Effect of retinoic acid on growth and morphological differentiation of mouse NB2a neuroblastoma cells in culture

Transcriptional-regulatory convergence across functional MDD risk variants identified by massively parallel reporter assays

Family and population studies indicate clear heritability of major depressive disorder (MDD), though its underlying biology remains unclear. The majority of single-nucleotide polymorphism (SNP) linkage blocks associated with MDD by genome-wide association studies (GWASes) are believed to alter transcriptional regulators (e.g., enhancers, promoters) based on enrichment of marks correlated with these functions. A key to understanding MDD pathophysiology will be elucidation of which SNPs are functional and how such functional variants biologically converge to elicit the disease. Furthermore, retinoids can elicit MDD in patients and promote depressive-like behaviors in rodent models, acting via a regulatory system of retinoid receptor transcription factors (TFs). We therefore sought to simultaneously identify functional genetic variants and assess retinoid pathway regulation of MDD risk loci. Using Massively Parallel Reporter Assays (MPRAs), we functionally screened over 1000 SNPs prioritized from 39 neuropsychiatric trait/disease GWAS loci, selecting SNPs based on overlap with predicted regulatory features-including expression quantitative trait loci (eQTL) and histone marks-from human brains and cell cultures. We identified >100 SNPs with allelic effects on expression in a retinoid-responsive model system. Functional SNPs were enriched for binding sequences of retinoic acid-receptive transcription factors (TFs), with additional allelic differences unmasked by treatment with all-trans retinoic acid (ATRA). Finally, motifs overrepresented across functional SNPs corresponded to TFs highly specific to serotonergic neurons, suggesting an in vivo site of action. Our application of MPRAs to screen MDD-associated SNPs suggests a shared transcriptional-regulatory program across loci, a component of which is unmasked by retinoids.

DCC regulates astroglial development essential for telencephalic morphogenesis and corpus callosum formation

The forebrain hemispheres are predominantly separated during embryogenesis by the interhemispheric fissure (IHF). Radial astroglia remodel the IHF to form a continuous substrate between the hemispheres for midline crossing of the corpus callosum (CC) and hippocampal commissure (HC). Deleted in colorectal carcinoma (DCC) and netrin 1 (NTN1) are molecules that have an evolutionarily conserved function in commissural axon guidance. The CC and HC are absent in Dcc and Ntn1 knockout mice, while other commissures are only partially affected, suggesting an additional aetiology in forebrain commissure formation. Here, we find that these molecules play a critical role in regulating astroglial development and IHF remodelling during CC and HC formation. Human subjects with DCC mutations display disrupted IHF remodelling associated with CC and HC malformations. Thus, axon guidance molecules such as DCC and NTN1 first regulate the formation of a midline substrate for dorsal commissures prior to their role in regulating axonal growth and guidance across it.

Purinergic signaling in nervous system health and disease: Focus on pannexin 1

Purinergic signaling encompasses the cycle of adenosine 5' triphosphate (ATP) release and its metabolism into nucleotide and nucleoside derivatives, the direct release of nucleosides, and subsequent receptor-triggered downstream intracellular pathways. Since the discovery of nerve terminal and glial ATP release into the neuropil, purinergic signaling has been implicated in the modulation of nervous system development, function, and disease. In this review, we detail our current understanding of the roles of the pannexin 1 (PANX1) ATP-release channel in neuronal development and plasticity, glial signaling, and neuron-glial-immune interactions. We additionally provide an overview of PANX1 structure, activation, and permeability to orientate readers and highlight recent research developments. We identify areas of convergence between PANX1 and purinergic receptor actions. Additional highlights include data on PANX1's participation in the pathophysiology of nervous system developmental, degenerative, and inflammatory disorders. Our aim in combining this knowledge is to facilitate the movement of our current understanding of PANX1 in the context of other nervous system purinergic signaling mechanisms one step closer to clinical translation.

SUMOylation of α-tubulin is a novel modification regulating microtubule dynamics

Microtubules (MTs) are regulated by a number of known posttranslational modifications (PTMs) on α/β-tubulin to fulfill diverse cellular functions. Here, we showed that SUMOylation is a novel PTM on α-tubulin in vivo and in vitro. The SUMOylation on α-tubulin mainly occurred at Lys 96 (K96), K166, and K304 of soluble α-tubulin and could be removed by small ubiquitin-related modifier (SUMO)-specific peptidase 1. In vitro experiments showed that tubulin SUMOylation could reduce interprotofilament interaction, promote MT catastrophe, and impede MT polymerization. In cells, mutation of the SUMOylation sites on α-tubulin reduced catastrophe frequency and increased the proportion of polymerized α-tubulin, while upregulation of SUMOylation with fusion of SUMO1 reduced α-tubulin assembly into MTs. Additionally, overexpression of SUMOylation-deficient α-tubulin attenuated the neurite extension in Neuro-2a cells. Thus, SUMOylation on α-tubulin represents a new player in the regulation of MT properties.

Stearoyl-CoA desaturase 5 (SCD5), a Δ-9 fatty acyl desaturase in search of a function

A large body of research has demonstrated that human stearoyl-CoA desaturase 1 (SCD1), a universally expressed fatty acid Δ9-desaturase that converts saturated fatty acids (SFA) into monounsaturated fatty acids (MUFA), is a central regulator of metabolic and signaling pathways involved in cell proliferation, differentiation, and survival. Unlike SCD1, stearoyl-CoA desaturase 5 (SCD5), a second SCD isoform found in a variety of vertebrates, including humans, has received considerably less attention but new information on the catalytic properties, regulation and biological functions of this enzyme has begun to emerge. This review will examine the new evidence that supports key metabolic and biological roles for SCD5, as well as the potential implication of this desaturase in the mechanisms of human diseases.

Neurofilaments form flexible bundles during neuritogenesis in culture and in mature axons in situ

Neurofilaments (NFs) undergo cation-dependent phospho-mediated associations with each other and other cytoskeletal elements that support axonal outgrowth. Progressive NF-NF associations generate a resident, bundled population that undergoes exchange with transporting NFs. We examined the properties of bundled NFs. Bundles did not always display a fully linear profile but curved and twisted at various points along the neurite length. Bundles retracted faster than neurites and retracted bundles did not expand following extraction with Triton, indicating that they coiled passively rather than due to pressure from the cell. Bundles consisted of helically wound NFs, which may provide flexibility necessary for turning of growing axons during pathfinding. Interactions between NFs and other cytoskeletal elements may be disrupted en masse during neurite retraction or regionally during remodeling. It is suggested that bundles within long axons that cannot be fully retracted into the soma could provide maintain proximal support yet still allow more distal flexibility for remodeling and changing direction during pathfinding.

Beneficial and Deleterious Impact of a Nutritional Supplementation for Inhibition of Proliferation of Neuroblastoma in Culture

Neuroblastoma, a cancer of the sympathetic nervous system, primarily affects infants and children ≤10 yr of age. High-risk neuroblastoma is associated with low survival rates and increased risks of treatment-related side-effects. Therefore, effective treatments that increase survival and reduce adverse side-effects are crucial. Cucurbitacin E (CucE), a nutritional supplement shown to have potential as an alternative to chemotherapy, was investigated for potential impact on neuroblastoma alone and in combination with the standard chemotherapeutic agent, paclitaxel, (PAC). CucE and PAC each inhibited proliferation of murine neuroblastoma cells in culture. Combined treatment with CucE and PAC also induced morphological differentiation. However, both differentiation and antiproliferative effects were reversible. Consequently, while nutritional supplementation represents a potential therapeutic approach toward treatment of cancer, certain nutritional/chemotherapeutic combinations may induce transient rather than permanent effects. Transient inhibition of proliferation by nutritional supplementation could inadvertently protect carcinogenic cells from toxicity otherwise induced by a chemotherapeutic agent. Combinatorial treatments involving nutritional supplements should therefore be utilized with caution.

The long non-coding RNA Dali is an epigenetic regulator of neural differentiation

Many intergenic long noncoding RNA (lncRNA) loci regulate the expression of adjacent protein coding genes. Less clear is whether intergenic lncRNAs commonly regulate transcription by modulating chromatin at genomically distant loci. Here, we report both genomically local and distal RNA-dependent roles of Dali, a conserved central nervous system expressed intergenic lncRNA. Dali is transcribed downstream of the Pou3f3 transcription factor gene and its depletion disrupts the differentiation of neuroblastoma cells. Locally, Dali transcript regulates transcription of the Pou3f3 locus. Distally, it preferentially targets active promoters and regulates expression of neural differentiation genes, in part through physical association with the POU3F3 protein. Dali interacts with the DNMT1 DNA methyltransferase in mouse and human and regulates DNA methylation status of CpG island-associated promoters in trans. These results demonstrate, for the first time, that a single intergenic lncRNA controls the activity and methylation of genomically distal regulatory elements to modulate large-scale transcriptional programmes.

Choline kinase alpha expression during RA-induced neuronal differentiation: role of C/EBPβ

Neuronal differentiation is a complex process characterized by a halt in proliferation and extension of neurites from the cell body. This process is accompanied by changes in gene expression that mediate the redirection leading to neurite formation and function. Acceleration of membrane phospholipids synthesis is associated with neurite elongation, and phosphatidylcholine (PtdCho) is the major membrane phospholipid in mammalian cells. The transcription of two genes in particular encoding key enzymes in the CDP-choline pathway for PtdCho biosynthesis are stimulated; the Chka gene for choline kinase (CK) alpha isoform and the Pcyt1a gene for the CTP:phosphocholine cytidylyltransferase (CCT) alpha isoform. We report that the stimulation of CKα expression during retinoic acid (RA) induced differentiation depends on a promoter region that contains two CCAAT/Enhancer-binding Protein-β (C/EBPβ) sites. We demonstrate that during neuronal differentiation of Neuro-2a cells, RA induces Chka expression by a mechanism that involves ERK1/2 activation which triggers C/EBPβ expression. Elevated levels of C/EBPβ bind to the Chka proximal promoter (Box1) inducing CKα expression. In addition we identified a downstream sequence named Box2 which together with Box1 is required for the promoter to reach the full induction. This is the first elucidation of the mechanism by which the expression of Chka is coordinately regulated during neuronal differentiation.

EFA6A, a guanine nucleotide exchange factor for Arf6, interacts with sorting nexin-1 and regulates neurite outgrowth

The membrane trafficking and actin cytoskeleton remodeling mediated by ADP ribosylation factor 6 (Arf6) are functionally linked to various neuronal processes including neurite formation and maintenance, neurotransmitter release, and receptor internalization. EFA6A is an Arf6-specific guanine nucleotide exchange factor that is abundantly expressed in the brain. In this study, we identified sorting nexin-1 (SNX1), a retromer component that is implicated in endosomal sorting and trafficking, as a novel interacting partner for EFA6A by yeast two-hybrid screening. The interaction was mediated by the C-terminal region of EFA6A and a BAR domain of SNX1, and further confirmed by pull-down assay and immunoprecipitation from mouse brain lysates. In situ hybridization analysis demonstrated the widespread expression of SNX1 in the mouse brain, which overlapped with the expression of EFA6A in the forebrain. Immunofluorescent analysis revealed the partial colocalization of EFA6A and SNX1 in the dendritic fields of the hippocampus. Immunoelectron microscopic analysis revealed the overlapping subcellular localization of EFA6A and SNX1 at the post-synaptic density and endosomes in dendritic spines. In Neuro-2a neuroblastoma cells, expression of either EFA6A or SNX1 induced neurite outgrowth, which was further enhanced by co-expression of EFA6A and SNX1. The present findings suggest a novel mechanism by which EFA6A regulates Arf6-mediated neurite formation through the interaction with SNX1.

The atypical guanine nucleotide exchange factor Dock4 regulates neurite differentiation through modulation of Rac1 GTPase and actin dynamics

Precise regulation of neurite growth and differentiation determines accurate formation of synaptic connections, whose disruptions are frequently associated with neurological disorders. Dedicator of cytokinesis 4 (Dock4), an atypical guanine nucleotide exchange factor for Rac1, is found to be associated with neuropsychiatric diseases, including autism and schizophrenia. Nonetheless, the neuronal function of Dock4 is only beginning to be understood. Using mouse neuroblastoma (Neuro-2a) cells as a model, this study identifies that Dock4 is critical for neurite differentiation and extension. This regulation is through activation of Rac1 and modulation of the dynamics of actin-enriched protrusions on the neurites. In cultured hippocampal neurons, Dock4 regulates the establishment of the axon-dendrite polarity and the arborization of dendrites, two critical processes during neural differentiation. Importantly, a microdeletion Dock4 mutant linked to autism and dyslexia that lacks the GEF domain leads to defective neurite outgrowth and neuronal polarization. Further analysis reveals that the SH3 domain-mediated interaction of Dock4 is required for its activity toward neurite differentiation, whereas its proline-rich C terminus is not essential for this regulation. Together, our findings reveal an important role of Dock4 for neurite differentiation during early neuronal development.

Corticotropin-releasing hormone stimulates mitotic kinesin-like protein 1 expression via a PLC/PKC-dependent signaling pathway in hippocampal neurons

Corticotropin-releasing hormone (CRH) has been shown to modulate dendritic development in hippocampus. Mitotic kinesin-like protein 1 (MKLP1) plays key roles in dendritic differentiation. In the present study, we examined the effects of CRH on MKLP1 expression in cultured hippocampal neurons and determine subsequent signaling pathways involved. CRH dose-dependently increased MKLP1 mRNA and protein expression. This effect can be reversed by CRHR1 antagonist but not by CRHR2 antagonist. CRHR1 knockdown impaired this effect of CRH. CRH stimulated GTP-bound Gαs protein and phosphorylated phospholipase C (PLC)-β3 expression, which were blocked by CRHR1 antagonist. Transfection of GP antagonist-2A, an inhibitory peptide of Gαq protein, blocked CRH-induced phosphorylated PLC-β3 expression. PLC and PKC inhibitors completely blocked whereas adenylyl cyclase (AC) and PKA inhibitors did not affect CRH-induced MKLP1 expression. Our results indicate that CRH act on CRHR1 to induce MKLP1 expression via PLC/PKC signaling pathway. CRH may regulate MKLP1 expression, thereby modulating dendritic development.

Pannexin 1 regulates postnatal neural stem and progenitor cell proliferation

Background

Pannexin 1 forms ion and metabolite permeable hexameric channels and is abundantly expressed in the brain. After discovering pannexin 1 expression in postnatal neural stem and progenitor cells we sought to elucidate its functional role in neuronal development.

Results

We detected pannexin 1 in neural stem and progenitor cells in vitro and in vivo. We manipulated pannexin 1 expression and activity in Neuro2a neuroblastoma cells and primary postnatal neurosphere cultures to demonstrate that pannexin 1 regulates neural stem and progenitor cell proliferation likely through the release of adenosine triphosphate (ATP).

Conclusions

Permeable to ATP, a potent autocrine/paracine signaling metabolite, pannexin 1 channels are ideally suited to influence the behavior of neural stem and progenitor cells. Here we demonstrate they play a robust role in the regulation of neural stem and progenitor cell proliferation. Endogenous postnatal neural stem and progenitor cells are crucial for normal brain health, and their numbers decline with age. Furthermore, these special cells are highly responsive to neurological injury and disease, and are gaining attention as putative targets for brain repair. Therefore, understanding the fundamental role of pannexin 1 channels in neural stem and progenitor cells is of critical importance for brain health and disease.

R-Ras controls axon branching through afadin in cortical neurons

Regulation of axon growth, guidance, and branching is essential for constructing a correct neuronal network. R-Ras, a Ras-family small GTPase, has essential roles in axon formation and guidance. During axon formation, R-Ras activates a series of phosphatidylinositol 3-kinase signaling, inducing activation of a microtubule-assembly promoter-collapsin response mediator protein-2. However, signaling molecules linking R-Ras to actin cytoskeleton-regulating axonal morphology remain obscure. Here we identify afadin, an actin-binding protein harboring Ras association (RA) domains, as an effector of R-Ras inducing axon branching through F-actin reorganization. We observe endogenous interaction of afadin with R-Ras in cortical neurons during the stage of axonal development. Ectopic expression of afadin increases axon branch number, and the RA domains and the carboxyl-terminal F-actin binding domain are required for this action. RNA interference knockdown experiments reveal that knockdown of endogenous afadin suppressed both basal and R-Ras-mediated axon branching in cultured cortical neurons. Subcellular localization analysis shows that active R-Ras-induced translocation of afadin and its RA domains is responsible for afadin localizing to the membrane and inducing neurite development in Neuro2a cells. Overall, our findings demonstrate a novel signaling pathway downstream of R-Ras that controls axon branching.

Assembly properties of lamprey neurofilament subunits and their expression after spinal cord transection

In mammals neurofilaments (NF) are formed by coassembly of three subunits: NFL, NFM, and NFH (light, medium, and heavy). It had been believed that lampreys have only one subunit, NF180. However, a previous study showed that NF180 could not self-assemble but could coassemble with rat NFL, suggesting the existence of additional NF subunits in lamprey. More recently, we cloned three additional NF subunits. These new subunits and NF180 have now been transfected in combinations into SW13cl.2Vim(-) cells, which lack endogenous cytoplasmic intermediate filaments. None of the subunits could self-assemble. No combination of NF subunits could form filaments in the absence of lamprey NFL (L-NFL). Assembly occurred at 28°C, but not at 37°C. L-NFL could form thick NF bundles with NF180 but not with NF132 and NF95, which formed only fine filamentous arrays. To determine which parts of the NF subunits are required for filament or bundle formation, we constructed deletion mutants of NF180 and cotransfected them with L-NFL. As with mammalian NF, only constructs with intact head and core domains could form filaments with L-NFL. However, the full length of NF180 was required to form NF bundles. As with NF180, in situ hybridization indicated that mRNA for L-NFL and NF132 was downregulated in identified reticulospinal neurons by 5 weeks after spinal cord transection, but was reexpressed at 10 weeks selectively in those neurons whose axons have a high probability of regenerating. This is consistent with a possible role of NFs in the mechanism of axon regeneration.

Proliferating versus differentiating stem and cancer cells exhibit distinct midbody-release behaviour

The central portion of the midbody, a cytoplasmic bridge between nascent daughter cells at the end of cell division, has generally been thought to be retained by one of the daughter cells, but has, recently, also been shown to be released into the extracellular space. The significance of midbody-retention versus -release is unknown. Here we show, by quantitatively analysing midbody-fate in various cell lines under different growth conditions, that the extent of midbody-release is significantly greater in stem cells than cancer-derived cells. Induction of cell differentiation is accompanied by an increase in midbody-release. Knockdown of the endosomal sorting complex required for transport family members, Alix and tumour-suppressor gene 101, or of their interaction partner, centrosomal protein 55, impairs midbody-release, suggesting mechanistic similarities to abscission. Cells with such impaired midbody-release exhibit enhanced responsiveness to a differentiation stimulus. Taken together, midbody-release emerges as a characteristic feature of cells capable of differentiation.

During cell division, a cytoplasmic bridge—the midbody—forms between the nascent daughter cells, but it has been unclear under which conditions this is retained by a daughter cell or released. Now, Ettinger and colleagues show that midbody-release occurs more frequently in stem cells compared with cancer cells.

The all-trans retinoic acid (atRA)-regulated gene Calmin (Clmn) regulates cell cycle exit and neurite outgrowth in murine neuroblastoma (Neuro2a) cells

The vitamin A metabolite all-trans retinoic acid (atRA) functions in nervous system development and regulates cell proliferation and differentiation. Neuroblastoma cells (SH-SY5Y and Neuro2a or N2A) exposed to atRA undergo growth inhibition and neuronal differentiation, both of which are preceded by an increase in Clmn mRNA. Treatment of N2A cells with atRA produces a reduction in phosphohistone 3 immunostaining and BrdU incorporation, both indicators of a reduction in cell proliferation. These effects are nearly eliminated in atRA-treated shClmn knockdown cells. Loss of Clmn in the mouse N2A cell line also results in a significant reduction of atRA-mediated neurite outgrowth, a response that can be rescued by reintroduction of the Clmn sequence. In contrast, ectopic overexpression of Clmn produces an increase in the cyclin dependent kinase inhibitor, p21(Cip1), a decrease in cyclin D1 protein and an increase in hypophosphorylated Rb, showing that Clmn participates in G(1)/S arrest. Clmn overexpression alone is sufficient to inhibit N2A cell proliferation, whereas both Clmn and atRA must be present to induce neurite outgrowth. This study shows that the atRA-responsive gene Clmn promotes exit from the cell cycle, a requisite event for neuronal differentiation.

Tropomodulins are negative regulators of neurite outgrowth

Regulation of the actin cytoskeleton is critical for neurite formation. Tropomodulins (Tmods) regulate polymerization at actin filament pointed ends. Previous experiments using a mouse model deficient for the neuron specific isoform Tmod2 suggested a role for Tmods in neuronal function by impacting processes underlying learning and memory. However, the role of Tmods in neuronal function on the cellular level remains unknown. Immunofluorescence localization of the neuronal isoforms Tmod1 and Tmod2 in cultured rat primary hippocampal neurons revealed that Tmod1 is enriched along the proximal part of F-actin bundles in lamellipodia of spreading cells and in growth cones of extending neurites, while Tmod2 appears largely cytoplasmic. Functional analysis of these Tmod isoforms in a mouse neuroblastoma N2a cell line showed that knockdown of Tmod2 resulted in a significant increase in the number of neurite-forming cells and in neurite length. While N2a cells compensated for Tmod2 knockdown by increasing Tmod1 levels, over-expression of exogenous Tmod1 had no effect on neurite outgrowth. Moreover, knockdown of Tmod1 increased the number of neurites formed per cell, without effect on the number of neurite-forming cells or neurite length. Taken together, these results indicate that Tmod1 and Tmod2 have mechanistically distinct inhibitory roles in neurite formation, likely mediated via different effects on F-actin dynamics and via differential localizations during early neuritogenesis.

Phosphatidylcholine biosynthesis during neuronal differentiation and its role in cell fate determination

Neuronal differentiation is characterized by neuritogenesis and neurite outgrowth, processes that are dependent on membrane biosynthesis. Thus, the production of phosphatidylcholine (PtdCho), the major membrane phospholipid, should be stimulated during neuronal differentiation. We demonstrate that during retinoic acid (RA)-induced differentiation of Neuro-2a cells, PtdCho synthesis was promoted by an ordered and sequential activation of choline kinase alpha (CK(alpha)) and choline cytidylyltransferase alpha (CCT(alpha)). Early after RA stimulation, the increase in PtdCho synthesis is mainly governed by the biochemical activation of CCT(alpha). Later, the transcription of CK(alpha)- and CCT(alpha)-encoding genes was induced. Both PtdCho biosynthesis and neuronal differentiation are dependent on ERK activation. A novel mechanism is proposed by which PtdCho biosynthesis is coordinated during neuronal differentiation. Enforced expression of either CK(alpha) or CCTalpha increased the rate of synthesis and the amount of PtdCho, and these cells initiated differentiation without RA stimulation, as evidenced by cell morphology and the expression of genes associated with neuritogenesis. The differentiation resulting from enforced expression of CCT(alpha) or CK(alpha) was dependent on persistent ERK activation. These results indicate that elevated PtdCho synthesis could mimic the RA signals and thus determine neuronal cell fate. Moreover, they could explain the key role that PtdCho plays during neuronal regeneration.

The nystagmus-associated FRMD7 gene regulates neuronal outgrowth and development

Mutations in the gene encoding FERM domain-containing 7 protein (FRMD7) are recognized as an important cause of X-linked idiopathic infantile nystagmus (IIN). However, the precise role of FRMD7 and its involvement in the pathogenesis of IIN are not understood. In the present study, we have explored the role of FRMD7 in neuronal development. Using in situ hybridization and immunohistochemistry, we reveal that FRMD7 expression is spatially and temporally regulated in both the human and mouse brain during embryonic and fetal development. Furthermore, we show that FRMD7 expression is up-regulated upon retinoic acid (RA)-induced differentiation of mouse neuroblastoma NEURO2A cells, suggesting FRMD7 may play a role in this process. Indeed, we demonstrate, for the first time, that knockdown of FRMD7 during neuronal differentiation results in altered neurite development. Taken together, our data suggest that FRMD7 is involved in multiple aspects of neuronal development, and have direct importance to further understanding the pathogenesis of IIN.

Vesicular trafficking and secretion of matrix metalloproteinases-2, -9 and tissue inhibitor of metalloproteinases-1 in neuronal cells

Matrix metalloproteinases (MMPs) are endopeptidases that cleave matrix, soluble and membrane-bound proteins and are regulated by their endogenous inhibitors the tissue inhibitors of MMPs (TIMPs). Nothing is known about MMP/TIMP trafficking and secretion in neuronal cells. We focussed our attention on the gelatinases MMP-2 and MMP-9, and their inhibitor TIMP-1. MMPs and TIMP-1 fused to GFP were expressed in N2a neuroblastoma and primary neuronal cells to study trafficking and secretion using real time video-microscopy, imaging, electron microscopy and biochemical approaches. We show that MMPs and TIMP-1 are secreted in 160-200 nm vesicles in a Golgi-dependent pathway. These vesicles distribute along microtubules and microfilaments, co-localise differentially with the molecular motors kinesin and myosin Va and undergo both anterograde and retrograde trafficking. MMP-9 retrograde transport involves the dynein/dynactin molecular motor. In hippocampal neurons, MMP-2 and MMP-9 vesicles are preferentially distributed in the somato-dendritic compartment and are found in dendritic spines. Non-transfected hippocampal neurons also demonstrate vesicular secretion of MMP-2 in both its pro- and active forms and gelatinolytic activity localised within dendritic spines. Our results show differential trafficking of MMP and TIMP-1-containing vesicles in neuronal cells and suggest that these vesicles could play a role in neuronal and synaptic plasticity.

Developmental neurotoxicity testing in vitro: models for assessing chemical effects on neurite outgrowth

In vitro models may be useful for the rapid toxicological screening of large numbers of chemicals for their potential to produce toxicity. Such screening could facilitate prioritization of resources needed for in vivo toxicity testing towards those chemicals most likely to result in adverse health effects. Cell cultures derived from nervous system tissue have proven to be powerful tools for elucidating cellular and molecular mechanisms of nervous system development and function, and have been used to understand the mechanism of action of neurotoxic chemicals. Recently, it has been suggested that in vitro models could be used to screen for chemical effects on critical cellular events of neurodevelopment, including differentiation and neurite growth. This review examines the use of neuronal cell cultures as an in vitro model of neurite outgrowth. Examples of the cell culture systems that are commonly used to examine the effects of chemicals on neurite outgrowth are provided, along with a description of the methods used to quantify this neurodevelopmental process in vitro. Issues relating to the relevance of the methods and models currently used to assess neurite outgrowth are discussed in the context of hazard identification and chemical screening. To demonstrate the utility of in vitro models of neurite outgrowth for the evaluation of large numbers of chemicals, efforts should be made to: (1) develop a set of reference chemicals that can be used as positive and negative controls for comparing neurite outgrowth between model systems, (2) focus on cell cultures of human origin, with emphasis on the emerging area of neural progenitor cells, and (3) use high-throughput methods to quantify endpoints of neurite outgrowth.

Valproic acid induces caspase 3-mediated apoptosis in microglial cells

Valproic acid is widely used for the treatment of epilepsy and mood disorders, but its mode of action is unclear. Treatment of neuronal cells with valproic acid promotes neurite sprouting, is neuroprotective and drives neurogenesis; however its effects on non-neuronal brain cells are less clear. We report that valproic acid induces apoptosis in the mouse microglial cell line, BV-2, at concentrations within the therapeutic range. When BV-2 cells were incubated for 24 h with 500-1000 microM valproic acid we observed a reduction in cell number, the appearance of apoptotic morphology and increased caspase 3 cleavage. Exposure of a macrophage cell line (RAW 264.7) to similar concentrations of valproic acid also led to reduced cell number but no caspase 3 cleavage, suggesting these cells responded to valproic acid with reduced proliferation rather than apoptosis. This was confirmed using bromodeoxyuridine incorporation studies. Similar concentrations of valproic acid added to Neuro-2a, SK-N-SH and C6 cell lines as well as human NTera-2 astrocytes did not evoke cell death. The caspase 3 inhibitor DEVD-CHO inhibited valproic acid-induced apoptosis in BV-2 cells whereas the MEK inhibitor U0126 potentiated valproic acid-mediated apoptosis. These results demonstrate that valproic acid selectively induces apoptosis in BV-2 cells by way of a caspase 3-mediated action. As activated microglia secrete neurotoxins in neurodegenerative diseases such as Alzheimer's, Parkinson's, and HIV dementia, valproic acid may alleviate these diseases by selectively killing microglia.

SRY-box containing gene 11 (Sox11) transcription factor is required for neuron survival and neurite growth

The transcription factor Sox11 is expressed at high levels in developing sensory neurons and injured adult neurons but little is known about its transcriptional targets and function. In this study we examined the role of Sox11 using Neuro2a neuroblastoma cells and cultured mouse dorsal root ganglia (DRG) neurons. Results show Sox11 has an essential role in regulation of neuron survival and neurite outgrowth in Neuro2a cells and primary sensory neurons. Neuro2a cells increase expression of Sox11 as they differentiate in culture. Following addition of 20 microM retinoic acid (RA), a stimulus for differentiation that enhances neurite growth and differentiation, Sox11 level rises. RNAi-mediated knockdown of Sox11 in RA-differentiated Neuro2a cells caused a decrease in neurite growth and an increase in the percent of apoptotic cells. RNA expression analysis showed that Sox11 knockdown modulated the level of mRNAs encoding several genes related to cell survival and death. Further validation in the Neuro2a model showed Sox11 knockdown increased expression of the pro-apoptotic gene BNIP3 (BclII interacting protein 1 NIP3) and decreased expression of the anti-apoptotic gene TANK (TNF receptor-associated factor family member-associated NFkappaB activator). Cultured primary DRG neurons also express Sox11 and treatment with Sox11 small interfering RNA (siRNA) caused a significant decrease in neurite growth and branching and a decrease in mRNA encoding actin-related protein complex 3 (Arpc3), an actin organizing protein that may be involved in axon growth. The percent of apoptotic neurons also increased in cultures of DRG neurons treated with Sox11 siRNA. Similar to Neuro2a cells, a decrease in TANK gene expression occurred, suggesting at least some overlap in Sox11 transcriptional targets in Neuro2a and DRG neurons. These data are consistent with a central role for Sox11 in regulating events that promote neurite growth and neuron survival.

Manufacture of multimicrotubule chitosan nerve conduits with novel molds and characterization in vitro

Multimicrotubule chitosan conduits (M-conduits) were fabricated using novel molds and a thermal-induced phase-separation technique. Hollow chitosan conduits (H-conduits) with an inner diameter of 1-5 mm and a wall thickness of 0.2-1.0 mm were made, and then a novel mold composed of a styrofoam insulating pedestal with several holes and a stainless steel cover plate was used to make M-conduits. In brief, corresponding H-conduits were inserted upright into the holes of the styrofoam pedestal, and filled with chitosan solution, then rapidly covered with the precooled stainless steel cover plate, and then placed in a freezer. The styrofoam insulating pedestal enclosing the conduits could reduce the heat transfer through the side wall of the conduits. Gradual phase separation then occurred uniaxially in the presence of a unidirectional temperature gradient from the top end to the bottom end of the chitosan conduits. The phase-separated polymer/solvent systems were then dried in a freeze-dryer. The microtubule diameters were controlled by adjusting the polymer concentration and cooling temperature. In vitro characterization demonstrated that the mold-based multimicrotubule chitosan conduits possessed suitable mechanical strength, microtubule diameter distribution, porosity, swelling, biodegradability, and nerve cell affinity, and so they showed potential for application as nerve tissue engineering scaffolds.

The Rho-family guanine nucleotide exchange factor GEFT enhances retinoic acid- and cAMP-induced neurite outgrowth

The Rho GTPases are important regulators of neurite outgrowth and pathfinding. We have recently reported that a Rho-family guanine nucleotide exchange factor, GEFT, modulates dendrite spine morphology and basal neurite outgrowth in primary hippocampal neurons and Neuro2A cells, respectively. Here we demonstrate that GEFT protein is highly expressed in all regions of the brain and is highly up-regulated upon treatment of Neuro2A cells with retinoic acid and dibutyric cAMP, which promote dendrite and axon-like neurite extensions, respectively. Within retinoic acid-induced neurite extensions, GEFT is localized to actin-enriched regions in the primary neurites, with little or no expression from secondary branches. Dibutyric cAMP-induced neurite extensions are highly concentrated for GEFT at the actin-rich distal tip of the growth cone. Additionally, we demonstrate that GEFT promotes neurite outgrowth in undifferentiated as well as differentiated Neuro2A cells. Together, our data provide new evidence suggesting that GEFT is an important regulator of multiple processes involved in axon and dendrite formation.

Tissue inhibitor of metalloproteinase-2 (TIMP-2) expression is regulated by multiple neural differentiation signals

Neuronal differentiation requires exquisitely timed cell cycle arrest for progenitors to acquire an appropriate neuronal cell fate and is achieved by communication between soluble signals, such as growth factors and extracellular matrix molecules. Here we report that the expression of TIMP-2, a matrix metalloproteinase inhibitor, is up-regulated by signals that control proliferation (bFGF and EGF) and differentiation (retinoic acid and NGF) in neural progenitor and neuroblastoma cell lines. TIMP-2 expression coincides with the appearance of neurofilament-positive neurons, indicating that TIMP-2 may play a role in neurogenesis. The up-regulation of TIMP-2 expression by proliferate signals suggests a role in the transition from proliferation to neuronal differentiation. Live labeling experiments demonstrate TIMP-2 expression only on alpha(3) integrin-positive cells. Thus, TIMP-2 function may be mediated via interaction with integrin receptor(s). We propose that TIMP-2 represents a component of the neurogenic signaling cascade induced by mitogenic stimuli that may withdraw progenitor cells from the cell cycle permitting their terminal neuronal differentiation.

Defective axonal transport of neurofilament proteins in neurons overexpressing peripherin

Peripherin is a type III neuronal intermediate filament detected in motor neuron inclusions of amyotrophic lateral sclerosis (ALS) patients. We previously reported that overexpression of peripherin provokes late-onset motor neuron dysfunction in transgenic mice. Here, we show that peripherin overexpression slows down axonal transport of neurofilament (NF) proteins, and that the transport defect precedes by several months the appearance of axonal spheroids in adult mice. Defective NF transport by peripherin up-regulation was further confirmed with dorsal root ganglia (DRG) neurons cultured from peripherin transgenic embryos. Immunofluorescence microscopy and western blotting revealed that excess peripherin provokes reduction in levels of hyperphosphorylated NF-H species in DRG neurites. Similarly the transport of a green fluorescent protein (GFP)-tagged NF-M, delivered by means of a lentiviral construct, was impaired in DRG neurites overexpressing peripherin. These results demonstrate that peripherin overexpression can cause defective transport of type IV NF proteins, a phenomenon that may account for the progressive formation of ALS-like spheroids in axons.

Temporal change in mKIAA gene expression during the early stage of retinoic acid-induced neurite outgrowth

mKIAA genes are mouse counterparts of human KIAA genes, which were isolated in our cDNA project and were functionally unknown at the time they were sequenced. Because KIAA/mKIAA genes were isolated mainly from cDNA libraries derived from brain tissues, they are thought to be important for the organization and function of the brain. To investigate the participation of mKIAA genes in neuronal phenomena, we analyzed retinoic acid-induced neurite outgrowth using an mKIAA oligonucleotide microarray. Focusing on the early stage of this outgrowth phenomenon, we analyzed temporal gene expression changes 1-24 h after treatment with retinoic acid and found several change patterns in 38 mKIAA genes. Among them, six were upregulated at 3 h and subsequently returned to the steady state. Supposing that these genes had important roles, we performed semi-quantitative RT-PCR analysis and confirmed the existence of temporal expression patterns in two genes (mKIAA0182 and mKIAA1039). Further computational analysis of the 38 genes enabled us to find the cellular pathway associated with 6 of them with high confidence. These results indicate that some mKIAA genes are apparently relevant to retinoic acid-induced neurite outgrowth.

The single neurofilament subunit of lamprey may need another element for filament assembly

Regenerating axon tips in transected lamprey spinal cord contain dense accumulations of neurofilaments (NFs), suggesting that NFs may play a role in the mechanism of axonal regeneration. Compared with heteropolymeric assemblies of NF triplet proteins in mammals, NF in lampreys has been thought to contain only a single subunit (NF180). This would imply that NF180 self-assembles, which would be important for manipulating its expression in studies of axonal regeneration. In order to study the possible role of NF in process outgrowth and to determine whether NF180 can self-assemble, its gene was transfected into mammalian and fish cell lines that either contain or lack vimentin. In transfected NIH3T3 cells, NF180 was poorly phosphorylated and its expression did not alter the length or number of cell processes. Nor did it appear to form typical intermediate filaments, suggesting that it may not self-assemble. NF180 also did not form typical filaments in SW13cl cells that either possessed or lacked vimentin, nor in transfected fish cells that were cultured at 18 degrees C. In vitro, NF180 could not self-assemble but interacted with NF-L to interrupt its self-assembly. When cotransfected with rat NF-L into SW13c1.2vim(-) cells, NF180 did form thick, rod-like filamentous structures on immunofluorescence. More typical NFs were observed when NF180 was cotransfected with both NF-L and NF-M. Thus, NF180 cannot self-assemble but appears to require one or more additional elements for incorporation into NFs.

ATF3 enhances c-Jun-mediated neurite sprouting

The AP-1 transcription factor c-Jun is induced in axotomized neurons of the peripheral and central nervous systems, and in both cases upregulation of c-Jun expression has been correlated with axonal regeneration. More recently there has been interest in the c-Jun-related bZIP transcription factor, ATF3, and its function in neurons. ATF3 is also induced in nerve cells in response to axotomy and there is a correlation between increased ATF3 expression and upregulation of c-Jun in surviving neurons. Moreover, c-Jun is able to induce expression of ATF3. We investigated the effect of co-expressing c-Jun and ATF3 in two neuronal-like cell lines to model transcriptional events occurring in axotomized neurons undergoing regeneration. We show that expression of ATF3 with c-Jun significantly enhances c-Jun-mediated neurite sprouting, and that this phenotype is most likely mediated by a physical association of these two transcription factors. Our results suggest that a program of axonal regeneration is initiated when both c-Jun and ATF3 are upregulated in neurons in response to axotomy.

Peptide-modified alginate surfaces as a growth permissive substrate for neurite outgrowth

Different strategies are being investigated for treatment of spinal cord injuries, one of the most promising being application of neurotrophic factors, which have been shown to prevent neuronal death and stimulate regeneration of injured axons. Ex vivo gene therapy has emerged as the leading delivery method at the site of the injury, and we have shown previously that encapsulating genetically engineered fibroblasts in an immunoprotective alginate capsule can permit implantation of the factor-secreting cells without need for immunosuppression. This strategy could be greatly enhanced by providing the sprouting neurons with a permissive substrate upon which to attach and grow. We report here studies on the modification of an alginate gel surface by either coating it with laminin or by covalent attachment of YIGSR peptide. Using NB2a neuroblastoma cells, we found that native alginate elicited minimal cell attachment ( approximately 1.5%); however, YIGSR-alginate conjugate elicited a fivefold increase in numbers of cells attached using peptide ratios of 0.5 and 1 mg/g alginate, ranging from 9.5% of the cells at the lower ratio, to about 44% at the higher. Only a further 19% increase was obtained at an increased peptide density of 2 mg/g alginate ( approximately 63% over control). Laminin-coated gels showed approximately 60% cell attachment. However, laminin coating did not stimulate differentiation and neurite growth, whereas both numbers and lengths of outgrowths increased with increasing peptide density on peptide-modified alginate. We demonstrate here the ability of the peptide-modified alginate gels to allow adhesion of NB2a neuroblastoma cells and to promote neurite outgrowth from these cells when attached to the peptide-modified alginate surface. Also, we show that the adhesion of NB2a neuroblastoma cells and neurite outgrowth from the attached cells is a function of the peptide density on the gel surface.

Enhanced expression and activation of CTP:phosphocholine cytidylyltransferase beta2 during neurite outgrowth

During differentiation neurons increase phospholipid biosynthesis to provide new membrane for neurite growth. We studied the regulation of phosphatidylcholine (PC) biosynthesis during differentiation of two neuronal cell lines: PC12 cells and Neuro2a cells. We hypothesized that in PC12 cells nerve growth factor (NGF) would up-regulate the activity and expression of the rate-limiting enzyme in PC biosynthesis, CTP:phosphocholine cytidylyltransferase (CT). During neurite outgrowth, NGF doubled the amount of cellular PC and CT activity. CTbeta2 mRNA increased within 1 day of NGF application, prior to the formation of visible neurites, and continued to increase during neurite growth. When neurites retracted in response to NGF withdrawal, CTbeta2 mRNA, protein, and CT activity decreased. NGF specifically activated CTbeta2 by promoting its translocation from cytosol to membranes. In contrast, NGF did not alter CTalpha expression or translocation. The increase in both CTbeta2 mRNA and CT activity was inhibited by U0126, an inhibitor of mitogen-activated kinase/extracellular signal-regulated kinase kinase 1/2 (MEK1/2). In Neuro2a cells, retinoic acid significantly increased CT activity (by 54%) and increased CTbeta2 protein, coincident with neurite outgrowth but did not change CTalpha expression. Together, these data suggest that the CTbeta2 isoform of CT is specifically up-regulated and activated during neuronal differentiation to increase PC biosynthesis for growing neurites.

The role of microtubule-associated protein 2c in the reorganization of microtubules and lamellipodia during neurite initiation

During neurite initiation, cells surrounded by a flattened, actin-rich lamellipodium transform to produce thin, microtubule-filled neurite shafts tipped by actin-rich growth cones, but little is known about this transformation. Our detailed time-lapse analyses of cultured hippocampal neurons, a widely used model system for neuronal development, revealed that neurites emerge from segmented lamellipodia, which then gradually extend from the cell body to become nascent growth cones. This suggests that actin- and microtubule-rich structures are reorganized in a coordinated manner. We hypothesized that proteins such as microtubule-associated protein 2 (MAP2), which can interact with both cytoskeletal components, might be critically involved in neurite initiation. Live-cell video and fluorescence microscopy in Neuro-2a cells showed that expression of MAP2c triggers neurite formation via rapid accumulation and bundling of stable, MAP2c-bound microtubules, concurrent with a gradual transformation of lamellipodia into nascent growth cones. The microtubule-stabilizing agent Taxol did not mimic this effect, suggesting that the ability of MAP2c to stabilize microtubules is not sufficient for neurite initiation. However, combination of Taxol treatment with actin disruption induced robust process formation, suggesting that inhibitory effects of F-actin need to be overcome as well. Neurite initiation by MAP2c required its microtubule-binding domain and was enhanced by its binding domain for cAMP-dependent protein kinase (PKA). MAP2c mutants defective in both PKA and microtubule binding acted as dominant negative inhibitors of neurite initiation in neuroblastoma cells and primary hippocampal neurons. Together, these data suggest that MAP2c bears functions that both stabilize microtubules and directly or indirectly alter actin organization during neurite initiation.

Transcription regulator LMO4 interferes with neuritogenesis in human SH-SY5Y neuroblastoma cells

LMO4 is a transcription regulator interacting with proteins involved, among else, in tumorigenesis. Its function in the nervous system, and particularly in the adult nervous system, has however still to be elucidated. We decided to modify its expression in a neuronal model, human SH-SY5Y neuroblastoma cells, by permanent transfection of sense or anti-sense Lmo4 cDNAs. Generated clones overexpressing the Lmo4 transcript in sense orientation tended to aggregate. They showed significantly reduced average number of neurites per cell and average neuritic length per cell. The opposite was observed with clones overexpressing the anti-sense Lmo4 transcript. Furthermore, selected clones were subjected to 72 h long-term treatments with retinoic acid and phorbol ester (TPA), two biochemicals known to stimulate differentiation of non-transfected SH-SY5Y cells and other neuroblastoma cells. Neuritogenesis occurred after retinoic acid stimulation in all cases. The inhibitory effect of sense Lmo4 RNA overexpression on neuritic outgrowth was indeed prevented. The protein kinase C activator TPA could not induce neuritogenesis in SH-SY5Y cells overexpressing sense Lmo4 RNA. Thus, sense Lmo4 RNA overexpression, not Lmo4 endogenous transcription, overrides the stimulatory effect of TPA upon neuritic outgrowth. We also showed that Lmo4-dependent neuritic retraction and outgrowth correspond to altered phosphorylation of cytoskeletal proteins. Overall, Lmo4 RNA overexpression interferes with neuritic outgrowth, whereas anti-sense Lmo4 RNA expression favors neuritogenesis in SH-SY5Y cells. Consequently, changes in Lmo4 RNA expression levels might alter the rate of neuritic outgrowth in the developing and adult nervous system.

Automated screening of neurite outgrowth

Outgrowth of neurites in culture is used for assessing neurotrophic activity. Neurite measurements have been performed very slowly using manual methods or more efficiently with interactive image analysis systems. In contrast, medium-throughput and noninteractive image analysis of neurite screens has not been well described. The authors report the performance of an automated image acquisition and analysis system (IN Cell Analyzer 1000) in the neurite assay. Neuro-2a (N2a) cells were plated in 96-well plates and were exposed to 6 conditions of retinoic acid. Immunofluorescence labeling of the cytoskeleton was used to detect neurites and cell bodies. Acquisition of the images was automatic. The image set was then analyzed by both manual tracing and automated algorithms. On 5 relevant parameters (number of neurites, neurite length, total cell area, number of cells, neurite length per cell), the authors did not observe a difference between the automated analysis and the manual analysis done by tracing. These data suggest that the automated system addresses the same biology as human scorers and with the same measurement precision for treatment effects. However, throughput of the automated system is orders of magnitude higher than with manual methods.

Uptake of recombinant iduronate-2-sulfatase into neuronal and glial cells in vitro

Mucopolysaccharidosis type II (MPS II, Hunter syndrome) is a congenital storage disorder resulting from mutations on the iduronate-2-sulfatase (IDS) gene. The disease shows variable clinical phenotypes from severe to mild with progressive neurological dysfunction. The therapeutic options for treatment of MPS II are limited and currently no specific therapies are available; the problem is further compounded by difficulties in delivering therapeutic agents to the central nervous system (CNS). In this work, as a potential treatment for this disease, the transfer of the recombinant IDS enzyme into brain cells has been studied in vitro. Two different approaches to obtain recombinant IDS have been utilized: production of the recombinant enzyme by a transfected human clone (Bosc 23 cells); production of the recombinant enzyme by adenoviral transduction of neuronal (SK-N-BE) or glial (C6) cells. Our data indicate that the transfected as well as the infected cells produce a large amount of the IDS enzyme, which is efficiently endocytosed into neuronal and glial cells through the mannose 6-phosphate (M6P) receptor system. Somatic gene therapy appears therefore to be suitable to correct IDS deficiency in brain cells.

Accelerated Differentiation in Response to Retinoic Acid After Retrovirally Mediated Gene Transfer of GAP-43 into Mouse Neuroblastoma Cells

Although substantial evidence exists for the involvement of growth-associated protein-43 (GAP-43) in neuronal development and regeneration, the precise role of this protein in neurite outgrowth is currently debated. To investigate the role of GAP-43 in the initiation of neurite outgrowth, we transfected a full-length cDNA coding for GAP-43 into a mouse neuroblastoma cell line (Neuro-2a) which can be differentiated to a neuronal phenotype using retinoic acid (RA). We show that the consequent overexpression of GAP-43 results in a change in the basic morphology of these cells, but is not in itself sufficient to induce the extension of neurites. However, overexpression of GAP-43 results in a marked acceleration of neurite formation in response to RA. We propose that while GAP-43 does not trigger the initiation of neurite extension, its expression is rate-limiting for neurite outgrowth in response to differentiation agents such as RA.

Role and distribution of retinoic acid during CNS development

Retinoic acid (RA), the biologically active derivative of vitamin A, induces a variety of embryonal carcinoma and neuroblastoma cell lines to differentiate into neurons. The molecular events underlying this process are reviewed with a view to determining whether these data can lead to a better understanding of the normal process of neuronal differentiation during development. Several transcription factors, intracellular signaling molecules, cytoplasmic proteins, and extracellular molecules are shown to be necessary and sufficient for RA-induced differentiation. The evidence that RA is an endogenous component of the developing central nervous system (CNS) is then reviewed, data which include high-pressure liquid chromotography (HPLC) measurements, reporter systems and the distribution of the enzymes that synthesize RA. The latter is particularly relevant to whether RA signals in a paracrine fashion on adjacent tissues or whether it acts in an autocrine manner on cells that synthesize it. It seems that a paracrine system may operate to begin early patterning events within the developing CNS from adjacent somites and later within the CNS itself to induce subsets of neurons. The distribution of retinoid-binding proteins, retinoid receptors, and RA-synthesizing enzymes is described as well as the effects of knockouts of these genes. Finally, the effects of a deficiency and an excess of RA on the developing CNS are described from the point of view of patterning the CNS, where it seems that the hindbrain is the most susceptible part of the CNS to altered levels of RA or RA receptors and also from the point of view of neuronal differentiation where, as in the case of embryonal carcinoma (EC) cells, RA promotes neuronal differentiation. The crucial roles played by certain genes, particularly the Hox genes in RA-induced patterning processes, are also emphasized.

Telomerase and differentiation in multicellular organisms: turn it off, turn it on, and turn it off again

Telomerase is a ribonucleoprotein complex that catalyses the addition of TTAGGG repeats onto telomeres, repetitive DNA structures found at the ends of linear chromosomes. The majority of human somatic tissues do not display telomerase activity and undergo telomeric shortening with consecutive divisions. This telomeric shortening results in replicative senescence in vitro and likely in vivo. Telomerase activity is present in the vast majority of tumors, preventing telomeric shortening and thereby enabling indefinite cell divisions. Telomerase activity is regulated throughout human development, undergoing silencing in almost all organ systems from embryogenesis onwards. However, regulated telomerase activity is seen in basal/stem cell compartments of highly regenerative tissues, such as those of the immune system, skin, and intestine. Avian species display telomerase repression and telomeric shortening similar to that seen in humans. However, rodents retain telomerase-competency throughout their lifespan and have not been shown to display division-dependent telomere shortening. The regulation of telomerase activity in plants is less well understood, although early indications suggest ubiquitous competency. The aim of this review is to present current data regarding developmental regulation of telomerase in humans, mice, chickens and flowering plants. Differentiation, quiescence and telomerase activity regulation will then be addressed in three human representative tissue systems; blood, skin, and intestine. We will also highlight similarities, differences and misconceptions in the developing field of telomere and telomerase biology.

Up-regulation of cyclooxygenase-1 in neuroblastoma cell lines by retinoic acid and corticosteroids

Cyclooxygenases-1 and -2 are both expressed in neuronal cells in vivo. In the neuroblastoma cell lines NG108 and N2a, however, only cyclooxygenase-1 was detectable. Differentiation of the cells with retinoic acid increased cyclooxygenase-1 mRNA and protein expression within 24 and 48 h, respectively. A further increase was observed when the cells were concomitantly treated with the glucocorticoid dexamethasone (a 2-3-fold increase compared with retinoic acid alone). In the absence of retinoic acid, dexamethasone only slightly up-regulated cyclooxygenase-1 expression. The inhibitor of protein synthesis cycloheximide abrogated the effect of dexamethasone, indicating the involvement of newly synthesised proteins. Retinoic acid increased the transcription of cyclooxygenase-1 mRNA, determined with a luciferase-coupled promoter construct. Dexamethasone only slightly augmented cyclooxygenase-1-promoter activity but increased cyclooxygenase-1 mRNA stability. Other corticosteroids, hydrocortisone and aldosterone, also up-regulated cyclooxygenase-1 whereas neurosteroids or oestrogen were ineffective. Up-regulation was mediated primarily by the glucocorticoid receptor, because the receptor antagonist RU486 strongly reduced the effects of all corticosteroids. This indicated that in NG108 cells, the mineralocorticoid aldosterone may bind to the glucocorticoid receptor. Treatment of NG108 or N2a cells with corticosteroids did not alter the morphological phenotype obtained during differentiation. We thus show that corticosteroids, which down-regulate cyclooxygenase expression in most cell types, up-regulate cyclooxygenase-1 during neuronal differentiation.

p190 RhoGAP is the principal Src substrate in brain and regulates axon outgrowth, guidance and fasciculation

The Src tyrosine kinases have been implicated in several aspects of neural development and nervous system function; however, their relevant substrates in brain and their mechanism of action in neurons remain to be established clearly. Here we identify the potent Rho regulatory protein, p190 RhoGAP (GTPase-activating protein), as the principal Src substrate detected in the developing and mature nervous system. We also find that mice lacking functional p190 RhoGAP exhibit defects in axon guidance and fasciculation. p190 RhoGAP is co-enriched with F-actin in the distal tips of axons, and overexpressing p190 RhoGAP in neuroblastoma cells promotes extensive neurite outgrowth, indicating that p190 RhoGAP may be an important regulator of Rho-mediated actin reorganization in neuronal growth cones. p190 RhoGAP transduces signals downstream of cell-surface adhesion molecules, and we find that p190-RhoGAP-mediated neurite outgrowth is promoted by the extracellular matrix protein laminin. Together with the fact that mice lacking neural adhesion molecules or Src kinases also exhibit defects in axon outgrowth, guidance and fasciculation, our results suggest that p190 RhoGAP mediates a Src-dependent adhesion signal for neuritogenesis to the actin cytoskeleton through the Rho GTPase.

Neuronal intermediate filament protein alpha-internexin facilitates axonal neurite elongation in neuroblastoma cells

We examined the localization and role of alpha-IN vs. other neuronal intermediate filaments before and during differentiation. Vimentin but not alpha-IN localized within filopodia-like neurites of undifferentiated cells. During differentiation, alpha-IN immunoreactivity accumulated within axonal neurites following vimentin but, as previously describe in neurons in situ, before the appearance of NF-L. We therefore manipulated alpha-IN synthesis, accumulation, and function in attempts to determine whether or not this intermediate filament species played a role in axonal development. Intracellular delivery of anti-alpha-IN antisense oligonucleotides and antibodies was permissive for neuritogenesis, yet compromised neurite elongation; this effect was further reflected in diminished levels of stabilized axonal microtubules. These data suggest that alpha-IN plays a role in the development of neuronal polarity. Relatively more alpha-IN than NF-L accumulated within the plastic axonal neurites induced following serum-deprivation, while stable, dbcAMP-induced neurites treatment contained equivalent levels of each. Protease inhibition increased NF-L and NF-H but not alpha-IN immunoreactivity within serum-deprived neurites, suggesting that proteolysis restricts NF-L accumulation pending neurite stabilization. To test the possibility that NF-H accumulation is dependent upon NF-L and cannot be mediated by alpha-IN, we examined levels of NF-H co-precipitated from cells with alpha-IN and NF-L. Virtually all newly synthesized NF-H co-precipitated with NF-L, while only a small percentage co-precipitated with alpha-IN. Finally, NF-H or NF-M were absent from the axon hillock or perikaryal area at the base of neurites, where alpha-IN immunoreactivity is prominent. These data extend earlier cell-free demonstrations that NF-H preferentially associates with NF-L rather than alpha-IN.