No description available.

Muscle satellite cells are a distinct population of myogenic progenitors that mediate the postnatal growth and regeneration of skeletal muscle. To gain insight into the genetic regulation of satellite cell function during muscle regeneration, genes expressed specifically in these cells were identified by representational difference analysis of cDNAs. Notably, the paired-box transcription factor Pax7 was isolated as a gene specifically expressed in quiescent and activated satellite cells. Cell culture and histological analysis of PaxZ-deficient muscle revealed a complete absence of satellite cells. This result demonstrates a requirement for Pax7 upstream of MyoD and Myf5 in the specification of muscle satellite cells. Consistent with their lack of satellite cells, adult PaxT mice displayed an aggravated muscle wasting phenotype characterized by spinal kyphosis and reduced muscle mass. Acute muscle damage led to extensive calcification and deposition of adipose and fibrotic tissues with the appearance of rare regenerated myofibers. Importantly, analysis of Pax7 muscle suspensions indicated that myogenic cells expressing Pax3 and MyoD were responsible for this low level of regeneration. To characterize the role of adult stem cells in skeletal muscle, we investigated the myogenic potential of muscle-derived CD45+:Sca1+ cells in vivo during regeneration and in vitro using coculture assays. CD45+ and Sca1+ cells isolated from uninjured muscle were uniformly non-myogenic. Strikingly, 7-10% of CD45+:Sca1+ cells purified from regenerating muscle activated the myogenic program by a Pax7-dependent mechanism in response to activation of the Wnt signaling pathway. Furthermore, expression of Pax7 was sufficient to induce myogenic commitment in CD45f+Scal cells from uninjured muscle. This result demonstrates that non-satellite cell derived myogenic progenitors possess a physiological role in muscle regeneration and tissue homeostasis. Taken together, this work establishes a requirement for Pax7 in the specification of muscle satellite cells and for the myogenic recruitment of adult stem cells populations during tissue repair. Importantly, these studies also suggest that targeted therapies to activate Wnt signaling and Pax7 expression in adult stem cells will be effective for promoting muscle regeneration in patients with degenerative neuromuscular diseases or muscular dystrophies. / Thesis / Doctor of Philosophy (PhD)

Receptor-like kinases (RLKs) make up one of the largest gene families in Arabidopsis thaliana. These genes are required for various biological processes, including response to biotic stress, cell elongation, cell proliferation, and cell fate patterning. An emerging theme in Arabidopsis and other plants is that networks of RLKs are required to regulate a specific process throughout development involving spatial and temporal regulation of transcription factors. However, there are still many RLKs (>50%) with no known function.Several RLKs regulate epidermal development by contributing to early embryonic epidermal maintenance or to epidermal differentiation. In my first analysis, I characterize the role of two related RLKs GASSHO1 (GSO1) and GSO2 in epidermal differentiation. gso1 gso2 double mutants initially form an epidermis during embryogenesis, but analysis of post-embryonic root development indicates the mis-expression of epidermal-specific genes. Three previously characterized RLKs that are involved in epidermal development are also involved in meristem maintenance. In order to decipher the RLK gene networks controlling epidermal development and meristem maintenance, it is necessary to identify additional RLKs involved in both of these processes. I further identified roles for GSO1 and GSO2 in maintaining root growth and root apical meristem (RAM) activity. A future goal will be to elucidate the networks of RLKs, including GSO1 and GSO2 in regulating epidermal and RAM development.The development of the vasculature in plants is controlled by a vascular meristem, the procambium. Oriented cell divisions from the procambium produce phloem, to the periphery, and xylem, to the center of the plant. In a reverse genetic screen to determine to roles of the remaining RLKs with unknown function, we identified the RLK XYLEM INTERMIXED WITH PHLOEM1 (XIP1) that is required for vascular development. We show XIP1 is required for regulating the differentiation of the phloem and for the organization of xylem vessel elements. Our analysis indicates that XIP1 is part of a vascular meristem network, further emphasizing the importance of social networks of RLKs regulating a specific process in development.

This dissertation investigates the embryonic specification of a specific group of cells: the germ cells. Germ cells, which give rise to sperm and egg, are the only cells in sexually-reproducing animals that directly contribute hereditary information to the next generation. Germ cells are therefore a universal cell type across animals, and represent a profound novelty that likely arose near the base of the animal phylogeny. Yet despite their conserved, essential function in all animals, there is surprising diversity in the mechanisms that specify these cells during embryonic development. In this dissertation, I address the diversity of germ cell specification mechanisms in insects. I focus on two species, the milkweed bug Oncopeltus fasciatus (Hemiptera) and the cricket Gryllus bimaculatus (Orthoptera), which both branch basally to the Holometabola (those insects which undergo metamorphosis, including the well-studied fruit fly Drosophila melanogaster), and thus provide important phylogenetic breadth to our understanding of germ cell specification across insects. Using functional genetic approaches, I show that germ cell specification in both Oncopeltus and Gryllus differs fundamentally from germ cell specification in Drosophila. Specifically, I provide evidence that germ cells arise via inductive cell signaling during mid-embryogenesis, rather than via maternally-supplied cytoplasmic determinants localized in the oocyte, as is the case for Drosophila. These data suggest that Drosophila employs an evolutionarily derived mode of germ cell specification. In further support of this hypothesis, I show that several of the genes required for Drosophila germ cell specification perform other functions in both Oncopeltus and Gryllus. I demonstrate that one of these genes, oskar, which is the only gene both necessary and sufficient for germ cell specification in Drosophila, instead functions in nervous system of the cricket, both during embryonic development and in the adult brain. I suggest that the evolution of the derived mode of germ cell specification seen in Drosophila may have involved co-opting oskar into the germ cell specification pathway from an ancestral role in the nervous system.

The mammalian central nervous system (CNS) has a high degree of complexity and cell type diversity that enables sophisticated processing of sensory information, circuit formation, and behaviour. While much is known about the patterning and specification of the major neuronal classes in the CNS, through processes such as morphogen gradient signaling and transcription factor combinatorial coding, much less is known about how subtypes within each cell class are specified. Bipolar cells are one of the main classes of interneurons in the vertebrate retina and consist of fifteen different subtypes based on their physiological function, morphology, and unique gene expression. The cellular mechanisms behind the specification of these subtypes are not fully known. In this thesis, I examine these mechanisms by investigating the role of extrinsic and intrinsic factors on the specification and differentiation of bipolar cell subtypes. We hypothesize that the specification of bipolar cell subtypes occurs in a multi-step manner and is dependent on non-cell autonomous (extrinsic) signals. To test this hypothesis, I conducted a series of experiments on the early postnatal mouse retina, which is the period when bipolar cells are generated. First, I examined whether bipolar cell marker onset was temporally ordered as would be predicted in a multi-step model. Postnatal day 3 (P3) mice were injected with EdU (5-ethynyl-2’-deoxyuridine), a thymidine analog that labels proliferating cells and then dissociated and fixed the retinal cells 24-120 hours after injection. My results show that Vsx2-5.3-PRE-Cre, a marker of pan-bipolar cells specification, is first detected 36 hrs after cell cycle exit, whereas specialized bipolar subtype-specific markers are expressed 48-60 hrs post-EdU injection. This observation is consistent with the idea that bipolar cells develop in a stepwise manner, first as an unspecified, pan-bipolar cell intermediate and then into one of the 15 subtypes. To further investigate this possibility, I developed a novel dissociated retinal culture assay that enabled me to accurately track retinal progenitor cells and postmitotic precursor cells and determine the requirement of cell autonomous and non-cell autonomous mechanisms during bipolar cell subtype specification. This assay involves culturing dissociated retinal cells from P3 EdU-injected mice at high density (abundant cell contact) or low density (scarce cell contact) at various timepoints, thereby allowing me to probe the role of these mechanisms in RPCs, early postmitotic cells, and late postmitotic cells. My findings revealed the first 24-48 hrs post cell cycle exit to be a critical, cell contact-dependent period for the specification of bipolar cell subtypes. This assay also allowed us to test the effect of blocking or activating the Notch and the Sonic Hedgehog (Shh) signal transduction pathways by using pharmacological compounds and recombinant ligands. Co-activation of Notch and Shh pathways increased the specification of Vsx1+ subtypes suggesting they play a role in their specification. Altogether, our results suggest that bipolar cell subtype specification follows a multi-step model, through an undifferentiated bipolar cell intermediate, and that cell contact plays a role in the specification mechanisms of bipolar cell subtype development. This is a novel finding that provides insight into the mechanisms underlying retinal neuronal subtype development and possibly in other neuronal cell types throughout the CNS. / Graduate / 10000-01-01

No description available.

The human central nervous system (CNS) contains a daunting number of cells and tremendous cellular diversity. A fundamental challenge of developmental neurobiology is to address the questions of how so many different types of neurons and glia can be generated at the precise time and place, making precisely the right connections. Resolving this issue involves dissecting the elaborate genetic networks that act within neurons and glia, as well as in the neural progenitor cells that generates them, to specify their identities. My PhD project has involved addressing a number of unresolved issues pertaining to how neural progenitor cells are specified to generate different types of neurons and glial cells in different temporal and spatial domains, and also how these early temporal and spatial cues are integrated to activate late cell fate determinants, which act in post-mitotic neural cells to activate distinct batteries of terminal differentiation genes. Analyzing the development of a specific Drosophila melanogaster (Drosophila) CNS stem cell – the neuroblast 5-6 (NB5-6) – we have identified several novel mechanisms of cell fate specification in the Drosophila CNS. We find that, within this lineage, the differential specification of a group of sequentially generated neurons – the Ap cluster neurons – is critically dependent upon the simultaneous triggering of two opposing feed-forward loops (FFLs) within the neuroblast. The first FFL involves cell fate determinants and progresses within the post-mitotic neurons to establish a highly specific combinatorial code of regulators, which activates a distinct battery of terminal differentiation genes. The second loop, which progresses in the neuroblast, involves temporal and sub-temporal genes that together oppose the progression of the first FFL. This leads to the establishment of an alternative code of regulators in late-born Ap cluster neurons, whereby alternative cell fates are specified. Furthermore, we find that the generation and specification of the Ap cluster neurons is modulated along the neuraxis by two different mechanisms. In abdominal segments, Hox genes of the Bithorax cluster integrates with Pbx/Meis factors to instruct NB5-6 to leave the cell cycle before the Ap cluster neurons are generated. In brain segments, Ap cluster neuron equivalents are generated, but improperly specified due to the absence of the proper Hox and temporal code. Additionally, in thoracic segments we find that the specification of the Ap cluster neurons is critically dependent upon the integration of the Hox, Pbx/Meis, and the temporal genes, in the activation of the critical cell fate determinant FFL. We speculate that the developmental principles of (i) feed-forward combinatorial coding; (ii) simultaneously triggered yet opposing feed-forward loops; and (iii) integration of different Hox, Pbx/Meis, and temporal factors, at different axial levels to control inter-segmental differences in lineage progression and specification; might be used widely throughout the animal kingdom to generate cell type diversity in the CNS.

During cell specification, temporal and spatially restricted gene expression programs are set up, forming different cell types and ultimately a multicellular organism. In this thesis, we have studied the molecular mechanisms by which sequence specific transcription factors and coactivators regulate RNA polymerase II (Pol II) transcription to establish specific gene expression programs and what epigenetic patterns that follows. We found that the transcription factor Dorsal is responsible for establishing discrete epigenetic patterns in the presumptive mesoderm, neuroectoderm and dorsal ectoderm, during early Drosophila embryo development. In addition, these different chromatin states can be linked to distinct modes of Pol II regulation. Our results provide novel insights into how gene regulatory networks form an epigenetic landscape and how their coordinated actions specify cell identity. CBP/p300 is a widely used co-activator and histone acetyltransferase (HAT) involved in transcriptional activation. We discovered that CBP occupies the genome preferentially together with Dorsal, and has a specific role during development in coordinating the dorsal-ventral axis of the Drosophila embryo. While CBP generally correlates with gene activation we also found CBP in H3K27me3 repressed chromatin. Previous studies have shown that CBP has an important role at transcriptional enhancers. We provide evidence that the regulatory role of CBP does not stop at enhancers, but is extended to many genomic regions. CBP binds to insulators and regulates their activity by acetylating histones to prevent spreading of H3K27me3. We further discovered that CBP has a direct regulatory role at promoters. Using a highly potent CBP inhibitor in combination with ChIP and PRO-seq we found that CBP regulates promoter proximal pausing of Pol II. CBP promotes Pol II recruitment to promoters via a direct interaction with TFIIB, and promotes transcriptional elongation by acetylating the first nucleosome. CBP is regulating Pol II activity of nearly all expressed genes, however, either recruitment or release of Pol II is the rate-limiting step affected by CBP. Taken together, these results reveal mechanistic insights into cell specification and transcriptional control during development. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.</p><p> </p>

The olfactory system is important for social behaviors, feeding and avoiding predators. Detection of odorous molecules is made by odorant receptors on specialized sensory neurons in the olfactory epithelial sheet. The olfactory sensory neurons are organized into a few regions or “zones” based on the spatially limited expression of odorant receptors. In this thesis the zonal division and functional specificity of olfactory sensory neurons have been studied in the mouse. We find that zones 2-4 show overlapping expression of odorant receptors while the border between the regions that express a zone 1 and a zone 2 odorant receptor, respectively, is sharp. This result indicates that zone 1 and zones 2-4 are inherently different from each other. In cDNA screens, aimed at finding genes whose expression correlate to the zonal expression pattern of odorant receptors, we have identified a number of signaling proteins implicated in neural-tissue organogenesis in other systems. The differential expression pattern of identified genes suggests that regional organization is maintained during the continuous neurogenesis in the olfactory epithelium as a result of counter gradients of positional information. We show that the gene c-fos is induced in olfactory sensory neurons as a result of cell activation by odorant exposure. A zonal and scattered distribution of c-Fos-positive neurons resembled the pattern of odorant receptor expression and a change of odorant results in a switch in which zone that is activated. Whereas earlier studies suggest that the odorant receptors are relatively broadly tuned with regard to ligand specificity, the restricted patterns of c-Fos induction suggests that low concentrations of odorous molecules activate only one or a few ORs. Studies on olfactory detection abilities of mice with zonal-restricted lesions in the olfactory epithelium show that loss of a zone has severe effects on the detection of some odorants but not others. These findings lend support to a hypothesis that odorant receptors are tuned to more limited numbers of odorants. Regional differences in gene expression and differences in response to toxic compounds between the zones indicate that there may be differences in tissue homeostasis within the epithelium. We have found that there are differences in proliferation and survival of olfactory sensory neurons in regions correlating to receptor expression zones. Identified differences with regard to gene expression, tissue homeostasis and odorant detection show that the olfactory epithelium is divided into regions that transduce different stimulus features.

L’acide rétinoïque (AR), dérivé actif de la vitamine A (rétinol) circulante, est une petite molécule lipophile contrôlant divers aspects de la mise en place du système nerveux central des vertébrés. L'AR influence notamment le développement précoce du cerveau antérieur, où il contrôle la prolifération et la survie des cellules progénitrices dans l'épithélium neural prosencéphalique. Le développement neural est un processus qui s'articule en trois grandes étapes : la phase d'expansion latérale (E9,5-E10,5 chez la souris), la phase de neurogenèse (E11,5-stades périnataux) et la phase de gliogenèse (stades périnataux-adulte). Nous avons montré que l'AR produit par les méninges à partir de E13 influence la spécification et la migration neuronale au cours de la phase de neurogenèse. De plus, nos travaux suggèrent un rôle plus précoce de l'AR pour la formation et la prolifération des populations progénitrices et neuronales avant et au début de la phase de neurogenèse. Une combinaison de signaux intrinsèques et extrinsèques contrôle divers aspects du développement neural cortical. Nos travaux placent l'AR parmi ces facteurs modulateurs de la neurogenèse corticale. / Retinoic acid (RA), an active vitamin A (retinol) metabolite, is a small lipophilic molecule controlling numerous events during central nervous system development in vertebrates. RA is involved in early forebrain development by controlling cell proliferation and survival in the prosencephalic neuroepithelium. Neural development is a process progressing through three key steps: a phase of lateral expansion (E9.5-E10.5 in the mouse), a phase of neurogenesis (E11.5-perinatal stages) and a gliogenic phase (perinatal stages-adult). My work has shown that RA produced by the developing meninges from E13 influences neuronal specification and migration during the phase of neurogenesis. Moreover, our data suggest an earlier role of RA during the production and proliferation of progenitor and neuronal populations, before and at the onset of the neurogenic phase. A combination of extrinsic and intrinsic signals is required to orchestrate the various aspects of cortical development. RA is likely to be one of such extrinsic factors modulating cortical neurogenesis.