Nitrosylation of proteins is emerging as a key post-translational modification important in both normal physiology and a wide spectrum of diseases, including neurodegenerative diseases. Physiological levels of nitric oxide (NO) can be neuroprotective, in part at least, by inhibiting caspase activity, but excess NO production leads to activation of cell death signalling cascades involved in many neurodegenerative disorders. Neuronal cell injury and death, which are prominent features of disorders such as Alzheimer’s, Huntington’s, and Parkinson’s diseases, are often mediated by the caspase family of cysteine proteases. Caspase activity is inhibited by S-nitrosylation and is also regulated by inhibitors of apoptosis such as X-linked inhibitor of apoptosis (XIAP) which associates with active caspases and represses their catalytic activity. XIAP also functions as an E3 ubiquitin ligase, targeting caspases for degradation by the proteasome.

A team of scientists led by Sanford-Burnham researchers have now discovered a new twist in caspase regulation. They showed that S-nitrosylation of XIAP (forming SNO-XIAP) inhibits the protein’s E3 ligase and antiapoptotic activity and also found that XIAP can be transnitrosylated by SNO-caspase but not vice versa. They found significant amounts of SNO-XIAP, but not SNO-caspase, in the brains of individuals with neurodegenerative diseases, suggesting that SNO-XIAP contributes to neuronal injury or death. The team hope that their study, which is published in the journal Molecular Cell, might lead to better biomarkers and earlier diagnosis for neurodegenerative diseases.

Over 30% of the 50 million people who are affected by epilepsy do not have their seizures adequately controlled, even with the best available medicines. The 29 amino acid neuropeptide, galanin, is a potent endogenous anticonvulsant that activates galanin receptors type 1 (GalR1) and type 2 (GalR2) and a number of groups, including researchers at the Scripps Research Institute, have been trying to develop drugs that mimic the effects of galanin as novel anticonvulsants. Galnon is an example of a systemically active nonpeptide galanin receptor ligand with affinity for the three galanin receptors which has been shown to reduce seizures in animal models. The Scripps team have now identified a compound that acts appears to act as a selective positive allosteric modulator of the galanin receptor type 2 (GalR2) which they hope will have a reduced potential for side effects compared with galnon. The compound, CYM2503, potentiated the effects of galanin in cells stably expressing the GalR2 receptor, but had no detectable affinity for the galanin binding site. In rodent models of epilepsy, intraperitoneal administration of CYM2503 increased the time to seizure, reduced the duration of seizures, and increased survival rate at 24 h.

The study is published in the Proceedings of the National Academy of Sciences.

Huntington’s disease is an inherited neurodegenerative disorder associated with mutations in the huntingtin gene on human chromosome 4. Although the functions of normal huntingtin protein are not entirely clear, it is known that abnormal huntingtin (mutantHtt, or mHtt) – and especially small proteolytic fragments of the protein – are toxic to neurons, particularly those in the striatum and cortex. Previous studies into the cleavage of huntingtin have focussed on the role of the cysteine protease families of caspases and calpains, but scientists at the Buck Institute for Age Research have now discovered that a metalloprotease also cleaves huntingtin into highly toxic fragments.

The team used a set of small interfering RNA (siRNA) pools targeting the 514 known and predicted human protease genes to identify those proteases involved in the cleavage of huntingtin. Eleven proteases were found to alter huntingtin fragment accumulation, and knockdown of the nine which are expressed in striatal cells significantly reduced huntingtin-mediated striatal cell death in a cellular toxicity screen. Amongst the proteases associated with huntingtin cleavage were three metalloproteases, MMP-10, MMP-14 and MMP-23B. Subsequent experiments showed that only MMP-10 directly cleaves huntingtin, suggesting that knockdown of MMP-10 reduces toxicity by directly altering proteolysis of huntingtin whereas knockdown of MMP-14 or MMP-23B modulates toxicity indirectly through proteolysis of cytokines or components of the extracellular matrix. Whilst matrix metalloproteases are generally thought to be secreted as proenzymes which are processed to the active forms extracellularly, MMP-10 was found to be activated inside the cell and to co-localise with huntingtin, suggesting that cleavage may occur intracellularly.

The study, which is published in the journal Neuron, suggests a role for matrix metalloproteases in the progression of Huntington’s disease and that development of inhibitors of MMP-10 may be a useful therapeutic strategy.

Following from positive phase II results, the announcement earlier this year that Dimebon (latrepirdine) failed to show a significant effect in a phase III clinical trial in Alzheimer’s patients was a major blow to patients, families and doctors. A study by researchers at UT Southwestern Medical Center has now shown that Dimebon can increase neurogenesis in adult rodent brains and have identified other, more potent compounds.

An in vivo screen of 1000 small molecules in adult mice identified eight compounds that were able to enhance neuron formation in the subgranular zone of the hippocampal dentate gyrus. One of the compounds, P7C3, was selected for further study on the basis of favourable ADME predictions. Daily administration of P7C3 to aged rats for 7 days was shown to enhance hippocampal neurogenesis relative to control animals and, after 2 months, treated rats performed significantly better in the Morris water maze test which provides a measure of learning and memory.

P7C3 exerts its proneurogenic effects by protecting newborn neurons from apoptosis and the team next compared the activity of P7C3 with that of Dimebon, which is also believed to have anti-apoptotic activity. Dimebon was found to be proneurogenic in vivo, albeit at levels 10-30 times higher than P7C3, raising the possibility that the two compounds may share a common mechanistic pathway. Although this idea can only be rigorously tested after identification of the molecular target(s), the study raises the hope that more potent analogues of Dimebon with improved clinical efficacy could be identified and also provides appropriate assays.

The study is published in the journal Cell.

Amyotrophic lateral sclerosis (ALS), or Lou Gehrig’s disease, is one of the most common neuromuscular diseases worldwide. The disease results in progressive loss of motor neurones leading to muscle weakening and atrophy, and is usually fatal within 5 years of the onset of symptoms. Most cases (90-95%) of ALS are sporadic with no evidence of inheritance but 5-10% of cases are familial and do have a hereditary component. Familial ALS has been linked to mutations in a number of genes including those encoding superoxide dismutase (SOD1) (~20% of familial cases), TAR DNA binding protein and, most recently, FUS (fused in sarcoma) protein (~5% of familial cases). The FUS protein is believed to be involved in DNA repair and transcription as well as RNA splicing and transport of RNA from the nucleus to the cytoplasm. Although normal FUS protein is largely confined to the nucleus, mutated FUS protein forms aggregates in the cytoplasm and these deposits have been correlated with degeneration of nerve cells.

Researchers from Northwestern University Feinberg School of Medicine have now shown that FUS protein forms characteristic skein-like cytoplasmic inclusions in spinal motor neurones in most cases of ALS, not just familial cases. Post-mortem examination of spinal cords and brains from 78 ALS sufferers and 22 controls showed that FUS pathology was present in all the ALS samples (except for those from patients with SOD1 mutations) but not in the control samples.

Although mutations in FUS account for only a small fraction of ALS, the study suggests that FUS protein may be a common component of the cellular inclusions in non-SOD1 ALS, whether familial or sporadic. Although the cause of ALS remains unknown, the identification of a common pathway in familial and sporadic ALS may spur the development of new cell-based and animal models of disease, and could eventually lead to new therapies for motor neurone diseases.

The study is published in the Annals of Neurology.

Epileptic seizures are caused by sudden bursts of excess electrical activity in the brain, leading to a temporary breakdown in normal communication between brain cells. Although it was originally believed that neurones were solely responsible for signalling in the nervous system, it has become increasingly clear that non-neuronal cells called glia – which provide structural and nutritional support for neurones – are also able to modulate neurotransmission. Star-shaped glia known as astrocytes have increasingly been recognised to play a key role in the excessive neuronal synchrony that occurs in epilepsy and researchers at Tufts University and the Children’s Hospital of Philadelphia have now added strong evidence to the case against astrocytes.

The team focussed on reactive astrocytosis, a condition which occurs prominently in response to CNS injury or disease and which has, so far, been difficult to study. Using a virus to selectively cause reactive astrocytosis in mice without triggering broader inflammation and brain injury, the researchers were able to study how the altered astrocytes affected specific synapses in neurones in brain slices from the animals. Normally, neurotransmission is a delicate balance between excitation and inhibition, with the astrocytic enzyme, glutamine synthetase, playing a key role in regulating this balance. In reactive astrocytosis, the astrocytes produce less glutamine synthetase which, in turn, decreases inhibition and leads to the uncontrolled signalling characteristic of epileptic seizures. By adding glutamine – which is depleted as a result of reduced glutamine synthetase activity – the researchers were able to dampen neuronal excitability in the brain slices. The team are continuing to investigate how their research may contribute to developing new treatments for epilepsy and other neurological disorders as well as stroke and traumatic brain injury.

The study is published in the journal Nature Neuroscience.

Huntington’s disease (HD) is a genetic disorder caused by mutations in the huntingtin gene. The altered huntingtin protein (htt) causes gradual neurological damage; HD usually develops between the ages of 30 and 50 and symptoms get worse over the next 20 or more years. Defects in macroautophagy – a lysosomal system for removing toxic and unwanted proteins – have been suggested to play a role in the cell’s inability to clear mutant htt, but the exact mechanisms are poorly understood.

Researchers at the Albert Einstein College of Medicine have now shown that mutant htt interferes directly with the function of the autophagosome. Normally, cellular debris is sequestered in double-membraned autophagosomes and delivered to lysosomes for degradation following fusion of the vesicles. In cells from two mouse models of HD and in cells from people with the disease, fusion of the two vesicles and enzymatic activity of the lysosome were unaffected but mutant htt was found to stick to the inner membrane of the autophagosome and prevent normal loading of proteins destined for recycling. As a result, the autophagosomes are empty when they arrive at the lysosomes and cellular debris accumulates and probably contributes to cell death. Although defects in autophagy have been linked to other neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease, the researchers believe that the defect in cargo loading seen in the present study has not been described before.

The study, which is published in Nature Neuroscience, also suggests that proposed treatments for HD which involve activating lysosomes are unlikely to be effective. Professor Ana Maria Cuervo, the senior author of the study, likened autophagosomes to ‘garbage bags’ and lysosomes to ‘garbage trucks’ – there is no point in more trucks if the bags are not being filled.

Conventional wisdom, supported by in vitro experiments, has previously suggested that phosphoinositide 3-kinase (PI3K) plays a protective role in Alzheimer’s disease. However, a team led by researchers at Cold Spring Harbor has now implicated PI3K in the pathogenesis of the disorder.

The team used fruit-flies (Drosophila) that were engineered to produce human β-amyloid in their brains – a model that mimics many of the features of Alzheimer’s, including age-dependent memory loss, neurodegeneration, β-amyloid deposits and plaque formation. In the model, the presence of β-amyloid enhances long term depression (LTD), a process in which nerve signal transmission at particular synapses is depressed for an extended period. The research demonstrated that the enhanced LTD was a consequence of increased PI3K activity and could be abrogated by genetic silencing or pharmacological inhibition of PI3K.

PI3K inhibition restored LTD to a normal level, rescued β-amyloid peptide (Aβ)-induced memory loss and reduced β-amyloid deposits in the Drosophila brain. The data suggest that Aβ42 stimulates PI3K, which in turn causes memory loss in association with increased accumulation of Aβ42 aggregates.

The researchers note that the up-regulation of PI3K may also explain the insulin-resistance observed in the brains of Alzheimer’s victims. Insulin is one of the molecules that normally induce PI3K activity, which in turn mediates the cell’s response to insulin. Since PI3K is already hyperactivated in response to β-amyloid, it may no longer be able to respond to insulin.

The study is published in PNAS.

Puberty is a time of great physical and emotional changes and studies have shown that some skills are most easily acquired before – or after – puberty. Working with mice, scientists at SUNY Downstate Medical Center in Brooklyn have now been able to suggest a reason for the observed reduction in learning ability seen in adolescents. They have shown that there is a temporary increase in levels of the α4βδ GABA-A receptor in the hippocampus during puberty. The hippocampus plays key roles in spatial learning and memory and increases in levels of the receptor reduce brain excitability and impair spatial learning. Levels of α4βδ GABA-A receptor were found to increase at puberty, falling back to an intermediate level when the animals reached maturity. Pubescent mice were found to be much less able to master a test of spatial learning than prepubescent animals. The team also showed that the learning disability could be reversed by administration of the stress steroid, THP. In human children and adults, THP reduces brain activity and has a tranquilizing effect; in pubescent mice, the hormone increases activity in the hippocampus and has the opposite effect.

The study suggests that intrinsic learning mechanisms alter during adolescence and that the temporary decline in learning ability might be reversed in middle school by different teaching and motivation strategies which involve mild stress. If findings from the mouse studies are applicable to human teenagers, compounds targeting the α4βδ GABA-A receptor may also prove useful, especially for adolescents with learning difficulties.

The study is published in the journal Science.

The dopamine reuptake inhibitor, Ritalin® (methylphenidate) has been used for almost 50 years to treat children with attention-deficit hyperactivity disorder (ADHD) and, more recently – and controversially, has been used by students to enhance academic performance and as a recreational drug. Although Ritalin® has been prescribed for millions of children, the mechanisms by which it modifies behavioural performance remain poorly understood. Researchers at the University of California, San Francisco have now shown, in animals at least, that Ritalin® improves ability to focus on tasks and directly enhances speed of learning by distinct dopamine receptor-mediated mechanisms.

By co-administering Ritalin® with either the dopamine D₁ receptor antagonist, SCH-23390, or the dopamine D₂ receptor antagonist, raclopride, the team were able to show the well-known benefit of improved focus was mediated through D₂ receptor-dependent mechanisms whereas learning efficiency was enhanced through D₁ receptor-dependent mechanisms. The study also established that Ritalin® strengthens synapses and enhances neuroplasticity. A better understanding of the way that Ritalin® improves focus and enhances learning could lead to the development of more targeted drugs for ADHD and learning enhancement.

The study is published in Nature Neuroscience.