Fruit Flies Lead to Insights Into Function of Sleep
Two studies of fruit flies published in 2009 provide experimental support for a recent hypothesis on why we sleep.1 The “synaptic homeostasis” hypothesis suggests that sleep plays an important role in “downscaling” or reducing synapses, the connections between brain cells, which if allowed to increase without control, would take up too much space in the brain. The hypothesis is based on the assumption that synaptic strength is generally increased during waking periods, and that this increase in strength requires both energy and physical space in the brain and would be unsustainable without an opposing process of reduction.
A group of researchers from the University of Wisconsin, including Giulio Tononi and Chiara Cirelli, who initially proposed the synaptic homeostasis theory, looked at levels of synaptic proteins in fruit flies after a period of wakefulness and after a period of sleep.2 They discovered that levels of these proteins, thought to be markers of synaptic activity, were high after waking and progressively declined during sleep. This study adds to earlier work showing similar results in rats.
The other study, conducted by Paul Shaw and colleagues from Washington University in St. Louis, Missouri, looked at the effects of daytime activities on sleep.3 The study was a follow-up to previous research showing that fruit flies sleep longer after exposure to socially enriched environments (consisting of groups of thirty or more male and female flies). Shaw and his fellow researchers identified three genes necessary for this increased sleep response. Flies without any of these genes did not show an increase in sleep after stimulation. Replacement of the genes in ventral lateral neurons, which are in the part of the fly brain that controls behavior in response to the circadian clock, was enough to restore the sleep response. This result suggests that these neurons may be involved with regulating the need for sleep.
The researchers also discovered an increase in the number of synapses in the flies after social stimulation, along with a decrease after sleep, supporting the hypothesis that sleep is involved in downscaling synapses. “For me it was a surprising outcome,” said Shaw, who admitted that he was previously skeptical of the synaptic homeostasis hypothesis. “I still think it’s probably more complicated than just [Tononi’s] model, but it’s the data that matters the most.”
Giulio Tononi and Chiara Cirelli propose that sleep allows the brain to reduce synapses that take up space in the brain. Studies showed that the level of synaptic proteins, thought to indicate synaptic activity, is low after a period of sleep (left) and high after being awake (right) in most regions of the fruit fly brain. (Courtesy of Chiara Cirelli / University of Wisconsin)
However, other research conducted by a team from the University of Pennsylvania, published in the February 2009 issue of Neuron, appears to contradict the theory.4 Looking at the consolidation of learning and memory in cats, researchers discovered that sleep strengthened synaptic connections in the visual cortex after a waking period when the cat was deprived of sight in one eye.
The authors of the study suggested that differences between their findings and previous results supporting synaptic homeostasis may be related to the different types of plasticity being studied in each case, as well as the nature of the changes taking place when the measurements were made.
New Targets for Treating Age-related Macular Degeneration
Age-related macular degeneration (AMD) is one of the leading causes of blindness worldwide. Now increased understanding of the disease is helping scientists to develop new ways to diagnose and treat it.
There are two forms of macular degeneration, so called because the disease affects the macula, the central part of the retina where the visual receptors are most dense and provide the highest visual acuity. In the “dry” form of AMD, the macula thins and dries out, causing the loss of central vision. The more severe neovascular or “wet” form is accompanied by the growth of new blood vessels in the choroid, a layer of vessels below the retina, a process known as choroidal neovascularization (CNV), a form of angiogenesis. The new blood vessels leak blood and fluid into the retina, disturbing vision and in many cases resulting in blindness.
Over the past few years, treatment approaches for neovascular AMD have focused on anti-angiogenesis therapy, in particular agents that block a molecule called vascular endothelial growth factor (VEGF), which stimulates angiogenesis in the cells lining the blood vessels. A 2008 Cochrane Review of anti-VEGF agents in the treatment of neovascular AMD found that the two anti-VEGF agents included in the review, pegaptanib and ranibizumab, both reduced the risk of visual acuity loss in patients with neovascular AMD.5 The report concluded that “anti-angiogenesis therapy modalities provide a promising means of treating the potentially devastating problem of AMD.”
In a 2009 paper published in Retina, Martin Friedlander from the Scripps Research Institute reported on a study of neovascularization in mice showing that a combination of anti-angiogenesis therapies, targeting different pathways of new vessel formation, was more effective in inhibiting neovascularization than a single angiogenic inhibitor, even a VEGF inhibitor.6 He wrote: “If the goal or one of the goals in AMD treatment is to fully inhibit choroidal neovascularization, then combination therapies will be essential.”
Another potential therapeutic target for AMD was identified by a team of researchers led by Jayakrishna Ambati, from the University of Kentucky, Lexington.7 They discovered a protein receptor, CCR3, on the surface of blood vessels in CNV tissue taken from patients who had the neovascular form of the disease. CCR3 is not found in normal vascular tissue, suggesting that it is a biological marker for CNV. Ambati and colleagues then looked at the effects of CCR3-blockade (induced either genetically or pharmacologically) in mice with CNV and found that it suppressed neovascularization. They also found CCR3 inhibition to be slightly more effective at reducing CNV than VEGF inhibition.
More interesting to Ambati and colleagues was the potential for using CCR3 as a marker for CNV before vision loss occurs. Using a method to track CCR3 antibodies in the mouse model, they were able to detect the new blood vessels before they penetrated the retina, something that was not previously possible. The results of the study were reported in Nature in June 2009.
Hope for early diagnosis or risk assessment also comes from recent genetic studies of AMD. In a 2009 review paper published in Current Opinion in Ophthalmology, Ian MacDonald and colleagues from the University of Alberta wrote: “First-degree relatives of patients with AMD tend to have a higher risk of AMD. Recognizing an inherent genetic risk of AMD in these patients will improve their management and potentially help prevent blindness.”8
A Step Back for New Drug Class for Treating Schizophrenia
A key trial for a new class of schizophrenia drugs yielded inconclusive results in 2009, a disappointing outcome for researchers studying this challenging disease.
Schizophrenia is a complex mental disorder, with symptoms that include hallucinations, delusions, social withdrawal and problems with attention and memory. Many of the currently prescribed antipsychotics have significant side effects and are not very effective in improving cognitive and psychological function.
An industry announcement at the International Congress on Schizophrenia in March 2009 reported that a promising candidate for a new class of drugs showed inconclusive results in a Phase II clinical trial, dampening hopes that a new treatment option for schizophrenia was within close reach.9
Antipsychotics have traditionally adjusted the regulation of the neurotransmitter dopamine in the brain, although there is no solid evidence that schizophrenia is the result of a primary dopamine abnormality. In 2007 a drug affecting activity of the neurotransmitter glutamate, thought to be an important element in the pathology of schizophrenia, showed promise in a “proof-of-concept” clinical trial. The new compound, known as LY2140023, activates specific glutamate receptors, the metabotropic glutamate receptors, or mGlur; it is thought to work in part by reducing the release of glutamate at the synapses of brain circuits regulating emotional and motivational behavior. The results of the 2007 study by Darryl Schoepp and colleagues at Eli Lilly were published in Nature Medicine.10
The results of the initial trial showed improvement in outcomes for LY2140023 compared with placebo, and comparable safety and tolerance with an existing antipsychotic, olanzapine. Daniel Weinberger from the National Institute of Mental Health noted in an accompanying commentary that this was the “first credible evidence” of an effective antipsychotic drug that did not target dopamine.11
New compounds being tested for treatment of schizophrenia work by activating specific metabotropic glutamate receptors (mGlur), and reducing the release of glutamate at specific synapses. (Reprinted by permission from Macmillan Publishers, Ltd: Weinbered, DR, Nature Medicine 2007 13: 1018-1019, doi:10.1038/nm0907-1018. Image created by Katie Ris-Vicari)
In the follow-up Phase II trial, involving a larger number of patients, in 2009, however, LY2140023 did no better than a placebo. A higher than normal placebo response was seen both against LY2140023 and against olanzapine, which was used as a control drug in the trial. Despite the inconclusive results, Eli Lilly has said it will go ahead with drug development. An additional Phase II study is being planned to test the molecule in hopes of validating the proof-of-concept trial results. Lilly expects the trial to begin sometime in the first half of 2010.
Piecing Together a Complex Puzzle
Autism spectrum disorders (ASDs) are characterized by a broad set of symptoms that include varying degrees of social, cognitive and communication dysfunctions that usually emerge within the first few years of life. New studies of large groups of children with ASDs and their families are allowing researchers further insights into the biological nature and heritability of these disorders.
A research team led by Hakon Hakonarson of the Children’s Hospital of Philadelphia conducted a genome-wide analysis of 912 families with more than one affected child and compared the results to DNA from families without such disorders.12 They identified twenty-seven different genetic regions with rare copy number variations (CNVs), genetic variations associated with missing or extra copies of DNA segments. Some CNVs had been previously associated with ASDs. The new study also identified several genes potentially involved in the pathophysiology of ASDs, including a previously reported gene involved in brain development and known as neuronaladhesion gene NRNX1, and two new genes, BZRAP1 and MDGA2, that also are believed to be important in neuronal development.
An earlier 2009 study by Hakonarson and colleagues, looking at the DNA of 780 families with children affected by an ASD, identified common genetic variations in ASD cases on a region on chromosome 5 (named 5p14.1), between the neuronal-adhesion genes, cadherin 10 and cadherin 9.13 The same results were also achieved in a smaller independent study led by Margaret Pericak-Vance from the Miami Institute for Human Genomics.14
| Hakon Hakonarson of the Children’s Hospital of Philadelphia analyzed the DNA of nearly a thousand families in which a child was affected by an autism spectrum disorder to identify copy number variations and genes associated with the disorder. (Courtesy of Hakon Hakonarson / Children’s Hospital of Philadelphia)|
While this gave some evidence that common variants are involved in ASDs, Pericak-Vance and colleagues wrote that it is “highly unlikely” that a strong single-gene association, such as the association of the APOE gene in Alzheimer’s, will be identified in autism. Rather, they concluded, “our results, in combination with the multiple rare variants already identified, suggest that the genetic architecture of autism is as exquisitely complex as its clinical phenotype.”
Both of these studies added to a growing body of evidence implicating neuronal-adhesion genes in ASDs. Neuronal cell-adhesion molecules play a vital role in various cell processes, including enabling neurons to connect with each other. Given these genetic findings, along with anatomical and functional imaging studies, Hakonarson and his fellow researchers postulate that ASDs may represent a neuronal disconnection syndrome.
Researchers and clinicians are also working to clarify and standardize the clinical criteria for ASDs and to better understand the onset of these disorders. Typically, onset of autism is categorized as “early,” when symptoms such as delayed speech development appear in the first year or so of life, or “regressive,” when initial normal development is followed by the loss of skills in the second year of life. A 2008 review of the literature on autism onset suggested that this classification is too narrow to accommodate the variety of ways in which autism can emerge.15 The review also highlighted the challenges to categorizing onset, including the unreliability of parents’ memories and reporting skills in retrospective studies.
A new endeavor by a group of autism researchers, called the Early Autism Risk Longitudinal Investigation (EARLI), is planning to focus on more than one thousand pregnant women who already have a child with autism and follow them through their pregnancies and deliveries and the first three years of the new child’s life.16 The study, which is being coordinated by the Drexel University School of Public Health in Philadelphia, will look at early risk factors for autism and will include genetic analyses as well as behavioral and developmental assessments in an attempt to put some of the pieces of this complex puzzle in place.