. A neuroprotective role for microRNA miR-1000 mediated by limiting glutamate excitotoxicity. Nat Neurosci. 2015 Mar;18(3):379-85. Epub 2015 Feb 2 PubMed.


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  1. Glutamate excitotoxicity has been put forward as one of the mechanisms at play in Alzheimer’s disease and other neurodegenerative conditions. In this paper, Verma et al. describe a very interesting microRNA-mediated mechanism for fine-tuning glutamatergic signaling that, when disturbed, can lead to neurodegeneration. They demonstrate how dysregulation of the activity-dependent miR-1000 in Drosophila, and of its homologue miR-137 in mice, controls presynaptic expression of the vesicular glutamate receptor. Increases in these microRNAs lower the glutamatergic tonus in various biological settings, also protecting neurons against excitotoxicity. The authors show that loss of miR-1000 in fruit flies leads to a progressive neurodegenerative phenotype. This paper presents a tremendous amount of elegant work, highly important for our field because it highlights the significance of microRNA “micromanagers” that keep major signaling pathways, relevant to neurodegenerative disorders, in fine-tuned check.

  2. This is an interesting study in Drosophila that supports a neuroprotective role for microRNAs in regulating presynaptic glutamate release and protecting against excitotoxicity. The paper highlights the multifaceted targets of microRNAs and reinforces the idea that a number of processes are likely dysregulated leading to neurodegenerative diseases such as AD.

  3. Micro-RNAs play a significant role in animal development and in a number of diseases related to neurodegeneration, cancer, and chronic ailments in humans. They are the master regulators of human gene machinery and are small molecules with big impacts, carrying immense therapeutic potential. With the advent of genome-wide studies (next-generation sequencing, small miRNA sequencing, and microarrays), we are beginning to visualize how miRNAs work. While not much is known about the genomic basis of neurodegenerative diseases in humans, genome-wide studies have the potential to provide a holistic view of complex interactions involving miRNAs that may lead to a more profound understanding of these and other diseases that afflict humans. Such holistic approaches are sorely needed for the development of a new generation of genome-based therapies to treat NDs and for possible early correction of functional genetic/functional deficits during developmental stages of an embryo.

    In this article, Verma et al. have presented a thought-provoking study in the area of neurodegeneration and developmental biology that may have considerable clinical and scientific implications.

    Most exciting is the demonstration that miR-1000 in Drosophila, and its homologue miR-137 in mammals, point to a significant functional analogy among animals. The authors demonstrate that these miRNAs control presynaptic expression of the vesicular glutamate receptor. This has not been shown before and is exciting given that glutamate excitotoxicity is believed to be one of the key mechanisms in the development of Alzheimer’s (AD) and a variety of neurodegenerative diseases (NDs). Although the study is largely relevant to neurodegeneration in Drosophila, it is bound to spark further investigations on NDs in humans, because the findings intersect with genomic and biochemical  links to NDs that affect mammals.

    Elegant and intelligent experimental design used for generating a panoply of functional mutants of miR-1000 through homologous recombination clearly validated each of the functional phenotypes associated with neurodegeneration, which may be important in the context of NDs in mammals. It was interesting to note that the miR-1000 mutant flies displayed reduced lifespan, with survival declining rapidly by 2 weeks of age, which was rescued by restoring miR-1000 expression using a rescue allele. In climbing performance tests, miR-1000 mutant flies showed early onset of movement disorder, such that by day 10, 80 percent of them had impaired movement. Interestingly, this climbing defect was also corrected and rescued by restoring miR-1000 expression using a rescue allele.

    Apoptotic cell death forms the central core of NDs in mammals and the authors probed the role of this neurodegenerative process in Drosophila. They unambiguously demonstrated that cells dying from apoptosis had more caspase-3 positive in miR-1000 mutants as opposed to controls. Again, cell death was rescued by restoring miR-1000 expression using a rescue allele. Further, Drosophila inhibitor of apoptosis protein (DIAP1) reduced the number of caspase-3 positive cells and neuronal death in the mutant brain and improved fly survival, suggesting a link between apoptotic cell death during neurodegenerative disease and reduced life span. Age-related neurodegeneration accompanied by elevated levels of oxidative stress and vacuolization of the Drosophila brain suggests miR-1000 mutants suffered from early onset of neurodegeneration—an analogous scenario to neurodegeneration in mammals. Together, the experiments demonstrate the possible role of miR-1000 in age-related neurodegeneration and its importance in brain developmental stages in Drosophila.

    Micro RNAs typically repress their gene targets. Verma et al. used a bioinformatic approach to reveal 374 predicted miR-1000 targets. Although they only analyzed gene targets relevant to the CNS, one of the key genes was the vesicular glutamate transporter, which increased fourfold in RNA isolated from the heads of miR-1000 mutants. This increase was halved in rescued mutants, consistent with partial reduction of miRNA expression. Subsequent deletion of the V-glutamate gene confirmed that overexpression of V-Glut contributed to defects in miR-1000 mutants. Glutamate is the predominant excitatory neurotransmitter in Drosophila CNS and miR-1000 is expressed in presynaptic motor neurons. The authors were able to show that an increase in V-Glut expression was responsible for the increased excitatory synaptic signaling. Memantine, a glutamate receptor antagonist used to treat AD, improved climbing performance in the miR-1000 mutants. Further, reducing NMDA glutamate receptor levels by mutating 1 copy of the Nmdar1 gene improved the survival of miR-1000 mutant flies and suppressed apoptosis. Notably, glutamate receptors are also found in muscles and reducing the muscle-specific glutamate receptors (GluRIIA and IIB) was ineffective, suggesting that these motor problems were a direct effect of glutamate receptors in the brain and that neurodegeneration observed in the miR-1000 mutants could largely be attributed to glutamate excitotoxicity mediated through NMDA, AMPA and metabolic glutamate receptors.

    Since it is already known that miR-1000 is not found in mammals, how are these studies by Verma et al. relevant to human disease? Micro RNA-137, a structural homologue of miR-1000, is conserved throughout mammalian species. The homology extends to functional targets. For instance, VGlutT1, T2, and T3 are also predicted to be major targets of miR-137, which is expressed in the dentate gyrus region of the hippocampus in mammals. In a preliminary study, the authors found that VGlutT2 was elevated in the hippocampus on the miR-137-depleted side of the brain.

    Finally, the authors show that miR-1000 not only presynaptically regulates VGlut and controls synaptic glutamate release, but also is controlled by light in vivo, presumably reflecting photoreceptor activity in the eye. This, in turn, leads to light-mediated regulation of VGlut receptors levels. What relevance might this have in the context of NDs in mammals? The circadian system, which has a vital role in sleep disorders and wakefulness seen in Alzheimer’s disease and various forms of dementias, is modulated by light. Moreover, it has been shown that light therapy tailored to increase circadian stimulation during the day has benefits for patients with Alzheimer's disease and related dementias living in long-term care facilities. Thus, these studies may have provided a genetic/biochemical lead into this aspect of NDs in mammals, hinting at a potential way to treat it, and should be followed up with more in-depth investigation. Moreover, increased levels of VGlut transporters have been also shown in epilepsy, traumatic brain injury, stroke, and chronic injury. Whether these levels have relevance in the development of vascular dementia remains to be confirmed, as a considerable number of stroke patients experience memory loss and gradual neurodegeneration.

    The take-home message from this study is that we need intense investigation at the genome-wide level to better understand the role of miRNAs in mammalian NDs. As a single miRNA can control expression of hundreds to thousands of genes, only large collaborative ventures can determine how the different miRNAs interact with each other and with their cognate targets to modulate function. Such efforts may not only reveal the relevance of miR-137 to human NDs, but may also unveil therapeutically relevant miRNAs that may be used as candidates in treating NDs, leading to tailored therapies for patients in the genomic medicine area.

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This paper appears in the following:


  1. In Fruit Flies, MicroRNA Protects Synapses Against Excitotoxicity