Inhibition of the nerve growth factor-induced outgrowth of neurites by trichostatin A requires protein synthesis de novo in PC12D cells

The expression of HDAC1 and HDAC2 during cerebellar cortical development

Histone deacetylases (HDACs) are epigenetic regulatory proteins that repress gene transcription by changing DNA conformation. The regulation of gene expression through histone deacetylation is an important mechanism for the development of the central nervous system. Although the disruption of the balance in epigenetic gene regulation has been implicated in many CNS developmental abnormalities and diseases, the expression pattern of HDACs in various cell types in the brain during its maturation process has had limited exploration. Therefore, in this study, we investigate the cell type-specific and developmental stage-specific expression pattern of HDAC1 and HDAC2 in the mouse cerebellum. Our experimental results show that the cerebellar progenitors and glial cells express high levels of HDAC1 and low levels of HDAC2. On the other hand, the post-mitotic migrating neuronal cells of the cerebellar cortex show strong HDAC2 and weak HDAC1 expressions. In more differentiated neuronal cells, including Purkinje cells, granule cells, unipolar brush cells, and GABAergic interneurons, we found a consistent expression pattern, high levels of HDAC2 and low levels of HDAC1. Therefore, our data provide support for the potential important roles of HDAC1 in cell proliferation and HDAC2 in migration and differentiation.

SIRT2-mediated protein deacetylation: An emerging key regulator in brain physiology and pathology

Protein function is considerably altered by posttranslational modification. In recent years, cycles of acetylation/deacetylation emerged as fundamental regulators adjusting biological activity of many proteins. Particularly, protein deacetylation by Sirtuins, a family of atypical histone deacetylases (HDACs), was demonstrated to regulate fundamental cell biological processes including gene expression, genome stability, mitosis, nutrient metabolism, aging, mitochondrial function and cell motility. Given this wealth of biological functions, perhaps not unexpectedly then, pharmacological compounds targeting Sirtuin activity are now prime therapeutic agents for alleviating severity of major diseases encompassing diabetes, cancer, cardiovascular and neurodegenerative disorders in many organs. In this review, we will focus on the brain and its physiological and pathological processes governed by Sirtuin-mediated deacetylation. Besides discussing Sirtuin function in neurodegenerative diseases, emphasis will be given on the mounting evidence deciphering key developmental brain functions for Sirtuins in neuronal motility, neuroprotection and oligodendrocyte differentiation. In this respect, we will particularly highlight functions of the unconventional family member SIRT2 in post-mitotic neurons and glial cells.

Characterization of NKIP: a novel, Na+/K+-ATPase interacting protein mediates neural differentiation and apoptosis

Cellular differentiation and programmed cell death are tightly controlled to maintain tissue homeostasis and proper organ function. In a screen for apoptosis specific gene products, we isolated an immediate early apoptosis response gene from myelomonocytic stem cells that appears to play a key regulatory role in a number of cell types and may be of particular importance in cells of the central nervous system. The gene's 28 kDa protein product interacts with the C-terminal ectodomain of the Na+/K+-ATPase (NKA) beta 1 subunit and was therefore named NKIP (NKA Interacting Protein). NKIP is coexpressed with NKA, localizes to lysosomes and the endoplasmic reticulum and is predominantly expressed in excitable tissues including polarized epithelia and the central nervous system. NKIP has been characterized as an endogenous suppressor of the NKA as reduction of NKIP in PC12 cells significantly increases NKA activity. In pluripotent NT2 progenitor cells, NKIP induced rapidly K+-level-dependent cell death. NKIP overexpression induced growth factor-independent neurite outgrowth, which was associated with MEK-independent phosphorylation of the transcription factor ERK1/2. Thus, we have identified NKIP as an important novel protein that interacts to the NKA complex, influencing cellular ion balance, induction of apoptosis and neuronal differentiation.

NGF-dependent formation of ruffles in PC12D cells required a different pathway from that for neurite outgrowth

Two signaling pathways, phosphoinositide 3-kinase (PI-3k)/Akt and Ras/MAPK, are major effectors triggered by nerve growth factor (NGF). Rac1, Cdc42 and GSK-3beta are reported to be targets of PI-3k in the signal transduction for neurite outgrowth. Immediately after NGF was added, broad ruffles were observed temporarily around the periphery of PC12 cells prior to neurite growth. As PC12D cells are characterized by a very rapid extension of neurites in response to various agents, the signaling pathways described above were studied in relation to the NGF-induced formation of ruffles and outgrowth of neurites. Wortmannin, an Akt inhibitor (V), and GSK-3beta inhibitor (SB425286) suppressed the neurite growth in NGF-treated cells, but not in dbcAMP-treated cells. The outgrowth of neurites induced by NGF but not by dbcAMP was inhibited with the expression of mutant Ras. But upon the expression of dominant-negative Rac1, cells often extended protrusions, incomplete neurites, lacking F-actin. Intact neurites were observed in cells with dominant-negative Cdc42. These results suggest that NGF-dependent neurite outgrowth occurs via a mechanism involving activation of the Ras/PI-3K/Akt/GSK-3beta pathway, while dbcAMP-dependent neurite growth might be induced in a distinct manner. However, inhibitors for GSK-3beta and PI-3k (wortmannin) did not suppress the NGF-dependent formation of ruffles. In addition, the formation of ruffles was not inhibited by the expression of mutant Ras. On the other hand, it was suppressed by the expression of dominant-negative Rac1 or Cdc42. These results suggest that the NGF-induced ruffling requires activation of Rac1 and Cdc42, but does not require Ras, PI-3k, Akt and GSK-3beta. Taken together, the NGF-dependent formation of ruffles might not require Ras/PI-3k/Akt/GSK-3beta, but these pathways might contribute to the formation of intact neurites due to combined actions including Rac1.

Differential regulation of the tyrosine hydroxylase and enkephalin neuropeptide transmitter genes in rat PC12 cells by short chain fatty acids: Concentration-dependent effects on transcription and RNA stability

At physiologic concentrations, butyrate regulates the expression of individual genes involving at least three mechanisms: (i) through induction of cis- and trans-acting butyrate-dependent transcription factors for selected genes, (ii) by inhibition of histone deacetylation and attendant chromatin remodeling and (iii) by affecting turnover of mRNAs. Our previous work illustrated gradual accumulation of mRNA for tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis and the neuropeptide transmitter proenkephalin (ppEnk) in butyrate-differentiated PC12 cells (Nankova, B.B., Chua, J., Mishra, R., Kobasiuk, C.D., La Gamma, E.F. 2003. Nicotinic induction of preproenkephalin and tyrosine hydroxylase gene expression in butyrate-differentiated rat PC12 cells: a model for adaptation to gut-derived environmental signals. Pediatr. Res. 53, 113-118.). However, at higher physiological concentrations (6 mM), TH mRNA levels are significantly reduced while ppEnk mRNA transcripts remained elevated. These differential effects suggest suppression of endogenous TH gene transcription, targeted degradation of TH mRNA or both. By using nuclear run-on assays, we found that transcription increased for both endogenous TH and ppEnk genes, even at time points and concentrations when reduced steady-state levels of TH mRNA were observed. The reduction in TH mRNA was blocked by cycloheximide consistent with a protein-dependent mechanism. We also observed a dose-dependent accumulation of luciferase reporter molecules driven by TH promoter in transient transfection experiments, data that provide additional support for separate regulatory pathways. Significantly, butyrate-dependent decreases in TH mRNA were also reflected in a reduction in TH protein. Our results suggest a novel mode of regulation for TH by butyrate operating via both transcriptional and post-transcriptional mechanisms. We speculate that, depending on plasma concentrations of butyrate, this naturally occurring signaling molecule can function as an in vivo molecular switch to alter levels of TH mRNA, its protein and thus the biosynthesis of endogenous catecholamines.

Epigenotypes of latent herpesvirus genomes

Epigenotypes are modified cellular or viral genotypes which differ in transcriptional activity in spite of having an identical (or nearly identical) DNA sequence. Restricted expression of latent, episomal herpesvirus genomes is also due to epigenetic modifications. There is no virus production (lytic viral replication, associated with the expression of all viral genes) in tight latency. In vitro experiments demonstrated that DNA methylation could influence the activity of latent (and/or crucial lytic) promoters of prototype strains belonging to the three herpesvirus subfamilies (alpha-, beta-, and gamma-herpesviruses). In vivo, however, DNA methylation is not a major regulator of herpes simplex virus type 1 (HSV-1, a human alpha-herpesvirus) latent gene expression in neurons of infected mice. In these cells, the promoter/enhancer region of latency-associated transcripts (LATs) is enriched with acetyl histone H3, suggesting that histone modifications may control HSV-1 latency in terminally differentiated, quiescent neurons. Epstein-Barr virus (EBV, a human gamma-herpesvirus) is associated with a series of neoplasms. Latent, episomal EBV genomes are subject to host cell-dependent epigenetic modifications (DNA methylation, binding of proteins and protein complexes, histone modifications). The distinct viral epigenotypes are associated with distinct EBV latency types, i.e., cell type-specific usage of latent EBV promoters controlling the expression of latent, growth transformation-associated EBV genes. The contribution of major epigenetic mechanisms to the regulation of latent EBV promoters is variable. DNA methylation contributes to silencing of Wp and Cp (alternative promoters for transcripts coding for the nuclear antigens EBNA 1-6) and LMP1p, LMP2Ap, and LMP2Bp (promoters for transcripts encoding transmembrane proteins). DNA methylation does not control, however, Qp (a promoter for EBNA1 transcripts only) in lymphoblastoid cell lines (LCLs), although in vitro methylated Qp-reporter gene constructs are silenced. The invariably unmethylated Qp is probably switched off by binding of a repressor protein in LCLs.

Enhancement of NGF- and cholera toxin-induced neurite outgrowth by butyrate in PC12 cells

It has been shown that sodium butyrate (NaBu) does not elicit neurite outgrowth of PC12, one of the most widely used cell lines as a model of neuronal differentiation. In this study, the effects of NaBu on nerve growth factor (NGF)- and cholera toxin-induced neurite outgrowth in PC12 cells were examined. NaBu dose-dependently enhanced neurite formation induced by both agents. The maximum responses obtained at 0.5 mM NaBu were nearly twice those of the inducers alone. Propionate and valerate were also effective, but acetate and caproate were ineffective. Among the butyrate analogs with a moiety of three to five carbon atoms tested, isobutyrate, isovalerate, vinylacetate and 3-chloropropionate enhanced neurite outgrowth promoted by both inducers. However, neither alpha-, beta-, and gamma-aminobutyrates nor alpha-, beta-, and gamma-hydroxybutyrates were effective. All of the effective short-chain fatty acids and their analogs increased the level of histone acetylation, while ineffective ones did not. Furthermore, Helminthosporium carbonum toxin (HC toxin), a structurally dissimilar inhibitor of histone deacetylase, mimicked the effect of butyrate. These results suggest that NaBu enhances neurite outgrowth induced by NGF and cholera toxin in PC12 cells through a mechanism involving an increase in the level of histone acetylation.

Herpes simplex virus type 1 promoter activity during latency establishment, maintenance, and reactivation in primary dorsal root neurons in vitro

A neonatal rat dorsal root ganglion-derived neuronal culture system has been utilized to study herpes simplex virus (HSV) latency establishment, maintenance, and reactivation. We present our initial characterization of viral gene expression in neurons following infection with replication-defective HSV recombinants carrying beta-galactosidase and/or green fluorescent protein reporter genes under the control of lytic cycle- or latency-associated promoters. In this system lytic virus reporter promoter activity was detected in up to 58% of neurons 24 h after infection. Lytic cycle reporter promoters were shut down over time, and long-term survival of neurons harboring latent virus genomes was demonstrated. Latency-associated promoter-driven reporter gene expression was detected in neurons from early times postinfection and was stably maintained in up to 83% of neurons for at least 3 weeks. In latently infected cultures, silent lytic cycle promoters could be activated in up to 53% of neurons by nerve growth factor withdrawal or through inhibition of histone deacetylases by trichostatin A. We conclude that the use of recombinant viruses containing reporter genes, under the regulation of lytic and latency promoter control in neuronal cultures in which latency can be established and reactivation can be induced, is a potentially powerful system in which to study the molecular events that occur during HSV infection of neurons.

Activation of mitogen-activated protein kinases is not required for the extension of neurites from PC12D cells triggered by nerve growth factor

Numerous studies with PC12 cells have suggested that the mitogen-activated protein (MAP) kinase pathway might play a major role in the neuronal differentiation that is induced by nerve growth factor (NGF). Cells of the PC12D subline extend neurites within several hours in response to NGF in the presence of inhibitors of the synthesis of RNA and protein. We examined the effects of a specific inhibitor 2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one (PD98059) of the MAP kinase kinase (MEK)/MAP kinase pathway on the NGF-induced outgrowth of neurites in PC12D cells. The increase in MAP kinase activity in response to NGF was reduced by 80% upon treatment of PC12D cells with 50 microM PD98059, whereas the NGF-dependent formation of ruffles and the subsequent outgrowth of neurites were not blocked by PD98059 at this concentration. The outgrowth of neurites from conventional PC12 cells by NGF was suppressed by the addition of 50 microM PD98059 as reported by Pang et al. [L. Pang, T. Sawada, J. Stuart,S.J. Decker, A.R. Saltiel, Inhibition of MAP kinase kinase blocks the differentiation of PC12 cells induced by nerve growth factor, J. Biol. Chem. 270 (1995) 13585-13588]. In contrast, the rapid regeneration of neurites from PC12 cells primed with NGF, was not altered in the presence of the same dose of the inhibitor of MEK. It appeared, therefore, that the activation of the MAP kinase pathway was not necessarily required for the NGF-dependent extension of neurites. When PC12D cells were transfected with the dominant inhibitory Ha-ras Asn-17 gene, the induction of the mutant Ras protein led the suppression of the rapid outgrowth of neurites in response to NGF but not to dibutyryl cyclic AMP (dbcAMP). The result implies a direct involvement of Ras protein in the NGF-induced signal transduction that lead to the formation of neurites in PC12D cells. We can conclude that the activation of MAP kinase and selective gene expression are required for the differentiation of conventional PC12 cells to sympathetic neuron-like cells and that activation of Ras protein and, subsequently, of a MAP kinase-independent pathway might be involved in the extension of neurites from PC12D cells or in the regeneration of neurites from primed PC12 cells in response to NGF.