Dendritic spine geometry: functional implication and regulation

Excitatory to inhibitory synaptic ratios are unchanged at presymptomatic stages in multiple models of ALS

Hyperexcitability of motor neurons and spinal cord motor circuitry has been widely reported in the early stages of Amyotrophic Lateral Sclerosis (ALS). Changes in the relative amount of excitatory to inhibitory inputs onto a neuron (E:I synaptic ratio), possibly through a developmental shift in synapse formation in favour of excitatory transmission, could underlie pathological hyperexcitability. Given that astrocytes play a major role in early synaptogenesis and are implicated in ALS pathogenesis, their potential contribution to disease mechanisms involving synaptic imbalances and subsequent hyperexcitability is also of great interest. In order to assess E:I ratios in ALS, we utilised a novel primary spinal neuron / astrocyte co-culture system, derived from neonatal mice, in which synapses are formed in vitro. Using multiple ALS mouse models we found that no combination of astrocyte or neuron genotype produced alterations in E:I synaptic ratios assessed using pre- and post-synaptic anatomical markers. Similarly, we observed that ephrin-B1, a major contact-dependent astrocytic synaptogenic protein, was not differentially expressed by ALS primary astrocytes. Further to this, analysis of E:I ratios across the entire grey matter of the lumbar spinal cord in young (post-natal day 16-19) ALS mice revealed no differences versus controls. Finally, analysis in co-cultures of human iPSC-derived motor neurons and astrocytes harbouring the pathogenic C9orf72 hexanucleotide repeat expansion showed no evidence of a bias toward excitatory versus inhibitory synapse formation. We therefore conclude, utilising multiple ALS models, that we do not observe significant changes in the relative abundance of excitatory versus inhibitory synapses as would be expected if imbalances in synaptic inputs contribute to early hyperexcitability.

Maintenance of Lognormal-Like Skewed Dendritic Spine Size Distributions in Dentate Granule Cells of TNF, TNF-R1, TNF-R2, and TNF-R1/2-Deficient Mice

Dendritic spines are sites of synaptic plasticity and their head size correlates with the strength of the corresponding synapse. We recently showed that the distribution of spine head sizes follows a lognormal-like distribution even after blockage of activity or plasticity induction. As the cytokine tumor necrosis factor (TNF) influences synaptic transmission and constitutive TNF and receptor (TNF-R)-deficiencies cause changes in spine head size distributions, we tested whether these genetic alterations disrupt the lognormality of spine head sizes. Furthermore, we distinguished between spines containing the actin-modulating protein synaptopodin (SP-positive), which is present in large, strong and stable spines and those lacking it (SP-negative). Our analysis revealed that neither TNF-deficiency nor the absence of TNF-R1, TNF-R2 or TNF-R 1 and 2 (TNF-R1/R2) degrades the general lognormal-like, skewed distribution of spine head sizes (all spines, SP-positive spines, SP-negative spines). However, TNF, TNF-R1 and TNF-R2-deficiency affected the width of the lognormal distribution, and TNF-R1/2-deficiency shifted the distribution to the left. Our findings demonstrate the robustness of the lognormal-like, skewed distribution, which is maintained even in the face of genetic manipulations that alter the distribution of spine head sizes. Our observations are in line with homeostatic adaptation mechanisms of neurons regulating the distribution of spines and their head sizes.

Alzheimer's therapeutic development: shifting neurodegeneration to neuroregeneration

Alzheimer's disease (AD), similar to AD-related dementias, is characterized by impaired/lost neuronal structures and functions due to a long progression of neurodegeneration. Derailed endogenous signal pathways and disease processes have critical roles in neurodegeneration and are pharmacological targets in inducing neuroregeneration. Pharmacologically switching/shifting the brain status from neurodegeneration to neuroregeneration is emerging as a new therapeutic concept, one that is not only achievable, but also essential for effective therapy for AD. The results of the pharmacological-induced shift from neurodegeneration to neuroregeneration are twofold: arresting cognitive deterioration (and directing the brain toward cognitive recovery) in established AD, and preventing neurodegeneration through building up cognitive resilience in patients with preclinical or probable AD. In this review, we discuss these new developments in AD pharmacology and relevant clinical trials.

Cadherin-13 Maintains Retinotectal Synapses via Transneuronal Interactions

Maintaining precise synaptic contacts between neuronal partners is critical to ensure the proper functioning of the mammalian central nervous system (CNS). Diverse cell recognition molecules, such as classic cadherins (Cdhs), are part of the molecular machinery mediating synaptic choices during development and synaptic maintenance. Yet, the principles governing neuron-neuron wiring across diverse CNS neuron types remain largely unknown. The retinotectal synapses, connections from the retinal ganglion cells (RGCs) to the superior collicular (SC) neurons, offer an ideal experimental system to reveal molecular logic underlying synaptic choices and formation. This is due to the retina's unidirectional and laminar-restricted projections to the SC and the large databases of presynaptic RGC subtypes and postsynaptic SC neuronal types. Here, we focused on determining the role of Type II Cdhs in wiring the retinotectal synapses. We surveyed Cdhs expression patterns at neuronal resolution and revealed that Cdh13 is enriched in the wide-field neurons in the superficial SC (sSC). In either the Cdh13 null mutant or selective adult deletion within the wide-field neurons, there is a significant reduction of spine densities in the distal dendrites of these neurons in both sexes. Additionally, Cdh13 removal from presynaptic RGCs reduced dendritic spines in the postsynaptic wide-field neurons. Cdh13-expressing RGCs use differential mechanisms than αRGCs and On-Off Direction-Selective Ganglion Cells (ooDSGCs) to form specific retinotectal synapses. The results revealed a selective transneuronal interaction mediated by Cdh13 to maintain proper retinotectal synapses in vivo.

Engineering memory with an extrinsically disordered kinase

Synaptic plasticity plays a crucial role in memory formation by regulating the communication between neurons. Although actin polymerization has been linked to synaptic plasticity and dendritic spine stability, the causal link between actin polymerization and memory encoding has not been identified yet. It is not clear whether actin polymerization and structural changes in dendritic spines are a driver or a consequence of learning and memory. Using an extrinsically disordered form of the protein kinase LIMK1, which rapidly and precisely acts on ADF/cofilin, a direct modifier of actin, we induced long-term enlargement of dendritic spines and enhancement of synaptic transmission in the hippocampus on command. The activation of extrinsically disordered LIMK1 in vivo improved memory encoding and slowed cognitive decline in aged mice exhibiting reduced cofilin phosphorylation. The engineered memory by an extrinsically disordered LIMK1 supports a direct causal link between actin-mediated synaptic transmission and memory.

Antidepressant-like effects of the Punica granatum and citalopram combination are associated with structural changes in dendritic spines of granule cells in the dentate gyrus of rats

Introduction: Natural products such as phytoestrogens-enriched foods or supplements have been considered as an alternative therapy to reduce depressive symptoms associated with menopause. It is known that the aqueous extract of Punica granatum (AE-PG) exerts antidepressant-like effects by activating β-estrogen receptors and facilitates the antidepressant response of the clinical drug citalopram (CIT). However, the effects on neuroplasticity are unknown. Objectvie investigated the antidepressant-like response of combining AE-PG and CIT at sub-optimal doses, analyzing their effects on the formation and maturation of dendrite spines in granule cells as well as on the dendrite complexity. Methods: Ovariectomized Wistar rats (3-month-old) were randomly assigned to one of the following groups: A) control (saline solution as vehicle of CIT and AE-PG, B) AE-PG at a sub-threshold dose (vehicle of CIT plus AE-PG at 0.125 mg/kg), C) CIT at a sub-threshold dose (0.77 mg/kg plus vehicle of AE-PG), and D) a combination of CIT plus AE-PG (0.125 mg/kg and 0.77 mg/kg, respectively). All rats were treated intraperitoneally for 14 days. Antidepressant-like effects were evaluated using the force swimming test test (FST). The complexity of dendrites and the number and morphology of dendrite spines of neurons were assessed in the dentate gyrus after Golgi-Cox impregnation. The expressions of the mature brain-derived neurotrophic factor (mBDNF) in plasma and of mBDNF and synaptophysin in the hippocampus, as markers of synaptogenesis, were also determined. Results: Administration of CIT combined with AE-PG, but not alone, induced a significant antidepressant-like effect in the FST with an increase in the dendritic complexity and the number of dendritic spines in the dentate gyrus (DG) of the hippocampus, revealed by the thin and stubby categories of neurons at the granular cell layer. At the same time, an increase of mBDNF and synaptophysin expression was observed in the hippocampus of rats that received the combination of AE-PG and CIT.

Spatiotemporal modelling reveals geometric dependence of AMPAR dynamics on dendritic spine morphology

The modification of neural circuits depends on the strengthening and weakening of synaptic connections. Synaptic strength is often correlated to the density of the ionotropic, glutamatergic receptors, AMPARs, (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors) at the postsynaptic density (PSD). While AMPAR density is known to change based on complex biological signalling cascades, the effect of geometric factors such as dendritic spine shape, size and curvature remain poorly understood. In this work, we developed a deterministic, spatiotemporal model to study the dynamics of AMPARs during long-term potentiation (LTP). This model includes a minimal set of biochemical events that represent the upstream signalling events, trafficking of AMPARs to and from the PSD, lateral diffusion in the plane of the spine membrane, and the presence of an extrasynaptic AMPAR pool. Using idealized and realistic spine geometries, we show that the dynamics and increase of bound AMPARs at the PSD depends on a combination of endo- and exocytosis, membrane diffusion, the availability of free AMPARs and intracellular signalling interactions. We also found non-monotonic relationships between spine volume and the change in AMPARs at the PSD, suggesting that spines restrict changes in AMPARs to optimize resources and prevent runaway potentiation. KEY POINTS: Synaptic plasticity involves dynamic biochemical and physical remodelling of small protrusions called dendritic spines along the dendrites of neurons. Proper synaptic functionality within these spines requires changes in receptor number at the synapse, which has implications for downstream neural functions, such as learning and memory formation. In addition to being signalling subcompartments, spines also have unique morphological features that can play a role in regulating receptor dynamics on the synaptic surface. We have developed a spatiotemporal model that couples biochemical signalling and receptor trafficking modalities in idealized and realistic spine geometries to investigate the role of biochemical and biophysical factors in synaptic plasticity. Using this model, we highlight the importance of spine size and shape in regulating bound AMPA receptor dynamics that govern synaptic plasticity, and predict how spine shape might act to reset synaptic plasticity as a built-in resource optimization and regulation tool.

SpineTool is an open-source software for analysis of morphology of dendritic spines

Dendritic spines form most excitatory synaptic inputs in neurons and these spines are altered in many neurodevelopmental and neurodegenerative disorders. Reliable methods to assess and quantify dendritic spines morphology are needed, but most existing methods are subjective and labor intensive. To solve this problem, we developed an open-source software that allows segmentation of dendritic spines from 3D images, extraction of their key morphological features, and their classification and clustering. Instead of commonly used spine descriptors based on numerical metrics we used chord length distribution histogram (CLDH) approach. CLDH method depends on distribution of lengths of chords randomly generated within dendritic spines volume. To achieve less biased analysis, we developed a classification procedure that uses machine-learning algorithm based on experts' consensus and machine-guided clustering tool. These approaches to unbiased and automated measurements, classification and clustering of synaptic spines that we developed should provide a useful resource for a variety of neuroscience and neurodegenerative research applications.

Mechanical regulation of synapse formation and plasticity

Dendritic spines are small protrusions arising from dendrites and constitute the major compartment of excitatory post-synapses. They change in number, shape, and size throughout life; these changes are thought to be associated with formation and reorganization of neuronal networks underlying learning and memory. As spines in the brain are surrounded by the microenvironment including neighboring cells and the extracellular matrix, their protrusion requires generation of force to push against these structures. In turn, neighboring cells receive force from protruding spines. Recent studies have identified BAR-domain proteins as being involved in membrane deformation to initiate spine formation. In addition, forces for dendritic filopodium extension and activity-induced spine expansion are generated through cooperation between actin polymerization and clutch coupling. On the other hand, force from expanding spines affects neurotransmitter release from presynaptic terminals. Here, we review recent advances in our understanding of the physical aspects of synapse formation and plasticity, mainly focusing on spine dynamics.

Integrated Proteomics to Understand the Role of Neuritin (NRN1) as a Mediator of Cognitive Resilience to Alzheimer's Disease

The molecular mechanisms and pathways enabling certain individuals to remain cognitively normal despite high levels of Alzheimer's disease (AD) pathology remain incompletely understood. These cognitively normal people with AD pathology are described as preclinical or asymptomatic AD (AsymAD) and appear to exhibit cognitive resilience to the clinical manifestations of AD dementia. Here we present a comprehensive network-based approach from cases clinically and pathologically defined as asymptomatic AD to map resilience-associated pathways and extend mechanistic validation. Multiplex tandem mass tag MS (TMT-MS) proteomic data (n = 7787 proteins) was generated on brain tissue from Brodmann area 6 and Brodmann area 37 (n = 109 cases, n = 218 total samples) and evaluated by consensus weighted gene correlation network analysis. Notably, neuritin (NRN1), a neurotrophic factor previously linked to cognitive resilience, was identified as a hub protein in a module associated with synaptic biology. To validate the function of NRN1 with regard to the neurobiology of AD, we conducted microscopy and physiology experiments in a cellular model of AD. NRN1 provided dendritic spine resilience against amyloid-β (Aβ) and blocked Aβ-induced neuronal hyperexcitability in cultured neurons. To better understand the molecular mechanisms of resilience to Aβ provided by NRN1, we assessed how exogenous NRN1 alters the proteome by TMT-MS (n = 8238 proteins) of cultured neurons and integrated the results with the AD brain network. This revealed overlapping synapse-related biology that linked NRN1-induced changes in cultured neurons with human pathways associated with cognitive resilience. Collectively, this highlights the utility of integrating the proteome from the human brain and model systems to advance our understanding of resilience-promoting mechanisms and prioritize therapeutic targets that mediate resilience to AD.

The effects of regular swimming exercise and melatonin on the neurons localized in the striatum of hemiparkinsonian rats

Parkinson's disease is a progressive neurodegenerative movement disorder. We aimed to investigate the effects of regular swimming exercise and melatonin applied in the 6-Hydroxydopamine-induced Parkinson's disease rats by analysing dendritic spine of striatal neurons. Twenty-four male Wistar albino rats were used. 6-Hydroxydopamine unilaterally injected four (control, exercise, melatonin and exercise + melatonin) groups were included in the study. Tyrosine hydroxylase expression was detected by immunohistochemistry. Neurons and structures were identified from three-dimensional images by Neurolucida software. There was not any apparent difference for tyrosine hydroxylase positive neurons in the substantia nigra pars compacta and fibres in the striatum between the lesion sides of hemiparkinsonian groups. The treatment groups blocked the apomorphine-induced increase in rotations compared to the control group. In stepping test, the treatment groups prevented the loss of stepping in the contralateral side of hemiparkinsonian groups. The melatonin mostly had a positive effect on motor activity tests. In morphological analyses, the 6-Hydroxydopamine-induced lesion led to the reduction of the total dendritic length and number of branches. In the treatment groups, the reduction of the dendritic parameters was not observed. 6-Hydroxydopamine lesion led to a decrease in the total spine density, spine densities of thin and mushroom types. The exercise and melatonin treatments prevented the loss of spine density. The exercise treatment prevented the loss of spine density of mushroom type spines. The melatonin treatment blocked the loss of spine density of stubby type. In conclusion, these results provide evidence for effective additional protective therapeutic strategies for Parkinson's disease. In conclusion, results from the current study provide evidence for swimming exercise and melatonin as a promising candidate for effective additional protective strategies for PD.

Butyrate ameliorates quinolinic acid-induced cognitive decline in obesity models

Obesity is a risk factor for neurodegenerative disease associated with cognitive dysfunction, including Alzheimer's disease. Low-grade inflammation is common in obesity, but the mechanism between inflammation and cognitive impairment in obesity is unclear. Accumulative evidence shows that quinolinic acid (QA), a neuroinflammatory neurotoxin, is involved in the pathogenesis of neurodegenerative processes. We investigated the role of QA in obesity-induced cognitive impairment and the beneficial effect of butyrate in counteracting impairments of cognition, neural morphology, and signaling. We show that in human obesity, there was a negative relationship between serum QA levels and cognitive function and decreased cortical gray matter. Diet-induced obese mice had increased QA levels in the cortex associated with cognitive impairment. At single-cell resolution, we confirmed that QA impaired neurons, altered the dendritic spine's intracellular signal, and reduced brain-derived neurotrophic factor (BDNF) levels. Using Caenorhabditis elegans models, QA induced dopaminergic and glutamatergic neuron lesions. Importantly, the gut microbiota metabolite butyrate was able to counteract those alterations, including cognitive impairment, neuronal spine loss, and BDNF reduction in both in vivo and in vitro studies. Finally, we show that butyrate prevented QA-induced BDNF reductions by epigenetic enhancement of H3K18ac at BDNF promoters. These findings suggest that increased QA is associated with cognitive decline in obesity and that butyrate alleviates neurodegeneration.

Layer-specific impairment in the developing lateral entorhinal cortex of immune-challenged Disc1+/- mice

Cognitive deficits in mental disorders result from dysfunctional activity in large-scale brain networks centred around the hippocampus and the prefrontal cortex. Dysfunctional activity emerges early during development and precedes the cognitive disabilities. The prefrontal-hippocampal network is driven by a prominent input from the lateral entorhinal cortex. We have previously shown that during early development, the entorhinal drive of the prefrontal-hippocampal network is impaired in a mouse model of mental disorders, yet the cellular substrate of this impairment is still poorly understood. Here, we address this question by a detailed characterization of projection neurons across the layers of the lateral entorhinal cortex in immune-challenged Disc1^+/- mice at the beginning of the second postnatal week. We found that the activity and morphology of neurons in layers 2b and 3, which project to the hippocampus, are impaired. Neurons in layer 2b show increased spike-frequency adaptation, whereas neurons in layer 3 have reduced dendritic complexity but increased spike density. These findings identify the developmental alterations of entorhinal-hippocampal communication that underlie network dysfunction in immune-challenged Disc1^+/- mice. KEY POINTS: Neonatal immune-challenged Disc1^+/- mice show layer-specific changes in the lateral entorhinal cortex. Entorhinal layer 2b pyramidal neurons have increased spike-frequency adaptation. Reduced dendritic complexity but increased spine density characterize layer 3 pyramidal neurons.

APOE4 homozygote females are resistant to the beneficial effects of 17β-estradiol on memory and CA1 dendritic spine density in the EFAD mouse model of Alzheimer's disease

Female APOE4 carriers are at greatest risk of Alzheimer's disease (AD). The potent estrogen 17β-estradiol (E2) may mediate AD risk, as the onset of memory decline coincides with the menopausal transition. Whether APOE genotype mediates E2's effects on memory and neuronal morphology is poorly understood. We used the APOE+/+/5xFAD+/- (EFAD) mouse model to examine how APOE3 homozygote (E3FAD), APOE3/4 heterozygote (E3/4FAD), and APOE4 homozygote (E4FAD) genotypes modulate effects of E2 on object and spatial memory consolidation, dendritic spine density, and dorsal hippocampal estrogen receptor expression in 6-month-old ovariectomized EFAD mice. Dorsal hippocampal E2 infusion enhanced memory consolidation and increased CA1 apical spine density in E3FAD and E3/4FAD, but not E4FAD, mice. CA1 basal mushroom spines were also increased by E2 in E3FADs. E4FAD mice exhibited reduced CA1 and mPFC basal spine density, and increased dorsal hippocampal ERα protein, independent of E2. Overall, E2 benefitted hippocampal memory and structural plasticity in females bearing one or no APOE4 allele, whereas two APOE4 alleles impeded the memory-enhancing and spinogenic effects of E2.

Comparison of Golgi-Cox and Intracellular Loading of Lucifer Yellow for Dendritic Spine Density and Morphology Analysis in the Mouse Brain

Dendritic spines are small protrusions on dendrites that serve as the postsynaptic site of the majority of excitatory synapses. These structures are important for normal synaptic transmission, and alterations in their density and morphology have been documented in various disease states. Over 130 years ago, Ramón y Cajal used Golgi-stained tissue sections to study dendritic morphology. Despite the array of technological advances, including iontophoretic microinjection of Lucifer yellow (LY) fluorescent dye, Golgi staining continues to be one of the most popular approaches to visualize dendritic spines. Here, we compared dendritic spine density and morphology among pyramidal neurons in layers 2/3 of the mouse medial prefrontal cortex (mPFC) and pyramidal neurons in hippocampal CA1 using three-dimensional digital reconstructions of (1) brightfield microscopy z-stacks of Golgi-impregnated dendrites and (2) confocal microscopy z-stacks of LY-filled dendrites. Analysis of spine density revealed that the LY microinjection approach enabled detection of approximately three times as many spines as the Golgi staining approach in both brain regions. Spine volume measurements were larger using Golgi staining compared to LY microinjection in both mPFC and CA1. Spine length was mostly comparable between techniques in both regions. In the mPFC, head diameter was similar for Golgi staining and LY microinjection. However, in CA1, head diameter was approximately 50% smaller on LY-filled dendrites compared to Golgi staining. These results indicate that Golgi staining and LY microinjection yield different spine density and morphology measurements, with Golgi staining failing to detect dendritic spines and overestimating spine size.

Changes in Dendritic Spine Morphology and Density of Granule Cells in the Olfactory Bulb of Anguilla anguilla (L., 1758): A Possible Way to Understand Orientation and Migratory Behavior

Simple Summary

The olfactory bulb can process odour cues through granular cells (GCs) and dendritic spines, changing their synaptic plasticity properties and their morphology. The GCs’ dendritic spines density and morphology were analysed in Anguilla anguilla, considering the olfaction as a driver involved in fish orientation and migration. For the head and neck morphology, spines were classified as mushroom, long thin, stubby, and filopodia. Spines’ density decreased from juvenile migrants to no-migrant stages and increased in the adult migrants. Spines’ density was comparable between glass and silver eels as an adaptation to migration, while at non-migrating phases, spines’ density decreased. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb. Considering the role of olfaction in the orientation and migration of A. anguilla, the modification of environmental stimuli (ocean alterations and climate change) could represent contributing factors that threaten this critically endangered species.

Abstract

Olfaction could represent a pivotal process involved in fish orientation and migration. The olfactory bulb can manage olfactive signals at the granular cell (GC) and dendritic spine levels for their synaptic plasticity properties and changing their morphology and structural stability after environmental odour cues. The GCs’ dendritic spine density and morphology were analysed across the life stages of the catadromous Anguilla anguilla. According to the head and neck morphology, spines were classified as mushroom (M), long thin (LT), stubby (S), and filopodia (F). Total spines’ density decreased from juvenile migrants to no-migrant stages, to increase again in the adult migrant stage. Mean spines’ density was comparable between glass and silver eels as an adaptation to migration. At non-migrating phases, spines’ density decreased for M and LT, while M, LT, and S density increased in silver eels. A great dendritic spine development was found in the two migratory phases, regressing in trophic phases, but that could be recreated in adults, tracing the migratory memory of the routes travelled in juvenile phases. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb as an index of synaptic plasticity involved in the modulation of olfactory stimuli. If olfaction is involved in the orientation and migration of A. anguilla and if eels possess a memory, these processes could be influenced by the modification of environmental stimuli (ocean alterations and rapid climate change) contributing to threatening this critically endangered species.

CaMKIIβ knockdown decreases store-operated calcium entry in hippocampal dendritic spines

Calcium/calmodulin-dependent protein kinase II (CaMKII) and neuronal store-operated calcium entry (nSOCE) have been implicated in the development of Alzheimer's disease (AD). nSOCE is involved in regulation of dendritic spine shape, particularly in stability of mushroom spines that play role in formation of strong synapses. CaMKII is involved in regulation of induction of long-term potentiation, that is needed for shaping of memory. In the present study, we demonstrated that inhibition of kinase activity of CaMKII by KN-62 decreases nSOCE amplitude in soma of primary hippocampal neurons. We have shown that knockdown of CaMKIIβ leads to the downregulation of nSOCE in dendritic spines. In agreement with previously published data, we have also observed that CaMKIIβ knockdown causes mushroom spine loss in primary hippocampal culture. The effect of CaMKIIβ knockdown on the nSOCE may be associated with a decrease of dendritic spine head size.

The Homer1 family of proteins at the crossroad of dopamine-glutamate signaling: An emerging molecular "Lego" in the pathophysiology of psychiatric disorders. A systematic review and translational insight

Once considered only scaffolding proteins at glutamatergic postsynaptic density (PSD), Homer1 proteins are increasingly emerging as multimodal adaptors that integrate different signal transduction pathways within PSD, involved in motor and cognitive functions, with putative implications in psychiatric disorders. Regulation of type I metabotropic glutamate receptor trafficking, modulation of calcium signaling, tuning of long-term potentiation, organization of dendritic spines' growth, as well as meta- and homeostatic plasticity control are only a few of the multiple endocellular and synaptic functions that have been linked to Homer1. Findings from preclinical studies, as well as genetic studies conducted in humans, suggest that both constitutive (Homer1b/c) and inducible (Homer1a) isoforms of Homer1 play a role in the neurobiology of several psychiatric disorders, including psychosis, mood disorders, neurodevelopmental disorders, and addiction. On this background, Homer1 has been proposed as a putative novel target in psychopharmacological treatments. The aim of this review is to summarize and systematize the growing body of evidence on Homer proteins, highlighting the role of Homer1 in the pathophysiology and therapy of mental diseases.

Mild Traumatic Brain Injury-Induced Disruption of the Blood-Brain Barrier Triggers an Atypical Neuronal Response

Mild TBI (mTBI), which affects 75% of TBI survivors or more than 50 million people worldwide each year, can lead to consequences including sleep disturbances, cognitive impairment, mood swings, and post-traumatic epilepsy in a subset of patients. To interrupt the progression of these comorbidities, identifying early pathological events is key. Recent studies have shown that microbleeds, caused by mechanical impact, persist for months after mTBI and are correlated to worse mTBI outcomes. However, the impact of mTBI-induced blood-brain barrier damage on neurons is yet to be revealed. We used a well-characterized mouse model of mTBI that presents with frequent and widespread but size-restricted damage to the blood-brain barrier to assess how neurons respond to exposure of blood-borne factors in this pathological context. We used immunohistochemistry and histology to assess the expression of neuronal proteins in excitatory and inhibitory neurons after mTBI. We observed that the expression of NeuN, Parvalbumin, and CamKII was lost within minutes in areas with blood-brain barrier disruption. Yet, the neurons remained alive and could be detected using a fluorescent Nissl staining even 6 months later. A similar phenotype was observed after exposure of neurons to blood-borne factors due to endothelial cell ablation in the absence of a mechanical impact, suggesting that entrance of blood-borne factors into the brain is sufficient to induce the neuronal atypical response. Changes in postsynaptic spines were observed indicative of functional changes. Thus, this study demonstrates That exposure of neurons to blood-borne factors causes a rapid and sustained loss of neuronal proteins and changes in spine morphology in the absence of neurodegeneration, a finding that is likely relevant to many neuropathologies.

Cytoskeleton Protein EB3 Contributes to Dendritic Spines Enlargement and Enhances Their Resilience to Toxic Effects of Beta-Amyloid

EB3 protein is expressed abundantly in the nervous system and transiently enters the dendritic spines at the tip of the growing microtubule, which leads to spine enlargement. Nevertheless, the role of dynamic microtubules, and particularly EB3 protein, in synapse function is still elusive. By manipulating the EB3 expression level, we have shown that this protein is required for a normal dendritogenesis. Nonetheless, EB3 overexpression also reduces hippocampal neurons dendritic branching and total dendritic length. This effect likely occurs due to the speeding neuronal development cycle from dendrite outgrowth to the step when dendritic spines are forming. Implementing direct morphometric characterization of dendritic spines, we showed that EB3 overexpression leads to a dramatic increase in the dendritic spine head area. EB3 knockout oppositely reduces spine head area and increases spine neck length and spine neck/spine length ratio. The same effect is observed in conditions of amyloid-beta toxicity, modeling Alzheimer`s disease. Neck elongation is supposed to be a common detrimental effect on the spine's shape, which makes them biochemically and electrically less connected to the dendrite. EB3 also potentiates the formation of presynaptic protein Synapsin clusters and CaMKII-alpha preferential localization in spines rather than in dendrites of hippocampal neurons, while its downregulation has an opposite effect and reduces the size of presynaptic protein clusters Synapsin and PSD95. EB3's role in spine development and maturation determines its neuroprotective effect. EB3 overexpression makes dendritic spines resilient to amyloid-beta toxicity, restores altered PSD95 clustering, and reduces CaMKII-alpha localization in spines observed in this pathological state.

Disease predictability review using common biomarkers appearing in diabetic nephropathy and neurodegeneration of experimental animals

It is recently known that the kidney and brain have a very rich distribution of blood vessels, and the histological structures of micro-vessels are very similar. Therefore, a number of studies have reported that renal diseases like chronic kidney disease (CKD) caused by various causes have a very close relationship with the occurrence of neurodegenerative diseases. On the other hand, since diabetic nephropathy, which is caused by chronic inflammation, such as diabetes, often shows very different prognoses even in patients at the same clinical stage, the judgment of their disease prognosis will have a critical meaning in clinical practice. Recently, many studies of cerebro-renal interaction have been reported using experimental animals. The discovery of common biomarkers found in both organs can predict the prognosis of renal disease and the possibility of neurodegenerative disease progression. More associations can be found with novel common biomarkers found in the brain and kidneys that seem entirely unrelated. In that case, it will ultimately be a research field that can expand predictive models of patients' complex diseases through these biomarkers in clinical practice. It is presented biomarkers such as α-klotho, Nephrin, and Synaptopodin. These markers are observed in both the brain and kidney, and it has been reported that both organs show a very significant change in function according to their expression. Even though the brain and kidneys perform very independent functions, it is thought that it has a crucial diagnostic significance that the genes commonly expressed in both organs are functionally effective. With the discovery of novel biomarkers that share cerebro-renal interactions at the early stage of diabetic nephropathy, physicians can predict post-clinical symptoms and prevent severe neurodegenerative and cerebrovascular diseases. Therefore, further study for the diseases of these two organs in laboratory animals means that the field of research on this relationship can be expanded in the future. In the future, more attention and research will be needed on the possibility of prediction for the prevention of neurological diseases caused by CKD in disease animal models.

Engineering a switchable single‐chain TEV protease to control protein maturation in living neurons

Engineered proteases are promising tools to address physiological and pathophysiological questions as well as to develop new therapeutic approaches. Here we introduce a new genetically encoded engineered single‐chain tobacco etch virus protease, allowing to control proprotein cleavage in different compartments of living mammalian cells. We demonstrated a set of controllable proteolytic effects, including cytosolic protein cleavage, inducible gene expression, and maturation of brain‐derived neurotrophic factor (BDNF) in the secretory pathway thus showing the versatility of this technique. Of note, the secretory pathway exhibits different characteristics from the cytosol and it is difficult to target because inaccessible to some small molecules. We were able to induce ligand‐mediated BDNF maturation and monitor its effects on dendritic spines in hippocampal pyramidal cells and in the mouse brain. This strategy paves the way to dissect proteolytic cleavage product signaling in various processes as well as for future therapeutic applications.

Genetic Mechanisms Underlying the Evolution of Connectivity in the Human Cortex

One of the most salient features defining modern humans is our remarkable cognitive capacity, which is unrivaled by any other species. Although we still lack a complete understanding of how the human brain gives rise to these unique abilities, the past several decades have witnessed significant progress in uncovering some of the genetic, cellular, and molecular mechanisms shaping the development and function of the human brain. These features include an expansion of brain size and in particular cortical expansion, distinct physiological properties of human neurons, and modified synaptic development. Together they specify the human brain as a large primate brain with a unique underlying neuronal circuit architecture. Here, we review some of the known human-specific features of neuronal connectivity, and we outline how novel insights into the human genome led to the identification of human-specific genetic modifiers that played a role in the evolution of human brain development and function. Novel experimental paradigms are starting to provide a framework for understanding how the emergence of these human-specific genomic innovations shaped the structure and function of neuronal circuits in the human brain.

Dendritic and behavioral changes in rats neonatally treated with homocysteine; A proposal as an animal model to study the attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHAD) is a neurobehavioral disorder that affects children and adolescents with a high prevalence. Despite its prevalence and an unclear etiology, previous reports suggest that it is closely related to homocysteine metabolism. Male Sprague Dawley rats were administered with homocysteine from postnatal day (PD) 2 to PD 16. Locomotor activity was evaluated at 35 PD (prepuberal age) and 60 PD (adult age) before and after amphetamine administration. In rats evaluated at both ages, homocysteine induced hyperactivity, and the amphetamine administration reduced hyperactivity significantly at 35 PD, but not at 60 PD. In the social interaction test, homocysteine reduced the number of contacts and increased the latency to the first contact only in rats at 35 PD. Homocysteine also had an effect on short term memory at 35D and 60 PD and long-term memory at 60 PD. Morphological changes were found mainly in the shape of dendritic spines in the prefrontal cortex (PFC-3), dorsal hippocampus (CA1), dentate gyrus (DG) and nucleus accumbens (NAcc), in rats administered neonatally with homocysteine at both ages studied. In prepuberal and adult rats, there was an increase in dendritic length in DG and NAcc, respectively. The dendritic spine morphology also was altered at both ages, mainly decreasing the number of mushroom spines in NAcc and CA1 at 30 PD and in all the areas studied at 60 PD rats. Those areas are associated with the processes of attention, learning and memory that were studied, and those alterations are possibly related to changes observed in the behavioral tests. These behavioral and morphological changes in rats at 35 PD administered with homocysteine could be similar to changes found in children diagnosed with ADHD. Moreover, half to two thirds of children diagnosed with ADHD reach adulthood with this disorder. In this study we found similarities with ADHD, finding alterations in both rats at 35 PD and 60 PD. So, this may be proposed as an animal model to study this disorder present in children, adolescents and adults.

Conditional deletion of ROCK2 induces anxiety-like behaviors and alters dendritic spine density and morphology on CA1 pyramidal neurons

Rho-associated kinase isoform 2 (ROCK2) is an attractive drug target for several neurologic disorders. A critical barrier to ROCK2-based research and therapeutics is the lack of a mouse model that enables investigation of ROCK2 with spatial and temporal control of gene expression. To overcome this, we generated ROCK2fl/fl mice. Mice expressing Cre recombinase in forebrain excitatory neurons (CaMKII-Cre) were crossed with ROCK2fl/fl mice (Cre/ROCK2fl/fl), and the contribution of ROCK2 in behavior as well as dendritic spine morphology in the hippocampus, medial prefrontal cortex (mPFC), and basolateral amygdala (BLA) was examined. Cre/ROCK2fl/fl mice spent reduced time in the open arms of the elevated plus maze and increased time in the dark of the light-dark box test compared to littermate controls. These results indicated that Cre/ROCK2fl/fl mice exhibited anxiety-like behaviors. To examine dendritic spine morphology, individual pyramidal neurons in CA1 hippocampus, mPFC, and the BLA were targeted for iontophoretic microinjection of fluorescent dye, followed by high-resolution confocal microscopy and neuronal 3D reconstructions for morphometry analysis. In dorsal CA1, Cre/ROCK2fl/fl mice displayed significantly increased thin spine density on basal dendrites and reduced mean spine head volume across all spine types on apical dendrites. In ventral CA1, Cre/ROCK2fl/fl mice exhibited significantly increased spine length on apical dendrites. Spine density and morphology were comparable in the mPFC and BLA between both genotypes. These findings suggest that neuronal ROCK2 mediates spine density and morphology in a compartmentalized manner among CA1 pyramidal cells, and that in the absence of ROCK2 these mechanisms may contribute to anxiety-like behaviors.

Dynamics of Dendritic Spines in Dorsal Striatum after Retrieval of Moderate and Strong Inhibitory Avoidance Learning

In marked contrast to the ample literature showing that the dorsal striatum is engaged in memory consolidation, little is known about its involvement in memory retrieval. Recent findings demonstrated significant increments in dendritic spine density and mushroom spine counts in dorsal striatum after memory consolidation of moderate inhibitory avoidance (IA) training; further increments were found after strong training. Here, we provide evidence that in this region spine counts were also increased as a consequence of retrieval of moderate IA training, and even higher mushroom spine counts after retrieval of strong training; by contrast, there were fewer thin spines after retrieval. Similar changes in mushroom and thin spine populations were found in the ventral striatum (nucleus accumbens), but they were related to the aversive stimulation and not to memory retrieval. These results suggest that memory retrieval is a dynamic process which produces neuronal structural plasticity that might be necessary for maintaining or strengthening assemblies that encode stored information.

SIRT1 Regulates Tau Expression and Tau Synaptic Pathology

BACKGROUND

Amyloid plaques and neurofibrillary tangles are two pathological hallmarks of Alzheimer's disease (AD). However, synaptic deficits occur much earlier and correlate stronger with cognitive decline than amyloid plaques and neurofibrillary tangles. Mislocalization of tau is an early hallmark of neurodegeneration and precedes aggregations. Sirtuin type 1 (SIRT1) is a deacetylase which acts on proteins including transcriptional factors and associates closely with AD.

OBJECTIVE

The present study investigated the association between SIRT1 and tau expression/tau localization in cells and in mice brains.

METHODS

Western blot was performed to detected tau, SIRT1, C/EBPα, and GAPDH protein levels. Immunological fluorescence assay was used to assess tau localization in primary cortical neuronal cells. Golgi staining was performed to evaluated dendritic spine morphology in mice brains.

RESULTS

In the present study, we found that SIRT1 negatively regulates expression of tau at the transcriptional level through transcriptional factor C/EBPα. Inhibition of the activity of SIRT1 limits the distribution of tau to the neurites. In the meantime, the alteration of dendritic spine morphology is also observed in the brains of SIRT1+/- mice.

CONCLUSION

SIRT1 may be a potential drug target for early intervention in AD.

Dendritic Spines: Mediators of Cognitive Resilience in Aging and Alzheimer's Disease

Cognitive resilience is often defined as the ability to remain cognitively normal in the face of insults to the brain. These insults can include disease pathology, such as plaques and tangles associated with Alzheimer's disease, stroke, traumatic brain injury, or other lesions. Factors such as physical or mental activity and genetics may contribute to cognitive resilience, but the neurobiological underpinnings remain ill-defined. Emerging evidence suggests that dendritic spine structural plasticity is one plausible mechanism. In this review, we highlight the basic structure and function of dendritic spines and discuss how spine density and morphology change in aging and Alzheimer's disease. We note evidence that spine plasticity mediates resilience to stress, and we tackle dendritic spines in the context of cognitive resilience to Alzheimer's disease. Finally, we examine how lifestyle and genetic factors may influence dendritic spine plasticity to promote cognitive resilience before discussing evidence for actin regulatory kinases as therapeutic targets for Alzheimer's disease.

Amphetamine sensitization alters hippocampal neuronal morphology and memory and learning behaviors

It is known that continuous abuse of amphetamine (AMPH) results in alterations in neuronal structure and cognitive behaviors related to the reward system. However, the impact of AMPH abuse on the hippocampus remains unknown. The aim of this study was to determine the damage caused by AMPH in the hippocampus in an addiction model. We reproduced the AMPH sensitization model proposed by Robinson et al. in 1997 and performed the novel object recognition test (NORt) to evaluate learning and memory behaviors. After the NORt, we performed Golgi-Cox staining, a stereological cell count, immunohistochemistry to determine the presence of GFAP, CASP3, and MT-III, and evaluated oxidative stress in the hippocampus. We found that AMPH treatment generates impairment in short- and long-term memories and a decrease in neuronal density in the CA1 region of the hippocampus. The morphological test showed an increase in the total dendritic length, but a decrease in the number of mature spines in the CA1 region. GFAP labeling increased in the CA1 region and MT-III increased in the CA1 and CA3 regions. Finally, we found a decrease in Zn concentration in the hippocampus after AMPH treatment. An increase in the dopaminergic tone caused by AMPH sensitization generates oxidative stress, neuronal death, and morphological changes in the hippocampus that affect cognitive behaviors like short- and long-term memories.

Molecular mechanism of hippocampal long-term potentiation - Towards multiscale understanding of learning and memory

Long-term potentiation (LTP) of synaptic transmission is considered to be a cellular counterpart of learning and memory. Activation of postsynaptic NMDA type glutamate receptor (NMDA-R) induces trafficking of AMPA type glutamate receptors (AMPA-R) and other proteins to the synapse in sequential fashion. At the same time, the dendritic spine expands for long-term and modulation of actin underlies this (structural LTP or sLTP). How these changes persist despite constant diffusion and turnover of the component proteins have been the central focus of the current LTP research. Signaling triggered by Ca²⁺-influx via NMDA-R triggers kinase including Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). CaMKII can sustain longer-term biochemical signaling by forming a reciprocally-activating kinase-effector complex with its substrate proteins including Tiam1, thereby regulating persistence of the downstream signaling. Furthermore, activated CaMKII can condense at the synapse through the mechanism of liquid-liquid phase separation (LLPS). This increases the binding capacity at the synapse, thereby contributing to the maintenance of enlarged protein complexes. It may also serve as the synapse tag, which captures newly synthesized proteins.

Stress induces microglia-associated synaptic circuit alterations in the dorsomedial prefrontal cortex

The mammalian dorsomedial prefrontal cortex (dmPFC) receives diverse inputs and plays important roles in adaptive behavior and cognitive flexibility. Stress, a major risk factor for many psychiatric disorders, compromises the structure and function of multiple brain regions and circuits. Here we show that 7-day restraint stress impairs reversal learning in the 4-choice odor discrimination test, a decision-making task requiring an intact dmPFC. In vivo two-photon imaging further reveals that stress increases dmPFC dendritic spine elimination, particularly those of the mushroom morphology, without affecting spine formation. In addition, stress alters dmPFC microglial branching complexity and elevates their terminal process dynamics. In stressed mice, dmPFC microglia contact dendrites more frequently, and dendritic spines with microglial contact are prone to elimination. In summary, our work suggests that stress-induced changes in glial-synapse interaction contributes to synaptic loss in dmPFC, resulting in neuronal circuit deficits and impaired cognitive flexibility.

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Restraint stress impairs cognitive flexibility in adolescent mice.

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Stress leads to synapse loss on pyramidal neurons in the dorsomedial prefrontal cortex.

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Stress decreases microglial complexity but increases their terminal dynamics and contacts with dendritic spines.

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Dendritic spines contacted by microglial processes are more prone to elimination.

Muscone Ameliorates Synaptic Dysfunction and Cognitive Deficits in APP/PS1 Mice

BACKGROUND

Dysfunction of synaptic plasticity leads to memory impairment in Alzheimer's disease (AD). Muscone (Mus) has shown neuroprotective effects in cerebral ischemic models. However, little is known of Mus effects on AD.

OBJECTIVE

To investigate the effects of Mus on memory functions and synaptic plasticity in 6-month-old APP/PS1 double-transgenic mice and explore the potential mechanisms.

METHODS

Mus was intraperitoneally injected into APP/PS1 or wild-type mice, and cognitive function was assessed by Novel object recognition and Morris water maze tests. The levels of amyloid-β (Aβ) were evaluated by immunofluorescence staining and ELISA. Synaptic morphology and plasticity were evaluated by Golgi staining and long-term potentiation. Cell viability was examined by Cell Counting Kit-8 assay. The protein levels of histone deacetylase 2 (HDAC2) were accessed by western blotting and Immunofluorescence staining. The protein levels of microtubule associated protein 2 and synaptophysin were analyzed by immunofluorescence staining. The ubiquitination of HDAC2 was examined by co-immunoprecipitation. The interaction of Mus with HDAC2 was predicted by molecular docking analysis.

RESULTS

Mus treatment attenuated memory dysfunction, reduced Aβ level, and enhanced synaptic plasticity in APP/PS1 mice. In addition, Mus treatment decreased the level of HDAC2 in the hippocampus of APP/PS1 mice and Aβ1-42-induced primary neurons, which might be associated with increased HDAC2 ubiquitination induced by HDAC2 and Mus interaction.

CONCLUSION

Mus protected against synaptic plasticity and memory impairment in APP/PS1 mice, and enhanced HDAC2 degradation via ubiquitination, indicating that Mus was a potential drug for AD treatment.

Shootin1a-mediated actin-adhesion coupling generates force to trigger structural plasticity of dendritic spines

Dendritic spines constitute the major compartments of excitatory post-synapses. They undergo activity-dependent enlargement, which is thought to increase the synaptic efficacy underlying learning and memory. The activity-dependent spine enlargement requires activation of signaling pathways leading to promotion of actin polymerization within the spines. However, the molecular machinery that suffices for that structural plasticity remains unclear. Here, we demonstrate that shootin1a links polymerizing actin filaments in spines with the cell-adhesion molecules N-cadherin and L1-CAM, thereby mechanically coupling the filaments to the extracellular environment. Synaptic activation enhances shootin1a-mediated actin-adhesion coupling in spines. Promotion of actin polymerization is insufficient for the plasticity; the enhanced actin-adhesion coupling is required for polymerizing actin filaments to push against the membrane for spine enlargement. By integrating cell signaling, cell adhesion, and force generation into the current model of actin-based machinery, we propose molecular machinery that is sufficient to trigger the activity-dependent spine structural plasticity.

Dendritic Spine Remodeling and Synaptic Tau Levels in PS19 Tauopathy Mice

Synapse or dendritic spine loss is the strongest correlate of cognitive decline in Alzheimer's disease (AD), and neurofibrillary tangles (NFTs), but not amyloid-β plaques, associate more closely with transition to mild cognitive impairment. Yet, how dendritic spine architecture is affected by hyperphosphorylated tau is still an ongoing question. To address this, we combined cell and biochemical analyses of the Tau P301S mouse line (PS19). Individual pyramidal neurons in the hippocampus and medial prefrontal cortex (mPFC) were targeted for iontophoretic microinjection of fluorescent dye, followed by high-resolution confocal microscopy and 3D morphometry analysis. In the hippocampus, PS19 mice and non-transgenic (NTG) littermates displayed equivalent spine density at 6 and 9 months, but both genotypes exhibited age-related thin spine loss. PS19 mice exhibited significant increases in synaptic tau protein levels and mean dendritic spine head diameter with age. This suggests that CA1 pyramidal neurons in PS19 mice may undergo spine remodeling in response to tau accumulation and age. In the mPFC, spine density was similar among PS19 mice and NTG littermates at 6 and 9 months, but age-related reductions in synaptic tau levels were observed among PS19 mice. Collectively, these studies reveal brain region-specific changes in dendritic spine density and morphology in response to age and the presence of hyperphosphorylated tau in the PS19 mouse line.

Comparative 2D and 3D Ultrastructural Analyses of Dendritic Spines from CA1 Pyramidal Neurons in the Mouse Hippocampus

Three-dimensional (3D) reconstruction from electron microscopy (EM) datasets is a widely used tool that has improved our knowledge of synapse ultrastructure and organization in the brain. Rearrangements of synapse structure following maturation and in synaptic plasticity have been broadly described and, in many cases, the defective architecture of the synapse has been associated to functional impairments. It is therefore important, when studying brain connectivity, to map these rearrangements with the highest accuracy possible, considering the affordability of the different EM approaches to provide solid and reliable data about the structure of such a small complex. The aim of this work is to compare quantitative data from two dimensional (2D) and 3D EM of mouse hippocampal CA1 (apical dendrites), to define whether the results from the two approaches are consistent. We examined asymmetric excitatory synapses focusing on post synaptic density and dendritic spine area and volume as well as spine density, and we compared the results obtained with the two methods. The consistency between the 2D and 3D results questions the need—for many applications—of using volumetric datasets (costly and time consuming in terms of both acquisition and analysis), with respect to the more accessible measurements from 2D EM projections.

Sculpting Dendritic Spines during Initiation and Maintenance of Neuropathic Pain

Accumulating evidence has established a firm role for synaptic plasticity in the pathogenesis of neuropathic pain. Recent advances have highlighted the importance of dendritic spine remodeling in driving synaptic plasticity within the CNS. Identifying the molecular players underlying neuropathic pain induced structural and functional maladaptation is therefore critical to understanding its pathophysiology. This process of dynamic reorganization happens in unique phases that have diverse pathologic underpinnings in the initiation and maintenance of neuropathic pain. Recent evidence suggests that pharmacological targeting of specific proteins during distinct phases of neuropathic pain development produces enhanced antinociception. These findings outline a potential new paradigm for targeted treatment and the development of novel therapies for neuropathic pain. We present a concise review of the role of dendritic spines in neuropathic pain and outline the potential for modulation of spine dynamics by targeting two proteins, srGAP3 and Rac1, critically involved in the regulation of the actin cytoskeleton.

Dysfunction of Mitochondrial Ca2+ Regulatory Machineries in Brain Aging and Neurodegenerative Diseases

Calcium ions (Ca²⁺) play critical roles in neuronal processes, such as signaling pathway activation, transcriptional regulation, and synaptic transmission initiation. Therefore, the regulation of Ca²⁺ homeostasis is one of the most important processes underlying the basic cellular viability and function of the neuron. Multiple components, including intracellular organelles and plasma membrane Ca²⁺-ATPase, are involved in neuronal Ca²⁺ control, and recent studies have focused on investigating the roles of mitochondria in synaptic function. Numerous mitochondrial Ca²⁺ regulatory proteins have been identified in the past decade, with studies demonstrating the tissue- or cell-type-specific function of each component. The mitochondrial calcium uniporter and its binding subunits are major inner mitochondrial membrane proteins contributing to mitochondrial Ca²⁺ uptake, whereas the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX) and mitochondrial permeability transition pore (mPTP) are well-studied proteins involved in Ca²⁺ extrusion. The level of cytosolic Ca²⁺ and the resulting characteristics of synaptic vesicle release properties are controlled via mitochondrial Ca²⁺ uptake and release at presynaptic sites, while in dendrites, mitochondrial Ca²⁺ regulation affects synaptic plasticity. During brain aging and the progress of neurodegenerative disease, mitochondrial Ca²⁺ mishandling has been observed using various techniques, including live imaging of Ca²⁺ dynamics. Furthermore, Ca²⁺ dysregulation not only disrupts synaptic transmission but also causes neuronal cell death. Therefore, understanding the detailed pathophysiological mechanisms affecting the recently discovered mitochondrial Ca²⁺ regulatory machineries will help to identify novel therapeutic targets. Here, we discuss current research into mitochondrial Ca²⁺ regulatory machineries and how mitochondrial Ca²⁺ dysregulation contributes to brain aging and neurodegenerative disease.

Loss of TREM2 Confers Resilience to Synaptic and Cognitive Impairment in Aged Mice

Triggering receptor expressed on myeloid cells 2 (TREM2), a receptor exclusively expressed by microglia in the brain, modulates microglial immune homeostasis. Human genetic studies have shown that the loss-of-function mutations in TREM2 signaling are strongly associated with an elevated risk of age-related neurodegenerative diseases including Alzheimer's disease (AD). Numerous studies have investigated the impact of TREM2 deficiency in the pathogenic process of AD. However, the role of TREM2 in shaping neuronal and cognitive function during normal aging is underexplored. In the present study, we employed behavioral, electrophysiological, and biochemical approaches to assess cognitive and synaptic function in male and female young and aged TREM2-deficient (Trem2-/-) mice compared with age-matched, sex-matched, and genetic background-matched wild-type (WT) C57BL/6J controls. Young Trem2-/- mice exhibited normal cognitive function and synaptic plasticity but had increased dendritic spine density compared with young WT. Unexpectedly, aged Trem2-/- mice showed superior cognitive performance compared with aged WT controls. Consistent with the behavioral data, aged Trem2-/- mice displayed significantly enhanced hippocampal long-term potentiation (LTP) and increased dendritic spine density and synaptic markers compared with aged WT mice. Taken together, these findings suggest that loss of TREM2 affects the neuronal structure and confers resilience to age-related synaptic and cognitive impairment during non-pathogenic aging.SIGNIFICANCE STATEMENT Microglia are innate immune cells of the brain that orchestrates neurodevelopment, synaptic function, and immune response to environmental stimuli. Microglial triggering receptor expressed on myeloid cells 2 (TREM2) signaling plays pivotal roles in regulating these functions and loss of TREM2 signaling leads to increased risk of developing age-related neurologic disorders. However, the neurologic role of TREM2 in normal aging is poorly understood. The results of the present study unveil the positive impacts of TREM2 deficiency on cognitive and synaptic function during aging and suggest that TREM2 may exert detrimental effects on neuronal function. The possibility of age-related negative impacts from TREM2 is critically important since TREM2 has emerged as a major therapeutic target for Alzheimer's dementia.

NeuroBox: Computational Mathematics in Multiscale Neuroscience

The brain is a complex organ operating on multiple scales. From molecular events that inform electrical and biochemical cellular responses, the brain interconnects processes all the way up to the massive network size of billions of brain cells. This strongly coupled, nonlinear, system has been subject to research that has turned increasingly multidisciplinary. The seminal work of Hodgkin and Huxley in the 1950s made use of experimental data to derive a coherent physical model of electrical signaling in neurons, which can be solved using mathematical and computational methods, thus bringing together neuroscience, physics, mathematics, and computer science. Over the last decades numerous projects have been dedicated to modeling and simulation of specific parts of molecular dynamics, neuronal signaling, and neural network behavior. Simulators have been developed around a specific objective and scale, in order to cope with the underlying computational complexity. Often times a dimension reduction approach allows larger scale simulations, this however has the inherent drawback of losing insight into structure-function interplay at the cellular level. This paper gives an overview of the project NeuroBox that has the objective of integrating multiple brain scales and associated physical models into one unified framework. NeuroBox hosts geometry and anatomical reconstruction methods, such that detailed three-dimensional domains can be integrated into numerical simulations of models based on partial differential equations. The project further focusses on deriving numerical methods for handling complex computational domains, and to couple multiple spatial dimensions. The latter allows the user to specify in which parts of the biological problem high-dimensional representations are necessary and where low-dimensional approximations are acceptable. NeuroBox offers workflow user interfaces that are automatically generated with VRL-Studio and can be controlled by non-experts. The project further uses uG4 as the numerical backend, and therefore accesses highly advanced discretization methods as well as hierarchical and scalable numerical solvers for very large neurobiological problems.

Dendritic Spines Shape Analysis-Classification or Clusterization? Perspective

Dendritic spines are small protrusions from the dendrite membrane, where contact with neighboring axons is formed in order to receive synaptic input. Changes in size, shape, and density of synaptic spines are associated with learning and memory, and observed after drug abuse in a variety of neurodegenerative, neurodevelopmental, and psychiatric disorders. Due to the preeminent importance of synaptic spines, there have been major efforts into developing techniques that enable visualization and analysis of dendritic spines in cultured neurons, in fixed slices and in intact brain tissue. The classification of synaptic spines into predefined morphological groups is a standard approach in neuroscience research, where spines are divided into fixed categories such as thin, mushroom, and stubby subclasses. This study examines accumulated evidence that supports the existence of dendritic spine shapes as a continuum rather than separated classes. Using new approaches and software tools we reflect on complex dendritic spine shapes, positing that understanding of their highly dynamic nature is required to perform analysis of their morphology. The study discusses and compares recently developed algorithms that rely on clusterization rather than classification, therefore enabling new levels of spine shape analysis. We reason that improved methods of analysis may help to investigate a link between dendritic spine shape and its function, facilitating future studies of learning and memory as well as studies of brain disorders.

Dysregulation of protein synthesis and dendritic spine morphogenesis in ASD: studies in human pluripotent stem cells

Autism spectrum disorder (ASD) is a brain disorder that involves changes in neuronal connections. Abnormal morphology of dendritic spines on postsynaptic neurons has been observed in ASD patients and transgenic mice that model different monogenetic causes of ASD. A number of ASD-associated genetic variants are known to disrupt dendritic local protein synthesis, which is essential for spine morphogenesis, synaptic transmission, and plasticity. Most of our understanding on the molecular mechanism underlying ASD depends on studies using rodents. However, recent advance in human pluripotent stem cells and their neural differentiation provides a powerful alternative tool to understand the cellular aspects of human neurological disorders. In this review, we summarize recent progress on studying mRNA targeting and local protein synthesis in stem cell-derived neurons, and discuss how perturbation of these processes may impact synapse development and functions that are relevant to cognitive deficits in ASD.

Consequences of hyperphosphorylated tau on the morphology and excitability of hippocampal neurons in aged tau transgenic mice

The intracellular accumulation of hyperphosphorylated tau characterizes many neurodegenerative diseases such as Alzheimer's disease and frontotemporal dementia. A critical role for tau is supported by studies in transgenic mouse models expressing the P301L mutation with accumulation of hyperphosphorylated human tau in hippocampal pyramidal neurons of aged mice. Especially, the somatodendritic mislocalization of hyperphosphorylated tau seems to affect the neuronal network of the hippocampus. To show the consequences of aggregation of hyperphosphorylated tau within hippocampal neurons of aged mice, the CA1 pyramidal cells were analyzed morphologically and electrophysiologically. Here we demonstrate in the P301L pR5 mouse model that hyperphosphorylated tau leads to an increase in stubby spines and filopodia, as well as a decrease in total dendritic length of hippocampal pyramidal neurons due to a decrease in apical dendritic length and nodes. This atrophy is in line with the significant reduction in CA1 long-term potentiation. Furthermore, mutant tau induced a depolarized threshold for action potential initiation and an increased current of inward rectifying potassium channels, which should lead, together with the long-term potentiation decrease, to a decreased excitability of CA1 neurons.

The SETD6 Methyltransferase Plays an Essential Role in Hippocampus-Dependent Memory Formation

BACKGROUND

Epigenetic mechanisms are critical for hippocampus-dependent memory formation. Building on previous studies that implicate the N-lysine methyltransferase SETD6 in the activation of nuclear factor-κB RELA (also known as transcription factor p65) as an epigenetic recruiter, we hypothesized that SETD6 is a key player in the epigenetic control of long-term memory.

METHODS

Using a series of molecular, biochemical, imaging, electrophysiological, and behavioral experiments, we interrogated the effects of short interfering RNA-mediated knockdown of Setd6 in the rat dorsal hippocampus during memory consolidation.

RESULTS

Our findings demonstrate that SETD6 is necessary for memory-related nuclear factor-κB RELA methylation at lysine 310 and associated increases in H3K9me2 (histone H3 lysine 9 dimethylation) in the dorsal hippocampus and that SETD6 knockdown interferes with memory consolidation, alters gene expression patterns, and disrupts spine morphology.

CONCLUSIONS

Together, these findings suggest that SETD6 plays a critical role in memory formation and may act as an upstream initiator of H3K9me2 changes in the hippocampus during memory consolidation.

Early Effects of Aβ Oligomers on Dendritic Spine Dynamics and Arborization in Hippocampal Neurons

Alzheimer’s disease (AD) is a neurodegenerative disorder that leads to impaired memory and cognitive deficits. Spine loss as well as changes in spine morphology correlates with cognitive impairment in this neurological disorder. Many studies in animal models and ex vivo cultures indicate that amyloid β-peptide (Aβ) oligomers induce synaptic damage early during the progression of the disease. Here, in order to determine the events that initiate synaptic alterations, we acutely applied oligomeric Aβ to primary hippocampal neurons and an ex vivo model of organotypic hippocampal cultures from a mouse after targeted expression of EGFP to allow high-resolution imaging and algorithm-based evaluation of spine changes. Dendritic spines were classified as thin, stubby or mushroom, based on morphology. In vivo, time-lapse imaging showed that the three spine types were relatively stable, although their stability significantly decreased after treatment with Aβ oligomers. Unexpectedly, we observed that the density of total dendritic spines increased in organotypic hippocampal slices treated with Aβ compared to control cultures. Specifically, the fraction of stubby spines significantly increased, while mushroom and thin spines remained unaltered. Pharmacological tools revealed that acute Aβ oligomers induced spine changes through mechanisms involving CaMKII and integrin β1 activities. Additionally, analysis of dendritic complexity based on a 3D reconstruction of the whole neuron morphology showed an increase in the apical dendrite length and branching points in CA1 organotypic hippocampal slices treated with Aβ. In contrast to spines, the morphological changes were affected by integrin β1 but not by CaMKII inhibition. Altogether, these data indicate that the Aβ oligomers exhibit early dual effects by acutely enhancing dendritic complexity and spine density.

c-Abl Deficiency Provides Synaptic Resiliency Against Aβ-Oligomers

Spine pathology has been implicated in the early onset of Alzheimer’s disease (AD), where Aβ-Oligomers (AβOs) cause synaptic dysfunction and loss. Previously, we described that pharmacological inhibition of c-Abl prevents AβOs-induced synaptic alterations. Hence, this kinase seems to be a key element in AD progression. Here, we studied the role of c-Abl on dendritic spine morphological changes induced by AβOs using c-Abl null neurons (c-Abl-KO). First, we characterized the effect of c-Abl deficiency on dendritic spine density and found that its absence increases dendritic spine density. While AβOs-treatment reduces the spine number in both wild-type (WT) and c-Abl-KO neurons, AβOs-driven spine density loss was not affected by c-Abl. We then characterized AβOs-induced morphological changes in dendritic spines of c-Abl-KO neurons. AβOs induced a decrease in the number of mushroom spines in c-Abl-KO neurons while preserving the populations of immature stubby, thin, and filopodia spines. Furthermore, synaptic contacts evaluated by PSD95/Piccolo clustering and cell viability were preserved in AβOs-exposed c-Abl-KO neurons. In conclusion, our results indicate that in the presence of AβOs c-Abl participates in synaptic contact removal, increasing susceptibility to AβOs damage. Its deficiency increases the immature spine population reducing AβOs-induced synapse elimination. Therefore, c-Abl signaling could be a relevant actor in the early stages of AD.

Geometric Control of Frequency Modulation of cAMP Oscillations due to Calcium in Dendritic Spines

The spatiotemporal regulation of cyclic adenosine monophosphate (cAMP) and its dynamic interactions with other second messengers such as calcium are critical features of signaling specificity required for neuronal development and connectivity. cAMP is known to contribute to long-term potentiation and memory formation by controlling the formation and regulation of dendritic spines. Despite the recent advances in biosensing techniques for monitoring spatiotemporal cAMP dynamics, the underlying molecular mechanisms that attribute to the subcellular modulation of cAMP remain unknown. In this work, we model the spatiotemporal dynamics of calcium-induced cAMP signaling pathway in dendritic spines. Using a three-dimensional reaction-diffusion model, we investigate the effect of different spatial characteristics of cAMP dynamics that may be responsible for subcellular regulation of cAMP concentrations. Our model predicts that the volume/surface ratio of the spine, regulated through the spine head size, spine neck size, and the presence of physical barriers (spine apparatus), is an important regulator of cAMP dynamics. Furthermore, localization of the enzymes responsible for the synthesis and degradation of cAMP in different compartments also modulates the oscillatory patterns of cAMP through exponential relationships. Our findings shed light on the significance of complex geometric and localization relationships for cAMP dynamics in dendritic spines.

Traumatic Stress Produces Delayed Alterations of Synaptic Plasticity in Basolateral Amygdala

Acute traumatic event exposure is a direct cause of post-traumatic stress disorder (PTSD). Amygdala is suggested to be associated with the development of PTSD. In our previous findings, different activation patterns of GABAergic neurons and glutamatergic neurons in early or late stages after stress were found. However, the neural plastic mechanism underlying the role of basolateral amygdala (BLA) in post-traumatic stress disorder remains unclear. Therefore, this study mainly aimed at investigating time-dependent morphologic and electrophysiological changes in BLA during the development of PTSD. We used single prolonged stress (SPS) procedure to establish PTSD model of rats. The rats showed no alterations in anxiety behavior as well as in dendritic spine density or synaptic transmission in BLA 1 day after SPS. However, 10 days after SPS, rats showed enhancement of anxiety behavior, and spine density and frequency of miniature excitatory and inhibitory postsynaptic currents in BLA. Our results suggested that after traumatic stress, BLA displayed delayed increase in both spinogenesis and synaptic transmission, which seemed to facilitate the development of PTSD.

A Confocal Reflection Super-Resolution Technique to Image Golgi-Cox Stained Neurons

Metal-based Golgi-Cox (GC) staining is an established method used to visualise neurons with great morphological detail. Although GC stained samples are imaged routinely under transmitted light microscopy, this method is unable to yield information on the three-dimensional structure of dendrites and neurons and thus help reveal the connective properties of the central nervous system. Although a few studies have attempted simultaneous visualisation of GC staining and antigen-specific fluorescent labelling under a confocal reflection technique, the resolution of both confocal reflection and fluorescence modalities used to acquire GC reflection and fluorescently stained antibody signals are still limited by the diffraction limit of light at about 220 nm. Here, we report a confocal reflection super-resolution technique (CRSR) to break this diffraction barrier, which is achieved by minimising the pinhole size from 1 airy unit (AU) to 0.1 AU. This is achieved by minimising or closing the confocal pinhole size and is possible in this reflection modality, unlike fluorescence, because it is not a photon limited technique. Utilising the lowest wavelength of light available in the system (405 nm), the CRSR technique results in ∼30% lateral and axial resolution improvement. We also show that the CRSR technique can be used in conjunction to visualise both GC and immunofluorescence targets to create precise and improved three-dimensional visualisation and analysis. In addition, using these superresolution confocal reflection data sets from GC in CRSR mode significantly reduced the data overestimation, improving the accuracy of statistical analysis of dendritic spine density and average spine dimensions. Combining the 0.1 AU setting with deconvolution routines, the signal-to-noise ratio and resolution could further be improved an additional ∼20-25%, yielding CRSR images with resolutions up to 2-fold over the diffraction limit both laterally and axially. The improved precision of both visualisation and quantification of subdiffraction limited dendritic spines using the CRSR technique may prove to be critical in investigations that concern changes in detailed neuron morphology under central nervous system disease conditions such as multiple sclerosis and Alzheimer's disease. LAY DESCRIPTION: For over a century, Golgi-Cox (GC) has been a leading staining technique in the field of neuroscience, used to visualise neurons with great morphological detail. GC stained brain or spinal cord samples are conventionally visualised under transmitted light techniques. This limits the view of Golgi-staining to a two-dimensional image. A recent report showed that Golgi staining can be visualised in three-dimensions using the reflection modality of the confocal microscope. This visualisation also allows for the simultaneous acquisition of immunofluorescence signals. However, the reported resolution of Golgi staining confocal reflection is limited by the diffraction limit of light, which is around 220 nm. Here, we report a superresolution confocal reflection technique (CRSR) that achieves superresolution by minimising the pinhole size used in confocal microscopy. The CRSR technique results in ∼30% lateral and axial resolution improvement. Adding a deconvolution step in the final processing could improve the SNR and resolution even further up to 2-fold improvement in resolution over the diffraction limit both laterally and axially. We hope that this improved visualisation will help in investigations that concern changes in detailed neuron morphology under central nervous system disease conditions such as multiple sclerosis and Alzheimer's disease.

Open Up to Make New Contacts: Caldendrin Senses Postsynaptic Calcium Influx to Dynamically Organize Dendritic Spines

In this issue of Neuron, Mikhaylova et al. (2018) report how the Ca²⁺ sensor caldendrin interacts in dendritic spines with cortactin to control actin remodeling. Combining molecular and functional approaches, this work gains insights into postsynaptic dynamics relevant for synaptic plasticity.