Fragile X syndrome (FXS) results from a genetic mutation in a single gene yet produces a phenotypically complex disorder with a range of neurological and psychiatric problems. Efforts to decipher how ...perturbations in signaling pathways lead to the myriad alterations in synaptic and cellular functions have provided insights into the molecular underpinnings of this disorder. From this large body of data, the theme of circuit hyperexcitability has emerged as a potential explanation for many of the neurological and psychiatric symptoms in FXS. The mechanisms for hyperexcitability range from alterations in the expression or activity of ion channels to changes in neurotransmitters and receptors. Contributions of these processes are often brain region and cell type specific, resulting in complex effects on circuit function that manifest as altered excitability. Here, we review the current state of knowledge of the molecular, synaptic, and circuit-level mechanisms underlying hyperexcitability and their contributions to the FXS phenotypes.
Contractor, Klyachko, and Portera-Cailliau review accumulating evidence of changes in channels, neurotransmitters, synapses, and circuits in Fmr1 knockout mice that ultimately cause hyperexcitability. They propose that such alterations in neuronal and circuit excitability could be exploited to treat Fragile X syndrome.
The promise of using reprogrammed human neurons for disease modeling and regenerative medicine relies on the ability to induce patient-derived neurons with high efficiency and subtype specificity. We ...have previously shown that ectopic expression of brain-enriched microRNAs (miRNAs), miR-9/9∗ and miR-124 (miR-9/9∗-124), promoted direct conversion of human fibroblasts into neurons. Here we show that coexpression of miR-9/9∗-124 with transcription factors enriched in the developing striatum, BCL11B (also known as CTIP2), DLX1, DLX2, and MYT1L, can guide the conversion of human postnatal and adult fibroblasts into an enriched population of neurons analogous to striatal medium spiny neurons (MSNs). When transplanted in the mouse brain, the reprogrammed human cells persisted in situ for over 6 months, exhibited membrane properties equivalent to native MSNs, and extended projections to the anatomical targets of MSNs. These findings highlight the potential of exploiting the synergism between miR-9/9∗-124 and transcription factors to generate specific neuronal subtypes.
•Striatal transcription factors with miRNAs robustly converts human fibroblasts to MSNs•Converted cells show a gene expression profile analogous to primary human MSNs•Transplanted MSNs converted from human fibroblasts can function in vivo•Engrafted cells wire into the circuit properly and are capable of long-term survival
Victor et al. demonstrate that microRNAs (miR-9/9∗ and miR-124) and striatal transcription factors directly convert human fibroblasts into striatal medium spiny neurons. When transplanted in the mouse brain, converted human cells functionally integrate into the striatum.
The number and availability of vesicle release sites at the synaptic active zone (AZ) are critical factors governing neurotransmitter release; yet, these fundamental synaptic parameters have remained ...undetermined. Moreover, how neural activity regulates the spatiotemporal properties of the release sites within individual central synapses is unknown. Here, we combined a nanoscale imaging approach with advanced image analysis to detect individual vesicle fusion events with ∼27 nm localization precision at single hippocampal synapses under physiological conditions. Our results revealed the presence of multiple distinct release sites within individual hippocampal synapses. Release sites were distributed throughout the AZ and underwent repeated reuse. Furthermore, the spatiotemporal properties of the release sites were activity dependent with a reduction in reuse frequency and a shift in location toward the AZ periphery during high-frequency stimulation. These findings have revealed fundamental spatiotemporal properties of individual release sites in small central synapses and their activity-dependent modulation.
•Individual release events are detected with ∼27 nm precision in hippocampal synapses•Multiple distinct release sites are present within individual hippocampal synapses•Spatiotemporal properties of release sites are modulated in an activity-dependent manner
Maschi and Klyachko used nanoscale-precision detection of individual synaptic vesicle release events to reveal the presence of multiple distinct release sites within individual hippocampal synapses and their activity-dependent modulation.
Loss of FMRP causes fragile X syndrome (FXS), but the physiological functions of FMRP remain highly debatable. Here we show that FMRP regulates neurotransmitter release in CA3 pyramidal neurons by ...modulating action potential (AP) duration. Loss of FMRP leads to excessive AP broadening during repetitive activity, enhanced presynaptic calcium influx, and elevated neurotransmitter release. The AP broadening defects caused by FMRP loss have a cell-autonomous presynaptic origin and can be acutely rescued in postnatal neurons. These presynaptic actions of FMRP are translation independent and are mediated selectively by BK channels via interaction of FMRP with BK channel’s regulatory β4 subunits. Information-theoretical analysis demonstrates that loss of these FMRP functions causes marked dysregulation of synaptic information transmission. FMRP-dependent AP broadening is not limited to the hippocampus, but also occurs in cortical pyramidal neurons. Our results thus suggest major translation-independent presynaptic functions of FMRP that may have important implications for understanding FXS neuropathology.
► FMRP regulates action potential duration via BK channels in hippocampal neurons ► FMRP acts via regulatory β4 subunit to modulate BK channel calcium sensitivity ► FMRP regulates presynaptic calcium influx, neurotransmitter release, and STP ► FMRP actions are cell autonomous presynaptic and translation independent
Deng et al. find that FMRP, a protein implicated in fragile X syndrome, regulates neurotransmitter release and information transmission by modulating action potential duration in hippocampal and cortical neurons. These actions are presynaptic, translation independent, and mediated by interaction with BK channels.
Peripheral sensory neurons regenerate their axon after nerve injury to enable functional recovery. Intrinsic mechanisms operating in sensory neurons are known to regulate nerve repair, but whether ...satellite glial cells (SGC), which completely envelop the neuronal soma, contribute to nerve regeneration remains unexplored. Using a single cell RNAseq approach, we reveal that SGC are distinct from Schwann cells and share similarities with astrocytes. Nerve injury elicits changes in the expression of genes related to fatty acid synthesis and peroxisome proliferator-activated receptor (PPARα) signaling. Conditional deletion of fatty acid synthase (Fasn) in SGC impairs axon regeneration. The PPARα agonist fenofibrate rescues the impaired axon regeneration in mice lacking Fasn in SGC. These results indicate that PPARα activity downstream of FASN in SGC contributes to promote axon regeneration in adult peripheral nerves and highlight that the sensory neuron and its surrounding glial coat form a functional unit that orchestrates nerve repair.
A synaptic active zone (AZ) can release multiple vesicles in response to an action potential. This multi-vesicular release (MVR) occurs at most synapses, but its spatiotemporal properties are ...unknown. Nanoscale-resolution detection of individual release events in hippocampal synapses revealed unprecedented heterogeneity among vesicle release sites within a single AZ, with a gradient of release probability decreasing from AZ center to periphery. Parallel to this organization, MVR events preferentially overlap with uni-vesicular release (UVR) events at sites closer to an AZ center. Pairs of fusion events comprising MVR are also not perfectly synchronized, and the earlier event tends to occur closer to AZ center. The spatial features of release sites and MVR events are similarly tightened by buffering intracellular calcium. These observations revealed a marked heterogeneity of release site properties within individual AZs, which determines the spatiotemporal features of MVR events and is controlled, in part, by non-uniform calcium elevation across the AZ.
Fragile X syndrome (FXS) is the most common inherited form of intellectual disability and the leading genetic cause of autism. It is associated with the lack of fragile X mental retardation protein ...(FMRP), a regulator of protein synthesis in axons and dendrites. Studies on FXS have extensively focused on the postsynaptic changes underlying dysfunctions in long-term plasticity. In contrast, the presynaptic mechanisms of FXS have garnered relatively little attention and are poorly understood. Activity-dependent presynaptic processes give rise to several forms of short-term plasticity (STP), which is believed to control some of essential neural functions, including information processing, working memory, and decision making. The extent of STP defects and their contributions to the pathophysiology of FXS remain essentially unknown, however. Here we report marked presynaptic abnormalities at excitatory hippocampal synapses in Fmr1 knock-out (KO) mice leading to defects in STP and information processing. Loss of FMRP led to enhanced responses to high-frequency stimulation. Fmr1 KO mice also exhibited abnormal synaptic processing of natural stimulus trains, specifically excessive enhancement during the high-frequency spike discharges associated with hippocampal place fields. Analysis of individual STP components revealed strongly increased augmentation and reduced short-term depression attributable to loss of FMRP. These changes were associated with exaggerated calcium influx in presynaptic neurons during high-frequency stimulation, enhanced synaptic vesicle recycling, and enlarged readily-releasable and reserved vesicle pools. These data suggest that loss of FMRP causes abnormal STP and information processing, which may represent a novel mechanism contributing to cognitive impairments in FXS.
Synaptic vesicle exo- and endocytosis are usually driven by neuronal activity but can also occur spontaneously. The identity and differences between vesicles supporting evoked and spontaneous ...neurotransmission remain highly debated. Here we combined nanometer-resolution imaging with a transient motion analysis approach to examine the dynamics of individual synaptic vesicles in hippocampal terminals under physiological conditions. We found that vesicles undergoing spontaneous and stimulated endocytosis differ in their dynamic behavior, particularly in the ability to engage in directed motion. Our data indicate that such motional differences depend on the myosin family of motor proteins, particularly myosin II. Analysis of synaptic transmission in the presence of myosin II inhibitor confirmed a specific role for myosin II in evoked, but not spontaneous, neurotransmission and also suggested a functional role of myosin II-mediated vesicle motion in supporting vesicle mobilization during neural activity.
► Motion dynamics of individual synaptic vesicles examined with nanometer resolution ► Spontaneous and evoked vesicles differ in motion dynamics during recycling ► Spontaneous and evoked vesicles differentially engage in active motion ► Vesicles' motional differences depended on myosin II
The properties of synaptic vesicles supporting activity-evoked and spontaneous neurotransmission remain highly debated. Here, Peng et al. use nanometer-resolution imaging to show that synaptic vesicle categories differ in dynamic behavior, particularly in the ability to engage in motor-mediated transport.
Glucose has long been considered a primary energy source for synaptic function. However, it remains unclear to what extent alternative fuels, such as lactate/pyruvate, contribute to powering synaptic ...transmission. By detecting individual release events in hippocampal synapses, we find that mitochondrial ATP production regulates basal vesicle release probability and release location within the active zone (AZ), evoked by single action potentials. Mitochondrial inhibition shifts vesicle release closer to the AZ center and alters the efficiency of vesicle retrieval by increasing the occurrence of ultrafast endocytosis. Furthermore, we uncover that terminals can use oxidative fuels to maintain the vesicle cycle during trains of activity. Mitochondria are sparsely distributed along hippocampal axons, and we find that terminals containing mitochondria display enhanced vesicle release and reuptake during high-frequency trains. Our findings suggest that mitochondria not only regulate several fundamental features of synaptic transmission but may also contribute to modulation of short-term synaptic plasticity.
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•Synapses can utilize oxidative fuels to sustain various activity levels•Mitochondria regulate probability and nanoscale organization of vesicle release•Mitochondrial inhibition increases the occurrence of ultrafast endocytosis•Mitochondrial localization in synapses enhances vesicle release and retrieval
Synaptic transmission is energetically costly. Myeong et al. demonstrate that hippocampal synapses can bypass glycolysis using oxidative fuels. Mitochondria, rather than glycolysis, control the spatiotemporal properties of vesicle release and ultrafast endocytosis. Mitochondrial localization in nerve terminals enhances vesicle release and reuptake, thus representing a form of synaptic plasticity.
Altered neuronal excitability is one of the hallmarks of fragile X syndrome (FXS), but the mechanisms underlying this critical neuronal dysfunction are poorly understood. Here, we find that pyramidal ...cells in the entorhinal cortex of Fmr1 KO mice, an established FXS mouse model, display a decreased AP threshold and increased neuronal excitability. The AP threshold changes in Fmr1 KO mice are caused by increased persistent sodium current (INaP). Our results indicate that this abnormal INaP in Fmr1 KO animals is mediated by increased mGluR5-PLC-PKC (metabotropic glutamate receptor 5/phospholipase C/protein kinase C) signaling. These findings identify Na+ channel dysregulation as a major cause of neuronal hyperexcitability in cortical FXS neurons and uncover a mechanism by which abnormal mGluR5 signaling causes neuronal hyperexcitability in a FXS mouse model.
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•AP threshold is decreased in EC layer III pyramidal neurons of Fmr1 KO mice•AP threshold changes are caused by increased INaP in Fmr1 KO mice•Abnormal threshold is mediated by increased mGluR5-PLC-PKC pathway signaling
Deng and Klyachko demonstrate that hyperexcitability of cortical excitatory neurons in a mouse model of fragile X syndrome is caused by a reduced action potential threshold. This excitability defect results from dysregulation of sodium channels by an exaggerated mGluR5-PLC-PKC pathway signaling.
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