Techniques for gene transfer into neurons

Neuropeptide gene therapy for epilepsy: viral vectors, stem cells and neurogenesis

Gene therapy for epilepsy is a relatively novel concept compared with previous approaches, which have relied on primary embryonic cells to deliver gene products of interest into localized brain regions. In vivo and ex vivo gene transfer offer promising, but yet insufficiently explored, possibilities to inhibit seizures, either by genetically modifying postmitotic neurons of the brain using viral vectors, or by transplanting genetically modified and in vitro tested cell lines, particularly stem cell lines, to produce and release gene products of interest. In this regard, neuropeptides are discussed as emerging candidates for such gene therapy approaches. Selective modification of newly generated neurons in the dentate gyrus by retroviral vector-based gene delivery opens novel possibilities in gene therapy for epilepsy. However, the limited number of new neurons targeted remains a main obstacle. Despite its early stage, gene therapy for epilepsy might not be a remote prospect for clinical trials, particularly in patients with intractable temporal lobe epilepsy. Ex vivo gene transfer using encapsulated genetically modified cells could be of particular value for such initial trials.

Optical and pharmacological tools to investigate the role of mitochondria during oxidative stress and neurodegeneration

Mitochondria are critical for cellular adenosine triphosphate (ATP) production; however, recent studies suggest that these organelles fulfill a much broader range of tasks. For example, they are involved in the regulation of cytosolic Ca(2+) levels, intracellular pH and apoptosis, and are the major source of reactive oxygen species (ROS). Various reactive molecules that originate from mitochondria, such as ROS, are critical in pathological events, such as ischemia, as well as in physiological events such as long-term potentiation, neuronal-vascular coupling and neuronal-glial interactions. Due to their key roles in the regulation of several cellular functions, the dysfunction of mitochondria may be critical in various brain disorders. There has been increasing interest in the development of tools that modulate mitochondrial function, and the refinement of techniques that allow for real time monitoring of mitochondria, particularly within their intact cellular environment. Innovative imaging techniques are especially powerful since they allow for mitochondrial visualization at high resolution, tracking of mitochondrial structures and optical real time monitoring of parameters of mitochondrial function. The techniques discussed include classic imaging techniques, such as rhodamine-123, the highly advanced semi-conductor nanoparticles (quantum dots), and wide field microscopy as well as high-resolution multiphoton imaging. We have highlighted the use of these techniques to study mitochondrial function in brain tissue and have included studies from our laboratories in which these techniques have been successfully applied.

Gene transfer into Purkinje cells using herpesviral amplicon vectors in cerebellar cultures

Purkinje cells play a crucial role in sensory motor coordination since they are the only output projection neurons in the cerebellar cortex and are affected in most spinocerebellar ataxias. They stand out in the central nervous system due to their large size and their profusely branched dendritic arbor. However, molecular and cellular studies on Purkinje cells are often hampered by the difficulty of maintaining these cells in culture. Here we report an easy, robust and reproducible method to obtain Purkinje-enriched mixed cerebellar cell cultures from day 16 mouse embryos using papain digestion and a semi-defined culture medium, being the composition of the culture approximately 20% Purkinje cells, 70% non-Purkinje neurons and 10% glial cells. We demonstrate that efficient gene transfer into Purkinje cells (as well as into other cerebellar populations) is possible using herpes simplex virus-1 (HSV-1)-derived vectors. Indeed, up to 50% of the Purkinje cells can be transduced and gene expression may persist for at least 14 days. As a result, this procedure permits functional gene expression studies to be carried out on cultured Purkinje neurons. To demonstrate this, we show that the expression of a dominant-negative form of glycogen synthase kinase-3 protects Purkinje neurons against cell death triggered by a chemical inhibitor of phosphatidylinositol-3 kinase. In summary, we have established reproducible and reliable cerebellar cell cultures enriched for Purkinje cells which enables gene transfer studies to be carried out using herpesviral vectors.

Stage-specific conditional mutagenesis in mouse embryonic stem cell-derived neural cells and postmitotic neurons by direct delivery of biologically active Cre recombinase

Conditional mutagenesis using Cre/loxP recombination is a powerful tool to investigate genes involved in neural development and function. However, the efficient delivery of biologically active Cre recombinase to neural cells, particularly to postmitotic neurons, represents a limiting factor. In this study, we devised a protocol enabling highly efficient conditional mutagenesis in ESC-derived neural progeny. Using a stepwise in vitro differentiation paradigm, we demonstrate that recombinant cell-permeable Cre protein can be used to efficiently induce recombination at defined stages of neural differentiation. Recombination rates of more than 90% were achieved in multipotent pan-neural and glial precursors derived from the Z/EG reporter mouse ESC line, in which Cre recombination activates enhanced green fluorescent proteinexpression. Recombined precursor cells displayed a normal phenotype and were able to differentiate into neurons and/or glial cells, indicating that Cre treatment has no overt side effects on proliferation and neural differentiation. Our data further demonstrate that recombination via Cre protein transduction is not restricted to dividing cells but can even be applied to postmitotic neurons. The ability to conduct Cre/loxP recombination at defined stages of stem cell differentiation in an expression-independent manner provides new prospects for studying the role of individual genes under stringent temporal control.

Vasoactive Intestinal Peptide and PACAP38 Control N-Methyl-D-aspartic Acid-induced Dendrite Motility by Modifying the Activities of Rho GTPases and Phosphatidylinositol 3-Kinases

Dendrite morphogenesis is highly dynamic and characterized by the addition and elongation of processes and also by their selective maintenance, retraction, and elimination. Glutamate can influence these events via N-methyl-d-aspartic acid (NMDA) receptors. The neuropeptides vasoactive intestinal peptide and pituitary adenylyl cyclase-activating polypeptide-38 (PACAP38) affect neurogenesis and differentiation in the developing nervous system. We report here that the peptides and NMDA acted synergistically on dendrite and branch formation. In stage III hippocampal neurons, NMDA increased not only the addition but also the elimination of new dendrites and branches by activating Rac and Cdc42 and phosphatidylinositol 3-kinases, respectively. When applied alone, the neuropeptides did not influence dendrite or branch formation. However, they reduced the elimination of newly formed dendrites and branches caused by NMDA by preventing the NMDA-induced activation of phosphatidylinositol 3-kinases. This led to the formation of persistent dendrites and branches. Additional timelapse studies on the dynamics of dendrite elongation showed alternating periods of elongation and retraction. Phosphatidylinositol 3-kinases increased the velocities of dendrite elongation and retraction, whereas the neuropeptides prolonged the periods of elongation. By modifying NMDA-induced activation of Rho GTPases and phosphatidylinositol 3-kinases, vasoactive intestinal peptide and PACAP38 could play an important role in the control of dendrite growth and branching during development and in response to neuronal activity.

Effective transfection of cells with multi-shell calcium phosphate-DNA nanoparticles

Coated calcium phosphate nanoparticles were prepared for cell transfection. A calcium phosphate nanoparticle served as core which was then coated with DNA for colloidal stabilisation. The efficiency of transfection could be considerably increased by adding another layer of calcium phosphate on the surface, thereby incorporating DNA into the particle and preventing its degradation within the cell by lysosomes. A subsequent outermost layer of DNA on the calcium phosphate gave a colloidal stabilisation. The efficiency of such multi-shell particles was significantly higher than that of simple DNA-coated calcium phosphate nanoparticles. The transfection efficiency of EGFP-encoding DNA was tested with different cell lines (T-HUVEC, HeLa, and LTK). The dispersions were stable and could be used for transfection after 2 weeks of storage at 4 degrees C without loss of efficiency.

An epoch-making application of discharge plasma phenomenon to gene-transfer

We discovered an epoch-making gene transfer method utilizing discharge plasma. Although an electroporation method is commonly used in present gene transfer experiments, it cannot transfer genes into primary cells sufficiently. The atmospheric pressure discharge plasma employed in this study was originally used for surface treatment of non-biological materials. We hypothesized that it could provide a suitable effect on the surface of target cells and applied it to gene transfer into various types of cells. The plasma technology succeeded in the efficient transfer of green fluorescence protein (GFP) plasmid into post-mitotic neuronal cells obtained from cerebral cortices of rats, into which an electroporation with conventional equipment cannot transfer genes sufficiently, as the cells were attached. After the transfection of rat pheochromocytoma PC12 cells with the GFP gene by plasma treatment, the cells retained their function, that is, nerve growth factor-induced differentiation. Furthermore, gene transfer with the plasma technology was also applicable to other types of cell lines such as HeLa cells and Chinese hamster lung (CHL) cells as adherent cell lines, and Jurkat cells as a suspended cell line, and another type of primary cell, human umbilical vein endothelial cells (HUVEC). In conclusion, the plasma method is an epoch-making gene transfer technology which efficiently transfers genes into primary cells into which electroporation cannot transfer genes. Moreover, the method is able to universally transfer genes into various types of cells as the function of the cells was maintained.

Differentiation of a human neuroblastoma into neuron-like cells increases their susceptibility to transduction by herpesviral vectors

Gene transfer is a powerful tool for functional gene analysis in human cells. In this respect, there is a need to develop experimental models that involve homogeneous cultures of human neuron-like cells susceptible to gene transduction and that are easy to handle. Here we describe an optimized and reproducible procedure to differentiate human SH-SY5Y neuroblastoma cells into a homogeneous population of neuron-like cells. The fully differentiated cells are postmitotic and resemble primary cultured neurons in terms of their cytoskeletal polarity. Notably, differentiated SH-SY5Y cells are far more susceptible to transduction by herpes simplex virus (HSV-1)-based vectors than proliferating SH-SY5Y cells. This increase in transduction efficiency after neuronal differentiation may be due to the up-regulation of cell surface receptors for herpesvirus entry. In summary, we propose that fully differentiated human neuron-like cells obtained from the SH-SY5Y neuroblastoma may constitute an excellent and versatile experimental tool for gene transfer and functional genomic studies with HSV-1 vectors.

Plasmid electrotransfer of eye ciliary muscle: principles and therapeutic efficacy using hTNF‐α soluble receptor in uveitis

Due to its small size and particular isolating barriers, the eye is an ideal target for local therapy. Recombinant protein ocular delivery requires invasive and painful repeated injections. Alternatively, a transfected tissue might be used as a local producer of transgene-encoded therapeutic protein. We have developed a nondamaging electrically mediated plasmid delivery technique (electrotransfer) targeted to the ciliary muscle, which is used as a reservoir tissue for the long-lasting expression and secretion of therapeutic proteins. High and long-lasting reporter gene expression was observed, which was restricted to the ciliary muscle. Chimeric TNF-alpha soluble receptor (hTNFR-Is) electrotransfer led to elevated protein secretion in aqueous humor and to drastic inhibition of clinical and histological inflammation scores in rats with endotoxin-induced uveitis. No hTNFR-Is was detected in the serum, demonstrating the local delivery of proteins using this method. Plasmid electrotransfer to the ciliary muscle, as performed in this study, did not induce any ocular pathology or structural damage. Local and sustained therapeutic protein production through ciliary muscle electrotransfer is a promising alternative to repeated intraocular protein administration for a large number of inflammatory, degenerative, or angiogenic diseases.

Nerve growth factor expression by PLG-mediated lipofection

Biomaterials capable of efficient gene delivery provide a fundamental tool for basic and applied research models, such as promoting neural regeneration. We developed a system for the encapsulation and sustained release of plasmid DNA complexed with a cationic lipid and investigated their efficacy using in vitro models of neurite outgrowth. Sustained lipoplex release was obtained for up to 50 days, with rates controlled by the fabrication conditions. Released lipoplexes retained their activity, transfecting 48.2+/-8.3% of NIH3T3 cells with luciferase activity of 3.97x10(7)RLU/mg. Expression of nerve growth factor (NGF) was employed in two models of neurite outgrowth: PC12 and primary dorsal root ganglia (DRG) co-culture. Polymer-mediated lipofection of PC12 produced bioactive NGF, eliciting robust neurite outgrowth. An EGFP/NGF dual-expression vector identified transfected cells (GFP-positive) while neurite outgrowth verified NGF secretion. A co-culture model examined the ability of NGF secretion by an accessory cell population to stimulate DRG neurite outgrowth. Polymer-mediated transfection of HEK293T with an NGF-encoding plasmid induced outgrowth by DRG neurons. This system could be fabricated as implants or nerve guidance conduits to support cellular and tissue regeneration. Combining this physical support with the ability to locally express neurotrophic factors will potentiate regeneration in nerve injury and disease models.

Ultrasound-mediated gene transfer into neuronal cells

A new field of gene transfer is emerging as a simple, effective means to drive the expression foreign genes in cells: ultrasound-mediated gene transfer or sonoporation. We report here that sonoporation is an effective means of gene transfer for cultured neurons, a cell type that has been difficult to transfect. Neuronal cell types that are effectively sonoporated include chick retinal neurons, chick dorsal forebrain, chick optic tectum, PC12 cells, rat cerebellar neurons and mouse hippocampal neurons. Depending on the type of cell and conditions of sonoporation the transfection efficacy was as high as 20%. Sonoporation of plasmid DNA was effective for cells adherent to a substrate and for free-floating cells that were freshly dissociated. In the free-floating preparations, between 60 and 95% of the cells that were transfected were neuronal, as much as 90% higher than that observed for other methods of gene transfer including adenovirus and lipid-based transfection methods. We conclude that sonoporation is a simple, effective and inexpensive means by which to preferentially transfect DNA into neuronal cells.

Novel cationic amphiphilic derivatives from vernonia oil: synthesis and self-aggregation into bilayer vesicles, nanoparticles, and DNA complexants

Self-assembling nanostructures were prepared from novel cationic amphiphilic compounds synthesized from vernonia oil, a natural epoxydized triglyceride. The presence of a 12,13-epoxy group on the C18 unsaturated fatty acid, vernolic acid, which is the main constituent of vernonia oil, permitted the synthesis of novel amphiphilic derivatives with a hydrogen-bonding hydroxyl and a cationic headgroup moiety on adjacent carbon atoms. The amphiphiles were prepared in a two-stage synthesis that comprised opening of the epoxy groups with a haloacetic acid, followed by quaternization of the halo group with a tertiary amine containing a C12 aliphatic chain. Intact vernonia oil as the starting material gave a triple-headed cationic amphiphile, containing three vernolic acid derived moieties connected through a glycerol backbone. A single-headed amphiphile with two alkyl chains and a single quaternary ammonium headgroup was synthesized from the methyl ester of vernolic acid as the starting material. The triple-headed derivative could form nonencapsulating structures. Cholesterol was required in the formulation (1:1) to make spherical vesicles that could encapsulate a water-soluble marker. The single-headed derivative, however, formed spherical encapsulating vesicles without cholesterol. TEM, NMR, and FT-IR were used to characterize the vesicles, and molecular structure vs morphology relationships were postulated on the basis of these data. The triple-headed amphiphile also formed a DNA complex that was highly resistant to hydrolysis by DNase. This amphiphile-DNA complex was used as vector for gene transfer in cell culture demonstrating efficient DNA transfection.

Effective gene delivery to adult neurons by a modified form of electroporation

Non-viral methods of transfection of cDNAs into adult neurons and other post-mitotic cells are generally very inefficient. However, the recent development of Nucleofector technology developed by Amaxa Biosystems allows direct delivery of cDNAs into the nucleus, enabling transfection of non-dividing cells. In this study, we describe a reliable method for culturing large numbers of retinal cells from adult rats and using Nucleofection, we were able to transfect cDNA-encoding GFP (jellyfish green fluorescent protein) into retinal ganglion cells (RGCs) with relatively high efficiency (up to 28%). Neuronal GFP expression was observed within 18 h and continued for up to 14 days. This compares with values up to 60% of RGCs expressing GFP following infection with an HSV-1 vector. Adult rat dorsal root ganglion (DRG) neurons were also successfully transfected. Thus, in summary, Nucleofection provides the possibility for a fast and efficient method for cDNA delivery and study of gene function in adult mammalian neurons.

Enhancing protein expression in single HEK 293 cells

Recombinant proteins are routinely expressed in heterologous expression systems such as human embryonic kidney 293 (HEK 293) cells. The efficiency of the expression is critical when the expressed protein must be characterized at the single-cell level. Here we describe a simple method by which the protein expression efficiency in single HEK 293 cells is enhanced by coexpressing simian virus 40 large T antigen (TAg), a powerful oncoprotein. Using the GluR2 ionotropic glutamate receptor as an example, we found that the receptor expression in single HEK 293S cells increased approximately seven-fold. The ratio of the plasmid amount of TAg to that of the receptor was optimized at 1:10, while the receptor function was unaffected in the presence of TAg. We further used fluorescence imaging from a population of cells as an independent detection method and found a similar increase in expression of green fluorescent protein (GFP) by TAg coexpression. This method is thus applicable for enhancing the expression of both membrane and soluble proteins at the single-cell level. More importantly, the function of a protein can be studied directly in intact cells, a feature particularly useful for studying membrane proteins.

Delivery systems for small molecule drugs, proteins, and DNA: the neuroscience/biomaterial interface

Manipulation of cellular processes in vivo by the delivery of drugs, proteins or DNA is of paramount importance to neuroscience research. Methods for the presentation of these molecules vary widely, including direct injection (either systemic or stereotactic), osmotic pump-mediated chronic delivery, or even implantation of cells engineered to indefinitely secrete a factor of interest. Biomaterial-based delivery systems represent an alternative to more traditional approaches, with the possibility of increased efficacy. Drug-releasing biomaterials, either as injectable microspheres or as three-dimensional implants, can deliver a molecule of interest (including small molecule drugs, biologically active proteins, or DNA) over a more prolonged period of time than by standard bolus injection, avoiding the need for repeated administration. Furthermore, sustained-release systems can maintain therapeutic concentrations at a target site, thus reducing the chance for toxicity. This review summarizes applications of polymer-based delivery of small molecule drugs, proteins, and DNA specifically relevant to neuroscience research. We detail the fabrication procedures for the polymeric systems and their utility in various experimental models. The biomaterial field offers unique experimental tools with downstream clinical application for the study and treatment of neurologic disease.

Alternative modalities of adenovirus-mediated gene expression in hippocampal neurons cultured on microisland substrate

Previously, we have used CsCl gradient-purified recombinant adenovirus (AdV) to successfully transfer genes into hippocampal neurons cultured on microisland substrate. Here, we report that purification of AdV particles is not required and efficient gene expression can be achieved using either crude AdV lysates or HEK 293 cells infected with AdV. The advantages of the simplified procedure are greatly reduced preparation time and reduced requirements for equipment and expertise.

Endogenous RGS proteins regulate presynaptic and postsynaptic function: functional expression of RGS-insensitive Galpha subunits in central nervous system neurons

Regulators of G-protein signaling (RGS)-insensitive (RGSi) G-protein alpha subunits can be used to indirectly determine the function of endogenous RGS proteins in native cells. This article describes the application of RGSi Galpha subunits to the study of endogenous RGS function in central nervous system (CNS) neurons. Presynaptic inhibition of neurotransmitter release was reconstituted in primary neurons using RGSi Galpha(i/o) subunits, whereas postsynaptic regulation of potassium channels was reconstituted using RGSi chimeras of Galpha(q) and Galpha(i). These studies have shown that endogenous RGS proteins are essential for the rapid termination of some G-protein-mediated signals in CNS neurons, whereas these proteins are much less important for the regulation of other signals. Together, these techniques have helped reveal the complexity of RGS regulation of CNS function.

Labeling neurons in vivo for morphological and functional studies

Increasingly sophisticated strategies for labeling cells in vivo are providing unprecedented opportunities to study neurons in living animals. Transgenic expression of genetically encoded reporters enables us to monitor changes in neuronal activity in response to sensory stimuli, and the labeling of single neurons with fluorescent proteins allows the dynamics of neuronal connectivity to be observed in transgenic animals over periods ranging from minutes to months. Advances in transient labeling techniques such as viral infection and electroporation provide a rapid means by which to analyze neuronal gene function in vivo. These new approaches to labeling, manipulating and imaging neurons in intact organisms are transforming the way in which the nervous system is studied.

A model of organotypic rat spinal slice culture and biolistic transfection to elucidate factors that drive the preprotachykinin-A promoter

The tachykinin substance P (SP) is a neuropeptide that is expressed in some nociceptive primary sensory afferents and in discrete populations of spinal cord neurons. Expression of spinal SP and the preprotachykinin-A (PPT-A) gene that encodes SP exhibits plasticity in response to conditions such as peripheral inflammation but the mechanisms that regulate expression are poorly understood. We have developed a spinal cord organotypic culture system that is suitable for the analysis of PPT-A gene promoter activity following biolistic transfection of recombinant DNA constructs. Spinal cord organotypic slices showed good viability over a 7-day culture period. Immunostaining for phenotypic markers such as NeuN and beta-III tubulin demonstrated preservation of neurons and their structure, although there was evidence of axotomy-induced down-regulation of NeuN in certain neuronal populations. Neurokinin-1 receptor (NK-1R) immunostaining in laminae I and III was similar to that seen in acute slices. Biolistic transfection was used to introduce DNA constructs into neurons of these organotypic cultures. Following transfection with a construct in which expression of enhanced green fluorescent protein (EGFP) is controlled by the PPT-A promoter, we showed that induction of neuronal activity by administration of a forskolin analogue/high K(+) (10 microM/10 mM) for 24 h resulted in a fourfold increase in the number of EGFP-positive cells. Similarly, a twofold increase was obtained after treatment with the NK-1R-specific agonist [Sar(9),Met (O(2))(11)]-substance P (10 microM). These data demonstrate the usefulness of this model to study physiological and pharmacological factors relevant to nociceptive processing that can modulate PPT-A promoter activity.

High Ca(2+)-phosphate transfection efficiency enables single neuron gene analysis

Introducing exogenous genes into cells is one of the most important molecular techniques to study gene functions. Comparing to other type of cells, neurons are more difficult to transfect with cDNAs because they are very sensitive to microenvironmental changes. Among various gene transfer techniques, the Ca(2+)-phosphate transfection method is one of the most popular tools in neuroscience research because of its low cell toxicity and easiness to use. However, it is well known that the Ca(2+)-phosphate transfection efficiency in neurons is very low, typically in the range of 1-5%, which has limited its applications in gene functional analyses. Here we report a novel Ca(2+)-phosphate transfection protocol that dramatically increased the transfection efficiency by 10-fold, up to 60%, while maintaining low cell toxicity. The critical factors are the formation of homogenous snow-like precipitate with particle size about 1-3 microm and the subsequent removal of the precipitate. Using this new transfection protocol, we were able to routinely transfect single autaptic neurons in hippocampal microisland cultures and combine it with electrophysiology and fluorescent imaging methods to study gene functions. This high efficiency, low toxicity, and simple to use gene transfer method will have a broad application in gene research at the single cell level.

Cell specificity and efficiency of the Semliki forest virus vector- and adenovirus vector-mediated gene expression in mouse cerebellum

Establishing efficient gene transfer and expression in post-mitotic neurons is important in understanding the genetic basis of neural circuits with cellular complexity. This study evaluates the properties of exogenous green fluorescent protein (GFP) expression mediated by the Semliki forest virus (SFV) and adenovirus (Ad) vectors in dissociated and slice cultures of the mouse cerebellum. Infection with SFV-GFP resulted in early-onset and high-level GFP expression in about 90% of Purkinje cells and in about 40% of granule cells in dissociated cultures at 1 day after infection. Two days after infection, GFP-positive cells showed signs of SFV-derived cytotoxicity. Ad-GFP infected almost all astrocytes and granule cells in dissociated cultures, and showed a steady increase in GFP fluorescence with a plateau at around 2 days post-infection. Ad vector-mediated GFP expression lasted for several weeks with no significant cell damage. In the slice cultures, both viral vectors mainly infected astroglial cells, but also showed a similar cell preference as that in dissociated cultures. These data indicate that the use of different viral vectors and infection conditions offers a powerful means of expressing exogenous genes in cerebellar cultures with different cell-type specificity and timing and duration of expression.

Chemically controlled formation of a DNA/calcium phosphate coprecipitate: Application for transfection of mature hippocampal neurons

Numerous methods exist for transfecting postmitotic neurons, for example, DNA/calcium phosphate coprecipitation, cationic lipids, viruses, and physical methods such as microinjection, electroporation, and biolistics. Most methods, however, are either toxic to the cell, yield only poor transfection efficiencies, or cells have to be electroporated before plating. In this article, we present a standardized and fast transfection method using DNA/calcium phosphate coprecipitates that efficiently transfer DNA into mature, postmitotic hippocampal neurons. Shifting to CO(2)-independent media with a well-defined pH allows for the tight control of the coprecipitate formation and for adjusting the transfection parameters for the individual DNA plasmid used. The two critical parameters for reproducible and efficient transfections are: the precise pH during crystal formation, and the incubation time of the cells with the coprecipitate. This improved procedure now enables biochemical approaches. By transfecting a dominant-positive Ras mutant, we activate the Erk/MAP kinase signal transduction pathway. Furthermore, using a siRNA plasmid directed against MAP2, the level of an endogenously expressed protein is down-regulated upon transfection. These two approaches demonstrate that the presented transient transfection method can now be used to address questions on a biochemical level in hippocampal neurons.

The sequential activity of the GTPases Rap1B and Cdc42 determines neuronal polarity

The establishment of a polarized morphology is an essential step in the differentiation of neurons with a single axon and multiple dendrites. In cultured rat hippocampal neurons, one of several initially indistinguishable neurites is selected to become the axon. Both phosphatidylinositol 3,4,5-trisphosphate and the evolutionarily conserved Par complex (comprising Par3, Par6 and an atypical PKC (aPKC) such as PKClambda or PKCzeta) are involved in axon specification. However, the initial signals that establish cellular asymmetry and the pathways that subsequently translate it into structural changes remain to be elucidated. Here we show that localization of the GTPase Rap1B to the tip of a single neurite is a decisive step in determining which neurite becomes the axon. Using GTPase mutants and RNA interference, we found that Rap1B is necessary and sufficient to initiate the development of axons upstream of Cdc42 and the Par complex.

Live Optical Imaging of Nervous System Development

Although development of the nervous system is inherently a process of dynamic change, until recently it has generally been investigated by inference from static images. However, advances in live optical imaging are now allowing direct observation of growth, synapse formation, and even incipient function in the developing nervous system, at length scales from molecules to cortical regions, and over timescales from milliseconds to months. In this review, we provide technical background and present examples of how these new methods, including confocal and two-photon microscopy, GFP-based markers, and functional indicators, are being applied to provide fresh insight into long-standing questions of neural development.

Local Effects of BDNF on Dendritic Growth

The development of dendrites is a crucial step in the formation of cortical circuitry. The morphogen brain-derived neurotrophic factor (BDNF) may mediate the effects of activity on dendritic morphology since its expression and release are thought to be activity-dependent. Using two-photon microscopy, the autocrine and paracrine effects of BDNF on dendritic morphology were assessed. Overexpression of BDNF profoundly altered the form and stability of basal dendritic arbors via an autocrine mechanism. Paracrine BDNF also altered dendritic branching, though in a highly local fashion. BDNF is capable of acting as an intercellular morphogen, and could hypothetically shape dendritic arbors to best fit the developing structure and function of the pre-synaptic circuit.

Rapid and efficient electroporation-based gene transfer into primary dissociated neurons

Non-viral gene transfer into neurons has proved to be a formidable task. Here, we describe an electroporation-based method that allows efficient and reliable DNA transfer into dissociated neural cells before they are plated and cultured. In hippocampal neural cells derived from either neonatal mouse or embryonic chicken brains, a high transfection rate was already observed 5 h after transfection, and reached 40-80% in 24 h, as monitored by expression of enhanced green fluorescent protein (eGFP). The level of eGFP expression per cell depended on the amount of DNA used in a gene transfer experiment. The survival and neuritic length of transfected cells resembled that of non-electroporated cells. The transfected neurons showed normal immunostaining for endogenous synaptic protein synaptophysin and the neural cell adhesion molecule (NCAM). Furthermore, efficient gene transfer of the NCAM isoform NCAM140 and eGFP-tagged NCAM140 could be achieved, allowing visualization of NCAM140 expression. Also, a glycosylphosphatidylinositol-anchored eGFP could be efficiently expressed, highlighting lipid rafts without altering electrophysiological properties of transfected neurons. When neurons transfected with green and red fluorescent proteins were cocultured, fine details of their interactions could be revealed in time-lapse experiments. Thus, the method provides a useful tool for elucidation of genes involved in different neuronal functions, including neurite outgrowth, synaptogenesis and synaptic transmission.

Coupling the iron-responsive element to GFP--an inducible system to study translation in a single living cell

Local protein synthesis in a cell represents an elegant mechanism to achieve important biological phenomena such as cell migration, body axis formation during embryonic development and establishment of cell polarity. A prerequisite to studying translation in a restricted cellular compartment is the ability to unambiguously discriminate between proteins that arise through local protein synthesis and those that reach the site of interest by diffusion or transport. To tackle this problem, we set up a green fluorescent protein (GFP)-based reporter system that allows one to uncouple the translation of reporter gene mRNA from its subcellular localization. The system is based on the iron-responsive element, which regulates the translation of both endogenous ferritin and transferrin transcripts in response to changes in iron concentration. Translation of the reporter messenger RNA (mRNA) is thus dependent on iron in the medium; both its transcription and localization, however, are unaffected. Known targeting sequences can be used to direct the mRNA transcript to a subcellular compartment of interest. For instance, the full-length 3'-untranslated region of calcium/calmodulin-dependent protein kinase IIalpha mRNA can be added to the construct, after the stop codon of the GFP sequence, to selectively target the transcript into the dendrites of transiently transfected hippocampal neurons. This novel fluorescent assay will allow us to address a number of important biological questions in living mammalian cells.

A modified Sindbis vector for prolonged gene expression in neurons

Sindbis viruses have been widely used in neurobiology to express a variety of genes in cultured neurons, in cultured slices, and in vivo. They provide fast onset and high levels of expression of foreign genes, but the expression is limited to a short time window due to a shut-off of host protein synthesis. We have used a mutation in an essential gene (nsP2) of the life cycle of Sindbis, which allows the functional analysis of changes in protein expression for >/=6 days after infection. This Sindbis mutant (nsP2) was used to express enhanced green fluorescent protein (EGFP) in hippocampal neurons in culture and in vivo without any sign of toxicity, based on two-photon imaging and electrophysiology. In addition, the EGFP mutant virus can be injected in vivo to visualize spines and other details of neuronal structure. The Sindbis mutant described here provides an improved tool in neurobiology with reduced cytotoxicity and a prolonged time window of expression for novel applications in imaging and behavior. In addition, the use of this vector for the functional expression of mammalian voltage-gated ion channels in organotypic slices is demonstrated.

Reducing GABA receptors

A number of important drugs act on GABA(A) receptors, pentameric GABA-gated chloride channels assembled from among 19 known subunits. In trying to discover the roles in the brain of the subunits and their combinations, with the goal of developing more selective drugs, one tool has been to reduce expression of the subunits and examine the functional consequences. After briefly examining the properties of GABA(A) receptors, this review surveys the means available for receptor subunit reduction, and some of the observations to which their application has led. The methods discussed include radiation-induced deletion, gene knockout, knock-in mutations, antisense, ribozymes, RNA interference, dominant negative constructs, and transcriptional regulation, e.g., via decoy oligonucleotides.