Effects on the serotoninergic system in sub-acute transmissible spongiform encephalopathies: current data, hypotheses, suggestions for experimentation

A roadmap for investigating the role of the prion protein in depression associated with neurodegenerative disease

The physiological properties of the native, endogenous prion protein (PrP(C)) is a matter of concern, due to its pleiotropic functions and links to neurodegenerative disorders and cancer. In line with our hypothesis that the basic function of PrP(C) is to serve as a cell surface scaffold for the assembly of signaling modules, multiple interactions have been identified of PrP(C) with signaling molecules, including neurotransmitter receptors. We recently reported evidence that PrP(C) may modulate monoaminergic neurotransmission, as well as depressive-like behavior in mice. Here, we discuss how those results, together with a number of other studies, including our previous demonstration that both inflammatory and behavioral stress modulate PrP(C) content in neutrophils, suggest a distributed role of PrP(C) in clinical depression and inflammation associated with neurodegenerative diseases. An overarching understanding of the multiple interventions of PrP(C) upon physiological events may both shed light on the pathogenesis of, as well as help the identification of novel therapeutic targets for clinical depression, Prion and Alzheimer's Diseases.

Prion Protein Modulates Monoaminergic Systems and Depressive-like Behavior in Mice

We sought to examine interactions of the prion protein (PrP(C)) with monoaminergic systems due to: the role of PrP(C) in both Prion and Alzheimer diseases, which include clinical depression among their symptoms, the implication of monoamines in depression, and the hypothesis that PrP(C) serves as a scaffold for signaling systems. To that effect we compared both behavior and monoaminergic markers in wild type (WT) and PrP(C)-null (PrP(-/-)) mice. PrP(-/-) mice performed poorly when compared with WT in forced swimming, tail suspension, and novelty suppressed feeding tests, typical of depressive-like behavior, but not in the control open field nor rotarod motor tests; cyclic AMP responses to stimulation of D1 receptors by dopamine was selectively impaired in PrP(-/-) mice, and responses to serotonin, but not to norepinephrine, also differed between genotypes. Contents of dopamine, tyrosine hydroxylase, and the 5-HT5A serotonin receptor were increased in the cerebral cortex of PrP(-/-), as compared with WT mice. Microscopic colocalization, as well as binding in overlay assays were found of PrP(C) with both the 5HT5A and D1, but not D4 receptors. The data are consistent with the scaffolding of monoaminergic signaling modules by PrP(C), and may help understand the pathogenesis of clinical depression and neurodegenerative disorders.

PrP(C) signalling in neurons: from basics to clinical challenges

The cellular prion protein PrP(C) was identified over twenty-five years ago as the normal counterpart of the scrapie prion protein PrP(Sc), itself the main if not the sole component of the infectious agent at the root of Transmissible Spongiform Encephalopathies (TSEs). PrP(C) is a ubiquitous cell surface protein, abundantly expressed in neurons, which constitute the targets of PrP(Sc)-mediated toxicity. Converging evidence have highlighted that neuronal, GPI-anchored PrP(C) is absolutely required for prion-induced neuropathogenesis, which warrants investigating into the normal function exerted by PrP(C) in a neuronal context. It is now well-established that PrP(C) can serve as a cell signalling molecule, able to mobilize transduction cascades in response to interactions with partners. This function endows PrP(C) with the capacity to participate in multiple neuronal processes, ranging from survival to synaptic plasticity. A diverse array of data have allowed to shed light on how this function is corrupted by PrP(Sc). Recently, amyloid Aβ oligomers, whose accumulation is associated with Alzheimer's disease (AD), were shown to similarly instigate toxic events by deviating PrP(C)-mediated signalling. Here, we provide an overview of the various signal transduction cascades ascribed to PrP(C) in neurons, summarize how their subversion by PrP(Sc) or Aβ oligomers contributes to TSE or AD neuropathogenesis and discuss the ensuing clinical implications.

The cellular prion protein interacts with the tissue non-specific alkaline phosphatase in membrane microdomains of bioaminergic neuronal cells

Background

The cellular prion protein, PrP^C, is GPI anchored and abundant in lipid rafts. The absolute requirement of PrP^C in neurodegeneration associated to prion diseases is well established. However, the function of this ubiquitous protein is still puzzling. Our previous work using the 1C11 neuronal model, provided evidence that PrP^C acts as a cell surface receptor. Besides a ubiquitous signaling function of PrP^C, we have described a neuronal specificity pointing to a role of PrP^C in neuronal homeostasis. 1C11 cells, upon appropriate induction, engage into neuronal differentiation programs, giving rise either to serotonergic (1C11^5-HT) or noradrenergic (1C11^NE) derivatives.

Methodology/Principal Findings

The neuronal specificity of PrP^C signaling prompted us to search for PrP^C partners in 1C11-derived bioaminergic neuronal cells. We show here by immunoprecipitation an association of PrP^C with an 80 kDa protein identified by mass spectrometry as the tissue non-specific alkaline phosphatase (TNAP). This interaction occurs in lipid rafts and is restricted to 1C11-derived neuronal progenies. Our data indicate that TNAP is implemented during the differentiation programs of 1C11^5-HT and 1C11^NE cells and is active at their cell surface. Noteworthy, TNAP may contribute to the regulation of serotonin or catecholamine synthesis in 1C11^5-HT and 1C11^NE bioaminergic cells by controlling pyridoxal phosphate levels. Finally, TNAP activity is shown to modulate the phosphorylation status of laminin and thereby its interaction with PrP.

Conclusion/Significance

The identification of a novel PrP^C partner in lipid rafts of neuronal cells favors the idea of a role of PrP in multiple functions. Because PrP^C and laminin functionally interact to support neuronal differentiation and memory consolidation, our findings introduce TNAP as a functional protagonist in the PrP^C-laminin interplay. The partnership between TNAP and PrP^C in neuronal cells may provide new clues as to the neurospecificity of PrP^C function.

Review of lithium effects on brain and blood

Clinicians have long used lithium to treat manic depression. They have also observed that lithium causes granulocytosis and lymphopenia while it enhances immunological activities of monocytes and lymphocytes. In fact, clinicians have long used lithium to treat granulocytopenia resulting from radiation and chemotherapy, to boost immunoglobulins after vaccination, and to enhance natural killer activity. Recent studies revealed a mechanism that ties together these disparate effects of lithium. Lithium acts through multiple pathways to inhibit glycogen synthetase kinase-3beta (GSK3 beta). This enzyme phosphorylates and inhibits nuclear factors that turn on cell growth and protection programs, including the nuclear factor of activated T cells (NFAT) and WNT/beta-catenin. In animals, lithium upregulates neurotrophins, including brain-derived neurotrophic factor (BDNF), nerve growth factor, neurotrophin-3 (NT3), as well as receptors to these growth factors in brain. Lithium also stimulates proliferation of stem cells, including bone marrow and neural stem cells in the subventricular zone, striatum, and forebrain. The stimulation of endogenous neural stem cells may explain why lithium increases brain cell density and volume in patients with bipolar disorders. Lithium also increases brain concentrations of the neuronal markers n-acetyl-aspartate and myoinositol. Lithium also remarkably protects neurons against glutamate, seizures, and apoptosis due to a wide variety of neurotoxins. The effective dose range for lithium is 0.6-1.0 mM in serum and >1.5 mM may be toxic. Serum lithium levels of 1.5-2.0 mM may have mild and reversible toxic effects on kidney, liver, heart, and glands. Serum levels of >2 mM may be associated with neurological symptoms, including cerebellar dysfunction. Prolonged lithium intoxication >2 mM can cause permanent brain damage. Lithium has low mutagenic and carcinogenic risk. Lithium is still the most effective therapy for depression. It "cures" a third of the patients with manic depression, improves the lives of about a third, and is ineffective in about a third. Recent studies suggest that some anticonvulsants (i.e., valproate, carbamapazine, and lamotrigene) may be useful in patients that do not respond to lithium. Lithium has been reported to be beneficial in animal models of brain injury, stroke, Alzheimer's, Huntington's, and Parkinson's diseases, amyotrophic lateral sclerosis (ALS), spinal cord injury, and other conditions. Clinical trials assessing the effects of lithium are under way. A recent clinical trial suggests that lithium stops the progression of ALS.

Prions impair bioaminergic functions through serotonin- or catecholamine-derived neurotoxins in neuronal cells

The conversion of the cellular prion protein, PrP(C), to an abnormal isoform, PrP(Sc), is a central event leading to neurodegeneration in prion diseases. Deciphering the molecular and cellular changes imparted by PrP(Sc) accumulation remains an arduous task due to the small number of cell lines supporting prion replication. Here we introduce the 1C11 cell line as a new in vitro model to investigate prion pathogenesis. This cell line is a committed neuroectodermal progenitor able to differentiate into fully functional serotonergic or catecholaminergic neurons. 1C11 cells, which naturally express PrP(C) from the undifferentiated state, can be chronically infected with various prion strains. Prion infection does not promote any noticeable phenotypic change in the progenitor cells nor prevent the onset of the serotonergic and catecholaminergic differentiation programs. Pathogenic prions, however, deviate the overall neurotransmitter-metabolism in both pathways by decreasing bioamine synthesis, storage, and transport, and enhancing catabolism. Noteworthy, oxidized derivatives of both serotonin and catecholamines are selectively detected in the differentiated progenies of infected cells and contribute to irreversible impairment in bioamine synthesis. Finally, the level of PrP(Sc) accumulation, that of infectivity, and the extent of all prion-induced changes in infected cells appear to be correlated. The report of such specific effects of infection on neuronal functions provides a foundation for dissecting the events underlying loss of neuronal homeostasis in prion diseases.

Increase of monoamine oxidase-B activity in the brain of scrapie-infected hamsters

In the present study, the purpose is to determine activities of monoamine oxidases (MAO) in the brain of 263K scrapie-infected hamsters during the development of this experimental prion disease. Indeed, MAO activity modifications which have already been related in aging and neurodegenerations is suspected to be involved in the neuron loss process by elevated hydrogen peroxide formation. Monoamine oxidase type A (MAO-A) and B (MAO-B) activities were followed in the brain at different stages of the disease. MAO-A activity did not change significantly during the evolution of the disease. However, concerning the MAO-B activity, a significant increase was observed from 50 days post-infection and through the course of the disease and reached 42.9+/-5.3% at its ultimate stage. Regarding these results, MAO-B could be a potential therapeutic target then we have performed a pre-clinical treatment with irreversible (Selegiline or L-deprenyl) or and reversible (MS-9510) MAO-B inhibitors used alone or in association with an anti-scrapie drug such as MS-8209, an amphotericin B derivative. Our results show that none of the MAO-B inhibitors used was able to delay the onset of the disease. Neither these MAO-B inhibitors nor R-NMDA inhibitors (MK-801) can enhance the effects of MS-8209. The present findings clearly indicate a significant increase of cerebral MAO-B activity in scrapie-infected hamsters. Furthermore, inhibitors of MAO-B do not have any curative or palliative effect on this experimental model indicating that the raise of this activity is probably more a consequence rather than a causal event of the neurodegenerative process.

The neurochemical nature of PrPc-containing cells in the rat brain

The cellular prion protein (PrP(C)) is a membrane-bound glycoprotein abundantly expressed in neurons and glial cells within the CNS. The scrapie prion protein (PrP(Sc)) is a conformationally altered isoform of PrP(C) that is responsible for prion diseases, also termed transmissible spongiform encephalopathies (TSE), a group of neurodegenerative diseases that affect a wide variety of mammal species, including humans. The presence of the cellular isoform of PrP is necessary for the establishment and further evolution of prion diseases and the physiological conditions where PrP(C) is present seems to modulate the alterations in TSE. In this work, the presence of PrP(C) in GABAergic, glutamatergic, nitrergic, cholinergic, serotoninergic and orexinergic populations of cells within the rat brain is examined. Our observations show that PrP(C) is widely expressed in a subset of neurons that contain markers of inhibitory populations of cells throughout the rat brain. The presence of PrP(C) in other cells types containing important neurotransmitters for the overall brain function is congruent with the imbalances reported for some of them in TSE. Within the cerebral cortex, PrP(C) is scarcely located in a subset of cells expressing the laminin receptor precursor (LRP) to such a low extent that suggests that other LRP-independent mechanisms actively participate during the pathogenic process. Taken together, our data demonstrate that investigation of the chemical partners of PrP(C) within cells gives a rational basis for the interpretation of the histopathological alterations in TSE and might help analyze some pathogenic mechanisms of PrP(Sc).

Cellular Prion Protein Signaling in Serotonergic Neuronal Cells

The cellular prion protein PrP(C) is the normal counterpart of the scrapie prion protein PrP(Sc), the main component of the infectious agent of transmissible spongiform encephalopathies (TSEs). It is a ubiquitous cell-surface glycoprotein, abundantly expressed in neurons, which constitute the targets of TSE pathogenesis. Taking advantage of the 1C11 neuroectodermal cell line, endowed with the capacity to convert into 1C11(5-HT) serotonergic or 1C11(NE) noradrenergic neuronal cells, allowed us to ascribe a signaling function to PrP(C). Antibody-mediated ligation of PrP(C) recruits transduction pathways, which involve nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent reactive oxygen species production and target the extracellular-regulated kinases ERK1/2. In fully differentiated cells only, these effectors are under the control of a PrP(C)-caveolin-Fyn platform, located on neuritic extensions. In addition to its proper signaling activity, PrP(C) modulates the agonist-induced response of the three serotonergic G protein-coupled receptors present on the 1C11(5-HT) differentiated cells. The impact of PrP(C) ligation on the receptor couplings depends on the receptor subtype and the pathway considered. The implementation of the PrP(C)-caveolin complex again is mandatory for PrP(C) to exert its action on 5-HT receptor signaling. Our current data argue that PrP(C) interferes with the intensities and/or dynamics of G protein activation by agonist-bound 5-HT receptors. By mobilizing transduction cascades controlling the cellular redox state and the ERK1/2 kinases and by altering 5-HT receptor-mediated intracellular response, PrP(C) takes part in the homeostasis of serotonergic neuronal cells. These findings may have implications for future research aiming at understanding the fate of serotonergic neurons in prion diseases.

Cytoarchitecture, morphology, and lumbosacral spinal cord projections of the red nucleus in cattle

OBJECTIVE

To analyze the morphology, cytoarchitecture, and lumbosacral spinal cord projections of the red nucleus (RN) in cattle.

ANIMALS

8 healthy Friesian male calves.

PROCEDURES

Anesthetized calves underwent a dorsal laminectomy at L5. Eight bilateral injections (lateral to the midline) of the neuronal retrograde fluorescent tracer fast blue (FB) were administered into the exposed lumbosacral portion of the spinal cord. A postsurgical calf survival time of 38 to 55 days was used. Following euthanasia, the midbrain and the L5-S2 spinal cord segments were removed. Nissl's method of staining was applied on paraffin-embedded and frozen sections of the midbrain.

RESULTS

The mean length of the RN from the caudal to cranial end ranged from 6,680 to 8,640 microm. The magnocellular and parvicellular components of the RN were intermixed throughout the nucleus, but the former predominate at the caudal portion of the nucleus and the latter at the cranial portion with a gradual transitional zone. The FB-labeled neurons were found along the entire craniocaudal extension of the nucleus, mainly in its ventrolateral part. The number of FB-labeled neurons was determined in 4 calves, ranging from 191 to 1,469 (mean, 465). The mean cross-sectional area of the FB-labeled neurons was approximately 1,680 microm2.

CONCLUSIONS AND CLINICAL RELEVANCE

In cattle, small, medium, and large RN neurons, located along the entire craniocaudal extension of the RN, contribute to the rubrospinal tract reaching the L6-S1 spinal cord segments. Thus, in cattle, as has been shown in cats, the RN parvicellular population also projects to the spinal cord.

Unaltered susceptibility to scrapie in serotonin transporter deficient mice

The serotonergic system has been hypothesized to play an important role in prion diseases. Specifically, hyperactivity of the serotonergic system in prion diseases is suggested by an increase in the turnover rate of the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) in human and experimental prion diseases. The 5-HT transporter (5-HTT) determines the duration of serotonergic neurotransmission by way of reuptake of 5-HT from the extracellular space. 5-HTT availability is reduced in brains of patients with the human prion disease familial fatal insomnia. To further clarify a possible role of the 5-HTT in prion diseases we investigated whether mice lacking the 5-HTT display an altered susceptibility to experimental scrapie infection. Surprisingly, 5-HTT knockout mice developed mouse scrapie in a time course similar to wildtype control mice with accumulation of the pathological prion protein, PrP(Sc) and with typical pathological hallmarks of the disease. These findings argue against a major role of the 5-HTT in the pathogenesis of prion diseases in mice.

Regulation of PrPC expression: Nerve growth factor (NGF) activates the prion gene promoter through the MEK1 pathway in PC12 cells

A high expression of PrP(C) in cells is one factor that increases the risk of conversion to the misfolded, disease-associated form (PrP(Sc)) characteristic of transmissible spongiform encephalopathies. Thus, developing a method to control the level of PrP(C) expression in cells could be one way to delay or prevent the onset of clinical signs of these diseases. In this study the mechanisms controlling the expression of the Prnp gene in PC12 cells and in rat brain were examined. We observed a slight activation of a cloned fragment of the human PRNP gene promoter using the luciferase reporter system in PC12 cells stimulated with nerve growth factor (NGF). The activating effect of NGF was enhanced by mitogen-activated protein kinase (MEK1) and suppressed by myristylated serine/threonine kinase (myrAKT). These results suggest that MEK1 is a positive activator of the PRNP promoter that inhibits the AKT pathway. Independent experiments suggested that high expression of PrP(C) in the brain depends on the rate of translation and/or the efficiency of PrP(C) stabilization. We also investigated the epigenic status of the Prnp promoter. We observed no increase of PrP(C) or Prnp mRNA levels in PC12 cells after treatment with the DNA-demethylating agent. The Prnp promoter did not display methylation either in NGF-treated and untreated PC12 cells, or in the rat brain. These results improve the understanding of the regulation of the Prnp gene promoter, a DNA regulatory element controlling the expression of PrP(C), a protein involved in several neurological diseases.