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In JoVE (1)
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- Annals of Neurology
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- Cell Cycle (Georgetown, Tex.)
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Articles by Simone Di Giovanni in JoVE
JoVE Neuroscience
Chromatine immunoprecipitatie van achterwortelganglia Tissue volgende axonale schade
1Laboratory for NeuroRegeneration and Repair, Department of Neurology, Hertie Institute for Clinical Brain Research, University of Tuebingen, 2Graduate School for Cellular and Molecular Neuroscience, University of Tuebingen
We presenteren een methode voor het chromatine immunoprecipitatie van achterwortelganglia weefsel na axonale schade. De aanpak kan worden gebruikt om specifieke transcriptiefactor bindingsplaatsen en epigenetische modificatie van histon en DNA belangrijk voor de regeneratie van axonen gewonden in zowel het perifere en het centrale zenuwstelsel te identificeren.
Other articles by Simone Di Giovanni on PubMed
Gene Profiling in Spinal Cord Injury Shows Role of Cell Cycle in Neuronal Death
Spinal cord injury causes secondary biochemical changes leading to neuronal cell death. To clarify the molecular basis of this delayed injury, we subjected rats to spinal cord injury and identified gene expression patterns by high-density oligonucleotide arrays (8,800 genes studied) at 30 minutes, 4 hours, 24 hours, or 7 days after injury (total of 26 U34A profiles). Detailed analyses were limited to 4,300 genes consistently expressed above background. Temporal clustering showed rapid expression of immediate early genes (30 minutes), followed by genes associated with inflammation, oxidative stress, DNA damage, and cell cycle (4 and 24 hours). Functional clustering showed a novel pattern of cell cycle mRNAs at 4 and 24 hours after trauma. Quantitative reverse transcription polymerase chain reaction verified mRNA changes in this group, which included gadd45a, c-myc, cyclin D1 and cdk4, pcna, cyclin G, Rb, and E2F5. Changes in their protein products were quantified by Western blot, and cell-specific expression was determined by immunocytochemistry. Cell cycle proteins showed an increased expression 24 hours after injury and were, in part, colocalized in neurons showing morphological evidence of apoptosis. These findings suggest that cell cycle-related genes, induced after spinal cord injury, are involved in neuronal damage and subsequent cell death.
Constitutive Activation of MAPK Cascade in Acute Quadriplegic Myopathy
Acute quadriplegic myopathy (AQM; also called "critical illness myopathy") shows acute muscle wasting and weakness and is experienced by some patients with severe systemic illness, often associated with administration of corticosteroids and/or neuroblocking agents. Key aspects of AQM include muscle atrophy and myofilament loss. Although these features are shared with neurogenic atrophy, myogenic atrophy in AQM appears mechanistically distinct from neurogenic atrophy. Using muscle biopsies from AQM, neurogenic atrophy, and normal controls, we show that both myogenic and neurogenic atrophy share induction of myofiber-specific ubiquitin/proteosome pathways (eg, atrogin-1). However, AQM patient muscle showed a specific strong induction of transforming growth factor (TGF)-beta/MAPK pathways. Atrophic AQM myofibers showed coexpression of TGF-beta receptors, p38 MAPK, c-jun, and c-myc, including phosphorylated active forms, and these same fibers showed apoptotic features. Our data suggest a model of AQM pathogenesis in which stress stimuli (sepsis, corticosteroids, pH imbalance, osmotic imbalance) converge on the TGF-beta pathway in myofibers. The acute stimulation of the TGF-beta/MAPK pathway, coupled with the inactivity-induced atrogin-1/proteosome pathway, leads to the acute muscle loss seen in AQM patients.
BOK and NOXA Are Essential Mediators of P53-dependent Apoptosis
Cellular stress leads to DNA damage and activation of the intrinsic apoptotic pathway in which translocation of mitochondrial cytochrome c to the cytosol plays a critical role. Previous studies have suggested alternative mechanisms responsible for this process. We examined initiation mechanisms of the intrinsic apoptotic pathway using human neuroblastoma and breast cancer cells. Results indicated that translocation of cytochrome c does not require prior activation of caspases but rather depends on activation of specific BCL-2 family members, depending upon the type of death signal. Thus, DNA damage-induced apoptosis requires new protein synthesis, accumulation of p53 tumor suppressor protein, and p53-dependent induction of BOK and NOXA genes, while a role for BAX in this pathway is not essential. In contrast, apoptosis induced by staurosporine does not require protein synthesis but is characterized by translocation of BAX. Based on these findings, we propose a model of the intrinsic apoptotic cascade induced by DNA damage where proapoptotic BOK substitutes for a function of BAX.
In Vivo and in Vitro Characterization of Novel Neuronal Plasticity Factors Identified Following Spinal Cord Injury
Following spinal cord injury, there are numerous changes in gene expression that appear to contribute to either neurodegeneration or reparative processes. We utilized high density oligonucleotide microarrays to examine temporal gene profile changes after spinal cord injury in rats with the goal of identifying novel factors involved in neural plasticity. By comparing mRNA changes that were coordinately regulated over time with genes previously implicated in nerve regeneration or plasticity, we found a gene cluster whose members are involved in cell adhesion processes, synaptic plasticity, and/or cytoskeleton remodeling. This group, which included the small GTPase Rab13 and actin-binding protein Coronin 1b, showed significantly increased mRNA expression from 7-28 days after trauma. Overexpression in vitro using PC-12, neuroblastoma, and DRG neurons demonstrated that these genes enhance neurite outgrowth. Moreover, RNAi gene silencing for Coronin 1b or Rab13 in NGF-treated PC-12 cells markedly reduced neurite outgrowth. Coronin 1b and Rab13 proteins were expressed in cultured DRG neurons at the cortical cytoskeleton, and at growth cones along with the pro-plasticity/regeneration protein GAP-43. Finally, Coronin 1b and Rab13 were induced in the injured spinal cord, where they were also co-expressed with GAP-43 in neurons and axons. Modulation of these proteins may provide novel targets for facilitating restorative processes after spinal cord injury.
Neuronal Plasticity After Spinal Cord Injury: Identification of a Gene Cluster Driving Neurite Outgrowth
Functional recovery after spinal cord injury (SCI) may result in part from axon outgrowth and related plasticity through coordinated changes at the molecular level. We employed microarray analysis to identify a subset of genes the expression patterns of which were temporally coregulated and correlated to functional recovery after SCI. Steady-state mRNA levels of this synchronously regulated gene cluster were depressed in both ventral and dorsal horn neurons within 24 h after injury, followed by strong re-induction during the following 2 wk, which paralleled functional recovery. The identified cluster includes neuritin, attractin, microtubule-associated protein 1a, and myelin oligodendrocyte protein genes. Transcriptional and protein regulation of this novel gene cluster was also evaluated in spinal cord tissue and in single neurons and was shown to play a role in axonal plasticity. Finally, in vitro transfection experiments in primary dorsal root ganglion cells showed that cluster members act synergistically to drive neurite outgrowth.
Cell Cycle Inhibition Provides Neuroprotection and Reduces Glial Proliferation and Scar Formation After Traumatic Brain Injury
Traumatic brain injury (TBI) causes neuronal apoptosis, inflammation, and reactive astrogliosis, which contribute to secondary tissue loss, impaired regeneration, and associated functional disabilities. Here, we show that up-regulation of cell cycle components is associated with caspase-mediated neuronal apoptosis and glial proliferation after TBI in rats. In primary neuronal and astrocyte cultures, cell cycle inhibition (including the cyclin-dependent kinase inhibitors flavopiridol, roscovitine, and olomoucine) reduced up-regulation of cell cycle proteins, limited neuronal cell death after etoposide-induced DNA damage, and attenuated astrocyte proliferation. After TBI in rats, flavopiridol reduced cyclin D1 expression in neurons and glia in ipsilateral cortex and hippocampus. Treatment also decreased neuronal cell death and lesion volume, reduced astroglial scar formation and microglial activation, and improved motor and cognitive recovery. The ability of cell cycle inhibition to decrease both neuronal cell death and reactive gliosis after experimental TBI suggests that this treatment approach may be useful clinically.
Gene Expression Profiling of Experimental Traumatic Spinal Cord Injury As a Function of Distance from Impact Site and Injury Severity
Changes in gene expression contribute to pathophysiological alterations following spinal cord injury (SCI). We examined gene expression over time (4 h, 24 h, 7 days) at the impact site, as well as rostral and caudal regions, following mild, moderate, or severe contusion SCI in rats. High-density oligonucleotide microarrays were used that included approximately 27,000 genes/ESTs (Affymetrix RG-U34; A, B and C arrays), together with multiple analyses (MAS 5.0, dChip). Alterations after mild injury were relatively rapid (4 and 24 h), whereas they were delayed and prolonged after severe injury (24 h and 7 days). The number and magnitude of gene expression changes were greatest at the injury site after moderate injury and increased in rostral and caudal regions as a function of injury severity. Sham surgery resulted in expression changes that were similar to mild injury, suggesting the importance of using time-linked surgical controls as well as naive animals for these kinds of studies. Expression of many genes and ESTs was altered; these were classified functionally based on ontology. Overall representation of these functional classes varied with distance from the site of injury and injury severity, as did the individual genes that contributed to each functional class. Different clustering approaches were used to identify changes in neuronal-specific genes and several transcription factors that have not previously been associated with SCI. This study represents the most comprehensive evaluation of gene expression changes after SCI to date. The results underscore the power of microarray approaches to reveal global genomic responses as well as changes in particular gene clusters and/or families that may be important in the secondary injury cascade.
Role of the Cell Cycle in the Pathobiology of Central Nervous System Trauma
Upregulation of cell cycle proteins occurs in both mitotic and post-mitotic neural cells after central nervous system (CNS) injury in adult animals. In mitotic cells, such as astroglia and microglia, they induce proliferation, whereas in post-mitotic cells such as neurons they initiate caspase-related apoptosis. We recently reported that early central administration of the cell cycle inhibitor flavopiridol after experimental traumatic brain injury (TBI) significantly reduced lesion volume, scar formation and neuronal cell death, while promoting near complete behavioral recovery. Here we show that in primary neuronal or astrocyte cultures structurally different cell cycle inhibitors (flavopiridol, roscovitine, and olomoucine) significantly reduce upregulation of cell cycle proteins, attenuate neuronal cell death induced by etoposide, and decrease astrocyte proliferation. Flavopiridol, in a concentration dependent manner, also attenuates proliferation/activation of microglia. In addition, we demonstrate that central administration of flavopiridol improves functional outcome in dose-dependent manner after fluid percussion induced brain injury in rats. Moreover, delayed systemic administration of flavopiridol significantly reduces brain lesion volume and edema development after TBI. These data provide further support for the therapeutic potential of cell cycle inhibitors for the treatment of clinical CNS injury and that protective mechanisms likely include reduction of neuronal cell death, inhibition of glial proliferation and attenuation of microglial activation.
Functional Recovery of Glycine Receptors in Spastic Murine Model of Startle Disease
Clinical variability is common in inherited gene defects of the central nervous system in humans and in animal models of human disorders. Here, we used the homozygous spastic (spa) mutant mice, which resemble human hereditary hyperekplexia, to determine the molecular remodeling of the spinal cord through the course of the disease, and develop a model for clinical disparity between littermates. The spa mutation is an insertion of a LINE-1 element in the gene for the beta subunit of the glycine receptor, Glrb. The insertion causes aberrant splicing in the beta subunit of glycine receptor gene with a consequent important reduction of glycine receptors. At young ages, all homozygous spa animals were spastic, showed loss of glycine receptors, increased expression of vesicular glycine/GABA transporter and NMDA receptors, induction of activated caspase3, and preferential loss of glycinergic interneurons consistent with neurotransmitter toxicity model. Those littermates that recovered from symptoms showed strong over-expression of the glycine receptor alpha 1 subunit (Glra1), and increased myelination and synaptic plasticity. Littermates that showed a deteriorating clinical course failed to over-express Glra1, and also showed relative loss of gephyrin (receptor clustering). These molecular changes were associated with a preferential loss of GABAergic interneurons, and extensive motorneuron loss. These data suggest that functional recovery is likely due to expression of homomeric glycine receptors, rescue from excitotoxicity, and subsequent neuronal remodeling. We propose that human patients with hyperekplexia show remodeling similar to that of the recovering spa mice, as human patients also show a lessening of symptoms as a function of age.
Multiple Sclerosis in Twins from Continental Italy and Sardinia: a Nationwide Study
Knowledge about the balance between heritable and nonheritable risk in multiple sclerosis (MS) is based on twin studies in high-prevalence areas. In a study that avoided ascertainment limitations and directly compared continental Italy (medium-prevalence) and Sardinia (high-prevalence), we ascertained 216 pairs from 34,549 patients. This gives a twinning rate of 0.62% among MS patients, significantly less than that of the general population. In continental Italy, probandwise concordance was 14.5% (95% confidence interval, 5.1-23.8) for monozygotic and 4.0% (95% confidence interval, 0.8-7.1) for dizygotic twins. Results in Sardinia resemble those in northern populations but in limited numbers. Monozygotic concordance was 22.2% (95% confidence interval, 0-49.3) probandwise, but no concordant dizygotic pairs were identified. A questionnaire on 80 items possibly related to disease cause was administered to 70 twin pairs, 135 sporadic patients, and 135 healthy volunteers. Variables positively (7) or negatively (2) associated with predisposition and concordance in twins largely overlapped and were mainly linked to infection. If compared with previous studies, our data demonstrate that penetrance in twins appears to correlate with MS prevalence. They highlight the relevance of nonheritable variables in Mediterranean areas. The apparent underrepresentation of MS among Italian twins draws attention to protective factors, shared by twins, that may influence susceptibility.
Expression of Two Temporally Distinct Microglia-related Gene Clusters After Spinal Cord Injury
The dual role of microglia in cytotoxicity and neuroprotection is believed to depend on the specific, temporal expression of microglial-related genes. To better clarify this issue, we used high-density oligonucleotide microarrays to examine microglial gene expression after spinal cord injury (SCI) in rats. We compared expression changes at the lesion site, as well as in rostral and caudal regions after mild, moderate, or severe SCI. Using microglial-associated anchor genes, we identified two clusters with different temporal profiles. The first, induced by 4 h postinjury to peak between 4 and 24 h, included interleukin-1beta, interleukin-6, osteopontin, and calgranulin, among others. The second was induced 24 h after SCI, and peaked between 72 h and 7 days; it included C1qB, Galectin-3, and p22(phox). These two clusters showed similar expression profiles regardless of injury severity, albeit with slight decreases in expression in mild or severe injury vs. moderate injury. Expression was also decreased rostral and caudal to the lesion site. We validated the expression of selected cluster members at the mRNA and protein levels. In addition, we demonstrated that stimulation of purified microglia in culture induces expression of C1qB, Galectin-3, and p22(phox). Finally, inhibition of p22(phox) activity within microglial cultures significantly suppressed proliferation in response to stimulation, confirming that this gene is involved in microglial activation. Because microglial-related factors have been implicated both in secondary injury and recovery, identification of temporally distinct clusters of genes related to microglial activation may suggest distinct roles for these groups of factors.
Regeneration Following Spinal Cord Injury, from Experimental Models to Humans: Where Are We?
Regeneration in the adult CNS following injury is extremely limited. Traumatic spinal cord injury causes a permanent neurological deficit followed by a very limited recovery due to failed regeneration attempts. In fact, it is now clear that the spinal cord intrinsically has the potential to regenerate, but cellular loss and the presence of an inhibitory environment strongly limit tissue regeneration and functional recovery. The molecular mechanisms responsible for failed regeneration are starting to be unveiled. This gain in knowledge led to the design of therapeutic strategies aimed to limit the tissue scar, to enhance the proregeneration versus the inhibitory environment, and to replace tissue loss, including the use of stem cells. They have been very successful in several animal models, although results are still controversial in humans. Nonetheless, novel experimental approaches hold great promise for use in humans.
Distinct P53 Acetylation Cassettes Differentially Influence Gene-expression Patterns and Cell Fate
The activity of the p53 gene product is regulated by a plethora of posttranslational modifications. An open question is whether such posttranslational changes act redundantly or dependently upon one another. We show that a functional interference between specific acetylated and phosphorylated residues of p53 influences cell fate. Acetylation of lysine 320 (K320) prevents phosphorylation of crucial serines in the NH(2)-terminal region of p53; only allows activation of genes containing high-affinity p53 binding sites, such as p21/WAF; and promotes cell survival after DNA damage. In contrast, acetylation of K373 leads to hyperphosphorylation of p53 NH(2)-terminal residues and enhances the interaction with promoters for which p53 possesses low DNA binding affinity, such as those contained in proapoptotic genes, leading to cell death. Further, acetylation of each of these two lysine clusters differentially regulates the interaction of p53 with coactivators and corepressors and produces distinct gene-expression profiles. By analogy with the "histone code" hypothesis, we propose that the multiple biological activities of p53 are orchestrated and deciphered by different "p53 cassettes," each containing combination patterns of posttranslational modifications and protein-protein interactions.
The Tumor Suppressor Protein P53 is Required for Neurite Outgrowth and Axon Regeneration
Axon regeneration is substantially regulated by gene expression and cytoskeleton remodeling. Here we show that the tumor suppressor protein p53 is required for neurite outgrowth in cultured cells including primary neurons as well as for axonal regeneration in mice. These effects are mediated by two newly identified p53 transcriptional targets, the actin-binding protein Coronin 1b and the GTPase Rab13, both of which associate with the cytoskeleton and regulate neurite outgrowth. We also demonstrate that acetylation of lysine 320 (K320) of p53 is specifically involved in the promotion of neurite outgrowth and in the regulation of the expression of Coronin 1b and Rab13. Thus, in addition to its recognized role in neuronal apoptosis, surprisingly, p53 is required for neurite outgrowth and axonal regeneration, likely through a different post-translational pathway. These observations may suggest a novel therapeutic target for promoting regenerative responses following peripheral or central nervous system injuries.
Gene Expression Profiles Reveal Homeostatic Dynamics During Interferon-beta Therapy in Multiple Sclerosis
Understanding the mechanisms that sustain the effects of disease modifying drugs in multiple sclerosis (MS) may help refine current therapies and improve our knowledge of disease pathogenesis. By using cDNA microarrays, we investigated gene expression in the peripheral blood mononuclear cells (PBMC) of 7 MS patients, at baseline (T0) as well as after 1 (T1) and 3 months (T3) of interferon beta-1a (IFN-beta-1a; Rebif 44 microg) therapy. Gene expression changes involved genes of both immunological and non-immunological significance. We validated IL-10 up-regulation, which is in accordance with previous reports, and other novel changes that underscore the capacity of IFN-beta to impair antigen presentation and migration of inflammatory elements into the central nervous system (up-regulation of filamin B and down-regulation of IL-16 and rab7). Overall, gene expression changes became less pronounced after 3 months of therapy, suggesting a homeostatic response to IFN-beta. This may be of use for the design of new treatment schedules.
Cell Cycle Activation Contributes to Post-mitotic Cell Death and Secondary Damage After Spinal Cord Injury
Spinal cord injury (SCI) causes delayed secondary biochemical alterations that lead to tissue loss and associated neurological dysfunction. Up-regulation of cell cycle proteins occurs in both neurons and glia after SCI and may contribute to these changes. The present study examined the role of cell cycle activation on secondary injury after severe SCI in rat. SCI caused cell cycle protein up-regulation associated with neuronal and oligodendroglial apoptosis, glial scar formation and microglial activation. Treatment with the cell cycle inhibitor flavopiridol reduced cell cycle protein induction and significantly improved functional recovery versus vehicle-treated controls at 21 and 28 days post-injury. Treatment also significantly reduced lesion volume, as measured by MRI and histology, decreased astrocytic reactivity, attenuated neuronal and oligodendroglial apoptosis and reduced the production of factors associated with microglial activation. Thus, flavopiridol treatment improves outcome after SCI by inhibiting cell cycle pathways, resulting in beneficial multifactorial actions on neurons and glia.
NFAT Signaling in Neural Development and Axon Growth
The NFAT (nuclear factor of activated T-cells) family of transcription factors functions as integrators of multiple signaling pathways by binding to chromatin in combination with other transcription factors and coactivators to regulate genes central for many developmental systems. Recent experimental evidence has shown that the calcineurin/NFAT signaling pathway is important in axonal growth and guidance during vertebrate development. In fact, studies with triple NFATc2/c3/c4 mutant mice demonstrate that the extension and organization of sensory axon projection and commissural axon growth are both dependent upon NFAT activity. Neurotrophin and L-type calcium channel signaling modulate intracellular calcium levels to regulate the nuclear import and transcriptional activity of NFAT by activating the phosphatase calcineurin. The rephosphorylation and subsequent export of NFAT from the nucleus is mediated by several kinases, including GSK-3beta, which contribute to the fine tuning of NFAT transcriptional activity in neurons. However, currently, no direct transcriptional targets for NFAT have been identified in a chromatin environment in the nervous system. Undiscovered are also the binding partners of NFAT that might combinatorially regulate specific genes important for neuronal development. This review will discuss the current knowledge related to NFAT signaling in the nervous system development and the potential for future research directions.
The Non-apoptotic Role of P53 in Neuronal Biology: Enlightening the Dark Side of the Moon
The transcription factor p53 protects neurons from transformation and DNA damage through the induction of cell-cycle arrest, DNA repair and apoptosis in a range of in vitro and in vivo conditions. Indeed, p53 has a crucial role in eliciting neuronal cell death during development and in adult organisms after exposure to a range of stressors and/or DNA damage. Nevertheless, accumulating evidence challenges this one-sided view of the role of p53 in the nervous system. Here, we discuss how-unexpectedly-p53 can regulate the proliferation and differentiation of neural progenitor cells independently of its role in apoptosis, and p53 post-translational modifications might promote neuronal maturation, as well as axon outgrowth and regeneration, following neuronal injury. We hope to encourage a more comprehensive view of the non-apoptotic functions of p53 during neural development, and to warn against oversimplifications regarding its role in neurons. In addition, we discuss how further insight into the p53-dependent modulation of these mechanisms is necessary to elucidate the decision-making processes between neuronal cell death and differentiation during development, and between neuronal degeneration and axonal regeneration after injury.
NFAT-3 is a Transcriptional Repressor of the Growth-associated Protein 43 During Neuronal Maturation
Transcription is essential for neurite and axon outgrowth during development. Recent work points to the involvement of nuclear factor of activated T cells (NFAT) in the regulation of genes important for axon growth and guidance. However, NFAT has not been reported to directly control the transcription of axon outgrowth-related genes. To identify transcriptional targets, we performed an in silico promoter analysis and found a putative NFAT site within the GAP-43 promoter. Using in vitro and in vivo experiments, we demonstrated that NFAT-3 regulates GAP-43, but unexpectedly, does not promote but represses the expression of GAP-43 in neurons and in the developing brain. Specifically, in neuron-like PC-12 cells and in cultured cortical neurons, the overexpression of NFAT-3 represses GAP-43 activation mediated by neurotrophin signaling. Using chromatin immunoprecipitation assays, we also show that prior to neurotrophin activation, endogenous NFAT-3 occupies the GAP-43 promoter in PC-12 cells, in cultured neurons, and in the mouse brain. Finally, we observe that NFAT-3 is required to repress the physiological expression of GAP-43 and other pro-axon outgrowth genes in specific developmental windows in the mouse brain. Taken together, our data reveal an unexpected role for NFAT-3 as a direct transcriptional repressor of GAP-43 expression and suggest a more general role for NFAT-3 in the control of the neuronal outgrowth program.
Molecular Targets for Axon Regeneration: Focus on the Intrinsic Pathways
Axonal damage and degeneration are prominent components of acute neurological disorders such as stroke, brain and spinal cord injuries, leading to the dysfunction of neuronal networks, which is largely responsible for the impaired neurological function. In the CNS, injured axons not only degenerate but are unable to regenerate and have a limited capacity to sprout and re-establish lost connections. Therefore, axonal damage often results in long term disability. Strategies aimed at fostering neurological recovery by promoting axonal sprouting and regeneration have largely targeted the glial inhibitory environment that develops following central nervous system injury. However, experimental evidence suggests that providing a favorable environment may not be the sole and sufficient means for functional regeneration, and that activating the limited intrinsic potential of neurons to sprout and regenerate may represent an alternative and complementary therapeutic approach. Experimental data that show how the modulation of the intrinsic potential of neurons can promote axonal sprouting and regeneration in the CNS are presented and discussed. These data may suggest future therapeutic opportunities to promote recovery in acute neurological disorders.
The Tumor Suppressor P53 Transcriptionally Regulates CGKI Expression During Neuronal Maturation and is Required for CGMP-dependent Growth Cone Collapse
The cGMP-dependent protein kinase type I (cGKI) has multiple functions including a role in axonal growth and pathfinding of sensory neurons, and counteracts Semaphorin 3A (Sema3A)-induced growth cone collapse. Within the nervous system, however, the transcriptional regulation of cGKI is still obscure. Recently, the transcription factor and tumor suppressor p53 has been reported to promote neurite outgrowth by regulating the gene expression of factors that promote growth cone extension, but specific p53 targets genes that may counteract growth cone collapse have not been identified so far. Here, we show that p53 promotes cGKI expression in neuronal-like PC-12 cells and primary neurons by occupying specific regulatory elements in a chromatin environment during neuronal maturation. Importantly, we demonstrate that p53-dependent expression of cGKI is required for the ability of cGMP to counteract growth cone collapse. Growth cone retraction mediated by Sema3A is overcome by cGMP only in wild-type, but not in p53-null dorsal root ganglia. Reconstitution of p53 levels is sufficient to recover both cGKI expression and the ability of cGMP to counteract growth cone collapse, while cGKI overexpression rescues growth cone collapse in p53-null primary neurons. In conclusion, this study identifies p53 as a transcription factor that regulates the expression of cGKI during neuronal maturation and cGMP-dependent inhibition of growth cone collapse.
Valproic Acid-mediated Neuroprotection and Regeneration in Injured Retinal Ganglion Cells
Valproic acid (VPA) has been demonstrated to have neuroprotective effects in neurodegenerative conditions. VPA inhibits histone-deacetylases (HDAC) and delays apoptosis in degenerating neurons. The authors investigated whether VPA delays retinal ganglion cell (RGC) death and enhances axonal regeneration after optic nerve crush (ONC). Furthermore, potential molecular targets involved in VPA-mediated protection were analyzed.
RA-RAR-β Counteracts Myelin-dependent Inhibition of Neurite Outgrowth Via Lingo-1 Repression
After an acute central nervous system injury, axonal regeneration is limited as the result of a lack of neuronal intrinsic competence and the presence of extrinsic inhibitory signals. The injury fragments the myelin neuronal insulating layer, releasing extrinsic inhibitory molecules to signal through the neuronal membrane-bound Nogo receptor (NgR) complex. In this paper, we show that a neuronal transcriptional pathway can interfere with extrinsic inhibitory myelin-dependent signaling, thereby promoting neurite outgrowth. Specifically, retinoic acid (RA), acting through the RA receptor β (RAR-β), inhibited myelin-activated NgR signaling through the transcriptional repression of the NgR complex member Lingo-1. We show that suppression of Lingo-1 was required for RA-RAR-β to counteract extrinsic inhibition of neurite outgrowth. Furthermore, we confirm in vivo that RA treatment after a dorsal column overhemisection injury inhibited Lingo-1 expression, specifically through RAR-β. Our findings identify a novel link between RA-RAR-β-dependent proaxonal outgrowth and inhibitory NgR complex-dependent signaling, potentially allowing for the development of molecular strategies to enhance axonal regeneration after a central nervous system injury.
The Histone Acetyltransferase P300 Promotes Intrinsic Axonal Regeneration
Axonal regeneration and related functional recovery following axonal injury in the adult central nervous system are extremely limited, due to a lack of neuronal intrinsic competence and the presence of extrinsic inhibitory signals. As opposed to what occurs during nervous system development, a weak proregenerative gene expression programme contributes to the limited intrinsic capacity of adult injured central nervous system axons to regenerate. Here we show, in an optic nerve crush model of axonal injury, that adenoviral (cytomegalovirus promoter) overexpression of the acetyltransferase p300, which is regulated during retinal ganglion cell maturation and repressed in the adult, can promote axonal regeneration of the optic nerve beyond 0.5 mm. p300 acetylates histone H3 and the proregenerative transcription factors p53 and CCAAT-enhancer binding proteins in retinal ganglia cells. In addition, it directly occupies and acetylates the promoters of the growth-associated protein-43, coronin 1 b and Sprr1a and drives the gene expression programme of several regeneration-associated genes. On the contrary, overall increase in cellular acetylation using the histone deacetylase inhibitor trichostatin A, enhances retinal ganglion cell survival but not axonal regeneration after optic nerve crush. Therefore, p300 targets both the epigenome and transcription to unlock a post-injury silent gene expression programme that would support axonal regeneration.
Retinoic Acid Signaling in Axonal Regeneration
Following an acute central nervous system (CNS) injury, axonal regeneration and functional recovery are extremely limited. This is due to an extrinsic inhibitory growth environment and the lack of intrinsic growth competence. Retinoic acid (RA) signaling, essential in developmental dorsoventral patterning and specification of spinal motor neurons, has been shown through its receptor, the transcription factor RA receptor β2 (RARβ2), to induce axonal regeneration following spinal cord injury (SCI). Recently, it has been shown that in dorsal root ganglion neurons (DRGs), cAMP levels were greatly increased by lentiviral RARβ2 expression and contributed to neurite outgrowth. Moreover, RARβagonists, in cerebellar granule neurons (CGN) and in the brain in vivo, induced phosphoinositide 3-kinase dependent phosphorylation of AKT that was involved in RARβ-dependent neurite outgrowth. More recently, RA-RARβpathways were shown to directly transcriptionally repress a member of the inhibitory Nogo receptor (NgR) complex, Lingo-1, under an axonal growth inhibitory environment in vitro as well as following spinal injury in vivo. This perspective focuses on these newly discovered molecular mechanisms and future directions in the field.
P53-dependent Pathways in Neurite Outgrowth and Axonal Regeneration
The tumor suppressor p53 is a multifunctional sensor of a number of cellular signals and pathways essential for cell biology, including DNA damage, cell cycle regulation, apoptosis, angiogenesis and cell metabolism. In the last few years, a novel role for p53 in neurobiology has emerged, which includes a role in the regulation of neurite outgrowth and axonal regeneration. p53 integrates a number of extracellular signals that involve neurotrophins and axon guidance cues to modulate the cytoskeletal response associated with neurite outgrowth at both the transcriptional and post-translational level. Here, we review our current knowledge of this topic and speculate about future research directions that involve p53 and related molecular pathways and that might advance our understanding of neurite outgrowth and axonal regeneration at the molecular level.
