Adrenocorticotropic hormone activation of adenylate cyclase in raphe neurons: multiple regulatory pathways control serotonergic neuronal differentiation

Serotonin transporter regulation by cholesterol-independent lipid signaling

Serotonin neurotransmission is largely governed by the regulation of the serotonin transporter (SERT). SERT is modulated in part by cholesterol, but the role of cholesterol and lipid signaling intermediates in regulating SERT are unknown. Serotonergic neurons were treated with statins to decrease cholesterol and lipid signaling intermediates. Contrary to reported decreases in 5-HT uptake after cholesterol depletion, biochemical and imaging methods both showed that statins increased 5-HT uptake in a fluoxetine-dependent manner. Simvastatin lowered the Km without changing Vmax for 5-HT or SERT distribution to the plasma membrane. Cholesterol repletion did not block enhanced 5-HT uptake by simvastatin but the enhanced uptake was blocked by lipid isoprenylation intermediates farnesyl pyrophosphate and geranylgeranyl pyrophosphate. Blockade of geranylgeranylation alone without statins also enhanced 5-HT uptake. Overall, this study revealed a specific neuronal effect of statin drugs and identified lipid signaling through geranylgeranylation within the isoprenylation pathway regulates SERT in a cholesterol-independent manner.

Review of the history and current status of cell-transplant approaches for the management of neuropathic pain

Treatment of sensory neuropathies, whether inherited or caused by trauma, the progress of diabetes, or other disease states, are among the most difficult problems in modern clinical practice. Cell therapy to release antinociceptive agents near the injured spinal cord would be the logical next step in the development of treatment modalities. But few clinical trials, especially for chronic pain, have tested the transplant of cells or a cell line to treat human disease. The history of the research and development of useful cell-transplant-based approaches offers an understanding of the advantages and problems associated with these technologies, but as an adjuvant or replacement for current pharmacological treatments, cell therapy is a likely near future clinical tool for improved health care.

Potential for Cell-Transplant Therapy with Human Neuronal Precursors to Treat Neuropathic Pain in Models of PNS and CNS Injury: Comparison of hNT2.17 and hNT2.19 Cell Lines

Effective treatment of sensory neuropathies in peripheral neuropathies and spinal cord injury (SCI) is one of the most difficult problems in modern clinical practice. Cell therapy to release antinociceptive agents near the injured spinal cord is a logical next step in the development of treatment modalities. But few clinical trials, especially for chronic pain, have tested the potential of transplant of cells to treat chronic pain. Cell lines derived from the human neuronal NT2 cell line parentage, the hNT2.17 and hNT2.19 lines, which synthesize and release the neurotransmitters gamma-aminobutyric acid (GABA) and serotonin (5HT), respectively, have been used to evaluate the potential of cell-based release of antinociceptive agents near the lumbar dorsal (horn) spinal sensory cell centers to relieve neuropathic pain after PNS (partial nerve and diabetes-related injury) and CNS (spinal cord injury) damage in rat models. Both cell lines transplants potently and permanently reverse behavioral hypersensitivity without inducing tumors or other complications after grafting. Functioning as cellular minipumps for antinociception, human neuronal precursors, like these NT2-derived cell lines, would likely provide a useful adjuvant or replacement for current pharmacological treatments for neuropathic pain.

Identification and functional characterization of three novel alleles for the serotonin transporter-linked polymorphic region

A promoter polymorphism in the serotonin transporter gene (5-HTTLPR) has been reported to confer relative risk for phenotypes (depression/anxiety) and endophenotypes (amygdala reactivity). In this report, we identify and characterize three rare 5-HTTLPR alleles not previously described in the human literature. The three novel alleles were identified while genotyping 5-HTTLPR in a family-based attention deficit hyperactivity disorder clinical population. Two of the novel alleles are longer than the common 16-repeat long (L) allele (17 and 18 repeats) and the third is significantly smaller than the 14-repeat short (S) allele (11 repeats). The sequence and genetic architecture of each novel allele is described in detail. We report a significant decrease in the expression between the XL₁₇ (17r) allele and the L(A) (16r) allele. The XS₁₁ (11r) allele showed similar expression with the S (14r) allele. A 1.8-fold increase in expression was observed with the L(A)(16r) allele compared with the L(G) (16r) allele, which replicates results from earlier 5-HTTLPR expression experiments. In addition, transcription factor binding site (TFBS) analysis was performed using MatInspector (Genomatix) that showed the presence or absence of different putative TFBSs between the novel alleles and the common L (16r) and S (14r) alleles. The identification of rare variants and elucidation of their functional impact could potentially lead to understanding the contribution that the rare variant may have on the inheritance/susceptibility of multifactorial common diseases.

Transcriptional dysregulation of 5-HT1A autoreceptors in mental illness

The serotonin-1A (5-HT1A) receptor is among the most abundant and widely distributed 5-HT receptors in the brain, but is also expressed on serotonin neurons as an autoreceptor where it plays a critical role in regulating the activity of the entire serotonin system. Over-expression of the 5-HT1A autoreceptor has been implicated in reducing serotonergic neurotransmission, and is associated with major depression and suicide. Extensive characterization of the transcriptional regulation of the 5-HT1A gene (HTR1A) using cell culture systems has revealed a GC-rich "housekeeping" promoter that non-selectively drives its expression; this is flanked by a series of upstream repressor elements for REST, Freud-1/CC2D1A and Freud-2/CC2D1B factors that not only restrict its expression to neurons, but may also regulate the level of expression of 5-HT1A receptors in various subsets of neurons, including serotonergic neurons. A separate set of allele-specific factors, including Deaf1, Hes1 and Hes5 repress at the HTR1A C(-1019)G (rs6295) polymorphism in serotonergic neurons in culture, as well as in vivo. Pet1, an obligatory enhancer for serotonergic differentiation, has been identified as a potent activator of 5-HT1A autoreceptor expression. Taken together, these results highlight an integrated regulation of 5-HT1A autoreceptors that differs in several aspects from regulation of post-synaptic 5-HT1A receptors, and could be selectively targeted to enhance serotonergic neurotransmission.

Serotonin transporter promoter gain-of-function genotypes are linked to obsessive-compulsive disorder

A functional serotonin transporter promoter polymorphism, HTTLPR, alters the risk of disease as well as brain morphometry and function. Here, we show that HTTLPR is functionally triallelic. The L(G) allele, which is the L allele with a common G substitution, creates a functional AP2 transcription-factor binding site. Expression assays in 62 lymphoblastoid cell lines representing the six genotypes and in transfected raphe-derived cells showed codominant allele action and low, nearly equivalent expression for the S and L(G) alleles, accounting for more variation in HTT expression than previously recognized. The gain-of-function L(A)L(A) genotype was approximately twice as common in 169 whites with obsessive-compulsive disorder (OCD) than in 253 ethnically matched controls. We performed a replication study in 175 trios consisting of probands with OCD and their parents. The L(A) allele was twofold overtransmitted to the patients with OCD. The HTTLPR L(A)L(A) genotype exerts a moderate (1.8-fold) effect on risk of OCD, which crystallizes the evidence that the HTT gene has a role in OCD.

Somato-dendritic distribution of 5-HT(1A) and 5-HT(1B) autoreceptors in the BDNF- and cAMP-differentiated RN46A serotoninergic raphe cell line

The rapid differentiating effects of brain-derived neurotrophic factor (BDNF) or dibutyryl-cAMP (dBcAMP) were characterized on RN46A, a rat raphe-derived neuronal cell line. After BDNF treatment, RN46A cells were serotonin-immunopositive and bipolar, and expressed the microtubule-associated-protein 2 (Map2). After dBcAMP treatment, the cells often became multipolar, bearing very long processes strongly immunopositive for serotonin and Map2. Under both conditions, the expression and distribution of 5-HT(1A) and 5-HT(1B) autoreceptors remained identical. 5-HT(1A) and Map2 immunolabelings were superimposable, as expected of their somato-dendritic targeting. Surprisingly, the distribution of 5-HT(1B) immunoreactivity was similar, in contrast with its usual localization in axons and nerve terminals in the brain. In conclusion, both BDNF and cAMP-differentiated RN46A cells towards a neuronal serotoninergic-like phenotype without the typical differential targeting of the 5-HT(1) autoreceptors, an interesting model to study the molecular mechanisms ensuing the targeting of 5-HT(1) autoreceptors to somas and dendrites.

Coupling of 5-HT1A autoreceptors to inhibition of mitogen-activated protein kinase activation via G beta gamma subunit signaling

The 5-HT1A receptor is expressed presynaptically as the primary somatodendritic autoreceptor on serotonergic raphe neurons, and postsynaptically in several brain regions. Signaling of the 5-HT1A autoreceptor was studied in RN46A cells, a model of serotonergic raphe neurons that express endogenous 5-HT1A receptors. In undifferentiated RN46A cells stably transfected with the wild-type 5-HT1A receptor, 5-HT1A receptor activation inhibited forskolin-induced cyclic adenosine monophosphate (cAMP) formation (by 50%), increased [Ca2+]i, and induced a novel inhibition (up to 60%) of phospho-p42/p44-mitogen-activated protein kinase (MAPK). Upon differentiation of non-transfected or 5-HT1A-transfected RN46A cells, agonist-mediated inhibition of MAPK was enhanced. These actions were blocked by pretreatment with pertussis toxin indicating mediation via Gi/Go proteins and the calcium response was blocked by preactivation of protein kinase C (PKC). In cells overexpressing the G beta gamma scavenger carboxyl-terminal domain of G protein receptor kinase 2 (GRK-CT), 5-HT1A receptor activation inhibited cAMP formation, but coupling to calcium mobilization and inhibition of MAPK was abolished. The activity of 5-HT1A receptors containing mutations of PKC sites in the second (i2: T149A) or third intracellular loop (i3: T229A/S253G/T343A) was tested. At comparable levels of receptor expression, the signaling of the 5-HT1A i3 mutant was similar to the 5-HT1A wild-type receptor, while the i2 and quadruple (i2/i3) mutants failed to couple to G beta gamma-mediated increase in [Ca2+]i or inhibition of MAPK, but did couple to G alpha i-mediated inhibition of cAMP. Thus, the i2-domain of the 5-HT1A autoreceptor is crucial for coupling to G beta gamma subunits and their subsequent responses (e.g. calcium mobilization and inhibition of MAPK activity).

Characterization of reproductive steroid receptors and response to estrogen in a rat serotonergic cell line

Study of the cellular and molecular consequences of steroid hormone action in the serotonin neural system will provide new avenues for pharmacotherapeutic intervention in mental illness related to reproductive function. However, it is difficult to probe intracellular mechanisms with whole animal models. We sought the steroid receptor compliment and estrogen response of two rat serotonin cell lines in order to determine if they could be of future assistance in this matter. Immunohistochemistry with a panel of antibodies, RT-PCR and a serotonin ELISA were utilized to characterize the RN46A-V1 cells (herein called RN46A), and the subclone RN46A-B14 (herein called B14) that is stably transfected with brain derived neurotrophic factor (BDNF). RN46A and B14 cells express estrogen receptor beta (ERbeta), androgen receptors (AR) and nuclear factor kappa B (NFkappaB) but not estrogen receptor alpha (ERalpha) or progestin receptors (PR). RT-PCR confirmed the presence of ERbeta and the absence of ERalpha and PR in both cell lines. B14 cells contain more immunodetectable BDNF and serotonin than the RN46A parent line. In addition, immunofluorescence for the serotonin reuptake transporter (SERT) was observed in the cell body region of undifferentiated B14 cells. After differentiation at a nonpermissive temperature, SERT immunostaining was observed in both the cell body region and along the extent of the axons. Serotonin content as determined by ELISA was higher in B14 than RN46A cells. Estrogen (0.1 and 1.0 nM) stimulated serotonin in the B14 cells in serum free medium. In summary, the RN46A cells and the B14 subclone contain the same compliment of nuclear steroid receptors as rat raphe serotonin neurons and thus may provide a convenient in vitro model for study of intracellular mechanisms of action of steroid hormones in the context of a serotonin neuron.

Novel variants of murine serotonin transporter mRNA and the promoter activity of its upstream site

Three variants of murine serotonin transporter (5-HTT) mRNA, which consist of a different exon-one (exon 1a, exon 1b or exon 1c) and the same exon-two to exon-five, were identified. The promoter region for each exon 1 (p1a, p1b and p1c, respectively), ligated to pGL-3 enhancer vector, had activities significantly higher than the empty vector in all cell lines tested except p1c in PC-12, whereas the activity of p1c was significantly lower than the others. Effects of the treatment of dibutyryl-cyclic AMP, human interferon-alpha or mouse interferon-gamma have different profiles among COS-7, PC-12, C6 glioma and immortalized rat serotonergic raphe neurons, RN46A. These three promoter regions may play a role in the transcription of the 5-HTT and could offer a model of the regulation of 5-HTT production in humans and further the pathogenesis of depression.

Divergent regulation of GAP-43 expression and CNS neurite outgrowth by cyclic AMP

Robust process outgrowth and high expression of the growth-associated protein GAP-43 seem to be intrinsic features of neurons, but both are down-regulated after axonal contact of target cells. We report that chronic exposure of the serotonergic CNS cell line RN46A to cyclic AMP analogs, forskolin, or cholera toxin represses GAP-43 expression in a dose dependent manner. Thus, cAMP could mediate a GAP-43 repressive signal that is initiated extracellularly. Activation of the cyclic AMP pathway by these same reagents, however, enhances the rate that RN46A cells extend neurites. This stimulation of neurite growth can occur during inhibition of new transcription, and in the absence of high levels of GAP-43. These findings demonstrate that a GAP-43-repressing intracellular signaling pathway exists, that repression of GAP-43 expression by cAMP is not directly coupled to inhibition of neurite growth, and that acceleration of growth cone advancement by cAMP is not dependent on the presence of GAP-43.

Emerging cell and molecular strategies for the study and treatment of painful peripheral neuropathies

Pharmacologic treatment for the symptoms of painful neuropathy has been problematic, because there has been limited understanding of the underlying etiologies and systemic levels that an effective dose can have on multiple side effects. The use of molecular methods, such as gene deletion from knockout mice and cellular minipumps for delivery of biologic antinociceptive molecules, has led to a better understanding of the underlying mechanisms of the induction of intractable neuropathic pain. The initiation of an excitatory cascade after injury or disease leads to the induction of various second messenger systems, loss or down-regulation of the endogenous inhibitory spinal GABA system and central sensitization, causing such pain. The development and use of cellular minipumps, immortalized cell lines bioengineered to secrete various antinociceptive molecules for the reversal of neuropathic pain, makes cellular therapy a strategy for clinical use in the next few years. The development of molecular "disimmortalization" technologies will make the use of such engineered cell lines safe for human use. Direct somatic gene transfer for neuropathic pain will eventually overcome the problems associated with transplantation of non-autologous and xenogenic cells. These virus-mediated methods, although at the early stages of evolution and use, offer large-scale production of biologic agents that can be conveniently and confidently used for the long-term relief of chronic neuropathic pain in a clinical setting, without systemic effects or surgical interventions.

Protein kinase C and cAMP-dependent protein kinase regulate the neuronal differentiation of immortalized raphe neurons

These studies examined the extent to which protein kinase C (PKC) and cAMP-dependent protein kinase (PKA) regulate the neuronal differentiation of the raphe-derived neuronal cell line, RN33B. A differentiation-specific 2.25-fold increase in soluble PKA activity was observed. Neither membrane-associated-PKA, -PKC, or soluble PKC activities changed concomitant with differentiation. The PKC activity was derived from PKC alpha, gamma, epsilon, and theta isoenzymes. Activation of PKC inhibited the immunocytochemical expression of low and medium molecular weight neurofilament proteins, an effect due at least in part to decreased steady-state levels of protein. PKC activation also decreased glutamate immunoreactivity and increased cell number, protein synthesis, and bromodeoxyuridine uptake by 2.4-fold, 25%, and 32%, respectively. Coupled with the decrease in mature neuronal antigen expression, these data suggest that PKC activation inhibits neuronal differentiation by inducing proliferation. Inhibition of PKC markedly upregulated glutamate immunoreactivity. PKA activation potentiated the glutamatergic phenotype of RN33B cells, but inhibition of PKA was without effect on the expression of all neuronal antigens examined. Thus, both PKC and PKA regulate the differentiation of RN33B cells, although neither is absolutely necessary for expression of the differentiated neuronal phenotype. These results suggest the existence of parallel pathways regulating raphe neuronal differentiation.

Advances in the molecular characterization of tryptophan hydroxylase

The neurotransmitter serotonin has been implicated in numerous physiological functions and pathophysiological disorders. The hydroxylation of the aromatic amino acid tryptophan is rate-limiting in the synthesis of serotonin. Tryptophan hydroxylase (TPH), as the rate-limiting enzyme, determines the concentrations of serotonin in vivo. Relative serotonin concentrations are clearly important in neural transmission, but serotonin has also been reported to function as a local antioxidant. Identification of the mechanisms regulating TPH activity has been hindered by its low levels in tissues and the instability of the enzyme. Several TPH expression systems have been developed to circumvent these problems. In addition, eukaryotic expressions systems are currently being developed and represent a new avenue of research for identifying TPH regulatory mechanisms. Recombinant DNA technology has enabled the synthesis of TPH deletions, chimeras, and point mutations that have served as tools for identifying structural and functional domains within TPH. Notably, the experiments have proven long-held hypotheses that TPH is organized into N-terminal regulatory and C-terminal catalytic domains, that serine-58 is a site for PKA-mediated phosphorylation, and that a C-terminal leucine zipper is involved in formation of the tetrameric holoenzyme. Several new findings have also emerged regarding regulation of TPH activity by posttranslational phosphorylation, kinetic inhibition, and covalent modification. Inhibition of TPH by L-DOPA may have implications for depression in Parkinson's disease (PD) patients. In addition, TPH inactivation by nitric oxide may be involved in amphetamine-induced toxicity. These regulatory concepts, in conjunction with new systems for studying TPH activity, are the focus of this article.

Stimulation of tryptophan hydroxylase production in a serotonin-producing cell line (RBL2H3) by intracellular calcium mobilizing reagents

RBL2H3 cells showed a remarkable increase in their level of tryptophan hydroxylase (up to 25-fold), the rate-limiting enzyme in serotonin biosynthesis, by stimulation with intracellular calcium mobilizers A23187, thapsigargin, and tBuBHQ as well as by stimulation with an antigen in the presence of IgE. The increase in the enzyme protein was visualized by Western blot analysis using anti-tryptophan hydroxylase antiserum. The enzyme turnover (Hasegawa et al., FEBS Lett., 368 (1995) 151-154) was not slowed down during the rise in tryptophan hydroxylase. Actinomycin D prevented the stimulation-induced elevation of the enzyme. These findings strongly suggest that this stimulation was achieved by the accelerated biosynthesis of tryptophan hydroxylase.

Physiological relevance and functional potential of central nervous system-derived cell lines

Central nervous system (CNS)-derived neural cell lines have proven to be extremely useful for delineating mechanisms controlling such diverse phenomena as cell lineage choice and differentiation, synaptic maturation, neurotransmitter synthesis and release, and growth factor signalling. In addition, there has been hope that such lines might play pivotal roles in CNS gene therapy and repair. The ability of some neural cell lines to integrate normally into the CNS following transplantation and to express foreign, often corrective gene products in situ might offer potential therapeutic approaches to certain neurodegenerative diseases. Five general strategies have evolved to develop neural cell lines: isolation and cloning of spontaneous or mutagenically induced malignancies, targeted oncogenesis in transgenic mice, somatic cell fusion, growth factor mediated expansion of CNS progenitor or stem cells, and retroviral transduction of neuroepithelial precursors. in this article, we detail recent progress in these areas, focusing on those cell lines that have enabled novel insight into the mechanisms controlling neuronal cell lineage choice and differentiation, both in vitro and in vivo.