortex and MGE, which utilize both common and region-specific sets of genes to control the proliferative capacity of neural progenitors, ensuring the generation of correct numbers of cortical neurons.

Development of Ependymal and Postnatal Neural Stem Cells and Their Origin from a Common Embryonic Progenitor

The adult mouse brain contains an extensive neurogenic niche in the lateral walls of the lateral ventricles. This epithelium, which has a unique pinwheel organization, contains multiciliated ependymal (E1) cells and neural stem cells (B1). This postnatal germinal epithelium develops from the embryonic ventricular zone, but the lineage relationship between E1 and B1 cells remains unknown. Distinct subpopulations of radial glia (RG) cells in late embryonic and early postnatal development either expand their apical domain >11-fold to form E1 cells or retain small apical domains that coalesce into the centers of pinwheels to form B1 cells. Using independent methods of lineage tracing, we show that individual RG cells can give rise to clones containing E1 and B1 cells. This study reveals key developmental steps in the formation of the postnatal germinal niche and the shared cellular origin of E1 and B1 cells.

Linking neuronal lineage and wiring specificity

Brain function requires precise neural circuit assembly during development. Establishing a functional circuit involves multiple coordinated steps ranging from neural cell fate specification to proper matching between pre- and post-synaptic partners. How neuronal lineage and birth timing influence wiring specificity remains an open question. Recent findings suggest that the relationships between lineage, birth timing, and wiring specificity vary in different neuronal circuits. In this review, we summarize our current understanding of the cellular, molecular, and developmental mechanisms linking neuronal lineage and birth timing to wiring specificity in a few specific systems in Drosophila and mice, and review different methods employed to explore these mechanisms.

Wide Dispersion and Diversity of Clonally Related Inhibitory Interneurons

The mammalian neocortex is composed of two major neuronal cell types with distinct origins: excitatory pyramidal neurons and inhibitory interneurons, generated in dorsal and ventral progenitor zones of the embryonic telencephalon, respectively. Thus, inhibitory neurons migrate relatively long distances to reach their destination in the developing forebrain. The role of lineage in the organization and circuitry of interneurons is still not well understood. Utilizing a combination of genetics, retroviral fate mapping, and lineage-specific retroviral barcode labeling, we find that clonally related interneurons can be widely dispersed while unrelated interneurons can be closely clustered. These data suggest that migratory mechanisms related to the clustering of interneurons occur largely independent of their clonal origin.

Decoding astrocyte heterogeneity: New tools for clonal analysis

The importance of astrocyte heterogeneity came out as a hot topic in neurosciences especially over the last decades, when the development of new methodologies allowed demonstrating the existence of big differences in morphological, neurochemical and physiological features between astrocytes. However, although the knowledge about the biology of astrocytes is increasing rapidly, an important characteristic that remained unexplored, until the last years, has been the relationship between astrocyte lineages and cell heterogeneity. To fill this gap, a new method called StarTrack was recently developed, a powerful genetic tool that allows tracking astrocyte lineages forming cell clones. Using StarTrack, a single astrocyte progenitor and its progeny can be specifically labeled from its generation, during embryonic development, to its final fate in the adult brain. Because of this specific labeling, astrocyte clones, exhibiting heterogeneous morphologies and features, can be easily analyzed in relation to their ontogenetic origin. This review summarizes how astrocyte heterogeneity can be decoded studying the embryonic development of astrocyte lineages and their clonal relationship. Finally, we discuss about some of the challenges and opportunities emerging in this exciting area of investigation.

CLoNe is a new method to target single progenitors and study their progeny in mouse and chick

Cell lineage analysis enables us to address pivotal questions relating to: the embryonic origin of cells and sibling cell relationships in the adult body; the contribution of progenitors activated after trauma or disease; and the comparison across species in evolutionary biology. To address such fundamental questions, several techniques for clonal labelling have been developed, each with its shortcomings. Here, we report a novel method, CLoNe that is designed to work in all vertebrate species and tissues. CLoNe uses a cocktail of labelling, targeting and transposition vectors that enables targeting of specific subpopulations of progenitor types with a combination of fluorophores resulting in multifluorescence that describes multiple clones per specimen. Furthermore, transposition into the genome ensures the longevity of cell labelling. We demonstrate the robustness of this technique in mouse and chick forebrain development, and show evidence that CLoNe will be broadly applicable to study clonal relationships in different tissues and species.

Lung epithelial healing: a modified seed and soil concept

Airway epithelial healing is defined as restoration of health or soundness; to cure. Our research indicates that two types of progenitor cells participate in this process: the tissue-specific stem cell (TSC) and the facultative basal progenitor (FBP). The TSC restores the epithelium to its normal structure and function. Thus, the TSC regenerates the epithelium. In contrast, the FBP-derived epithelium is characterized by regions of cellular hyperplasia and hypoplasia. Since the FBP-derived epithelium deviates from normal, we term the FBP-mediated process repair. Our work indicates that the TSC responds to signals from other epithelial cells, including the FBP. These signals instruct the TSC to proliferate or to select one of several differentiation pathways. We interpret these data in the context of Stephen Padget's "seed and soil" paradigm. Therein, Padget explained that metastasis of a tumor, the seed, to a specific site, the soil, was determined by the growth and differentiation requirements of the tumor cell. By extending the seed and soil paradigm to airway epithelial healing, we suggest that proliferation and differentiation of the TSC, the seed, is determined by its interactions with other cell types, the soil. Based on this concept, we provide a set of suggestions for development of cell-based therapies that are directed toward chronic airways disease.

Dynamic lineage analysis of embryonic morphogenesis using transgenic quail and 4D multispectral imaging

We describe the development of transgenic quail that express various fluorescent proteins in targeted manners and their use as a model system that integrates advanced imaging approaches with conventional and emerging molecular genetics technologies. We also review the progression and complications of past fate mapping techniques that led us to generate transgenic quail, which permit dynamic imaging of amniote embryogenesis with unprecedented subcellular resolution.

Mathematical and experimental approaches to identify and predict the effects of chemotherapy on neuroglial precursors

The adverse effects of chemotherapy on normal cells of the body create substantial clinical problems for many cancer patients. However, relatively little is known about the effects, other than promotion of cell death, of such agents on the function of normal precursor cells critical in tissue homeostasis and repair. We have combined mathematical and experimental analyses to identify the effects of sublethal doses of chemotherapy on glial precursor cells of the central nervous system. We modeled the temporal development of a population of precursor and terminally differentiated cells exposed to sublethal doses of carmustine (BCNU), a classic alkylating chemotherapeutic agent used in treatment of gliomas and non-Hodgkin's lymphomas, as a multitype age-dependent branching process. We fitted our model to data from in vitro clonal experiments using the method of pseudo-likelihood. This approach identifies several novel drug effects, including modification of the cell cycle length, the time between division and differentiation, and alteration in the probability of undergoing self-renewal division in precursor cells. These changes of precursor cell function in the chemotherapy-exposed brain may have profound clinic implications.

MAJOR FINDINGS

We applied our computational approach to analyze the effects of BCNU on clonal cultures of oligodendrocyte progenitor cells-one of the best-characterized neural progenitor cells in the mammalian brain. Our analysis reveals that transient exposures to BCNU increased the cell cycle length of progenitor cells and decreased their time to differentiation, while also decreasing the likelihood that they will undergo self-renewing divisions. By investigating the behavior of our mathematical model, we demonstrate that precursor cell populations should recover spontaneously from transient modifications of the timing of division and of differentiation, but such recovery will not happen after alteration of cell fate. These studies identify means by which precursor cell function can be critically compromised by transient exposure to chemotherapy with long-term consequences on the progenitor cell pool even in the absence of drug-induced apoptosis. These analyses also provide novel tools that apply broadly to identify effects of chemotherapeutic agents and other physiological stressors.

Integration of embryonic and fetal skeletal myogenic programs at the myosin light chain 1f/3f locus

The genetic control of skeletal muscle differentiation at the onset of myogenesis in the embryo is relatively well understood compared to the formation of muscle during the fetal period giving rise to the bulk of skeletal muscle fibers at birth. The Mlc1f/3f (Myl1) locus encodes two alkali myosin light chains, Mlc1f and Mlc3f, from two promoters that are differentially regulated during development. The Mlc1f promoter is active in embryonic, fetal and adult fast skeletal muscle whereas the Mlc3f promoter is upregulated during fetal development and remains on in adult fast skeletal muscle. Two enhancer elements have been identified at the mammalian Mlc1f/3f locus, a 3' element active at all developmental stages and an intronic enhancer activated during fetal development. Here, using transgenesis, we demonstrate that these enhancers act combinatorially to confer the spatial, temporal and quantitative expression profile of the endogenous Mlc3f promoter. Using double reporter transgenes we demonstrate that each enhancer can activate both Mlc1f and Mlc3f promoters in vivo, revealing enhancer sharing rather than exclusive enhancer-promoter interactions. Finally, we demonstrate that the fetal activated enhancer contains critical E-box myogenic regulatory factor binding sites and that enhancer activation is impaired in vivo in the absence of myogenin but not in the absence of innervation. Together our observations provide insights into the regulation of fetal myogenesis and the mechanisms by which temporally distinct genetic programs are integrated at a single locus.

Protocols for studying adult neurogenesis: insights and recent developments

The first evidence that neurogenesis occurs in the adult brain was reported in rodents in the early 1960s, using [(3)H]-thymidine autoradiography. In the 1980s and 90s, the advent of new techniques and protocols for studying cell proliferation in situ, and particularly bromodeoxyuridine labeling, helped to confirm that neurogenesis occurs in the adult brain and neural stem cells reside in the adult CNS, including in humans. Bromodeoxyuridine labeling is currently the method most commonly used for studying neurogenesis in the adult brain. However, this procedure is not without limitations, and controversies. In this article, I will review recent protocols for studying adult neurogenesis, particularly new protocols for studying cell kinetics and cell proliferative history, using halopyrimidines. I will review these techniques, and discuss their implications for the field of adult neurogenesis.

Pdm and Castor specify late-born motor neuron identity in the NB7-1 lineage

Embryonic development requires generating cell types at the right place (spatial patterning) and the right time (temporal patterning). Drosophila neuroblasts undergo stem cell-like divisions to generate an ordered sequence of neuronal progeny, making them an attractive system to study temporal patterning. Embryonic neuroblasts sequentially express Hunchback, Krüppel, Pdm1/Pdm2 (Pdm), and Castor (Cas) transcription factors. Hunchback and Krüppel specify early-born temporal identity, but the role of Pdm and Cas in specifying temporal identity has never been addressed. Here we show that Pdm and Cas regulate late-born motor neuron identity within the NB7-1 lineage: Pdm specifies fourth-born U4 motor neuron identity, while Pdm/Cas together specify fifth-born U5 motor neuron identity. We conclude that Pdm and Cas specify late-born neuronal identity; that Pdm and Cas act combinatorially to specify a temporal identity distinct from either protein alone, and that Cas repression of pdm expression regulates the generation of neuronal diversity.

Transgenic analysis of the stem cell leukemia +19 stem cell enhancer in adult and embryonic hematopoietic and endothelial cells

Appropriate transcriptional regulation is critical for the biological functions of many key regulatory genes, including the stem cell leukemia (SCL) gene. As part of a systematic dissection of SCL transcriptional regulation, we have previously identified a 5,245-bp SCL +18/19 enhancer that targeted embryonic endothelium together with embryonic and adult hematopoietic progenitors and stem cells (HSCs). This enhancer is proving to be a powerful tool for manipulating hematopoietic progenitors and stem cells, but the design and interpretation of such transgenic studies require a detailed understanding of enhancer activity in vivo. In this study, we demonstrate that the +18/19 enhancer is active in mast cells, megakaryocytes, and adult endothelium. A 644-bp +19 core enhancer exhibited similar temporal and spatial activity to the 5,245-bp +18/19 fragment both during development and in adult mice. Unlike the +18/19 enhancer, the +19 core enhancer was only active in adult mice when linked to the eukaryotic reporter gene human placental alkaline phosphatase. Activity of a single core enhancer in HSCs, endothelium, mast cells, and megakaryocytes suggests possible overlaps in their respective transcriptional programs. Moreover, activity in a proportion of thymocytes and other SCL-negative cell types suggests the existence of a silencer elsewhere in the SCL locus.

A subset of mouse tracheal epithelial basal cells generates large colonies in vitro

Airway epithelial stem cells are not well characterized. To examine clonal growth potential, we diluted single, viable B6.129S7-Gtrosa26 (Rosa26) mouse tracheal epithelial cells that constitutively express -galactosidase into non-Rosa26 cells in an air-liquid interface cell culture model; 1.7% of the cells formed colonies of varying size, and, on average, 0.1% of the cells formed large colonies. Thus only a small subset of cells displayed progenitorial capacity suggestive of stem or early transient amplifying cells. Prior studies identified cells with high keratin 5 (K5) promoter activity in specific niches in the mouse trachea and these cells corresponded to the location of bromodeoxyuridine label-retaining cells, thought to be stem cells (Borthwick DW, Shahbazian M, Todd KQ, Dorin JR, and Randell SH, Am J Respir Cell Mol Biol: 24: 662-670, 2001). To explore the hypothesis that stem cells were present in the K5-expressing compartment, we created transgenic mice in which enhanced green fluorescent protein (EGFP) was driven by the K5 promoter. These mice expressed EGFP in most basal cells of the body including a subset of tracheal basal cells apparently located in positions similar to previously identified stem cell niches. Flow cytometrically purified EGFP-positive cells had an overall colony-forming efficiency 4.5-fold greater than EGFP-negative cells, but the ability to generate large colonies was 12-fold greater. Thus adult mouse tracheal epithelial cells with progenitorial capacity sufficient to generate large colonies reside in the basal cell compartment. These studies are a first step toward purification and characterization of airway epithelial stem cells.

Expression of transgenes in primary neurons from chick peripheral and central nervous systems by retroviral infection of early embryos

Many cell biological studies require the expression of transgenes in cells in culture, but it is difficult to obtain uniform, stable, and efficient expression of transgenes in primary neurons. We have approached this problem by adapting from developmental biologists the avian retroviral vector, RCAS. This vector allows the introduction of a transgene by infection early in chick embryonic development. Transgenes that are less than 2.6 kb in size can be cloned through an adapter vector, SLAX 12 NCO, and into the RCAS retroviral vector with relative ease. The vector is then used to produce active virus, and the virus is injected into the neural tube or ventricles of stage 10 embryos. By infecting the neuronal precursor cells while they are still mitotic, the retrovirus and accompanying transgene are introduced into the genome and subsequently spread by replication, shedding of new virus, and infection of other cells. Embryos are incubated from the time of injection until E9-E12 and peripheral and central nervous system neurons are dissected out and grown in culture using standard techniques. In this manner, the majority of the sympathetic and dorsal root ganglion neurons can be induced to express the trangene. A similar result, at lower efficiencies, is obtained for central nervous system neurons.

Caspase processing activates atypical protein kinase C zeta by relieving autoinhibition and destabilizes the protein

Treatment of HeLa cells with tumour necrosis factor alpha (TNFalpha) induced caspase processing of ectopic PKC (protein kinase C) zeta, which converted most of the holoenzyme into the freed kinase domain and increased immune-complex kinase activity. The goal of the present study was to determine the basis for the increased kinase activity that is associated with caspase processing of PKC zeta. Atypical PKC iota is largely identical with PKC zeta, except for a 60-amino-acid segment that lacks the caspase-processing sites of the zeta isoform. Replacement of this segment of PKC zeta with the corresponding segment of PKC iota prevented caspase processing and activation of the kinase function. Processing of purified recombinant PKC zeta by caspase 3 in vitro markedly increased its kinase activity. Caspase processing activated PKC zeta in vitro or intracellularly without increasing the phosphorylation of Thr410 of PKC zeta, which is required for catalytic competency. The freed kinase domain of PKC zeta had a much shorter half-life than the holoenzyme in transfected HeLa cells and in non-transfected kidney epithelial cells. Treatment with TNF-alpha shortened the half-life of the kinase domain protein, and proteasome blockade stabilized the protein. Studies of kinase-domain mutants indicate that a lack of negative charge at Thr410 can shorten the half-life of the freed kinase domain. The present findings indicate that the freed kinase domain has substantially higher kinase activity and a much shorter half-life than the holoenzyme because of accelerated degradation by the ubiquitin-proteasome system.

Preservation of caspase-3 subunits from degradation contributes to apoptosis evoked by lactacystin: any single lysine or lysine pair of the small subunit is sufficient for ubiquitination

Procaspase-3 (p32) is processed by upstream caspases to p12 and p20 subunits, which heterodimerize. Concomitant with formation of the active heterotetramer, p20 is autoprocessed to p17. Treatment of HL-60 cells with lactacystin, a selective inhibitor of the proteasome, exponentially increased caspase-3-like hydrolytic activity and induced apoptosis but had little or no effect on the activity of upstream caspase-8, caspase-9, or granzyme B. Lactacystin treatment decreased the p32 zymogen and evoked the accumulation of the p17 and p12 subunits. Treatment of transfected human retinoblast 911 cells with a proteasome inhibitor evoked the accumulation of epitope-tagged p12, p17, and p20 but had no effect on p32 zymogen. This result suggests that caspase-3 subunits, in contrast to the zymogen, are unstable because of degradation by the ubiquitin-proteasome system. Ubiquitin conjugates of p12 and p17 accumulated in cells that were cotransfected with p12 and a caspase inactive mutant of p17. Substitution of arginine for all eight lysines of p12 almost abolished its ubiquitination. Any single lysine or lysine pair was sufficient for p12 ubiquitination. Lactacystin treatment of HL-60 cells induced proteolytic processing of the X-linked inhibitor of apoptosis (XIAP) and decreased full-length XIAP, which is known to have ubiquitin-protein ligase activity for active caspase-3. These findings indicate that caspase-3 subunits can be degraded by the ubiquitin-proteasome system and suggest that lactacystin induces apoptosis in part by disabling the ubiquitin-protein ligase function of XIAP and by stabilizing active caspase-3 subunits.

A review of inner ear fate maps and cell lineage studies

A renewed interest in the development of the inner ear has provided more data on the fate and cell lineage relationships of the tissues making up this complex structure. The inner ear develops from a simple ectodermal thickening of the head called the otic placode, which undergoes a great deal of growth and differentiation to form a multichambered nonsensory epithelium that houses the six to nine sensory organs of the inner ear. Despite a large number of studies examining otic development, there have been surprisingly few fate maps generated. The published fate maps encompass four species and range from preotic to otocyst stages. Although some of these studies were consistent with a compartment and boundary model, other studies reveal extensive cell mixing during development. Cell lineage studies have been done in fewer species. At the single cell level the resulting clones in both chicks and frogs appear somewhat restricted in terms of distribution. We conclude that up until late placode stages there are no clear lineage restriction boundaries, meaning that cells seem to mix extensively at these early stages. At late placode stages, when the otic cup has formed, there are at least two boundaries located dorsally in the forming otocyst but none ventrally. These conclusions are consistent with all the fate maps and reconciles the chick and frog data. These results suggest that genes involved in patterning the inner ear may have dynamic and complex expression patterns.

Lack of constitutive activity of the free kinase domain of protein kinase C zeta. Dependence on transphosphorylation of the activation loop

Following the induction of apoptosis in mammalian cells, protein kinase C zeta (PKC zeta) is processed between the regulatory and catalytic domains by caspases, which increases its kinase activity. The catalytic domain fragments of PKC isoforms are considered to be constitutively active, because they lack the autoinhibitory amino-terminal regulatory domain, which includes a pseudosubstrate segment that plugs the active site. Phosphorylation of the activation loop at Thr(410) is known to be sufficient to activate the kinase function of full-length PKC zeta, apparently by inducing a conformational change, which displaces the amino-terminal pseudosubstrate segment from the active site. Amino acid substitutions for Thr(410) of the catalytic domain of PKC zeta (CAT zeta) essentially abolished the kinase function of ectopically expressed CAT zeta in mammalian cells. Similarly, substitution of Ala for a Phe of the docking motif for phosphoinositide-dependent kinase-1 prevented activation loop phosphorylation and abolished the kinase activity of CAT zeta. Treatment of purified CAT zeta with the catalytic subunit of protein phosphatase 1 decreased activation loop phosphorylation and kinase activity. Recombinant CAT zeta from bacteria lacked detectable kinase activity. Phosphoinositide-dependent kinase-1 phosphorylated the activation loop and activated recombinant CAT zeta from bacteria. Treatment of HeLa cells with fetal bovine serum markedly increased the phosphothreonine 410 content of CAT zeta and stimulated its kinase activity. These findings indicate that the catalytic domain of PKC zeta is intrinsically inactive and dependent on the transphosphorylation of the activation loop.

Size distribution of retrovirally marked lineages matches prediction from population measurements of cell cycle behavior

Mechanisms that regulate neuron production in the developing mouse neocortex were examined by using a retroviral lineage marking method to determine the sizes of the lineages remaining in the proliferating population of the ventricular zone during the period of neuron production. The distribution of clade sizes obtained experimentally in four different injection-survival paradigms (E11-E13, E11-E14, E11-E15, and E12-E15) from a total of over 500 labeled lineages was compared with that obtained from three models in which the average behavior of the proliferating population [i.e., the proportion of cells remaining in the proliferative population (P) vs. that exiting the proliferative population (Q)] was quantitatively related to lineage size distribution. In model 1, different proportions of asymmetric, symmetric terminal, and symmetric nonterminal cell divisions coexisted during the entire developmental period. In model 2, the developmental period was divided into two epochs: During the first, asymmetric and symmetric nonterminal cell divisions occurred, but, during the second, asymmetric and symmetric terminal cell divisions occurred. In model 3, the shifts in P and Q are accounted for by changes in the proportions of the two types of symmetric cell divisions without the inclusion of any asymmetric cell divisions. The results obtained from the retroviral experiments were well accounted for by model 1 but not by model 2 or 3. These findings demonstrate that: 1) asymmetric and both types of symmetric cell divisions coexist during the entire period of neurogenesis in the mouse, 2) neuron production is regulated in the proliferative population by the independent decisions of the two daughter cells to reenter S phase, and 3) neurons are produced by both asymmetric and symmetric terminal cell divisions. In addition, the findings mean that cell death and/or tangential movements of cells in the proliferative population occur at only a low rate and that there are no proliferating lineages "reserved" to make particular laminae or cell types.

Evidence of common progenitors and patterns of dispersion in rat striatum and cerebral cortex

To correlate clonal patterns in the rat striatum with adult neuronal phenotypes, we labeled striatal progenitors between embryonic day 14 (E14) and E19 with a retroviral library encoding alkaline phosphatase. In the adult striatum, the majority of E14-labeled neurons (87%) were members of discrete horizontal or radial cell clusters. Radial clusters accounted for only 23% of cell clusters but >34% of labeled cells. Striatal clones also demonstrated an unexpected widespread pattern of clonal dispersion. The majority of striatal clones were widely dispersed within the striatum, and 80% of clones were part of even larger clones that included cortical interneurons. Finally, we observed that PCR-positive cortical interneurons were members of clones containing both interneurons and pyramids (44%), exclusively interneuron clones (24%), or combined striatal-cortical clones (16%), consistent with the view that cortical interneurons have multiple origins in differentially behaving progenitor cells. Our data are also consistent with the notion that similar mechanisms underpin striatal and cortical development.

Prolonged seizures increase proliferating neuroblasts in the adult rat subventricular zone-olfactory bulb pathway

Neuronal precursors in the adult rodent forebrain subventricular zone (SVZ) proliferate, migrate to the olfactory bulb in a restricted pathway known as the rostral migratory stream (RMS), and differentiate into neurons. The effects of injury on this neurogenic region of the mature brain are poorly understood. To determine whether seizure-induced injury modulates SVZ neurogenesis, we induced status epilepticus (SE) in adult rats by systemic chemoconvulsant administration and examined patterns of neuronal precursor proliferation and migration in the SVZ-olfactory bulb pathway. Within 1-2 weeks after pilocarpine-induced SE, bromodeoxyuridine (BrdU) labeling and Nissl staining increased in the rostral forebrain SVZ. These changes were associated with an increase in cells expressing antigenic markers of SVZ neuroblasts 2-3 weeks after prolonged seizures. At these same time points the RMS expanded and contained more proliferating cells and immature neurons. BrdU labeling and stereotactic injections of retroviral reporters into the SVZ showed that prolonged seizures also increased neuroblast migration to the olfactory bulb and induced a portion of the neuronal precursors to exit the RMS prematurely. These findings indicate that SE expands the SVZ neuroblast population and alters neuronal precursor migration in the adult rat forebrain. Identification of the mechanisms underlying the response of neural progenitors to seizure-induced injury may help to advance brain regenerative therapies by using either transplanted or endogenous neural precursor cells.

Is it all DNA repair? Methodological considerations for detecting neurogenesis in the adult brain

Since the early 1960s, in vivo observations have shown the generation of new neurons from dividing precursor cells. Nevertheless, these experiments suffered from skepticism, suggesting that the prevailing labeling method, which incorporates tagged thymidine analogs, such as [3H]-thymidine or bromodeoxyuridine (BrdU), may not be detecting a proliferative event, but could rather mark DNA repair in postmitotic neurons. Even today many scientists outside the field are still skeptical, because the question of specificity for thymidine labeling has not been sufficiently answered. This current paper aims at evaluating the arguments that are used by proponents and skeptics of this method by (i) presenting histological evidence of specificity of BrdU labeling for neural stem cell/progenitor activity in the adult brain; (ii) validating and comparing BrdU labeling with other histological methods; and (iii) combining BrdU and labeling methods for apoptosis to argue against DNA repair being a major contribution of BrdU-positive cells.

Evidence for stem-cell niches in the tracheal epithelium

It is generally important to elucidate airway epithelial cell lineages and to identify multipotent progenitors as targets for gene therapy. Stem (S) cells are typically present in specialized compartments spatially proximal to their differentiated progeny, but an equivalent paradigm has not been demonstrated in the airway. We discovered a distinct population of cells displaying high levels of keratin expression in murine tracheal submucosal gland ducts, and tested the hypothesis that bromodeoxyuridine (BrdU) label-retaining cells (LRCs), thought to represent the S-cells, were present in this compartment. Mice received weekly epithelial damage by intratracheal detergent or SO(2) inhalation for 4 wk and received intraperitoneal injections of BrdU every 48 h during the injury and repair period. At 3 and 6 d after injury, BrdU-positive epithelial cells were noted along the entire tracheal length in both basal and lumenal cell positions. At later time points (20 and 95 d) LRCs were localized to gland ducts in the upper trachea and to systematically arrayed foci in the lower trachea, typically near the cartilage-intercartilage junction. LRCs were not pulmonary neuroendocrine cells. Heterotopic tracheal grafts after surface epithelial removal demonstrated reconstitution of a surface-like epithelium from gland remnants. These results suggest that airway epithelial S cells are localized to specific niches.

Spatiotemporal selectivity of response to Notch1 signals in mammalian forebrain precursors

The olfactory bulb, neocortex and archicortex arise from a common pool of progenitors in the dorsal telencephalon. We studied the consequences of supplying excess Notch1 signal in vivo on the cellular and regional destinies of telencephalic precursors using bicistronic replication defective retroviruses. After ventricular injections mid-neurogenesis (E14.5), activated Notch1 retrovirus markedly inhibited the generation of neurons from telencephalic precursors, delayed the emergence of cells from the subventricular zone (SVZ), and produced an augmentation of glial progeny in the neo- and archicortex. However, activated Notch1 had a distinct effect on the progenitors of the olfactory bulb, markedly reducing the numbers of cells of any type that migrated there. To elucidate the mechanism of the cell fate changes elicited by Notch1 signals in the cortical regions, short- and long-term cultures of E14.5 telencephalic progenitors were examined. These studies reveal that activated Notch1 elicits a cessation of proliferation that coincides with an inhibition of the generation of neurons. Later, during gliogenesis, activated Notch1 triggers a rapid cellular proliferation with a significant increase in the generation of cells expressing GFAP. To examine the generation of cells destined for the olfactory bulb, we used stereotaxic injections into the early postnatal anterior subventricular zone (SVZa). We observed that precursors of the olfactory bulb responded to Notch signals by remaining quiescent and failing to give rise to differentiated progeny of any type, unlike cortical precursor cells, which generated glia instead of neurons. These data show that forebrain precursors vary in their response to Notch signals according to spatial and temporal cues, and that Notch signals influence the composition of forebrain regions by modulating the rate of proliferation of neural precursor cells.

Gene transfer into chicken embryos by retrovirus vectors

While chickens have many properties that are advantageous for embryological studies, their genetic analysis has been restricted. However, by using retrovirus vector systems in combination with classical techniques of experimental developmental biology, it has recently become possible to analyze the function of genes involved in the development of this organism. Avian retrovirus vectors are unique in that they can be divided into two categories: replication-competent and replication-defective (replication-incompetent). By choosing the vectors correctly, there are many experimental applications of these vectors such as induction of constitutive (or regulated) gene expression in a restricted region of tissues, organs and embryos; cell lineage analysis; and formation of concentration gradients of morphogens in micromass cultures. In this paper, several retrovirus vectors available for the chicken will be introduced and their applications in developmental biology will be reviewed.

New approach to cell lineage analysis in mammals using the Cre-loxP system

The Cre-loxP site-specific recombination system was used for cell lineage analysis in mammals. We constructed an expression plasmid, pCETZ-17, which consists of cytomegalovirus enhancer/chicken beta-actin promoter (CAG), a portion of the rabbit beta-globin gene, loxP-flanked DNA sequence (containing enhanced green fluorescent protein (EGFP) cDNA), and lacZ gene encoding E. coli beta-galactosidase (beta-gal). When circular pCETZ-17 plasmid DNA was microinjected into the pronuclei of fertilized eggs and these eggs were allowed to develop to two-cell stage, 62.8% (59/94) of the two-cell embryos exhibited distinct fluorescence in one or both blastomeres, but never expressed lacZ protein, as evaluated by histochemical staining with X-Gal, a substrate for beta-gal. When both circular plasmids, pCETZ-17 and pCAG/NCre (containing CAG and DNA sequences encoding nuclear location signal and Cre), were co-injected into fertilized eggs, almost all (87.0%, 47/54) embryos exhibited low or no fluorescence, but 51.9% (27/52) exhibited positive staining for beta-gal activity. This indicates that transient expression of the Cre recombinase gene removed the loxP-flanked DNA sequence in pCETZ-17 and then caused expression of the downstream lacZ sequence. We next microinjected pCETZ-17 into the pronuclei of fertilized eggs, cultured these injected eggs for 1 day, and collected only two-cell embryos expressing EGFP in both blastomeres. One blastomere of the EGFP-expressing two-cell embryos was microinjected with pCAG/NCre, and these treated embryos were cultured for 1 day up to four-cell stage. When the developing four-cell embryos were subjected to staining with X-Gal, cell lineage-related staining pattern for beta-gal activity was observed in most (77.8%, 7/9) embryos. These findings were further confirmed using two-cell embryos derived from a transgenic mouse line carrying CETZ-17 transgene. Thus, our system, which is based on transient expression of the Cre recombinase gene directly introduced into nuclei of embryonic cells by microinjection, is a powerful means for cell lineage analysis in mammals.

Autonomous and non-autonomous regulation of mammalian neurite development by Notch1 and Delta1

BACKGROUND

On the basis of experiments suggesting that Notch and Delta have a role in axonal development in Drosophila neurons, we studied the ability of components of the Notch signaling pathway to modulate neurite formation in mammalian neuroblastoma cells in vitro.

RESULTS

We observed that N2a neuroblastoma cells expressing an activated form of Notch, Notch1(IC), produced shorter neurites compared with controls, whereas N2a cell lines expressing a dominant-negative Notch1 or a dominant-negative Delta1 construct extended longer neurites with a greater number of primary neurites. We then compared the effects on neurites of contacting Delta1 on another cell and of overexpression of Delta1 in the neurite-extending cell itself. We found that N2a cells co-cultured with Delta1-expressing quail cells produced fewer and shorter neuritic processes. On the other hand, high levels of Delta1 expressed in the N2a cells themselves stimulated neurite extension, increased numbers of primary neurites and induced expression of Jagged1 and Notch1.

CONCLUSIONS

These studies show that Notch signals can antagonize neurite outgrowth and that repressing endogenous Notch signals enhances neurite outgrowth in neuroblastoma cells. Notch signals therefore act as regulators of neuritic extension in neuroblastoma cells. The response of neuritic processes to Delta1 expressed in the neurite was opposite to that to Delta1 contacted on another cell, however. These results suggest a model in which developing neurons determine their extent of process outgrowth on the basis of the opposing influences on Notch signals of ligands contacted on another cell and ligands expressed in the same cell.

Inhibition of beta1-integrin expression reduces clone size during early retinogenesis

We have used an antisense retrovirus strategy to test the role of beta1-integrin in the early period of clone development in the embryonic chick neural retina. Analyses of clone size and dispersion demonstrated a marked effect of reducing the levels of expressed beta1-integrin on this critical phase of retina cell and tissue development.

p21(WAF1/Cip1) functions as a suppressor of malignant skin tumor formation and a determinant of keratinocyte stem-cell potential

p21(WAF1/Cip1) is one of the best characterized downstream targets of p53, and the growth suppressing function of this cyclin-dependent kinase inhibitor is well established. However, whether p21 exerts a tumor-suppressing function of its own remains to be established. We report here that, similarly to loss of p53, disruption of the p21(WAF1/Cip1) gene results in a markedly increased susceptibility to chemically induced skin carcinoma formation, whereas the number of papillomas is reduced. Previous evidence indicates that malignant versus benign keratinocyte tumor formation is likely to involve distinct target-cell populations with a different commitment to differentiation. In parallel with the increased susceptibility to carcinoma formation, loss of p21(WAF1/Cip1) was found to promote keratinocyte subpopulations with increased growth/differentiation potential, including clonal growth capability, reversible commitment to differentiation, and capability to generate all types of terminally differentiated keratinocytes present in vivo, not only in the interfollicular epidermis but also in hair follicles. Thus, these findings have revealed a function of p21 as a suppressor of malignant but not benign skin-tumor formation and a determinant of the growth/differentiation potential of keratinocyte subpopulations.

Origins, functions, and potential of adult neural stem cells

In recent years, the existence of neural stem cells (NSCs) in the adult mammalian brain has been confirmed. The generation of new neurons from these cells is regulated by growth factors, hormones, and environmental cues; however, the function of newly generated neurons in the adult brain remains elusive. Two recent articles emphasize the impact of motor activity and learning on in situ hippocampal neurogenesis,((1,2)) suggesting a close link to hippocampal function. Adult NSCs can be isolated and expanded in vitro. It was presumed that the origins of the NSCs were within subependyma of the lateral ventricle; however, new evidence suggests that the "real" stem cells may reside in the ependymal lining.((3)) In a related study, these same cells were transplanted into irradiated mice and were able to integrate into the bone marrow and produce various blood cell types,((4)) challenging the limits of neural cell fate determination.

Lineage and pluripotentiality of epithelial precursor cells in developing chicken skin

How do epithelial cells in developing skin accommodate the constantly growing embryo? Where do cells in skin appendages come from? Are they derivatives of a single appendage stem cell, or are they polyclonal? Here we analyze these issues in developing chicken skin using a replication-defective virus carrying beta-galactosidase and DiI microinjections. The results demonstrate that in early skin, epithelial cells labelled near the spine show a parallel linear stripe distribution pattern that is perpendicular to the midline of the trunk. This is similar to the human lines of Blaschko, a linear pattern on the skin, which many skin nevoid or acquired disorders follow. In later skin, feather buds form and contain a mixture of labeled and unlabeled cells, attesting to their polyclonal origin. When cells are traced for shorter time intervals, the labeled progeny appear to follow certain rules. The degree of cell dispersion and mixing increases with a longer incubation period between the time of labeling and detection. The spatial maturation sequence of skin appendages is not regulated by the order in which epithelial cells are generated. Epithelial cells at this developmental stage are pluripotent and competent to respond to new signals to assume appropriate fates according to their micro-environment. The results suggest that local interactions act upon the originally linearly deposited pluripotential epithelial cells to form skin appendages.

Modelling T-cell memory by genetic marking of memory T cells in vivo

Immunological memory is the ability of the immune system to respond with enhanced vigour to pathogens that have been encountered in the past. Following infection or immunization, most effector T cells undergo apoptotic cell death, but a small fraction of these cells, proportional to the early antigen load and initial clonal burst size, persist in the host as a stable pool of memory T cells. The existence of immunological memory has been recognized for over 2,000 years, but our understanding of this phenomenon is limited, primarily because memory lymphocytes cannot be unequivocally identified as they lack specific, permanent markers. Here we have developed a transgenic mouse model system whereby memory T cells and their precursors can be irreversibly marked with a reporter gene and thus can be unambiguously identified. Adoptive transfer of marked CD8+ T cells specific for lymphocytic choriomeningitis virus protected naive recipients following viral challenge, demonstrating that we have marked memory T cells. We also show that cytotoxic effector lymphocytes that develop into memory T cells can be identified in the primary response.

Transcription factors Mash-1 and Prox-1 delineate early steps in differentiation of neural stem cells in the developing central nervous system

Like other tissues and organs in vertebrates, multipotential stem cells serve as the origin of diverse cell types during genesis of the mammalian central nervous system (CNS). During early development, stem cells self-renew and increase their total cell numbers without overt differentiation. At later stages, the cells withdraw from this self-renewal mode, and are fated to differentiate into neurons and glia in a spatially and temporally regulated manner. However, the molecular mechanisms underlying this important step in cell differentiation remain poorly understood. In this study, we present evidence that the expression and function of the neural-specific transcription factors Mash-1 and Prox-1 are involved in this process. In vivo, Mash-1- and Prox-1-expressing cells were defined as a transient proliferating population that was molecularly distinct from self-renewing stem cells. By taking advantage of in vitro culture systems, we showed that induction of Mash-1 and Prox-1 coincided with an initial step of differentiation of stem cells. Furthermore, forced expression of Mash-1 led to the down-regulation of nestin, a marker for undifferentiated neuroepithelial cells, and up-regulation of Prox-1, suggesting that Mash-1 positively regulates cell differentiation. In support of these observations in vitro, we found specific defects in cellular differentiation and loss of expression of Prox-1 in the developing brain of Mash-1 mutant mice in vivo. Thus, we propose that induction of Mash-1 and Prox-1 is one of the critical molecular events that control early development of the CNS.

A mutant E2F-1 transcription factor that affects the phenotype of NIH3T3 fibroblasts inefficiently associates with cyclin A - cdk2

The amino-terminal domain of the E2F1 transcription factor is the site of association with cyclin A - cdk2, mapping to residues 87-94. A mutant of E2F1 lacking the first 87 amino acids (termed E2F1d87) has a number of potent effects on cellular phenotype when constitutively expressed in NIH3T3 fibroblasts. For example, in these fibroblasts the duration of S phase and the sensitivity to S phase chemotherapeutic agents are both increased. Since E2F1d87 only partially truncates the cyclin A - cdk2 binding domain, it was important to determine the level of cyclin A - cdk2 association with this mutant to correlate any reduction in association with the observed effects on the cell cycle. It was found that cyclin A - cdk2 binds E2F1d87 in an in vitro assay but that this binding is reduced approximately 8 fold compared with binding to full-length E2F1, whereas no detectable binding was seen to a mutant E2F1 that lacks the first 117 amino acids. Correspondingly, H1 kinase activity in E2F1d87 immunoprecipitates from E2F1d87-expressing cells was significantly reduced compared with that seen for full-length E2F1. From these data it appears that E2F1 with reduced cyclin A - cdk2 binding activity mediates the alteration in cell cycle parameters seen in these cells.Key words: E2F1, apoptosis, cyclin A, cell cycle.