Mice heterozygous for neurotrophin-3 display enhanced vulnerability to excitotoxicity in the striatum through increased expression of N-methyl-D-aspartate receptors

Pharmacological Co-Activation of TrkB and TrkC Receptor Signaling Ameliorates Striatal Neuropathology and Motor Deficits in Mouse Models of Huntington's Disease

BACKGROUND

Loss of neurotrophic support in the striatum, particularly reduced brain-derived neurotrophic factor (BDNF) levels, contributes importantly to Huntington's disease (HD) pathogenesis. Another neurotrophin (NT), NT-3, is reduced in the cortex of HD patients; however, its role in HD is unknown. BDNF and NT-3 bind with high affinity to the tropomyosin receptor-kinases (Trk) B and TrkC, respectively. Targeting TrkB/TrkC may be an effective HD therapeutic strategy, as multiple links exist between their signaling pathways and HD degenerative mechanisms. We developed a small molecule ligand, LM22B-10, that activates TrkB and TrkC to promote cell survival.

OBJECTIVE

This study aimed to determine if upregulating TrkB/TrkC signaling with LM22B-10 would alleviate the HD phenotype in R6/2 and Q140 mice.

METHODS

LM22B-10 was delivered by concomitant intranasal-intraperitoneal routes to R6/2 and Q140 mice and then motor performance and striatal pathology were evaluated.

RESULTS

NT-3 levels, TrkB/TrkC phosphorylation, and AKT signaling were reduced in the R6/2 striatum; LM22B-10 counteracted these deficits. LM22B-10 also reduced intranuclear huntingtin aggregates, dendritic spine loss, microglial activation, and degeneration of dopamine- and cyclic AMP-regulated phosphoprotein with a molecular weight of 32 kDa (DARPP-32) and parvalbumin-containing neurons in the R6/2 and/or Q140 striatum. Moreover, both HD mouse models showed improved motor performance after LM22B-10 treatment.

CONCLUSIONS

These results reveal an NT-3/TrkC signaling deficiency in the striatum of R6/2 mice, support the idea that targeting TrkB/TrkC alleviates HD-related neurodegeneration and motor dysfunction, and suggest a novel, disease-modifying, multi-target strategy for treating HD.

Neurotrophin-3 restores synaptic plasticity in the striatum of a mouse model of Huntington's disease

AIMS

Neurotrophin-3 (NT-3) is expressed in the mouse striatum; however, it is not clear the NT-3 role in striatal physiology. The expression levels of mRNAs and immune localization of the NT-3 protein and its receptor TrkC are altered in the striatum following damage induced by an in vivo treatment with 3-nitropropionic acid (3-NP), a mitochondrial toxin used to mimic the histopathological hallmarks of Huntington's disease (HD). The aim of this study was to evaluate the role of NT-3 on corticostriatal synaptic transmission and its plasticity in both the control and damaged striatum.

METHODS

Corticostriatal population spikes were electrophysiologically recorded and striatal synaptic plasticity was induced by high-frequency stimulation. Further, the phosphorylation status of Trk receptors was tested under conditions that imitated electrophysiological experiments.

RESULTS

NT-3 modulates both synaptic transmission and plasticity in the striatum; nonetheless, synaptic plasticity was modified by the 3-NP treatment, where instead of producing striatal long-term depression (LTD), long-term potentiation (LTP) was obtained. Moreover, the administration of NT-3 in the recording bath restored the plasticity observed under control conditions (LTD) in this model of striatal degeneration.

CONCLUSION

NT-3 modulates corticostriatal transmission through TrkB stimulation and restores striatal LTD by signaling through its TrkC receptor.

Conditional BDNF release under pathological conditions improves Huntington's disease pathology by delaying neuronal dysfunction

Background

Brain-Derived Neurotrophic Factor (BDNF) is the main candidate for neuroprotective therapy for Huntington's disease (HD), but its conditional administration is one of its most challenging problems.

Results

Here we used transgenic mice that over-express BDNF under the control of the Glial Fibrillary Acidic Protein (GFAP) promoter (pGFAP-BDNF mice) to test whether up-regulation and release of BDNF, dependent on astrogliosis, could be protective in HD. Thus, we cross-mated pGFAP-BDNF mice with R6/2 mice to generate a double-mutant mouse with mutant huntingtin protein and with a conditional over-expression of BDNF, only under pathological conditions. In these R6/2:pGFAP-BDNF animals, the decrease in striatal BDNF levels induced by mutant huntingtin was prevented in comparison to R6/2 animals at 12 weeks of age. The recovery of the neurotrophin levels in R6/2:pGFAP-BDNF mice correlated with an improvement in several motor coordination tasks and with a significant delay in anxiety and clasping alterations. Therefore, we next examined a possible improvement in cortico-striatal connectivity in R62:pGFAP-BDNF mice. Interestingly, we found that the over-expression of BDNF prevented the decrease of cortico-striatal presynaptic (VGLUT1) and postsynaptic (PSD-95) markers in the R6/2:pGFAP-BDNF striatum. Electrophysiological studies also showed that basal synaptic transmission and synaptic fatigue both improved in R6/2:pGAP-BDNF mice.

Conclusions

These results indicate that the conditional administration of BDNF under the GFAP promoter could become a therapeutic strategy for HD due to its positive effects on synaptic plasticity.

Heterozygous deletion of NR1 subunit of the NMDA receptor alters ethanol-related behaviors and regional expression of NR2 subunits in the brain

NMDA receptors have been hypothesized to play a role in various aspects of ethanol-related phenotypes, notably in ethanol withdrawal. However, the role of each of the specific subunits remains unclear. To address this issue, mice that are heterozygous for the NR1 deletion, and thus have a reduction in functional NMDA receptors, were examined for ethanol consumption and acute ethanol withdrawal. Additionally, mice were examined for the level of vocalization following footshock, and behavior in an elevated plus maze, to determine their responses to stress. In these behavioral tests, NR1 heterozygous mice were shown to consume significantly higher levels of ethanol in the two bottle-choice test showing a possible role for this receptor in ethanol consumption. Analysis of acute withdrawal found that the heterozygous mice exhibit lower levels of handling-induced convulsions consistent with a role in ethanol sensitivity or withdrawal. In contrast, no effects on stress-related phenotypes were detected. Levels of NR2A-NR2D subunits of the NMDA receptor in specific brain regions were compared between NR1+/- mice and wild-type controls to assess whether the behavioral responses were specific to the diminution in NR1 expression or whether these changes could be due to secondary changes in expression of other NMDA subunits. Real-time quantitative PCR, Western blot and immunohistochemistry were used to examine expression levels in the hippocampus, neocortex, striatum and cerebellum. For the majority of the subunits, no differences were found between the wild-type and heterozygous mice in any of the brain regions. However, the NR2B subunit exhibited differences in expression of RNA in the hippocampus and protein levels in multiple brain regions, between wild-type and NR1+/- mice. These results show that NR1 plays a role, through mechanisms as yet unknown, in the expression of NR2 subunits in a region and subtype specific manner. This provides evidence of the effects of altered levels of NR1 expression on ethanol withdrawal and consumption, and suggests that concomitant changes in the levels of NR2B may contribute to that effect.

Somatostatin receptor 1 and 5 double knockout mice mimic neurochemical changes of Huntington's disease transgenic mice

BACKGROUND

Selective degeneration of medium spiny neurons and preservation of medium sized aspiny interneurons in striatum has been implicated in excitotoxicity and pathophysiology of Huntington's disease (HD). However, the molecular mechanism for the selective sparing of medium sized aspiny neurons and vulnerability of projection neurons is still elusive. The pathological characteristic of HD is an extensive reduction of the striatal mass, affecting caudate putamen. Somatostatin (SST) positive neurons are selectively spared in HD and Quinolinic acid/N-methyl-D-aspartic acid induced excitotoxicity, mimic the model of HD. SST plays neuroprotective role in excitotoxicity and the biological effects of SST are mediated by five somatostatin receptor subtypes (SSTR1-5).

METHODS AND FINDINGS

To delineate subtype selective biological responses we have here investigated changes in SSTR1 and 5 double knockout mice brain and compared with HD transgenic mouse model (R6/2). Our study revealed significant loss of dopamine and cAMP regulated phosphoprotein of 32 kDa (DARPP-32) and comparable changes in SST, N-methyl-D-aspartic acid receptors subtypes, calbindin and brain nitric oxide synthase expression as well as in key signaling proteins including calpain, phospho-extracellular-signal-regulated kinases1/2, synapsin-IIa, protein kinase C-α and calcineurin in SSTR1/5(-/-) and R6/2 mice. Conversely, the expression of somatostatin receptor subtypes, enkephalin and phosphatidylinositol 3-kinases were strain specific. SSTR1/5 appears to be important in regulating NMDARs, DARPP-32 and signaling molecules in similar fashion as seen in HD transgenic mice.

CONCLUSIONS

This is the first comprehensive description of disease related changes upon ablation of G- protein coupled receptor gene. Our results indicate that SST and SSTRs might play an important role in regulation of neurodegeneration and targeting this pathway can provide a novel insight in understanding the pathophysiology of Huntington's disease.

Motor neuron trophic factors: therapeutic use in ALS?

The modest effects of neurotrophic factor (NTF) treatment on lifespan in both animal models and clinical studies of Amyotropic Lateral Sclerosis (ALS) may result from any one or combination of the four following explanations: 1.) NTFs block cell death in some physiological contexts but not in ALS; 2.) NTFs do not rescue motoneurons (MNs) from death in any physiological context; 3.) NTFs block cell death in ALS but to no avail; and 4.) NTFs are physiologically effective but limited by pharmacokinetic constraints. The object of this review is to critically evaluate the role of both NTFs and the intracellular cell death pathway itself in regulating the survival of spinal and cranial (lower) MNs during development, after injury and in response to disease. Because the role of molecules mediating MN survival has been most clearly resolved by the in vivo analysis of genetically engineered mice, this review will focus on studies of such mice expressing reporter, null or other mutant alleles of NTFs, NTF receptors, cell death or ALS-associated genes.

Alterations of interneurons in the striatum and frontal cortex of mice during postnatal development

We investigated the postnatal alterations of neuronal nuclei (NeuN)-positive neurons, parvalbumin (PV)-positive interneurons, neuronal nitric oxide synthase (nNOS)-positive interneurons, and neurotrophic factors in the mouse striatum and frontal cortex using immunohistochemistry. NeuN, PV, nNOS, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) immunoreactivity were measured in 1-, 2-, 4- and 8-week-old mice. Total number of NeuN-positive neurons was unchanged in the mouse striatum and frontal cortex from 1 up to 8 weeks of age. In contrast, a significant decrease in the number of PV-positive interneurons was observed in the striatum and frontal cortex of 1-, 2- and 4-week-old mice. Furthermore, a significant increase of nNOS-positive interneurons was found in the striatum and frontal cortex of 1- and/or 2-week-old mice. NGF-positive neurons were unchanged in the mouse striatum from 1 up to 8 weeks of age. In the frontal cortex, a significant increase in the number of NGF-positive neurons was observed only in 1-week-old mice. In contrast, a significant increase in the number of NGF-positive glia 1 cells was found in the striatum and frontal cortex of 4-week-old mice. Our double-labeled immunostaining showed that nNOS immunoreactivity was not found in PV-immunopositive interneurons. Furthermore, BDNF immunoreactivity was observed in both nNOS-positive and PV-positive interneurons in the striatum of 1- or 2-week-old mice. These results show that the maturation of nNOS-immunopositive interneurons precedes the maturation of PV-immunopositive interneurons in the striatum and frontal cortex during postnatal development. Furthermore, our results demonstrate that the expression of BDNF may play some role in the maturation of interneurons in the striatum and frontal cortex during postnatal development. Moreover, our findings suggest that the expression of NGF in glia cells may play some role in the maturation of glial cells and PV-positive interneurons in the striatum and frontal cortex during postnatal development.

Ikaros-1 couples cell cycle arrest of late striatal precursors with neurogenesis of enkephalinergic neurons

During central nervous system development, several transcription factors regulate the differentiation of progenitor cells to postmitotic neurons. Here we describe a novel role for Ikaros-1 in the generation of late-born striatal neurons. Our results show that Ikaros-1 is expressed in the boundary of the striatal germinal zone (GZ)/mantle zone (MZ), where it induces cell cycle arrest of neural progenitors by up-regulation of the cyclin-dependent kinase inhibitor (CDKi) p21(Cip1/Waf1). This effect is coupled with the neuronal differentiation of late precursors, which in turn is critical for the second wave of striatal neurogenesis that gives rise to matrix neurons. Consistently, Ikaros(-/-) mice had fewer striatal projecting neurons and, in particular, enkephalin (ENK)-positive neurons. In addition, overexpression of Ikaros-1 in primary striatal cultures increases the number of calbindin- and ENK-positive neurons. Our results also show that Ikaros-1 acts downstream of the Dlx family of transcription factors, insofar as its expression is lost in Dlx1/2 double knockout mice. However, we demonstrate that Ikaros-1 and Ebf-1 independently regulate the final determination of the two populations of striatal projection neurons of the matrix compartment, ENK- and substance P-positive neurons. In conclusion, our findings identify Ikaros-1 as a modulator of cell cycle exit of neural progenitors that gives rise to the neurogenesis of ENK-positive striatal neurons.

Colocalization of dopamine receptor subtypes with dopamine and cAMP-regulated phosphoprotein (DARPP-32) in rat brain

In the present study using indirect immunofluorescence immunohistochemistry, co-immunoprecipitation and western blot analysis we determined the colocalization of dopamine receptors 1-5 and dopamine and cAMP-regulated phosphoprotein (DARPP-32) in rat brain cortex and striatum. All five DR subtypes and DARPP-32 were expressed in rat brain cortex and striatum. DARPP-32 positive neurons displayed comparative colocalization with DR1-5. In cingulate cortex, the colocalization of DR subtypes was greatly different from frontal or temporal cortex. D1R is one of the most predominant subtypes which colocalized with DARPP-32 in cortex as well as striatum and followed by D2R, D3R, D4R and D5R. Amongst all DR subtypes D5R was coexpressed the least with DARPP-32 positive neurons. Consistent with immunohistochemical data, western blot analysis also reveals comparable distribution of DR subtypes and DARPP-32 in cortex and striatum. Colocalization studies were also supported by using co-immunoprecipitate assay displaying DARPP-32 expression in DR immunoprecipitate from tissue lysate prepared from cortex and striatum. Taken together our data support receptor specific association of DARPP-32 with DR subtypes that might shed new information in drugs of abuse and pathophysiology of neurodegenerative diseases as well as neuropsychiatric disorders such as schizophrenia.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Targeting of neurotrophic factors, their receptors, and signaling pathways in the developmental neurotoxicity of organophosphates in vivo and in vitro

Neurotrophic factors control neural cell differentiation and assembly of neural circuits. We previously showed that organophosphate pesticides differentially regulate members of the fibroblast growth factor (fgf) gene family. We administered chlorpyrifos and diazinon to neonatal rats on postnatal days 1-4 at doses devoid of systemic toxicity or growth impairment, and spanning the threshold for barely-detectable cholinesterase inhibition. We evaluated the impact on gene families for different classes of neurotrophic factors. Using microarrays, we examined the regional expression of mRNAs encoding the neurotrophins (ntfs), brain-derived neurotrophic factor (bdnf), nerve growth factor (ngf), the wnt and fzd gene families and the corresponding receptors. Chlorpyrifos and diazinon both had widespread effects on the fgf, ntf, wnt and fzd families but much less on the bdnf and ngf groups. However, the two organophosphates showed disparate effects on a number of key neurotrophic factors. To determine if the actions were mediated directly on differentiating neurons, we tested chlorpyrifos in PC12 cells, an in vitro model of neural cell development. Effects in PC12 cells mirrored many of those for members of the fgf, ntf and wnt families, as well as the receptors for the ntfs, especially during early differentiation, the stage known to be most susceptible to disruption by organophosphates. Our results suggest that actions on neurotrophic factors provide a mechanism for the developmental neurotoxicity of low doses of organophosphates, and, since effects on expression of the affected genes differed with test agent, may help explain regional disparities in effects and critical periods of vulnerability.

BH3-only proteins Bid and Bim(EL) are differentially involved in neuronal dysfunction in mouse models of Huntington's disease

Apoptosis, a cell death mechanism regulated by Bcl-2 family members, has been proposed as one of the mechanisms leading to neuronal loss in Huntington's disease (HD). Here we examined the regulation of Bcl-2 family proteins in three different mouse models of HD with exon 1 mutant huntingtin: the R6/1, the R6/1:BDNF+/-, and the Tet/HD94 in which the huntingtin transgene is controlled by the tetracycline-inducible system. Our results disclosed an increase in the levels of the BH3-only proteins Bid and Bim(EL) in the striatum of HD mouse models that was different depending on the stage of the disease. At 16 weeks of age, Bid was similarly enhanced in the striatum of R6/1 and R6/1:BDNF+/- mice, whereas Bim(EL) protein levels were enhanced only in R6/1:BDNF+/- mice. In contrast, at later stages of the disease, both genotypes displayed increased levels of Bid and Bim(EL) proteins. Furthermore, Bax, Bak, Bad, Bcl-2, and Bcl-x(L) proteins were not modified in any of the points analyzed. We next explored the potential reversibility of this phenomenon by analyzing conditional Tet/HD94 mice. Constitutive expression of the transgene resulted in increased levels of Bid and Bim(EL) proteins, and only the Bid protein returned to wild-type levels 5 months after mutant huntingtin shutdown. In conclusion, our results show that enhanced Bid protein levels represent an early mechanism linked to the continuous expression of mutant huntingtin that, together with enhanced Bim(EL), may be a reporter of the progress and severity of neuronal dysfunction.

Disruption of striatal glutamatergic transmission induced by mutant huntingtin involves remodeling of both postsynaptic density and NMDA receptor signaling

We study the striatal susceptibility to NMDA receptor (NMDAR)-mediated injury of two Huntington's disease (HD) transgenic mice: R6/1 and R6/1:BDNF(+/-). We found that R6/1:BDNF(+/-) mice--which express reduced levels of BDNF--were more resistant than R6/1 mice to intrastriatal injection of quinolinate. This increased resistance is related to a differential reduction in expression of NMDAR scaffolding proteins, MAGUKs (PSD-95, PSD-93, SAP-102 and SAP-97) but not to altered levels or synaptic location of NMDAR. A robust reorganization of postsynaptic density (PSD) was detected in HD transgenic mice, shown by a switch of PSD-93 by PSD-95 in PSD. Furthermore, NMDAR signaling pathways were affected by different BDNF levels in HD mice; we found a reduction of synaptic alpha CaMKII (but not of nNOS) in R6/1:BDNF(+/-) compared to R6/1 mice. The specific regulation of MAGUKs and alpha CaMKII in striatal neurons may reflect a protective mechanism against expression of mutant huntingtin exon-1.