Synapse Turnover Research Articles

Morphological correlates of synaptic protein turnover in the mouse brain.

Synaptic proteins need to be replaced regularly, to maintain function and to prevent damage. It is unclear whether this process, known as protein turnover, relates to synaptic morphology. To test this, we relied on nanoscale secondary ion mass spectrometry, to detect newly synthesized synaptic components in the brains of young adult (6 mo old) and aged mice (24 mo old), and on transmission electron microscopy, to reveal synapse morphology. Several parameters correlated to turnover, including pre- and postsynaptic size, the number of synaptic vesicles and the presence of a postsynaptic nascent zone. In aged mice, the turnover of all brain compartments was reduced by ∼20%. The turnover rates of the pre- and postsynapses correlated well in aged mice, suggesting that they are subject to common regulatory mechanisms. This correlation was poorer in young adult mice, in line with their higher synaptic dynamics. We conclude that synapse turnover is reflected by synaptic morphology.

Development and Application of Technology for Neural Circuit Visualization - Secondary Publication.

The dynamics of neurite extension and synaptic connections are central issues in neural circuit research. The development of technologies for labeling purified cytoskeletal proteins with fluorescent dyes and introducing them into living neurons using microinjection greatly facilitated our understanding of cytoskeletal dynamics in neuronal axons. Imaging data showed that the cytoskeleton repeatedly polymerized and depolymerized within the axon, and elongation was driven by the new cytoskeleton formed at the axon tip. This finding significantly revised previously proposed models that explained slow axonal transport. After the discovery of green fluorescent protein (GFP), its potential application to the live imaging of neurons was recognized in the 1990s, and a new method for visualizing synapses using GFP-tagged postsynaptic scaffolding molecules was established. This method revealed the continuous turnover of synapses during development, which overturned the established theory that synapses are highly stable once they are formed. Live imaging of synapses also demonstrated that the molecular composition of synapses changes rapidly, driven by the rapid replacement of synaptic molecules. Fluorescence measurement of single GFP molecules enabled estimation of the absolute number of postsynaptic molecules in a single synapse. Furthermore, in multiple mouse models of autism spectrum disorders (ASDs), enhanced synapse turnover was detected as a common circuit-level phenotype. This study provides solid experimental evidence that an increase in synapse dynamics underlies the pathophysiology in mouse models of ASDs. The introduction of fluorescence imaging in neurobiology revealed that the neuronal cytoskeleton and synaptic structure are not static but dynamic cellular components. Imaging technology is expected to further advance our understanding of the dynamic properties of neurons and neural circuits.

Single-cell long-read sequencing-based mapping reveals specialized splicing patterns in developing and adult mouse and human brain

RNA isoforms influence cell identity and function. However, a comprehensive brain isoform map was lacking. We analyze single-cell RNA isoforms across brain regions, cell subtypes, developmental time points and species. For 72% of genes, full-length isoform expression varies along one or more axes. Splicing, transcription start and polyadenylation sites vary strongly between cell types, influence protein architecture and associate with disease-linked variation. Additionally, neurotransmitter transport and synapse turnover genes harbor cell-type variability across anatomical regions. Regulation of cell-type-specific splicing is pronounced in the postnatal day 21-to-postnatal day 28 adolescent transition. Developmental isoform regulation is stronger than regional regulation for the same cell type. Cell-type-specific isoform regulation in mice is mostly maintained in the human hippocampus, allowing extrapolation to the human brain. Conversely, the human brain harbors additional cell-type specificity, suggesting gain-of-function isoforms. Together, this detailed single-cell atlas of full-length isoform regulation across development, anatomical regions and species reveals an unappreciated degree of isoform variability across multiple axes.

Repetitive transcranial magnetic stimulation increases synaptic plasticity of cortical axons in the APP‐PS1 mouse model of amyloidosis

AbstractBackgroundRepetitive transcranial magnetic stimulation (rTMS) is non‐invasive brain stimulation with therapeutic potential for use in diseases causing dementia. We previously demonstrated that low intensity (LI) rTMS induced functional improvements in the accuracy and rate of learning in adult mouse skilled reaching paradigms (Tang, Bennett et al, Scientific Reports, 2018) and induced dendritic spine changes in cortical motor neurons after a single application of LI‐rTMS (Tang, Bennett et al, Brain Stimulation, 2021). Here, we investigate the effect of LI‐rTMS on axonal boutons of excitatory axons in the presence of amyloid pathology in the APP‐PS1 mouse.MethodAPP/PS1 mice were crossed with Thy1‐GFPM mice to visualise excitatory axon circuitry in the presence of amyloid plaque pathology. Multiphoton imaging through a cranial window was used to identify cortical axons in the upper layers of the sensorimotor cortex in adult animals (10‐12 months old) and monitor the stability and turnover (gains and losses as a fraction of total number of synapses) of axonal boutons in the presence (APP‐GFPM) or absence (WT‐GFPM) of amyloid pathology. Axons were imaged at 48‐hour intervals for 8 days either side of a single session of LI‐rTMS.ResultThe density of axonal boutons along the axon shaft was stable and comparable in APP‐GFPM and WT‐GFP mice, before and after LI‐rTMS. The turnover of synapses was significantly (p < 0.05) reduced in APP‐GFPM cortical axons compared to WT‐GFPM prior to stimulation. A single session of LI‐rTMS induced a significant (p < 0.01)and sustained increase in synaptic turnover in both genotypes. Interestingly the initial lower turnover of boutons in the APP‐GFPM cortex was increased to the levels observed in WT‐GFPM axons at baseline.ConclusionIn the presence of amyloid pathology, synaptic density in excitatory cortical axons is unchanged, however the dynamic fraction of synapses is reduced in compared to aged‐matched controls. LI‐rTMS boosts synaptic plasticity in mature cortical axons in both the presence and absence of plaque pathology. Together with our findings of improved functional performance following stimulation, these results suggest not only possible mechanisms of action but also a potential clinical benefit in the management of Alzheimer’s disease.

Oxygen and the Spark of Human Brain Evolution: Complex Interactions of Metabolism and Cortical Expansion across Development and Evolution

Scientific theories on the functioning and dysfunction of the human brain require an understanding of its development—before and after birth and through maturation to adulthood—and its evolution. Here we bring together several accounts of human brain evolution by focusing on the central role of oxygen and brain metabolism. We argue that evolutionary expansion of human transmodal association cortices exceeded the capacity of oxygen delivery by the vascular system, which led these brain tissues to rely on nonoxidative glycolysis for additional energy supply. We draw a link between the resulting lower oxygen tension and its effect on cytoarchitecture, which we posit as a key driver of genetic developmental programs for the human brain—favoring lower intracortical myelination and the presence of biosynthetic materials for synapse turnover. Across biological and temporal scales, this protracted capacity for neural plasticity sets the conditions for cognitive flexibility and ongoing learning, supporting complex group dynamics and intergenerational learning that in turn enabled improved nutrition to fuel the metabolic costs of further cortical expansion. Our proposed model delineates explicit mechanistic links among metabolism, molecular and cellular brain heterogeneity, and behavior, which may lead toward a clearer understanding of brain development and its disorders.

Mechanisms of microglia-mediated synapse turnover and synaptogenesis.

Microglia shape synaptic connections among neurons of the central nervous system (CNS) during development and adulthood. In this review, the actions by which they facilitate pruning, refinement, and new synaptic development throughout the lifespan are considered, along with the molecular mechanisms by which neurons and microglia communicate to guide these actions. Microglia survey neuronal activity and selectively modify synaptic connections at the level of individual dendrites and synapses. This is important given that microglia are necessary for a healthy nervous system capable of learning and other neural phenomena based on synaptic modifications and can also cause pathological synaptic disfunctions in immunologically driven neurodegenerative diseases. Understanding how microglia directly shape synaptic connections between neurons yields a more complete understanding of normal neuroplasticity and provides new routes for understanding disease states.

AMPA receptors in the synapse turnover by monomer diffusion

The number and subunit compositions of AMPA receptors (AMPARs), hetero- or homotetramers composed of four subunits GluA1–4, in the synapse is carefully tuned to sustain basic synaptic activity. This enables stimulation-induced synaptic plasticity, which is central to learning and memory. The AMPAR tetramers have been widely believed to be stable from their formation in the endoplasmic reticulum until their proteolytic decomposition. However, by observing GluA1 and GluA2 at the level of single molecules, we find that the homo- and heterotetramers are metastable, instantaneously falling apart into monomers, dimers, or trimers (in 100 and 200 ms, respectively), which readily form tetramers again. In the dendritic plasma membrane, GluA1 and GluA2 monomers and dimers are far more mobile than tetramers and enter and exit from the synaptic regions. We conclude that AMPAR turnover by lateral diffusion, essential for sustaining synaptic function, is largely done by monomers of AMPAR subunits, rather than preformed tetramers.

Rehabilitative Training Interacts with Ischemia-Instigated Spine Dynamics to Promote a Lasting Population of New Synapses in Peri-Infarct Motor Cortex

After subtotal infarcts of primary motor cortex (M1), motor rehabilitative training (RT) promotes improvements in paretic forelimb function that have been linked with its promotion of structural and functional reorganization of peri-infarct cortex, but how the reorganization unfolds is scantly understood. Cortical infarcts also instigate a prolonged period of dendritic spine turnover in peri-infarct cortex. Here we investigated the possibility that synaptic structural responses to RT in peri-infarct cortex reflect, in part, interactions with ischemia-instigated spine turnover. This was tested after artery-targeted photothrombotic M1 infarcts or Sham procedures in adult (4 months) C57BL/6 male and female GFP-M line (n = 24) and male yellow fluorescent protein-H line (n = 5) mice undergoing RT in skilled reaching or no-training control procedures. Regardless of training condition, spine turnover was increased out to 5 weeks postinfarct relative to Sham, as was the persistence of new spines formed within a week postinfarct. However, compared with no-training controls, new spines formed during postinfarct weeks 2-4 in mice undergoing RT persisted in much greater proportions to later time points, by a magnitude that predicted behavioral improvements in the RT group. These results indicate that RT interacts with ischemia-instigated spine turnover to promote preferential stabilization of newly formed spines, which is likely to yield a new population of mature synapses in peri-infarct cortex that could contribute to cortical functional reorganization and behavioral improvement. The findings newly implicate ischemia-instigated spine turnover as a mediator of cortical synaptic structural responses to RT and newly establish the experience dependency of new spine fates in the postischemic turnover context.SIGNIFICANCE STATEMENT Motor rehabilitation, the main treatment for motor impairments after stroke, is far from sufficient to normalize function. A better understanding of neural substrates of rehabilitation-induced behavioral improvements could be useful for understanding how to optimize it. Here, we investigated the nature and time course of synaptic responses to motor rehabilitative training in vivo Focal ischemia instigated a period of synapse turnover in peri-infarct motor cortex of mice. Rehabilitative training increased the stability of new synapses formed during the initial weeks after the infarct, the magnitude of which was correlated with improvements in skilled motor performance. Therefore, the maintenance of new synapses formed after ischemia could represent a structural mechanism of rehabilitative training efficacy.

The Stability of Glutamatergic Synapses Is Independent of Activity Level, but Predicted by Synapse Size.

Neuronal activity is thought to drive the remodeling of circuits in the mammalian cerebral cortex. However, its precise function in the underlying formation and elimination of glutamatergic synapses has remained controversial. To clarify the role of activity in synapse turnover, we have assessed the effects of inhibition of glutamate release from a sparse subset of cultured hippocampal neurons on synapse turnover. Sustained chemogenetic attenuation of release through presynaptic expression of a designer receptor exclusively activated by designer drugs (DREADD) had no effect on the formation or elimination of glutamatergic synapses. Sparse expression of tetanus neurotoxin light chain (TeNT-LC), a synaptobrevin-cleaving protease that completely abolishes neurotransmitter release, likewise did not lead to changes in the rate of synapse elimination, although it reduced the rate of synapse formation. The stability of active and silenced synapses correlated with measures of synapse size. While not excluding a modulatory role in synapse elimination, our findings show that synaptic activity is neither required for the removal nor the maintenance of glutamatergic synapses between hippocampal neurons. Our results also demonstrate that the stability of glutamatergic synapses scales with their size irrespective of their activity.

Stable memory and computation in randomly rewiring neural networks.

Our brains must maintain a representation of the world over a period of time much longer than the typical lifetime of the biological components producing that representation. For example, recent research suggests that dendritic spines in the adult mouse hippocampus are transient with an average lifetime of ~10 days. If this is true, and if turnover is equally likely for all spines, ~95% of excitatory synapses onto a particular neuron will turn over within 30 days; however, a neuron's receptive field can be relatively stable over this period. Here, we use computational modeling to ask how memories can persist in neural circuits such as the hippocampus and visual cortex in the face of synapse turnover. We demonstrate that Hebbian plasticity during replay of presynaptic activity patterns can integrate newly formed synapses into pre-existing memories. Furthermore, we find that Hebbian plasticity during replay is sufficient to stabilize the receptive fields of hippocampal place cells in a model of the grid-cell-to-place-cell transformation in CA1 and of orientation-selective cells in a model of the center-surround-to-simple-cell transformation in V1. Together, these data suggest that a simple plasticity rule, correlative Hebbian plasticity of synaptic strengths, is sufficient to preserve neural representations in the face of synapse turnover, even in the absence of activity-dependent structural plasticity. NEW & NOTEWORTHY Recent research suggests that synapses turn over rapidly in some brain structures; however, memories seem to persist for much longer. We show that Hebbian plasticity of synaptic strengths during reactivation events can preserve memory in computational models of hippocampal and cortical networks despite turnover of all synapses. Our results suggest that memory can be stored in the correlation structure of a network undergoing rapid synaptic remodeling.

Nano-scale analysis of synapse morphology in an autism mouse model with 15q11-13 copy number variation using focused ion beam milling and scanning electron microscopy.

Circuit-level alternations in patients of autism spectrum disorder (ASD) is under active investigation and detailed characterization of synapse morphology in ASD model mice should be informative. We utilized focused ion beam milling and scanning electron microscopy (FIB-SEM) to obtain three-dimensional images of synapses in the layer 2/3 of the somatosensory cortex from a mouse model for ASD with human 15q11-13 chromosomal duplication (15q dup mice). We found a trend of higher spine density and a higher fraction of astrocytic contact with both spine and shaft synapses in 15q dup mice. Measurement of spine synapse structure indicated that the size of the post-synaptic density (PSD), spine head volume, spine head width and spine neck width were smaller in 15q dup mice. Categorization of spine synapses into five classes suggested a trend of less frequent mushroom spines in 15q dup mice. These results suggest relative increase in excitatory synapses with immature morphology but more astrocytic contacts in 15q dup mice, which may be linked to enhanced synapse turnover seen in ASD mouse models.

Redefining neuroendocrinology: Epigenetics of brain-body communication over the life course.

The brain is the central organ of stress and adaptation to stress that perceives and determines what is threatening, as well as the behavioral and physiological responses to the stressor, and it does so somewhat differently in males and females. The expression of steroid hormone receptors throughout the brain has broadened the definition of 'neuroendocrinology' to include the reciprocal communication between the entire brain and body via hormonal and neural pathways. Mediated in part via systemic hormonal influences, the adult and developing brain possess remarkable structural and functional plasticity in response to stress, including neuronal replacement, dendritic remodeling, and synapse turnover. This article is both an account of an emerging field elucidating brain-body interactions at multiple levels, from molecules to social organization, as well as a personal account of my laboratory's role and, most importantly, the roles of trainees and colleagues, along with my involvement in interdisciplinary groups working on this topic.

Synapse formation in the brain can be enhanced by co-administering three specific nutrients

The memory impairments of early Alzheimer's disease [AD] are thought to result from a deficiency in synapses within the hippocampus and related brain regions. This deficiency could result from an acceleration in synapse turnover – perhaps caused by an endogenous neurotoxin like A-beta oligomers – or from a decrease in the production of the synaptic membrane needed to form new synapses. An AD-associated decrease in synaptogenesis almost certainly does occur, inasmuch as major decreases are also observed in the numbers of hippocampal dendritic spines, the immediate cytologic precursor of glutamatergic synapses. The syntheses of new dendritic spines and synapses can, however, be increased by concurrently raising brain levels of three circulating nutrients – uridine, omega-3 fatty acids docosahexaenoic acid (DHA) or eicosapentaenoic acid (EPA), and choline. This could provide an additional strategy for restoring synapses and thereby memory. The three nutrients are rate-limiting precursors in the Kennedy Cycle, the pathway which forms the phosphatides that are the major component of synaptic membranes. Uridine also increases the production of synaptic proteins, the other major membrane constituent, and the outgrowth of neurites. Hence administering the three nutrients accelerates synapse formation.These actions of uridine are largely mediated by uridine triphosphate (UTP), which can be released as a neurotransmitter from presynaptic terminals and can then activate P2Y2 receptors. The UTP in neurons can also be converted to cytidylyl triphosphate, CTP, the intermediate utilized in the Kennedy Cycle.

Patrolling monocytes sense peripheral infection and induce cytokine-mediated neuronal dysfunction.

A new study reveals that virus-associated activation of a subset of circulating monocytes results in the release of the proinflammatory cytokine tumor necrosis factor (TNF)-α, which induces increased turnover of synapses in the brain and learning deficits in mice.

Altered Synapse Stability in the Early Stages of Tauopathy

SummarySynapse loss is a key feature of dementia, but it is unclear whether synaptic dysfunction precedes degenerative phases of the disease. Here, we show that even before any decrease in synapse density, there is abnormal turnover of cortical axonal boutons and dendritic spines in a mouse model of tauopathy-associated dementia. Strikingly, tauopathy drives a mismatch in synapse turnover; postsynaptic spines turn over more rapidly, whereas presynaptic boutons are stabilized. This imbalance between pre- and post-synaptic stability coincides with reduced synaptically driven neuronal activity in pre-degenerative stages of the disease.

Short Latency Gray Matter Changes in Voxel-Based Morphometry following High Frequent Visual Stimulation.

Magnetic resonance imaging studies using voxel-based morphometry (VBM) detected structural changes in the human brain within periods of months or weeks. The underlying molecular mechanisms of VBM findings remain unresolved. We showed that simple visual stimulation by an alternating checkerboard leads to instant, short-lasting alterations of the primary and secondary visual cortex detected by VBM. The rapidness of occurrence (i.e., within 10 minutes) rather excludes most of the proposed physiological mechanism such as neural or glial cell genesis/degeneration or synapse turnover. We therefore favour cerebral fluid shifts to be the underlying correlate of the here observed VBM gray matter changes. Fast onset gray matter changes might be one important explanation for the inconsistency of VBM study results that often raise concern in regard to the validity of presented data. This study shows that changes detectable by VBM may occur within a few minutes after physiological stimulation and must be considered in future VBM experiments to avoid misinterpretation of results.

Fluorescence imaging of synapse dynamics in normal circuit maturation and in developmental disorders.

One of the most fundamental questions in neurobiology is how proper synaptic connections are established in the developing brain. Live-cell imaging of the synaptic structure and functional molecules can reveal the time course of synapse formation, molecular dynamics, and functional maturation. Using postsynaptic scaffolding proteins as a marker of synapse development, fluorescence time-lapse imaging revealed rapid formation of individual synapses that occurred within hours and their remodeling in culture preparations. In vivo two-photon excitation microscopy development enabled us to directly measure synapse turnover in living animals. In vivo synapse dynamics were suppressed in the adult rodent brain, but were maintained at a high level during the early postnatal period. This transition in synapse dynamics is biologically important and can be linked to the pathology of juvenile-onset psychiatric diseases. Indeed, the upregulation of synapse dynamics was observed in multiple mouse models of autism spectrum disorders. Fluorescence imaging of synapses provides new information regarding the physiology and pathology of neural circuit construction.

Piecing together a different picture: A host of new studies on autism have begun decoding the longstanding puzzle of its causes.

“A developmental disability that hinders the normal functioning of the brain, affecting, in varying degrees, communication skills and social interaction. Repetitive behaviours, and different ways of learning, paying attention, or reacting to things are often distinctive signs”. This standard definition of autism fails to describe the complexity of a condition that ranges in its manifestations from severe intellectual impairment to superior cognitive skills, like in the Asperger syndrome. To comprise such diversity, autism disorders are now covered under the umbrella term “autism spectrum disorder” (ASD). In most cases, ASD manifests during the first 5 years of life, with boys significantly more likely to be diagnosed than girls. ASD usually goes together with several other problems that frequently include anxiety, sleep disorders, or epilepsy. No cure exists; treatment, such as speech therapy, just attempts to alleviate specific deficits of autistic patients. > Nothing is simple in autism. Even the real number of people affected is uncertain. Nothing is simple in autism. Even the real number of people affected is uncertain. The US CDC estimates that about 1 in 68 (or 1.5%) of children in the USA are living with ASD (http://www.cdc.gov/ncbddd/autism/data.html). The WHO has a more conservative estimate, last revised in January this year, of 1 in 160 children, based on a larger set of epidemiological surveys (http://www.who.int/mediacentre/factsheets/autism-spectrum-disorders/en/). Needless to say, most studies were conducted in developed countries, and the prevalence of ASD in many low‐ and middle‐income countries remains largely unknown. > Along the years, many potential causes have been indicated, including genetic and environmental factors, exposure to toxins during pregnancy, wide gaps between parent ages, and so on Although the general consensus is that prevalence rates are increasing globally, this point is debated too. Some analyses indicate that a large percentage of the increase in ASD owes to improved awareness and …

Early Activation of Experience-Independent Dendritic Spine Turnover in a Mouse Model of Alzheimer's Disease.

Synaptic loss is critical in Alzheimer's disease (AD), but the dynamics of synapse turnover are poorly defined. We imaged dendritic spines in transgenic APPswe/PSen1∆E9 (APP/PS1) cerebral cortex. Dendritic spine turnover is increased far from plaque in aged APP/PS1 mice, and in young APP/PS1 mice prior to plaque formation. Dysregulation occurs in the presence of soluble Aβ oligomer and requires cellular prion protein (PrPC). APP/PS1 mice lack responsiveness of spine turnover to sensory stimulation. Critically, enhanced spine turnover is coupled with the loss of persistent spines starting early and continuing with age. To evaluate mechanisms of experience-independent supranormal spine turnover, we analyzed the transcriptome of young APP/PS1 mouse brain when turnover is altered but synapse density and memory are normal, and plaque and inflammation are absent. Early PrPC-dependent expression changes occur in synaptic and lipid-metabolizing genes. Thus, pathologic synaptic dysregulation underlying AD begins at a young age prior to Aβ plaque.

C2-P-05In vivo two-photon imaging of synapse dynamics in mouse models of autism

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by abnormal social communication and repetitive behaviour. Although previous genetic researches have reported many risk genes for ASD, it is still unclear what happens in the developing brains of ASD patients. Formation and remodelling of synapses are thought to be one of the key events for precise neural circuit development [ 1 ]. By using the technique of in vivo two-photon imaging [ 2 ], we examined the synapse turnover in multiple mouse models of ASD; patDp/+ mice which mimicked the human 15q11-13 duplication, NLG R451C mice which have a neuroligin-3 R451C point mutation, and BTBR mice. Time-lapse in vivo imaging revealed that synapse turnover is enhanced at the similar degree in three mouse models of ASD [ 3 ]. Importantly, these three ASD models have quite different genetic backgrounds, suggesting that enhanced spine turnover is a phenotype commonly seen in the developing neural circuits of ASD. Furthermore, classification of dendritic spines by gephyrin-GFP revealed that enhanced spine turnover is restricted to the spines without gephyrin-GFP clusters [ 3 ]. Immunohistochemical analysis showed that gephyrin-positive and -negative spines received the inputs from thalamic and cortical neurons, respectively, indicating that abnormal spine turnover in ASD mouse models is restricted to cortico-cortical circuits. Our data suggest that selective enhancement of synapse turnover during the development underlies abnormal neural circuits in ASD.