Exploring the links of skeletal muscle mitochondrial oxidative capacity, physical functionality, and mental well-being of cancer survivors.

Physical impairments following cancer treatment have been linked with the toxic effects of these treatments on muscle mass and strength, through their deleterious effects on skeletal muscle mitochondrial oxidative capacity. Accordingly, we designed the present study to explore relationships of skeletal muscle mitochondrial oxidative capacity with physical performance and perceived cancer-related psychosocial experiences of cancer survivors. We assessed skeletal muscle mitochondrial oxidative capacity using in vivo phosphorus-31 magnetic resonance spectroscopy (31P MRS), measuring the postexercise phosphocreatine resynthesis time constant, τPCr, in 11 post-chemotherapy participants aged 34-70 years. During the MRS procedure, participants performed rapid ballistic knee extension exercise to deplete phosphocreatine (PCr); hence, measuring the primary study outcome, which was the recovery rate of PCr (τPCr). Patient-reported outcomes of psychosocial symptoms and well-being were assessed using the Patient-Reported Outcomes Measurement Information System and the 36-Item Short Form health survey (SF-36). Rapid bioenergetic recovery, reflected through a smaller value of τPCr was associated with worse depression (rho ρ = - 0.69, p = 0.018, and Cohen's d = - 1.104), anxiety (ρ = - 0.61, p = .046, d = - 0.677), and overall mental health (ρ = 0.74, p = 0.010, d = 2.198) scores, but better resilience (ρ = 0.65, p = 0.029), and coping-self efficacy (ρ = 0.63, p = 0.04) scores. This is the first study to link skeletal muscle mitochondrial oxidative capacity with subjective reports of cancer-related behavioral toxicities. Further investigations are warranted to confirm these findings probing into the role of disease status and personal attributes in these preliminary results.

The mediation roles of intermuscular fat and inflammation in muscle mitochondrial associations with cognition and mobility.

BACKGROUND: Mitochondrial dysfunction may contribute to brain and muscle health through inflammation or fat infiltration in the muscle, both of which are associated with cognitive function and mobility. We aimed to examine the association between skeletal muscle mitochondrial function and cognitive and mobility outcomes and tested the mediation effect of inflammation or fat infiltration. METHODS: We analysed data from 596 Baltimore Longitudinal Study of Aging participants who had concurrent data on skeletal muscle oxidative capacity and cognitive and mobility measures of interest (mean age: 66.1, 55% women, 24% Black). Skeletal muscle oxidative capacity was assessed as post-exercise recovery rate (kPCr) via P31 MR spectroscopy. Fat infiltration was measured as intermuscular fat (IMF) via CT scan and was available for 541 participants. Inflammation markers [IL-6, C-reactive protein (CRP), total white blood cell (WBC), neutrophil count, erythrocyte sedimentation rate (ESR), or albumin] were available in 594 participants. We examined the association of kPCr and cognitive and mobility measures using linear regression and tested the mediation effect of IMF or inflammation using the mediation package in R. Models were adjusted for demographics and PCr depletion. RESULTS: kPCr and IMF were both significantly associated with specific cognitive domains (DSST, TMA-A, and pegboard dominant hand performance) and mobility (usual gait speed, HABCPPB, 400 m walk time) (all P < 0.05). IMF significantly mediated the relationship between kPCr and these cognitive and mobility measures (all P < 0.05, proportion mediated 13.1% to 27%). Total WBC, neutrophil count, and ESR, but not IL-6 or CRP, also mediated at least one of the cognitive and mobility outcomes (all P < 0.05, proportion mediated 9.4% to 15.3%). CONCLUSIONS: Skeletal muscle mitochondrial function is associated with cognitive performance involving psychomotor speed. Muscle fat infiltration and specific inflammation markers mediate the relationship between muscle mitochondrial function and cognitive and mobility outcomes. Future studies are needed to confirm these associations longitudinally and to understand their mechanistic underpinnings.

Energizing Mitochondria to Prevent Mobility Loss in Aging: Rationale and Hypotheses.

Based on recent studies from our group and others, we hypothesize that mitochondrial dysfunction during aging may be the root cause of mobility decline through deficits in the musculoskeletal and central nervous systems. Mitochondrial dysfunction could be a therapeutic target to prevent mobility decline in aging.

Activity-Dependent Gene Expression in Neurons.

The function of the nervous system in conveying and processing information necessary to interact with the environment confers unique aspects on how the expression of genes in neurons is regulated. Three salient factors are that (1) neurons are the largest and among the most morphologically complex of all cells, with strict polarity, subcellular compartmentation, and long-distant transport of gene products, signaling molecules, and other materials; (2) information is coded in the temporal firing pattern of membrane depolarization; and (3) neurons must maintain a stable homeostatic level of activation to function so stimuli do not normally drive intracellular signaling to steady state. Each of these factors can require special methods of analysis differing from approaches used in non-neuronal cells. This review considers these three aspects of neuronal gene expression and the current approaches being used to analyze these special features of how the neuronal transcriptome is modulated by action potential firing.

Motor learning requires myelination to reduce asynchrony and spontaneity in neural activity.

Myelination increases the conduction velocity in long-range axons and is prerequisite for many brain functions. Impaired myelin regulation or impairment of myelin itself is frequently associated with deficits in learning and cognition in neurological and psychiatric disorders. However, it has not been revealed what perturbation of neural activity induced by myelin impairment causes learning deficits. Here, we measured neural activity in the motor cortex during motor learning in transgenic mice with a subtle impairment of their myelin. This deficit in myelin impaired motor learning, and was accompanied by a decrease in the amplitude of movement-related activity and an increase in the frequency of spontaneous activity. Thalamocortical axons showed variability in axonal conduction with a large spread in the timing of postsynaptic cortical responses. Repetitive pairing of forelimb movements with optogenetic stimulation of thalamocortical axon terminals restored motor learning. Thus, myelin regulation helps to maintain the synchrony of cortical spike-time arrivals through long-range axons, facilitating the propagation of the information required for learning. Our results revealed the pathological neuronal circuit activity with impaired myelin and suggest the possibility that pairing of noninvasive brain stimulation with relevant behaviors may ameliorate cognitive and behavioral abnormalities in diseases with impaired myelination.

Differential nucleosome spacing in neurons and glia.

Eukaryotic chromosomes are composed of chromatin, in which regularly spaced nucleosomes containing ∼147 bp of DNA are separated by linker DNA. Most eukaryotic cells have a characteristic average nucleosome spacing of ∼190 bp, corresponding to a ∼45 bp linker. However, cortical neurons have a shorter average spacing of ∼165 bp. The significance of this atypical global chromatin organization is unclear. We have compared the chromatin structures of purified mouse dorsal root ganglia (DRG) neurons, cortical oligodendrocyte precursor cells (OPCs) and cortical astrocytes. DRG neurons have short average spacing (∼165 bp), whereas OPCs (∼182 bp) and astrocytes (∼183 bp) have longer spacing. We measured nucleosome positions by MNase-seq and gene expression by RNA-seq. Most genes in all three cell types have a promoter chromatin organization typical of active genes: a nucleosome-depleted region at the promoter flanked by regularly spaced nucleosomes phased relative to the transcription start site. In DRG neurons, the spacing of phased nucleosomes downstream of promoters (∼182 bp) is longer than expected from the genomic average for DRG neurons, whereas phased nucleosome spacing in OPCs and astrocytes is similar to the global average for these cells (∼183 bp). Thus, the atypical nucleosome spacing of neuronal chromatin does not extend to promoter-proximal regions.

Coordinated Activity of Transcriptional Networks Responding to the Pattern of Action Potential Firing in Neurons.

Transcriptional responses to the appropriate temporal pattern of action potential firing are essential for long-term adaption of neuronal properties to the functional activity of neural circuits and environmental experience. However, standard transcriptome analysis methods can be too limited in identifying critical aspects that coordinate temporal coding of action potential firing with transcriptome response. A Pearson correlation analysis was applied to determine how pairs of genes in the mouse dorsal root ganglion (DRG) neurons are coordinately expressed in response to stimulation producing the same number of action potentials by two different temporal patterns. Analysis of 4728 distinct gene-pairs related to calcium signaling, 435,711 pairs of transcription factors, 820 pairs of voltage-gated ion channels, and 86,862 pairs of calcium signaling genes with transcription factors indicated that genes become coordinately activated by distinct action potential firing patterns and this depends on the duration of stimulation. Moreover, a measure of expression variance revealed that the control of transcripts abundances is sensitive to the pattern of stimulation. Thus, action potentials impact intracellular signaling and the transcriptome in dynamic manner that not only alter gene expression levels significantly (as previously reported) but also affects the control of their expression fluctuations and profoundly remodel the transcriptional networks.

Regulation of myelin structure and conduction velocity by perinodal astrocytes.

The speed of impulse transmission is critical for optimal neural circuit function, but it is unclear how the appropriate conduction velocity is established in individual axons. The velocity of impulse transmission is influenced by the thickness of the myelin sheath and the morphology of electrogenic nodes of Ranvier along axons. Here we show that myelin thickness and nodal gap length are reversibly altered by astrocytes, glial cells that contact nodes of Ranvier. Thrombin-dependent proteolysis of a cell adhesion molecule that attaches myelin to the axon (neurofascin 155) is inhibited by vesicular release of thrombin protease inhibitors from perinodal astrocytes. Transgenic mice expressing a dominant-negative fragment of VAMP2 in astrocytes, to reduce exocytosis by 50%, exhibited detachment of adjacent paranodal loops of myelin from the axon, increased nodal gap length, and thinning of the myelin sheath in the optic nerve. These morphological changes alter the passive cable properties of axons to reduce conduction velocity and spike-time arrival in the CNS in parallel with a decrease in visual acuity. All effects were reversed by the thrombin inhibitor Fondaparinux. Similar results were obtained by viral transfection of tetanus toxin into astrocytes of rat corpus callosum. Previously, it was unknown how the myelin sheath could be thinned and the functions of perinodal astrocytes were not well understood. These findings describe a form of nervous system plasticity in which myelin structure and conduction velocity are adjusted by astrocytes. The thrombin-dependent cleavage of neurofascin 155 may also have relevance to myelin disruption and repair.

Systemic Delivery of MicroRNA Using Recombinant Adeno-associated Virus Serotype 9 to Treat Neuromuscular Diseases in Rodents.

RNA interference via the endogenous miRNA pathway regulates gene expression by controlling protein synthesis through post-transcriptional gene silencing. In recent years, miRNA-mediated gene regulation has shown potential for treatment of neurological disorders caused by a toxic gain of function mechanism. However, efficient delivery to target tissues has limited its application. Here we used a transgenic mouse model for spinal and bulbar muscular atrophy (SBMA), a neuromuscular disease caused by polyglutamine expansion in the androgen receptor (AR), to test gene silencing by a newly identified AR-targeting miRNA, miR-298. We overexpressed miR-298 using a recombinant adeno-associated virus (rAAV) serotype 9 vector to facilitate transduction of non-dividing cells. A single tail-vein injection in SBMA mice induced sustained and widespread overexpression of miR-298 in skeletal muscle and motor neurons and resulted in amelioration of the neuromuscular phenotype in the mice.

Decreased Motor Neuron Support by SMA Astrocytes due to Diminished MCP1 Secretion.

Spinal muscular atrophy (SMA) is an autosomal-recessive disorder characterized by severe, often fatal muscle weakness due to loss of motor neurons. SMA patients have deletions and other mutations of the survival of motor neuron 1 (SMN1) gene, resulting in decreased SMN protein. Astrocytes are the primary support cells of the CNS and are responsible for glutamate clearance, metabolic support, response to injury, and regulation of signal transmission. Astrocytes have been implicated in SMA as in in other neurodegenerative disorders. Astrocyte-specific rescue of SMN protein levels has been shown to mitigate disease manifestations in mice. However, the mechanism by which SMN deficiency in astrocytes may contribute to SMA is unclear and what aspect of astrocyte activity is lacking is unknown. Therefore, it is worthwhile to identify defects in SMN-deficient astrocytes that compromise normal function. We show here that SMA astrocyte cultures derived from mouse spinal cord of both sexes are deficient in supporting both WT and SMN-deficient motor neurons derived from male, female, and mixed-sex sources and that this deficiency may be mitigated with secreted factors. In particular, SMN-deficient astrocytes have decreased levels of monocyte chemoactive protein 1 (MCP1) secretion compared with controls and MCP1 restoration stimulates outgrowth of neurites from cultured motor neurons. Correction of MCP1 deficiency may thus be a new therapeutic approach to SMA.SIGNIFICANCE STATEMENT Spinal muscular atrophy (SMA) is caused by the loss of motor neurons, but astrocyte dysfunction also contributes to the disease in mouse models. Monocyte chemoactive protein 1 (MCP1) has been shown to be neuroprotective and is released by astrocytes. Here, we report that MCP1 levels are decreased in SMA mice and that replacement of deficient MCP1 increases differentiation and neurite length of WT and SMN-deficient motor-neuron-like cells in cell culture. This study reveals a novel aspect of astrocyte dysfunction in SMA and indicates a possible approach for improving motor neuron growth and survival in this disease.

Gene networks activated by specific patterns of action potentials in dorsal root ganglia neurons.

Gene regulatory networks underlie the long-term changes in cell specification, growth of synaptic connections, and adaptation that occur throughout neonatal and postnatal life. Here we show that the transcriptional response in neurons is exquisitely sensitive to the temporal nature of action potential firing patterns. Neurons were electrically stimulated with the same number of action potentials, but with different inter-burst intervals. We found that these subtle alterations in the timing of action potential firing differentially regulates hundreds of genes, across many functional categories, through the activation or repression of distinct transcriptional networks. Our results demonstrate that the transcriptional response in neurons to environmental stimuli, coded in the pattern of action potential firing, can be very sensitive to the temporal nature of action potential delivery rather than the intensity of stimulation or the total number of action potentials delivered. These data identify temporal kinetics of action potential firing as critical components regulating intracellular signalling pathways and gene expression in neurons to extracellular cues during early development and throughout life.

BDNF mRNA abundance regulated by antidromic action potentials and AP-LTD in hippocampus.

Action-potential-induced LTD (AP-LTD) is a form of synaptic plasticity that reduces synaptic strength in CA1 hippocampal neurons firing antidromically during sharp-wave ripples. This firing occurs during slow-wave sleep and quiet moments of wakefulness, which are periods of offline replay of neural sequences learned during encoding sensory information. Here we report that rapid and persistent down-regulation of different mRNA transcripts of the BDNF gene accompanies AP-LTD, and that AP-LTD is abolished in mice with the BDNF gene knocked out in CA1 hippocampal neurons. These findings increase understanding of the mechanism of AP-LTD and the cellular mechanisms of memory consolidation.

Blast shockwaves propagate Ca(2+) activity via purinergic astrocyte networks in human central nervous system cells.

In a recent study of the pathophysiology of mild, blast-induced traumatic brain injury (bTBI) the exposure of dissociated, central nervous system (CNS) cells to simulated blast resulted in propagating waves of elevated intracellular Ca(2+). Here we show, in dissociated human CNS cultures, that these calcium waves primarily propagate through astrocyte-dependent, purinergic signaling pathways that are blocked by P2 antagonists. Human, compared to rat, astrocytes had an increased calcium response and prolonged calcium wave propagation kinetics, suggesting that in our model system rat CNS cells are less responsive to simulated blast. Furthermore, in response to simulated blast, human CNS cells have increased expressions of a reactive astrocyte marker, glial fibrillary acidic protein (GFAP) and a protease, matrix metallopeptidase 9 (MMP-9). The conjoint increased expression of GFAP and MMP-9 and a purinergic ATP (P2) receptor antagonist reduction in calcium response identifies both potential mechanisms for sustained changes in brain function following primary bTBI and therapeutic strategies targeting abnormal astrocyte activity.

MiR-298 Counteracts Mutant Androgen Receptor Toxicity in Spinal and Bulbar Muscular Atrophy.

Spinal and bulbar muscular atrophy (SBMA) is a currently untreatable adult-onset neuromuscular disease caused by expansion of a polyglutamine repeat in the androgen receptor (AR). In SBMA, as in other polyglutamine diseases, a toxic gain of function in the mutant protein is an important factor in the disease mechanism; therefore, reducing the mutant protein holds promise as an effective treatment strategy. In this work, we evaluated a microRNA (miRNA) to reduce AR expression. From a list of predicted miRNAs that target human AR, we selected microRNA-298 (miR-298) for its ability to downregulate AR mRNA and protein levels when transfected in cells overexpressing wild-type and mutant AR and in SBMA patient-derived fibroblasts. We showed that miR-298 directly binds to the 3'-untranslated region of the human AR transcript, and counteracts AR toxicity in vitro. Intravenous delivery of miR-298 with adeno-associated virus serotype 9 vector resulted in efficient transduction of muscle and spinal cord and amelioration of the disease phenotype in SBMA mice. Our findings support the development of miRNAs as a therapeutic strategy for SBMA and other neurodegenerative disorders caused by toxic proteins.

Nonsynaptic junctions on myelinating glia promote preferential myelination of electrically active axons.

The myelin sheath on vertebrate axons is critical for neural impulse transmission, but whether electrically active axons are preferentially myelinated by glial cells, and if so, whether axo-glial synapses are involved, are long-standing questions of significance to nervous system development, plasticity and disease. Here we show using an in vitro system that oligodendrocytes preferentially myelinate electrically active axons, but synapses from axons onto myelin-forming oligodendroglial cells are not required. Instead, vesicular release at nonsynaptic axo-glial junctions induces myelination. Axons releasing neurotransmitter from vesicles that accumulate in axon varicosities induces a local rise in cytoplasmic calcium in glial cell processes at these nonsynaptic functional junctions, and this signalling stimulates local translation of myelin basic protein to initiate myelination.

Mutation in CPT1C Associated With Pure Autosomal Dominant Spastic Paraplegia.

IMPORTANCE: The family of genes implicated in hereditary spastic paraplegias (HSPs) is quickly expanding, mostly owing to the widespread availability of next-generation DNA sequencing methods. Nevertheless, a genetic diagnosis remains unavailable for many patients. OBJECTIVE: To identify the genetic cause for a novel form of pure autosomal dominant HSP. DESIGN, SETTING, AND PARTICIPANTS: We examined and followed up with a family presenting to a tertiary referral center for evaluation of HSP for a decade until August 2014. Whole-exome sequencing was performed in 4 patients from the same family and was integrated with linkage analysis. Sanger sequencing was used to confirm the presence of the candidate variant in the remaining affected and unaffected members of the family and screen the additional patients with HSP. Five affected and 6 unaffected participants from a 3-generation family with pure adult-onset autosomal dominant HSP of unknown genetic origin were included. Additionally, 163 unrelated participants with pure HSP of unknown genetic cause were screened. MAIN OUTCOME AND MEASURE: Mutation in the neuronal isoform of carnitine palmitoyl-transferase (CPT1C) gene. RESULTS: We identified the nucleotide substitution c.109C>T in exon 3 of CPT1C, which determined the base substitution of an evolutionarily conserved Cys residue for an Arg in the gene product. This variant strictly cosegregated with the disease phenotype and was absent in online single-nucleotide polymorphism databases and in 712 additional exomes of control participants. We showed that CPT1C, which localizes to the endoplasmic reticulum, is expressed in motor neurons and interacts with atlastin-1, an endoplasmic reticulum protein encoded by the ATL1 gene known to be mutated in pure HSPs. The mutation, as indicated by nuclear magnetic resonance spectroscopy studies, alters the protein conformation and reduces the mean (SD) number (213.0 [46.99] vs 81.9 [14.2]; P < .01) and size (0.29 [0.01] vs 0.26 [0.01]; P < .05) of lipid droplets on overexpression in cells. We also observed a reduction of mean (SD) lipid droplets in primary cortical neurons isolated from Cpt1c-/- mice as compared with wild-type mice (1.0 [0.12] vs 0.44 [0.05]; P < .001), suggesting a dominant negative mechanism for the mutation. CONCLUSIONS AND RELEVANCE: This study expands the genetics of autosomal dominant HSP and is the first, to our knowledge, to link mutation in CPT1C with a human disease. The association of the CPT1C mutation with changes in lipid droplet biogenesis supports a role for altered lipid-mediated signal transduction in HSP pathogenesis.

Systematic identification of 3'-UTR regulatory elements in activity-dependent mRNA stability in hippocampal neurons.

Ongoing neuronal activity during development and plasticity acts to refine synaptic connections and contributes to the induction of plasticity and ultimately long-term memory storage. Activity-dependent, post-transcriptional control of mRNAs occurs through transport to axonal and dendritic compartments, local translation and mRNA stability. We have identified a mechanism that contributes to activity-dependent regulation of mRNA stability during synaptic plasticity in rat hippocampal neurons. In this study, we demonstrate rapid, post-transcriptional control over process-enriched mRNAs by neuronal activity. Systematic analysis of the 3'-UTRs of destabilized transcripts, identifies enrichment in sequence motifs corresponding to microRNA (miRNA)-binding sites. The miRNAs that were identified, miR-326-3p/miR-330-5p, miR-485-5p, miR-666-3p and miR-761 are predicted to regulate networks of genes important in plasticity and development. We find that these miRNAs are developmentally regulated in the hippocampus, many increasing by postnatal day 14. We further find that miR-485-5p controls NGF-induced neurite outgrowth in PC12 cells, tau expression and axonal development in hippocampal neurons. miRNAs can function at the synapse to rapidly control and affect short- and long-term changes at the synapse. These processes likely occur during refinement of synaptic connections and contribute to the induction of plasticity and learning and memory.

Metallothionein promotes regenerative axonal sprouting of dorsal root ganglion neurons after physical axotomy.

Prior studies have reported that metallothionein I/II (MT) promote regenerative axonal sprouting and neurite elongation of a variety of central nervous system neurons after injury. In this study, we evaluated whether MT is capable of modulating regenerative axon outgrowth of neurons from the peripheral nervous system. The effect of MT was firstly investigated in dorsal root ganglion (DRG) explants, where axons were scratch-injured in the presence or absence of exogenous MT. The application of MT led to a significant increase in regenerative sprouting of neurons 16 h after injury. We show that the pro-regenerative effect of MT involves an interaction with the low-density lipoprotein receptor megalin, which could be blocked using the competitive antagonist RAP. Pre-treatment with the mitogen-activated protein kinase (MAPK) inhibitor PD98059 also completely abrogated the effect of exogenous MT in promoting axonal outgrowth. Interestingly, we only observed megalin expression in neuronal soma and not axons in the DRG explants. To investigate this matter, an in vitro injury model was established using Campenot chambers, which allowed the application of MT selectively into either the axonal or cell body compartments after scratch injury was performed to axons. At 16 h after injury, regenerating axons were significantly longer only when exogenous MT was applied solely to the soma compartment, in accordance with the localized expression of megalin in neuronal cell bodies. This study provides a clear indication that MT promotes axonal regeneration of DRG neurons, via a megalin- and MAPK-dependent mechanism.

Control of local protein synthesis and initial events in myelination by action potentials.

Formation of myelin, the electrical insulation on axons produced by oligodendrocytes, is controlled by complex cell-cell signaling that regulates oligodendrocyte development and myelin formation on appropriate axons. If electrical activity could stimulate myelin induction, then neurodevelopment and the speed of information transmission through circuits could be modified by neural activity. We find that release of glutamate from synaptic vesicles along axons of mouse dorsal root ganglion neurons in culture promotes myelin induction by stimulating formation of cholesterol-rich signaling domains between oligodendrocytes and axons, and increasing local synthesis of the major protein in the myelin sheath, myelin basic protein, through Fyn kinase-dependent signaling. This axon-oligodendrocyte signaling would promote myelination of electrically active axons to regulate neural development and function according to environmental experience.

MicroRNA regulation of homeostatic synaptic plasticity.

Homeostatic mechanisms are required to control formation and maintenance of synaptic connections to maintain the general level of neural impulse activity within normal limits. How genes controlling these processes are co-coordinately regulated during homeostatic synaptic plasticity is unknown. MicroRNAs (miRNAs) exert regulatory control over mRNA stability and translation and may contribute to local and activity-dependent posttranscriptional control of synapse-associated mRNAs. However, identifying miRNAs that function through posttranscriptional gene silencing at synapses has remained elusive. Using a bioinformatics screen to identify sequence motifs enriched in the 3'UTR of rapidly destabilized mRNAs, we identified a developmentally and activity-regulated miRNA (miR-485) that controls dendritic spine number and synapse formation in an activity-dependent homeostatic manner. We find that many plasticity-associated genes contain predicted miR-485 binding sites and further identify the presynaptic protein SV2A as a target of miR-485. miR-485 negatively regulated dendritic spine density, postsynaptic density 95 (PSD-95) clustering, and surface expression of GluR2. Furthermore, miR-485 overexpression reduced spontaneous synaptic responses and transmitter release, as measured by miniature excitatory postsynaptic current (EPSC) analysis and FM 1-43 staining. SV2A knockdown mimicked the effects of miR-485, and these effects were reversed by SV2A overexpression. Moreover, 5 d of increased synaptic activity induced homeostatic changes in synaptic specializations that were blocked by a miR-485 inhibitor. Our findings reveal a role for this previously uncharacterized miRNA and the presynaptic protein SV2A in homeostatic plasticity and nervous system development, with possible implications in neurological disorders (e.g., Huntington and Alzheimer's disease), where miR-485 has been found to be dysregulated.