P2Y6 Receptor-Dependent Microglial Phagocytosis of Synapses during Development Regulates Synapse Density and Memory.

During brain development, excess synapses are pruned (i.e., removed), in part by microglial phagocytosis, and dysregulated synaptic pruning can lead to behavioral deficits. The P2Y6 receptor (P2Y6R) is known to regulate microglial phagocytosis of neurons, and to regulate microglial phagocytosis of synapses in cell culture and in vivo during aging. However, currently it is unknown whether P2Y6R regulates synaptic pruning during development. Here, we show that P2Y6R KO mice of both sexes had strongly reduced microglial internalization of synaptic material, measured as Vglut1 within CD68-staining lysosomes of microglia at postnatal day 30 (P30), suggesting reduced microglial phagocytosis of synapses. Consistent with this, we found an increased density of synapses in the somatosensory cortex and the CA3 region and dentate gyrus of the hippocampus at P30. We also show that adult P2Y6R KO mice have impaired short- and long-term spatial memory and impaired short- and long-term recognition memory compared with WT mice, as measured by novel location recognition, novel object recognition, and Y-maze memory tests. Overall, this indicates that P2Y6R regulates microglial phagocytosis of synapses during development, and this contributes to memory capacity.SIGNIFICANCE STATEMENT The P2Y6 receptor (P2Y6R) is activated by uridine diphosphate released by neurons, inducing microglial phagocytosis of such neurons or synapses. We tested whether P2Y6R regulates developmental synaptic pruning in mice and found that P2Y6R KO mice have reduced synaptic material within microglial lysosomes, and increased synaptic density in the brains of postnatal day 30 mice, consistent with reduced synaptic pruning during development. We also found that adult P2Y6R KO mice had reduced memory, consistent with persistent deficits in brain function, resulting from impaired synaptic pruning. Overall, the results suggest that P2Y6R mediates microglial phagocytosis of synapses during development, and the absence of this results in memory deficits in the adult.

Ikzf1 as a novel regulator of microglial homeostasis in inflammation and neurodegeneration.

In the last two decades, microglia have emerged as key contributors to disease progression in many neurological disorders, not only by exerting their classical immunological functions but also as extremely dynamic cells with the ability to modulate synaptic and neural activity. This dynamic behavior, together with their heterogeneous roles and response to diverse perturbations in the brain parenchyma has raised the idea that microglia activation is more diverse than anticipated and that understanding the molecular mechanisms underlying microglial states is essential to unravel their role in health and disease from development to aging. The Ikzf1 (a.k.a. Ikaros) gene plays crucial roles in modulating the function and maturation of circulating monocytes and lymphocytes, but whether it regulates microglial functions and states is unknown. 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.

Wild-type sTREM2 blocks Aß aggregation and neurotoxicity, but the Alzheimer's R47H mutant increases Aß aggregation.

TREM2 is a pattern recognition receptor, expressed on microglia and myeloid cells, detecting lipids and Aß and inducing an innate immune response. Missense mutations (e.g., R47H) of TREM2 increase risk of Alzheimer's disease (AD). The soluble ectodomain of wild-type TREM2 (sTREM2) has been shown to protect against AD in vivo, but the underlying mechanisms are unclear. We show that Aß oligomers bind to cellular TREM2, inducing shedding of the sTREM2 domain. Wild-type sTREM2 bound to Aß oligomers (measured by single-molecule imaging, dot blots, and Bio-Layer Interferometry) inhibited Aß oligomerization and disaggregated preformed Aß oligomers and protofibrils (measured by transmission electron microscopy, dot blots, and size-exclusion chromatography). Wild-type sTREM2 also inhibited Aß fibrillization (measured by imaging and thioflavin T fluorescence) and blocked Aß-induced neurotoxicity (measured by permeabilization of artificial membranes and by loss of neurons in primary neuronal-glial cocultures). In contrast, the R47H AD-risk variant of sTREM2 is less able to bind and disaggregate oligomeric Aß but rather promotes Aß protofibril formation and neurotoxicity. Thus, in addition to inducing an immune response, wild-type TREM2 may protect against amyloid pathology by the Aß-induced release of sTREM2, which blocks Aß aggregation and neurotoxicity. In contrast, R47H sTREM2 promotes Aß aggregation into protofibril that may be toxic to neurons. These findings may explain how wild-type sTREM2 apparently protects against AD in vivo and why a single copy of the R47H variant gene is associated with increased AD risk.

Microglial phagocytosis of neurons in neurodegeneration, and its regulation.

There is growing evidence that excessive microglial phagocytosis of neurons and synapses contributes to multiple brain pathologies. RNA-seq and genome-wide association (GWAS) studies have linked multiple phagocytic genes to neurodegenerative diseases, and knock-out of phagocytic genes has been found to protect against neurodegeneration in animal models, suggesting that excessive microglial phagocytosis contributes to neurodegeneration. Here, we review recent evidence that microglial phagocytosis of live neurons and synapses causes neurodegeneration in animal models of Alzheimer's disease and other tauopathies, Parkinson's disease, frontotemporal dementias, multiple sclerosis, retinal degeneration and neurodegeneration induced by ischaemia, infection or ageing. We also review factors regulating microglial phagocytosis of neurons, including: nucleotides, frackalkine, phosphatidylserine, calreticulin, UDP, CD47, sialylation, complement, galectin-3, Apolipoprotein E, phagocytic receptors, Siglec receptors, cytokines, microglial epigenetics and expression profile. Some of these factors may be potential treatment targets to prevent neurodegeneration mediated by excessive microglial phagocytosis of live neurons and synapses.

Inflammatory neuronal loss in the substantia nigra induced by systemic lipopolysaccharide is prevented by knockout of the P2Y6 receptor in mice.

Inflammation may contribute to multiple brain pathologies. One cause of inflammation is lipopolysaccharide/endotoxin (LPS), the levels of which are elevated in blood and/or brain during bacterial infections, gut dysfunction and neurodegenerative diseases, such as Parkinson's disease. How inflammation causes neuronal loss is unclear, but one potential mechanism is microglial phagocytosis of neurons, which is dependent on the microglial P2Y6 receptor. We investigated here whether the P2Y6 receptor was required for inflammatory neuronal loss. Intraperitoneal injection of LPS on 4 successive days resulted in specific loss of dopaminergic neurons (measured as cells staining with tyrosine hydroxylase or NeuN) in the substantia nigra of wild-type mice, but no neuronal loss in cortex or hippocampus. This supports the hypothesis that neuronal loss in Parkinson's disease may be driven by peripheral LPS. By contrast, there was no LPS-induced neuronal loss in P2Y6 receptor knockout mice. In vitro, LPS-induced microglial phagocytosis of cells was prevented by inhibition of the P2Y6 receptor, and LPS-induced neuronal loss was reduced in mixed glial-neuronal cultures from P2Y6 receptor knockout mice. This supports the hypothesis that microglial phagocytosis contributes to inflammatory neuronal loss, and can be prevented by blocking the P2Y6 receptor, suggesting that P2Y6 receptor antagonists might be used to prevent inflammatory neuronal loss in Parkinson's disease and other brain pathologies involving inflammatory neuronal loss.

Activated microglia desialylate their surface, stimulating complement receptor 3-mediated phagocytosis of neurons.

The glycoproteins and glycolipids of the cell surface have sugar chains that normally terminate in a sialic acid residue, but inflammatory activation of myeloid cells can cause sialidase enzymes to remove these residues, resulting in desialylation and altered activity of surface receptors, such as the phagocytic complement receptor 3 (CR3). We found that activation of microglia with lipopolysaccharide (LPS), fibrillar amyloid beta (Aß), Tau or phorbol myristate acetate resulted in increased surface sialidase activity and desialylation of the microglial surface. Desialylation of microglia by adding sialidase, stimulated microglial phagocytosis of beads, but this was prevented by siRNA knockdown of CD11b or a blocking antibody to CD11b (a component of CR3). Desialylation of microglia by a sialyl-transferase inhibitor (3FAx-peracetyl-Neu5Ac) also stimulated microglial phagocytosis of beads. Desialylation of primary glial-neuronal co-cultures by adding sialidase or the sialyl-transferase inhibitor resulted in neuronal loss that was prevented by inhibiting phagocytosis with cytochalasin D or the blocking antibody to CD11b. Adding desialylated microglia to glial-neuronal cultures, in the absence of neuronal desialylation, also caused neuronal loss prevented by CD11b blocking antibody. Adding LPS or Aß to primary glial-neuronal co-cultures caused neuronal loss, and this was prevented by inhibiting endogenous sialidase activity with N-acetyl-2,3-dehydro-2-deoxyneuraminic acid or blockage of CD11b. Thus, activated microglia release a sialidase activity that desialylates the cell surface, stimulating CR3-mediated phagocytosis of neurons, making extracellular sialidase and CR3 potential treatment targets to prevent inflammatory loss of neurons.

A role for Kalirin-7 in corticostriatal synaptic dysfunction in Huntington's disease.

Cognitive dysfunction is an early clinical hallmark of Huntington's disease (HD) preceding the appearance of motor symptoms by several years. Neuronal dysfunction and altered corticostriatal connectivity have been postulated to be fundamental to explain these early disturbances. However, no treatments to attenuate cognitive changes have been successful: the reason may rely on the idea that the temporal sequence of pathological changes is as critical as the changes per se when new therapies are in development. To this aim, it becomes critical to use HD mouse models in which cognitive impairments appear prior to motor symptoms. In this study, we demonstrate procedural memory and motor learning deficits in two different HD mice and at ages preceding motor disturbances. These impairments are associated with altered corticostriatal long-term potentiation (LTP) and specific reduction of dendritic spine density and postsynaptic density (PSD)-95 and spinophilin-positive clusters in the cortex of HD mice. As a potential mechanism, we described an early decrease of Kalirin-7 (Kal7), a guanine-nucleotide exchange factor for Rho-like small GTPases critical to maintain excitatory synapse, in the cortex of HD mice. Supporting a role for Kal7 in HD synaptic deficits, exogenous expression of Kal7 restores the reduction of excitatory synapses in HD cortical cultures. Altogether, our results suggest that cortical dysfunction precedes striatal disturbances in HD and underlie early corticostriatal LTP and cognitive defects. Moreover, we identified diminished Kal7 as a key contributor to HD cortical alterations, placing Kal7 as a molecular target for future therapies aimed to restore corticostriatal function in HD.

Chelerythrine promotes Ca2+-dependent calpain activation in neuronal cells in a PKC-independent manner.

BACKGROUND: Chelerythrine is widely used as a broad range protein kinase C (PKC) inhibitor, but there is controversy about its inhibitory effect. Moreover, it has been shown to exert PKC-independent effects on non-neuronal cells. METHODS: In this study we investigated possible off-target effects of chelerythrine on cultured cortical rodent neurons and a neuronal cell line. RESULTS: We found that 10µM chelerythrine, a commonly used concentration in neuronal cultures, reduces PKC and cAMP-dependent protein kinase substrates phosphorylation in mouse cultured cortical neurons, but not in rat primary cortical neurons or in a striatal cell line. Furthermore, we found that incubation with chelerythrine increases pERK1/2 levels in all models studied. Moreover, our results show that chelerythrine promotes calpain activation as assessed by the cleavage of spectrin, striatal-enriched protein tyrosine phosphatase and calcineurin A. Remarkably, chelerythrine induces a concentration-dependent increase in intracellular Ca2+ levels that mediates calpain activation. In addition, we found that chelerythrine induces ERK1/2- and calpain-independent caspase-3 activation that can be prevented by the Ca2+ chelator BAPTA-AM. CONCLUSIONS: This is the first report showing that chelerythrine promotes Ca2+-dependent calpain activation in neuronal cells, which has consequences for the interpretation of studies using this compound. GENERAL SIGNIFICANCE: Chelerythrine is still marketed as a specific PKC inhibitor and extensively used in signal transduction studies. We believe that the described off-target effects should preclude its use as a PKC inhibitor in future works.

Cdk5-mediated mitochondrial fission: A key player in dopaminergic toxicity in Huntington's disease.

The molecular mechanisms underlying striatal vulnerability in Huntington's disease (HD) are still unknown. However, growing evidence suggest that mitochondrial dysfunction could play a major role. In searching for a potential link between striatal neurodegeneration and mitochondrial defects we focused on cyclin-dependent kinase 5 (Cdk5). Here, we demonstrate that increased mitochondrial fission in mutant huntingtin striatal cells can be a consequence of Cdk5-mediated alterations in Drp1 subcellular distribution and activity since pharmacological or genetic inhibition of Cdk5 normalizes Drp1 function ameliorating mitochondrial fragmentation. Interestingly, mitochondrial defects in mutant huntingtin striatal cells can be worsened by D1 receptor activation a process also mediated by Cdk5 as down-regulation of Cdk5 activity abrogates the increase in mitochondrial fission, the translocation of Drp1 to the mitochondria and the raise of Drp1 activity induced by dopaminergic stimulation. In sum, we have demonstrated a new role for Cdk5 in HD pathology by mediating dopaminergic neurotoxicity through modulation of Drp1-induced mitochondrial fragmentation, which underscores the relevance for pharmacologic interference of Cdk5 signaling to prevent or ameliorate striatal neurodegeneration in HD.

P2Y6 receptor-dependent microglial phagocytosis of synapses mediates synaptic and memory loss in aging.

Aging causes loss of brain synapses and memory, and microglial phagocytosis of synapses may contribute to this loss. Stressed neurons can release the nucleotide UTP, which is rapidly converted into UDP, that in turn activates the P2Y6 receptor (P2Y6 R) on the surface of microglia, inducing microglial phagocytosis of neurons. However, whether the activation of P2Y6 R affects microglial phagocytosis of synapses is unknown. We show here that inactivation of P2Y6 R decreases microglial phagocytosis of isolated synapses (synaptosomes) and synaptic loss in neuronal-glial co-cultures. In vivo, wild-type mice aged from 4 to 17 months exhibited reduced synaptic density in cortical and hippocampal regions, which correlated with increased internalization of synaptic material within microglia. However, this aging-induced synaptic loss and internalization were absent in P2Y6 R knockout mice, and these mice also lacked any aging-induced memory loss. Thus, P2Y6 R appears to mediate aging-induced loss of synapses and memory by increasing microglial phagocytosis of synapses. Consequently, blocking P2Y6 R has the potential to prevent age-associated memory impairment.

The microglial P2Y6 receptor mediates neuronal loss and memory deficits in neurodegeneration.

Microglia are implicated in neurodegeneration, potentially by phagocytosing neurons, but it is unclear how to block the detrimental effects of microglia while preserving their beneficial roles. The microglial P2Y6 receptor (P2Y6R) - activated by extracellular UDP released by stressed neurons - is required for microglial phagocytosis of neurons. We show here that injection of amyloid beta (Aß) into mouse brain induces microglial phagocytosis of neurons, followed by neuronal and memory loss, and this is all prevented by knockout of P2Y6R. In a chronic tau model of neurodegeneration (P301S TAU mice), P2Y6R knockout prevented TAU-induced neuronal and memory loss. In vitro, P2Y6R knockout blocked microglial phagocytosis of live but not dead targets and reduced tau-, Aß-, and UDP-induced neuronal loss in glial-neuronal cultures. Thus, the P2Y6 receptor appears to mediate Aß- and tau-induced neuronal and memory loss via microglial phagocytosis of neurons, suggesting that blocking this receptor may be beneficial in the treatment of neurodegenerative diseases.

Sialylation and Galectin-3 in Microglia-Mediated Neuroinflammation and Neurodegeneration.

Microglia are brain macrophages that mediate neuroinflammation and contribute to and protect against neurodegeneration. The terminal sugar residue of all glycoproteins and glycolipids on the surface of mammalian cells is normally sialic acid, and addition of this negatively charged residue is known as "sialylation," whereas removal by sialidases is known as "desialylation." High sialylation of the neuronal cell surface inhibits microglial phagocytosis of such neurons, via: (i) activating sialic acid receptors (Siglecs) on microglia that inhibit phagocytosis and (ii) inhibiting binding of opsonins C1q, C3, and galectin-3. Microglial sialylation inhibits inflammatory activation of microglia via: (i) activating Siglec receptors CD22 and CD33 on microglia that inhibit phagocytosis and (ii) inhibiting Toll-like receptor 4 (TLR4), complement receptor 3 (CR3), and other microglial receptors. When activated, microglia release a sialidase activity that desialylates both microglia and neurons, activating the microglia and rendering the neurons susceptible to phagocytosis. Activated microglia also release galectin-3 (Gal-3), which: (i) further activates microglia via binding to TLR4 and TREM2, (ii) binds to desialylated neurons opsonizing them for phagocytosis via Mer tyrosine kinase, and (iii) promotes Aß aggregation and toxicity in vivo. Gal-3 and desialylation may increase in a variety of brain pathologies. Thus, Gal-3 and sialidases are potential treatment targets to prevent neuroinflammation and neurodegeneration.

Modulation of dopamine D1 receptors via histamine H3 receptors is a novel therapeutic target for Huntington's disease.

Early Huntington's disease (HD) include over-activation of dopamine D1 receptors (D1R), producing an imbalance in dopaminergic neurotransmission and cell death. To reduce D1R over-activation, we present a strategy based on targeting complexes of D1R and histamine H3 receptors (H3R). Using an HD mouse striatal cell model and HD mouse organotypic brain slices we found that D1R-induced cell death signaling and neuronal degeneration, are mitigated by an H3R antagonist. We demonstrate that the D1R-H3R heteromer is expressed in HD mice at early but not late stages of HD, correlating with HD progression. In accordance, we found this target expressed in human control subjects and low-grade HD patients. Finally, treatment of HD mice with an H3R antagonist prevented cognitive and motor learning deficits and the loss of heteromer expression. Taken together, our results indicate that D1R - H3R heteromers play a pivotal role in dopamine signaling and represent novel targets for treating HD.

Calreticulin and Galectin-3 Opsonise Bacteria for Phagocytosis by Microglia.

Opsonins are soluble, extracellular proteins, released by activated immune cells, and when bound to a target cell, can induce phagocytes to phagocytose the target cell. There are three known classes of opsonin: antibodies, complement factors and secreted pattern recognition receptors, but these have limited access to the brain. We identify here two novel opsonins of bacteria, calreticulin, and galectin-3 (both lectins that can bind lipopolysaccharide), which were released by microglia (brain-resident macrophages) when activated by bacterial lipopolysaccharide. Calreticulin and galectin-3 both bound to Escherichia coli, and when bound increased phagocytosis of these bacteria by microglia. Furthermore, lipopolysaccharide-induced microglial phagocytosis of E. coli bacteria was partially inhibited by: sugars, an anti-calreticulin antibody, a blocker of the calreticulin phagocytic receptor LRP1, a blocker of the galectin-3 phagocytic receptor MerTK, or simply removing factors released from the microglia, indicating this phagocytosis is dependent on extracellular calreticulin and galectin-3. Thus, calreticulin and galectin-3 are opsonins, released by activated microglia to promote clearance of bacteria. This innate immune response of microglia may help clear bacterial infections of the brain.

Effective Knockdown of Gene Expression in Primary Microglia With siRNA and Magnetic Nanoparticles Without Cell Death or Inflammation.

Microglia, the resident immune cells of the brain, have multiple functions in physiological and pathological conditions, including Alzheimer's disease (AD). The use of primary microglial cell cultures has proved to be a valuable tool to study microglial biology under various conditions. However, more advanced transfection methodologies for primary cultured microglia are still needed, as current methodologies provide low transfection efficiency and induce cell death and/or inflammatory activation of the microglia. Here, we describe an easy, and effective method based on the Glial-Mag method (OZ Biosciences) using magnetic nanoparticles and a magnet to successfully transfect primary microglia cells with different small interfering RNAs (siRNAs). This method does not require specialist facilities or specific training and does not induce cell toxicity or inflammatory activation. We demonstrate that this protocol successfully decreases the expression of two key genes associated with AD, the triggering receptor expressed in myeloid cells 2 (TREM2) and CD33, in primary microglia cell cultures.

Cdk5 Contributes to Huntington's Disease Learning and Memory Deficits via Modulation of Brain Region-Specific Substrates.

Cognitive deficits are a major hallmark of Huntington's disease (HD) with a great impact on the quality of patient's life. Gaining a better understanding of the molecular mechanisms underlying learning and memory impairments in HD is, therefore, of critical importance. Cdk5 is a proline-directed Ser/Thr kinase involved in the regulation of synaptic plasticity and memory processes that has been associated with several neurodegenerative disorders. In this study, we aim to investigate the role of Cdk5 in learning and memory impairments in HD using a novel animal model that expresses mutant huntingtin (mHtt) and has genetically reduced Cdk5 levels. Genetic reduction of Cdk5 in mHtt knock-in mice attenuated both corticostriatal learning deficits as well as hippocampal-dependent memory decline. Moreover, the molecular mechanisms by which Cdk5 counteracts the mHtt-induced learning and memory impairments appeared to be differentially regulated in a brain region-specific manner. While the corticostriatal learning deficits are attenuated through compensatory regulation of NR2B surface levels, the rescue of hippocampal-dependent memory was likely due to restoration of hippocampal dendritic spine density along with an increase in Rac1 activity. This work identifies Cdk5 as a critical contributor to mHtt-induced learning and memory deficits. Furthermore, we show that the Cdk5 downstream targets involved in memory and learning decline differ depending on the brain region analyzed suggesting that distinct Cdk5 effectors could be involved in cognitive impairments in HD.

Singular Location and Signaling Profile of Adenosine A2A-Cannabinoid CB1 Receptor Heteromers in the Dorsal Striatum.

The dorsal striatum is a key node for many neurobiological processes such as motor activity, cognitive functions, and affective processes. The proper functioning of striatal neurons relies critically on metabotropic receptors. Specifically, the main adenosine and endocannabinoid receptors present in the striatum, ie, adenosine A2A receptor (A2AR) and cannabinoid CB1 receptor (CB1R), are of pivotal importance in the control of neuronal excitability. Facilitatory and inhibitory functional interactions between striatal A2AR and CB1R have been reported, and evidence supports that this cross-talk may rely, at least in part, on the formation of A2AR-CB1R heteromeric complexes. However, the specific location and properties of these heteromers have remained largely unknown. Here, by using techniques that allowed a precise visualization of the heteromers in situ in combination with sophisticated genetically modified animal models, together with biochemical and pharmacological approaches, we provide a high-resolution expression map and a detailed functional characterization of A2AR-CB1R heteromers in the dorsal striatum. Specifically, our data unveil that the A2AR-CB1R heteromer (i) is essentially absent from corticostriatal projections and striatonigral neurons, and, instead, is largely present in striatopallidal neurons, (ii) displays a striking G protein-coupled signaling profile, where co-stimulation of both receptors leads to strongly reduced downstream signaling, and (iii) undergoes an unprecedented dysfunction in Huntington's disease, an archetypal disease that affects striatal neurons. Altogether, our findings may open a new conceptual framework to understand the role of coordinated adenosine-endocannabinoid signaling in the indirect striatal pathway, which may be relevant in motor function and neurodegenerative diseases.

Cognitive dysfunction in Huntington's disease: mechanisms and therapeutic strategies beyond BDNF.

One of the main focuses in Huntington's disease (HD) research, as well as in most neurodegenerative diseases, is the development of new therapeutic strategies, as currently there is no treatment to delay or prevent the progression of the disease. Neuronal dysfunction and neuronal death in HD are caused by a combination of interrelated pathogenic processes that lead to motor, cognitive and psychiatric symptoms. Understanding how mutant huntingtin impacts on a plethora of cellular functions could help to identify new molecular targets. Although HD has been classically classified as a neurodegenerative disease affecting voluntary movement, lately cognitive dysfunction is receiving increased attention as it is very invalidating for patients. Thus, an ambitious goal in HD research is to find altered molecular mechanisms that contribute to cognitive decline. In this review, we have focused on those findings related to corticostriatal and hippocampal cognitive dysfunction in HD, as well as on the underlying molecular mechanisms, which constitute potential therapeutic targets. These include alterations in synaptic plasticity, transcriptional machinery and neurotrophic and neurotransmitter signaling.

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.