Stress-induced activation of c-Jun N-terminal kinase in sensory ganglion neurons: accumulation in nuclear domains enriched in splicing factors and distribution in perichromatin fibrils

Neuronal accumulation of unrepaired DNA in a novel specific chromatin domain: structural, molecular and transcriptional characterization

There is growing evidence that defective DNA repair in neurons with accumulation of DNA lesions and loss of genome integrity underlies aging and many neurodegenerative disorders. An important challenge is to understand how neurons can tolerate the accumulation of persistent DNA lesions without triggering the apoptotic pathway. Here we study the impact of the accumulation of unrepaired DNA on the chromatin architecture, kinetics of the DNA damage response and transcriptional activity in rat sensory ganglion neurons exposed to 1-to-3 doses of ionizing radiation (IR). In particular, we have characterized the structural, molecular and transcriptional compartmentalization of unrepaired DNA in persistent DNA damaged foci (PDDF). IR induced the formation of numerous transient foci, which repaired DNA within the 24 h post-IR, and a 1-to-3 PDDF. The latter concentrate DNA damage signaling and repair factors, including γH2AX, pATM, WRAP53 and 53BP1. The number and size of PDDF was dependent on the doses of IR administered. The proportion of neurons carrying PDDF decreased over time of post-IR, indicating that a slow DNA repair occurs in some foci. The fine structure of PDDF consisted of a loose network of unfolded 30 nm chromatin fiber intermediates, which may provide a structural scaffold accessible for DNA repair factors. Furthermore, the transcription assay demonstrated that PDDF are transcriptionally silent, although transcription occurred in flanking euchromatin. Therefore, the expression of γH2AX can be used as a reliable marker of gene silencing in DNA damaged neurons. Moreover, PDDF were located in repressive nuclear environments, preferentially in the perinucleolar domain where they were frequently associated with Cajal bodies or heterochromatin clumps forming a structural triad. We propose that the sequestration of unrepaired DNA in discrete PDDF and the transcriptional silencing can be essential to preserve genome stability and prevent the synthesis of aberrant mRNA and protein products encoded by damaged genes.

Effect of ionizing radiation in sensory ganglion neurons: organization and dynamics of nuclear compartments of DNA damage/repair and their relationship with transcription and cell cycle

Neurons are very sensitive to DNA damage induced by endogenous and exogenous genotoxic agents, as defective DNA repair can lead to neurodevelopmental disorders, brain tumors and neurodegenerative diseases with severe clinical manifestations. Understanding the impact of DNA damage/repair mechanisms on the nuclear organization, particularly on the regulation of transcription and cell cycle, is essential to know the pathophysiology of defective DNA repair syndromes. In this work, we study the nuclear architecture and spatiotemporal organization of chromatin compartments involved in the DNA damage response (DDR) in rat sensory ganglion neurons exposed to X-ray irradiation (IR). We demonstrate that the neuronal DDR involves the formation of two categories of DNA-damage processing chromatin compartments: transient, disappearing within the 1 day post-IR, and persistent, where unrepaired DNA is accumulated. Both compartments concentrate components of the DDR pathway, including γH2AX, pATM and 53BP1. Furthermore, DNA damage does not induce neuronal apoptosis but triggers the G0-G1 cell cycle phase transition, which is mediated by the activation of the ATM-p53 pathway and increased protein levels of p21 and cyclin D1. Moreover, the run on transcription assay reveals a severe inhibition of transcription at 0.5 h post-IR, followed by its rapid recovery over the 1 day post-IR in parallel with the progression of DNA repair. Therefore, the response of healthy neurons to DNA damage involves a transcription- and cell cycle-dependent but apoptosis-independent process. Furthermore, we propose that the segregation of unrepaired DNA in a few persistent chromatin compartments preserves genomic stability of undamaged DNA and the global transcription rate in neurons.

Nucleolar disruption and cajal body disassembly are nuclear hallmarks of DNA damage-induced neurodegeneration in purkinje cells

The Purkinje cell (PC) degeneration (pcd) phenotype results from mutation in nna1 gene and is associated with the degeneration and death of PCs during the postnatal life. Although the pcd mutation is a model of the ataxic mouse, it shares clinical and pathological characteristics of inherited human spinocerebellar ataxias. PC degeneration in pcd mice provides a useful neuronal system to study nuclear mechanisms involved in DNA damage-dependent neurodegeneration, particularly the contribution of nucleoli and Cajal bodies (CBs). Both nuclear structures are engaged in housekeeping functions for neuronal survival, the biogenesis of ribosomes and the maturation of snRNPs and snoRNPs required for pre-mRNA and pre-rRNA processing, respectively. In this study, we use ultrastructural analysis, in situ transcription assay and molecular markers for DNA damage, nucleoli and CB components to demonstrate that PC degeneration involves the progressive accumulation of nuclear DNA damage associated with disruption of nucleoli and CBs, disassembly of polyribosomes into monoribosomes, ribophagy and shut down of nucleolar and extranucleolar transcription. Microarray analysis reveals that four genes encoding repressors of nucleolar rRNA synthesis (p53, Rb, PTEN and SNF2) are upregulated in the cerebellum of pcd mice. Collectively, these data support that nucleolar and CB alterations are hallmarks of DNA damage-induced neurodegeneration.

Signal transduction pathways associated with ATP-induced proliferation of colon adenocarcinoma cells

BACKGROUND

In previous work, we have demonstrated that extracellular adenosine 5'-triphosphate (ATP) acts on intestinal Caco-2 cell P2Y receptors promoting a rapid increase in the phosphorylation of ERK1/2, p46 JNK and p38 MAP kinases (MAPKs).

METHODS AND RESULTS

In this study, we investigated whether the extracellular ATP-P2Y receptor signalling pathways were required for the proliferation of Caco-2 cells. Confocal microscopy and immunobloting studies showed that ERK1/2 and JNK translocate into the nucleus of the cells stimulated by ATP, where they participate, together with p38 MAPK, in the phosphorylation of JunD, ATF-1 and ATF-2 transcription factors. In addition, ATP through the activation of MAPKs induces the expression of the immediate early genes products of the Jun family, c-Fos and MAP kinase phosphatase-1 (MKP-1). Moreover, ERK1/2 and p38 MAPK are involved in the phosphorylation of MKP-1 in Caco-2 cells. Of physiological significance, in agreement with the mitogenic role of the MAPK cascade, ATP increased Caco-2 cell proliferation, and this effect was blocked by UO126, SB203580 and SP600125, the specific inhibitors of ERK1/2, p38 MAPK and JNK1/2, respectively.

CONCLUSION

Extracellular ATP induces proliferation of Caco-2 human colonic cancer cells by activating MAPK cascades and modulation of transcription factors.

GENERAL SIGNIFICANCE

These findings and identification of the specific P2Y subtype receptors involved in the mitogenic effect of ATP on Caco-2 cells might be relevant for understanding tumor cell development, resistance to treatment regimens and the design of new therapeutic strategies.

Nucleolar disruption ensures nuclear accumulation of p21 upon DNA damage

p21(cip1) is a protein with a dual function in oncogenesis depending mainly on its intracellular localization: tumor suppressor in the nucleus and oncogenic in the cytoplasm. After DNA damage, p21(cip1) increases and accumulates in the nucleus to ensure cell cycle arrest. We show here that the nuclear accumulation of p21(cip1) is not only a consequence of its increased levels but to a DNA damage cellular response, which is ataxia telangiectasia and Rad3 related (ATR)/ataxia telangiectasia mutated (ATM) and p53 independent. Furthermore, after DNA damage, p21(cip1) not only accumulates in the nucleoplasm but also in the disrupted nucleolus. Inside the nucleolus, it is found in spherical structures, which are not a protrusion of the nucleoplasm. The steady-state distribution of p21(cip1) in the nucleolus resulted from a highly dynamic equilibrium between nucleoplasmic and nucleolar p21(cip1) and correlated with the inhibition of p21(cip1) nuclear export. Most interestingly, inhibition of ribosomal export after expressing a dominant-negative mutant of nucleophosmin induced p21(cip1) accumulation in the nucleus and the nucleolus in the absence of DNA damage. This proved the existence of a nucleolar export route to the cytoplasm for p21(cip1) in control conditions that would be inhibited upon DNA damage leading to nuclear and nucleolar accumulation of p21(cip1).

Bortezomib induces the formation of nuclear poly(A) RNA granules enriched in Sam68 and PABPN1 in sensory ganglia neurons

The ubiquitin-dependent proteasome system (UPS) is the major pathway responsible for selective nuclear and cytoplasmic protein degradation. Bortezomib, a boronic acid dipeptide, is a reversible 20S proteasome inhibitor used as novel anticancer drug, particularly in the treatment of multiple myeloma and certain lymphomas. Bortezomib-induced peripheral neuropathy (BIPN) is a widely recognized dose-limiting neurotoxicity of this proteasome inhibitor, which causes a significant negative impact on the quality of life. The pathogenic mechanisms underlying bortezomib neurotoxicity are little known. In this study a rat was used as our animal model to investigate the bortezomib-induced nuclear changes in dorsal root ganglia (DRG) neurons. Our results indicate that this neuronal population is an important target of bortezomib neurotoxicity. Nuclear changes include accumulation of ubiquitin-protein conjugates, reduction of transcriptional activity, and nuclear retention of poly(A) RNAs in numerous spherical or ring-shaped dense granules. They also contained the RNA-binding proteins PABPN1 (poly(A) binding protein nuclear 1) and Sam68, but lacked the mRNA nuclear export factors REF and Y14. At the cytoplasmic level, most neurons exhibited chromatolysis, supporting the inhibition of mRNA translation. Our results indicate that bortezomib interferes with transcription, nuclear processing and transport, and cytoplasmic translation of mRNAs in DRG neurons. They also support that this neuronal dysfunction is an essential pathogenic mechanism in the BIPN, which is characterized by sensory impairment including sensory ataxia.

Pre-neurodegeneration of mitral cells in the pcd mutant mouse is associated with DNA damage, transcriptional repression, and reorganization of nuclear speckles and Cajal bodies

DNA damage and impairment of its repair underlie several neurodegenerative diseases. The Purkinje cell degeneration (pcd) mutation causes the loss of Nna1 expression and is associated with a selective and progressive degeneration of specific neuronal populations, including mitral cells in the olfactory bulb. Using an in situ transcription assay, molecular markers for both nuclear compartments and components of the DNA damage/repair pathway, and ultrastructural analysis, here we demonstrate that the pcd mutation induces the formation of DNA damage/repair foci in mitral cells. Furthermore, this effect is associated with transcriptional inhibition, heterochromatinization, nucleolar segregation and the reorganization of nuclear speckles of splicing factors and Cajal bodies. The most significant cytoplasmic alteration observed was a partial replacement of rough endoplasmic reticulum cisternae by a larger amount of free ribosomes, while other organelles were structurally preserved. The tools employed in this work may be of use for the early detection of predegenerative processes in neurodegenerative disorders and for validating rescue strategies.

Nuclear organization and dynamics of transcription sites in rat sensory ganglia neurons detected by incorporation of 5'-fluorouridine into nascent RNA

In this study we have used the transcription assay with 5'-fluorouridine incorporation into nascent RNA to analyze the nuclear organization and dynamics of transcription sites in rat trigeminal ganglia neurons. The 5'-FU administrated by i.p. injection was successfully incorporated into nuclear domains containing actively transcribing genes of trigeminal neurons. 5'-Fluorouridine RNA-labeling was detected with immunocytochemistry at light and electron microscopy levels. The 5'-fluorouridine incorporation sites were detected in the nucleolus, particularly on the dense fibrillar component, and in numerous transcription foci spread throughout the euchromatin regions, without preferential positioning at the nuclear periphery or in the nuclear interior. Double labeling experiments to combine 5'-fluorouridine incorporation with molecular markers of nuclear compartments showed the absence of transcription sites in Cajal bodies and nuclear speckles of splicing factors. Similarly, no 5'-fluorouridine labeling was detected in well-characterized chromatin silencing domain, the telomeric heterochromatin. The specificity and sensitivity of the run-on transcription assay in trigeminal ganglia neurons was verified by the i.p. administration of the transcription inhibitor actinomycin D. The dramatic reduction in RNA synthesis upon actinomycin D treatment was associated with two important cellular events, heterochromatin silencing and formation of DNA damage/repair nuclear foci, demonstrated by the expression of tri-methylated histone H4 and phosphorylated H2AX, respectively. 5'-Fluorouridine incorporation in animal models provides a useful tool to investigate the organization of gene expression in mammalian neurons in both normal physiology and experimental pathology systems.

Transcriptional regulation of the LAT-1/CD98 light chain

LAT-1/CD98 amino acid transporter expression and activity are induced in hepatic cells deprived of arginine. The promoter dependency of this regulation was investigated. LAT-1 expression, in contrast to that of CD98 heavy chain 4F2, was actinomycin D sensitive in cells cultured without arginine. Transient transfection analysis with promoter reporter constructs including the 2 kbp LAT-1 promoter or a sub-sequence containing multiple potential amino acid response elements failed to show significant amino acid sensitivity in various cell types. Chromatin-dependency did not appear to account for this result as hepatic cell clones stably transfected with the promoter constructs showed little or no arginine or leucine responsive promoter activity. These studies suggest that while amino acid sensitivity of LAT-1 expression is transcriptionally regulated, cis elements within the proximal promoter do not directly mediate this regulation. Understanding mechanisms by which this gene responds to amino acid availability will contribute to our knowledge of how eukaryotic cells sense and respond to their environment.

A role for c-Jun N-terminal kinase 1 (JNK1), but not JNK2, in the beta-amyloid-mediated stabilization of protein p53 and induction of the apoptotic cascade in cultured cortical neurons

beta-Amyloid (A beta) peptide has been shown to induce neuronal apoptosis; however, the mechanisms underlying A beta-induced neuronal cell death remain to be fully elucidated. The stress-activated protein kinase, c-Jun N-terminal kinase (JNK), is activated in response to cellular stress and has been identified as a proximal mediator of cell death. In the present study, expression of active JNK was increased in the nucleus and cytoplasm of A beta-treated cells. Evaluation of the nature of the JNK isoforms activated by A beta revealed a transient increase in JNK1 activity that reached its peak at 1 h and a later activation (at 24 h) of JNK2. The tumour suppressor protein, p53, is a substrate for JNK and can serve as a signalling molecule in apoptosis. In cultured cortical neurons, we found that A beta increased p53 protein expression and phosphorylation of p53 at Ser(15). Thus it appears that A beta increases p53 expression via phosphorylation-mediated stabilization of the protein. Given the lack of availability of a JNK inhibitor that can distinguish between JNK1- and JNK2-mediated effects, we employed antisense technology to deplete cells of JNK1 or JNK2 selectively. Using this strategy, the respective roles of JNK1 and JNK2 on the A beta-mediated activation of the apoptotic cascade (i.e. p53 stabilization, caspase 3 activation and DNA fragmentation) were examined. The results obtained demonstrate a role for JNK1 in the A beta-induced stabilization of p53, activation of caspase 3 and DNA fragmentation. In contrast, depletion of JNK2 had no effect on the proclivity of A beta to activate capase 3 or induce DNA fragmentation. These results demonstrate a significant role for JNK1 in A beta-mediated induction of the apoptotic cascade in cultured cortical neurons.

PSKH1, a novel splice factor compartment-associated serine kinase

Small nuclear ribonucleoprotein particles (snRNPs) and non-snRNP splicing factors containing a serine/arginine-rich domain (SR proteins) concentrate in splicing factor compartments (SFCs) within the nucleus of interphase cells. Nuclear SFCs are considered mainly as storage sites for splicing factors, supplying splicing factors to active genes. The mechanisms controlling the interaction of the various spliceosome constituents, and the dynamic nature of the SFCs, are still poorly understood. We show here that endogenous PSKH1, a previously cloned kinase, is located in SFCs. Migration of PSKH1-FLAG into SFCs is enhanced during co-expression of T7-tagged ASF/SF2 as well as other members of the SR protein family, but not by two other non-SR nuclear proteins serving as controls. Similar to the SR protein kinase family, overexpression of PSKH1 led to reorganization of co-expressed T7-SC35 and T7-ASF/SF2 into a more diffuse nuclear pattern. This redistribution was not dependent on PSKH1 kinase activity. Different from the SR protein kinases, the SFC-associating features of PSKH1 were located within its catalytic kinase domain and within its C-terminus. Although no direct interaction was observed between PSKH1 and any of the SR proteins tested in pull-down or yeast two-hybrid assays, forced expression of PSKH1-FLAG was shown to stimulate distal splicing of an E1A minigene in HeLa cells. Moreover, a GST-ASF/SF2 fusion was not phosphorylated by PSKH1, suggesting an indirect mechanism of action on SR proteins. Our data suggest a mutual relationship between PSKH1 and SR proteins, as they are able to target PSKH1 into SFCs, while forced PSKH1 expression modulates nuclear dynamics and the function of co-expressed splicing factors.

Increased expression and activation of c-Jun contributes to human amylin-induced apoptosis in pancreatic islet beta-cells

The role of c-Jun in apoptosis evoked by human amylin was investigated using human and rat insulinoma beta-cell lines. Two transient increases in the levels of c-jun mRNA were detected at 30 minutes and eight hours after treatment with human amylin. The level of c-Jun protein was also up-regulated in a time-dependent manner, reaching maximal levels after eight hours of exposure. However, no c-Jun induction was detected in cells treated with vehicle only or with rat amylin, indicating that the amyloidogenic feature of the human peptide may be important for c-Jun induction. We found that c-Jun was activated by phosphorylation specifically at Ser63 at four hours, but not at Ser73, after treatment with human amylin, preceding increased c-Jun protein. Furthermore, expression of an antisense c-jun (AS-c-jun), which suppressed protein levels of both c-Jun and phosphorylated-c-Jun, caused a marked reduction in apoptotic cell death, whereas the corresponding sense c-jun (S-c-jun) had no effect on changes of either c-Jun production or apoptosis. This indicated that increased expression and activation of c-Jun is required for human amylin-induced apoptosis. Immunocytochemical studies showed a significant increase in nuclear staining for c-Jun, phosphorylated-c-Jun (Ser63) and phosphorylated-JNK, suggesting that c-Jun may be activated through activation of JNK. In addition, electrophoretic mobility-shift assays showed that the increase in expression and phosphorylation of c-Jun was associated with increased AP-1 DNA binding activity. Supershift assays demonstrated that c-Jun, c-Fos and ATF-2 are part of the AP-1 complex, indicating that activated c-Jun is dimerized with c-Fos or ATF-2 for control of its target gene expression. Finally, we showed that human amylin triggers AP-1-mediated transcriptional activation. Our results suggest strongly that human amylin induces apoptosis through stimulation of expression and activation of c-Jun, and that co-expression and dimerization of c-Jun and c-fos or ATF-2 may be important for activation of the downstream apoptotic process.

MAPKs entry into the nucleus at specific interchromatin domains in plant differentiation and proliferation processes

Mitogen-activated protein kinases (MAPKs) are involved in the signaling of extracellular stimuli in eukaryotes, including plants. Different MAPKs have recently been shown to be expressed during plant cell proliferation and developmental processes such as pollen development and embryogenesis, but the structural subdomain where these MAPKs are targeted in the nucleus has not yet been characterized. We have determined the changes in the expression and subcellular localization of ERK homologues, proteins belonging to the MAPK family, and MAPK-active forms in two plant developmental processes which involved differentiation (pollen maturation) and proliferation (the initials of pollen embryogenesis). Immunofluorescence and immunogold labeling in the species studied showed that the progression of differentiation and proliferation was accompanied by an increase in the expression of ERKs and MAPK activation together with a translocation to the nucleus. Combining ultrastructural cytochemistry and immunogold for RNA and phosphorylated proteins we have identified the nuclear sites housing these MAPKs in areas of the interchromatin region enriched in RNA and phosphoproteins that include clusters of interchromatin granules. This could suggest a role of these MAPKs in the early events of activation of the transcription and processing machinery, via phosphorylation, which subsequently would be recruited to the transcription sites. The association of the nuclear localization of MAPKs with the progression through the cell cycle and the commitment toward differentiation in the two plant developmental processes can be correlated.

Cell cycle regulation of c-Jun N-terminal kinase activity at the centrosomes

The c-Jun N-terminal kinase (JNK), a subgroup of the mitogen-activated protein kinase (MAPK) family of serine/threonine kinases, has established functions in cell growth and apoptosis. While the mechanisms are unclear, JNK has also been also implicated in signaling pathways that initiate cell cycle checkpoints and cell cycle progression. By following the localization of active and inactive JNK during the cell cycle, we have found that the majority of cellular JNK is soluble and present in the cytoplasm and the nucleus. Interestingly, insoluble fractions of JNK are also localized in nuclear and cytoplasmic speckles, and to the centrosomes. While JNK is associated with the centrosome throughout the cell cycle, it is only active at the centrosome from S phase through anaphase. This novel localization of centrosomal JNK is a possible link between JNK-activating stimuli and centrosome or cell cycle events.

AP-1 proteins in the adult brain: facts and fiction about effectors of neuroprotection and neurodegeneration

Jun and Fos proteins are induced and activated following most physiological and pathophysiological stimuli in the brain. Only few data allow conclusions about distinct functions of AP-1 proteins in neurodegeneration and neuroregeneration, and these functions mainly refer to c-Jun and its activation by JNKs. Apoptotic functions of activated c-Jun affect hippocampal, nigral and primary cultured neurons following excitotoxic stimulation and destruction of the neuron-target-axis including withdrawal of trophic molecules. The inhibition of JNKs might exert neuroprotection by subsequent omission of c-Jun activation. Besides endogenous neuronal functions, the c-Jun/AP-1 proteins can damage the nervous system by upregulation of harmful programs in non-neuronal cells (e.g. microglia) with release of neurodegenerative molecules. In contrast, the differentiation with neurite extension and maturation of neural cells in vitro indicate physiological and potentially neuroprotective functions of c-Jun and JNKs including sensoring for alterations in the cytoskeleton. This review summarizes the multiple molecular interfunctions which are involved in the shift from the physiological role to degenerative effects of the Jun/JNK-axis such as cell type-specific expression and intracellular localization of scaffold proteins and upstream activators, antagonistic phosphatases, interaction with other kinase systems, or the activation of transcription factors competing for binding to JNK proteins and AP-1 DNA elements.

Activation of p44/p42 MAP kinase in striatal neurons via kainate receptors and PI3 kinase

The members of the mitogen-activated protein (MAP) kinase family -- p44/p42 MAP kinase (ERK), c-jun N-terminal kinase (JNK) and p38 MAP kinase (p38) are known to be important mediators of the physiological plasticity or neurotoxicity induced in the striatum by activation of ionotropic glutamate receptors. However, our knowledge of the class of glutamate receptor and the intracellular pathways involved derives totally from studies on embryonic neurons, where the mechanisms are likely to be totally different from those operating in mature neurons. In superfused striatal slices from adult rats, NMDA and kainate, but not AMPA, were found to activate ERK. No activation of p38 or JNK was detected following treatment with any ionotropic glutamate receptor agonist. The activation of ERK by kainate was blocked by the ERK kinase (MEK) inhibitor PD98059, and the PI3 kinase inhibitor wortmannin, but not by the p38 MAP kinase inhibitor SB203580. This provides evidence for a novel pathway linking striatal kainate receptors to ERK activation via PI3 kinase and MEK.

Mitogen-activated protein (MAP) kinase pathways: regulation and physiological functions

Mitogen-activated protein (MAP) kinases comprise a family of ubiquitous proline-directed, protein-serine/threonine kinases, which participate in signal transduction pathways that control intracellular events including acute responses to hormones and major developmental changes in organisms. MAP kinases lie in protein kinase cascades. This review discusses the regulation and functions of mammalian MAP kinases. Nonenzymatic mechanisms that impact MAP kinase functions and findings from gene disruption studies are highlighted. Particular emphasis is on ERK1/2.

Jun, Fos and Krox in the hippocampus after noxious stimulation: simultaneous-input-dependent expression and nuclear speckling

Stimulation of sensory C-fibres produces extensive expression of the Fos, Jun and Krox families of inducible transcription factors (ITFs) in many nociceptive CNS areas [28]. In the hippocampus, however, c-Fos is only weakly induced by such stimulation, and expression of the other ITFs has not been studied. Here we examine the effects of single, repeated and simultaneous C-fibre inputs on ITF expressions in the rat hippocampus. A brief, strong electrical stimulation of sciatic nerve C-fibres induced little or no expression of c-Fos or Krox-20. In contrast, FosB was induced and continued to rise in all areas, whereas the basal expressions of c-Jun and Krox-24 were initially reduced but then returned during the subsequent 36 h. A weak noxious cutaneous stimulus applied to one hindpaw induced only weak expressions of the ITFs. However, if the sciatic stimulation was applied contralaterally and 6 h beforehand, this weak stimulus strongly induced Krox-24, but not other ITFs, i.e. there was a potentiation of Krox-24 expression. When these two stimuli were applied simultaneously a few c-Fos labelled cells did appear, and there was and an increased Krox-24 expression. There was also a strong potentiation of FosB and a strong reduction in c-Jun expression. This simultaneous stimulation was the only type of stimulation to induce expression of Krox-20. Also after simultaneous stimulation the majority of the nuclear labelling for FosB, but not of the other ITFs, had a speckled appearance. MK-801 blocked these changes in ITF expressions, but it could also cause the C-fibre stimulations to induce c-Fos and c-Jun in specific areas of the hippocampus. Thus C-fibre stimulation does affect transcription factor activity in the hippocampus; and the strong responses of some ITFs to simultaneous inputs points to their having a role as 'genetic coincidence detectors' in the hippocampus.

Neuronal body size correlates with the number of nucleoli and Cajal bodies, and with the organization of the splicing machinery in rat trigeminal ganglion neurons

Trigeminal ganglion neurons comprise three main cell body-size types. This cell size heterogeneity provides an excellent neuronal model to study the cell size-dependent organization and dynamics of the nucleoli, Cajal (coiled) bodies (CBs), and nuclear speckles of pre-mRNA splicing factors, nuclear structures that play a key role in the normal neuronal physiology. We have analyzed the number of nucleoli and CBs and the structural and molecular organization of CBs and nuclear speckles in the three neuronal types by using immunofluorescence with antibodies that recognize nucleoli (fibrillarin), CBs (coilin), and nuclear speckles (snRNPs), confocal microscopy, and electron microscopy. Whereas the mean number of nucleoli per neuron decreases as a function of cell size, the number of CBs per cell significantly increases in large neurons in comparison with the small ones. In addition, large neurons have a higher proportion of CBs associated with the nucleolus. In all neuronal types, CBs concentrate coilin, fibrillarin, snRNPs, and the survival motor neuron protein (SMN). Immunostaining for snRNPs shows small speckle domains and extensive areas of diffuse nucleoplasmic signal in large neurons, in contrast with the large nuclear speckles found in small neurons. Furthermore, flow cytometric analysis shows that all neurons are in the range of diploid cells. These findings indicate that the fusion behavior of nucleoli, the formation of CBs and their relationships with the nucleolus, as well as the compartmentalization of the pre-mRNA splicing machinery, is related to cell body size in the trigeminal ganglion neurons. Because transcriptional activity is a basic determinant mechanism of cell size in diploid cells, we suggest that our findings reflect a distinct transcription-dependent organization of the nucleolus and splicing machinery in the three cell types of trigeminal ganglion neurons.