Microglia, the resident immune cells of the brain, rapidly change states in response to their environment, but we lack molecular and functional signatures of different microglial populations. Here, ...we analyzed the RNA expression patterns of more than 76,000 individual microglia in mice during development, in old age, and after brain injury. Our analysis uncovered at least nine transcriptionally distinct microglial states, which expressed unique sets of genes and were localized in the brain using specific markers. The greatest microglial heterogeneity was found at young ages; however, several states—including chemokine-enriched inflammatory microglia—persisted throughout the lifespan or increased in the aged brain. Multiple reactive microglial subtypes were also found following demyelinating injury in mice, at least one of which was also found in human multiple sclerosis lesions. These distinct microglia signatures can be used to better understand microglia function and to identify and manipulate specific subpopulations in health and disease.
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•Mouse microglia are heterogenous cells that are most diverse in the developing brain•Unique microglia transcriptional states can be localized to many brain regions•Small subsets of varied inflammatory microglia found in the aged brain•Diverse activated microglia subpopulations found in mouse demyelinated lesions and human MS
Hammond et al. redefine mouse microglia states using single-cell RNA-seq and in situ brain mapping. They find that microglia are most diverse in the developing, aged, and injured brain. Using focal demyelination, they show that microglia activation states are transcriptionally and spatially distinct within the lesion environment.
•MDGAs suppress synapse development by binding to neuroligin-occluding neurexin interaction.•MDGA1 and MDGA2 interact with all neuroligins with differential affinities in the physiological range.•In ...vivo loss of MDGA1 elevates inhibitory synapse numbers rendering networks resistant to excitation.•In vivo loss of MDGA2 elevates excitatory synapse numbers and strength.•MDGA1 also functions in basal progenitor aggregation in the subventricular zone.
Synapse development depends on a dynamic balance between synapse promoters and suppressors. MDGAs, immunoglobulin superfamily proteins, negatively regulate synapse development through blocking neuroligin–neurexin interactions. Recent analyses of MDGA–neuroligin complexes revealed the structural basis of this activity and indicate that MDGAs interact with all neuroligins with differential affinities. Surprisingly, analyses of mouse mutants revealed a functional divergence, with targeted mutation of Mdga1 and Mdga2 elevating inhibitory and excitatory synapses, respectively, on hippocampal pyramidal neurons. Further research is needed to determine the synapse-specific organizing properties of MDGAs in neural circuits, which may depend on relative levels and subcellular distributions of each MDGA, neuroligin and neurexin. Behavioral deficits in Mdga mutant mice support genetic links to schizophrenia and autism spectrum disorders and raise the possibility of harnessing these interactions for therapeutic purposes.
Selective synapse development determines how complex neuronal networks in the brain are formed. Complexes of postsynaptic neuroligins and LRRTMs with presynaptic neurexins contribute widely to ...excitatory synapse development, and mutations in these gene families increase the risk of developing psychiatric disorders. We find that LRRTM4 has distinct presynaptic binding partners, heparan sulfate proteoglycans (HSPGs). HSPGs are required to mediate the synaptogenic activity of LRRTM4. LRRTM4 shows highly selective expression in the brain. Within the hippocampus, we detected LRRTM4 specifically at excitatory postsynaptic sites on dentate gyrus granule cells. LRRTM4−/− dentate gyrus granule cells, but not CA1 pyramidal cells, exhibit reductions in excitatory synapse density and function. Furthermore, LRRTM4−/− dentate gyrus granule cells show impaired activity-regulated AMPA receptor trafficking. These results identifying cell-type-specific functions and multiple presynaptic binding partners for different LRRTM family members reveal an unexpected complexity in the design and function of synapse-organizing proteins.
•The excitatory postsynaptic protein LRRTM4 binds heparan sulfate proteoglycans•Axonal HSPGs are necessary for LRRTM4 to induce presynaptic differentiation•LRRTM4 is selectively expressed in hippocampal dentate gyrus molecular layers•LRRTM4−/− mice show cell-specific deficits in excitatory synapse density and function
Siddiqui et al. show that LRRTM4 mediates cell-type-specific excitatory synapse development in hippocampal dentate gyrus granule cells through binding to heparan sulfate sugar chains of proteoglycans, a mechanism distinct from that of other synapse-organizing proteins.
Beyond canonical roles in mediating synapse development, synapse organizers directly contribute to activity-dependent changes in synaptic strength.Synapse organizers promote molecular reconfiguration ...of active synapses through regulation of neurotransmitter receptors, inter- and intracellular signaling, and macromolecular synthesis.This link between synapse organizers and activity-dependent refinement of synapses further establishes the role of these central regulators in neurodevelopmental and neuropsychiatric disorders.
Synapse organizing proteins are multifaceted molecules that coordinate the complex processes of brain development and plasticity at the level of individual synapses. Their importance is demonstrated by the major brain disorders that emerge when their function is compromised. The mechanisms whereby the various families of organizers govern synapses are diverse, but converge on the structure, function, and plasticity of synapses. Therefore, synapse organizers regulate how synapses adapt to ongoing activity, a process central for determining the developmental trajectory of the brain and critical to all forms of cognition. Here, we explore how synapse organizers set the conditions for synaptic plasticity and the associated molecular events, which eventually link to behavioral features of neurodevelopmental and neuropsychiatric disorders. We also propose central questions on how synapse organizers influence network function through integrating nanoscale and circuit-level organization of the brain.
Synapse organizing proteins are multifaceted molecules that coordinate the complex processes of brain development and plasticity at the level of individual synapses. Their importance is demonstrated by the major brain disorders that emerge when their function is compromised. The mechanisms whereby the various families of organizers govern synapses are diverse, but converge on the structure, function, and plasticity of synapses. Therefore, synapse organizers regulate how synapses adapt to ongoing activity, a process central for determining the developmental trajectory of the brain and critical to all forms of cognition. Here, we explore how synapse organizers set the conditions for synaptic plasticity and the associated molecular events, which eventually link to behavioral features of neurodevelopmental and neuropsychiatric disorders. We also propose central questions on how synapse organizers influence network function through integrating nanoscale and circuit-level organization of the brain.
A Place at the Table Connor, Steven A.; Wang, Yu Tian
The Neuroscientist,
08/2016, Volume:
22, Issue:
4
Book Review, Journal Article
Peer reviewed
Resolving how our brains encode information requires an understanding of the cellular processes taking place during memory formation. Since the 1970s, considerable effort has focused on determining ...the properties and mechanisms underlying long-term potentiation (LTP) at glutamatergic synapses and how these processes influence initiation of new memories. However, accumulating evidence suggests that long-term depression (LTD) of synaptic strength, particularly at glutamatergic synapses, is a bona fide learning and memory mechanism in the mammalian brain. The known range of mechanisms capable of inducing LTD has been extended to those including NMDAR-independent forms, neuromodulator-dependent LTD, synaptic depression following stress, and non-synaptically induced forms. The examples of LTD observed at the hippocampal CA1 synapse to date demonstrate features consistent with LTP, including homo- and heterosynaptic expression, extended duration beyond induction (several hours to weeks), and association with encoding of distinct types of memories. Canonical mechanisms through which synapses undergo LTD include activation of phosphatases, initiation of protein synthesis, and dynamic regulation of presynaptic glutamate release and/or postsynaptic glutamate receptor endocytosis. Here, we will discuss the pre- and postsynaptic changes underlying LTD, recent advances in the identification and characterization of novel mechanisms underlying LTD, and how engagement of these processes constitutes a cellular analog for the genesis of specific types of memories.
Encoding new information in the brain requires changes in synaptic strength. Neuromodulatory transmitters can facilitate synaptic plasticity by modifying the actions and expression of specific ...signaling cascades, transmitter receptors and their associated signaling complexes, genes, and effector proteins. One critical neuromodulator in the mammalian brain is norepinephrine (NE), which regulates multiple brain functions such as attention, perception, arousal, sleep, learning, and memory. The mammalian hippocampus receives noradrenergic innervation and hippocampal neurons express β-adrenergic receptors, which are known to play important roles in gating the induction of long-lasting forms of synaptic potentiation. These forms of long-term potentiation (LTP) are believed to importantly contribute to long-term storage of spatial and contextual memories in the brain. In this review, we highlight the contributions of noradrenergic signaling in general and β-adrenergic receptors in particular, toward modulating hippocampal LTP. We focus on the roles of NE and β-adrenergic receptors in altering the efficacies of specific signaling molecules such as NMDA and AMPA receptors, protein phosphatases, and translation initiation factors. Also, the roles of β-adrenergic receptors in regulating synaptic "tagging" and "capture" of LTP within synaptic networks of the hippocampus are reviewed. Understanding the molecular and cellular bases of noradrenergic signaling will enrich our grasp of how the brain makes new, enduring memories, and may shed light on credible strategies for improving mental health through treatment of specific disorders linked to perturbed memory processing and dysfunctional noradrenergic synaptic transmission.
The noradrenergic system is implicated in neuropathologies contributing to major disorders of the memory, including post-traumatic stress disorder and Alzheimer’s disease. Determining the impact of ...norepinephrine on cellular function and plasticity is thus essential for making inroads into our understanding of these brain conditions, while expanding our capacity for treating them. Norepinephrine is a neuromodulator within the mammalian central nervous system which plays important roles in cognition and associated synaptic plasticity. Specifically, norepinephrine regulates the formation of memory through the stimulation of β-ARs, increasing the dynamic range of synaptic modifiability. The mechanisms through which NE influences neural circuit function have been extended to the level of the epigenome. This review focuses on recent insights into how the noradrenergic recruitment of epigenetic modifications, including DNA methylation and post-translational modification of histones, contribute to homo- and heterosynaptic plasticity. These advances will be placed in the context of synaptic changes associated with memory formation and linked to brain disorders and neurotherapeutic applications.
Perturbations of cell surface synapse-organizing proteins, particularly α-neurexins, contribute to neurodevelopmental and psychiatric disorders. From an unbiased screen, we identify calsyntenin-3 ...(alcadein-β) as a synapse-organizing protein unique in binding and recruiting α-neurexins, but not β-neurexins. Calsyntenin-3 is present in many pyramidal neurons throughout cortex and hippocampus but is most highly expressed in interneurons. The transmembrane form of calsyntenin-3 can trigger excitatory and inhibitory presynapse differentiation in contacting axons. However, calsyntenin-3-shed ectodomain, which represents about half the calsyntenin-3 pool in brain, suppresses the ability of multiple α-neurexin partners including neuroligin 2 and LRRTM2 to induce presynapse differentiation. Clstn3−/− mice show reductions in excitatory and inhibitory synapse density by confocal and electron microscopy and corresponding deficits in synaptic transmission. These results identify calsyntenin-3 as an α-neurexin-specific binding partner required for normal functional GABAergic and glutamatergic synapse development.
•Calsyntenin-3 triggers excitatory and inhibitory presynapse differentiation•Calsyntenin-3 binds α-neurexins, but not β-neurexins•Shed calsyntenin-3 ectodomain has broad antisynaptogenic activity•Clstn3−/− mice are deficient in excitatory and inhibitory synapse development
Pettem et al. identify calsyntenin-3 as a synapse-organizing protein that selectively binds α-neurexins. The transmembrane form of calsyntenin-3 triggers excitatory and inhibitory synapse differentiation, whereas proteolytic cleavage of calsyntenin-3 generates a fragment that competes with other α-neurexin ligands.
Definitive diagnosis of Parkinson's disease (PD) and dementia with Lewy bodies (DLB) relies on postmortem finding of disease-associated alpha-synuclein (αSyn
) as misfolded protein aggregates in the ...central nervous system (CNS). The recent development of the real-time quaking induced conversion (RT-QuIC) assay for ultrasensitive detection of αSyn
aggregates has revitalized the diagnostic values of clinically accessible biospecimens, including cerebrospinal fluid (CSF) and peripheral tissues. However, the current αSyn RT-QuIC assay platforms vary widely and are thus challenging to implement and standardize the measurements of αSyn
across a wide range of biospecimens and in different laboratories. We have streamlined αSyn RT-QuIC assay based on a second generation assay platform that was assembled entirely with commercial reagents. The streamlined RT-QuIC method consisted of a simplified protocol requiring minimal hands-on time, and allowing for a uniform analysis of αSyn
in different types of biospecimens from PD and DLB. Ultrasensitive and specific RT-QuIC detection of αSyn
aggregates was achieved in million-fold diluted brain homogenates and in nanoliters of CSF from PD and DLB cases but not from controls. Comparative analysis revealed higher seeding activity of αSyn
in DLB than PD in both brain homogenates and CSF. Our assay was further validated with CSF samples of 214 neuropathologically confirmed cases from tissue repositories (88 PD, 58 DLB, and 68 controls), yielding a sensitivity of 98% and a specificity of 100%. Finally, a single RT-QuIC assay protocol was employed uniformly to detect seeding activity of αSyn
in PD samples across different types of tissues including the brain, skin, salivary gland, and colon. We anticipate that our streamlined protocol will enable interested laboratories to easily and rapidly implement the αSyn RT-QuIC assay for various clinical specimens from PD and DLB. The utilization of commercial products for all assay components will improve the robustness and standardization of the RT-QuIC assay for diagnostic applications across different sites. Due to ultralow sample consumption, the ultrasensitive RT-QuIC assay will facilitate efficient use and sharing of scarce resources of biospecimens. Our streamlined RT-QuIC assay is suitable to track the distribution of αSyn
in CNS and peripheral tissues of affected patients. The ongoing evaluation of RT-QuIC assay of αSyn
as a potential biomarker for PD and DLB in clinically accessible biospecimens has broad implications for understanding disease pathogenesis, improving early and differential diagnosis, and monitoring therapeutic efficacies in clinical trials.
Neuromodulators regulate higher‐order cognitive processes including learning and memory through modulation of synaptic transmission and plasticity. Norepinephrine is a neuromodulator that is secreted ...throughout the brain in response to novelty or increased arousal, which alters neural circuits by increasing the modifiability of CNS synapses. Norepinephrine activates metabotropic receptors, initiating complex intracellular signalling cascades that can promote enduring changes in synaptic strength including long‐term potentiation (LTP). In particular, activation of beta‐adrenergic receptors (β‐ARs) by norepinephrine enhances LTP through downstream engagement of signalling cascades which upregulate protein synthesis at synapses. Here, we sought to determine the select signalling pathways recruited by norepinephrine to promote homosynaptic LTP at hippocampal synapses in mice. Application of norepinephrine initiated a long‐lasting form of homosynaptic LTP that requires protein synthesis. Norepinephrine‐mediated enhancement of LTP was reduced by inhibition of mammalian target of rapamycin and exchange protein directly activated by cAMP (Epac) but not cAMP‐dependent protein kinase A, suggesting that the endogenous β‐AR ligand norepinephrine may preferentially recruit Epac signalling to promote enduring changes in synaptic strength. These findings advance our understanding of the mechanisms through which norepinephrine regulates synaptic plasticity associated with formation of new memories.
Application of the endogenous transmitter noradrenaline enhances synaptic strength through the cAMP sensor, exchange protein directly activated by cAMP (Epac). Protein synthesis is upregulated in response to Epac‐dependent recruitment of ERK and mammalian target of rapamycin, which boost long‐term potentiation of glutamatergic synapses in the hippocampus, a brain structure required for memory.
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