Protecting motor neurons from toxic insult by antagonism of adenosine A2a and Trk receptors

Transcriptional analysis reveals distinct subtypes in amyotrophic lateral sclerosis: implications for personalized therapy

Amyotrophic lateral sclerosis (ALS) is an incurable disease, caused by the loss of the upper and lower motor neurons. The lack of therapeutic progress is mainly due to the insufficient understanding of complexity and heterogeneity underlying the pathogenic mechanisms of ALS. Recently, we analyzed whole-genome expression profiles of motor cortex of sporadic ALS patients, classifying them into two subgroups characterized by differentially expressed genes and pathways. Some of the deregulated genes encode proteins, which are primary targets of drugs currently in preclinical or clinical studies for several clinical conditions, including neurodegenerative diseases. In this review, we discuss in-depth the potential role of these candidate targets in ALS pathogenesis, highlighting their possible relevance for personalized ALS treatments.

GGGGCC microsatellite RNA is neuritically localized, induces branching defects, and perturbs transport granule function

Microsatellite expansions are the leading cause of numerous neurodegenerative disorders. Here we demonstrate that GGGGCC and CAG microsatellite repeat RNAs associated with C9orf72 in amyotrophic lateral sclerosis/frontotemporal dementia and with polyglutamine diseases, respectively, localize to neuritic granules that undergo active transport into distal neuritic segments. In cultured mammalian spinal cord neurons, the presence of neuritic GGGGCC repeat RNA correlates with neuronal branching defects, and the repeat RNA localizes to granules that label with fragile X mental retardation protein (FMRP), a transport granule component. Using a Drosophila GGGGCC expansion disease model, we characterize dendritic branching defects that are modulated by FMRP and Orb2. The human orthologs of these modifiers are misregulated in induced pluripotent stem cell-differentiated neurons (iPSNs) from GGGGCC expansion carriers. These data suggest that expanded repeat RNAs interact with the messenger RNA transport and translation machinery, causing transport granule dysfunction. This could be a novel mechanism contributing to the neuronal defects associated with C9orf72 and other microsatellite expansion diseases.

Purine nucleosides in neuroregeneration and neuroprotection

In the present review, we stress the importance of the purine nucleosides, adenosine and guanosine, in protecting the nervous system, both centrally and peripherally, via activation of their receptors and intracellular signalling mechanisms. A most novel part of the review focus on the mechanisms of neuronal regeneration that are targeted by nucleosides, including a recently identified action of adenosine on axonal growth and microtubule dynamics. Discussion on the role of the purine nucleosides transversally with the most established neurotrophic factors, e.g. brain derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), is also focused considering the intimate relationship between some adenosine receptors, as is the case of the A2A receptors, and receptors for neurotrophins. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Purinergic contribution to amyotrophic lateral sclerosis

By signalling through purinergic receptors classified as ionotropic P2X (for ATP) and metabotropic P1 (for adenosine) and P2Y (mainly for ADP, UDP, UTP, ATP), the extracellular nucleotides and their metabolic derivatives originated by extracellular activity of several different ectonucleotidases, are involved in the functioning of the nervous system. Here they exert a central role during physiological processes, but also in the precarious balance between beneficial and noxious events. Indeed, in recent years, the dysregulation of extracellular purinergic homeostasis has been correlated to well-characterized acute and chronic neurodegenerative and neuroinflammatory diseases. Among these, we focus our attention on purinergic signalling occurring in amyotrophic lateral sclerosis (ALS), the most common late onset motoneuron disease, characterized by specific loss of motoneurons in brain stem and ventral horns of spinal cord. ALS is a progressive non-cell-autonomous and multifactorial neuroinflammatory disease, whose aetiology and pathological mechanisms are unidentified for most patients and initiate long before any sign or symptom becomes apparent. By combining purinergic with ALS knowledge, in this work we thus present and sustain a novel line of investigation on the purinergic contribution to ALS. In particular, here we recapitulate very early results about P2X4, P2X7 and P2Y6 receptor expression in tissues from ALS animal and cell models and patients, and more recent achievements about purinergic signalling mainly performed in vitro in microglia and lately in astrocytes and motoneurons. We finally highlight how purinergic signalling has progressively evolved up to preclinical trials, to the point of deserving now full consideration with reference to ALS. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Inhibition of Cytohesins Protects against Genetic Models of Motor Neuron Disease

Mutant genes that underlie Mendelian forms of amyotrophic lateral sclerosis (ALS) and biochemical investigations of genetic disease models point to potential driver pathophysiological events involving endoplasmic reticulum (ER) stress and autophagy. Several steps in these cell biological processes are known to be controlled physiologically by small ADP-ribosylation factor (ARF) signaling. Here, we investigated the role of ARF guanine nucleotide exchange factors (GEFs), cytohesins, in models of ALS. Genetic or pharmacological inhibition of cytohesins protects motor neurons in vitro from proteotoxic insults and rescues locomotor defects in a Caenorhabditis elegans model of disease. Cytohesins form a complex with mutant superoxide dismutase 1 (SOD1), a known cause of familial ALS, but this is not associated with a change in GEF activity or ARF activation. ER stress evoked by mutant SOD1 expression is alleviated by antagonism of cytohesin activity. In the setting of mutant SOD1 toxicity, inhibition of cytohesin activity enhances autophagic flux and reduces the burden of misfolded SOD1. These observations suggest that targeting cytohesins may have potential benefits for the treatment of ALS.

An introduction to the roles of purinergic signalling in neurodegeneration, neuroprotection and neuroregeneration

Purinergic signalling appears to play important roles in neurodegeneration, neuroprotection and neuroregeneration. Initially there is a brief summary of the background of purinergic signalling, including release of purines and pyrimidines from neural and non-neural cells and their ectoenzymatic degradation, and the current characterisation of P1 (adenosine), and P2X (ion channel) and P2Y (G protein-coupled) nucleotide receptor subtypes. There is also coverage of the localization and roles of purinoceptors in the healthy central nervous system. The focus is then on the roles of purinergic signalling in trauma, ischaemia, stroke and in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's diseases, as well as multiple sclerosis and amyotrophic lateral sclerosis. Neuroprotective mechanisms involving purinergic signalling are considered and its involvement in neuroregeneration, including the role of adult neural stem/progenitor cells. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Regulation of protein quality control by UBE4B and LSD1 through p53-mediated transcription

Protein quality control is essential for clearing misfolded and aggregated proteins from the cell, and its failure is associated with many neurodegenerative disorders. Here, we identify two genes, ufd-2 and spr-5, that when inactivated, synergistically and robustly suppress neurotoxicity associated with misfolded proteins in Caenorhabditis elegans. Loss of human orthologs ubiquitination factor E4 B (UBE4B) and lysine-specific demethylase 1 (LSD1), respectively encoding a ubiquitin ligase and a lysine-specific demethylase, promotes the clearance of misfolded proteins in mammalian cells by activating both proteasomal and autophagic degradation machineries. An unbiased search in this pathway reveals a downstream effector as the transcription factor p53, a shared substrate of UBE4B and LSD1 that functions as a key regulator of protein quality control to protect against proteotoxicity. These studies identify a new protein quality control pathway via regulation of transcription factors and point to the augmentation of protein quality control as a wide-spectrum antiproteotoxicity strategy.

A new protein quality control regulatory pathway is identified in which a ubiquitin ligase and a lysine-specific demethylase act together on the transcription factor p53 to control protein degradation systems.

To function properly, proteins must assume their correct three-dimensional shapes. There are numerous mechanisms within the cell, collectively referred to as protein quality control (PQC), that verify proper folding. If abnormal folding is detected, PQC can either help the protein to refold or target it for degradation. Failures in protein folding and PQC lead to the accumulation of misfolded proteins, which often self-associate into large aggregations that are thought to be the underlying cause of several neurodegenerative diseases. In this study, we use the roundworm Caenorhabditis elegans as a model to understand how cells handle disease-associated misfolded proteins. In a large-scale genetic screen, we discovered two suppressor genes, ufd-2 and spr-5, which encode a ubiquitin ligase and a lysine-specific demethylase, respectively. When these two proteins are inactivated, we observe a marked reduction in the toxicity of several misfolded proteins. ufd-2 and spr-5 are conserved in humans (UBE4B and LSD1, respectively), as are their effects on misfolded proteins. We show that UBE4B and LSD1 regulate the activity of protein degradation machineries including the proteasome and autophagosomes. Using microarrays and biochemical analyses, we identify p53 as a key downstream transcription factor that mediates the action of UBE4B and LSD1 on protein clearance. This work establishes p53 as a regulator of proteome integrity and uncovers a new protein quality control pathway that could potentially be exploited to increase the degradation of misfolded proteins in diseased cells.

Suppression of adenosine 2a receptor (A2aR)-mediated adenosine signaling improves disease phenotypes in a mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a rapidly progressing neurodegenerative disease in which the majority of upper and lower motor neurons are degenerated. Despite intensive efforts to identify drug targets and develop neuroprotective strategies, effective therapeutics for ALS remains unavailable. The identification and characterization of novel targets and pathways remain crucial in the development of ALS therapeutics. Adenosine is a major neuromodulator that actively regulates synaptic transmission. Interestingly, adenosine levels are significantly elevated in the cerebrospinal fluid (CSF) of progressing human ALS patients. In the current study, we showed that adenosine 2a receptor (A2aR), but not adenosine 1 receptor (A1R), is highly enriched in spinal (motor) neurons. A2aR expression is also selectively increased at the symptomatic onset in the spinal cords of SOD1G93A mice and end-stage human ALS spinal cords. Interestingly, we found that direct adenosine treatment is sufficient to induce embryonic stem cell-derived motor neuron (ESMN) cell death in cultures. Subsequent pharmacological inhibition and partial genetic ablation of A2aR (A2aR(+/-)) significantly protect ESMN from SOD1G93A(+) astrocyte-induced cell death and delay disease progression of SOD1G93A mice. Taken together, our results provide compelling novel evidence that A2aR-mediated adenosine signaling contributes to the selective spinal motor neuron degeneration observed in the SOD1G93A mouse model of ALS.

Lipid membrane domains in the brain

The brain is characterized by the presence of cell types with very different functional specialization, but with the common trait of a very high complexity of structures originated by their plasma membranes. Brain cells bear evident membrane polarization with the creation of different morphological and functional subcompartments, whose formation, stabilization and function require a very high level of lateral order within the membrane. In other words, the membrane specialization of brain cells implies the presence of distinct membrane domains. The brain is the organ with the highest enrichment in lipids like cholesterol, glycosphingolipids, and the most recently discovered brain membrane lipid, phosphatidylglucoside, whose collective behavior strongly favors segregation within the membrane leading to the formation of lipid-driven membrane domains. Lipid-driven membrane domains function as dynamic platforms for signal transduction, protein processing, and membrane turnover. Essential events involved in the development and in the maintenance of the functional integrity of the brain depend on the organization of lipid-driven membrane domains, and alterations in lipid homeostasis, leading to deranged lipid-driven membrane organization, are common in several major brain diseases. In this review, we summarize the forces behind the formation of lipid membrane domains and their biological roles in different brain cells. This article is part of a Special Issue entitled Brain Lipids.

Pathogenic role of ganglioside metabolism in neurodegenerative diseases

Ganglioside metabolism is altered in several neurodegenerative diseases, and this may participate in several events related to the pathogenesis of these diseases. Most changes occur in specific areas of the brain and their distinct membrane microdomains or lipid rafts. Antiganglioside antibodies may be involved in dysfunction of the blood-brain barrier and disease progression in these diseases. In lipid rafts, interactions of glycosphingolipids, including ganglioside, with proteins may be responsible for the misfolding events that cause the fibril and/or aggregate processing of disease-specific proteins, such as α-synuclein, in Parkinson's disease, huntingtin protein in Huntington's disease, and copper-zinc superoxide dismutase in amyotrophic lateral sclerosis. Targeting ganglioside metabolism may represent an underexploited opportunity to design novel therapeutic strategies for neurodegeneration in these diseases.

Regulation of TrkB receptor translocation to lipid rafts by adenosine A(2A) receptors and its functional implications for BDNF-induced regulation of synaptic plasticity

Brain-derived neurotrophic factor (BDNF) signalling is critical for neuronal development and transmission. Recruitment of TrkB receptors to lipid rafts has been shown to be necessary for the activation of specific signalling pathways and modulation of neurotransmitter release by BDNF. Since TrkB receptors are known to be modulated by adenosine A(2A) receptor activation, we hypothesized that activation of A(2A) receptors could influence TrkB receptor localization among different membrane microdomains. We found that adenosine A(2A) receptor agonists increased the levels of TrkB receptors in the lipid raft fraction of cortical membranes and potentiated BDNF-induced augmentation of phosphorylated TrkB levels in lipid rafts. Blockade of the clathrin-mediated endocytosis with monodansyl cadaverine (100 μM) did not modify the effects of the A(2A) receptor agonists, but significantly impaired BDNF effects on TrkB recruitment to lipid rafts. The effect of A(2A) receptor activation in TrkB localization was mimicked by 5 μM forskolin, an adenylyl cyclase activator. Also, it was blocked by the PKA inhibitors RpcAMPs and PKI-(14-22) and by the Src-family kinase inhibitor PP2. Moreover, removal of endogenous adenosine or disruption of lipid rafts reduced BDNF stimulatory effects on glutamate release from cortical synaptosomes. Lipid raft integrity was also required for the effects of BDNF upon hippocampal long-term potentiation at CA1 synapses. Our data demonstrate, for the first time, a BDNF-independent recruitment of TrkB receptors to lipid rafts, induced by the activation of adenosine A(2A) receptors, with functional consequences for TrkB phosphorylation and BDNF-induced modulation of neurotransmitter release and hippocampal plasticity.

Regulation of TrkB receptor translocation to lipid rafts by adenosine A(2A) receptors and its functional implications for BDNF-induced regulation of synaptic plasticity

Brain-derived neurotrophic factor (BDNF) signalling is critical for neuronal development and transmission. Recruitment of TrkB receptors to lipid rafts has been shown to be necessary for the activation of specific signalling pathways and modulation of neurotransmitter release by BDNF. Since TrkB receptors are known to be modulated by adenosine A(2A) receptor activation, we hypothesized that activation of A(2A) receptors could influence TrkB receptor localization among different membrane microdomains. We found that adenosine A(2A) receptor agonists increased the levels of TrkB receptors in the lipid raft fraction of cortical membranes and potentiated BDNF-induced augmentation of phosphorylated TrkB levels in lipid rafts. Blockade of the clathrin-mediated endocytosis with monodansyl cadaverine (100 μM) did not modify the effects of the A(2A) receptor agonists, but significantly impaired BDNF effects on TrkB recruitment to lipid rafts. The effect of A(2A) receptor activation in TrkB localization was mimicked by 5 μM forskolin, an adenylyl cyclase activator. Also, it was blocked by the PKA inhibitors RpcAMPs and PKI-(14-22) and by the Src-family kinase inhibitor PP2. Moreover, removal of endogenous adenosine or disruption of lipid rafts reduced BDNF stimulatory effects on glutamate release from cortical synaptosomes. Lipid raft integrity was also required for the effects of BDNF upon hippocampal long-term potentiation at CA1 synapses. Our data demonstrate, for the first time, a BDNF-independent recruitment of TrkB receptors to lipid rafts, induced by the activation of adenosine A(2A) receptors, with functional consequences for TrkB phosphorylation and BDNF-induced modulation of neurotransmitter release and hippocampal plasticity.

Neuronal and glial purinergic receptors functions in neuron development and brain disease

Brain development requires the interaction of complex signaling pathways, involving different cell types and molecules. For a long time, most attention has focused on neurons in a neuronocentric conceptualization of central nervous system development, these cells fulfilling an intrinsic program that establishes the brain’s morphology and function. By contrast, glia have mainly been studied as support cells, offering guidance or as the cells that react to brain injury. However, new evidence is appearing that demonstrates a more fundamental role of glial cells in the control of different aspects of neuronal development and function, events in which the influence of neurons is at best weak. Moreover, it is becoming clear that the function and organization of the nervous system depends heavily on reciprocal neuron–glia interactions. During development, neurons are often generated far from their final destination and while intrinsic mechanisms are responsible for neuronal migration and growth, they need support and regulatory influences from glial cells in order to migrate correctly. Similarly, the axons emitted by neurons often have to reach faraway targets and in this sense, glia help define the way that axons grow. Moreover, oligodendrocytes and Schwann cells ultimately envelop axons, contributing to the generation of nodes of Ranvier. Finally, recent publications show that astrocytes contribute to the modulation of synaptic transmission. In this sense, purinergic receptors are expressed widely by glial cells and neurons, and recent evidence points to multiple roles of purines and purinergic receptors in neuronal development and function, from neurogenesis to axon growth and functional axonal maturation, as well as in pathological conditions in the brain. This review will focus on the role of glial and neuronal secreted purines, and on the purinergic receptors, fundamentally in the control of neuronal development and function, as well as in diseases of the nervous system.

A2A adenosine receptors are up-regulated in lymphocytes from amyotrophic lateral sclerosis patients

Adenosine, a purine nucleoside interacting with A1, A2A, A2B and A3 adenosine receptors (ARs), is a potent endogenous modulator of inflammatory and neuronal processes involved in the pathophysiology of several neurodegenerative diseases. In the present study, ARs were investigated in lymphocytes from patients with amyotrophic lateral sclerosis (ALS) and compared with age-matched healthy subjects. In ALS patients A2AARs were analysed by using RT-PCR, Western blotting and saturation binding experiments. The effect of A2AAR stimulation on cyclic AMP levels was evaluated in lymphocytes from ALS patients and healthy subjects. An up-regulation of A2AARs was observed in ALS patients with respect to healthy subjects while A1, A2B and A3AR affinity and density did not change. In ALS patients, the A2AAR density values correlated with the Amyotrophic Lateral Sclerosis Functional Rating Scale-Revised (ALSFRS-R) scores. Furthermore, the stimulation of A2AARs mediated a significant increase in cyclic AMP levels in lymphocytes from ALS patients, with a higher potency than in lymphocytes from healthy subjects. In conclusion, the positive correlation between A2AAR density and ALSFRS-R scores could indicate a possible protective effect of this receptor subtype, representing an interesting starting point for the study of alternative therapeutic approaches for ALS based on A2AAR modulation.

Three-dimensional structure of CAP-gly domain of mammalian dynactin determined by magic angle spinning NMR spectroscopy: conformational plasticity and interactions with end-binding protein EB1

Microtubules and their associated proteins play important roles in vesicle and organelle transport, cell motility and cell division. Perturbation of these processes by mutation typically gives rise to severe pathological conditions. In our efforts to obtain atomic information on microtubule-associated protein/microtubule interactions with the goal to understand mechanisms that might potentially assist in the development of treatments for these diseases, we have determined the three-dimensional structure of CAP-Gly (cytoskeleton-associated protein, glycine-rich) domain of mammalian dynactin by magic angle spinning NMR spectroscopy. We observe two conformations in the β2 strand encompassing residues T43-V44-A45, residues that are adjacent to the disease-associated mutation, G59S. Upon binding of CAP-Gly to microtubule plus-end tracking protein EB1, the CAP-Gly shifts to a single conformer. We find extensive chemical shift perturbations in several stretches of residues of CAP-Gly upon binding to EB1, from which we define accurately the CAP-Gly/EB1 binding interface. We also observe that the loop regions may exhibit unique flexibility, especially in the GKNDG motif, which participates in the microtubule binding. This study in conjunction with our previous reports suggests that conformational plasticity is an intrinsic property of CAP-Gly likely due to its unusually high loop content and may be required for its biological functions.

Effects of chronic caffeine intake in a mouse model of amyotrophic lateral sclerosis

Caffeine is a nonselective adenosine receptor antagonist; chronic consumption has proved protective toward neurodegenerative diseases such as Parkinson's and Alzheimer's diseases. The present study was designed to determine whether caffeine intake affected survival and/or motor performance in a transgenic model of amyotrophic lateral sclerosis (ALS). SOD1(G93A) mice received caffeine through drinking water from 70 days of age until death. Body weight, motor performance and survival were evaluated. Furthermore, the expression of adenosine A(2A) receptors (A(2A) Rs), glial glutamate transporter (GLT1), and glial fibrillar acidic protein (GFAP) were evaluated by Western blotting. The results showed that caffeine intake significantly shortened the survival of SOD1(G93A) mice (log rank test, P = 0.01) and induced a nonsignificant advancing of disease onset. The expression of A(2A) R, GLT1, and GFAP was altered in the spinal cords of ALS mice, but caffeine did not influence their expression in either wild-type or SOD1(G93) mice. These data indicate that adenosine receptors may play an important role in ALS.

Voluntary exercise does not ameliorate spatial learning and memory deficits induced by chronic administration of nandrolone decanoate in rats

Chronic exposure to the anabolic androgenic steroids (AAS) nandrolone decanoate (ND) in supra-physiological doses is associated with learning and memory impairments. Given the well-known beneficial effects of voluntary exercise on cognitive functions, we examined whether voluntary exercise would improve the cognitive deficits induced by chronic administration of ND. We also investigated the effects of ND and voluntary exercise on hippocampal BDNF levels. The rats were randomly distributed into 4 experimental groups: the vehicle-sedentary group, the ND-sedentary group, the vehicle-exercise group, and the ND-exercise group. The vehicle-exercise and the ND-exercise groups were allowed to freely exercise in a running wheel for 15 days. The vehicle-sedentary and the ND-sedentary groups were kept sedentary for the same period. Vehicle or ND injections were started 14 days prior to the voluntary exercise and continued throughout the 15 days of voluntary exercise. After the 15-day period, the rats were trained and tested on a water maze spatial task using four trials per day for 5 consecutive days followed by a probe trial two days later. Exercise significantly improved performance during both the training and retention of the water maze task, and enhanced hippocampal BDNF. ND impaired spatial learning and memory, and this effect was not rescued by exercise. ND also potentiated the exercise-induced increase in hippocampal BDNF levels. These results seem to indicate that voluntary exercise is unable to improve the disruption of cognitive functions by chronic ND. Moreover, increased levels of BDNF may play a role in ND-induced impairments in learning and memory. The harmful effects of ND and other AAS on learning and memory should be taken into account when athletes decide to use AAS for performance or body image improvement.

Brain-derived neurotrophic factor and androgen interactions in spinal neuromuscular systems

Neurotrophic factors and steroid hormones interact to regulate a variety of neuronal processes such as neurite outgrowth, differentiation, and neuroprotection. The coexpression of steroid hormone and neurotrophin receptor mRNAs and proteins, as well as their reciprocal regulation provides the necessary substrates for such interactions to occur. This review will focus on androgen brain-derived neurotrophic factor (BDNF) interactions in the spinal cord, describing androgen regulation of BDNF in neuromuscular systems following castration, androgen manipulation, and injury. Androgens interact with BDNF during development to regulate normally-occurring motoneuron death, and in adulthood, androgen-BDNF interactions are involved in the maintenance of several features of neuromuscular systems. Androgens regulate BDNF and trkB expression in spinal motoneurons. Androgens also regulate BDNF levels in the target musculature, and androgenic action at the muscle regulates BDNF levels in motoneurons. These interactions have important implications for the maintenance of motoneuron morphology. Finally, androgens interact with BDNF after injury, influencing soma size, dendritic morphology, and axon regeneration. Together, these findings provide further insight into the development and maintenance of neuromuscular systems and have implications for the neurotherapeutic/neuroprotective roles of androgens and trophic factors in the treatment of motoneuron disease and recovery from injury.

Lipid rafts, synaptic transmission and plasticity: impact in age-related neurodegenerative diseases

The synapse is a crowded area. In the last years, the concept that proteins can be organized in different membrane domains according to their structure has emerged. Cholesterol-rich membrane domains, or lipid rafts, form an organized portion of the membrane that is thought to concentrate signaling molecules. Accumulating evidence has shown that both the pre-synaptic and post-synaptic sites are highly enriched in lipid rafts, which are likely to organize and maintain synaptic proteins in their precise localization. Here we review recent studies highlighting the importance of lipid rafts for synaptic function and plasticity, as well as their relevance for age or disease-related cognitive impairment. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

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.

Ras family small GTPase-mediated neuroprotective signaling in stroke

Selective neuronal cell death is one of the major causes of neuronal damage following stroke, and cerebral cells naturally mobilize diverse survival signaling pathways to protect against ischemia. Importantly, therapeutic strategies designed to improve endogenous anti-apoptotic signaling appear to hold great promise in stroke treatment. While a variety of complex mechanisms have been implicated in the pathogenesis of stroke, the overall mechanisms governing the balance between cell survival and death are not well-defined. Ras family small GTPases are activated following ischemic insults, and in turn, serve as intrinsic switches to regulate neuronal survival and regeneration. Their ability to integrate diverse intracellular signal transduction pathways makes them critical regulators and potential therapeutic targets for neuronal recovery after stroke. This article highlights the contribution of Ras family GTPases to neuroprotective signaling cascades, including mitogen-activated protein kinase (MAPK) family protein kinase- and AKT/PKB-dependent signaling pathways as well as the regulation of cAMP response element binding (CREB), Forkhead box O (FoxO) and hypoxiainducible factor 1(HIF1) transcription factors, in stroke.

Reduced activity of AMP-activated protein kinase protects against genetic models of motor neuron disease

A growing body of research indicates that amyotrophic lateral sclerosis (ALS) patients and mouse models of ALS exhibit metabolic dysfunction. A subpopulation of ALS patients possesses higher levels of resting energy expenditure and lower fat-free mass compared to healthy controls. Similarly, two mutant copper zinc superoxide dismutase 1 (mSOD1) mouse models of familial ALS possess a hypermetabolic phenotype. The pathophysiological relevance of the bioenergetic defects observed in ALS remains largely elusive. AMP-activated protein kinase (AMPK) is a key sensor of cellular energy status and thus might be activated in various models of ALS. Here, we report that AMPK activity is increased in spinal cord cultures expressing mSOD1, as well as in spinal cord lysates from mSOD1 mice. Reducing AMPK activity either pharmacologically or genetically prevents mSOD1-induced motor neuron death in vitro. To investigate the role of AMPK in vivo, we used Caenorhabditis elegans models of motor neuron disease. C. elegans engineered to express human mSOD1 (G85R) in neurons develops locomotor dysfunction and severe fecundity defects when compared to transgenic worms expressing human wild-type SOD1. Genetic reduction of aak-2, the ortholog of the AMPK α2 catalytic subunit in nematodes, improved locomotor behavior and fecundity in G85R animals. Similar observations were made with nematodes engineered to express mutant tat-activating regulatory (TAR) DNA-binding protein of 43 kDa molecular weight. Altogether, these data suggest that bioenergetic abnormalities are likely to be pathophysiologically relevant to motor neuron disease.

Purinergic signalling: from normal behaviour to pathological brain function

Purinergic neurotransmission, involving release of ATP as an efferent neurotransmitter was first proposed in 1972. Later, ATP was recognised as a cotransmitter in peripheral nerves and more recently as a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the CNS. Both ATP, together with some of its enzymatic breakdown products (ADP and adenosine) and uracil nucleotides are now recognised to act via P2X ion channels and P1 and P2Y G protein-coupled receptors, which are widely expressed in the brain. They mediate both fast signalling in neurotransmission and neuromodulation and long-term (trophic) signalling in cell proliferation, differentiation and death. Purinergic signalling is prominent in neurone-glial cell interactions. In this review we discuss first the evidence implicating purinergic signalling in normal behaviour, including learning and memory, sleep and arousal, locomotor activity and exploration, feeding behaviour and mood and motivation. Then we turn to the involvement of P1 and P2 receptors in pathological brain function; firstly in trauma, ischemia and stroke, then in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's, as well as multiple sclerosis and amyotrophic lateral sclerosis. Finally, the role of purinergic signalling in neuropsychiatric diseases (including schizophrenia), epilepsy, migraine, cognitive impairment and neuropathic pain will be considered.

The in vivo contribution of motor neuron TrkB receptors to mutant SOD1 motor neuron disease

Brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB) are widely expressed in the vertebrate nervous system and play a central role in mature neuronal function. In vitro BDNF/TrkB signaling promotes neuronal survival and can help neurons resist toxic insults. Paradoxically, BDNF/TrkB signaling has also been shown, under certain in vitro circumstances, to render neurons vulnerable to insults. We show here that in vivo conditional deletion of TrkB from mature motor neurons attenuates mutant superoxide dismutase 1 (SOD1) toxicity. Mutant SOD1 mice lacking motor neuron TrkB live a month longer than controls and retain motor function for a longer period, particularly in the early phase of the disease. These effects are subserved by slowed motor neuron loss, persistence of neuromuscular junction integrity and reduced astrocytic and microglial reactivity within the spinal cord. These results suggest that manipulation of BDNF/TrkB signaling might have therapeutic efficacy in motor neuron diseases.

Cannabinoid receptor signalling in neurodegenerative diseases: a potential role for membrane fluidity disturbance

Type-1 cannabinoid receptor (CB(1)) is the most abundant G-protein-coupled receptor (GPCR) in the brain. CB(1) and its endogenous agonists, the so-called 'endocannabinoids (eCBs)', belong to an ancient neurosignalling system that plays important functions in neurodegenerative and neuroinflammatory disorders like Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis. For this reason, research on the therapeutic potential of drugs modulating the endogenous tone of eCBs is very intense. Several GPCRs reside within subdomains of the plasma membranes that contain high concentrations of cholesterol: the lipid rafts. Here, the hypothesis that changes in membrane fluidity alter function of the endocannabinoid system, as well as progression of particular neurodegenerative diseases, is described. To this end, the impact of membrane cholesterol on membrane properties and hence on neurodegenerative diseases, as well as on CB(1) signalling in vitro and on CB(1) -dependent neurotransmission within the striatum, is discussed. Overall, present evidence points to the membrane environment as a critical regulator of signal transduction triggered by CB(1) , and calls for further studies aimed at better clarifying the contribution of membrane lipids to eCBs signalling. The results of these investigations might be exploited also for the development of novel therapeutics able to combat disorders associated with abnormal activity of CB(1).

Identification of anti-inflammatory targets for Huntington's disease using a brain slice-based screening assay

Huntington's disease (HD) is a late-onset, neurodegenerative disease for which there are currently no cures nor disease-modifying treatments. Here we report the identification of several potential anti-inflammatory targets for HD using an ex vivo model of HD that involves the acute transfection of human mutant huntingtin-based constructs into rat brain slices. This model recapitulates key components of the human disease, including the formation of intracellular huntingtin protein (HTT)-containing inclusions and the progressive neurodegeneration of striatal neurons-both occurring within the native tissue context of these neurons. Using this "high-throughput biology" screening platform, we conducted a hypothesis-neutral screen of a collection of drug-like compounds which identified several anti-inflammatory targets that provided neuroprotection against HTT fragment-induced neurodegeneration. The nature of these targets provide further support for non-cell autonomous mechanisms mediating significant aspects of neuropathogenesis induced by mutant HTT fragment proteins.

ALS: focus on purinergic signalling

Amyotrophic lateral sclerosis (ALS) is one of the most common neuromuscular diseases. It is devastating and fatal, causing progressive paralysis of all voluntary muscles and eventually death, while sparing cognitive functions. A pathological hallmark of ALS is neuroinflammation mediated by non-neuronal cells in the nervous system, such as microglia and astrocytes that accelerate the disease progression. Scientists have neither found a unique key mechanism, nor an effective treatment against ALS, supposedly because it is a multi-factorial and multi-systemic disease. Extracellular purines and pyrimidines are widespread and powerful physiopathological molecules, signalling to most cell types and directing cell-to-cell communication networks. They are instrumental for instance for neurotransmission, muscle contraction and immune surveillance. Recent work has reported the crucial involvement of purinergic pathways in many neurodegenerative and neuroinflammatory diseases, comprising ALS. Especially P2 receptors for ATP, P1 receptors for adenosine, and nucleotide transporters were found to be modulated in ALS cells and tissues, playing a potential role in the disease. Given the composite cellular cross-talk occurring during ALS and the established action of extracellular purines/pyrimidines as neuron-to-glia alarm signal in the nervous system, a mutual query in these two fields should now be whether, how and when purinergic would meet ALS. In this review, we will highlight the early cellular and molecular purinergic cross-talk that participates to ALS etiopathology, with the conviction that better understanding of purinergic dynamics might provide original research perspectives, stimulate alternative disease modelling, and the design and testing of more powerful targeted therapeutics against this relentlessly progressive disorder.

Pharmacological characterization of six trkB antibodies reveals a novel class of functional agents for the study of the BDNF receptor

BACKGROUND AND PURPOSE

By interacting with trkB receptors, brain-derived neurotrophic factor (BDNF) triggers various signalling pathways responsible for neurone survival, differentiation and modulation of synaptic transmission. Numerous reports have implicated BDNF and trkB in the pathogenesis of various central nervous system affections and in cancer, thus representing trkB as a promising therapeutic target. In this study, we used an antibody-based approach to search for trkB-selective functional reagents.

EXPERIMENTAL APPROACH

Six commercially available polyclonal and monoclonal antibodies were tested on recombinant and native, human and rodent trkB receptors. Functional and pharmacological characterization was performed using a modified version of the KIRA-elisa method and radioligand binding studies. Western blot analyses and neurite outgrowth assays were carried out to determine the specificity and selectivity of antibody effects. The survival properties of one antibody were further assessed on cultured neurones in a serum-deprived paradigm.

KEY RESULTS

The functional trkB-selective antibodies showed distinct pharmacological profiles, ranging from partial agonists to antagonists, acting on trkB receptors through allosteric modulations. The same diversity of effects was observed on the mitogen-activated protein kinase signalling pathway downstream of trkB and on the subsequent neurite outgrowth. One antibody with partial agonist activity demonstrated cell survival properties by activating the Akt pathway. Finally, these antibodies were functionally validated as true trkB-selective ligands because they failed activating trkA or trkC, and contrary to BDNF, none of them bind to p75(NTR).

CONCLUSIONS AND IMPLICATIONS

These trkB-selective antibodies represent a novel class of pharmacological tools to explore the pathophysiological roles of trkB and its potential therapeutic relevance for the treatment of various disorders.

Adenosine receptors and membrane microdomains

Adenosine receptors are a member of the large family of seven transmembrane spanning G protein coupled receptors. The four adenosine receptor subtypes-A(1), A(2a), A(2b), A(3)-exert their effects via the activation of one or more heterotrimeric G proteins resulting in the modulation of intracellular signaling. Numerous studies over the past decade have documented the complexity of G protein coupled receptor signaling at the level of protein-protein interactions as well as through signaling cross talk. With respect to adenosine receptors, the activation of one receptor subtype can have profound direct effects in one cell type but little or no effect in other cells. There is significant evidence that the compartmentation of subcellular signaling plays a physiological role in the fidelity of G protein coupled receptor signaling. This compartmentation is evident at the level of the plasma membrane in the form of membrane microdomains such as caveolae and lipid rafts. This review will summarize and critically assess our current understanding of the role of membrane microdomains in regulating adenosine receptor signaling.

Circulating levels of brain-derived neurotrophic factor: correlation with mood, cognition and motor function

Brain-derived neurotrophic factor (BDNF) is the most widely distributed neurotrophin in the CNS, where it plays several pivotal roles in synaptic plasticity and neuronal survival. As a consequence, BDNF has become a key target in the physiopathology of several neurological and psychiatric diseases. Recent studies have consistently reported altered levels of BDNF in the circulation (i.e., serum or plasma) of patients with major depression, bipolar disorder, Alzheimer's disease, Huntington's disease and Parkinson's disease. Correlations between serum BDNF levels and affective, cognitive and motor symptoms have also been described. BDNF appears to be an unspecific biomarker of neuropsychiatric disorders characterized by neurodegenerative changes.

Adenosine A2A receptors control neuroinflammation and consequent hippocampal neuronal dysfunction

The blockade of adenosine A(2A) receptors (A2AR) affords a robust neuroprotection in different noxious brain conditions. However, the mechanisms underlying this general neuroprotection are unknown. One possible mechanism could be the control of neuroinflammation that is associated with brain damage, especially because A2AR efficiently control peripheral inflammation. Thus, we tested if the intracerebroventricular injection of a selective A2AR antagonist (SCH58261) would attenuate the changes in the hippocampus triggered by intraperitoneal administration of lipopolysaccharide (LPS) that induces neuroinflammation through microglia activation. LPS administration triggers an increase in inflammatory mediators like interleukin-1β that causes biochemical changes (p38 and c-jun N-terminal kinase phosphorylation and caspase 3 activation) contributing to neuronal dysfunction typified by decreased long-term potentiation, a form of synaptic plasticity. Long-term potentiation, measured 30 min after the tetanus, was significantly lower in LPS-treated rats compared with control-treated rats, while SCH58261 attenuated the LPS-induced change. The LPS-induced increases in phosphorylation of c-jun N-terminal kinase and p38 and activation of caspase 3 were also prevented by SCH58261. Significantly, SCH58261 also prevented the LPS-induced recruitment of activated microglial cells and the increase in interleukin-1β concentration in the hippocampus, indicating that A2AR activation is a pivotal step in mediating the neuroinflammation triggered by LPS. These results indicate that A2AR antagonists prevent neuroinflammation and support the hypothesis that this mechanism might contribute for the ability of A2AR antagonists to control different neurodegenerative diseases known to involve neuroinflammation.

Spinal 5-HT7 receptor activation induces long-lasting phrenic motor facilitation

Acute intermittent hypoxia elicits a form of serotonin-dependent respiratory plasticity known as phrenic long term facilitation (pLTF). Episodic spinal serotonin-2 (5-HT2) receptor activation on or near phrenic motor neurons is necessary for pLTF. A hallmark of pLTF is the requirement for serotonin-dependent synthesis of brain-derived neurotrophic factor (BDNF), and activation of its high affinity receptor, TrkB. Activation of spinal Gs protein-coupled adenosine 2A receptors (GsPCRs) elicits a unique form of long-lasting phrenic motor facilitation (PMF), but via unique mechanisms (BDNF independent TrkB trans-activation).We hypothesized that other GsPCRs elicit PMF, specifically serotonin-7 (5-HT7) receptors, which are expressed in phrenic motor neurons. Cervical spinal (C4) injections of a selective 5-HT7 receptor agonist, AS-19 (10 μM, 5 μl; 3 × 5 min), in anaesthetized, vagotomized and ventilated male Sprague-Dawley rats elicited long-lasting PMF (>120 min), an effect prevented by pretreatment with a 5-HT7 receptor antagonist (SB 269970; 5mM, 7 μl).GsPCR activation 'trans-activates'TrkB by increasing synthesis of an immature TrkB isoform. Spinal injection of a TrkB inhibitor (k252a) and siRNAs that prevent TrkB (but not BDNF) mRNA translation both blocked 5-HT7 agonist-induced PMF, confirming a requirement for TrkB synthesis and activity. k252a affected late PMF (≥ 90 min) only. Spinal inhibition of the PI3K/AKT pathway blocked 5-HT7 agonist-induced PMF, whereas MEK/ERK inhibition delayed, but did not block, PMF. An understanding of signalling mechanisms giving rise to PMF may guide development of novel therapeutic strategies to treat ventilatory control disorders associated with respiratory insufficiency, such as spinal injury and motor neuron disease.

Purinergic signalling at the plasma membrane: a multipurpose and multidirectional mode to deal with amyotrophic lateral sclerosis and multiple sclerosis

ATP is a widespread and multipurpose signalling molecule copiously released in the extracellular environment of the whole nervous system upon cell activation, stress, or damage. Extracellular ATP is also a multidirectional information molecule, given the concurrent presence at the plasma membrane of various targets for ATP. These include ectonucleotidases (metabolizing ATP down to adenosine), ATP/adenosine transporters, P2 receptors for purine/pyrimidine nucleotides (ligand-gated ion channels P2X receptors and G-protein-coupled P2Y receptors), in addition to metabotropic P1 receptors for nucleosides. All these targets rarely operate as single units, rather they associate with each other at the plasma membrane as multi-protein complexes. Altogether, they control the duration, magnitude and/or direction of the signals triggered and propagated by purine/pyrimidine ligands, and the impact that each single ligand has on a variety of short- and long-term functions. A strict control system allows assorted, even divergent, biological outcomes. Among these, we enumerate cell-to-cell communication, tropic, trophic, but also noxious actions causing the insurgence/progression of pathological conditions. Here, we show that purinergic signalling in the nervous system can be instrumental for instance to neurodegenerative and neuroinflammatory diseases such as amyotrophic lateral sclerosis and multiple sclerosis.

Signaling events in axons and/or dendrites render motor neurons vulnerable to mutant superoxide dismutase toxicity

The survival of dorsal root ganglion and sympathetic neurons is promoted whether nerve growth factor (NGF) activates TrkA receptors on the cell body or the axon. Yet other aspects of neurotrophic factor actions (i.e., ability to promote axon growth, selection of neurochemical phenotype and engagement of signaling modules) differ as a function of the location of the ligand-receptor interaction. The extent to which these observations are relevant to CNS neurons is unknown. This may be particularly relevant to neurodegenerative diseases such as amyotrophic lateral sclerosis, where beneficial axon-target interactions are disturbed early in the disease process. Here we characterize the growth of pure motor neurons in compartment cultures and show that brain-derived neurotrophic factor (BDNF) stimulation of the cell body or axons/dendrites promotes survival. Expression of G37R mutant superoxide dismutase (SOD) in motor neurons will lead to death and this depends on BDNF activation of TrkB on axons and/or dendrites. BDNF action depends upon endocytosis of the BDNF-TrkB complex and de novo protein synthesis. These results highlight the importance of signaling events occurring in axons/dendrites in mutant SOD toxicity.

Role of purinergic receptors in CNS function and neuroprotection

The purinergic receptor family contains some of the most abundant receptors in living organisms. A growing body of evidence indicates that extracellular nucleotides play important roles in the regulation of neuronal and glial functions in the nervous system through purinergic receptors. Nucleotides are released from or leaked through nonexcitable cells and neurons during normal physiological and pathophysiological conditions. Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system (CNS), participate in the synaptic processes, and mediate intercellular communications between neuron and gila and between glia and other glia. Glial cells in the CNS are classified into astrocytes, oligodendrocytes, and microglia. Astrocytes express many types of purinergic receptors, which are integral to their activation. Astrocytes release adenosine triphosphate (ATP) as a "gliotransmitter" that allows communication with neurons, the vascular walls of capillaries, oligodendrocytes, and microglia. Oligodendrocytes are myelin-forming cells that construct insulating layers of myelin sheets around axons, and using purinergic receptor signaling for their development and for myelination. Microglia also express many types of purinergic receptors and are known to function as immunocompetent cells in the CNS. ATP and other nucleotides work as "warning molecules" especially by activating microglia in pathophysiological conditions. Studies on purinergic signaling could facilitate the development of novel therapeutic strategies for disorder of the CNS.

Trophic effects of brain-derived neurotrophic factor blockade in an androgen-sensitive neuromuscular system

In adult male rats, androgens are necessary for the maintenance of the motoneurons and their target muscles of the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB) neuromuscular system, regulating motoneuron and muscle morphology, function, and expression of trophic factors. Castration of males results in somal, dendritic, and muscle atrophy as well as increases in brain-derived neurotrophic factor (BDNF) in the target musculature. Because BDNF can have either facilitative or inhibitory effects in other systems, we examined SNB neuromuscular morphology after BDNF blockade using a fusion protein (tyrosine kinase receptor type B IgG). Blockade of BDNF in gonadally intact males resulted in hypertrophy of SNB motoneuron dendrites and target musculature, suggesting that normal levels of BDNF are inhibitory in SNB neuromuscular system. BDNF blockade in castrated males prevented SNB motoneuron atrophy and attenuated target muscle weight loss. This is the first demonstration that the highly androgen-sensitive SNB motoneuron dendrites and target muscles can be maintained in the absence of gonadal hormones and, furthermore, that blocking BDNF can have trophic effects on skeletal muscle. These results suggest that whereas BDNF is involved in the signaling cascade mediating the androgenic support of SNB neuromuscular morphology, its action can be inhibitory. Furthermore, the elevations in BDNF after castration may be responsible for the castration-induced atrophy in SNB motoneurons and target muscles, and the trophic effects of androgens may be mediated in part through a suppression of BDNF. These results may have relevance to therapeutic approaches to the treatment of neurodegenerative disease or myopathies.

Use of laser microdissection in the investigation of facial motoneuron and neuropil molecular phenotypes after peripheral axotomy

The mechanism underlying axotomy-induced motoneuron loss is not fully understood, but appears to involve molecular changes within the injured motoneuron and the surrounding local microenvironment (neuropil). The mouse facial nucleus consists of six subnuclei which respond differentially to facial nerve transection at the stylomastoid foramen. The ventromedial (VM) subnucleus maintains virtually full facial motoneuron (FMN) survival following axotomy, whereas the ventrolateral (VL) subnucleus results in significant FMN loss with the same nerve injury. We hypothesized that distinct molecular phenotypes of FMN existed within the two subregions, one responsible for maintaining cell survival and the other promoting cell death. In this study, we used laser microdissection to isolate VM and VL facial subnuclear regions for molecular characterization. We discovered that, regardless of neuronal fate after injury, FMN in either subnuclear region respond vigorously to injury with a characteristic "regenerative" profile and additionally, the surviving VL FMN appear to compensate for the significant FMN loss. In contrast, significant differences in the expression of pro-inflammatory cytokine mRNA in the surrounding neuropil response were found between the two subnuclear regions of the facial nucleus that support a causative role for glial and/or immune-derived molecules in directing the contrasting responses of the FMN to axonal transection.

The effect of simvastatin on the proteome of detergent-resistant membrane domains: decreases of specific proteins previously related to cytoskeleton regulation, calcium homeostasis and cell fate

Cell death induced by over-activation of glutamate receptors occurs in different neuropathologies. Cholesterol depletors protect from neurotoxic over-activation of glutamate receptors, and we have recently reported that this neuroprotection is associated with a reduction of the N-methyl-D-aspartate subtype of glutamate receptors in detergent-resistant membrane domains (DRM). In the present study we used comparative proteomics to further identify which proteins, besides the N-methyl-D-aspartate receptor, change its percentage of association to DRM after treatment of neurons with simvastatin. We detected 338 spots in neuronal DRM subjected to 2-DE; eleven of these spots changed its intensity after treatment with simvastatin. All 11 differential spots showed reduced intensity in simvastatin-treated samples and were identified as adipocyte plasma membrane associated protein, enolase, calretinin, coronin 1a, f-actin capping protein alpha1, f-actin capping protein alpha2, heat shock cognate protein 71, malate dehydrogenase, n-myc downregulated gene 1, prohibitin 2, Rab GDP dissociation inhibitor, translationally controlled tumor protein and voltage dependent anion selective channel protein 1. The proteins tested colocalized with the lipid raft marker caveolin-1. Interestingly, the proteins we have identified in the present study had been previously reported to play a role in cell fate and, thus, they might represent novel targets for neuroprotection.

Activation of adenosine A2A receptors induces TrkB translocation and increases BDNF-mediated phospho-TrkB localization in lipid rafts: implications for neuromodulation

Brain-derived neurotrophic factor (BDNF) signaling is critical for neuronal development and transmission. Recruitment of TrkB receptors to lipid rafts has been shown to be necessary for the activation of specific signaling pathways and modulation of neurotransmitter release by BDNF. Since TrkB receptors are known to be modulated by adenosine A(2A) receptor activation, we hypothesized that activation of A(2A) receptors could influence TrkB receptor localization among different membrane microdomains. We found that adenosine A(2A) receptor agonists increased the levels of TrkB receptors in the lipid raft fraction of cortical membranes and potentiated BDNF-induced augmentation of phosphorylated TrkB levels in lipid rafts. Blockade of the clathrin-mediated endocytosis with monodansylcadaverine (100 microm) did not modify the effects of the A(2A) receptor agonists but significantly impaired BDNF effects on TrkB recruitment to lipid rafts. The effect of A(2A) receptor activation in TrkB localization was mimicked by 5 microm forskolin, an adenylyl cyclase activator. Also, it was blocked by the PKA inhibitors Rp-cAMPs and PKI-(14-22), and by the Src-family kinase inhibitor PP2. Moreover, removal of endogenous adenosine or disruption of lipid rafts reduced BDNF stimulatory effects on glutamate release from cortical synaptosomes. Lipid raft integrity was also required for the effects of BDNF on hippocampal long-term potentiation at CA1 synapses. Our data demonstrate, for the first time, a BDNF-independent recruitment of TrkB receptors to lipid rafts induced by activation of adenosine A(2A) receptors, with functional consequences for TrkB phosphorylation and BDNF-induced modulation of neurotransmitter release and hippocampal plasticity.

Modulation of brain-derived neurotrophic factor (BDNF) actions in the nervous system by adenosine A(2A) receptors and the role of lipid rafts

In this paper we review some novel aspects related to the way adenosine A(2A) receptors (A(2A)R) modulate the action of BDNF or its high-affinity receptors, the TrkB receptors, on synaptic transmission and plasticity, as well as upon cholinergic currents and GABA transporters. Evidence has been accumulating that adenosine A(2A)Rs are required for most of the synaptic actions of BDNF. In some cases, where A(2A)Rs are constitutively activated (e.g. by endogenous extracellular adenosine), the need for A(2A)R activation for the maintenance of the synaptic influences of BDNF can be envisaged from the loss of BDNF effects upon blockade of adenosine A(2A)Rs or upon removal of extracellular adenosine with adenosine deaminase. In some other cases, it is necessary to enhance extracellular adenosine levels (e.g. depolarization) or to further activate A(2A)Rs (e.g. with selective agonists) to trigger a BDNF neuromodulatory role at the synapses. Age- and cell-dependent differences may determine the above two possibilities, but in all cases it is quite clear that there is close interplay between adenosine A(2A)Rs and BDNF TrkB receptors at synapses. The role of lipid rafts in this cross-talk will be discussed. This article is part of a Special Issue entitled: "Adenosine Receptors".

Tuning and fine-tuning of synapses with adenosine

The ‘omnipresence’ of adenosine in all nervous system cells (neurons and glia) together with the intensive release of adenosine following insults, makes adenosine as a sort of ‘maestro’ of synapses leading to the homeostatic coordination of brain function. Besides direct actions of adenosine on the neurosecretory mechanisms, where adenosine operates to tune neurotransmitter release, receptor-receptor interactions as well as interplays between adenosine receptors and transporters occur as part of the adenosine’s attempt to fine tuning synaptic transmission. This review will focus on the different ways adenosine can use to trigger or brake the action of several neurotransmitters and neuromodulators. Adenosine receptors cross talk with other G protein coupled receptors (GPCRs), with ionotropic receptors and with receptor kinases. Most of these interactions occur through A2A receptors, which in spite their low density in some brain areas, such as the hippocampus, may function as metamodulators. Tonic adenosine A2A receptor activity is a required step to allow synaptic actions of neurotrophic factors, namely upon synaptic transmission at both pre- and post-synaptic level as well as upon synaptic plasticity and neuronal survival. The implications of these interactions in normal brain functioning and in neurologic and psychiatric dysfunction will be discussed.

Modulation and metamodulation of synapses by adenosine

The presence of adenosine in all nervous system cells (neurones and glia) together with its intensive release following insults makes adenosine as a sort of 'regulator' of synaptic communication, leading to the homeostatic coordination of brain function. Besides the direct actions of adenosine on the neurosecretory mechanisms, to tune neurotransmitter release, adenosine receptors interact with other receptors as well as with transporters as part of its attempt to fine-tune synaptic transmission. This review will focus on examples of the different ways adenosine can use to modulate or metamodulate synapses, in other words, to trigger or brake the action of some neurotransmitters and neuromodulators, to cross-talk with other G protein-coupled receptors, with ionotropic receptors and with receptor kinases as well as with transporters. Most of these interactions occur through A2A receptors, which in spite of their low density in some brain areas, such as the hippocampus, may function as amplifiers of the signalling of other mediators at synapses.

BDNF levels are not related with levodopa-induced dyskinesias in MPTP monkeys

Levodopa-induced dyskinesias (LIDs) are frequent in parkinsonian patients and may result from an aberrant plasticity. Brain-derived neurotrophic factor (BDNF) represents a likely candidate to subserve neuroadaptive processes encountered in LIDs. We compared striatal BDNF levels measured by ELISA in levodopa-treated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys having developed LIDs compared with animals where LIDs were prevented by the addition of CI-1041 (NR1A/2B NMDA receptor antagonist) or low doses of cabergoline (dopamine D2 receptor agonist). We observed reduced striatal BDNF concentrations in levodopa-treated MPTP monkeys with or without LIDs, suggesting that levodopa treatment is associated with reduced striatal BDNF levels and is independent of dyskinesias.

Cyclotraxin-B, the first highly potent and selective TrkB inhibitor, has anxiolytic properties in mice

In the last decades, few mechanistically novel therapeutic agents have been developed to treat mental and neurodegenerative disorders. Numerous studies suggest that targeting BDNF and its TrkB receptor could be a promising therapeutic strategy for the treatment of brain disorders. However, the development of potent small ligands for the TrkB receptor has proven to be difficult. By using a peptidomimetic approach, we developed a highly potent and selective TrkB inhibitor, cyclotraxin-B, capable of altering TrkB-dependent molecular and physiological processes such as synaptic plasticity, neuronal differentiation and BDNF-induced neurotoxicity. Cyclotraxin-B allosterically alters the conformation of TrkB, which leads to the inhibition of both BDNF-dependent and -independent (basal) activities. Finally, systemic administration of cyclotraxin-B to mice results in TrkB inhibition in the brain with specific anxiolytic-like behavioral effects and no antidepressant-like activity. This study demonstrates that cyclotraxin-B might not only be a powerful tool to investigate the role of BDNF and TrkB in physiology and pathology, but also represents a lead compound for the development of new therapeutic strategies to treat brain disorders.

Multipotent neurotrophin antagonist targets brain-derived neurotrophic factor and nerve growth factor

Brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) are members of the neurotrophin family that normally play a role in the development and maintenance of the nervous system. However, neurotrophin dysregulation has been implicated in several neurodegenerative diseases and psychiatric disorders including Alzheimer's disease, Parkinson's disease, neuropathic pain, depression, and substance abuse. Despite their central role in the nervous system, neurotrophins have proved to be an elusive pharmacological target. Here, we describe a novel multipotent neurotrophin antagonist, 3-[(5E)-4-oxo-5-[[5-(4-sulfamoylphenyl)-2-furyl]methylene]-2-thioxo-thiazolidin-3-yl]propanoic acid (Y1036). Y1036 binds BDNF (K(D) = 3.5 +/- 0.3 microM) and NGF (K(D) = 3.0 +/- 0.4 microM) preventing either BDNF or NGF from interacting with their obligate receptor(s). Y1036 prevents both BDNF- and NGF-mediated trk activation, downstream activation of the p44/42 mitogen-activated protein kinase pathway, and neurotrophin-mediated differentiation of dorsal-root ganglion sensory neurons. Identification of a BDNF- and NGF-specific antagonist is of considerable interest in the study and treatment of diseases where dysregulation of multiple neurotrophins has been implicated.