Nuclear Calcium-VEGFD Signaling Controls Maintenance of Dendrite Arborization Necessary for Memory Formation

Calcium release via IP3R/RyR channels contributes to the nuclear and mitochondrial Ca2+ signals elicited by neuronal stimulation

The brain constantly adapts to environmental changes by modifying the expression of genes that enable synaptic plasticity, learning and memory. The expression of several of these genes requires nuclear calcium (Ca2+) signals, which in turn requires that Ca2+ signals generated by neuronal activity at the synapses or the soma propagate to the nucleus. Since cytoplasmic Ca2+ diffusion is highly restricted, Ca2+ signal propagation to the nucleus requires the participation of other cellular mechanisms. The inositol trisphosphate receptor (IP3R) and the ryanodine receptor (RyR) channels, both of which reside in the endoplasmic reticulum (ER) membrane, play key roles in cellular Ca2+ signal generation. Yet, their roles in the generation of nuclear and mitochondrial Ca2+ signals induced by neuronal activity require further investigation. Here, the impact of IP3R1 or RyR2 knockdown on gabazine-induced nuclear and mitochondrial Ca2+ signals in neurons was evaluated. To this aim, recombinant adeno-associated viruses (rAAVs) were used to introduce small hairpin RNAs (shRNAs) to knockdown type-1 (IP3R1) and type-2 (RyR2) channel expression in cultured rat hippocampal neurons. Additionally, synaptic contact numbers were assessed through immunocytochemistry. Knockdown of IP3R1 or RyR2 channels significantly reduced their protein contents and the generation of gabazine-induced nuclear and mitochondrial Ca2+ signals, without altering synaptic contact numbers. Our results highlight the contribution of IP3R1 and RyR2 channels to the generation of nuclear and mitochondrial Ca2+ signal induced by neuronal activity, reinforcing the role that these Ca2+ release channels play in hippocampal synaptic plasticity and memory formation.

Beyond vessels: unraveling the impact of VEGFs on neuronal functions and structure

Neurons rely on the bloodstream for essential nutrients and oxygen, which is facilitated by an intricate coupling of the neuronal and vascular systems. Central to this neurovascular interaction is the vascular endothelial growth factor (VEGF) family, a group of secreted growth factors traditionally known for their roles in promoting endothelial cell proliferation, migration, and survival in the cardiovascular and lymphatic systems. However, emerging evidence shows that VEGFs also play indispensable roles in the nervous system, extending beyond their canonical angiogenic and lymphangiogenic functions. Over the past two decades, VEGFs have been found to exert direct effects on neurons, influencing key aspects of neuronal function independently of their actions on vascular cells. In particular, it has become increasingly evident that VEGFs also play crucial functions in the development, regulation, and maintenance of neuronal morphology. Understanding the roles of VEGFs in neuronal development is of high scientific and clinical interest because of the significance of precise neuronal morphology for neural connectivity and network function, as well as the association of morphological abnormalities with neurological and neurodegenerative disorders. This review begins with an overview of the VEGF family members, their structural characteristics, receptors, and established roles in vasculature. However, it then highlights and focuses on the exciting variety of neuronal functions of VEGFs, especially their crucial role in the development, regulation, and maintenance of neuronal morphology.

Nuclear Calcium Signaling in D1 Receptor-Expressing Neurons of the Nucleus Accumbens Regulates Molecular, Cellular, and Behavioral Adaptations to Cocaine

BACKGROUND

The persistence of cocaine-evoked adaptations relies on gene regulations within the reward circuit, especially in the ventral striatum (i.e., nucleus accumbens [NAc]). Notably, activation of the ERK (extracellular signal-regulated kinase) pathway in the striatum is known to trigger a transcriptional program shaping long-term responses to cocaine. Nuclear calcium signaling has also been shown to control multiple forms of transcription-dependent neuroadaptations, but the dynamics and roles of striatal nuclear calcium signaling in preclinical models of addiction remain unknown.

METHODS

A genetically encoded cell type-specific nuclear calcium probe has been developed to monitor calcium dynamics in the nuclei of striatal neurons, including in freely moving mice. A cell type-specific inhibitor of nuclear calcium signaling combined with 3-dimensional imaging of neuronal morphology, immunostaining, and behavior was used to disentangle the roles of nuclear calcium in NAc medium spiny neurons (MSNs) expressing the dopamine D1 receptor (D1R) or D2 receptor (D2R) on cocaine-evoked responses.

RESULTS

The D1R-mediated potentiation of calcium influx through glutamate NMDA receptors, which shapes cocaine effects, also drives nuclear calcium transients. Fiber photometry revealed that cocaine-treated mice showed a sustained nuclear calcium increase in NAc D1R-MSNs. Disrupting nuclear calcium in D1R-MSNs, but not D2R-MSNs, blocked cocaine-evoked morphological changes of MSNs and gene expression and blunted cocaine's rewarding effects.

CONCLUSIONS

Our study unravels the dynamics and roles of cocaine-induced nuclear calcium signaling increases in D1R-MSNs on molecular, cellular, and behavioral adaptations to cocaine and represents a significant breakthrough because it could contribute to the development of innovative strategies with therapeutic potential to alleviate addiction symptoms.

NMDA Receptors in Neurodevelopmental Disorders: Pathophysiology and Disease Models

N-methyl-D-aspartate receptors (NMDARs) are critical components of the mammalian central nervous system, involved in synaptic transmission, plasticity, and neurodevelopment. This review focuses on the structural and functional characteristics of NMDARs, with a particular emphasis on the GRIN2 subunits (GluN2A-D). The diversity of GRIN2 subunits, driven by alternative splicing and genetic variants, significantly impacts receptor function, synaptic localization, and disease manifestation. The temporal and spatial expression of these subunits is essential for typical neural development, with each subunit supporting distinct phases of synaptic formation and plasticity. Disruptions in their developmental regulation are linked to neurodevelopmental disorders, underscoring the importance of understanding these dynamics in NDD pathophysiology. We explore the physiological properties and developmental regulation of these subunits, highlighting their roles in the pathophysiology of various NDDs, including ASD, epilepsy, and schizophrenia. By reviewing current knowledge and experimental models, including mouse models and human-induced pluripotent stem cells (hiPSCs), this article aims to elucidate different approaches through which the intricacies of NMDAR dysfunction in NDDs are currently being explored. The comprehensive understanding of NMDAR subunit composition and their mutations provides a foundation for developing targeted therapeutic strategies to address these complex disorders.

VEGFD signaling balances stability and activity-dependent structural plasticity of dendrites

Mature neurons have stable dendritic architecture, which is essential for the nervous system to operate correctly. The ability to undergo structural plasticity, required to support adaptive processes like memory formation, is still present in mature neurons. It is unclear what molecular and cellular processes control this delicate balance between dendritic structural plasticity and stabilization. Failures in the preservation of optimal dendrite structure due to atrophy or maladaptive plasticity result in abnormal connectivity and are associated with various neurological diseases. Vascular endothelial growth factor D (VEGFD) is critical for the maintenance of mature dendritic trees. Here, we describe how VEGFD affects the neuronal cytoskeleton and demonstrate that VEGFD exerts its effects on dendrite stabilization by influencing the actin cortex and reducing microtubule dynamics. Further, we found that during synaptic activity-induced structural plasticity VEGFD is downregulated. Our findings revealed that VEGFD, acting on its cognate receptor VEGFR3, opposes structural changes by negatively regulating dendrite growth in cultured hippocampal neurons and in vivo in the adult mouse hippocampus with consequences on memory formation. A phosphoproteomic screening identified several regulatory proteins of the cytoskeleton modulated by VEGFD. Among the actin cortex-associated proteins, we found that VEGFD induces dephosphorylation of ezrin at tyrosine 478 via activation of the striatal-enriched protein tyrosine phosphatase (STEP). Activity-triggered structural plasticity of dendrites was impaired by expression of a phospho-deficient mutant ezrin in vitro and in vivo. Thus, VEGFD governs the equilibrium between stabilization and plasticity of dendrites by acting as a molecular brake of structural remodeling.

Toll-like receptor 4 deficiency in Purkinje neurons drives cerebellar ataxia by impairing the BK channel-mediated after-hyperpolarization and cytosolic calcium homeostasis

Toll-like receptor (TLR) 4 contributes to be the induction of neuroinflammation by recognizing pathology-associated ligands and activating microglia. In addition, numerous physiological signaling factors act as agonists or antagonists of TLR4 expressed by non-immune cells. Recently, TLR4 was found to be highly expressed in cerebellar Purkinje neurons (PNs) and involved in the maintenance of motor coordination through non-immune pathways, but the precise mechanisms remain unclear. Here we report that mice with PN specific TLR4 deletion (TLR4PKO mice) exhibited motor impairments consistent with cerebellar ataxia, reduced PN dendritic arborization and spine density, fewer parallel fiber (PF) - PN and climbing fiber (CF) - PN synapses, reduced BK channel expression, and impaired BK-mediated after-hyperpolarization, collectively leading to abnormal PN firing. Moreover, the impaired PN firing in TLR4PKO mice could be rescued with BK channel opener. The PNs of TLR4PKO mice also exhibited abnormal mitochondrial structure, disrupted mitochondrial endoplasmic reticulum tethering, and reduced cytosolic calcium, changes that may underly abnormal PN firing and ultimately drive ataxia. These results identify a previously unknown role for TLR4 in regulating PN firing and maintaining cerebellar function.

The VEGFs/VEGFRs system in Alzheimer's and Parkinson's diseases: Pathophysiological roles and therapeutic implications

The vascular endothelial growth factors (VEGFs) and their cognate receptors (VEGFRs), besides their well-known involvement in physiological angiogenesis/lymphangiogenesis and in diseases associated to pathological vessel formation, play multifaceted functions in the central nervous system (CNS). In addition to shaping brain development, by controlling cerebral vasculogenesis and regulating neurogenesis as well as astrocyte differentiation, the VEGFs/VEGFRs axis exerts essential functions in the adult brain both in physiological and pathological contexts. In this article, after describing the physiological VEGFs/VEGFRs functions in the CNS, we focus on the VEGFs/VEGFRs involvement in neurodegenerative diseases by reviewing the current literature on the rather complex VEGFs/VEGFRs contribution to the pathogenic mechanisms of Alzheimer's (AD) and Parkinson's (PD) diseases. Thereafter, based on the outcome of VEGFs/VEGFRs targeting in animal models of AD and PD, we discuss the factual relevance of pharmacological VEGFs/VEGFRs modulation as a novel and potential disease-modifying approach for these neurodegenerative pathologies. Specific VEGFRs targeting, aimed at selective VEGFR-1 inhibition, while preserving VEGFR-2 signal transduction, appears as a promising strategy to hit the molecular mechanisms underlying AD pathology. Moreover, therapeutic VEGFs-based approaches can be proposed for PD treatment, with the aim of fine-tuning their brain levels to amplify neurotrophic/neuroprotective effects while limiting an excessive impact on vascular permeability.

VEGFR-3 signaling in macrophages: friend or foe in disease?

Lymphatic vessels have been increasingly appreciated in the context of immunology not only as passive conduits for immune and cancer cell transport but also as key in local tissue immunomodulation. Targeting lymphatic vessel growth and potential immune regulation often takes advantage of vascular endothelial growth factor receptor-3 (VEGFR-3) signaling to manipulate lymphatic biology. A receptor tyrosine kinase, VEGFR-3, is highly expressed on lymphatic endothelial cells, and its signaling is key in lymphatic growth, development, and survival and, as a result, often considered to be "lymphatic-specific" in adults. A subset of immune cells, notably of the monocyte-derived lineage, have been identified to express VEGFR-3 in tissues from the lung to the gut and in conditions as varied as cancer and chronic kidney disease. These VEGFR-3+ macrophages are highly chemotactic toward the VEGFR-3 ligands VEGF-C and VEGF-D. VEGFR-3 signaling has also been implicated in dictating the plasticity of these cells from pro-inflammatory to anti-inflammatory phenotypes. Conversely, expression may potentially be transient during monocyte differentiation with unknown effects. Macrophages play critically important and varied roles in the onset and resolution of inflammation, tissue remodeling, and vasculogenesis: targeting lymphatic vessel growth and immunomodulation by manipulating VEGFR-3 signaling may thus impact macrophage biology and their impact on disease pathogenesis. This mini review highlights the studies and pathologies in which VEGFR-3+ macrophages have been specifically identified, as well as the activity and polarization changes that macrophage VEGFR-3 signaling may elicit, and affords some conclusions as to the importance of macrophage VEGFR-3 signaling in disease.

Overexpression of NT-3 in the hippocampus suppresses the early phase of the adult neurogenic process

The dentate gyrus (DG) of the hippocampus regulates stress-related emotional behaviors and ensures neurogenesis throughout life. Neurotrophin-3 (NT-3) is a neurotrophic factor that regulates neuronal differentiation, survival, and synaptic formation in both the peripheral and central nervous systems. NT-3 is expressed in the adult DG of the hippocampus; several chronic stress conditions enhance NT-3 expression in rodents. However, functional modulation of the adult DG by NT-3 signaling remains unclear. To directly investigate the impact of NT-3 on DG function, NT-3 was overexpressed in the hippocampal ventral DG by an adeno-associated virus carrying NT-3 (AAV-NT-3). Four weeks following the AAV-NT-3 injection, high NT-3 expression was observed in the ventral DG. We examined the influence of NT-3 overexpression on the neuronal responses and neurogenic processes in the ventral DG. NT-3 overexpression significantly increased the expression of the mature DG neuronal marker calbindin and immediate early genes, such as Fos and Fosb, thereby suggesting DG neuronal activation. During neurogenesis, the number of proliferating cells and immature neurons in the subgranular zone of the DG significantly decreased in the AAV-NT-3 group. Among the neurogenesis-related factors, Vegfd, Lgr6, Bmp7, and Drd1 expression significantly decreased. These results demonstrated that high NT-3 levels in the hippocampus regulate the activation of mature DG neurons and suppress the early phase of neurogenic processes, suggesting a possible role of NT-3 in the regulation of adult hippocampal function under stress conditions.

Long-term potentiation and spatial memory training stimulate the hippocampal expression of RyR2 calcium release channels

Introduction: Neuronal Ca²⁺ signals generated through the activation of Ca²⁺-induced Ca²⁺ release in response to activity-generated Ca²⁺ influx play a significant role in hippocampal synaptic plasticity, spatial learning, and memory. We and others have previously reported that diverse stimulation protocols, or different memory-inducing procedures, enhance the expression of endoplasmic reticulum-resident Ca²⁺ release channels in rat primary hippocampal neuronal cells or hippocampal tissue. Methods and Results: Here, we report that induction of long-term potentiation (LTP) by Theta burst stimulation protocols of the CA3-CA1 hippocampal synapse increased the mRNA and protein levels of type-2 Ryanodine Receptor (RyR2) Ca²⁺ release channels in rat hippocampal slices. Suppression of RyR channel activity (1 h preincubation with 20 μM ryanodine) abolished both LTP induction and the enhanced expression of these channels; it also promoted an increase in the surface expression of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits GluR1 and GluR2 and caused a moderate but significant reduction of dendritic spine density. In addition, training rats in the Morris water maze induced memory consolidation, which lasted for several days after the end of the training period, accompanied by an increase in the mRNA levels and the protein content of the RyR2 channel isoform. Discussion: We confirm in this work that LTP induction by TBS protocols requires functional RyR channels. We propose that the increments in the protein content of RyR2 Ca²⁺ release channels, induced by LTP or spatial memory training, play a significant role in hippocampal synaptic plasticity and spatial memory consolidation.

Cohort study on the differential expression of inflammatory and angiogenic factors in thrombi, cerebral and peripheral plasma following acute large vessel occlusion stroke

Inflammation plays an important role in the pathogenesis of stroke. The differential expression of inflammatory and angiogenic factors in thrombi and plasma remain undefined. In this observational cohort study, we evaluated angiogenic factors and inflammatory cytokines, in cerebral thrombi, local cerebral plasma (CP), and peripheral plasma (PP) in patients with acute ischemic stroke. Protein analysis of thrombi, CP and PP were used to measure angiogenic and inflammatory proteins using electrochemiluminescence. Our data indicate that VEGF-A, VEGF-C, bFGF, IL-4, IL-13, IL-1β, IL-2, IL-8, IL-16, IL-6 and IL-12p70 were higher in the thrombi of acute ischemic stroke patients than in the CP and PP of stroke patients. Moreover, the protein levels of GM-CSF were lower in the PP than in the CP and the clot. Moreover, VEGF-D, Flt-1, PIGF, TIE-2, IL-5, TNF-β, IL-15, IL-12/IL-23p40, IFN-γ and IL-17A were higher in PP and CP than in thrombi. Our results show that cytokines mediating the inflammatory response and proteins involved in angiogenesis are differentially expressed in thrombi within the cerebral and peripheral circulations. These data highlight the importance of identifying new biomarkers in different compartments of the circulatory system and in thrombi that may be used for the diagnosis and treatment of stroke patients.

Neurotransmitters, neuropeptides and calcium in oocyte maturation and early development

A primary reason behind the high level of complexity we embody as multicellular organisms is a highly complex intracellular and intercellular communication system. As a result, the activities of multiple cell types and tissues can be modulated resulting in a specific physiological function. One of the key players in this communication process is extracellular signaling molecules that can act in autocrine, paracrine, and endocrine fashion to regulate distinct physiological responses. Neurotransmitters and neuropeptides are signaling molecules that renders long-range communication possible. In normal conditions, neurotransmitters are involved in normal responses such as development and normal physiological aspects; however, the dysregulation of neurotransmitters mediated signaling has been associated with several pathologies such as neurodegenerative, neurological, psychiatric disorders, and other pathologies. One of the interesting topics that is not yet fully explored is the connection between neuronal signaling and physiological changes during oocyte maturation and fertilization. Knowing the importance of Ca²⁺ signaling in these reproductive processes, our objective in this review is to highlight the link between the neuronal signals and the intracellular changes in calcium during oocyte maturation and embryogenesis. Calcium (Ca²⁺) is a ubiquitous intracellular mediator involved in various cellular functions such as releasing neurotransmitters from neurons, contraction of muscle cells, fertilization, and cell differentiation and morphogenesis. The multiple roles played by this ion in mediating signals can be primarily explained by its spatiotemporal dynamics that are kept tightly checked by mechanisms that control its entry through plasma membrane and its storage on intracellular stores. Given the large electrochemical gradient of the ion across the plasma membrane and intracellular stores, signals that can modulate Ca²⁺ entry channels or Ca²⁺ receptors in the stores will cause Ca²⁺ to be elevated in the cytosol and consequently activating downstream Ca²⁺-responsive proteins resulting in specific cellular responses. This review aims to provide an overview of the reported neurotransmitters and neuropeptides that participate in early stages of development and their association with Ca²⁺ signaling.

Brain-enriched RagB isoforms regulate the dynamics of mTORC1 activity through GATOR1 inhibition

Mechanistic target of rapamycin complex 1 (mTORC1) senses nutrient availability to appropriately regulate cellular anabolism and catabolism. During nutrient restriction, different organs in an animal do not respond equally, with vital organs being relatively spared. This raises the possibility that mTORC1 is differentially regulated in different cell types, yet little is known about this mechanistically. The Rag GTPases, RagA or RagB bound to RagC or RagD, tether mTORC1 in a nutrient-dependent manner to lysosomes where mTORC1 becomes activated. Although the RagA and B paralogues were assumed to be functionally equivalent, we find here that the RagB isoforms, which are highly expressed in neurons, impart mTORC1 with resistance to nutrient starvation by inhibiting the RagA/B GTPase-activating protein GATOR1. We further show that high expression of RagB isoforms is observed in some tumours, revealing an alternative strategy by which cancer cells can retain elevated mTORC1 upon low nutrient availability.

Organic anion transporter 1 is an HDAC4-regulated mediator of nociceptive hypersensitivity in mice

Persistent pain is sustained by maladaptive changes in gene transcription resulting in altered function of the relevant circuits; therapies are still unsatisfactory. The epigenetic mechanisms and affected genes linking nociceptive activity to transcriptional changes and pathological sensitivity are unclear. Here, we found that, among several histone deacetylases (HDACs), synaptic activity specifically affects HDAC4 in murine spinal cord dorsal horn neurons. Noxious stimuli that induce long-lasting inflammatory hypersensitivity cause nuclear export and inactivation of HDAC4. The development of inflammation-associated mechanical hypersensitivity, but neither acute nor basal sensitivity, is impaired by the expression of a constitutively nuclear localized HDAC4 mutant. Next generation RNA-sequencing revealed an HDAC4-regulated gene program comprising mediators of sensitization including the organic anion transporter OAT1, known for its renal transport function. Using pharmacological and molecular tools to modulate OAT1 activity or expression, we causally link OAT1 to persistent inflammatory hypersensitivity in mice. Thus, HDAC4 is a key epigenetic regulator that translates nociceptive activity into sensitization by regulating OAT1, which is a potential target for pain-relieving therapies.

Chronic pain is sustained by alterations in gene transcription. Here, the authors show that increased expression of Organic Anionic Transporter 1 in the spinal cord is epigenetically controlled and key to hypersensitivity in pathological pain.

Disrupted expression of mitochondrial NCLX sensitizes neuroglial networks to excitotoxic stimuli and renders synaptic activity toxic

The mitochondrial sodium/calcium/lithium exchanger (NCLX) is an important mediator of calcium extrusion from mitochondria. In this study, we tested the hypothesis that physiological expression levels of NCLX are essential for maintaining neuronal resilience in the face of excitotoxic challenge. Using a short hairpin RNA (shRNA)-mediated approach, we showed that reduced NCLX expression exacerbates neuronal mitochondrial calcium dysregulation, mitochondrial membrane potential (ΔΨ_m) breakdown, and reactive oxygen species (ROS) generation during excitotoxic stimulation of primary hippocampal cultures. Moreover, NCLX knockdown-which affected both neurons and glia-resulted not only in enhanced neurodegeneration following an excitotoxic insult, but also in neuronal and astrocytic cell death under basal conditions. Our data also revealed that synaptic activity, which promotes neuroprotective signaling, can become lethal upon NCLX depletion; expression of NCLX-targeted shRNA impaired the clearance of mitochondrial calcium following action potential bursts and was associated both with ΔΨ_mbreakdown and substantial neurodegeneration in hippocampal cultures undergoing synaptic activity. Finally, we showed that NCLX knockdown within the hippocampal cornu ammonis 1 (CA1) region in vivo causes substantial neuro- and astrodegeneration. In summary, we demonstrated that dysregulated NCLX expression not only sensitizes neuroglial networks to excitotoxic stimuli but notably also renders otherwise neuroprotective synaptic activity toxic. These findings may explain the emergence of neuro- and astrodegeneration in patients with disorders characterized by disrupted NCLX expression or function, and suggest that treatments aimed at enhancing or restoring NCLX function may prevent central nervous system damage in these disease states.

The relationship between vascular endothelial growth factor (VEGF) and amnestic mild cognitive impairment among older adults living with HIV

Older people with HIV (PWH) experience increased risk of age-related neurodegenerative disorders and cognitive decline, such as amnestic mild cognitive impairment (aMCI). The objective of this study was to examine the relationship between aMCI and plasma VEGF biomarkers among older PWH. Data were collected at a university-based research center from 2011 to 2013. Participants were 67 antiretroviral therapy-treated, virally suppressed PWH. Participants completed comprehensive neurobehavioral and neuromedical evaluations. aMCI status was determined using adapted Jak/Bondi criteria, classifying participants as aMCI + if their performance was > 1 SD below the normative mean on at least two of four memory assessments. VEGF family plasma biomarkers (i.e., VEGF, VEGF-C, VEGF-D, and PIGF) were measured by immunoassay. Logistic regression models were conducted to determine whether VEGF biomarkers were associated with aMCI status. Participants were mostly non-Hispanic white (79%) men (85%) with a mean age of 57.7 years. Eighteen (26.9%) participants met criteria for aMCI. Among potential covariates, only antidepressant drug use differed by aMCI status, and was included as a covariate. VEGF-D was significantly lower in the aMCI + group compared to the aMCI - group. No other VEGF levels (VEGF, VEGF-C, PIGF) differed by aMCI classification (ps > .05). In a sample of antiretroviral therapy-treated, virally suppressed PWH, lower levels of VEGF-D were associated with aMCI status. Longitudinal analyses in a larger and more diverse sample are needed to support VEGF-D as a putative biological marker of aMCI in HIV.

Pathophysiological Ionotropic Glutamate Signalling in Neuroinflammatory Disease as a Therapeutic Target

Glutamate signalling is an essential aspect of neuronal communication involving many different glutamate receptors, and underlies the processes of memory, learning and synaptic plasticity. Despite neuroinflammatory diseases covering a range of maladies with very different biological causes and pathophysiologies, a central role for dysfunctional glutamate signalling is becoming apparent. This is not just restricted to the well-described role of glutamate in mediating neurodegeneration, but also includes a myriad of other influences that glutamate can exert on the vasculature, as well as immune cell and glial regulation, reflecting the ability of neurons to communicate with these compartments in order to couple their activity with neuronal requirements. Here, we discuss the role of pathophysiological glutamate signalling in neuroinflammatory disease, using both multiple sclerosis and Alzheimer's disease as examples, and how current steps are being made to harness our growing understanding of these processes in the development of neuroprotective strategies. This review focuses in particular on N-methyl-D-aspartate (NMDA) and 2-amino-3-(3-hydroxy-5-methylisooxazol-4-yl) propionate (AMPA) type ionotropic glutamate receptors, although metabotropic, G-protein-coupled glutamate receptors may also contribute to neuroinflammatory processes. Given the indispensable roles of glutamate-gated ion channels in synaptic communication, means of pharmacologically distinguishing between physiological and pathophysiological actions of glutamate will be discussed that allow deleterious signalling to be inhibited whilst minimising the disturbance of essential neuronal function.

Chronic VEGFR-3 signaling preserves dendritic arborization and sensitization under stress

Dendritic arborization is critical for the establishment and maintenance of precise neural circuits. Vascular endothelial growth factor D (VEGF-D), well-characterized as a "lymphangiogenic" growth factor, reportedly maintains dendritic arborization and synaptic strength in the hippocampus of adult mice through VEGF receptor (VEGFR-3) signaling. Here, we investigated the effect of chronic VEGFR-3-specific activation on adipose arbor morphometry using the Adipo-VD mouse, a model of inducible, adipose-specific VEGF-D overexpression. We examined whether adipose tissue innervation was preserved or functionally different in Adipo-VD mice during stress in vivo and if VEGFR-3 signaling afforded neuroprotection to challenged neurons in vitro. Chronic VEGFR-3 signaling in Adipo-VD subcutaneous adipose tissue resulted in a reduction in the dendrite length, dendritic terminal branches (filament length), and dendritic terminal branch volume (filament volume), but increased dendrite branching. We also identified reduced stimulus-evoked excitatory sympathetic nerve activity in Adipo-VD mice. Following 6-hydroxydopamine (6-OHDA) denervation, Adipo-VD dendritic arbors were preserved, including improved dendritic branch volume, length, and dendritic branches than in wildtype tissues. In vitro, we found that chronic elevation of VEGFR-3 signaling in developing mVC neurons changes the dendritic arbor complexity and improves stress-induced structure remodeling. Developing neurons are conferred neuroprotection against stress, potentially by upregulation of proteolytic conversion of pro-BDNF to mature BDNF. Mature neurons, however, display improved dendritic arbor complexity, and unaltered dendritic structural remodeling and improved resistance to stress with VEGFR-3 signaling. Overall, chronically increasing VEGFR-3 signaling in neurons has a synergistic impact on neurosensitization and neuroprotection during stress.

Increased Levels of Circulating Angiogenic Cells and Signaling Proteins in Older Adults With Cerebral Small Vessel Disease

Background: Cerebral small vessel disease (SVD) is associated with increased risk of stroke and dementia. Progressive damage to the cerebral microvasculature may also trigger angiogenic processes to promote vessel repair. Elevated levels of circulating endothelial progenitor cells (EPCs) and pro-angiogenic signaling proteins are observed in response to vascular injury. We aimed to examine circulating levels of EPCs and proangiogenic proteins in older adults with evidence of SVD.

Methods: Older adults (ages 55–90) free of dementia or stroke underwent venipuncture and brain magnetic resonance imaging (MRI). Flow cytometry quantified circulating EPCs as the number of cells in the lymphocyte gate positively expressing EPC surface markers (CD34+CD133+CD309+). Plasma was assayed for proangiogenic factors (VEGF-A, VEGF-C, VEGF-D, Tie-2, and Flt-1). Total SVD burden score was determined based on MRI markers, including white matter hyperintensities, cerebral microbleeds and lacunes.

Results: Sixty-four older adults were included. Linear regression revealed that older adults with higher circulating EPC levels exhibited greater total SVD burden [β = 1.0 × 10⁵, 95% CI (0.2, 1.9), p = 0.019], after accounting for age and sex. Similarly, a positive relationship between circulating VEGF-D and total SVD score was observed, controlling for age and sex [β = 0.001, 95% CI (0.000, 0.001), p = 0.048].

Conclusion: These findings suggest that elevated levels of circulating EPCs and VEGF-D correspond with greater cerebral SVD burden in older adults. Additional studies are warranted to determine whether activation of systemic angiogenic growth factors and EPCs represents an early attempt to rescue the vascular endothelium and repair damage in SVD.

RyR-mediated Ca2+ release elicited by neuronal activity induces nuclear Ca2+ signals, CREB phosphorylation, and Npas4/RyR2 expression

The expression of several hippocampal genes implicated in learning and memory processes requires that Ca²⁺ signals generated in dendritic spines, dendrites, or the soma in response to neuronal stimulation reach the nucleus. The diffusion of Ca²⁺ in the cytoplasm is highly restricted, so neurons must use other mechanisms to propagate Ca²⁺ signals to the nucleus. Here, we present evidence showing that Ca²⁺ release mediated by the ryanodine receptor (RyR) channel type-2 isoform (RyR2) contributes to the generation of nuclear Ca²⁺ signals induced by gabazine (GBZ) addition, glutamate uncaging in the dendrites, or high-frequency field stimulation of primary hippocampal neurons. Additionally, GBZ treatment significantly increased cyclic adenosine monophosphate response element binding protein (CREB) phosphorylation-a key event in synaptic plasticity and hippocampal memory-and enhanced the expression of Neuronal Per Arnt Sim domain protein 4 (Npas4) and RyR2, two central regulators of these processes. Suppression of RyR-mediated Ca²⁺ release with ryanodine significantly reduced the increase in CREB phosphorylation and the enhanced Npas4 and RyR2 expression induced by GBZ. We propose that RyR-mediated Ca²⁺ release induced by neuronal activity, through its contribution to the sequential generation of nuclear Ca²⁺ signals, CREB phosphorylation, Npas4, and RyR2 up-regulation, plays a central role in hippocampal synaptic plasticity and memory processes.

Mfn2 localization in the ER is necessary for its bioenergetic function and neuritic development

Mfn2 is a mitochondrial fusion protein with bioenergetic functions implicated in the pathophysiology of neuronal and metabolic disorders. Understanding the bioenergetic mechanism of Mfn2 may aid in designing therapeutic approaches for these disorders. Here we show using endoplasmic reticulum (ER) or mitochondria‐targeted Mfn2 that Mfn2 stimulation of the mitochondrial metabolism requires its localization in the ER, which is independent of its fusion function. ER‐located Mfn2 interacts with mitochondrial Mfn1/2 to tether the ER and mitochondria together, allowing Ca²⁺ transfer from the ER to mitochondria to enhance mitochondrial bioenergetics. The physiological relevance of these findings is shown during neurite outgrowth, when there is an increase in Mfn2‐dependent ER‐mitochondria contact that is necessary for correct neuronal arbor growth. Reduced neuritic growth in Mfn2 KO neurons is recovered by the expression of ER‐targeted Mfn2 or an artificial ER‐mitochondria tether, indicating that manipulation of ER‐mitochondria contacts could be used to treat pathologic conditions involving Mfn2.

Endophilin A1 drives acute structural plasticity of dendritic spines in response to Ca2+/calmodulin

Induction of long-term potentiation (LTP) in excitatory neurons triggers a large transient increase in the volume of dendritic spines followed by decays to sustained size expansion, a process termed structural LTP (sLTP) that contributes to the cellular basis of learning and memory. Although mechanisms regulating the early and sustained phases of sLTP have been studied intensively, how the acute spine enlargement immediately after LTP stimulation is achieved remains elusive. Here, we report that endophilin A1 orchestrates membrane dynamics with actin polymerization to initiate spine enlargement in NMDAR-mediated LTP. Upon LTP induction, Ca²⁺/calmodulin enhances binding of endophilin A1 to both membrane and p140Cap, a cytoskeletal regulator. Consequently, endophilin A1 rapidly localizes to the plasma membrane and recruits p140Cap to promote local actin polymerization, leading to spine head expansion. Moreover, its molecular functions in activity-induced rapid spine growth are required for LTP and long-term memory. Thus, endophilin A1 serves as a calmodulin effector to drive acute structural plasticity necessary for learning and memory.

The role of nuclear Ca2+ in maintaining neuronal homeostasis and brain health

Nuclear Ca2+ has emerged as one of the most potent mediators of the dialogue between neuronal synapses and the nucleus that regulates heterochromatin states, transcription factor activity, nuclear morphology and neuronal gene expression induced by synaptic activity. Recent studies underline the importance of nuclear Ca2+ signaling in long-lasting, activity-induced adaptation and maintenance of proper brain function. Diverse forms of neuroadaptation require transient nuclear Ca2+ signaling and cyclic AMP-responsive element-binding protein (CREB1, referred to here as CREB) as its prime target, which works as a tunable switch to drive and modulate specific gene expression profiles associated with memory, pain, addiction and neuroprotection. Furthermore, a reduction of nuclear Ca2+ levels has been shown to be neurotoxic and a causal factor driving the progression of neurodegenerative disorders, as well as affecting neuronal autophagy. Because of its central role in the brain, deficits in nuclear Ca2+ signaling may underlie a continuous loss of neuroprotection in the aging brain, contributing to the pathophysiology of Alzheimer's disease. In this Review, we discuss the principles of the 'nuclear calcium hypothesis' in the context of human brain function and its role in controlling diverse forms of neuroadaptation and neuroprotection. Furthermore, we present the most relevant and promising perspectives for future studies.

The impact of Semaphorin 4C/Plexin-B2 signaling on fear memory via remodeling of neuronal and synaptic morphology

Aberrant fear is a cornerstone of several psychiatric disorders. Consequently, there is large interest in elucidation of signaling mechanisms that link extracellular cues to changes in neuronal function and structure in brain pathways that are important in the generation and maintenance of fear memory and its behavioral expression. Members of the Plexin-B family of receptors for class 4 semaphorins play important roles in developmental plasticity of neurons, and their expression persists in some areas of the adult nervous system. Here, we aimed to elucidate the role of Semaphorin 4C (Sema4C) and its cognate receptor, Plexin-B2, in the expression of contextual and cued fear memory, setting a mechanistic focus on structural plasticity and exploration of contributing signaling pathways. We observed that Plexin-B2 and Sema4C are expressed in forebrain areas related to fear memory, such as the anterior cingulate cortex, amygdala and the hippocampus, and their expression is regulated by aversive stimuli that induce fear memory. By generating forebrain-specific Plexin-B2 knockout mice and analyzing fear-related behaviors, we demonstrate that Sema4C-PlexinB2 signaling plays a crucial functional role in the recent and remote recall of fear memory. Detailed neuronal morphological analyses revealed that Sema4C-PlexinB2 signaling largely mediates fear-induced structural plasticity by enhancing dendritic ramifications and modulating synaptic density in the adult hippocampus. Analyses on signaling-related mutant mice showed that these functions are mediated by PlexinB2-dependent RhoA activation. These results deliver important insights into the mechanistic understanding of maladaptive plasticity in fear circuits and have implications for novel therapeutic strategies against fear-related disorders.

Coupling of NMDA receptors and TRPM4 guides discovery of unconventional neuroprotectants

Excitotoxicity induced by NMDA receptors (NMDARs) is thought to be intimately linked to high intracellular calcium load. Unexpectedly, NMDAR-mediated toxicity can be eliminated without affecting NMDAR-induced calcium signals. Instead, excitotoxicity requires physical coupling of NMDARs to TRPM4. This interaction is mediated by intracellular domains located in the near-membrane portions of the receptors. Structure-based computational drug screening using the interaction interface of TRPM4 in complex with NMDARs identified small molecules that spare NMDAR-induced calcium signaling but disrupt the NMDAR/TRPM4 complex. These interaction interface inhibitors strongly reduce NMDA-triggered toxicity and mitochondrial dysfunction, abolish cyclic adenosine monophosphate-responsive element-binding protein (CREB) shutoff, boost gene induction, and reduce neuronal loss in mouse models of stroke and retinal degeneration. Recombinant or small-molecule NMDAR/TRPM4 interface inhibitors may mitigate currently untreatable human neurodegenerative diseases.

Genetically Encoded Calcium Indicators Can Impair Dendrite Growth of Cortical Neurons

A battery of genetically encoded calcium indicators (GECIs) with different binding kinetics and calcium affinities was developed over the recent years to permit long-term calcium imaging. GECIs are calcium buffers and therefore, expression of GECIs may interfere with calcium homeostasis and signaling pathways important for neuronal differentiation and survival. Our objective was to investigate if the biolistically induced expression of five commonly used GECIs at two postnatal time points (days 14 and 22–25) could affect the morphological maturation of cortical neurons in organotypic slice cultures of rat visual cortex. Expression of GCaMP3 in both time windows, and of GCaMP5G and TN-XXL in the later time window impaired apical and /or basal dendrite growth of pyramidal neurons. With time, the proportion of GECI transfectants with nuclear filling increased, but an only prolonged expression of TN-XXL caused higher levels of neurodegeneration. In multipolar interneurons, only GCaMP3 evoked a transient growth delay during the early time window. GCaMP6m and GCaMP6m-X_C were quite “neuron-friendly.” Since growth-impaired neurons might not have the physiological responses typical of age-matched wildtype neurons the results obtained after prolonged developmental expression of certain GECIs might need to be interpreted with caution.

VEGF-C Induces Alternative Activation of Microglia to Promote Recovery from Traumatic Brain Injury

Traumatic brain injury (TBI), a brain disorder that causes death and long-term disability in humans, is increasing in prevalence, though there is a lack of protective or therapeutic strategies for mitigating the damage after TBI and for preserving neurological functionality. Microglia cells play a key role in neuroinflammation following TBI, but their regulation and polarization by a member of the vascular endothelial growth factor (VEGF) family, VEGF-C, is unknown. Here, we show that VEGF-C induced M2 polarization in a murine microglia cell line, BV-2, in vitro, by a mechanism that required signaling from its unique receptor, VEGF receptor 3 (VEGFR3). Moreover, in a TBI model in rats, VEGF-C administration induced M2 polarization of microglia cells, significantly improved motor deficits after experimental TBI, and significantly improved neurological function following TBI, likely through a reduction in cell apoptosis. Together, our data reveal a previously unknown role of VEGF-C/VEGFR3 signaling in the regulation of post-TBI microglia cell polarization, which appears to be crucial for recovery from TBI.

Cyclase-associated protein 2 dimerization regulates cofilin in synaptic plasticity and Alzheimer's disease

Regulation of actin cytoskeleton dynamics in dendritic spines is crucial for learning and memory formation. Hence, defects in the actin cytoskeleton pathways are a biological trait of several brain diseases, including Alzheimer's disease. Here, we describe a novel synaptic mechanism governed by the cyclase-associated protein 2, which is required for structural plasticity phenomena and completely disrupted in Alzheimer's disease. We report that the formation of cyclase-associated protein 2 dimers through its Cys³² is important for cyclase-associated protein 2 binding to cofilin and for actin turnover. The Cys³²-dependent cyclase-associated protein 2 homodimerization and association to cofilin are triggered by long-term potentiation and are required for long-term potentiation-induced cofilin translocation into spines, spine remodelling and the potentiation of synaptic transmission. This mechanism is specifically affected in the hippocampus, but not in the superior frontal gyrus, of both Alzheimer's disease patients and APP/PS1 mice, where cyclase-associated protein 2 is down-regulated and cyclase-associated protein 2 dimer synaptic levels are reduced. Notably, cyclase-associated protein 2 levels in the cerebrospinal fluid are significantly increased in Alzheimer's disease patients but not in subjects affected by frontotemporal dementia. In Alzheimer's disease hippocampi, cofilin association to cyclase-associated protein 2 dimer/monomer is altered and cofilin is aberrantly localized in spines. Taken together, these results provide novel insights into structural plasticity mechanisms that are defective in Alzheimer's disease.

Nasally delivered VEGFD mimetics mitigate stroke-induced dendrite loss and brain damage

In the adult brain, vascular endothelial growth factor D (VEGFD) is required for structural integrity of dendrites and cognitive abilities. Alterations of dendritic architectures are hallmarks of many neurologic disorders, including stroke-induced damage caused by toxic extrasynaptic NMDA receptor (eNMDAR) signaling. Here we show that stimulation of eNMDARs causes a rapid shutoff of VEGFD expression, leading to a dramatic loss of dendritic structures. Using the mouse middle cerebral artery occlusion (MCAO) stroke model, we have established the therapeutic potential of recombinant mouse VEGFD delivered intraventricularly to preserve dendritic architecture, reduce stroke-induced brain damage, and facilitate functional recovery. An easy-to-use therapeutic intervention for stroke was developed that uses a new class of VEGFD-derived peptide mimetics and postinjury nose-to-brain delivery.

Functional Consequences of Calcium-Dependent Synapse-to-Nucleus Communication: Focus on Transcription-Dependent Metabolic Plasticity

In the nervous system, calcium signals play a major role in the conversion of synaptic stimuli into transcriptional responses. Signal-regulated gene transcription is fundamental for a range of long-lasting adaptive brain functions that include learning and memory, structural plasticity of neurites and synapses, acquired neuroprotection, chronic pain, and addiction. In this review, we summarize the diverse mechanisms governing calcium-dependent transcriptional regulation associated with central nervous system plasticity. We focus on recent advances in the field of synapse-to-nucleus communication that include studies of the signal-regulated transcriptome in human neurons, identification of novel regulatory mechanisms such as activity-induced DNA double-strand breaks, and the identification of novel forms of activity- and transcription-dependent adaptations, in particular, metabolic plasticity. We summarize the reciprocal interactions between different kinds of neuroadaptations and highlight the emerging role of activity-regulated epigenetic modifiers in gating the inducibility of signal-regulated genes.

A microRNA signature of toxic extrasynaptic N-methyl-D-aspartate (NMDA) receptor signaling

The cellular consequences of N-Methyl-D-Aspartate receptor (NMDAR) stimulation depend on the receptors' subcellular localization. Synaptic NMDARs promote plasticity and survival whereas extrasynaptic NMDARs mediate excitotoxicity and contribute to cell death in neurodegenerative diseases. The mechanisms that couple activation of extrasynaptic NMDARs to cell death remain incompletely understood. We here show that activation of extrasynaptic NMDARs by bath application of NMDA or L-glutamate leads to the upregulation of a group of 19 microRNAs in cultured mouse hippocampal neurons. In contrast, none of these microRNAs is induced upon stimulation of synaptic activity. Increased microRNA expression depends on the pri-miRNA processing enzyme Drosha, but not on de novo gene transcription. These findings suggest that toxic NMDAR signaling involves changes in the expression levels of particular microRNAs.

VEGFD Protects Retinal Ganglion Cells and, consequently, Capillaries against Excitotoxic Injury

In the central nervous system, neurons and the vasculature influence each other. While it is well described that a functional vascular system is trophic to neurons and that vascular damage contributes to neurodegeneration, the opposite scenario in which neural damage might impact the microvasculature is less defined. In this study, using an in vivo excitotoxic approach in adult mice as a tool to cause specific damage to retinal ganglion cells, we detected subsequent damage to endothelial cells in retinal capillaries. Furthermore, we detected decreased expression of vascular endothelial growth factor D (VEGFD) in retinal ganglion cells. In vivo VEGFD supplementation via neuronal-specific viral-mediated expression or acute intravitreal delivery of the mature protein preserved the structural and functional integrity of retinal ganglion cells against excitotoxicity and, additionally, spared endothelial cells from degeneration. Viral-mediated suppression of expression of the VEGFD-binding receptor VEGFR3 in retinal ganglion cells revealed that VEGFD exerts its protective capacity directly on retinal ganglion cells, while protection of endothelial cells is the result of upheld neuronal integrity. These findings suggest that VEGFD supplementation might be a novel, clinically applicable approach for neuronal and vascular protection.

EphrinB2 regulates VEGFR2 during dendritogenesis and hippocampal circuitry development

Vascular endothelial growth factor (VEGF) is an angiogenic factor that play important roles in the nervous system, although it is still unclear which receptors transduce those signals in neurons. Here, we show that in the developing hippocampus VEGFR2 (also known as KDR or FLK1) is expressed specifically in the CA3 region and it is required for dendritic arborization and spine morphogenesis in hippocampal neurons. Mice lacking VEGFR2 in neurons (Nes-cre Kdr^lox/-) show decreased dendritic arbors and spines as well as a reduction in long-term potentiation (LTP) at the associational-commissural – CA3 synapses. Mechanistically, VEGFR2 internalization is required for VEGF-induced spine maturation. In analogy to endothelial cells, ephrinB2 controls VEGFR2 internalization in neurons. VEGFR2-ephrinB2 compound mice (Nes-cre Kdr^lox/+ Efnb2^lox/+) show reduced dendritic branching, reduced spine head size and impaired LTP. Our results demonstrate the functional crosstalk of VEGFR2 and ephrinB2 in vivo to control dendritic arborization, spine morphogenesis and hippocampal circuitry development.

Conditions of limited calcium influx (CLCI) inhibits IL2 induction and favors expression of anergy-related genes in TCR/CD3 and CD28 costimulated primary human T cells

Calcium is a key regulator of the T cell immune response. Depending on the spatial properties (nucleus versus cytoplasm) of the calcium signals generated after CD3xCD28 stimulation, primary human T cells either mount a productive immune response or develop tolerance. Nuclear calcium acts as a genomic decision maker: during T cell activation, it drives expression of genes associated with a productive immune response while in its absence, stimulated T cells acquire an anergy-like gene profile. Selective inhibition of nuclear calcium signaling in stimulated T cells blocks the productive immune response and directs the cells towards an anergy-like state. Here we show that the two transcriptional programs that include, respectively, the 'activation gene', interleukin 2 (IL2) and 'anergy-related genes', EGR2, EGR3, and CREM have different requirements for transmembrane calcium flux. By either lowering extracellular calcium concentrations with EGTA or using low concentrations of the ORAI blockers, BTP2 or RO2959, we reduced transmembrane calcium flux in human primary T cells stimulated with CD3xCD28. These 'conditions of limited calcium influx' (CLCI) blocked CD3xCD28-induced IL2 expression but only moderately affected induction of the anergy-related genes EGR2, EGR3, and CREM. We observed no difference in NFAT2 nuclear translocation after CD3xCD28 stimulation between normal conditions and CLCI. These results indicate that CLCI favors expression of anergy-related genes in activated human T cells. CLCI may be used to develop novel means for pro-tolerance immunosuppressive treatments.

Beneficial Effects of Fingolimod in Alzheimer's Disease: Molecular Mechanisms and Therapeutic Potential

Alzheimer's disease (AD), the most common cause of dementia remains of unclear etiology with current pharmacological therapies failing to halt disease progression. Several pathophysiological mechanisms have been implicated in AD pathogenesis including amyloid-β protein (Aβ) accumulation, tau hyperphosphorylation, neuroinflammation and alterations in bioactive lipid metabolism. Sphingolipids, such as sphingosine-1-phosphate (S1P) and intracellular ceramide/S1P balance are highly implicated in central nervous system physiology as well as in AD pathogenesis. FTY720/Fingolimod, a structural sphingosine analog and S1P receptor (S1PR) modulator that is currently used in the treatment of relapsing-remitting multiple sclerosis (RRMS) has been shown to exert beneficial effects on AD progression. Recent in vitro and in vivo evidence indicate that fingolimod may suppress Aβ secretion and deposition, inhibit apoptosis and enhance brain-derived neurotrophic factor (BDNF) production. Furthermore, it regulates neuroinflammation, protects against N-methyl-D-aspartate (NMDA)-excitotoxicity and modulates receptor for advanced glycation end products signaling axis that is highly implicated in AD pathogenesis. This review discusses the underlying molecular mechanisms of the emerging neuroprotective role of fingolimod in AD and its therapeutic potential, aiming to shed more light on AD pathogenesis as well as direct future treatment strategies.

Pleiotropic neuroprotective effects of taxifolin in cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) results from amyloid-β deposition in the cerebrovasculature. It is frequently accompanied by Alzheimer's disease and causes dementia. We recently demonstrated that in a mouse model of CAA, taxifolin improved cerebral blood flow, promoted amyloid-β removal from the brain, and prevented cognitive dysfunction when administered orally. Here we showed that taxifolin inhibited the intracerebral production of amyloid-β through suppressing the ApoE-ERK1/2-amyloid-β precursor protein axis, despite the low permeability of the blood-brain barrier to taxifolin. Higher expression levels of triggering receptor expressed on myeloid cell 2 (TREM2) were associated with the exacerbation of inflammation in the brain. Taxifolin suppressed inflammation, alleviating the accumulation of TREM2-expressing cells in the brain. It also mitigated glutamate levels and oxidative tissue damage and reduced brain levels of active caspases, indicative of apoptotic cell death. Thus, the oral administration of taxifolin had intracerebral pleiotropic neuroprotective effects on CAA through suppressing amyloid-β production and beneficially modulating proinflammatory microglial phenotypes.

Increased Expression of Vascular Endothelial Growth Factor-D Following Brain Injury

Alterations in the expression of the vascular endothelial growth factors (VEGF) A and B occur during blood–brain barrier (BBB) breakdown and angiogenesis following brain injury. In this study, the temporal and spatial expression of VEGF-D and VEGF receptors-2 and -3 (VEGFR-2 and VEGFR-3, respectively) was determined at the mRNA and protein level in the rat cortical cold-injury model over a period of 0.5 to 6 days post-injury. In order to relate endothelial VEGF-D protein expression with BBB breakdown, dual labeling immunofluorescence was performed using antibodies to VEGF-D and to fibronectin, a marker of BBB breakdown. In control rats, VEGF-D signal was only observed in scattered perivascular macrophages in the cerebral cortex. The upregulation of VEGF-D mRNA expression was observed in the injury site between days 0.5 to 4, coinciding with the period of BBB breakdown and angiogenesis. At the protein level, intracerebral vessels with BBB breakdown to fibronectin in the lesion on days 0.5 to 4 failed to show endothelial VEGF-D. Between days 0.5 to 6, an increased VEGF-D immunoreactivity was noted in the endothelium of pial vessels overlying the lesion site, in neutrophils, macrophages, and free endothelial cells within the lesion. The upregulation of VEGFR-2 and -3 mRNA and protein expression was observed early post-injury on day 0.5. Although there was concurrent expression of VEGF-A, VEGF-B, and VEGF-D post-injury, differences in their spatial expression during BBB breakdown and angiogenesis suggest that they have specific and separate roles in these processes.

Epigenetic control of hypersensitivity in chronic inflammatory pain by the de novo DNA methyltransferase Dnmt3a2

Chronic pain is a pathological manifestation of neuronal plasticity supported by altered gene transcription in spinal cord neurons that results in long-lasting hypersensitivity. Recently, the concept that epigenetic regulators might be important in pathological pain has emerged, but a clear understanding of the molecular players involved in the process is still lacking. In this study, we linked Dnmt3a2, a synaptic activity-regulated de novo DNA methyltransferase, to chronic inflammatory pain. We observed that Dnmt3a2 levels are increased in the spinal cord of adult mice following plantar injection of Complete Freund's Adjuvant, an in vivo model of chronic inflammatory pain. In vivo knockdown of Dnmt3a2 expression in dorsal horn neurons blunted the induction of genes triggered by Complete Freund's Adjuvant injection. Among the genes whose transcription was found to be influenced by Dnmt3a2 expression in the spinal cord is Ptgs2, encoding for Cox-2, a prime mediator of pain processing. Lowering the levels of Dnmt3a2 prevented the establishment of long-lasting inflammatory hypersensitivity. These results identify Dnmt3a2 as an important epigenetic regulator needed for the establishment of central sensitization. Targeting expression or function of Dnmt3a2 may be suitable for the treatment of chronic pain.

Synaptic Activity Protects Neurons Against Calcium-Mediated Oxidation and Contraction of Mitochondria During Excitotoxicity

AIMS

Excitotoxicity triggered by extrasynaptic N-methyl-d-aspartate-type glutamate receptors has been implicated in many neurodegenerative conditions, including Alzheimer's disease, Huntington's disease, amyotrophic lateral sclerosis, and stroke. Mitochondrial calcium overload leading to mitochondrial dysfunction represents an early event in excitotoxicity. Neurons are rendered resistant to excitotoxicity by previous periods of synaptic activity that activates a nuclear calcium-driven neuroprotective gene program. This process, termed acquired neuroprotection, involves transcriptional repression of the mitochondrial calcium uniporter leading to a reduction in excitotoxcity-associated mitochondrial calcium load. As mitochondrial calcium and the production of reactive oxygen species may be linked, we monitored excitotoxicity-associated changes in the mitochondrial redox status using the ratiometric glutathione redox potential indicator, glutaredoxin 1 (GRX1)-redox-sensitive green fluorescent protein (roGFP)2, targeted to the mitochondrial matrix. Aim of this study was to investigate if suppression of oxidative stress underlies mitoprotection afforded by synaptic activity.

RESULTS

We found that synaptic activity protects primary rat hippocampal neurons against acute excitotoxicity-induced mitochondrial oxidative stress and mitochondrial contraction associated with it. Downregulation of the mitochondrial uniporter by genetic means mimics the protective effect of synaptic activity on mitochondrial redox status. These findings indicate that oxidative stress acts downstream of mitochondrial calcium overload in excitotoxicity. Innovation and Conclusion: We established mito-GRX1-roGFP2 as a reliable and sensitive tool to monitor rapid redox changes in mitochondria during excitotoxicity. Our results highlight the importance of developing means of blocking mitochondrial calcium overload for therapeutic targeting of oxidative stress and mitochondrial dysfunction in neurodegenerative diseases. Antioxid. Redox. Signal. 29, 1109-1124.

Histone deacetylase 4 shapes neuronal morphology via a mechanism involving regulation of expression of vascular endothelial growth factor D

Nucleo-cytoplasmic shuttling of class IIa histone deacetylases (i.e HDAC4, -5, -7, and -9) is a synaptic activity- and nuclear calcium-dependent mechanism important for epigenetic regulation of signal-regulated gene expression in hippocampal neurons. HDAC4 in particular has been linked to the regulation of genes important for both synaptic structure and plasticity. Here, using a constitutively nuclear-localized, dominant-active variant of HDAC4 (HDAC4 3SA), we demonstrate that HDAC4 accumulation in the nucleus severely reduces both the length and complexity of dendrites of cultured mature hippocampal neurons, but does not affect the number of dendritic spines. This phenomenon appeared to be specific to HDAC4, as increasing the expression of HDAC3 or HDAC11, belonging to class I and class IV HDACs, respectively, did not alter dendritic architecture. We also show that HDAC4 3SA decreases the expression of vascular endothelial growth factor D (VEGFD), a key protein required for the maintenance of dendritic arbors. The expression of other members of the VEGF family and their receptors was not affected by the nuclear accumulation of HDAC4. VEGFD overexpression or administration of recombinant VEGFD, but not VEGFC, the closest VEGFD homologue, rescued the impaired dendritic architecture caused by the nuclear-localized HDAC4 variant. These results identify HDAC4 as an epigenetic regulator of neuronal morphology that controls dendritic arborization via the expression of VEGFD.

Neuroprotective effect of acute ethanol intoxication in TBI is associated to the hierarchical modulation of early transcriptional responses

Ethanol intoxication is a risk factor for traumatic brain injury (TBI) but clinical evidence suggests that it may actually improve the prognosis of intoxicated TBI patients. We have employed a closed, weight-drop TBI model of different severity (2cm or 3cm falling height), preceded (-30min) or followed (+20min) by ethanol administration (5g/Kg). This protocol allows us to study the interaction of binge ethanol intoxication in TBI, monitoring behavioral changes, histological responses and the transcriptional regulation of a series of activity-regulated genes (immediate early genes, IEGs). We demonstrate that ethanol pretreatment before moderate TBI (2cm) significantly reduces neurological impairment and accelerates recovery. In addition, better preservation of neuronal numbers and cFos+cells was observed 7days after TBI. At transcriptional level, ethanol reduced the upregulation of a subset of IEGs encoding for transcription factors such as Atf3, c-Fos, FosB, Egr1, Egr3 and Npas4 but did not affect the upregulation of others (e.g. Gadd45b and Gadd45c). While a subset of IEGs encoding for effector proteins (such as Bdnf, InhbA and Dusp5) were downregulated by ethanol, others (such as Il-6) were unaffected. Notably, the majority of genes were sensitive to ethanol only when administered before TBI and not afterwards (the exceptions being c-Fos, Egr1 and Dusp5). Furthermore, while severe TBI (3cm) induced a qualitatively similar (but quantitatively larger) transcriptional response to moderate TBI, it was no longer sensitive to ethanol pretreatment. Thus, we have shown that a subset of the TBI-induced transcriptional responses were sensitive to ethanol intoxication at the instance of trauma (ultimately resulting in beneficial outcomes) and that the effect of ethanol was restricted to a certain time window (pre TBI treatment) and to TBI severity (moderate). This information could be critical for the translational value of ethanol in TBI and for the design of clinical studies aimed at disentangling the role of ethanol intoxication in TBI.

Emerging Roles for VEGF-D in Human Disease

Blood vessels and lymphatic vessels are located in many tissues and organs throughout the body, and play important roles in a wide variety of prevalent diseases in humans. Vascular endothelial growth factor-D (VEGF-D) is a secreted protein that can promote the remodeling of blood vessels and lymphatics in development and disease. Recent fundamental and translational studies have provided insight into the molecular mechanisms by which VEGF-D exerts its effects in human disease. Hence this protein is now of interest as a therapeutic and/or diagnostic target, or as a potential therapeutic agent, in a diversity of indications in cardiovascular medicine, cancer and the devastating pulmonary condition lymphangioleiomyomatosis. This has led to clinical trial programs to assess the effect of targeting VEGF-D signaling pathways, or delivering VEGF-D, in angina, cancer and ocular indications. This review summarizes our understanding of VEGF-D signaling in human disease, which is largely based on animal disease models and clinicopathological studies, and provides information about the outcomes of recent clinical trials testing agonists or antagonists of VEGF-D signaling.

Growth Factor Proteins and Treatment-Resistant Depression: A Place on the Path to Precision

Background: Since the neurotrophic hypothesis of depression was formulated, conflicting results have been reported regarding the role of growth factor proteins in depressed patients, including whether there are state or trait alterations found in patients compared to controls and whether they represent predictors of treatment response. Recently it has been hypothesized that heterogeneity of findings within this literature might be partly explained by participants' history of treatment-resistant depression. This study aimed to investigate the role of growth factor proteins in patients with treatment-resistant depression (TRD) undergoing an inpatient intervention. Methods: Blood samples were collected from 36 patients with TRD and 36 matched controls. Patients were assessed both at admission and discharge from a specialist inpatient program. We examined serum biomarker differences between patients and non-depressed matched controls, longitudinal changes after inpatient treatment and relationship to clinical outcomes. Additionally, the influence of potential covariates on biomarker levels were assessed. Results: Patients displayed lower serum levels of brain-derived neurotrophic factor (OR = 0.025; 95% CI = 0.001, 0.500) and vascular endothelial growth factor-C (VEGFC; OR = 0.083, 95% CI = 0.008, 0.839) as well as higher angiopoietin-1 receptor (Tie2; OR = 2.651, 95% CI = 1.325, 5.303) compared to controls. Patients were stratified into responders (56%) and non-responders (44%). Lower VEGFD levels at admission predicted subsequent non-response (OR = 4.817, 95% CI = 1.247, 11.674). During treatment, non-responders showed a decrease in VEGF and VEGFC levels, while responders showed no significant changes. Conclusion: TRD patients demonstrate a deficit of peripheral growth factors and our results suggest that markers of the VEGF family might decline over time in chronically depressed patients in spite of multidisciplinary treatment. The action of angiogenic proteins may play an important role in the pathophysiology of TRD, and pending comprehensive investigation may provide important insights for the future of precision psychiatry.

Interfacing Neural Network Components and Nucleic Acids

Translating neural activity into nucleic acid modifications in a controlled manner harbors unique advantages for basic neurobiology and bioengineering. It would allow for a new generation of biological computers that store output in ultra-compact and long-lived DNA and enable the investigation of animal nervous systems at unprecedented scales. Furthermore, by exploiting the ability of DNA to precisely influence neuronal activity and structure, it could be possible to more effectively create cellular therapy approaches for psychiatric diseases that are currently difficult to treat.

Tau-Induced Ca2+/Calmodulin-Dependent Protein Kinase-IV Activation Aggravates Nuclear Tau Hyperphosphorylation

Hyperphosphorylated tau is the major protein component of neurofibrillary tangles in the brains of patients with Alzheimer's disease (AD). However, the mechanism underlying tau hyperphosphorylation is not fully understood. Here, we demonstrated that exogenously expressed wild-type human tau40 was detectable in the phosphorylated form at multiple AD-associated sites in cytoplasmic and nuclear fractions from HEK293 cells. Among these sites, tau phosphorylated at Thr205 and Ser214 was almost exclusively found in the nuclear fraction at the conditions used in the present study. With the intracellular tau accumulation, the Ca²⁺ concentration was significantly increased in both cytoplasmic and nuclear fractions. Further studies using site-specific mutagenesis and pharmacological treatment demonstrated that phosphorylation of tau at Thr205 increased nuclear Ca²⁺ concentration with a simultaneous increase in the phosphorylation of Ca²⁺/calmodulin-dependent protein kinase IV (CaMKIV) at Ser196. On the other hand, phosphorylation of tau at Ser214 did not significantly change the nuclear Ca²⁺/CaMKIV signaling. Finally, expressing calmodulin-binding protein-4 that disrupts formation of the Ca²⁺/calmodulin complex abolished the okadaic acid-induced tau hyperphosphorylation in the nuclear fraction. We conclude that the intracellular accumulation of phosphorylated tau, as detected in the brains of AD patients, can trigger nuclear Ca²⁺/CaMKIV signaling, which in turn aggravates tau hyperphosphorylation. Our findings provide new insights for tauopathies: hyperphosphorylation of intracellular tau and an increased Ca²⁺ concentration may induce a self-perpetuating harmful loop to promote neurodegeneration.

Reciprocal Interaction of Dendrite Geometry and Nuclear Calcium-VEGFD Signaling Gates Memory Consolidation and Extinction

Nuclear calcium is an important signaling end point in synaptic excitation-transcription coupling that is critical for long-term neuroadaptations. Here, we show that nuclear calcium acting via a target gene, VEGFD, is required for hippocampus-dependent fear memory consolidation and extinction in mice. Nuclear calcium-VEGFD signaling upholds the structural integrity and complexity of the dendritic arbor of CA1 neurons that renders those cells permissive for the efficient generation of synaptic input-evoked nuclear calcium transients driving the expression of plasticity-related genes. Therefore, the gating of memory functions rests on the reciprocally reinforcing maintenance of an intact dendrite geometry and a functional synapse-to-nucleus communication axis. In psychiatric and neurodegenerative disorders, therapeutic application of VEGFD may help to stabilize dendritic structures and network connectivity, which may prevent cognitive decline and could boost the efficacy of extinction-based exposure therapies.SIGNIFICANCE STATEMENT This study uncovers a reciprocal relationship between dendrite geometry, the ability to generate nuclear calcium transients in response to synaptic inputs, and the subsequent induction of expression of plasticity-related and dendritic structure-preserving genes. Insufficient nuclear calcium signaling in CA1 hippocampal neurons and, consequently, reduced expression of the nuclear calcium target gene VEGFD, a dendrite maintenance factor, leads to reduced-complexity basal dendrites of CA1 neurons, which severely compromises the animals' consolidation of both memory and extinction memory. The structure-protective function of VEGFD may prove beneficial in psychiatric disorders as well as neurodegenerative and aging-related conditions that are associated with loss of neuronal structures, dysfunctional excitation-transcription coupling, and cognitive decline.

BAF53b, a Neuron-Specific Nucleosome Remodeling Factor, Is Induced after Learning and Facilitates Long-Term Memory Consolidation

Although epigenetic mechanisms of gene expression regulation have recently been implicated in memory consolidation and persistence, the role of nucleosome-remodeling is largely unexplored. Recent studies show that the functional loss of BAF53b, a postmitotic neuron-specific subunit of the BAF nucleosome-remodeling complex, results in the deficit of consolidation of hippocampus-dependent memory and cocaine-associated memory in the rodent brain. However, it is unclear whether BAF53b expression is regulated during memory formation and how BAF53b regulates fear memory in the amygdala, a key brain site for fear memory encoding and storage. To address these questions, we used viral vector approaches to either decrease or increase BAF53b function specifically in the lateral amygdala of adult mice in auditory fear conditioning paradigm. Knockdown of Baf53b before training disrupted long-term memory formation with no effect on short-term memory, basal synaptic transmission, and spine structures. We observed in our qPCR analysis that BAF53b was induced in the lateral amygdala neurons at the late consolidation phase after fear conditioning. Moreover, transient BAF53b overexpression led to persistently enhanced memory formation, which was accompanied by increase in thin-type spine density. Together, our results provide the evidence that BAF53b is induced after learning, and show that such increase of BAF53b level facilitates memory consolidation likely by regulating learning-related spine structural plasticity.SIGNIFICANCE STATEMENT Recent works in the rodent brain begin to link nucleosome remodeling-dependent epigenetic mechanism to memory consolidation. Here we show that BAF53b, an epigenetic factor involved in nucleosome remodeling, is induced in the lateral amygdala neurons at the late phase of consolidation after fear conditioning. Using specific gene knockdown or overexpression approaches, we identify the critical role of BAF53b in the lateral amygdala neurons for memory consolidation during long-term memory formation. Our results thus provide an idea about how nucleosome remodeling can be regulated during long-term memory formation and contributes to the permanent storage of associative fear memory in the lateral amygdala, which is relevant to fear and anxiety-related mental disorders.