Brain-derived neurotrophic factor regulates maturation of the DARPP-32 phenotype in striatal medium spiny neurons: studies in vivo and in vitro

hESC-derived striatal progenitors grafted into a Huntington's disease rat model support long-term functional motor recovery by differentiating, self-organizing and connecting into the lesioned striatum

BACKGROUND

Huntington's disease (HD) is a motor and cognitive neurodegenerative disorder due to prominent loss of striatal medium spiny neurons (MSNs). Cell replacement using human embryonic stem cells (hESCs) derivatives may offer new therapeutic opportunities to replace degenerated neurons and repair damaged circuits.

METHODS

With the aim to develop effective cell replacement for HD, we assessed the long-term therapeutic value of hESC-derived striatal progenitors by grafting the cells into the striatum of a preclinical model of HD [i.e., adult immunodeficient rats in which the striatum was lesioned by monolateral injection of quinolinic acid (QA)]. We examined the survival, maturation, self-organization and integration of the graft as well as its impact on lesion-dependent motor alterations up to 6 months post-graft. Moreover, we tested whether exposing a cohort of QA-lesioned animals to environmental enrichment (EE) could improve graft integration and function.

RESULTS

Human striatal progenitors survived up to 6 months after transplantation and showed morphological and neurochemical features typical of human MSNs. Donor-derived interneurons were also detected. Grafts wired in both local and long-range striatal circuits, formed domains suggestive of distinct ganglionic eminence territories and displayed emerging striosome features. Moreover, over time grafts improved complex motor performances affected by QA. EE selectively increased cell differentiation into MSN phenotype and promoted host-to-graft connectivity. However, when combined to the graft, the EE paradigm used in this study was insufficient to produce an additive effect on task execution.

CONCLUSIONS

The data support the long-term therapeutic potential of ESC-derived human striatal progenitor grafts for the replacement of degenerated striatal neurons in HD and suggest that EE can effectively accelerate the maturation and promote the integration of human striatal cells.

ASCL1-regulated DARPP-32 and t-DARPP stimulate small cell lung cancer growth and neuroendocrine tumour cell proliferation

Background

Small cell lung cancer (SCLC) is the most aggressive form of lung cancer, and new molecular insights are necessary for prognostic and therapeutic advances.

Methods

Dopamine and cAMP-regulated phosphoprotein, Mr 32000 (DARPP-32) and its N-terminally truncated splice variant, t-DARPP, were stably overexpressed or ablated in human DMS-53 and H1048 SCLC cells. Functional assays and immunoblotting were used to assess how DARPP-32 isoforms regulate SCLC cell growth, proliferation, and apoptosis. DARPP-32-modulated SCLC cells were orthotopically injected into the lungs of SCID mice to evaluate how DARPP-32 and t-DARPP regulate neuroendocrine tumour growth. Immunostaining for DARPP-32 proteins was performed in SCLC patient-derived specimens. Bioinformatics analysis and subsequent transcription assays were used to determine the mechanistic basis of DARPP-32-regulated SCLC growth.

Results

We demonstrate in mice that DARPP-32 and t-DARPP promote SCLC growth through increased Akt/Erk-mediated proliferation and anti-apoptotic signalling. DARPP-32 isoforms are overexpressed in SCLC patient-derived tumour tissue, but undetectable in physiologically normal lung. Achaete-scute homologue 1 (ASCL1) transcriptionally activates DARPP-32 isoforms in human SCLC cells.

Conclusions

We reveal new regulatory mechanisms of SCLC oncogenesis that suggest DARPP-32 isoforms may represent a negative prognostic indicator for SCLC and serve as a potential target for the development of new therapies.

Cellular Models: HD Patient-Derived Pluripotent Stem Cells

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by expanded polyglutamine (polyQ)-encoding repeats in the Huntingtin (HTT) gene. Traditionally, HD cellular models consisted of either patient cells not affected by disease or rodent neurons expressing expanded polyQ repeats in HTT. As these models can be limited in their disease manifestation or proper genetic context, respectively, human HD pluripotent stem cells (PSCs) are currently under investigation as a way to model disease in patient-derived neurons and other neural cell types. This chapter reviews embryonic stem cell (ESC) and induced pluripotent stem cell (iPSC) models of disease, including published differentiation paradigms for neurons and their associated phenotypes, as well as current challenges to the field such as validation of the PSCs and PSC-derived cells. Highlighted are potential future technical advances to HD PSC modeling, including transdifferentiation, complex in vitro multiorgan/system reconstruction, and personalized medicine. Using a human HD patient model of the central nervous system, hopefully one day researchers can tease out the consequences of mutant HTT (mHTT) expression on specific cell types within the brain in order to identify and test novel therapies for disease.

Darpp-32 and t-Darpp protein products of PPP1R1B: Old dogs with new tricks

The PPP1R1B gene is located on chromosome 17q12 (39,626,208-39,636,626[GRCh38/hg38]), which codes for multiple transcripts and two experimentally-documented proteins Darpp-32 and t-Darpp. Darpp-32 (Dopamine and cAMP Regulated Phosphoprotein), discovered in the early 1980s, is a protein whose phosphorylation is upregulated in response to cAMP in dopamine-responsive tissues in the brain. It's phosphorylation profile modulates its ability to bind and inhibit Protein Phosphatase 1 activity, which, in turn, controls the activity of hundreds of phosphorylated proteins. PPP1R1B knockout mice exhibit subtle learning defects. In 2002, the second protein product of PPP1R1B was discovered in gastric cancers: t-Darpp (truncated Darpp-32). The start codon of t-Darpp is amino acid residue 37 of Darpp-32 and it lacks the domain responsible for modulating Protein Phosphatase 1. Aside from gastric cancers, t-Darpp and/or Darpp-32 is overexpressed in tumor cells from breast, colon, esophagus, lung and prostate tissues. More than one research team has demonstrated that these proteins, through mechanisms that to date remain cloudy, activate AKT, a protein whose phosphorylation leads to cell survival and blocks apoptosis. Furthermore, in Her2 positive breast cancers (an aggressive form of breast cancer), t-Darpp/Darpp-32 overexpression causes resistance to the frequently-administered anti-Her2 drug, trastuzumab (Herceptin), likely through AKT activation. Here we briefly describe how Darpp-32 and t-Darpp were discovered and report on the current state of knowledge of their involvement in cancers. We present a case for the development of an anti-t-Darpp therapeutic agent and outline the unique challenges this endeavor will likely encounter.

Age-related changes in STriatal-Enriched protein tyrosine Phosphatase levels: Regulation by BDNF

Recent results indicate that STriatal-Enriched protein tyrosine Phosphatase (STEP) levels are regulated by brain-derived neurotrophic factor (BDNF), whose expression changes during postnatal development and aging. Here, we studied STEP ontogeny in mouse brain and changes in STEP with age with emphasis on the possible regulation by BDNF. We found that STEP expression increased during the first weeks of life, reaching adult levels by 2-3weeks of age in the striatum and cortex, and by postnatal day (P) 7 in the hippocampus. STEP protein levels were unaffected in BDNF^+/- mice, but were significantly reduced in the striatum and cortex, but not in the hippocampus, of BDNF^-/- mice at P7 and P14. In adult wild-type mice there were no changes in cortical and hippocampal STEP₆₁ levels with age. Conversely, striatal STEP levels were reduced from 12months of age, correlating with higher ubiquitination and increased BDNF content and signaling. Lower STEP levels in older mice were paralleled by increased phosphorylation of its substrates. Since altered STEP levels are involved in cellular malfunctioning events, its reduction in the striatum with increasing age should encourage future studies of how this imbalance might participate in the aging process.

Laquinimod treatment in the R6/2 mouse model

The transgenic mouse model R6/2 exhibits Huntington's disease (HD)-like deficits and basic pathophysiological similarities. We also used the pheochromocytoma-12 (PC12)-cell-line-model to investigate the effect of laquinimod on metabolic activity. Laquinimod is an orally administered immunomodulatory substance currently under development for the treatment of multiple sclerosis (MS) and HD. As an essential effect, increased levels of BDNF were observed. Therefore, we investigated the therapeutic efficacy of laquinimod in the R6/2 model, focusing on its neuroprotective capacity. Weight course and survival were not influenced by laquinimod. Neither were any metabolic effects seen in an inducible PC12-cell-line model of HD. As a positive effect, motor functions of R6/2 mice at the age of 12 weeks significantly improved. Preservation of morphologically intact neurons was found after treatment in the striatum, as revealed by NeuN, DARPP-32, and ubiquitin. Biochemical analysis showed a significant increase in the brain-derived neurotrophic factor (BDNF) level in striatal but not in cortical neurons. The number of mutant huntingtin (mhtt) and inducible nitric oxide synthase (iNOS) positive cells was reduced in both the striatum and motor cortex following treatment. These findings suggest that laquinimod could provide a mild effect on motor function and striatal histopathology, but not on survival. Besides influences on the immune system, influence on BDNF-dependent pathways in HD are discussed.

Pluripotent stem cell-derived neurons for transplantation in Huntington's disease

Pluripotent stem cells present a potentially unlimited source of cells for regenerative medicine, providing that they can be efficiently and accurately differentiated to the target cell type. The principle target cell for Huntington's disease is the striatal medium spiny neuron. In this chapter, we review strategies for directing medium spiny neuron differentiation, based on known developmental principles, and we discuss the remaining hurdles on the road to engineering such cells for therapeutic application in Huntington's disease.

The involvement of DARPP-32 in the pathophysiology of schizophrenia

Schizophrenia is one of the most devastating heterogeneous psychiatric disorders. The dopamine hypothesis is the longest standing pathoetiologic theory of schizophrenia based on neurochemical evidences of elevated brain striatal dopamine synthesis capacity and increased dopamine release in response to stress. Dopamine and cyclic AMP-regulated phosphoprotein of relative molecular mass 32,000 (DARPP-32) is a cytosolic protein highly enriched in the medium spiny neurons of the neostriatum, considered as the most important integrator between the cortical input and the basal ganglia, and associated with motor control. Accumulating evidences has indicated the involvement of DARPP-32 in the development of schizophrenia; i. DARPP-32 phosphorylation is regulated by several neurotransmitters, including dopamine and glutamate, neurotransmitters implicated in schizophrenia pathogenesis; ii. decrease of both total and phosphorylated DARPP-32 in the prefrontal cortex are observed in schizophrenic animal models; iii. postmortem brain studies indicated decreased expression of DARPP-32 protein in the superior temporal gyrus and dorsolateral prefrontal cortex in patients with schizophrenia; iv. DARPP-32 phosphorylation is increased upon therapy with antipsychotic drugs, such as haloperidol and risperidone which improve behavioral performance in experimental animal models and patients; v. Genetic analysis of the gene coding for DARPP-32 propose an association with schizophrenia. Cumulatively, these findings implicate DARPP-32 protein in schizophrenia and propose it as a potential therapeutic target. Here, we summarize the possible roles of DARPP-32 during the development of schizophrenia and make some recommendations for future research. We propose that DARPP-32 and its interacting proteins may serve as potential therapeutic targets in the treatment of schizophrenia.

Muscarinic acetylcholine M4 receptors play a critical role in oxotremorine-induced DARPP-32 phosphorylation at threonine 75 in isolated medium spiny neurons

Dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) play essential roles in dopamine (DA) transmission in the striatum. It is suggested that a link exists between muscarinic acetylcholine receptors (mAChRs) and DA/DARPP-32 signaling, but the molecular mechanisms mediating this relationship have not been elucidated. The predominant mAChRs subtypes in the striatum are M₁ and M₄. In this study, we investigated the functions of these two receptors, particularly M₄, in regulating cAMP production and DARPP-32 phosphorylation in rat striatal medium spiny neurons (MSNs). We used time-resolved fluorescence resonance energy transfer, immunofluorescence confocal microscopy, and western blot assays. In cultured intact MSNs, we confirmed that muscarinic M₁ and M₄ receptors were highly expressed. Notably, M₄ receptors were co-expressed with D₁ receptors in only a portion of the cultured MSNs. The nonselective muscarinic agonist oxotremorine M (OX) slightly enhanced cAMP production, but this effect was independent of M₁ or M₄ receptors. However, OX directly participated in DARPP-32 phosphorylation, phosphorylating DARPP-32 at Thr75 (the CDK5 site) and concomitantly de-phosphorylating DARPP-32 at Thr34 (the PKA site) in virtually cultured MSNs, whereas APO phosphorylated DARPP-32 at both Thr34 and Thr75. The OX-induced time-dependent increase in DARPP-32 phosphorylation at Thr75 was accompanied by increased p35 and CDK5 activity. Specifically, elevated immunoreactivity for phospho-DARPP-32-Thr75 and p35 was detected in M₄ receptor-expressing MSNs. Both genetic knockdown and pharmacologic inhibition of M₄ receptors with MT3, an M₄ receptor-selective antagonist, decreased the OX-induced DARPP-32-Thr75 phosphorylation in MSNs. These results indicate that the M₄ muscarinic receptor plays a critical role in modulating phosphorylation of DARPP-32-Thr75 in MSNs. The results suggest that M₄ receptor activation acts antagonistically with dopamine D₁-like receptors within the striatum, and indicate that M₄ receptors may be a potential target for the treatment of Parkinson's disease and other relevant central nervous system disorders.

Impaired TrkB Signaling Underlies Reduced BDNF-Mediated Trophic Support of Striatal Neurons in the R6/2 Mouse Model of Huntington's Disease

The principal projection neurons of the striatum are critically dependent on an afferent supply of brain derived neurotrophic factor (BDNF) for neurotrophic support. These neurons express TrkB, the cognate receptor for BDNF, which activates signaling pathways associated with neuronal survival and phenotypic maintenance. Impairment of the BDNF-TrkB pathway is suspected to underlie the early dysfunction and prominent degeneration of striatal neurons in Huntington disease (HD). Some studies in HD models indicate that BDNF supply is reduced, while others suggest that TrkB signaling is impaired earlier in disease progression. It remains important to determine whether a primary defect in TrkB signaling underlies reduced neurotrophic support and the early vulnerability of striatal neurons in HD. Using the transgenic R6/2 mouse model of HD we found that prior to striatal degeneration there are early deficits in striatal protein levels of activated phospho-TrkB and the downstream-regulated protein DARPP-32. In contrast, total-TrkB and BDNF protein levels remained normal. Primary neurons cultured from R6/2 striatum exhibited reduced survival in response to exogenous BDNF applications. Moreover, BDNF activation of phospho-TrkB and downstream signal transduction was attenuated in R6/2 striatal cultures. These results suggest that neurotrophic support of striatal neurons is attenuated early in disease progression due to defects in TrkB signal transduction in the R6/2 model of HD.

How to make striatal projection neurons

Medium spiny neurons (MSNs) are the main projection neurons of the striatum and are preferentially lost in Huntington's disease (HD). With no current cure for this neurodegenerative disorder, the specificity of neuronal loss in the striatum makes cell transplantation therapy an attractive avenue for its treatment. Also, given that MSNs are particularly vulnerable in HD, it is necessary to understand why these neurons degenerate in order to develop new therapeutic options. Both approaches require access to human MSN progenitors and their mature neuronal derivatives. Human embryonic stem cells and HD patient induced pluripotent stem cells (together referred to as hPSCs) may serve as an unlimited source of such tissue if they can be directed toward authentic striatal neuronal lineage. Understanding the MSN differentiation pathway in the brain is therefore of paramount importance for the generation of accurate protocols to obtain striatal cells in vitro. The focus of this mini review will be on striatal development and current methods to generate MSNs from hPSCs.

Dystonia type 6 gene product Thap1: identification of a 50 kDa DNA-binding species in neuronal nuclear fractions

Mutations in THAP1 result in dystonia type 6, with partial penetrance and variable phenotype. The goal of this study was to examine the nature and expression pattern of the protein product(s) of the Thap1 transcription factor (DYT6 gene) in mouse neurons, and to study the regional and developmental distribution, and subcellular localization of Thap1 protein. The goal was accomplished via overexpression and knock-down of Thap1 in the HEK293T cell line and in mouse striatal primary cultures and western blotting of embryonic Thap1-null tissue. The endogenous and transduced Thap1 isoforms were characterized using three different commercially available anti-Thap1 antibodies and validated by immunoprecipitation and DNA oligonucleotide affinity chromatography. We identified multiple, novel Thap1 species of apparent M_r 32 kDa, 47 kDa, and 50–52 kDa in vitro and in vivo, and verified the previously identified species at 29–30 kDa in neurons. The Thap1 species at the 50 kDa size range was exclusively detected in murine brain and testes and were located in the nuclear compartment. Thus, in addition to the predicted 25 kDa apparent M_r, we identified Thap1 species with greater apparent M_r that we speculate may be a result of posttranslational modifications. The neural localization of the 50 kDa species and its nuclear compartmentalization suggests that these may be key Thap1 species controlling neuronal gene transcription. Dysfunction of the neuronal 50 kDa species may therefore be implicated in the pathogenesis of DYT6.

Use of Genetically Altered Stem Cells for the Treatment of Huntington's Disease

Transplantation of stem cells for the treatment of Huntington’s disease (HD) garnered much attention prior to the turn of the century. Several studies using mesenchymal stem cells (MSCs) have indicated that these cells have enormous therapeutic potential in HD and other disorders. Advantages of using MSCs for cell therapies include their ease of isolation, rapid propagation in culture, and favorable immunomodulatory profiles. However, the lack of consistent neuronal differentiation of transplanted MSCs has limited their therapeutic efficacy to slowing the progression of HD-like symptoms in animal models of HD. The use of MSCs which have been genetically altered to overexpress brain derived neurotrophic factor to enhance support of surviving cells in a rodent model of HD provides proof-of-principle that these cells may provide such prophylactic benefits. New techniques that may prove useful for cell replacement therapies in HD include the use of genetically altering fate-restricted cells to produce induced pluripotent stem cells (iPSCs). These iPSCs appear to have certain advantages over the use of embryonic stem cells, including being readily available, easy to obtain, less evidence of tumor formation, and a reduced immune response following their transplantation. Recently, transplants of iPSCs have shown to differentiate into region-specific neurons in an animal model of HD. The overall successes of using genetically altered stem cells for reducing neuropathological and behavioral deficits in rodent models of HD suggest that these approaches have considerable potential for clinical use. However, the choice of what type of genetically altered stem cell to use for transplantation is dependent on the stage of HD and whether the end-goal is preserving endogenous neurons in early-stage HD, or replacing the lost neurons in late-stage HD. This review will discuss the current state of stem cell technology for treating the different stages of HD and possible future directions for stem-cell therapy in HD.

Induction of DARPP-32 by brain-derived neurotrophic factor in striatal neurons in vitro is modified by histone deacetylase inhibitors and Nab2

Neurotrophins and modifiers of chromatin acetylation and deacetylation participate in regulation of transcription during neuronal maturation and maintenance. The striatal medium spiny neuron is supported by cortically-derived brain derived neurotrophic factor and is the most vulnerable neuron in Huntington’s disease, in which growth factor and histone deacetylase activity are both disrupted. We examined the ability of three histone deacetylase inhibitors, trichostatin A, valproic acid and Compound 4 b, alone and combined with brain derived neurotrophic factor (BDNF), to promote phenotypic maturation of striatal medium spiny neurons in vitro. Exposure of these neurons to each of the three compounds led to an increase in overall histone H3 and H4 acetylation, dopamine and cyclic AMP-regulated phosphoprotein, 32 kDa (DARPP-32) mRNA and protein, and mRNA levels of other markers of medium spiny neuron maturation. We were, however, unable to prove that HDAC inhibitors directly lead to remodeling of Ppp1r1b chromatin. In addition, induction of DARPP-32 by brain-derived neurotrophic factor was inhibited by histone deacetylase inhibitors. Although BDNF-induced increases in pTrkB, pAkt, pERK and Egr-1 were unchanged by combined application with VPA, the increase in DARPP-32 was relatively diminished. Strikingly, the NGF1A-binding protein, Nab2, was induced by BDNF, but not in the presence of VPA or TSA. Gel shift analysis showed that α-Nab2 super-shifted a band that is more prominent with extract derived from BDNF-treated neurons than with extracts from cultures treated with VPA alone or VPA plus BDNF. In addition, overexpression of Nab2 induced DARPP-32. We conclude that histone deacetylase inhibitors inhibit the induction of Nab2 by BDNF, and thereby the relative induction of DARPP-32.

Small-molecule TrkB receptor agonists improve motor function and extend survival in a mouse model of Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative disease characterized by abnormal motor coordination, cognitive decline and psychiatric disorders. This disease is caused by an expanded CAG trinucleotide repeat in the gene encoding the protein huntingtin. Reduced levels of brain-derived neurotrophic factor (BDNF) in the brain, which results from transcriptional inhibition and axonal transport deficits mediated by mutant huntingtin, have been suggested as critical factors underlying selective neurodegeneration in both HD patients and HD mouse models. BDNF activates its high-affinity receptor TrkB and promotes neuronal survival; restoring BDNF signaling is thus of particular therapeutic interest. In the present study, we evaluated the ability of a small-molecule TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) and its synthetic derivative 4'-dimethylamino-7,8- dihydroxyflavone (4'-DMA-7,8-DHF) to protect neurons in the well-characterized N171-82Q HD mouse model. We found that chronic administration of 7, 8-DHF (5 mg/kg) or 4'-DMA-7,8-DHF (1 mg/kg) significantly improved motor deficits, ameliorated brain atrophy and extended survival in these N171-82Q HD mice. Moreover, 4'-DMA-7,8-DHF preserved DARPP32 levels in the striatum and rescued mutant huntingtin-induced impairment of neurogenesis in the N171-82Q HD mice. These data highlight consideration of TrkB as a therapeutic target in HD and suggest that small-molecule TrkB agonists that penetrate the brain have high potential to be further tested in clinical trials of HD.

Egr-1 induces DARPP-32 expression in striatal medium spiny neurons via a conserved intragenic element

DARPP-32 (dopamine and adenosine 3', 5'-cyclic monophosphate cAMP-regulated phosphoprotein, 32 kDa) is a striatal-enriched protein that mediates signaling by dopamine and other first messengers in the medium spiny neurons. The transcriptional mechanisms that regulate striatal DARPP-32 expression remain enigmatic and are a subject of much interest in the efforts to induce a striatal phenotype in stem cells. We report the identification and characterization of a conserved region, also known as H10, in intron IV of the gene that codes for DARPP-32 (Ppp1r1b). This DNA sequence forms multiunit complexes with nuclear proteins from adult and embryonic striata of mice and rats. Purification of proteins from these complexes identified early growth response-1 (Egr-1). The interaction between Egr-1 and H10 was confirmed in vitro and in vivo by super-shift and chromatin immunoprecipitation assays, respectively. Importantly, brain-derived neurotrophic factor (BDNF), a known inducer of DARPP-32 and Egr-1 expression, enhanced Egr-1 binding to H10 in vitro. Moreover, overexpression of Egr-1 in primary striatal neurons induced the expression of DARPP-32, whereas a dominant-negative Egr-1 blocked DARPP-32 induction by BDNF. Together, this study identifies Egr-1 as a transcriptional activator of the Ppp1r1b gene and provides insight into the molecular mechanisms that regulate medium spiny neuron maturation.

Huntington's disease and the striatal medium spiny neuron: cell-autonomous and non-cell-autonomous mechanisms of disease

Huntington's disease is an autosomal dominant disorder caused by a mutation in the gene encoding the protein huntingtin on chromosome 4. The mutation is an expanded CAG repeat in the first exon, encoding a polyglutamine tract. If the polyglutamine tract is > 40, penetrance is 100% and death is inevitable. Despite the widespread expression of huntingtin, HD has long been considered primarily as a disease of the striatum. It is characterized by selective vulnerability with dysfunction followed by death of the medium size spiny neuron. Considerable effort is being expended to determine whether striatal damage is cell-autonomous, non-cell-autonomous, requiring cell-cell and region to region communication, or both. We review data supporting both mechanisms. We also attempt to organize the data into common mechanisms that may arise outside the medium, spiny neuron, but ultimately have their greatest impact in the striatum.

Neuroprotective role of Sirt1 in mammalian models of Huntington's disease through activation of multiple Sirt1 targets

Huntington's disease is a fatal neurodegenerative disorder caused by an expanded polyglutamine repeat in huntingtin (HTT) protein. We previously showed that calorie restriction ameliorated Huntington's disease pathogenesis and slowed disease progression in mice that model Huntington's disease (Huntington's disease mice). We now report that overexpression of sirtuin 1 (Sirt1), a mediator of the beneficial metabolic effects of calorie restriction, protects neurons against mutant HTT toxicity, whereas reduction of Sirt1 exacerbates mutant HTT toxicity. Overexpression of Sirt1 improves motor function, reduces brain atrophy and attenuates mutant-HTT-mediated metabolic abnormalities in Huntington's disease mice. Further mechanistic studies suggested that Sirt1 prevents the mutant-HTT-induced decline in brain-derived neurotrophic factor (BDNF) concentrations and the signaling of its receptor, TrkB, and restores dopamine- and cAMP-regulated phosphoprotein, 32 kDa (DARPP32) concentrations in the striatum. Sirt1 deacetylase activity is required for Sirt1-mediated neuroprotection in Huntington's disease cell models. Notably, we show that mutant HTT interacts with Sirt1 and inhibits Sirt1 deacetylase activity, which results in hyperacetylation of Sirt1 substrates such as forkhead box O3A (Foxo3a), thereby inhibiting its pro-survival function. Overexpression of Sirt1 counteracts the mutant-HTT-induced deacetylase deficit, enhances the deacetylation of Foxo3a and facilitates cell survival. These findings show a neuroprotective role for Sirt1 in mammalian Huntington's disease models and open new avenues for the development of neuroprotective strategies in Huntington's disease.

Cellular therapy and induced neuronal replacement for Huntington's disease

Huntington's disease (HD) is an inherited, relentlessly progressive neurodegenerative disease with an invariably fatal outcome. HD is inherited in an autosomal dominant fashion, and is characterized pathologically by the loss of cortical and striatal neurons, and clinically by involuntary choreiform movements accompanied by progressive cognitive impairment and emotional lability. The disorder is caused by an expanded cystosine adenine guanine (CAG) tri-nucleotide repeat encoding polyglutamine (polyQ) in the first exon of the Huntingtin gene. There is a correlation between the number of CAG repeats and disease onset, such that in patients with CAG repeat lengths of 36 to 60, disease symptoms typically manifest after 35 years of age, whereas CAG repeat lengths >60 yield the more severe juvenile form of the disease. Even though mutant huntingtin is expressed throughout the brain, it is characterized by the selective degeneration of medium spiny neurons of the caudate and putamen, which heralds more widespread neuronal degeneration with disease progression. The mechanisms of cell dysfunction and death in HD have been the subjects of a number of studies, which have led to therapeutic strategies largely based on the amelioration of mutant huntingtin-related metabolic impairment and cellular toxicity. Each of these approaches has aimed to delay or stop the preferential degeneration of medium spiny neurons early in the disease course. Yet, in later stages of the disease, after cell death has become prominent, cell replacement therapy (whether by direct cell transplantation or by the mobilization of endogenous progenitors) may comprise a stronger potential avenue for therapy. In this review, we will consider recent progress in the transplantation of fetal striatal cells to the HD brain, as well as emerging alternative sources for human striatal progenitor cells. We will then consider the potential application of gene therapy toward the induction of striatal neurogenesis and neuronal recruitment, with an eye toward its potential therapeutic use in HD.

An intrastriatal brain-derived neurotrophic factor infusion restores striatal gene expression in Bdnf heterozygous mice

Reduction in the amount of brain-derived neurotrophic factor (BDNF) in corticostriatal afferents is thought to contribute to the vulnerability of medium spiny striatal neurons in Huntington's disease. In young Bdnf heterozygous ((+/-)) mice, striatal medium spiny neurons (MSNs) express less preprodynorphin (PPD), preproenkephalin (PPE), and D(3) receptor mRNA than wildtype mice. Further, in aged Bdnf (+/-) mice, opioid, trkB receptor, and glutamic acid decarboxylase gene expression, and the number of dendritic spines on MSNs are more affected than in wildtype or younger Bdnf (+/-) mice. In this study, the possibility that intrastriatal infusions of BDNF would elevate gene expression in the striatum of Bdnf (+/-) mice was investigated. Wildtype and Bdnf (+/-) mice received a single, bilateral microinjection of BDNF or PBS into the dorsal striatum. Mice were killed 24 h later and semi-quantitative in situ hybridization histochemical analysis confirmed that PPD, PPE, and D(3) receptor mRNA was less in the caudate-putamen (CPu) and nucleus accumbens (NAc) core of Bdnf (+/-) mice than in wildtype mice. A BDNF infusion increased PPD mRNA in the CPu and NAc core of wildtype mice and restored PPD mRNA levels in the NAc core of Bdnf (+/-) mice. BDNF also restored the gene expression of PPE in the CPu of Bdnf (+/-) mice to wildtype levels; however, PPE mRNA in the NAc did not differ among groups. Furthermore, BDNF increased D(3) receptor mRNA in the NAc core of wildtype and Bdnf (+/-) mice. These results demonstrate that exogenous BDNF restores striatal opioid and D(3)R gene expression in mice with genetically reduced levels of endogenous BDNF.

Assemblies of glutamate receptor subunits with post-synaptic density proteins and their alterations in Parkinson's disease

N-methyl-D-aspartate (NMDA) receptors have been implicated as a mediator of neuronal injury associated with many neurological disorders including ischemia, epilepsy, brain trauma, dementia and neurodegenerative disorders such as Parkinson's disease (PD) and Alzheimer's disease. To this, non-selective NMDA receptor antagonists have been tried and have been shown to be effective in many experimental animal models of disease, and some of these compounds have moved into clinical trials. However, the initial enthusiasm for this approach has waned, because the therapeutic index for most NMDA antagonists is quite poor, with significant adverse effects at clinically effective doses, thus limiting their utility. More recently, the concept that the exact pathways downstream NMDA receptor activation could represent a key variable element among neurological disorders has been put forward. In particular, variations in NMDA receptor subunit composition could be important in different disorders, both in the pathophysiological mechanisms of cell death and in the application of specific symptomatic therapies. As to PD, NMDA receptor complex has been shown to be altered in experimental models of parkinsonism and in PD in humans. Further, it has become increasingly evident that the NMDA receptor complex is intimately involved in the regulation of corticostriatal long-term potentiation, which is altered in experimental parkinsonism. The following sections will examine the modifications of specific NMDA receptor subunits as well as post-synaptic associated signalling complex including kinases and scaffolding proteins in experimental parkinsonism. These findings may allow the identification of specific molecular targets whose pharmacological or genetic manipulation might lead to innovative therapies for PD.

Calcium signaling cascade links dopamine D1-D2 receptor heteromer to striatal BDNF production and neuronal growth

Although the perturbation of either the dopaminergic system or brain-derived neurotrophic factor (BDNF) levels has been linked to important neurological and neuropsychiatric disorders, there is no known signaling pathway linking these two major players. We found that the exclusive stimulation of the dopamine D1-D2 receptor heteromer, which we identified in striatal neurons and adult rat brain by using confocal FRET, led to the activation of a signaling cascade that links dopamine signaling to BDNF production and neuronal growth through a cascade of four steps: (i) mobilization of intracellular calcium through Gq, phospholipase C, and inositol trisphosphate, (ii) rapid activation of cytosolic and nuclear calcium/calmodulin-dependent kinase IIalpha, (iii) increased BDNF expression, and (iv) accelerated morphological maturation and differentiation of striatal neurons, marked by increased microtubule-associated protein 2 production. These effects, although robust in striatal neurons from D5(-/-) mice, were absent in neurons from D1(-/-) mice. We also demonstrated that this signaling cascade was activated in adult rat brain, although with regional specificity, being largely limited to the nucleus accumbens. This dopaminergic pathway regulating neuronal growth and maturation through BDNF may have considerable significance in disorders such as drug addiction, schizophrenia, and depression.

Differential expression of neurotrophins in postnatal C57BL/6 mice striatum

Neurotrophin expression in early stages of development is crucial for brain assembly and function. In particular, postnatal expression of neurotrophins has not been well documented in the neostriatum and in general neurotrophins or their receptor mRNA's are normally reported, but not protein expression. In the present study, immunocytochemical expression of BDNF, NT-3 and NT-4/5 was characterized in striatal tissue of C57BL/6 mice at postnatal days 10^th (P10), 21^st (P21), 42^nd (P42) and 80^th (P80).

We found that the expression of BDNF diminished along the postnatal time course we evaluated, while staining for NT-4 increased up to age P42 and remained constant, thereafter in the cell's soma. In contrast, NT-3 was first expressed in the neostriatal bundles and later on, in neostriatal cell somas. These results provide information about differences in the spatial and temporal expression of each neurotrophin in the neostriatum during the first 80^th postnatal days.

RT-PCR procedures were also carried out to further determine whether protein levels of neurotrophins observed in the neostriatum were under control of gene expression. All neurotrophin mRNAs were expressed and only mRNA^BDNF was reduced during the postnatal evaluated days.

Differences in temporal expression of neurotrophins may be related to the heterochronic development of neostriatal cell populations, but also with the specificity of each neurotrophin modulating different neuronal targets.

Neocortical expression of mutant huntingtin is not required for alterations in striatal gene expression or motor dysfunction in a transgenic mouse

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disease caused by an expanded polyglutamine tract in the ubiquitously expressed huntingtin protein. Clinically, HD is characterized by motor, cognitive and psychiatric deficits. Striking degeneration of the striatum is observed in HD with the medium spiny neurons (MSNs) being the most severely affected neuronal subtype. Dysfunction of MSNs is marked by characteristic changes in gene expression which precede neuronal death. The ubiquitous expression of the huntingtin protein raises the question as to whether the selective vulnerability of the MSN is cell-autonomous, non-cell-autonomous, or a combination thereof. In particular, growing evidence suggests that abnormalities of the cortex and corticostriatal projections may be primary causes of striatal vulnerability. To examine this issue, we developed transgenic mice that, within the forebrain, selectively express a pathogenic huntingtin species in the MSNs, specifically excluding the neocortex. These mice develop a number of abnormalities characteristic of pan-cellular HD mouse models, including intranuclear inclusion bodies, motor impairment, and changes in striatal gene expression. As this phenotype develops in the presence of normal levels of brain-derived neurotrophic factor and its major striatal receptor, tropomyosin-related kinase B, these data represent the first demonstration of in vivo cell-autonomous transcriptional dysregulation in an HD mouse model. Furthermore, our findings suggest that therapies targeted directly to the striatum may be efficacious at reversing some of the molecular abnormalities present in HD.

Phosphatidylinositide 3-kinase and protein kinase C zeta mediate retinoic acid induction of DARPP-32 in medium size spiny neurons in vitro

Mature striatal medium size spiny neurons express the dopamine and cAMP-regulated phosphoprotein, 32 kDa (DARPP-32), but little is known about the mechanisms regulating its levels, or the specification of fully differentiated neuronal subtypes. Cell extrinsic molecules that increase DARPP-32 mRNA and/or protein levels include retinoic acid (RA), brain-derived neurotrophic factor, and estrogen (E(2)). We now demonstrate that RA regulates DARPP-32 mRNA and protein in primary striatal neuronal cultures. Furthermore, DARPP-32 induction by RA in vitro requires phosphatidylinositide 3-kinase, but is independent of tropomyosin-related kinase B, cyclin-dependent kinase 5, and protein kinase B. Using pharmacologic inhibitors of various isoforms of protein kinase C (PKC), we also demonstrate that DARPP-32 induction by RA in vitro is dependent on PKC zeta (PKCzeta). Thus, the signal transduction pathways mediated by RA are very different than those mediating DARPP-32 induction by brain-derived neurotrophic factor. These data support the presence of multiple signal transduction pathways mediating expression of DARPP-32 in vitro, including a novel, important pathway via which phosphatidylinositide 3-kinase regulates the contribution of PKCzeta.

Dopamine D1 receptor-induced signaling through TrkB receptors in striatal neurons

In addition to its role as a neurotransmitter, dopamine can stimulate neurite outgrowth and morphological effects upon primary neurons. To investigate the signal transduction mechanisms used by dopamine in developing striatal neurons, we focused upon the effects of activating the dopamine D1 receptor. Using the D1 receptor agonist SKF38393, we found that Trk neurotrophin receptors were activated in embryonic day 18 striatal neurons. K-252a, a Trk tyrosine kinase inhibitor, and a dopamine D1 receptor antagonist could block the effects of SKF38393. The increase in TrkB phosphorylation was not the result of increased neurotrophin production. Induction of TrkB activity by SKF38393 was accompanied by the phosphorylation of several Trk signaling proteins, including phospholipase Cgamma, Akt, and MAPK. Biotinylation experiments followed by immunostaining by phospho-TrkB-specific antibodies indicated that the mechanism involved increased TrkB surface expression by dopamine D1 receptor activation. This increase in cell surface TrkB expression was dependent upon an increase in intracellular Ca(2+). These results indicate that stimulation of dopamine D1 receptors can be coupled to the neurotrophin receptor signaling to mediate the effects of dopamine upon striatal neurons.

Trk: a neuromodulator of age-specific behavioral and neurochemical responses to cocaine in mice

Responses to psychostimulants vary with age, but the molecular etiologies of these differences are largely unknown. The goal of the present research was to identify age-specific behavioral and molecular adaptations to cocaine and to elucidate the mechanisms involved therein. Postweanling, periadolescent, and adult male CD-1 mice were exposed to cocaine (20 mg/kg) for 7 d. The rewarding effects of cocaine were assessed, as were the response to a Trk antagonist and the regulation of dopamine and cAMP-regulated phosphoprotein, 32 kDa (DARPP-32). Cocaine was rewarding in both periadolescent and adult mice using a conditioned place preference procedure. In contrast, postweanling mice failed to demonstrate significant cocaine-induced place preference. Because components of the neurotrophin system including brain-derived neurotrophic factor and TrkB are developmentally regulated, their role in the age-specific effects of cocaine was determined using the Trk receptor antagonist K252a. Postweanling mice that received K252a before daily cocaine showed a significant place preference to the cocaine-paired environment that was not seen in the absence of K252a. DARPP-32 protein levels were significantly upregulated in the lateral region of the caudate-putamen exclusively in postweanling mice after chronic cocaine. Daily pretreatment with K252a attenuated the induction of DARPP-32 in the postweanling striatum. These data indicate that Trk neurotransmission plays a role in age-specific behavioral and molecular responses to cocaine and concurrently modulates DARPP-32 levels.

Induction of neostriatal neurogenesis slows disease progression in a transgenic murine model of Huntington disease

Ependymal overexpression of brain-derived neurotrophic factor (BDNF) stimulates neuronal addition to the adult striatum, from subependymal progenitor cells. Noggin, by suppressing subependymal gliogenesis and increasing progenitor availability, potentiates this process. We asked whether BDNF/Noggin overexpression might be used to recruit new striatal neurons in R6/2 huntingtin transgenic mice. R6/2 mice injected with adenoviral BDNF and adenoviral Noggin (AdBDNF/AdNoggin) recruited BrdU(+)betaIII-tubulin(+) neurons, which developed as DARPP-32(+) and GABAergic medium spiny neurons that expressed either enkephalin or substance P and extended fibers to the globus pallidus. Only AdBDNF/AdNoggin-treated R6/2 mice harbored migrating doublecortin-defined neuroblasts in their striata, and the new neurons expressed p27 as a marker of mitotic quiescence after parenchymal integration. AdBDNF/AdNoggin-treated R6/2 mice sustained their rotarod performance and open-field activity and survived longer than did AdNull-treated and untreated controls. Neither motor performance nor survival improved in R6/2 mice treated only with AdBDNF, and intraventricular infusion of the mitotic inhibitor Ara-C completely blocked the performance and survival effects of AdBDNF/AdNoggin, suggesting that the benefits of AdBDNF/AdNoggin derived from neuronal addition. Thus, BDNF and Noggin induced striatal neuronal regeneration, delayed motor impairment, and extended survival in R6/2 mice, suggesting a new therapeutic strategy in Huntington disease.

Group I mGluR5 metabotropic glutamate receptors regulate proliferation of neuronal progenitors in specific forebrain developmental domains

Major classical neurotransmitters including GABA and glutamate play novel morphogenic roles during development of the mammalian CNS. During forebrain neurogenesis, glutamate regulates neuroblast proliferation in different germinal domains using receptor subtype-specific mechanisms. For example, ionotropic N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptors mediate distinct proliferative effects in ventral or dorsal forebrain germinal domains, and regulate the correct number of neurons that populate the striatum or cerebral cortex. Recent work suggests metabotropic receptors may also mediate glutamate's proliferative effects. Group I mGluR5 receptor subtypes are highly expressed in forebrain germinal zones. Using in vitro and in vivo methods, we demonstrate mGluR5 receptor activation plays an important role in neuroblast proliferation in the ventral telencephalon, and helps determine the complement of striatum projection neurons. mGluR5 receptor-mediated effects on striatal neuronal progenitors are restricted mainly to early cycling populations in the ventricular zone, with little effect on secondary proliferative populations in the subventricular zone. In contrast to proliferative effects in the ventral telencephalon, mGluR5 receptors do not modulate proliferation of dorsal telencephalon-derived cortical neuroblasts. Heterogeneous domain-specific proliferative effects of glutamate-mediated by specific receptor subtypes provide an important developmental mechanism allowing generation of the correct complement of neuronal subtypes that populate the mammalian forebrain.

AKT and CDK5/p35 Mediate Brain-derived Neurotrophic Factor Induction of DARPP-32 in Medium Size Spiny Neurons in Vitro

Mature striatal medium size spiny neurons express the dopamine and cyclic AMP-regulated phosphoprotein, 32 kDa (DARPP-32), but little is known about the mechanisms regulating its levels or the specification of fully differentiated neuronal subtypes. Cell extrinsic molecules that increase DARPP-32 mRNA and/or protein levels include brain-derived neurotrophic factor (BDNF), retinoic acid, and estrogen. DARPP-32 induction by BDNF in vitro requires phosphatidylinositide 3-kinase (PI3K), but inhibition of phosphorylation of protein kinase B/Akt does not entirely abolish expression of DARPP-32. Moreover, the requirement for Akt has not been established. Using pharmacologic inhibitors of PI3K, Akt, and cyclin-dependent kinase 5 (cdk5) and constitutively active and dominant negative PI3K, Akt, cdk5, and p35 viruses in cultured striatal neurons, we measured BDNF-induced levels of DARPP-32 protein and/or mRNA. We demonstrated that both the PI3K/Akt/mammalian target of rapamycin and the cdk5/p35 signal transduction pathways contribute to the induction of DARPP-32 protein levels by BDNF and that the effects are on both the transcriptional and translational levels. It also appears that PI3K is upstream of cdk5/p35, and its activation can lead to an increase in p35 protein levels. These data support the presence of multiple signal transduction pathways mediating expression of DARPP-32 in vitro, including a novel, important pathway via by which PI3K regulates the contribution of cdk5/p35.

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

The striatum is one of the brain areas most vulnerable to excitotoxicity, a lesion that can be prevented by neurotrophins. In the present study, intrastriatal injection of the N-methyl-d-aspartate receptor (NMDAR) agonist quinolinate (QUIN) was performed in mice heterozygous for neurotrophin-3 (NT3 +/-) or brain-derived neurotrophic factor (BDNF +/-) to analyze the role of endogenous neurotrophins on the regulation of striatal neurons susceptibility to excitotoxic injury. QUIN injection induced a decrease in dopamine- and cyclic AMP-regulated phosphoprotein of 32 kDa (DARPP-32) protein levels that was higher in NT-3 +/- than in BDNF+/- or wild type animals. This enhanced susceptibility was specific for enkephalin- and tachykinin-positive projection neurons, and also for parvalbumin-positive interneurons. However the excitotoxic damage in large interneurons was not modified in NT-3 +/- mice compared with wild type animals. This effect can be related to the regulation of NMDARs by endogenous NT-3. Thus, our results show that there is an age-dependent regulation of NMDAR subunits NR1 and NR2A, but not NR2B, in NT-3 +/- mice. The deficit of endogenous NT-3 induced a decrease in NR1 and NR2A subunits at postnatal day (P) 0 and P3 mice respectively, whereas an upregulation was observed in 12 week old NT-3 +/- mice. This differential effect was also observed after administration of exogenous NT-3. In primary striatal cultures, NT-3 treatment induced an enhancement in NR2A, but not NR2B, protein levels. However, intrastriatal grafting of NT-3 secreting-cells in adult wild type mice produced a down-regulation of NR2A subunit. In conclusion, NT-3 regulates the expression of NMDAR subunits modifying striatal neuronal properties that confers the differential vulnerability to excitotoxicity in projection neurons and interneurons in the striatum.

Enriched environment improves motor function in intact and unilateral dopamine-depleted rats

Previous studies have suggested that experience and environmental conditions can affect the progression and severity of symptoms in Parkinson's disease. Furthermore, earlier reports have indicated that enriched environment promotes the survival of dopaminergic grafts in a rat model of Parkinson's disease. Here we investigated whether environmental enrichment affects normal motor function and the severity of dopamine depletion in a rat model of Parkinson's disease. Adult female Long-Evans rats were pre-trained and tested daily in a skilled reaching task. One group of rats was placed in an enriched environment while one group was housed under standard conditions. During this time period, reaching success of animals exposed to the enriched environment improved as compared with animals living in standard housing. The animals remained in the two housing conditions for six weeks prior to receiving unilateral infusion of the neurotoxin 6-hydroxydopamine into the nigrostriatal bundle. The daily behavioral testing continued up to four weeks after lesion. The observations showed that rats housed in an enriched environment significantly improved in reaching success during the first three weeks after lesion as compared with rats housed in the standard condition. Qualitative movement analysis, drug-induced rotation and histological findings indicate that compensatory processes in particular might have accounted for the behavioral improvements. These data are discussed in relation to possible mechanisms of experience-dependent modulation of the pathology of Parkinson's disease.

Shedding light into the role of BDNF in the pharmacotherapy of Parkinson's disease

Parkinson's disease (PD) is a chronic, neurodegenerative disease with a 1% incidence in the population over 55 years of age. Movement impairments represent undoubtedly the hallmark of the disorder; however, extensive evidence implicates cognitive deficits as concomitant peculiar features. Brain-derived neurotrophic factor (BDNF) colocalizes with dopamine neurons in the substantia nigra, where dopaminergic cell bodies are located, and it has recently garnered attention as a molecule crucial for cognition, a function that is also compromised in PD patients. Thus, due to its colocalization with dopaminergic neurons and its role in cognition, BDNF might possess a dual role in PD, both as a neuroprotective molecule, since its inhibition leads to loss of nigral dopaminergic neurons, and as a neuromodulator, as its enhanced expression ameliorates cognitive processes. In this review, we discuss the mechanism of action of established as well as novel drugs for PD with a particular emphasis to those interfering with BDNF biosynthesis.

Glial cell line-derived neurotrophic factor promotes the arborization of cultured striatal neurons through the p42/p44 mitogen-activated protein kinase pathway

Glial cell line-derived neurotrophic factor (GDNF) promotes the survival or differentiation of several types of neurons. This study examines GDNF-induced signal transduction and biological effects in cultured striatal neurons. Results show that GDNF addition to striatal cultures transiently increased the protein levels of phosphorylated p42/p44, but did not change the levels of phosphorylated Akt. GDNF effects on phosphorylated p42/p44 levels were blocked by the mitogen-activated protein kinase (MAPK) pathway specific inhibitors (PD98059 and U0126). Activation of the p42/p44 MAPK pathway by GDNF led to an increase in the degree of dendritic arborization and axon length of both GABA- and calbindin-positive neurons but had no effect on their survival and maturation. These GDNF-mediated effects were suppressed in the presence of the inhibitor of the MAPK pathway (PD98059). Furthermore, the addition of the phosphatidylinositol 3-kinase pathway specific inhibitor (LY294002) blocked GDNF-mediated striatal cell differentiation suggesting that the basal activity of this pathway is needed for the effects of GDNF. Our results indicate that treatment of cultured striatal cells with GDNF specifically activates the p42/p44 MAPK pathway, leading to an increase in the arborization of GABA- and calbindin-positive neurons.

Regulation of multiple dopamine signal transduction molecules by retinoids in the developing striatum

Increasing evidence based on pharmacological and genetic studies suggests that retinoid signaling plays an important role in developmental control of striatal neurons. In the present report, we screened for genes that might be regulated by retinoids in the developing striatum. We cultured tissue explants from the lateral ganglionic eminence (striatal primordium), and for regional comparison, its adjacent structures of the cerebral cortex and the medial ganglionic eminence in embryonic day 15 rat telencephalon. Using the ribonuclease protection assay, we found that both all-trans retinoic acid and 9-cis retinoic acid significantly up-regulated dopamine D1 receptor, heterotrimeric G protein olfactory, adenylyl cyclase type V and dopamine- and cyclic adenosine 3':5'-monophosphate-regulated phosphoprotein mRNAs in the lateral ganglionic eminence culture. By contrast, neither all-trans retinoic acid nor 9-cis retinoic acid significantly altered D1 receptor, heterotrimeric G protein olfactory, adenylyl cyclase type V and dopamine- and cyclic adenosine 3':5'-monophosphate-regulated phosphoprotein mRNAs in the cortical and the medial ganglionic eminence cultures except that D1 receptor mRNA was dramatically induced in the medial ganglionic eminence by retinoic acid treatments. To test whether the induction of multiple dopamine signaling molecules in the lateral ganglionic eminence was due to a general enhancement of neuronal differentiation by retinoic acid, we assayed the effects of retinoic acid on other differentiation markers, including glutamate decarboxylase 65, NR1 subunit of glutamate NMDA receptor and microtubule-associated protein-2. None of these genes were significantly altered by retinoic acid treatments in the lateral ganglionic eminence culture, indicating the specificity of gene regulation by retinoic acid signaling. As D1 receptor, heterotrimeric G protein olfactory, adenylyl cyclase type V and dopamine- and cyclic adenosine 3':5'-monophosphate-regulated phosphoprotein are important molecules involved in propagation of striatal dopamine neurotransmission, our study raises the hypothesis that retinoid signaling may coordinately activate the transcriptional program that is associated with the dopamine signaling pathway in developing striatal neurons. Such coordinate regulation by retinoids may be part of the mechanisms by which the complex yet highly organized neurochemical constituents of the striatum are established during development.

DARPP-32 genomic fragments drive Cre expression in postnatal striatum

To direct Cre-mediated recombination to differentiated medium-size spiny neurons (MSNs) of the striatum, we generated transgenic mice that express Cre recombinase under the regulation of DARPP-32 genomic fragments. In this reported line, recombination of an R26R reporter allele occurred postnatally in the majority of medium-size spiny neurons of the dorsal and ventral striatum (caudate nucleus and nucleus accumbens/olfactory tubercle), as well as in the piriform cortex and choroid plexus. Although regulatory fragments were selected to target MSNs, low levels of Cre-recombinase expression, as detected by beta-galactosidase activity from the R26R reporter gene, were also apparent in widely dispersed areas or cells of the forebrain and hindbrain. These included the primary and secondary motor cortex, and association cortex, as well as in the olfactory bulb and cerebellar Purkinje cells. Notably, expression in these regions was well below that of endogenous DARPP-32. Analysis of colocalization of beta-galactosidase, as detected either by histochemistry or immunocytochemistry, and DARPP-32 revealed double-labeling in almost all DARPP-32-expressing MSNs in the postnatal striatum, but not in extrastriatal regions. The DARPP-32Cre transgenic mouse line thus provides a useful tool to specifically express and/or inactivate genes in mature MSNs of the striatum.

Optimising plasticity: environmental and training associated factors in transplant-mediated brain repair

With progressively ageing populations, degeneration of nerve cells of the brain, due to accident or disease, represents one of the major problems for health and welfare in the developed world. The molecular environment in the adult brain promotes stability limiting its ability to regenerate or to repair itself following injury. Cell transplantation aims to repair the nervous system by introducing new cells that can replace the function of the compromised or lost cells. Alternatives to primary embryonic tissue are actively being sought but this is at present the only source that has been shown reliably to survive grafting into the adult brain and spinal cord, connect with the host nervous system, and influence behaviour. Based on animal studies, several clinical trials have now shown that embryonic tissue grafts can partially alleviate symptoms in Parkinson's disease, and related strategies are under evaluation for Huntington's disease, spinal cord injury, stroke and other CNS disorders. The adult brain is at its most plastic in the period following injury, offering a window of opportunity for therapeutic intervention. Enriched environment, behavioural experience and grafting can each separately influence neuronal plasticity and recovery of function after brain damage, but the extent to which these factors interact is at present unknown. To improve the outcome following brain damage, transplantation must make use of the endogenous potential for plasticity of both the host and the graft and optimise the external circumstances associated with graft-mediated recovery. Our understanding of mechanisms of brain plasticity subsequent to brain damage needs to be associated with what we know about enhancing intrinsic recovery processes in order to improve neurobiological and surgical strategies for repair at the clinical level. With the proof of principle beginning to emerge from clinical trials, a rich area for innovative research with profound therapeutic application, even broader than the specific context of transplantation, is now opening for investigation.

RARβ isoform-specific regulation of DARPP-32 gene expression: an ectopic expression study in the developing rat telencephalon

Dopamine and adenosine 3':5'-monophosphate-regulated phosphoprotein (DARPP-32) is a key molecule for dopamine neurotransmission. The molecular mechanisms underlying the regulation of DARPP-32 in the developing brain remains elusive. Previous studies have shown that retinoids are capable of inducing DARPP-32 in striatal cell culture, suggesting that retinoids are candidate molecules for controlling DARPP-32 expression. In the present study, we first studied the expression profiles of retinoid receptors and their associated co-factors in the developing rat telencephalon by RT-PCR. The results showed that among the retinoid receptors, RARbeta and RXRgamma were nearly selectively expressed in the developing striatum. By contrast, the retinoid receptors associated transcriptional co-factors, including the co-repressors of N-CoR and SMRT, and the co-activators of SRC-1 and P/CAF, were ubiquitously expressed in the developing telencephalon. In light of the previous findings that DARPP-32 was inducible by retinoids in striatal culture, but not in cortical culture, we hypothesized that the striatum-selective RARbeta and RXRgamma may mediate DARPP-32 induction by retinoids. To test this hypothesis, we used the gain-of-function approach to ectopically express RARbeta and RXRgamma in the developing cerebral cortex that lacked these two retinoid receptors. Ectopic expression of RARbeta1, but not RXRgamma1, up-regulated DARPP-32 in the cortical explant culture. Notably, DARPP-32 was up-regulated only by the RARbeta1 isoform, but not by other RARbeta isoforms. Our study suggests that RARbeta signaling may regulate DARPP-32 gene expression by an isoform-specific mechanism in developing telencephalic neurons. The molecular diversity of RARbeta isoforms may underlie parts of the complex gene regulation by retinoids during neural development.

Brain-derived neurotrophic factor regulates the onset and severity of motor dysfunction associated with enkephalinergic neuronal degeneration in Huntington's disease

The mechanism that controls the selective vulnerability of striatal neurons in Huntington's disease is unclear. Brain-derived neurotrophic factor (BDNF) protects striatal neurons and is regulated by Huntingtin through the interaction with the neuron-restrictive silencer factor. Here, we demonstrate that the downregulation of BDNF by mutant Huntingtin depends on the length and levels of expression of the CAG repeats in cell cultures. To analyze the functional effects of these changes in BDNF in Huntington's disease, we disrupted the expression of bdnf in a transgenic mouse model by cross-mating bdnf(+/ -) mice with R6/1 mice. Thus, we compared transgenic mice for mutant Huntingtin with different levels of BDNF. Using this double mutant mouse line, we show that the deficit of endogenous BDNF modulates the pathology of Huntington's disease. The decreased levels of this neurotrophin advance the onset of motor dysfunctions and produce more severe uncoordinated movements. This behavioral pathology correlates with the loss of striatal dopamine and cAMP-regulated phosphoprotein-32-positive projection neurons. In particular, the insufficient levels of BDNF cause specific degeneration of the enkephalinergic striatal projection neurons, which are the most affected cells in Huntington's disease. This neuronal dysfunction can specifically be restored by administration of exogenous BDNF. Therefore, the decrease in BDNF levels plays a key role in the specific pathology observed in Huntington's disease by inducing dysfunction of striatal enkephalinergic neurons that produce severe motor dysfunctions. Hence, administration of exogenous BDNF may delay or stop illness progression.

Glutamate and Regulation of Proliferation in the Developing Mammalian Telencephalon

Increasing evidence suggests that classical neurotransmitters play an important morphogenetic role during development of the mammalian central nervous system. Using in vitro and in vivo models, we have previously identified a role for the N-methyl-D-aspartate (NMDA) subclass of glutamate receptors in the proliferation of striatal progenitors. Here, we compare the roles of ionotropic glutamate receptors in the proliferation of either striatal or cortical progenitors. In culture, glutamate receptor activation promoted proliferation of both striatal and cortical neuroblasts. However, cortical and striatal neuroblasts responded to distinct ionotropic receptors. Cortical cultures were sensitive to AMPA/KA receptor blockade, whereas striatal neuroblast proliferation was altered by NMDA antagonists. In vivo, BrdU uptake in the proliferative ventricular zone was reduced in embryos following acute administration of ionotropic glutamate receptor antagonists. In keeping with in vitro observations, proliferation in cortical and striatal ventricular regions was reduced, respectively, by either AMPA/KA or NMDA receptor blockade. We also determined whether forebrain-derived progenitors expanded as neurospheres in the presence of growth factors show similar ionotropic glutamatergic responses. Cells in neither dorsal nor ventral telencephalon-derived neurospheres showed altered proliferation following exposure to either class of ionotropic glutamate receptor antagonist. Together, these findings suggest that glutamate influences the proliferation of forebrain neuronal progenitors, but not more primitive populations represented in multipotential progenitors expanded in vitro. The effects on neuroblast proliferation in different forebrain domains are heterogeneous and are mediated by distinct subclasses of ionotropic glutamate receptors.

Striatal neuron differentiation from neurosphere-expanded progenitors depends on Gsh2 expression

Neural stem and progenitor cells from the embryonic forebrain can be expanded under growth factor stimulation in vitro, either as free-floating aggregates called neurospheres or as attached monolayer cultures. We have previously shown that despite the maintenance of important regulatory genes such as Gsh2, in vitro expansion of cells from the lateral ganglion eminence (LGE) restricts their differentiation potential. Specifically, their ability to differentiate into striatal projection neurons is compromised. It is not clear whether this restriction is caused by loss of progenitors with the ability to generate striatal projection neurons or whether the restricted differentiation potential is caused by factors lacking during in vitro differentiation. To address this, we have set up an in vitro system, in which expanded LGE-derived cells are differentiated in coculture with primary cells isolated from different regions of the embryonic brain. We provide evidence that the primary cells supply the expanded cells with contact-mediated region-specific developmental cues. Neurosphere-expanded LGE progenitors can, when presented with these cues, differentiate into neurons with characteristics of striatal projection neurons. Furthermore, we show that the ability of the expanded LGE cells to respond to the developmental cues presented by the primary cells depends on the maintained expression of Gsh2 in the expanded cells.

Early striatal dendrite deficits followed by neuron loss with advanced age in the absence of anterograde cortical brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, modulates neuronal survival, differentiation, and synaptic function. Reduced BDNF expression in the cortex caused by mutation of the huntingtin gene has been suggested to play a role in the striatal degeneration observed in Huntington's disease. BDNF expression rises dramatically in the cortex during the first few weeks of postnatal life in mice. Previously, it has been impossible to study the specific long-term effects of BDNF absence on CNS structures because of the early postnatal lethality of BDNF-/- mice. Mice harboring a floxed BDNF gene were bred with Emx1(IREScre/+) mice to generate Emx-BDNF(KO) mice that lack cortical BDNF but are viable. Adult Emx-BDNF(KO) mice display a hindlimb clasping phenotype similar to that observed in mouse models of Huntington's disease. The striatum of postnatal Emx-BDNF(KO) mice was reduced in volume compared with controls, and the most abundant neuron type of the striatum, medium spiny neurons (MSNs), had shrunken cell somas, thinner dendrites, and fewer dendritic spines at 35 d of age. Although significant striatal neuron losses were not detected at 35 or 120 d postnatal, 35% of striatal neurons were missing in Emx-BDNF(KO) mice aged beyond 1 year. Thus, cortical BDNF, although not required for the generation or near-term survival of MSN, is necessary for normal striatal neuron dendrite morphology during the period when BDNF expression rises in the cortex. Furthermore, a long-term in vivo requirement for cortical BDNF in supporting the survival of MSNs is revealed.

Environmental enrichment affects striatal graft morphology and functional recovery

Environmental conditions and behavioural experience can affect neuronal function and morphology. It is less well known whether such factors also influence the growth, integration and functional recovery provided by neural grafts placed within the damaged brain. Here we report on the effects of differential housing conditions on striatal graft morphology and functional recovery after striatal lesions. Rats were pretrained on a skilled bilateral forelimb task, the staircase test, and lesioned unilaterally in the lateral dorsal striatum with quinolinic acid. One group of lesioned animals was given suspension grafts of E15 whole ganglionic eminence implanted into the lesioned striatum. Following transplantation, the animals were housed either in standard cages (four per cage) or in enriched environment housing conditions (10 per cage) with tunnels, ladders and increased living space available for exploration, social interaction and play. The differentially housed animals were retested on the skilled staircase test at two separate time points. Repeated testing, environmental enrichment and transplantation positively influenced behavioural recovery. Partial recovery was observed bilaterally amongst the grafted animals in both housing conditions. Nevertheless, the grafted animals housed in the enriched environment performed significantly better in the final test compared with all of the other experimental groups. The grafts survived equally well under both housing conditions but the grafts of animals housed in the enriched environment contained larger projection neurons and were somewhat better reinnervated by dopaminergic afferents. An increased level of striatal brain-derived neurotrophic factor was observed in the control animals housed under the enriched compared with the standard conditions. The results indicate that an enriched environment can affect both graft function and graft morphology through as yet unknown mechanisms.

Differential involvement of phosphatidylinositol 3-kinase and p42/p44 mitogen activated protein kinase pathways in brain-derived neurotrophic factor-induced trophic effects on cultured striatal neurons

Brain-derived neurotrophic factor (BDNF) is a potent trophic factor for striatal cells that promotes survival and/or differentiation of GABAergic neurons in vitro. In the present study, we show that the stimulation of cultured striatal cells with BDNF increased the phosphorylation of Akt and p42/p44. This effect was specifically blocked by inhibitors of phosphatidylinositol 3-kinase (PI3-K) pathways (LY294002 and wortmannin) or p42/p44 mitogen-activated protein (MAP) kinase (PD98059 and U0126). BDNF treatment induced an increase in the number of calbindin-positive neurons but not in the number of GABAergic or total cells. Furthermore, BDNF increased the degree of dendritic arborization, soma area and axon length of striatal neurons. However, PD98059 was more effective blocking BDNF effects on calbindin- than on GABA-positive neurons, whereas LY294002 inhibited morphological differentiation in both neuronal populations. Moreover, BDNF induced neuronal survival only through the activation of the PI3-K pathway.

Pattern of corticostriatal innervation in organotypic cocultures is dependent on the age of the cortical tissue

The patch-matrix organization of the striatum is defined by the selective expression of neuronal markers and a semisegregated pattern of afferents and efferents that develops before birth in all mammals. Differential projections from 'limbic' and 'somatomotor' cortices contribute to the selective circuitry of patch ("striosome") and matrix compartments. Organotypic cultures were used to determine the pattern of early corticostriatal innervation as a first step toward understanding the role of cortical innervation in the development of striatal patch-matrix organization. Perinatal striatum (E19-P4) was cocultured with the cortex obtained from same-age or different-age rats in the presence or absence of substantia nigra obtained from E14-15 fetuses. After 4-21 days in vitro, crystals of biocytin were placed directly onto the cortical piece to trace cortical projections into the striatal piece. Cortex obtained from fetuses (E19-22) or neonatal (P0-1) rats gave rise to a dense innervation of both prenatal and postnatal striatal slices; however, the pattern of biocytin-labeled fibers was found to be highly dependent on the age of the cortical tissue used. Cortex derived from rats between E20 and P1 gave rise to a heterogeneous distribution of fibers indicative of striatal patches when combined with striatal slices from same-age or younger (E18-19) fetuses. Cortex from E18-19 fetuses produced a homogeneous innervation even when cocultured with older striatal tissue in which the striatal patches were already present. The postnatal cortex (P2-P5) gave rise to little to no innervation of striatum of all ages. Similar findings were obtained with the use of either prelimbic or somatosensory cortex. In double- and triple-labeled cultures, the distribution of corticostriatal fibers overlapped substantially with patches of developing striatal neurons, as revealed by DARPP-32 immunocytochemistry. Dopaminergic innervation present when the substantia nigra was included in the cocultures also distributed preferentially to the developing patch compartment, but it did not substantially alter the pattern of corticostriatal innervation. These findings suggest that the cortex provides directive signals to the developing striatum rather than simply responding to the presence of patches that have already formed.

17beta-Estradiol promotes striatal medium size spiny neuronal maturation in vitro

Gender differences exist in the development of the nigrostriatal dopamine system, and in the incidence and course of pediatric and adult neuropsychiatric diseases in which this system is implicated. The medium size spiny neuron (MSN) is the major output neuron of the caudate nucleus. It receives a large dopaminergic input from the substantia nigra, and 96% of the MSNs express DARPP-32, a dopamine and cyclic AMP-regulated phosphoprotein and key mediator of dopamine function. There are few examples, however, of direct effects of sex hormones, including 17beta-estradiol (E(2)), on the MSN. We report that in vitro, E(2) (10-50 nM) promotes MSN phenotypic maturation, as determined by increased soma size, neurite length, and DARPP-32 protein levels. Treatment with the 'anti-estrogen' ICI 182,780 or the partial-agonist tamoxifen also increases DARPP-32 levels, but when added to E(2), ICI 182,780 only prevents the increase in DARPP-32 levels and increase in soma size and neurite length. Surprisingly, maturation effects are more robust in cells derived exclusively from female embryos. Western blot analysis of protein lysates and immunocytochemistry of cultured MSNs reveals the presence of the estrogen receptor beta (ERbeta). These data suggest that ERbeta may mediate the differentiating effect of E(2) on embryonic MSNs, and provide new avenues of investigation for the role of sex hormones in the development of the striatum and in diseases affecting the basal ganglia.

Neurotrophic factors in Huntington's disease

Huntington's disease is a neurodegenerative disorder characterized by the selective loss of striatal neurons and, to a lesser extent, cortical neurons. The neurodegenerative process is caused by the mutation of huntingtin gene. Recent studies have established a link between mutant huntingtin, excitotoxicity and neurotrophic factors. Neurotrophic factors prevent cell death in degenerative processes but they can also enhance growth and function of neurons that are affected in Huntington's disease. The endogenous regulation of the expression of neurotrophic factors and their receptors in the striatum and its connections can be important to protect striatal cells and maintains basal ganglia connectivity. The administration of exogenous neurotrophic factors, in animal models of Huntington's disease, has been used to characterize the trophic requirements of striatal and cortical neurons. Neurotrophins, glial cell line-derived neurotrophic factor family members and ciliary neurotrophic factor have shown a potent neuroprotective effects on different neuronal populations of the striatum. Furthermore, they are also useful to maintain the integrity of the corticostriatal pathway. Thus, these neurotrophic factors may be suitable for the development of a neuroprotective therapy for neurodegenerative disorders of the basal ganglia.