Very low density lipoprotein receptor regulates dendritic spine formation in a RasGRF1/CaMKII dependent manner

Upregulation of RasGRF1 ameliorates spatial cognitive dysfunction in mice after chronic cerebral hypoperfusion

Chronic cerebral hypoperfusion (CCH)-mediated cognitive impairment is a serious problem worldwide. However, given its complexity, the underlying mechanisms by which CCH induces cognitive dysfunction remain unclear, resulting in a lack of effective treatments. In this study, we aimed to determine whether changes in the expression of RasGRF1, an important protein associated with cognition and synaptic plasticity, underlie the associated impairments in cognition after CCH. We found that RasGRF1 levels markedly decreased following CCH. Through prediction and validation studies, we observed that miRNA-323-3p was upregulated after CCH and could bind to the 3'-untranslated region of Rasgrf1 mRNA and regulate its expression in vitro. Moreover, the inhibition of miRNA-323-3p upregulated Rasgrf1 expression in the hippocampus after CCH, which was reversed by Rasgrf1 siRNA. This suggests that miRNA-323-3p is an important regulator of Rasgrf1. The Morris water maze and Y maze tests showed that miRNA-323-3p inhibition and Rasgrf1 upregulation improved spatial learning and memory, and electrophysiological measurements revealed deficits in long-term potentiation after CCH that were reversed by Rasgrf1 upregulation. Dendritic spine density and mature mushroom spine density were also improved after miRNA-323-3p inhibition and Rasgrf1 upregulation. Furthermore, Rasgrf1 upregulation by miRNA-323-3p inhibition improved dendritic spine density and mature mushroom spine density and ameliorated the deterioration of synapses and postsynaptic density. Overall, RasGRF1 regulation attenuated cognitive impairment, helped maintain structural and functional synaptic plasticity, and prevented synapse deterioration after CCH. These results suggest that Rasgrf1 downregulation by miRNA-323-3p plays an important role in cognitive impairment after CCH. Thus, RasGRF1 and miRNA-323-3p may represent potential therapeutic targets for cognitive impairment after CCH.

Cancer and Alzheimer's Inverse Correlation: an Immunogenetic Analysis

Numerous studies have demonstrated an inverse link between cancer and Alzheimer's disease (AD), with data suggesting that people with Alzheimer's have a decreased risk of cancer and vice versa. Although other studies have investigated mechanisms to explain this relationship, the connection between these two diseases remains largely unexplained. Processes seen in cancer, such as decreased apoptosis and increased cell proliferation, seem to be reversed in AD. Given the need for effective therapeutic strategies for AD, comparisons with cancer could yield valuable insights into the disease process and perhaps result in new treatments. Here, through a review of existing literature, we compared the expressions of genes involved in cell proliferation and apoptosis to establish a genetic basis for the reciprocal association between AD and cancer. We discuss an array of genes involved in the aforementioned processes, their relevance to both diseases, and how changes in those genes produce varying effects in either disease.

Bidirectional Control between Cholesterol Shuttle and Purine Signal at the Central Nervous System

Recent studies have highlighted the mechanisms controlling the formation of cerebral cholesterol, which is synthesized in situ primarily by astrocytes, where it is loaded onto apolipoproteins and delivered to neurons and oligodendrocytes through interactions with specific lipoprotein receptors. The “cholesterol shuttle” is influenced by numerous proteins or carbohydrates, which mainly modulate the lipoprotein receptor activity, function and signaling. These molecules, provided with enzymatic/proteolytic activity leading to the formation of peptide fragments of different sizes and specific sequences, could be also responsible for machinery malfunctions, which are associated with neurological, neurodegenerative and neurodevelopmental disorders. In this context, we have pointed out that purines, ancestral molecules acting as signal molecules and neuromodulators at the central nervous system, can influence the homeostatic machinery of the cerebral cholesterol turnover and vice versa. Evidence gathered so far indicates that purine receptors, mainly the subtypes P2Y2, P2X7 and A2A, are involved in the pathogenesis of neurodegenerative diseases, such as Alzheimer’s and Niemann–Pick C diseases, by controlling the brain cholesterol homeostasis; in addition, alterations in cholesterol turnover can hinder the purine receptor function. Although the precise mechanisms of these interactions are currently poorly understood, the results here collected on cholesterol–purine reciprocal control could hopefully promote further research.

Retinoic acid-induced protein 14 controls dendritic spine dynamics associated with depressive-like behaviors

Dendritic spines are the central postsynaptic machinery that determines synaptic function. The F-actin within dendritic spines regulates their dynamic formation and elimination. Rai14 is an F-actin-regulating protein with a membrane-shaping function. Here, we identified the roles of Rai14 for the regulation of dendritic spine dynamics associated with stress-induced depressive-like behaviors. Rai14-deficient neurons exhibit reduced dendritic spine density in the Rai14^+/- mouse brain, resulting in impaired functional synaptic activity. Rai14 was protected from degradation by complex formation with Tara, and accumulated in the dendritic spine neck, thereby enhancing spine maintenance. Concurrently, Rai14 deficiency in mice altered gene expression profile relevant to depressive conditions and increased depressive-like behaviors. Moreover, Rai14 expression was reduced in the prefrontal cortex of the mouse stress model, which was blocked by antidepressant treatment. Thus, we propose that Rai14-dependent regulation of dendritic spines may underlie the plastic changes of neuronal connections relevant to depressive-like behaviors.

New Strategies for the Treatment of Neuropsychiatric Disorders Based on Reelin Dysfunction

Reelin is an extracellular matrix protein that is mainly produced in Cajal-Retzius cells and controls neuronal migration, which is important for the proper formation of cortical layers in the developmental stage of the brain. In the adult brain, Reelin plays a crucial role in the regulation of N-methyl-D-aspartate receptor-dependent synaptic function, and its expression decreases postnatally. Clinical studies showed reductions in Reelin protein and mRNA expression levels in patients with psychiatric disorders; however, the causal relationship remains unclear. Reelin-deficient mice exhibit an abnormal neuronal morphology and behavior, while Reelin supplementation ameliorates learning deficits, synaptic dysfunctions, and spine loss in animal models with Reelin deficiency. These findings suggest that the neuronal deficits and brain dysfunctions associated with the down-regulated expression of Reelin are attenuated by enhancements in its expression and functions in the brain. In this review, we summarize findings on the role of Reelin in neuropsychiatric disorders and discuss potential therapeutic approaches for neuropsychiatric disorders associated with Reelin dysfunctions.

Reelin Functions, Mechanisms of Action and Signaling Pathways During Brain Development and Maturation

During embryonic development and adulthood, Reelin exerts several important functions in the brain including the regulation of neuronal migration, dendritic growth and branching, dendritic spine formation, synaptogenesis and synaptic plasticity. As a consequence, the Reelin signaling pathway has been associated with several human brain disorders such as lissencephaly, autism, schizophrenia, bipolar disorder, depression, mental retardation, Alzheimer’s disease and epilepsy. Several elements of the signaling pathway are known. Core components, such as the Reelin receptors very low-density lipoprotein receptor (VLDLR) and Apolipoprotein E receptor 2 (ApoER2), Src family kinases Src and Fyn, and the intracellular adaptor Disabled-1 (Dab1), are common to most but not all Reelin functions. Other downstream effectors are, on the other hand, more specific to defined tasks. Reelin is a large extracellular protein, and some aspects of the signal are regulated by its processing into smaller fragments. Rather than being inhibitory, the processing at two major sites seems to be fulfilling important physiological functions. In this review, I describe the various cellular events regulated by Reelin and attempt to explain the current knowledge on the mechanisms of action. After discussing the shared and distinct elements of the Reelin signaling pathway involved in neuronal migration, dendritic growth, spine development and synaptic plasticity, I briefly outline the data revealing the importance of Reelin in human brain disorders.

Fyn Tyrosine Kinase as Harmonizing Factor in Neuronal Functions and Dysfunctions

Fyn is a non-receptor or cytoplasmatic tyrosine kinase (TK) belonging to the Src family kinases (SFKs) involved in multiple transduction pathways in the central nervous system (CNS) including synaptic transmission, myelination, axon guidance, and oligodendrocyte formation. Almost one hundred years after the original description of Fyn, this protein continues to attract extreme interest because of its multiplicity of actions in the molecular signaling pathways underlying neurodevelopmental as well as neuropathologic events. This review highlights and summarizes the most relevant recent findings pertinent to the role that Fyn exerts in the brain, emphasizing aspects related to neurodevelopment and synaptic plasticity. Fyn is a common factor in healthy and diseased brains that targets different proteins and shapes different transduction signals according to the neurological conditions. We will primarily focus on Fyn-mediated signaling pathways involved in neuronal differentiation and plasticity that have been subjected to considerable attention lately, opening the fascinating scenario to target Fyn TK for the development of potential therapeutic interventions for the treatment of CNS injuries and certain neurodegenerative disorders like Alzheimer’s disease.

Immunoregulatory effects of very low density lipoprotein from healthy individuals and metabolic syndrome patients on glial cells

Epidemiological studies have reported that elderly patients with metabolic syndrome (MetS) are significantly more likely to develop neuronal degenerative diseases than those without MetS. Our previous study showed that patients with MetS had significantly higher levels of negatively charged very low density lipoproteins (VLDLs) in the plasma than healthy controls. Highly electronegative VLDL is a key risk factor for endothelial dysfunction and atrial fibrillation. However, the impact of negatively charged VLDL in brain immunity remains unclear. In this study, VLDLs were isolated from normal healthy (nVLDL) individuals or patients with MetS (metVLDL). Primary astroglia and microglia mixed cell cultures as well as microglial-enriched cultures were used to test the effects of VLDLs. Microglia/astroglia activation as evidenced by their morphological changes and production of pro-inflammatory factors, such as tumor necrosis factor-α (TNF-α) and prostaglandin E2 (PGE2), were assessed by immunofluorescence staining and ELISA, respectively. Our results showed that metVLDLs mainly act on the microglia, and not the astroglia, with low concentration (0.05-0.5 μg/mL) inducing cell morphological changes and decreased cellular processes in the microglia. However, nVLDL treatment at these concentrations had no effects on microglia and astroglia. Most importantly, TNF-α and PGE2 levels significantly increased in the microglia treated with metVLDL via a dose-dependent manner. Together, our data indicate that metVLDLs can contribute to MetS-associated brain disorders through microglia activation and neuroinflammation.

RIP at the Synapse and the Role of Intracellular Domains in Neurons

Regulated intramembrane proteolysis (RIP) occurs in a cell when transmembrane proteins are cleaved by intramembrane proteases such as secretases to generate soluble protein fragments in the extracellular environment and the cytosol. In the cytosol, these soluble intracellular domains (ICDs) have local functions near the site of cleavage or in many cases, translocate to the nucleus to modulate gene expression. While the mechanism of RIP is relatively well studied, the fate and function of ICDs for most substrate proteins remain poorly characterized. In neurons, RIP occurs in various subcellular compartments including at the synapse. In this review, we summarize current research on RIP in neurons, focusing specifically on synaptic proteins where the presence and function of the ICDs have been reported. We also briefly discuss activity-driven processing of RIP substrates at the synapse and the cellular machinery that support long-distance transport of ICDs from the synapse to the nucleus. Finally, we describe future challenges in this field of research in the context of understanding the contribution of ICDs in neuronal function.

Reelin is modulated by diet-induced obesity and has direct actions on arcuate proopiomelanocortin neurons

Objective

Reelin (RELN) is a large glycoprotein involved in synapse maturation and neuronal organization throughout development. Deficits in RELN signaling contribute to multiple psychological disorders, such as autism spectrum disorder, schizophrenia, and bipolar disorder. Nutritional stress alters RELN expression in brain regions associated with these disorders; however, the involvement of RELN in the neural circuits involved in energy metabolism is unknown. The RELN receptors apolipoprotein E receptor 2 (ApoER2) and very low-density lipoprotein receptor (VLDLR) are involved in lipid metabolism and expressed in the hypothalamus. Here we explored the involvement of RELN in hypothalamic signaling and the impact of diet-induced obesity (DIO) on this system.

Methods

Adult male mice were fed a chow diet or maintained on a high-fat diet (HFD) for 12–16 weeks. HFD-fed DIO mice exhibited decreased ApoER2 and VLDLR expression and increased RELN protein in the hypothalamus. Electrophysiology was used to determine the mechanism by which the central fragment of RELN (CF-RELN) acts on arcuate nucleus (ARH) satiety-promoting proopiomelanocortin (POMC) neurons and the impact of DIO on this circuitry.

Results

CF-RELN exhibited heterogeneous presynaptic actions on inhibitory inputs onto ARH-POMC-EGFP neurons and consistent postsynaptic actions. Additionally, central administration of CF-RELN caused a significant increase in ARH c-Fos expression and an acute decrease in food intake and body weight.

Conclusions

We conclude that RELN signaling is modulated by diet, that RELN is involved in synaptic signaling onto ARH-POMC neurons, and that altering central CF-RELN levels can impact food intake and body weight.

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Diet-induced obesity alters reelin protein levels and expression of ApoER2 and VLDLR.

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Reelin has direct, but divergent actions on GABAergic inputs onto POMC neurons.

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Central administration of reelin protein decreases food intake and body weight.

Identification of Blood Biomarkers for Alzheimer's Disease Through Computational Prediction and Experimental Validation

Background: Alzheimer's disease (AD) is the major cause of dementia in population aged over 65 years, accounting up to 70% dementia cases. However, validated peripheral biomarkers for AD diagnosis are not available up to present. In this study, we adopted a new strategy of combination of computational prediction and experimental validation to identify blood protein biomarkers for AD.

Methods: First, we collected tissue-based gene expression data of AD patients and healthy controls from GEO database. Second, we analyzed these data and identified differentially expressed genes for AD. Third, we applied a blood-secretory protein prediction program on these genes and predicted AD-related proteins in blood. Finally, we collected blood samples of AD patients and healthy controls to validate the potential AD biomarkers by using ELISA experiments and Western blot analyses.

Results: A total of 2754 genes were identified to express differentially in brain tissues of AD, among which 296 genes were predicted to encode AD-related blood-secretory proteins. After careful analysis and literature survey on these predicted blood-secretory proteins, ten proteins were considered as potential AD biomarkers, five of which were experimentally verified with significant change in blood samples of AD vs. controls by ELISA, including GSN, BDNF, TIMP1, VLDLR, and APLP2. ROC analyses showed that VLDLR and TIMP1 had excellent performance in distinguishing AD patients from controls (area under the curve, AUC = 0.932 and 0.903, respectively). Further validation of VLDLR and TIMP1 by Western blot analyses has confirmed the results obtained in ELISA experiments.

Conclusion: VLDLR and TIMP1 had better discriminative abilities between ADs and controls, and might serve as potential blood biomarkers for AD. To our knowledge, this is the first time to identify blood protein biomarkers for AD through combination of computational prediction and experimental validation. In addition, VLDLR was first reported here as potential blood protein biomarker for AD. Thus, our findings might provide important information for AD diagnosis and therapies.

Complexity and Selectivity of γ-Secretase Cleavage on Multiple Substrates: Consequences in Alzheimer's Disease and Cancer

The processing of the amyloid-β protein precursor (AβPP) by β- and γ-secretases is a pivotal event in the genesis of Alzheimer's disease (AD). Besides familial mutations on the AβPP gene, or upon its overexpression, familial forms of AD are often caused by mutations or deletions in presenilin 1 (PSEN1) and 2 (PSEN2) genes: the catalytic components of the proteolytic enzyme γ-secretase (GS). The "amyloid hypothesis", modified over time, states that the aberrant processing of AβPP by GS induces the formation of specific neurotoxic soluble amyloid-β (Aβ) peptides which, in turn, cause neurodegeneration. This theory, however, has recently evidenced significant limitations and, in particular, the following issues are debated: 1) the concept and significance of presenilin's "gain of function" versus "loss of function"; and 2) the presence of several and various GS substrates, which interact with AβPP and may influence Aβ formation. The latter consideration is suggestive: despite the increasing number of GS substrates so far identified, their reciprocal interaction with AβPP itself, even in the AD field, is significantly unexplored. On the other hand, GS is also an important pharmacological target in the cancer field; inhibitors or GS activity are investigated in clinical trials for treating different tumors. Furthermore, the function of AβPP and PSENs in brain development and in neuronal migration is well known. In this review, we focused on a specific subset of GS substrates that directly interact with AβPP and are involved in its proteolysis and signaling, by evaluating their role in neurodegeneration and in cell motility or proliferation, as a possible connection between AD and cancer.

A Putative Mechanism of Age-Related Synaptic Dysfunction Based on the Impact of IGF-1 Receptor Signaling on Synaptic CaMKIIα Phosphorylation

Insulin-like growth factor 1 receptor (IGF-1R) signaling regulates the activity and phosphorylation of downstream kinases linked to inflammation, neurodevelopment, aging and synaptic function. In addition to the control of Ca2+ currents, IGF-1R signaling modulates the activity of calcium-calmodulin-dependent kinase 2 alpha (CaMKIIα) and mitogen activated protein kinase (MAPK/ErK) through multiple signaling pathways. These proteins (CaMKIIα and MAPK) regulate Ca2+ movement and long-term potentiation (LTP). Since IGF-1R controls the synaptic activity of Ca2+, CaMKIIα and MAPK signaling, the possible mechanism through which an age-dependent change in IGF-1R can alter the synaptic expression and phosphorylation of these proteins in aging needs to be investigated. In this study, we evaluated the relationship between an age-dependent change in brain IGF-1R and phosphorylation of CaMKIIα/MAPK. Furthermore, we elucidated possible mechanisms through which dysregulated CaMKIIα/MAPK interaction may be linked to a change in neurotransmitter processing and synaptic function. Male C57BL/6 VGAT-Venus mice at postnatal days 80 (P80), 365 and 730 were used to study age-related neural changes in two brain regions associated with cognitive function: hippocampus and prefrontal cortex (PFC). By means of high throughput confocal imaging and quantitative immunoblotting, we evaluated the distribution and expression of IGF-1, IGF-1R, CaMKIIα, p-CaMKIIα, MAPK and p-MAPK in whole brain lysate, hippocampus and cortex. Furthermore, we compared protein expression patterns and regional changes at P80, P365 and P730. Ultimately, we determined the relative phosphorylation pattern of CaMKIIα and MAPK through quantification of neural p-CaMKIIα and p-MAPK/ErK, and IGF-1R expression for P80, P365 and P730 brain samples. In addition to a change in synaptic function, our results show a decrease in neural IGF-1/IGF-1R expression in whole brain, hippocampus and cortex of aged mice. This was associated with a significant upregulation of phosphorylated neural MAPK (p-MAPK) and decrease in total brain CaMKIIα (i.e., CaMKIIα and p-CaMKIIα) in the aged brain. Taken together, we showed that brain aging is associated with a change in neural IGF-1/IGF-1R expression and may be linked to a change in phosphorylation of synaptic kinases (CaMKIIα and MAPK) that are involved in the modulation of LTP.

The functions of Reelin in membrane trafficking and cytoskeletal dynamics: implications for neuronal migration, polarization and differentiation

Reelin is a large extracellular matrix protein with relevant roles in mammalian central nervous system including neurogenesis, neuronal polarization and migration during development; and synaptic plasticity with its implications in learning and memory, in the adult. Dysfunctions in reelin signaling are associated with brain lamination defects such as lissencephaly, but also with neuropsychiatric diseases like autism, schizophrenia and depression as well with neurodegeneration. Reelin signaling involves a core pathway that activates upon reelin binding to its receptors, particularly ApoER2 (apolipoprotein E receptor 2)/LRP8 (low-density lipoprotein receptor-related protein 8) and very low-density lipoprotein receptor, followed by Src/Fyn-mediated phosphorylation of the adaptor protein Dab1 (Disabled-1). Phosphorylated Dab1 (pDab1) is a hub in the signaling cascade, from which several other downstream pathways diverge reflecting the different roles of reelin. Many of these pathways affect the dynamics of the actin and microtubular cytoskeleton, as well as membrane trafficking through the regulation of the activity of small GTPases, including the Rho and Rap families and molecules involved in cell polarity. The complexity of reelin functions is reflected by the fact that, even now, the precise mode of action of this signaling cascade in vivo at the cellular and molecular levels remains unclear. This review addresses and discusses in detail the participation of reelin in the processes underlying neurogenesis, neuronal migration in the cerebral cortex and the hippocampus; and the polarization, differentiation and maturation processes that neurons experiment in order to be functional in the adult brain. In vivo and in vitro evidence is presented in order to facilitate a better understanding of this fascinating system.

Protein-Protein Interaction Among the FoxP Family Members and their Regulation of Two Target Genes, VLDLR and CNTNAP2 in the Zebra Finch Song System

The Forkhead transcription factor FOXP2 is implicated in speech perception and production. The avian homolog, FoxP2¹ contributes to song learning and production in birds. In human cell lines, transcriptional activity of FOXP2 requires homo-dimerization or dimerization with paralogs FOXP1 or FOXP4. Whether FoxP dimerization occurs in the brain is unknown. We recently showed that FoxP1, FoxP2 and FoxP4 (FoxP1/2/4) proteins are co-expressed in neurons of Area X, a song control region in zebra finches. We now report on dimer- and oligomerization of zebra finch FoxPs and how this affects transcription. In cell lines and in the brain we identify homo- and hetero-dimers, and an oligomer composed of FoxP1/2/4. We further show that FoxP1/2 but not FoxP4 bind to the regulatory region of the target gene Contactin-associated protein-like 2 (CNTNAP2). In addition, we demonstrate that FoxP1/4 bind to the regulatory region of very low density lipoprotein receptor (VLDLR), as has been shown for FoxP2 previously. Interestingly, FoxP1/2/4 individually or in combinations regulate the promoters for SV40, zebra finch VLDLR and CNTNAP2 differentially. These data exemplify the potential for complex transcriptional regulation of FoxP1/2/4, highlighting the need for future functional studies dissecting their differential regulation in the brain.

Lactucopicrin potentiates neuritogenesis and neurotrophic effects by regulating Ca2+/CaMKII/ATF1 signaling pathway

ETHNO-PHARMACOLOGICAL RELEVANCE

Lactucopicrin is one of constitutes in Cichorium intybus L, which is commonly known as chicory in worldwide. It has been used for traditional usage such as antianalgesics, antidepressants and antihyperglycemics AIM OF STUDY: We investigated the neurotrophin-mediated neuroprotective effect of lactucopicrin in in vitro and examined for the underlying mechanism.

MATERIALS AND METHOD

To verify the neuroprotective effect of lactucopicrin, we investigated the inhibitory AChE activity, neurite outgrowth-related downstream signaling in murine neuroblastoma N2a and neurotrophins secretion in rat C6 glioma cells.

RESULTS

Lactucopicrin inhibited the AChE activity and increased intracellular Ca²⁺ levels with a substantial rise in muscarinic acetylcholine receptor M1 (CHRM1) expression in N2a cells. Moreover, lactucopicrin actively promoted neurite outgrowth via Ca²⁺-mediated activation of Ca²⁺/calmodulin-dependent protein kinase-II (CaMKII). It further activates transcription factor 1 (ATF1) along with modulating the levels of tropomyosin receptor kinase A, extracellular signal-regulated kinase 1 and 2, AKT, and synaptophysin 1 in N2a cells. Additionally, the levels of neurotrophins including NGF, BDNF, and NT3 were increased by treatment of lactucopicrin in C6 cells. The effects of lactucopicrin on NGF secretion and neuritogenesis were maintained even in the presence of phosphatidylinositol-3-kinase (PI3K) inhibitor LY294002, indicating that lactucopicrin exerts its effect on neuritogenesis in a PI3K-independent manner.

CONCLUSION

Our results suggest that the natural compound lactucopicrin may be a promising neurotrophin-mediated neuroprotective candidate for neurodegenerative diseases.

A Mercaptoacetamide-Based Class II Histone Deacetylase Inhibitor Increases Dendritic Spine Density via RasGRF1/ERK Pathway

BACKGROUND

The accumulation of amyloid-β (Aβ) leads to the loss of dendritic spines and synapses, which is hypothesized to cause cognitive impairments in Alzheimer's disease (AD) patients. In our previous study, we demonstrated that a novel mercaptoacetamide-based class II histone deacetylase inhibitor (HDACI), known as W2, decreased Aβ levels and improved learning and memory in mice. However, the underlying mechanism of this effect is unknown.

OBJECTIVE

Because dendritic spine formation is associated with cognitive performance, here we investigated whether HDACI W2 regulates dendritic spine density and its molecular mechanism of action.

METHODS

To examine the effect of HDACI W2 on dendritic spine density, we conducted morphological analysis of dendritic spines using GFP transfection and Golgi staining. In addition, to determine the molecular mechanism of W2 effects on spines, we measured the levels of mRNAs and proteins involved in the Ras signaling pathway using quantitative real-time PCR, immunocytochemistry, and western analysis.

RESULTS

We found that HDACI W2 altered dendritic spine density and morphology in vitro and in vivo. Additionally, W2 increased the mRNA or protein levels of Ras GRF1 and phospho-ERK. Moreover, knockdown of RasGRF1 and inhibition of ERK activity prevented the W2-mediated spinogenesis in primary hippocampal neurons.

CONCLUSION

Our Class II-selective HDACI W2 promotes the formation and growth of dendritic spines in a RasGRF1 and ERK dependent manner in primary hippocampal neurons.

Canonical and Non-canonical Reelin Signaling

Reelin is a large secreted glycoprotein that is essential for correct neuronal positioning during neurodevelopment and is important for synaptic plasticity in the mature brain. Moreover, Reelin is expressed in many extraneuronal tissues; yet the roles of peripheral Reelin are largely unknown. In the brain, many of Reelin's functions are mediated by a molecular signaling cascade that involves two lipoprotein receptors, apolipoprotein E receptor-2 (Apoer2) and very low density-lipoprotein receptor (Vldlr), the neuronal phosphoprotein Disabled-1 (Dab1), and members of the Src family of protein tyrosine kinases as crucial elements. This core signaling pathway in turn modulates the activity of adaptor proteins and downstream protein kinase cascades, many of which target the neuronal cytoskeleton. However, additional Reelin-binding receptors have been postulated or described, either as coreceptors that are essential for the activation of the "canonical" Reelin signaling cascade involving Apoer2/Vldlr and Dab1, or as receptors that activate alternative or additional signaling pathways. Here we will give an overview of canonical and alternative Reelin signaling pathways, molecular mechanisms involved, and their potential physiological roles in the context of different biological settings.

New Insights into Reelin-Mediated Signaling Pathways

Reelin, a multifunctional extracellular protein that is important for mammalian brain development and function, is secreted by different cell types in the prenatal or postnatal brain. The spatiotemporal regulation of Reelin expression and distribution during development relates to its multifaceted function in the brain. Prenatally Reelin controls neuronal radial migration and proper positioning in cortical layers, whereas postnatally Reelin promotes neuronal maturation, synaptic formation and plasticity. The molecular mechanisms underlying the distinct biological functions of Reelin during and after brain development involve unique and overlapping signaling pathways that are activated following Reelin binding to its cell surface receptors. Distinct Reelin ligand isoforms, such as the full-length protein or fragments generated by proteolytic cleavage differentially affect the activity of downstream signaling pathways. In this review, we discuss recent advances in our understanding of the signaling transduction pathways activated by Reelin that regulate different aspects of brain development and function. A core signaling machinery, including ApoER2/VLDLR receptors, Src/Fyn kinases, and the adaptor protein Dab1, participates in all known aspects of Reelin biology. However, distinct downstream mechanisms, such as the Crk/Rap1 pathway and cell adhesion molecules, play crucial roles in the control of neuronal migration, whereas the PI3K/Akt/mTOR pathway appears to be more important for dendrite and spine development. Finally, the NMDA receptor (NMDAR) and an unidentified receptor contribute to the activation of the MEK/Erk1/2 pathway leading to the upregulation of genes involved in synaptic plasticity and learning. This knowledge may provide new insight into neurodevelopmental or neurodegenerative disorders that are associated with Reelin dysfunction.

FoxP2 directly regulates the reelin receptor VLDLR developmentally and by singing

Mutations of the transcription factor FOXP2 cause a severe speech and language disorder. In songbirds, FoxP2 is expressed in the medium spiny neurons (MSNs) of the avian basal ganglia song nucleus, Area X, which is crucial for song learning and adult song performance. Experimental downregulation of FoxP2 in Area X affects spine formation, prevents neuronal plasticity induced by social context and impairs song learning. Direct target genes of FoxP2 relevant for song learning and song production are unknown. Here we show that a lentivirally mediated FoxP2 knockdown in Area X of zebra finches downregulates the expression of VLDLR, one of the two reelin receptors. Zebra finch FoxP2 binds to the promoter of VLDLR and activates it, establishing VLDLR as a direct FoxP2 target. Consistent with these findings, VLDLR expression is co-regulated with FoxP2 as a consequence of adult singing and during song learning. We also demonstrate that knockdown of FoxP2 affects glutamatergic transmission at the corticostriatal MSN synapse. These data raise the possibility that the regulatory relationship between FoxP2 and VLDLR guides structural plasticity towards the subset of FoxP2-positive MSNs in an activity dependent manner via the reelin pathway.