Lineage analysis using retroviral vectors

Neural lineage tracing in the mammalian brain

Delineating the lineage of neural cells that captures the progressive steps in their specification is fundamental to understanding brain development, organization, and function. Since the earliest days of embryology, lineage questions have been addressed with methods of increasing specificity, capacity, and resolution. Yet, a full realization of individual cell lineages remains challenging for complex systems. A recent explosion of technical advances in genome-editing and single-cell sequencing has enabled lineage analysis in an unprecedented scale, speed, and depth across different species. In this review, we discuss the application of available as well as future genetic labeling techniques for tracking neural lineages in vivo in the mammalian nervous system.

Embryonic Origin of Postnatal Neural Stem Cells

Adult neural stem/progenitor (B1) cells within the walls of the lateral ventricles generate different types of neurons for the olfactory bulb (OB). The location of B1 cells determines the types of OB neurons they generate. Here we show that the majority of mouse B1 cell precursors are produced between embryonic days (E) 13.5 and 15.5 and remain largely quiescent until they become reactivated postnatally. Using a retroviral library carrying over 100,000 genetic tags, we found that B1 cells share a common progenitor with embryonic cells of the cortex, striatum, and septum, but this lineage relationship is lost before E15.5. The regional specification of B1 cells is evident as early as E11.5 and is spatially linked to the production of neurons that populate different areas of the forebrain. This study reveals an early embryonic regional specification of postnatal neural stem cells and the lineage relationship between them and embryonic progenitor cells.

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.

Molecular basis of intervertebral disc degeneration

Intervertebral disc (IVD) degeneration is a disease of the discs connecting adjoining vertebrae in which structural damage leads to degeneration of the disc and surrounding area. Degeneration of the disc is considered to be a normal process of aging, but can accelerate faster than expected or be precipitated by other factors. The scientific community has come a long way in understanding the biological basis and interpreting the lifestyle implications of IVD degeneration. Of all the diseases of the intervertebral disc, degeneration is the most common and has earned much attention due to its diversity in presentation and potential multiorgan involvement. We will provide a brief overview of the anatomic, cellular, and molecular structure of the IVD, and delve into the cellular and molecular pathophysiology surrounding IVD degeneration. We will then highlight some of the newest developments in stem cell, protein, and genetic therapy for IVD degeneration.

PiggyBac transgenic strategies in the developing chicken spinal cord

The chicken spinal cord is an excellent model for the study of early neural development in vertebrates. However, the lack of robust, stable and versatile transgenic methods has limited the usefulness of chick embryos for the study of later neurodevelopmental events. Here we describe a new transgenic approach utilizing the PiggyBac (PB) transposon to facilitate analysis of late-stage neural development such as axon targeting and synaptic connection in the chicken embryo. Using PB transgenic approaches we achieved temporal and spatial regulation of transgene expression and performed stable RNA interference (RNAi). With these new capabilities, we mapped axon projection patterns of V2b subset of spinal interneurons and visualized maturation of the neuromuscular junction (NMJ). Furthermore, PB-mediated RNAi in the chick recapitulated the phenotype of loss of agrin function in the mouse NMJ. The simplicity and versatility of PB-mediated transgenic strategies hold great promise for large-scale genetic analysis of neuronal connectivity in the chick.

Paraxial mesoderm contributes stromal cells to the developing kidney

The development of most, if not all, tubular organs is dependent on signaling between epithelial and stromal progenitor populations. Most often, these lineages derive from different germ layers that are specified during gastrulation, well in advance of organ condensation. Thus, one of the first stages of organogenesis is the integration of distinct progenitor populations into a single embryonic rudiment. In contrast, the stromal and epithelial lineages controlling renal development are both believed to derive from the intermediate mesoderm and to be specified as the kidney develops. In this study we directly analyzed the lineage of renal epithelia and stroma in the developing chick embryo using two independent fate mapping techniques. Results of these experiments confirm the hypothesis that nephron epithelia derive from the intermediate mesoderm. Most importantly, we discovered that large populations of renal stroma originate in the paraxial mesoderm. Collectively, these studies suggest that the signals that subdivide mesoderm into intermediate and paraxial domains may play a role in specifying nephron epithelia and a renal stromal lineage. In addition, these fate mapping data indicate that renal development, like the development of all other tubular organs, is dependent on the integration of progenitors from different embryonic tissues into a single rudiment.

Genetic modification of neural stem cells

Neural stem cells (NSCs) are the main vehicle for genetic and molecular therapies in the central nervous system (CNS). The sustainability of NSCs has been ensured through genetic manipulation both in vitro and in vivo. NSC lines have also been immortalized and controlled for cell growth in similar fashion. Their potential to differentiate and their genetic plasticity make them the modality of choice for cellular transplantation. After transplantation, NSCs also exhibit inherent long-distance migratory capabilities and a remarkable capacity to integrate into brain structures. This makes NSCs the ideal candidate for delivery and expression of therapeutic genes. Mouse models of CNS diseases have already demonstrated the efficacy of such NSC-mediated treatment, and further investigations are underway to bridge the gap into true clinical application. Finally, the imaging possibilities with NSC transplants are endless, and they will be a pivotal component to safe and effective human transplantation. This paper provides an overview on NSCs and the various methods in which they have been genetically manipulated for biological investigation.

Statistical analysis of data from retroviral clonal experiments in the developing retina

Retroviral lineage studies have been widely used over the past decade to study retinal development in vivo and in explant culture [Donovan S.L., Dyer, M.A., 2006. Preparation and Square Wave Electroporation of Retinal Explant Cultures, Nature Protocols 1, 2710-2718; Donovan, S.L., Schweers, B., Martins, R., Johnson D., Dyer, M.A., 2001. Compensation by tumor suppressor genes during retinal development in mice and humans, BMC Biol 4 , 14; Dyer M.A., Cepko, C.L., 2001. p27Kip1 and p57Kip2 regulate proliferation in distinct retinal progenitor cell populations, J. of Neurosci 21, 4259-4271; Dyer M.A., Cepko, C.L., 2000. p57(Kip2) regulates progenitor cell proliferation and amacrine interneuron development in the mouse retina, Development 127, 3593-3605; Dyer, M.A., Livesey, F.J., Cepko C.L., Oliver, G., 2003. Prox1 function controls progenitor cell proliferation and horizontal cell genesis in the mammalian retina, Nat Genet 34, 53-58]. These approaches can provide important data on the proliferation, cell fate specification, differentiation and survival of individual neurons and glia derived from single infected retinal progenitor cells. In some experiments, these parameters are compared in retinae from animals with different targeted deletions or transgenes. Alternatively, the effect of ectopic expression of virally encoded transgenes may be studied at the level of individual retinal progenitor cells in vivo and in explant culture. One of the challenges with interpreting retroviral lineage studies is determining the statistical significance of differences in the proliferation, cell fate specification, differentiation of survival of retinal progenitor cells between experimental and control samples. In this study, we provide a clear step-by-step guide to the application of statistical methods to retroviral lineage analyses actual data sets. We anticipate that this will serve as a guide for future statistical analyses of retroviral lineage studies and will help to provide a uniform standard in the field.

Transduction of human embryonic stem cells by ecotropic retroviral vectors

The steadily increasing availability of human embryonic stem (hES) cell lines has created strong interest in applying available tools for gene transfer in murine cells to human systems. Here we present a method for the transduction of hES cells with ecotropic retroviral vectors. hES cells were transiently transfected with a construct carrying the murine retrovirus receptor mCAT1. Subsequently, the cells were exposed to replication-deficient Moloney murine leukemia virus (MoMuLV) derivatives or pseudotyped lentiviral vectors. With oncoretroviral vectors, this procedure yields overall transduction efficiencies of up to 20% and permits selection of permanently transduced clones with high frequency. Selected clones maintained expression of pluripotency-associated markers and exhibited multi-germ layer differentiation both in vitro and in vivo. HES cell-derived somatic cells including neural progeny maintained high levels of transgene expression. Lentiviral vectors pseudotyped with the MoMuLV envelope could be introduced in the same manner with efficiencies of up to 33%. Transgene expression of lentivirally transduced hES cells remained permanent after differentiation even without selection pressure. Bypassing the regulatory issues associated with the use of amphotropic retroviral systems and exploiting the large pool of existing murine vectors, this method provides a safe and versatile tool for gene transfer and lineage analysis in hES cells and their progeny.

Notch activity is downregulated just prior to retinal ganglion cell differentiation

The Notch signaling pathway is important at several stages of retinal development including the differentiation of retinal ganglion cells and Muller glia. The downstream effectors of Notch signaling, Hes1 and Hes5, have been shown to be critical in the retina as well. While Notch activity directly regulates Hes1 and Hes5 elsewhere in the nervous system, it has been unclear whether Hes1 and/or Hes5 are directly regulated by Notch activity in the developing retina. Here, we report that both Hes1 and Hes5 are directly regulated by Notch activity during retinal development. Using fluorescence-based Hes1 and Hes5 reporter constructs, we can monitor Notch activity in progenitor cells in the intact retina, and we find that Notch activity is downregulated just prior to retinal ganglion cell differentiation.

Somitic origin of limb muscle satellite and side population cells

Repair of mature skeletal muscle is mediated by adult muscle progenitors. Satellite cells have long been recognized as playing a major role in muscle repair, whereas side population (SP) cells have more recently been identified as contributing to this process. The developmental source of these two progenitor populations has been considerably debated. We explicitly tested and quantified the contribution of embryonic somitic cells to these progenitor populations. Chick somitic cells were labeled by using replication-defective retroviruses or quail/chick chimeras, and mouse cells were labeled by crossing somite-specific, Pax3-derived Cre driver lines with a Cre-dependent reporter line. We show that the majority of, if not all, limb muscle satellite cells arise from cells expressing Pax3 specifically in the hypaxial somite and their migratory derivatives. We also find that a significant number of, but not all, limb muscle SP cells are derived from the hypaxial somite. Notably, the heterogeneity in the developmental origin of SP cells is reflected in their functional heterogeneity; somitically derived SP cells are intrinsically more myogenic than nonsomitically derived ones. Thus, we show that the somites, which supply embryonic and fetal myoblasts, are also an important source of highly myogenic adult muscle progenitors.

Drosophila neuroblast 7-3 cell lineage: a model system for studying programmed cell death, Notch/Numb signaling, and sequential specification of ganglion mother cell identity

Cell lineage studies provide an important foundation for experimental analysis in many systems. Drosophila neural precursors (neuroblasts) sequentially generate ganglion mother cells (GMCs), which generate neurons and/or glia, but the birth order, or cell lineage, of each neuroblast is poorly understood. The best-characterized neuroblast is NB7-3, in which GMC-1 makes the EW1 serotonergic interneuron and GW motoneuron; GMC-2 makes the EW2 serotonergic interneuron and a programmed cell death; and GMC-3 gives rise to the EW3 interneuron. However, the end of this lineage has not been determined. Here, we use positively marked genetic clones, bromodeoxyuridine (BrdU) labeling, mutations that affect Notch signaling, and antibody markers to further define the end of the cell lineage of NB7-3. We provide evidence that GMC-3 directly differentiates into EW3 and that the sibling neuroblast undergoes programmed cell death. Our results confirm and extend previous work on the early portion of the NB7-3 lineage (Novotny et al. [2002] Development 129:1027-1036; Lundell et al. [ 2003] Development 130:4109-4121).

Migration patterns of subventricular zone cells in adult mice change after cerebral cortex injury

The subventricular zone (SVZ) generates the largest number of migratory cells in the adult brain. SVZ neuroblasts migrate to the olfactory bulbs (OB) in the adult, whereas during development, SVZ cells migrate into many adjacent nuclei. Previously, we showed that cerebral cortex injury in the adult causes molecular and cellular changes which may recapitulate the developmental migratory directions. Consistent with this, growth factors, as well as models of illness or injury can cause adult SVZ cells to migrate into non-olfactory bulb nuclei. Here, we tested the hypothesis that cerebral cortex injury in the adult mouse induces changes in migration, by labeling adult SVZ cells with a retroviral vector and examining the distribution of cells 4 days and 3 weeks later. Four days after cortical lesions, disproportionately fewer retrovirally-labeled cells had migrated to the olfactory bulb in lesioned mice than in controls. Conversely, the number of cells found in non-olfactory bulb regions (primarily the area of the lesion and the corpus callosum) was increased in lesioned mice. The morphology of these emigrated cells suggested that they were differentiating into glial cells. Three weeks after cortical injury, the majority of retrovirally-labeled cells in both groups of mice had migrated into the granule and periglomerular layers of the olfactory bulb. At 3 weeks, we still observed retrovirally-labeled glial cells in the corpus callosum and in the area of the injury in lesioned mice. These results suggest that cortical lesions cause a transient change in migration patterns of SVZ progeny, which is characterized by decreases in migration to the olfactory bulb but increased migration towards the injury. Our studies also suggest that cortical lesions induce the production of new glial cells which survive for at least 3 weeks after injury. The data support the concept that in the adult, SVZ cells can generate progeny that migrate towards injured areas and thus potentially be harnessed for neural repair.

A Role for Ligand-Gated Ion Channels in Rod Photoreceptor Development

Neurotransmitter receptors are central to communication at synapses. Many components of the machinery for neurotransmission are present prior to synapse formation, suggesting a developmental role. Here, evidence is presented that signaling through glycine receptor alpha2 (GlyRalpha2) and GABA(A) receptors plays a role in photoreceptor development in the vertebrate retina. The signaling is likely mediated by taurine, which is present at high levels throughout the developing central nervous system (CNS). Taurine potentiates the production of rod photoreceptors, and this induction is inhibited by strychnine, an antagonist of glycine receptors, and bicuculline, an antagonist of GABA receptors. Gain-of-function experiments showed that signaling through GlyRalpha2 induced exit from mitosis and an increase in rod photoreceptors. Furthermore, targeted knockdown of GlyRalpha2 decreased the number of photoreceptors while increasing the number of other retinal cell types. These data support a previously undescribed role for these ligand-gated ion channels during the early stages of CNS development.

A noninvasive genetic/pharmacologic strategy for visualizing cell morphology and clonal relationships in the mouse

Analysis of cellular morphology is the most general approach to neuronal classification. With the increased use of genetically engineered mice, there is a growing need for methods that can selectively visualize the morphologies of specified subsets of neurons. This capability is needed both to define cell morphologic phenotypes and to mark cells in a noninvasive manner for lineage studies. To this end, we describe a bipartite genetic system based on a Cre-estrogen receptor (ER) fusion protein that irreversibly activates a plasma membrane-bound alkaline phosphatase reporter gene by site-specific recombination. Because the efficiency and timing of gene rearrangement is controlled pharmacologically, a sparse subset of labeled cells can be generated from the set of CreER-expressing cells at any time during development. Histochemical visualization of alkaline phosphatase activity reveals neuronal morphology with strong and uniform labeling of all processes.

Local extrinsic signals determine muscle and endothelial cell fate and patterning in the vertebrate limb

Both the muscle and endothelium of the vertebrate limb derive from somites. We have used replication-defective retroviral vectors to analyze the lineage relationships of these somite-derived cells in the chick. We find that myogenic precursors in the somites or proximal limb are not committed to forming slow or fast muscle fibers, particular anatomical muscles, or muscles within specific proximal/distal or dorsal/ventral limb regions. Somitic endothelial precursors are uncommitted to forming endothelium in particular proximal/distal or dorsal/ventral limb regions. Surprisingly, we also find that myogenic and endothelial cells are derived from a common somitic precursor. Thus, local extrinsic signals are critical for determining muscle and endothelial patterning as well as cell fate in the limb.

Distribution of gonadotropin-releasing hormone neurones in the chick forebrain is independent of lineage relationships among cells of the early nasal placode

The regulation of reproduction depends upon the successful migration of gonadotropin-releasing hormone (GnRH) neurones from the nasal placode to the ventral forebrain during embryogenesis. Within the central nervous system (CNS), these neurones migrate to stereotyped, highly reproducible locations in septal, preoptic and hypothalamic nuclei. We postulated that lineage relationships (descent from a common precursor) might predict the final location of these neurones. To test this hypothesis, a complex retroviral library was used to label dividing cells in the placode and subsequently to identify them by the presence of the alkaline phosphatase marker. GnRH was detected immunocytochemically and lineage relationships determined by single cell polymerase chain reaction and sequencing of the degenerate oligonucleotide component of the retrovirus. GnRH-positive and GnRH-negative neurones were confined to the side ipsilateral to the injection; many cells derived from the placode that entered the CNS did not contain GnRH. This precise method of identifying and mapping the progeny of single neurones revealed that GnRH cells in any given area were derived from multiple precursors. This developmental pattern may contribute to assuring that all CNS locations critical to the orchestration of reproductive events will be populated by GnRH neurones.

Boundaries in development: formation and function

Developing organisms may contain billions of cells destined to differentiate in numerous different ways. One strategy organisms use to simplify the orchestration of development is the separation of cell populations into distinct functional units. Our expanding knowledge of boundary formation and function in different systems is beginning to reveal general principles of this process. Fields of cells are subdivided by the interpretation of morphogen gradients, and these subdivisions are then maintained and refined by local cell-cell interactions. Sharp and stable separation between cell populations requires special mechanisms to keep cells segregated, which in many cases appear to involve the regulation of cell affinity. Once cell populations become distinct, specialized cells are often induced along the borders between them. These boundary cells can then influence the patterning of surrounding cells, which can result in progressively finer subdivisions of a tissue. Much has been learned about the signaling pathways that establish boundaries, but a key challenge for the future remains to elucidate the cellular and molecular mechanisms that actually keep cell populations separated.

p27Kip1 and p57Kip2 regulate proliferation in distinct retinal progenitor cell populations

In the developing vertebrate retina, progenitor cell proliferation must be precisely regulated to ensure appropriate formation of the mature tissue. Cyclin kinase inhibitors have been implicated as important regulators of proliferation during development by blocking the activity of cyclin-cyclin-dependent kinase complexes. We have found that the p27(Kip1) cyclin kinase inhibitor regulates progenitor cell proliferation throughout retinal histogenesis. p27(Kip1) is upregulated during the late G(2)/early G(1) phase of the cell cycle in retinal progenitor cells, where it interacts with the major retinal D-type cyclin-cyclin D1. Mice deficient for p27(Kip1) exhibited an increase in the proportion of mitotic cells throughout development as well as extensive apoptosis, particularly during the later stages of retinal histogenesis. Retroviral-mediated overexpression of p27(Kip1) in mitotic retinal progenitor cells led to premature cell cycle exit yet had no dramatic effects on Müller glial or bipolar cell fate specification as seen with the Xenopus cyclin kinase inhibitor, p27(Xic1). Consistent with the overexpression of p27(Kip1), mice lacking one or both alleles of p27(Kip1) maintained the same relative ratios of each major retinal cell type as their wild-type littermates. During the embryonic stages of development, when both p27(Kip1) and p57(Kip2) are expressed in retinal progenitor cells, they were found in distinct populations, demonstrating directly that different retinal progenitor cells are heterogeneous with respect to their expression of cell cycle regulators.

Control of Müller glial cell proliferation and activation following retinal injury

Müller glial cells are the major support cell for neurons in the vertebrate retina. Following neuronal damage, Müller cells undergo reactive gliosis, which is characterized by proliferation and changes in gene expression. We have found that downregulation of the tumor supressor protein p27Kip1 and re-entry into the cell cycle occurs within the first 24 hours after retinal injury. Shortly thereafter, Müller glial cells upregulate genes typical of gliosis and then downregulate cyclin D3, in concert with an exit from mitosis. Mice lacking p27Kip1 showed a constitutive form of reactive gliosis, which leads to retinal dysplasia and vascular abnormalities reminiscent of diabetic retinopathy. We conclude that p27Kip1 regulates Müller glial cell proliferation during reactive gliosis.

p57(Kip2) regulates progenitor cell proliferation and amacrine interneuron development in the mouse retina

A precise balance between proliferation and differentiation must be maintained during retinal development to obtain the correct proportion of each of the seven cell types found in the adult tissue. Cyclin kinase inhibitors can regulate cell cycle exit coincident with induction of differentiation programs during development. We have found that the p57(Kip2) cyclin kinase inhibitor is upregulated during G(1)/G(0) in a subset of retinal progenitor cells exiting the cell cycle between embryonic day 14.5 and 16.5 of mouse development. Retroviral mediated overexpression of p57(Kip2) in embryonic retinal progenitor cells led to premature cell cycle exit. Retinae from mice lacking p57(Kip2) exhibited inappropriate S-phase entry and apoptotic nuclei were found in the region where p57(Kip2) is normally expressed. Apoptosis precisely compensated for the inappropriate proliferation in the p57(Kip2)-deficient retinae to preserve the correct proportion of the major retinal cell types. Postnatally, p57(Kip2) was found to be expressed in a novel subpopulation of amacrine interneurons. At this stage, p57(Kip2)did not regulate proliferation. However, perhaps reflecting its role during this late stage of development, animals lacking p57(Kip2) showed an alteration in amacrine subpopulations. p57(Kip2) is the first gene to be implicated as a regulator of amacrine subtype/subpopulation development. Consequently, we propose that p57(Kip2) has two roles during retinal development, acting first as a cyclin kinase inhibitor in mitotic progenitor cells, and then playing a distinct role in neuronal differentiation.

Early cortical precursors do not undergo LIF-mediated astrocytic differentiation

Multipotential stem cells have been isolated from the developing and adult CNS. Similar identified factors control the differentiation of these cells. A striking example is the instructive action of CNTF/LIF activating the JAK/STAT pathway to induce astrocytic differentiation in both fetal and adult CNS stem cells. Here we show that E12 cortical precursors express functional LIF receptors but do not exhibit this differentiation response to CNTF/LIF either in explant or in dissociated cell culture. The lack of response to LIF-induced astrocytic differentiation is maintained in cocultures with LIF responsive cells derived from E15 cortex. This suggests cell intrinsic differences between early and late stage precursors in the interpretation of LIF-mediated signaling; however, the early nestin-positive precursor population differentiates into both neurons and neural crest derivatives. These data define differences between CNS stem cells from different stages of cortical development. J. Neurosci. Res. 59:301-311, 2000. Published 2000 Wiley-Liss, Inc.

Gene therapy in the CNS

Gene therapy for neurological disorder is currently an experimental concept. The goals for clinical utilization are the relief of symptoms, slowing of disease progression, and correction of genetic abnormalities. Experimental studies are realizing these goals in the development of gene therapies in animal models. Discoveries of the molecular basis of neurological disease and advances in gene transfer systems have allowed focal and global delivery of therapeutic genes for a wide variety of CNS disorders. Limitations are still apparent, such as stability and regulation of transgene expression, and safety of both vector and expressed transgene. In addition, the brain adds several challenges not seen in peripheral gene therapy paradigms, such as post-mitotic cells, heterogeneity of cell types and circuits, and limited access. Moreover, it is likely that several modes of gene delivery will be necessary for successful gene therapies of the CNS. Collaborative efforts between clinicians and basic researchers will likely yield effective gene therapy in the CNS.

Developmental evolution: going beyond the 'just so'

Two new protocols for infecting non-mammalian embryos with viruses, together with RNA inhibition, have provided evolutionary developmental biologists with the tools to study the effects of manipulating gene activity in a wide range of species, allowing them to test hypotheses rather than rely on inference from similarity.

Development of the Drosophila mushroom bodies: sequential generation of three distinct types of neurons from a neuroblast

The mushroom bodies (MBs) are prominent structures in the Drosophila brain that are essential for olfactory learning and memory. Characterization of the development and projection patterns of individual MB neurons will be important for elucidating their functions. Using mosaic analysis with a repressible cell marker (Lee, T. and Luo, L. (1999) Neuron 22, 451–461), we have positively marked the axons and dendrites of multicellular and single-cell mushroom body clones at specific developmental stages. Systematic clonal analysis demonstrates that a single mushroom body neuroblast sequentially generates at least three types of morphologically distinct neurons. Neurons projecting into the (gamma) lobe of the adult MB are born first, prior to the mid-3rd instar larval stage. Neurons projecting into the alpha' and beta' lobes are born between the mid-3rd instar larval stage and puparium formation. Finally, neurons projecting into the alpha and beta lobes are born after puparium formation. Visualization of individual MB neurons has also revealed how different neurons acquire their characteristic axon projections. During the larval stage, axons of all MB neurons bifurcate into both the dorsal and medial lobes. Shortly after puparium formation, larval MB neurons are selectively pruned according to birthdays. Degeneration of axon branches makes early-born gamma neurons retain only their main processes in the peduncle, which then project into the adult gamma lobe without bifurcation. In contrast, the basic axon projections of the later-born (alpha'/beta') larval neurons are preserved during metamorphosis. This study illustrates the cellular organization of mushroom bodies and the development of different MB neurons at the single cell level. It allows for future studies on the molecular mechanisms of mushroom body development.

A method for rapid gain-of-function studies in the mouse embryonic nervous system

We used ultrasound image-guided injections of high-titer retroviral vectors to obtain widespread introduction of genes into the mouse nervous system in utero as early as embryonic day 8.5 (E8.5). The vectors used included internal promoters that substantially improved proviral gene expression in the ventricular zone of the brain. To demonstrate the utility of this system, we extended our previous work in vitro by infecting the telencephalon in vivo as early as E8. 5 with a virus expressing Sonic Hedgehog. Infected embryos showed gross morphological brain defects, as well as ectopic expression of ventral telencephalic markers characteristic of either the medial or lateral ganglionic eminences.

Imaging cells in the developing nervous system with retrovirus expressing modified green fluorescent protein

To visualize the movements of cells and their processes in developing vertebrates, we constructed replication-incompetent retroviral vectors encoding green fluorescent protein (GFP) that can be detected as a single integrated copy per cell. To optimize GFP expression, the CMV enhancer and avian beta-actin promoter were incorporated within a retrovirus construct to drive transcription of redshifted (F64L, S65T) and codon-modified GFP (EGFP), EGFP tagged with GAP-43 sequences targeting the GFP to the cell membrane, or EGFP with additional mutations that increase its ability to fold properly at 37 degrees C (S147P or V163A, S175G). We have used these viruses to efficiently mark and follow the developmental progression of a large population of cells in rat neocortex and whole avian embryos. In the chick embryo, the migration and development of GFP-marked neural crest cells were monitored using time-lapse videomicroscopy. In the neocortex, GFP clearly delineates the morphology of a variety of neuronal and glial phenotypes. Cells expressing GFP display normal dendritic morphologies, and infected cells persist into adulthood. Cortical neurons appear to form normal local axonal and long-distance projections, suggesting that the presence of cytoplasmic or GAP-43-tagged GFP does not significantly interfere with normal development.