Strengthening the Case Against Astrocytes in Epilepsy

Image: Flickr - El Garza
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.

Faulty ‘Garbage Disposal’ in Huntington’s Disease

Image: Flickr - Adrian Miles
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.

Challenging the Role of PI3 Kinase in Alzheimer’s

Image: Flickr – Richard0
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.

Teenagers Really Are Different

Image: Flickr - erin MC hammer
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.

Ritalin® Circuits Revealed

circuit board
Image: Flickr - Quapan
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.

Narcolepsy Established as Autoimmune Disorder

Image: Flickr - Right About Me
Although the disorder is not very well known, narcolepsy is thought to affect 1 in 2000 individuals and this figure may be higher as a consequence of under-reporting and under-diagnosis. The most common symptom is excessive daytime sleepiness (EDS), which may be accompanied by sudden loss of muscular control (cataplexy) triggered by strong emotions. Narcoleptics may also experience sleep paralysis (short periods of paralysis when waking or falling asleep), hypnagogic or hypnopompic hallucinations (vivid images or sounds, respectively, when waking or falling asleep) or automatic behaviour (when routine activities are continued during a sleep episode).

For the last ten years it has been known that narcoleptics have a deficiency in hypocretin (orexin), a neurotransmitter involved in control of sleep/wakefulness. In parallel with the neurotransmitter deficiency there is a massive loss of hypothalamic neurons that produce hypocretin and it has been hypothesised that this results from an autoimmune response.

Swiss scientists have now identified autoantibodies to Tribbles homolog 2 (Trib2), an autoantigen previously identified in autoimmune uveitis, in narcolepsy patients. The team developed a transgenic mouse model to identify peptides enriched within hypocretin-producing neurons that could serve as potential autoimmune targets. Having identified enrichment of Trib2 in the mouse hypocretin neurons, the team went on to analyse sera from narcoleptics. Narcolepsy patients with cataplexy had higher Trib2-specific antibody titres compared with either normal controls or patients with other inflammatory neurological disorders. Trib2-specific antibody titres were highest early after narcolepsy onset, sharply decreased within 2–3 years, and then stabilized at levels substantially higher than that of controls for up to 30 years. Additionally, high Trib2-specific antibody titres correlated with the severity of cataplexy.

The study, published in the Journal of Clinical Investigation, provides the first evidence that narcolepsy is an autoimmune disorder.

Promise on the Horizon for Huntington’s

Image: Flickr – Dominic’s pics
Results from a phase II trial of the experimental drug Dimebon (latrepirdine) in people with Huntington’s disease have provided indications that it may improve cognition. The drug, being developed by Medivation, Inc., is also in Phase III trials for Alzheimer’s disease. In July 2009, Medivation and Pfizer, Inc. launched a Phase III clinical trial (HORIZON) of the drug for Huntington’s disease.

Huntington’s disease is a progressive neurodegenerative disorder that impacts movement, behaviour and cognition, generally resulting in death within 20 years of the disease’s onset. The disease steadily erodes memory and ability to think and learn. Over time, this cognitive impairment contributes to the loss of the ability to work and perform the activities of daily life. There are no treatments current available that effectively alter the course of the disease or improve cognition.

We have previously reported on the potential for Dimebon in Alzheimer’s disease (July 2008, July 2009), where the ability of the drug to stabilise and/or enhance mitochondrial function is believed to be of benefit. Mitochondria are also thought to play a role in the development of Huntington’s disease, suggesting that Dimebon could also have utility in this condition.

Karl Kieburtz, M.D., University of Rochester Medical Center neurologist and lead investigator on the Horizon trial, said:

“This is the first clinical trial that has focused on what is perhaps the most disabling aspect of the disease. While more investigation needs to be done, these results are encouraging and show, for the first time, a statistically significant benefit in terms of improved cognitive function in patients with Huntington’s disease.”

dimebon (latrepirdine) structure
Dimebon (latrepirdine)
In the phase II study, the impact of the drug on 91 patients over a 90 day period was assessed. Half were given the drug and the other half a placebo. The patients were then evaluated using a cognitive tool called the Mini-Mental State Examination. This test – which is used by clinicians to evaluate the stage and severity of dementia and Alzheimer’s disease – consists of questions used to evaluate an individual’s orientation, memory, and ability to follow commands. The researchers found that the drug on average improved the scores of people taking the drug compared to those who received the placebo. Although the treatment had no significant impact on the Unified Huntington’s Disease Rating Scale (UHDRS) or the Alzheimer Disease Assessment Scale–cognitive subscale (ADAS-cog), the results support further investigation in Huntington’s disease.

Results of the study are published in the Archives of Neurology.

Neuroprotective Small Molecule TrkB Agonist

Image: Benedict Campbell, Wellcome Images via Flickr - Hljod.Huskona
Roles have been suggested for brain-derived neurotrophic factor (BDNF) – which helps to support neurons and also stimulates and controls neurogenesis – in preventing or treating degenerative diseases such amyotrophic lateral sclerosis, Parkinson’s disease, and Alzheimer’s disease. The use of BDNF itself in therapy is limited by a poor pharmokinetic profile including rapid metabolism and poor CNS penetration. BDNF elicits at least some of its effects through binding to the high affinity tyrosine kinase receptor B, TrkB, and investigators at Emory University School of Medicine have now identified a small, high-affinity molecule that can also activate signalling through TrkB.

7,8-dihydroxy flavone structure
7,8-Dihydroxyflavone was shown to protect wild-type, but not TrkB-deficient, neurons from apoptosis. Following intraperitoneal administration, the compound was also found to activate TrkB in the brain and to be protective in animal models of seizure, stroke and Parkinson’s disease. The compound was also found to have low toxicity on chronic dosing. Although favonoids such as 7,8-dihydroxyflavone occur in a wide range of foodstuffs, levels obtained from a normal diet are believed to be insufficient for a sustained effect.

The study is published in the online early edition of PNAS.

Correct Protein Folding

Image: Flickr - hashashin
Neurons are particularly sensitive to the toxic effects of misfolded proteins and the accumulation of such species has been associated with neurodegenerative diseases including Parkinson’s disease, amyotropic lateral sclerosis (ALS), Alzheimer’s disease and transmissible spongiform encephalopathies (prion diseases). Hereditary protein conformational disorders such as Huntington’s disease are characterised by trinucleotide repeats that result in the insertion of poly-glutamine (polyQ) stretches which adopt β-sheet structures and make the protein prone to incorrect folding and aggregation. The ability to stabilise native protein conformations would likely prevent the neurotoxicity linked to misfolding and scientists at Duke University Medical Center have now discovered compounds that may be able to achieve this.

HSF-1A structure
Since increasing the levels of protein chaperones has been shown to suppress protein misfolding, the team focussed on identifying small molecule activators of heat shock transcription factor 1 (HSF1), the master regulator of protein chaperone gene transcription. HSF1A was discovered using a humanised yeast-based high throughput screen and shown to activate HSF1 in mammalian and fruit fly cells, to elevate protein chaperone expression, and to reduce protein misfolding. HSF1A was also shown to prevent cell death in polyQ-expressing neuronal precursor cells and to protect against cytotoxicity in a fruit fly model of polyQ-mediated neurodegeneration.

Previous screens that have identified activators of HSF1 have not been able to discriminate against compounds that promote HSF1 activation through the proteotoxic accumulation of unfolded proteins or through the inhibition of Hsp90, a central chaperone involved in cell growth, signalling, and proliferation. HSF1A is structurally distinct from other small molecule activators of HSF1 and, although the precise mechanism by which the compound activates human HSF1 is not yet understood, it could lead to new therapies for neurodegenerative diseases caused by protein misfolding.

The study is published in PLoS Biology.

Opposing Roles for Brain Macrophages

Personality and the mask
Image: Flickr - Vagamundos
Systemic infection and inflammation lead to release of cytokines, such as IL-1, which activate the brain’s stress response mechanisms, producing typical symptoms such as lethargy, fever, and lack of appetite. In response to inflammation or infection, the hypothalamus releases corticotropin-releasing factor which, in turn, stimulates the pituitary gland to secrete adrenocorticotropic hormone. This then causes the adrenal glands to increase production of glucocorticoids, which both mobilise energy reserves to cope with the inflammatory insult and also act as powerful immunosuppressants, preventing excessive cytokine production and immune cell proliferation. Since cytokines are not able to freely cross the blood-brain barrier, exactly how they initiate this cascade of events has not been clear but researchers at the Salk Institute for Biological Studies have now begun to unravel the process.

It had been suggested that cytokines might interact with epithelial cells in the brain’s vasculature to produce prostanoids which act as secondary messengers transmitting the signal onwards. Epithelial cells are ideally positioned to receive inflammatory signals from circulating blood but need a very strong signal to become activated. In contrast, perivascular macrophages, a subset of brain-resident macrophages, are much more sensitive to cytokines but are not in direct contact with the bloodstream. To clarify the roles of both cell types, liposomes containing clodronate, which specifically deplete macrophages, were injected into the lateral cerebral ventricles of rats. This procedure abolished responses to IL-1 which activates prostanoid synthesis only in perivascular cells, but enhanced responses to LPS which stimulates prostanoid synthesis by both perivascular cells and endothelial cells. Resident macrophages lined up along the blood-brain barrier thus play opposing roles in the transmission of immune signals to the brain depending on the nature of the stimulus.

As well as clarifying the cellular mechanisms of CNS responses to inflammatory insults, the team hope that a better understanding of how immune signals are transmitted across the blood-brain barrier may also lead ultimately to new treatments for chronic neurodegenerative diseases such as Amyotrophic Lateral Sclerosis, Parkinson’s disease, Alzheimer’s disease and prion diseases, in which inflammation is believed to play an important role.

The study is published in the January 14th issue of Neuron.