Manipulation of gene expression in the mammalian nervous system: application in the study of neurite outgrowth and neuroregeneration-related proteins

Isofuranodiene: A neuritogenic compound isolated from wild celery (Smyrnium olusatrum L., Apiaceae)

In the search for neuroactive compounds that mimic the nerve growth factor (NGF) activity for the protection against neurodegenerative diseases, the potential medicinal values of foods and plants attracts intense interest. Isofuranodiene is the major constituent of the essential oil of wild celery (Smyrnium olusatrum L., Apiaceae). The cytotoxic effects of isofuranodiene towards rat neuronal PC-12 pheochromocytoma cells were determined by MTT assay, while the cell differentiation was evaluated with xCELLigence real time cell analysis system (RTCA DP), and the neuritogenic activity was assessed by neurite outgrowth image analysis. Isofuranodiene at concentrations of 25 and 12.5 μM alone, or in combination with 50 nM NGF, showed a marked stimulation of neuritogenesis, but it was more effective at 12.5 μM with or without NGF. The present study reports the first evidence of the neuritogenic effects of isofuranodiene, which appears to be a promising neurotrophic and neuroprotective agent deserving further investigation.

Understanding the neural repair-promoting properties of olfactory ensheathing cells

Olfactory ensheathing glial cells (OECs) are a specialized type of glia that form a continuously aligned cellular pathway that actively supports unprecedented regeneration of primary olfactory axons from the periphery into the central nervous system. Implantation of OECs stimulates neural repair in experimental models of spinal cord, brain and peripheral nerve injury and delays disease progression in animal models for neurodegenerative diseases like amyotrophic lateral sclerosis. OECs implanted in the injured spinal cord display a plethora of pro-regenerative effects; they promote axonal regeneration, reorganize the glial scar, remyelinate axons, stimulate blood vessel formation, have phagocytic properties and modulate the immune response. Recently genome wide transcriptional profiling and proteomics analysis combined with classical or larger scale "medium-throughput" bioassays have provided novel insights into the molecular mechanism that endow OECs with their pro-regenerative properties. Here we review these studies and show that the gaps that existed in our understanding of the molecular basis of the reparative properties of OECs are narrowing. OECs express functionally connected sets of genes that can be linked to at least 10 distinct processes directly relevant to neural repair. The data indicate that OECs exhibit a range of synergistic cellular activities, including active and passive stimulation of axon regeneration (by secretion of growth factors, axon guidance molecules and basement membrane components) and critical aspects of tissue repair (by structural remodeling and support, modulation of the immune system, enhancement of neurotrophic and antigenic stimuli and by metabolizing toxic macromolecules). Future experimentation will have to further explore the newly acquired knowledge to enhance the therapeutic potential of OECs.

Imaging neocortical neurons through a chronic cranial window

The rich structural dynamics of axonal arbors and neuronal circuitry can only be revealed through direct and repeated observations of the same neuron(s) over time, preferably in vivo. This protocol describes a long-term, high-resolution method for imaging neocortical neurons in vivo, using a combination of two-photon laser scanning microscopy (2PLSM) and a surgically implanted chronic cranial window. The window is used because the skull of most mammals is too opaque to allow high-resolution imaging of cortical neurons. Using this method, it is feasible to image the smallest neuronal structures in the superficial layers of the neocortex, such as dendritic spines and axonal boutons. Because the surface area of the craniotomy is relatively large, this technique is even suitable for use when labeled neurons are relatively uncommon. The surgery and imaging procedures are illustrated with examples from our studies of structural plasticity in the developing or adult mouse brain. The protocol is optimized for adult mice; we have used mice up to postnatal day 511 (P511). With minor modifications, it is possible to image neurons in rats and mice from P2. Most of our studies have used the Thy1 promoter to drive expression of fluorophores in subsets of cortical neurons.

Molecular target discovery for neural repair in the functional genomics era

A comprehensive understanding of the molecular pathways activated by traumatic neural injury is of major importance for the development of treatments for spinal cord injury (SCI). High-throughput gene expression profiling is a powerful approach to reveal genome-wide changes in gene expression during a specific biological process. Microarray analysis of injured nerves or neurons would ideally generate new hypotheses concerning the progression or deregulation of injury- and repair-related biological processes, such as neural scar formation and axon regeneration. These hypotheses should subsequently be tested experimentally and would eventually provide the molecular substrates for the development of novel therapeutics. Over the last decade, this approach has elucidated numerous extrinsic (mostly neural scar-associated) as well as neuron-intrinsic genes that are regulated following an injury. To date, the main challenge is to translate the observed injury-induced gene expression changes into a mechanistic framework to understand their functional implications. To achieve this, research on neural repair will have to adopt the conceptual advances and analytical tools provided by the functional genomics and systems biology revolution. Based on progress made in bioinformatics, high-throughput and high-content functional cellular screening, and in vivo gene transfer technology, we propose a multistep "roadmap" that provides an integrated strategy for molecular target discovery for repair of the injured spinal cord.

Transcribing the path to neurological recovery-From early signals through transcription factors to downstream effectors of successful regeneration

The peripheral nervous system is known to regenerate comparatively well and this ability is mirrored in the de novo expression or upregulation of a wide variety of molecules involved in axonal outgrowth starting with transcription factors, but also including growth-stimulating substances, guidance and cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions. Recent studies using pharmacological agents, and global as well as neuron-selective gene inactivation techniques have shed light on those endogenous molecules that play a non-redundant role in mediating regenerative axonal outgrowth in vivo. The aim of the current review is to sketch the sequence of molecular events from early sensors of injury to transcription factors to downstream effectors that cooperate in successful regeneration and functional recovery.

Coupling of cAMP/PKA and MAPK signaling in neuronal cells is dependent on developmental stage

Neurite formation, an essential feature of neuronal development, is believed to involve participation of the ras-mitogen-activated protein kinase (MAPK) and cAMP-dependent protein kinase A (cAMP/PKA)-mediated signaling pathways. These pathways have been studied extensively in the rat pheochromocytoma cell line PC12, and current hypotheses suggest a single effector mechanism resulting from the convergence of cAMP/PKA and MAPK signaling. However, based on observations using a central neuronal progenitor cell line, AS583-8, there also exists evidence that the two signaling pathways may act independently resulting in neurites with differing dynamic features. In the present study, the upstream components of cAMP/PKA signaling were examined in AS583-8 cells as well as possible interactions with the MAPK pathway. We found that activation of PKA is both necessary and sufficient for the elaboration of rapidly forming processes, typical of the cAMP response. In addition, blockade of the MAPK pathway has no effect on the cAMP response, suggesting that activation of the cAMP/PKA pathway can stimulate neurite formation independent of the MAPK pathway. In order to evaluate which cell line model, PC12 vs AS583-8, best reflects the signaling features of developing central neurons, we examined interactions between cAMP/PKA and MAPK signaling in primary neuronal cultures from several brain regions. In immature cultures (1-day-old), at a point where the initiation of neurite formation is maximal, no interaction was observed. In more mature cultures (7 days old), where synaptic contacts have been established, we found a weak but reproducible activation of MAPK following stimulation of the cAMP/PKA pathway. These results suggest that cAMP/PKA and MAPK signaling act independently at the initiation of neuritogenesis but become coupled during later stages of neuronal development. Therefore, the interaction of the two pathways may be cell stage (younger vs older) specific and may participate in cellular functions that take place after initial neurite formation.

Vasopressin-induced neurotrophism in cultured neurons of the cerebral cortex: dependency on calcium signaling and protein kinase C activity

Neuronal process outgrowth has been postulated to be one of the fundamental steps involved in neuronal development. To test whether vasopressin can influence neuronal development by acting on the outgrowth of neuronal processes, we determined the neurotrophic action of the memory-enhancing peptide, vasopressin, in neurons derived from the cerebral cortex, a site of integrative cognitive function and long-term memory. Exposure to V(1) receptor agonist significantly increased multiple features of nerve cell morphology, including neurite length, number of branches, branch length, number of branch bifurcation points and number of microspikes. The dose-response profile of V(1) receptor agonist-induced neurotrophism exhibited a biphasic function, with lower concentrations inducing a significant increase while higher concentrations generally induced no significant effect. The neurotrophic effect of V(1) receptor activation did not require growth factors present in serum. Analysis of the regional selectivity of the vasopressin-induced neurotrophic effect revealed significant V(1) receptor agonist-induced neurotrophism in occipital and parietal neurons, whereas frontal and temporal neurons were unresponsive. Results of experiments to determine the mechanism of vasopressin-induced neurotrophism demonstrated that vasopressin-induced neurotrophism is dependent on V(1)a receptor activation, requires L-type calcium channel activation and activation of both pathways of the phosphatidylinositol signaling cascade, inositol trisphosphate and protein kinase C. These studies are the first to describe a functional cellular response for vasopressin in the cerebral cortex. The findings are discussed with respect to their implications for understanding the role of vasopressin-induced neurotrophism, the associated signaling pathways required for this response, and the ability of vasopressin to enhance memory function.

Extensive sprouting of sensory afferents and hyperalgesia induced by conditional expression of nerve growth factor in the adult spinal cord

Genetic transfer of growth-promoting molecules was proposed as a potential strategy to modify the nonpermissive nature of the adult CNS to induce axonal regeneration. To evaluate whether overexpression of neurotrophins or cellular adhesion molecules would effect axonal plasticity, adenoviruses encoding fibroblast growth factor-2 (FGF-2/Adts), nerve growth factor (NGF/Adts), neurotrophin-3, and the cell adhesion molecules N-cadherin and L1 were injected into the dorsal horn of the adult spinal cord. Transgene expression was primarily localized to astrocytes in the dorsal horn and motor neurons within the ventral horn. Overexpression of these factors, with the exception of NGF/Adts, failed to increase axonal sprouting. Eight days after NGF/Adts injections, axonal sprouting within the dorsal horn was apparent, and after 4 weeks, extensive spouting was observed throughout the entire dorsal horn, extending into the ventral horn and the white matter of the lateral funiculus. These axons were identified primarily as a subpopulation of nociceptive fibers expressing calcitonin gene-related peptide and substance-P. Behavioral analysis revealed thermal hyperalgesia and perturbation of accurate paw placement on grid-walking tasks for both FGF-2- and NGF-treated animals. These results indicate that the administration of growth-promoting molecules can induce robust axonal plasticity of normal adult primary sensory neurons into areas of transgene expression, causing significant alterations in behavioral responses. This observation also indicates that gene transfer protocols that aim to reconstruct diseased or injured pathways should also be designed to prevent the sprouting of the normal circuitry from adjacent unaffected neurons.

S(+)-ketamine up-regulates neuronal regeneration associated proteins following glutamate injury in cultured rat hippocampal neurons

In previous studies, racemic ketamine improved neurological outcome after experimental brain injury and S(+)-ketamine demonstrated neuroprotective effects in neurons after damage in vitro. We compared the expression of regeneration-associated proteins in rat hippocampal neurons after glutamate injury and treatment with S(+)-ketamine versus racemic ketamine. Following an 8 minute exposure to 100 microM glutamate, neurons were maintained untreated or in the presence of S(+)-ketamine or racemic ketamine (10(-4) M, 10(-5) M, 10(-6) M) for one week. Growth-associated protein-43 (GAP-43) and synaptosomal-associated protein-25 (SNAP-25) was analyzed by Western Blotting, the mitochondrial transmembrane potential (MTP) by fluorescence imaging, and [3H]2-deoxy-D-glucose ([3H]2-DG) uptake by scintillation spectrometry. Seven days after exposure, GAP-43 decreased to 15% and SNAP-25 to 30% in the glutamate-injured, untreated neurons. The MTP declined to 50% and [3H]2-DG to 30%. Both S(+)-ketamine and racemic ketamine at 10(-4) M and 10(-5) M minimized the decline in MTP, almost maintaining it at control value. Additionally, S(+)-ketamine and racemic ketamine decreased the reduction in [3H]2-DG. S(+)-ketamine at 10(-4) M and 10(-5) M and racemic ketamine at 10(-4) M reduced the decline in SNAP-25 to 60% of controls (P < .05). However, S(+)-ketamine at 10(-4) M and 10(-5) M only reversed the decrease in GAP-43 to 50% and 40% of controls, respectively (P < .05). We conclude that the synthesis of a growth-associated protein related to plasticity and repair in the adult nervous system is increased by S(+)-ketamine but is not increased by racemic ketamine.

Micro-scale chromophore-assisted laser inactivation of nerve growth cone proteins

Directed growth cone movement is crucial for the correct wiring of the nervous system. This movement is governed by the concerted actions of cell surface receptors, signaling proteins, cytoskeleton-associated molecules, and molecular motors. In order to investigate the molecular basis of growth cone motility, we applied a new technique to functionally inactivate proteins: micro-scale Chromophore-Assisted Laser Inactivation [Diamond et al. (1993) Neuron 11:409-421]. Micro-CALI uses laser light of 620 nm, focused through microscope optics into a 10-microm spot. The laser energy is targeted via specific Malachite green-labeled, non-function-blocking antibodies, that generate short-lived protein-damaging hydroxyl radicals [Liao et al. (1994) Proc Natl Acad Sci USA 91:2659-2663]. Micro-CALI mediates specific loss of protein function with unachieved spatial and temporal resolution. Combined with time-lapse video microscopy, it offers the possibility to induce and observe changes in growth cone dynamics on a real time base. We present here the effects of the acute and localized inactivation of selected growth cone molecules on growth cone behavior and morphology. Based on our observations, we propose specific roles for these proteins in growth cone motility and neurite outgrowth.

Biological interventions for spinal cord injury

Spinal cord injury is frequently followed by the loss of supraspinal control of sensory, autonomus and motor functions at sublesional level. To enhance recovery in patients with spinal cord injuries, three fundamental strategies have been developed in experimental models. These strategies involve three different time points for postlesional intervention in the spinal cord. Neuroprotection soon after injury uses pharmacological tools to reduce the progressive secondary injury processes that follow during the first week after the initial lesion occurs, in order to limit tissue damage. A second strategy, which is initiated shortly after the lesion occurs, aims at promoting axonal regeneration by acting pharmacologically on inhibitors or barriers of regeneration, or by the application of cell or gene therapy as a source of neurotrophic factors or as a bridge or support to enhance the regeneration of lesioned axons. Finally, a mid-term substitutive strategy is the management of the sublesional spinal cord by sensorimotor stimulation or the supply of missing key afferents, such as monoaminergic systems. These three strategies are reviewed. Only a combination of these different approaches can provide an optimal basis for potential therapeutic interventions aimed at functional recovery after spinal cord injury.