Imbalance of p75(NTR)/TrkB protein expression in Huntington's disease: implication for neuroprotective therapies

Neuroprotective therapies based on brain-derived neurotrophic factor (BDNF) administration have been proposed forHuntington's disease (HD) treatment. However, our group has recently reported reduced levels of TrkB in HD mouse models andHD human brain suggesting that besides a decrease on BDNF levels a reduction of TrkB expression could also contribute todiminished neurotrophic support in HD. BDNF can also bind to p75 neurotrophin receptor (p75NTR) modulating TrkB signaling.Therefore, in this study we have analyzed the levels of p75NTRin several HD models, as well as in HD human brain. Our datademonstrates a p75NTR/TrkB imbalance in the striatum of two different HD mouse models,HdhQ111/111homozygous knockin miceand R6/1 mice that was also manifested in the putamen of HD patients. The imbalance between TrkB and p75NTRlevels in a HDcellular model did not affect BDNF-mediated TrkB activation of prosurvival pathways but induced activation of apoptoticcascades as demonstrated by increased JNK phosphorylation. Moreover, BDNF failed to protect mutant huntingtin striatal cellstransfected with p75NTRagainst NMDA-mediated excitotoxicity, which was associated with decreased Akt phosphorylation.Interestingly, lack of Akt activation following BDNF and NMDA treatment correlated with increased PP1 levels. Accordingly,pharmacological inhibition of PP1 by okadaic acid (OA) prevented mutant huntingtin striatal cell death induced by NMDA andBDNF. Altogether, our findings demonstrate that the p75NTR/TrkB imbalance induced by mutant huntingtin in striatal cellsassociated with the aberrant activity of PP1 disturbs BDNF neuroprotection likely contributing to increasing striatal vulnerabilityin HD. On the basis of this data we hypothesize that normalization of p75NTRand/or TrkB expression or their signaling willimprove BDNF neuroprotective therapies in HD

BDNF Induces Striatal-Enriched Protein Tyrosine Phosphatase 61 Degradation Through the Proteasome

Brain-derived neurotrophic factor (BDNF) promotes synaptic strengthening through the regulation of kinase and phosphatase activity. Conversely, striatal-enriched protein tyrosine phosphatase (STEP) opposes synaptic strengthening through inactivation or internalization of signaling molecules. Here, we investigated whether BDNF regulates STEP levels/activity. BDNF induced a reduction of STEP61 levels in primary cortical neurons, an effect that was prevented by inhibition of tyrosine kinases, phospholipase C gamma, or the ubiquitin-proteasome system (UPS). The levels of pGluN2B(Tyr1472) and pERK1/2(Thr202/Tyr204), two STEP substrates, increased in BDNF-treated cultures, and blockade of the UPS prevented STEP61 degradation and reduced BDNF-induced GluN2B and ERK1/2 phosphorylation. Moreover, brief or sustained cell depolarization reduced STEP61 levels in cortical neurons by different mechanisms. BDNF also promoted UPS-mediated STEP61 degradation in cultured striatal and hippocampal neurons. In contrast, nerve growth factor and neurotrophin-3 had no effect on STEP61 levels. Our results thus indicate that STEP61 degradation is an important event in BDNF-mediated effects

Long-term memory deficits in Huntington's disease are associated with reduced CBP histone acetylase activity

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder caused by an expanded CAG/polyglutamine repeat in the coding region of the huntingtin (htt) gene. Although HD is classically considered a motor disorder, there is now considerable evidence that early cognitive deficits appear in patients before the onset of motor disturbances. Here we demonstrate early impairment of long-term spatial and recognition memory in heterozygous HD knock-in mutant mice (Hdh(Q7/Q111)), a genetically accurate HD mouse model. Cognitive deficits are associated with reduced hippocampal expression of CREB-binding protein (CBP) and diminished levels of histone H3 acetylation. In agreement with reduced CBP, the expression of CREB/CBP target genes related to memory, such c-fos, Arc and Nr4a2, was significantly reduced in the hippocampus of Hdh(Q7/Q111) mice compared with wild-type mice. Finally, and consistent with a role of CBP in cognitive impairment in Hdh(Q7/Q111) mice, administration of the histone deacetylase inhibitor trichostatin A rescues recognition memory deficits and transcription of selective CREB/CBP target genes in Hdh(Q7/Q111) mice. These findings demonstrate an important role for CBP in cognitive dysfunction in HD and suggest the use of histone deacetylase inhibitors as a novel therapeutic strategy for the treatment of memory deficits in this disease

Neurotrophin receptor p75NTR mediates Huntington's disease-associated synaptic and memory dysfunction

Learning and memory deficits are early clinical manifestations of Huntington's disease (HD). These cognitive impairments have been mainly associated with frontostriatal HD pathology; however, compelling evidence provided by several HD murine models suggests that the hippocampus may contribute to synaptic deficits and memory dysfunction in HD. The neurotrophin receptor p75(NTR) negatively regulates spine density, which is associated with learning and memory; therefore, we explored whether disturbed p75(NTR) function in the hippocampus could contribute to synaptic dysfunction and memory deficits in HD. Here, we determined that levels of p75(NTR) are markedly increased in the hippocampus of 2 distinct mouse models of HD and in HD patients. Normalization of p75(NTR) levels in HD mutant mice heterozygous for p75(NTR) prevented memory and synaptic plasticity deficits and ameliorated dendritic spine abnormalities, likely through normalization of the activity of the GTPase RhoA. Moreover, viral-mediated overexpression of p75(NTR) in the hippocampus of WT mice reproduced HD learning and memory deficits, while knockdown of p75(NTR) in the hippocampus of HD mice prevented cognitive decline. Together, these findings provide evidence of hippocampus-associated memory deficits in HD and demonstrate that p75(NTR) mediates synaptic, learning, and memory dysfunction in HD

Description and Validation of New Therapeutical Targets to Prevent Neurodegenertlion and Cognitive Deficits in Huntington's Disease

[cat] La malaltia de Huntington (MH) és un desordre neurodegeneratiu caracteritzat per la disfunció i mort neuronal de regions específiques del cervell. La regió més afectada és l’estriat (nuclis caudat i putamen en humans), tot i que en estadis més avançats de la malaltia s’ha descrit una atròfia i pèrdua neuronal del còrtex cerebral i hipocamp (Vonsattel et al., 1985;Vonsattel and DiFiglia, 1998). La temprana disfuncionalitat de les neurones hipocampals i corticals es creu crítica per restablir les deficiències cognitives i de memòria en aquesta patologia. La malaltia s’hereta de forma autosòmica dominant i és causada per la mutació del gen IT15, localitzat en el braç curt del cromosoma 4 (4p.16.3), que codifica per la proteïna anomenada huntingtina (htt). Aquesta mutació va ser identificada l’any 1993 com una expansió de repeticions del triplet CAG que codifiquen per una regió poliglutamínica (poliQ) a l’extrem N-terminal de la proteïna htt (350KDa) (HDCRG, 1993). En individus sans, el nombre de repeticions oscil·la de 6 a 35; quan el nombre de repeticions d’aquest triplet és superior a 40, l’individu desenvoluparà la malaltia. Les primeres manifestacions de la malaltia solen produir-se als 35 anys d’edat conduint a la mort 15-­20 anys després de l’aparició dels símptomes (Bates, 2003;Martin and Gusella, 1986). La simptomatologia inclou disfunció motora, associada majoritàriament a l’atròfia estriatal, acompanyada de trastorns cognitius i emocionals associats a l’afectació corticoestriatal i hipocampal que son de manifestació primerenca, fins i tot prèvia a la simptomatologia motora. Aquestes alteracions cognitives i emocionals constitueixen un dels pilars discapacitants en aquesta patologia, per això al llarg d’aquesta Tesi doctoral proposem un estudi dual que ens permeti definir diverses estratègies terapèutiques dirigides al tractament d’ambdues simptomatologies: motora i cognitiva. Si bé es coneix que aquesta mutació és la causant de la malaltia, avui en dia no es coneixen els mecanismes cel·lulars i moleculars responsables de la disfunció i mort neuronal en la MH. Diversos estudis han postulat que la pèrdua de funció de la proteïna wild-type i/o el guany de funció de la proteïna mutada (mhtt) juguen un paper clau en el desenvolupament de la malaltia. Així s’ha descrit que l’expressió de la proteïna huntingtina mutada resulta en l’alteració de diversos processos cel·lulars i moleculars, tals com l’agregació proteica, alteracions en el sistema ubiqüitina­proteosoma, desregulació en la maquinària transcripcional així com en la remodelació de la cromatina, alteracions en la síntesi proteica, reducció del suport tròfic, alteracions en les vies de senyalització intracel·lulars, alteració en la homeòstasis del calci, dany mitocondrial, excitotoxicitat, activació de caspases, alteracions en les interaccions proteïna-proteïna i alteració en la circuiteria neuronal (Cattaneo et al., 2005;Zuccato and Cattaneo, 2009). En aquesta Tesis ens hem centrat en estudiar alguns dels mecanismes moleculars implicats en la mort neuronal, així com en els dèficits cognitius i alteracions en la plasticitat sinàptica produïda per la presència de la huntingtina mutada, mitjançant l’estudi de les alteracions produïdes en: 1) maquinària transcripcional, 2) suport neurotròfic, 3) canvis estructurals en les sinapsis excitadores, 4) senyalització de proteïnes cinasa i fosfatasa i 5) formació d’heteròmers entre receptors acoblats a proteïnes G.[eng] Understanding the molecular underpinnings of neuronal dysfunction and degeneration involved in Huntington’s disease is a goal of increasing urgency for society and scientific community. In this Thesis we have studied different proteins and signaling pathways, specifically altered by the presence of mutant huntingtin, as new potential candidates to develop pharmacological strategies to treat or delay motor and cognitive deficits in Huntington’s disease. AIM 1. To study the contribution of CBP/CREB pathway in the cognitive deficits present in Huntington’s disease. AIM 2. To study the molecular mechanisms involved in neurotrophic support dysfunction in Huntington’s disease. 2.1. To analyze the role of p75NTR/TrkB receptors in the major striatal vulnerability in Huntington’s disease. 2.2. To study the role of p75NTR in cognitive deficits in Huntington’s disease. AIM 3. To study the molecular mechanisms involved in corticostriatal dysfunction in Huntington’s disease. 3.1. To characterize corticostriatal deficits in HD mouse models and analyze the role of Kalirin-7 in the alteration of corticostriatal excitatory synapses in HD. AIM 4. To study the role of Cdk5 in cognitive deficits in Huntington’s disease. AIM 5. To study the role of D1R-H3R heteromers in neuronal cell death and cognitive deficits in Huntington’s disease

Ikzf1 as a novel regulator of microglial homeostasis in inflammation<span style="background-color:rgb( 255 , 255 , 255 );color:rgb( 0 , 0 , 0 )"> and neurodegeneration</span>

Using genetic tools, here we describe that Ikzf1 is specifically expressed in the adult microglia in brain regions such as cortex and hippocampus. By characterizing the Ikzf1 deficient mice, we observed that these mice displayed spatial learning deficits, impaired hippocampal CA3-CA1 long-term potentiation, and decreased spine density in pyramidal neurons of the CA1, which correlates with an increased expression of synaptic markers within microglia. Additionally, these Ikzf1 deficient microglia exhibited a severe abnormal morphology in the hippocampus, which is accompanied by astrogliosis, an aberrant composition of the inflammasome, and an altered expression of disease-associated microglia molecules. Interestingly, the lack of Ikzf1 induced changes on histone 3 acetylation and methylation levels in the hippocampus. Since the lack of Ikzf1 in mice appears to induce the internalization of synaptic markers within microglia, and severe gliosis we then analyzed hippocampal Ikzf1 levels in several models of neurological disorders. Ikzf1 levels were increased in the hippocampus of these neurological models, as well as in postmortem hippocampal samples from Alzheimer’s disease patients. Finally, over-expressing Ikzf1 in cultured microglia made these cells hyporeactive upon treatment with lipopolysaccharide, and less phagocytic compared to control microglia. Altogether, these results suggest that altered Ikzf1 levels in the adult hippocampus are sufficient to induce synaptic plasticity and memory deficits via altering microglial state and function