Survival-promoting activity of inhibitors of cyclin-dependent kinases on primary neurons correlates with inhibition of c-Jun kinase-1

Inhibition of cyclin-dependent kinases by olomoucine and roscovitine reduces lipopolysaccharide-induced inflammatory responses via down-regulation of nuclear factor kappaB

OBJECTIVES

Initiation and maintenance of pro-inflammatory reactions elicited by bacterial lipopolysaccharide and/or cytokines in the macrophage lineage have been reported to play a crucial role in acute and chronic pathogenic effects. Whether pro-inflammatory responses triggered by lipopolysaccharide in growth arrested cells differ from those in proliferating cells remains unanswered.

MATERIALS AND METHODS

Olomoucine and roscovitine are cyclin-dependent kinase (CDK) inhibitors that prevent progression through the cell cycle. After treatment with CDK inhibitors, expression of pro-inflammatory genes was analysed by reverse transcriptase-polymerase chain reaction. Protein levels of inducible nitric oxide synthase (iNOS) and nuclear factor kappaB (NF-kappaB) were determined by Western blotting. Promoter activity of iNOS was measured by the luciferase activity assay.

RESULTS

In this study we have demonstrated that both olomoucine and roscovitine inhibit cell proliferation and diminish nitric oxide production and cytokine gene expression, in lipopolysaccharide-stimulated murine RAW264.7 macrophages. In addition, olomoucine reduces iNOS promoter activity and alleviates NF-kappaB transcription activation. After co-transfection with E2F1 interference RNA, suppression of lipopolysaccharide-mediated iNOS promoter activity and NF-kappaB activation was observed. Furthermore, we demonstrated that olomoucine-induced growth arrested cells reduce expression of the p65 subunit of NF-kappaB.

CONCLUSIONS

The findings of this study suggest that inhibition of cell-cycle progression is capable of reducing pro-inflammatory responses via down-regulation of NF-kappaB.

Cell cycle molecules define a pathway required for neuron death in development and disease

We review here evidence defining a molecular pathway that includes cell cycle-related molecules and that appears to play a required role in neuron death during normal development as well as in disease and trauma. The pathway starts with inappropriate activation of cyclin dependent kinase 4 (Cdk4) in neurons which leads to hyper-phosphorylation of the pRb family member p130. This in turn results in dissociation of p130 and its associated chromatin modifiers Suv39H1 and HDAC1 from the transcription factor E2F4. Dissociation of this complex results in de-repression of genes with E2F binding sites including those encoding the transcription factors B- and C-Myb. Once elevated in neurons, B- and C-Myb proteins bind to the promoter for the pro-apoptotic BH3-only protein Bim and promote its induction. Bim then interacts with the core cellular apoptotic machinery, leading to caspase activation and apoptotic death. This pathway is supported by a variety of observations and experimental findings that implicate it as a required element for neuron loss in development and in many nervous system traumas and disorders. The components of this pathway appear to represent potential therapeutic targets for prevention of disease-associated neuron death.

The cell cycle as a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease worldwide. It is a progressive, incurable disease whose predominant clinical manifestation is memory loss, and which always ends in death. The classic neuropathological diagnostic markers for AD are amyloid plaques and neurofibrillary tangles, but our understanding of the role that these features of AD play in the etiology and progression of the disease remains incomplete. Research over the last decade has revealed that cell cycle abnormalities also represent a major neuropathological feature of AD. These abnormalities appear very early in the disease process, prior to the appearance of plaques and tangles. Growing evidence suggests that neuronal cell cycle regulatory failure, leading to apoptosis, may be a significant component of the pathogenesis of AD. A number of signaling pathways with the potential to activate aberrant cell cycle re-entry in AD have been described. The relationships among these signaling cascades, which involve the amyloid precursor protein (APP), cyclin-dependent kinases (cdks), and the cell cycle protein Pin1, have not yet been fully elucidated, but details of the individual pathways are beginning to emerge. This review summarizes the current state of knowledge with respect to specific neuronal signaling events that are thought to underlie cell cycle regulatory failure in AD brain. The elements of these pathways that represent potential new therapeutic targets for AD are described. Drugs and peptides that can inhibit molecular steps leading to AD neurodegeneration by intervening in the activation of cell cycle re-entry in neurons represent an entirely new approach to the development of treatments for AD.

Protection of renal cells from cisplatin toxicity by cell cycle inhibitors

The optimal use of cisplatin as a chemotherapeutic drug has been limited by its nephrotoxicity. Murine models have been used to study cisplatin-induced acute renal failure. After cisplatin administration, cells of the S3 segment in the renal proximal tubule are especially sensitive and undergo extensive necrosis in vivo. Similarly, cultured proximal tubule cells undergo apoptosis in vitro after cisplatin exposure. We have shown in vivo that kidney cells enter the cell cycle after cisplatin administration but that cell cycle-inhibitory proteins p21 and 14-3-3sigma are also upregulated. These proteins coordinate the cell cycle, and deletion of either of the genes resulted in increased nephrotoxicity in vivo or increased cell death in vitro after exposure to cisplatin. However, it was not known whether cell cycle inhibition before acute renal failure could protect from cisplatin-induced cell death, especially in cells with functional p21 and 14-3-3sigma genes. Using several cell cycle inhibitors, including a p21 adenovirus, and the drugs roscovitine and olomoucine, we have been able to completely protect a mouse kidney proximal tubule cell culture from cisplatin-induced apoptosis. The protection by p21 was independent of an effect on the cell cycle and was likely caused by selective inhibition of caspase-dependent and -independent cell death pathways in the cells.

JNK-independent activation of c-Jun during neuronal apoptosis induced by multiple DNA-damaging agents

Activation of the JNK pathway and induction of the AP-1 transcription factor c-Jun are critical for neuronal apoptosis caused by a variety of insults. Ara-C-induced DNA damage caused rapid sympathetic neuronal death that was associated with an increase of c-jun expression. In addition, c-Jun was phosphorylated in its N-terminal transactivation domain, which is important for c-Jun-mediated gene transcription. Blocking c-Jun activation by JNK pathway inhibition prevented neuronal death after stress. In contrast, neither the JNK inhibitor SP600125 nor the mixed lineage kinase inhibitor CEP-1347 prevented cytosine arabinoside-induced neuronal death, demonstrating that the JNK pathway was not necessary for DNA damage-induced neuronal apoptosis. Surprisingly, SP600125 or CEP-1347 could not block c-Jun induction or phosphorylation after DNA damage. Pharmacological inhibitors of cyclin-dependent kinase (CDK) activity completely prevented c-Jun phosphorylation after DNA damage. These results demonstrate that c-Jun activation during DNA damage-induced neuronal apoptosis was independent of the classical JNK pathway and was mediated by a novel c-Jun kinase. Based on pharmacological criteria, DNA damage-induced neuronal c-Jun kinase may be a member of the CDK family or be activated by a CDK-like kinase. Activation of this novel kinase and subsequent phosphorylation of c-Jun may be important in neuronal death after DNA damage.

Differential effects of cyclin-dependent kinase blockers upon cell death in the developing retina

Pharmacological blockers of cyclin-dependent kinases (CDKs) can inhibit cell cycle progression. Deferoxamine (DFO) and mimosine (MIMO) arrest cells reversibly at the G1/S transition and olomoucine (OLO) inhibits the cell cycle at both G1/S and G2/M. We investigated the effect of these drugs upon cell death in histotypical explants taken from the retina of neonatal rats. Degeneration of retinal ganglions cells (RGC) induced by axotomy was inhibited by OLO (100 microM) but not by DFO (up to 2 mM) or MIMO (up to 1 mM). On the other hand, after 1 day in vitro, all cell cycle inhibitors induced cell death in the neuroblastic layer (NBL) of the explants. DFO and MIMO induced cell death only of proliferating cells, identified either by their incorporation of bromodeoxyuridine or by immunolabeling the proliferating cell nuclear antigen. In turn, OLO induced cell death of both proliferating and post-mitotic cells. However, the post-mitotic cells were unlabeled with markers of retinal differentiation. Our results indicate that cyclin-dependent kinases are involved in the control of sensitivity to cell death in the retina, and that retinal cells present differentiation-dependent responses to modulation of CDK activity.

Inhibition of mitochondrial permeability transition and release of cytochrome c by anti-apoptotic nucleoside analogues

We have investigated whether nucleoside drugs that induce or protect neurones against apoptosis might directly activate or inhibit mitochondrial permeability transition (mPT) since opening of the mPT pore can promote release of cytochrome c and apoptosis, while its closure can prevent these changes. We found that the pro-apoptotic pyrimidine analogues cytosine beta-D-arabinofuranoside and cytosine beta-D-arabinofuranoside 5'-triphosphate, which activated apoptosis in post-mitotic neurones without incorporation into nuclear DNA, induced rapid calcium-dependent mitochondrial swelling of isolated liver mitochondria in a dose-dependent manner. Induction of up to 50 and 80%, respectively, of maximal swelling induced by high calcium was obtained at 1mM concentrations, which also promoted a 17-fold increase in the release of cytochrome c. Both activities were inhibited by cyclosporine A to unstimulated levels; dCTP had no effect. In contrast, the anti-apoptotic adenine analogues, 3-methyladenine (3-MA) and olomoucine (but not iso-olomoucine), inhibited swelling induced by calcium or phenylarsine oxide in a dose-dependent manner at concentrations that protect neurones from apoptosis. Both compounds also inhibited the release of cytochrome c (by 82%, 20 mM 3-MA and 95%, 0.9 mM olomoucine), similar to the inhibition obtained with cyclosporine A, and 5mM ADP or ATP. Similar inhibitory effects with olomoucine and 3-MA were found in isolated heart mitochondria. These studies identify the mPT as an important target for hitherto untested pro- and anti-apoptotic nucleoside-based drugs and suggest that screening for mPT modulation is an important component in the validation of a drug's mechanism of action.

Regenerative and survival capabilities of Purkinje cells overexpressing c-Jun

Following axotomy, cerebellar Purkinje cells (PCs) do not elongate their axons, even in a favourable environment, and are resistant to death. They have no constitutive presence of common growth-associated proteins, such as GAP-43 and c-Jun. Previous experiments show that injured transgenic PCs overexpressing GAP-43 exhibit a profuse sprouting along the axon and at its severed end. Nevertheless, the lesioned axons are unable to regenerate either spontaneously or into growth-permissive environments. In addition, a considerable number of GAP-43 transgenic PCs degenerate after injury. c-Jun is an inducible transcription factor expressed in axotomized central neurons and regenerating peripheral neurons. It also contributes to programmed cell death during development. To test whether c-Jun could modify the response of PCs to axotomy or enhance the growth/death phenomena of GAP-43 Purkinje neurons, we generated transgenic mice overexpressing c-Jun in PCs. However, c-Jun upregulation did not affect the adult intact phenotype of these neurons and their regenerative and survival capabilities after axotomy. Also in the cross-bred GAP-43/c-Jun mice, c-Jun did not modify the response of GAP-43 PCs to axotomy. By contrast, in organotypic cultures of cerebellum taken from 9-day-old-pups, the survival capabilities of PCs overexpressing c-Jun decreased, in association with a consistent c-Jun phosphorylation. On the whole our data show that c-Jun alone is unable to trigger regenerative or degenerative phenomena in PCs and suggest that the cellular action of this early gene in developing and mature neurons strongly depends on interplaying intracellular signals.

Involvement of cyclin-dependent kinases in axotomy-induced retinal ganglion cell death

We have tested the role of cyclin-dependent kinases (CDKs) in the type 3B death of axotomized retinal ganglion cells, by injecting intraocularly olomoucine, roscovitine, or butyrolactone I. Each of these inhibits CDK1, CDK2, and CDK5; CDK1 and CDK2 are involved in cell proliferation, whereas CDK5 is involved in neuronal differentiation. The inhibitors partially protected ganglion cells against the effects of axotomy. These agents may affect the ganglion cells directly, because CDK1, its regulatory subunit cyclin B1, and CDK5 were identified immunohistochemically in the perikarya of ganglion cells, and this was confirmed for CDK1 and CDK5 in Western blots of the ganglion cell layer. These blots showed an axotomy-induced phosphorylation of CDK5 occurring remarkably quickly (within 6 hours of axotomy) but little if any change in the phosphorylation state of CDK1. In addition, we studied the expression of proliferation markers, including proliferating cell nuclear antigen (PCNA) and the synthesis of DNA, by immunohistochemical and autoradiographic methods. Normal or axotomized ganglion cells did not express PCNA and did not synthesize DNA. Although we cannot exclude the possibility that axotomized ganglion cells may leave their quiescent state, our data show that they did not progress beyond the G1 phase of the cell cycle. Finally, in contrast to inhibitors of CDKs, cell cycle blockers with different targets than CDKs did not protect ganglion cells. Globally, our results suggest that axotomy-induced death of ganglion cells involves the activation of CDK1, CDK2, or CDK5 (most probably CDK5) but not the full cell cycle machinery.

Cyclin-dependent kinases and stroke

Cyclin-dependent kinases (CDKs) are a group of enzymes predominately known for their role in cell cycle regulation in proliferating cell types. Increasing evidence, however, suggests that CDKs also promote death in neurones. These observations have lead to the notion that CDKs may serve as a therapeutic target for neuropathological conditions such as stroke. Accordingly, in this review, we will examine the evidence which indicates a role for CDKs in neuronal death and evaluate the potential of CDK inhibitors as a therapeutic target for stroke.

GSK3 takes centre stage more than 20 years after its discovery

Identified originally as a regulator of glycogen metabolism, glycogen synthase kinase-3 (GSK3) is now a well-established component of the Wnt signalling pathway, which is essential for setting up the entire body pattern during embryonic development. It may also play important roles in protein synthesis, cell proliferation, cell differentiation, microtubule dynamics and cell motility by phosphorylating initiation factors, components of the cell-division cycle, transcription factors and proteins involved in microtubule function and cell adhesion. Generation of the mouse knockout of GSK3beta, as well as studies in neurons, also suggest an important role in apoptosis. The substrate specificity of GSK3 is unusual in that efficient phosphorylation of many of its substrates requires the presence of another phosphorylated residue optimally located four amino acids C-terminal to the site of GSK3 phosphorylation. Recent experiments, including the elucidation of its three-dimensional structure, have enhanced our understanding of the molecular basis for the unique substrate specificity of GSK3. Insulin and growth factors inhibit GSK3 by triggering its phosphorylation, turning the N-terminus into a pseudosubstrate inhibitor that competes for binding with the 'priming phosphate' of substrates. In contrast, Wnt proteins inhibit GSK3 in a completely different way, by disrupting a multiprotein complex comprising GSK3 and its substrates in the Wnt signalling pathway, which do not appear to require a 'priming phosphate'. These latest findings have generated an enormous amount of interest in the development of drugs that inhibit GSK3 and which may have therapeutic potential for the treatment of diabetes, stroke and Alzheimer's disease.

c-Jun promotes neurite outgrowth and survival in PC12 cells

We investigated the function of c-Jun in PC12 cells by transfecting them with a plasmid containing a c-Jun cDNA transcription cassette. Transfected cells expressed high levels of c-Jun mRNA and protein and demonstrated an increase in both AP-1 DNA binding and gene activation. The c-Jun over-expressing cells showed marked neurite outgrowth but no evidence of spontaneous cell death. In fact, c-Jun over-expressing cells were more resistant to okadaic acid-induced apoptosis. The process outgrowth was not indicative of a full neuronal differentiation response as the transfected PC12 cells did not display action potentials when examined with whole-cell patch-clamping. The phosphorylation of c-Jun on serine 73 appears to be important for this neurite sprouting effect as mutagenesis at this site reduced sprouting whereas a serine 63 mutant tended to increase sprouting. Thus, in PC12 cells c-Jun expression does not induce apoptosis, but rather functions as a neurite outgrowth and neuronal survival signal.

A rapid method for determination of cell survival in primary neuronal DRG cultures

A simple and rapid enzyme-linked immunosorbent assay (ELISA) has been developed to provide an alternative to cell counting to detect increases in cell survival in primary neuronal cultures. This sensitive assay has the advantage of being less time consuming and labour intensive than cell counting, can be used to quantify cell survival and is more accurate than estimation methods of counting. The ELISA uses an antibody raised to GAP-43, a growth-associated protein which is strongly expressed by developing and regenerating neurones. The effects of nerve growth factor (NGF), neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) on GAP-43 immunoreactivity in dissociated primary cultures of rat and chick dorsal root ganglia have been compared to results obtained by cell counting. Data show that human NGF produced the greatest increase in GAP-43-immunoreactive neurones in both species; this increase in immunoreactivity correlated well with the increased survival shown by cell count data. Results prove that the ELISA can also be used to accurately detect small changes in cell survival as seen with NT-3 and BDNF, or potentiation of the effects obtained with the trophic factor NT-3. In conclusion, this ELISA may be a useful tool to detect neurotrophic effects of unknown agents or novel neurotrophins.

Transient hypoxia may lead to neuronal proliferation in the developing mammalian brain: from apoptosis to cell cycle completion

Cerebral hypoxia/ischemia was shown to induce delayed, apoptotic neuronal death occurring through biochemical pathways potentially sharing common events with cell proliferation. This study was designed to test the hypothesis that a sublethal hypoxia may promote mitotic activity in developing central neurons. After six days in vitro, cultured neurons from the forebrain of 14-day-old rat embryos were exposed to hypoxia (95% N2/5% CO2) for 3 h and re-oxygenated for up to 96 h. Controls were kept in normoxia. As a function of time, cell viability was measured by diphenyltetrazolium bromide, and rates of DNA and protein synthesis were monitored using [3H]thymidine and [3H]leucine, respectively. Morphological features of apoptosis, necrosis and mitosis were scored under fluorescence microscopy after nuclear staining with 4,6-diamidino-2-phenylindole, and the expression profile of proliferating cell nuclear antigen, a cofactor for DNA polymerase, was analysed by immunohistochemistry. Data were compared to those obtained after transient hypoxia for 6 h followed by re-oxygenation for 96 h and which was shown to induce apoptosis. Whereas a 6-h insult reduced cell viability, with 23% of the neurons exhibiting apoptosis by the end of re-oxygenation, a 3-h hypoxia led to a cycloheximide-sensitive increase in the final number of living neurons compared to controls (13%, P < 0.01), with no signs of apoptosis, significantly increased thymidine incorporation into acid-precipitable fraction, and persistent over-expression of proliferating cell nuclear antigen. Accordingly, final score of mitotic nuclei was significantly enhanced. In addition, the cell cycle inhibitor olomoucine (50 microM) prevented apoptosis consecutive to a 6-h hypoxia, but impaired the stimulatory effects of a 3-h insult. These findings support the conclusion that some neurons exposed to sublethal hypoxia may dodge apoptotic death by fully achieving the cell cycle.

Butyrolactone I induces cyclin B1 and causes G2/M arrest and skipping of mitosis in human prostate cell lines

Several naturally occurring cyclin-dependent kinase (CDK) inhibitors have been isolated from different lower organisms. In this report, we examined the effect of one of the CDK inhibitors, butyrolactone I (BL), on the expression of cyclins D2, A and B1 in three human prostatic cancer cell lines (DU145, PC-3, LNCaP) using two colored flow cytometric analysis. The percentage of DU145 cells in the 4C phase of the cell cycle were increased significantly at both 70 microM and 100 microM BL. Furthermore, an additional 8C peak was observed which had double the DNA content of the 4C phase at these concentrations of BL. The appearance of the 8C peak increased gradually and was more evident in DU145 and PC-3 than LNCaP. Cells in the 8C peak had either two nuclei or abnormal nuclei as observed by Papanicolaou stain. BL also increased the amount of cyclin B1 positive cells in the 4C phase. This increase was apparent on day 1 and returned to normal by day 3. Since BL selectively inhibits cyclin-dependent kinase, cyclin B1 might accumulate without being degraded. Other cyclins were not significantly changed by BL. The data demonstrate that BL inhibited Cdc2 of unsynchronized cultured prostate cancer cells, and interrupted the cell cycle progression toward cell division. The BL inhibition of Cdc2 led to the accumulation of cells in the 4C phase without mitosis resulting in an accumulation of cyclin B1. The appearance of cells in the 8C phase may be due to the progression of cells in the 4C phase through the cell cycle skipping mitosis. Cyclin B1 decreased in correlation with the progression through a new cell cycle. These results suggest that BL does not cause a complete arrest of the cell cycle in G2/M but that BL occasionally allows for the skipping of mitosis and subsequent progression through the cell cycle to occur.

The mechanism of Ara-C-induced apoptosis of differentiating cerebellar granule neurons

Neurotoxicity is one of the side-effects of the therapeutically useful antitumour agent, Ara-C (or 1-beta-d-arabinofuranosyl-cytosine, cytarabine). This agent is also reported to induce cell death of cultured neurons. In this study, we show that Ara-C-induced death of differentiating rat cerebellar granule neurons is prevented by cycloheximide at concentrations corresponding to its action in preventing protein synthesis. The death is accompanied by cleavage of the caspase substrate poly ADP ribose polymerase (PARP) and c-Abl-dependent activation of the stress-activated protein kinases c-Jun N-terminal kinase and p38. However, c-Jun levels do not rise and the activation of the stress-activated protein kinases is not required for this form of neuronal death. Cyclin-dependent kinase (cdk) activity and inappropriate cell-cycle re-entry have been implicated in some forms of death in differentiated neurons. Here we show that Ara-C-induced death of cerebellar granule neurons is prevented by an inhibitor of cdk4, whereas inhibition of cdk1, -2 and -5 mimics the death, and non-cdk4/6 cdks are inhibited by Ara-C treatment. Cdk1 and -2 are dramatically down-regulated during neuronal differentiation, and neither Ara-C nor inhibition of these cdks induces death in mature neurons. This mechanism could also play a significant role in the neurotoxicity associated with the therapeutic use of Ara-C, as cdk levels can be upregulated in stressed neurons of adult brain. We propose that the balance between cdk4/6 and cdk1/2/5 activity may determine the survival of early differentiating neurons, and that DNA-damaging agents may induce neuronal death by inhibiting cdk1/2/5 under conditions which require these activities for survival.

CDK inhibition and cancer therapy

The cell-division cycle is a tightly controlled process that is regulated by the cyclin/CDK family of protein kinase complexes. Stringent control of this process is essential to ensure that DNA synthesis and subsequent mitotic division are accurately and coordinately executed. There is now strong evidence that CDKs, their regulators, and substrates are the targets of genetic alteration in many human cancers. As a result of this, the CDKs have been targeted for drug discovery and a number of small molecule inhibitors of CDKs have been identified.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Retrograde regulation of growth-associated gene expression in adult rat Purkinje cells by myelin-associated neurite growth inhibitory proteins

Axon regeneration requires that injured neurons reinitiate long-distance growth and upregulate specific genes. To address the question of whether inhibitory environmental cues along the axon could exert a negative, tonic downregulation of growth-associated genes, we have examined adult rat Purkinje cells, which are endowed with poor regenerative capabilities. First we have compared their response to axotomy with that of neurons of the inferior olive, lateral reticular nucleus, and deep cerebellar nuclei, all of which vigorously regenerate into growth-permissive transplants. These injured neurons upregulate the transcription factors c-Jun and JunD, GAP-43, and NADPH diaphorase. In contrast, most axotomized Purkinje cells fail to express any of these markers, showing that the strength of this response parallels the regenerative potential of the examined neuron populations. However, strong upregulation of the same genes can be induced in Purkinje cells after colchicine injection into the uninjured adult cerebellum, indicating that their expression could be controlled by retrograde signals. To assess whether myelin-associated neurite growth inhibitory proteins contribute to this regulation, we applied the neutralizing antibodies IN-1 against one of the main inhibitory components of central myelin (NI-250) either in vivo or in vitro to organotypic cerebellar cultures. Application of IN-1 antibodies induces the upregulation of c-Jun, JunD, and NADPH diaphorase in Purkinje cells, showing that their expression is suppressed constitutively by myelin-associated neurite growth inhibitors. Thus, the inhibitory activity of the IN-1 antigen on axon growth is not restricted to the control of growth cone motility but also involves a retrograde regulation of gene expression in adult central neurons.