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

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.

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.

Dysregulation of Npas4 and Inhba expression and an altered excitation-inhibition balance are associated with cognitive deficits in DBA/2 mice

Differences in the learning associated transcriptional profiles between mouse strains with distinct learning abilities could provide insight into the molecular basis of learning and memory. The inbred mouse strain DBA/2 shows deficits in hippocampus-dependent memory, yet the transcriptional responses to learning and the underlying mechanisms of the impairments are unknown. Comparing DBA/2J mice with the reference standard C57BL/6N mouse strain we verify an enhanced susceptibility to kainic acid induced seizures, confirm impairments in hippocampus-dependent spatial memory tasks and uncover additional behavioral abnormalities including deficits in hippocampus-independent learning. Surprisingly, we found no broad dysfunction of the DBA/2J strain in immediate early gene (IEG) activation but instead report brain region-specific and gene-specific alterations. The learning-associated IEGs Arc, c-Fos, and Nr4a1 showed no DBA/2J deficits in basal or synaptic activity induced gene expression in hippocampal or cortical primary neuronal cultures or in the CA1, CA3, or retrosplenial cortex following spatial object recognition (SOR) training in vivo. However, the parietal cortex showed reduced and the dentate gyrus showed enhanced SOR-evoked induction of most IEGs. All DBA/2J hippocampal regions exhibited elevated basal expression of inhibin β A (Inhba) and a learning-associated superinduction of the transcription factor neuronal Per-Arnt-Sim domain protein 4 (Npas4) known to regulate the synaptic excitation-inhibition balance. In line with this, CA1 pyramidal neurons of DBA/2J mice showed fewer inhibitory and more excitatory miniature postsynaptic currents but no alteration in most other electrophysiological properties or gross dendritic morphology. The dysregulation of Npas4 and Inhba expression and synaptic connectivity may underlie the cognitive deficits and increased susceptibility to seizures of DBA/2J mice.

Npas4 regulates medium spiny neuron physiology and gates cocaine-induced hyperlocomotion

We show here that the transcription factor Npas4 is an important regulator of medium spiny neuron spine density and electrophysiological parameters and that it determines the magnitude of cocaine-induced hyperlocomotion in mice. Npas4 is induced by synaptic stimuli that cause calcium influx, but not dopaminergic or PKA-stimulating input, in mouse medium spiny neurons and human iPSC-derived forebrain organoids. This induction is independent of ubiquitous kinase pathways such as PKA and MAPK cascades, and instead depends on calcineurin and nuclear calcium signalling. Npas4 controls a large regulon containing transcripts for synaptic molecules, such as NMDA receptors and VDCC subunits, and determines in vivo MSN spine density, firing rate, I/O gain function and paired-pulse facilitation. These functions at the molecular and cellular levels control the locomotor response to drugs of abuse, as Npas4 knockdown in the nucleus accumbens decreases hyperlocomotion in response to cocaine in male mice while leaving basal locomotor behaviour unchanged.

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.

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.

Soluble CD163 Changes Indicate Monocyte Association With Cognitive Deficits in Parkinson's Disease

BACKGROUND

Parkinson's disease (PD) is a neurodegenerative disorder with a significant immune component, as demonstrated by changes in immune biomarkers in patients' biofluids. However, which specific cells are responsible for those changes is unclear because most immune biomarkers can be produced by various cell types.

OBJECTIVES

The aim of this study was to explore monocyte involvement in PD.

METHODS

We investigated the monocyte-specific biomarker sCD163, the soluble form of the receptor CD163, in cerebrospinal fluid (CSF) and serum in two experiments, and compared it with other biomarkers and clinical data. Potential connections between CD163 and alpha-synuclein were studied in vitro.

RESULTS

CSF-sCD163 increased in late-stage PD and correlated with the PD biomarkers alpha-synuclein, Tau, and phosphorylated Tau, whereas it inversely correlated with the patients' cognitive scores, supporting monocyte involvement in neurodegeneration and cognition in PD. Serum-sCD163 increased only in female patients, suggesting a sex-distinctive monocyte response. CSF-sCD163 also correlated with molecules associated with adaptive and innate immune system activation and with immune cell recruitment to the brain. Serum-sCD163 correlated with proinflammatory cytokines and acute-phase proteins, suggesting a relation to chronic systemic inflammation. Our in vitro study showed that alpha-synuclein activates macrophages and induces shedding of sCD163, which in turn enhances alpha-synuclein uptake by myeloid cells, potentially participating in its clearance.

CONCLUSIONS

Our data present sCD163 as a potential cognition-related biomarker in PD and suggest a role for monocytes in both peripheral and brain immune responses. This may be directly related to alpha-synuclein's proinflammatory capacity but could also have consequences for alpha-synuclein processing. © 2020 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

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.

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.

Neuronal ensemble-specific DNA methylation strengthens engram stability

Memories are encoded by memory traces or engrams, represented within subsets of neurons that are synchronously activated during learning. However, the molecular mechanisms that drive engram stabilization during consolidation and consequently ensure its reactivation by memory recall are not fully understood. In this study we manipulate, during memory consolidation, the levels of the de novo DNA methyltransferase 3a2 (Dnmt3a2) selectively within dentate gyrus neurons activated by fear conditioning. We found that Dnmt3a2 upregulation enhances memory performance in mice and improves the fidelity of reconstitution of the original neuronal ensemble upon memory retrieval. Moreover, similar manipulation in a sparse, non-engram subset of neurons does not bias engram allocation or modulate memory strength. We further show that neuronal Dnmt3a2 overexpression changes the DNA methylation profile of synaptic plasticity-related genes. Our data implicates DNA methylation selectively within neuronal ensembles as a mechanism of stabilizing engrams during consolidation that supports successful memory retrieval.

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.

Epigenetic regulation of immediate-early gene Nr4a2/Nurr1 in the medial habenula during reinstatement of cocaine-associated behavior

Propensity to relapse following long periods of abstinence is a key feature of substance use disorder. Drugs of abuse, such as cocaine, cause long-term changes in the neural circuitry regulating reward, motivation, and memory processes through dysregulation of various molecular mechanisms, including epigenetic regulation of activity-dependent gene expression. Underlying drug-induced changes to neural circuit function are the molecular mechanisms regulating activity-dependent gene expression. Of note, histone acetyltransferases and histone deacetylases (HDACs), powerful epigenetic regulators of gene expression, are dysregulated following both acute and chronic cocaine exposure and are linked to cocaine-induced changes in neural circuit function. To better understand the effect of drug-induced changes on epigenetic function and behavior, we investigated HDAC3-mediated regulation of Nr4a2/Nurr1 in the medial habenula, an understudied pathway in cocaine-associated behaviors. Nr4a2, a transcription factor critical in cocaine-associated behaviors and necessary for MHb development, is enriched in the cholinergic cell-population of the MHb; yet, the role of NR4A2 within the MHb in the adult brain remains elusive. Here, we evaluated whether epigenetic regulation of Nr4a2 in the MHb has a role in reinstatement of cocaine-associated behaviors. We found that HDAC3 disengages from Nr4a2 in the MHb in response to cocaine-primed reinstatement. Whereas enhancing HDAC3 function in the MHb had no effect on reinstatement, we found, using a dominant-negative splice variant (NURR2C), that loss of NR4A2 function in the MHb blocked reinstatement behaviors. These results show for the first time that regulation of NR4A2 function in the MHb is critical in relapse-like behaviors.

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.

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.

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.

Lineage divergence of activity-driven transcription and evolution of cognitive ability

Excitation-transcription coupling shapes network formation during brain development and controls neuronal survival, synaptic function and cognitive skills in the adult. New studies have uncovered differences in the transcriptional responses to synaptic activity between humans and mice. These differences are caused both by the emergence of lineage-specific activity-regulated genes and by the acquisition of signal-responsive DNA elements in gene regulatory regions that determine whether a gene can be transcriptionally induced by synaptic activity or alter the extent of its inducibility. Such evolutionary divergence may have contributed to lineage-related advancements in cognitive abilities.