Amy Kit Yu Fu

Blockade of EphA4 signaling ameliorates hippocampal synaptic dysfunctions in mouse models of Alzheimer's disease

Significance Synaptic loss and dysfunction is associated with cognitive impairment in Alzheimer’s disease (AD). However, the pathophysiological mechanisms underlying synaptic impairment are largely unknown. Here, we reveal a previously unidentified signaling pathway whereby activation of a receptor tyrosine kinase EphA4 is critical for synaptic dysfunctions in AD. Proof-of-concept studies undertaken in both in vitro and in vivo systems demonstrate its importance in mediating the deficit of synaptic transmission and hippocampal long-term potentiation in AD models. Specifically, blocking the EphA4-dependent pathway through knockdown studies or the use of small-molecule inhibitors effectively rescues the impaired synaptic transmission induced by Aβ and reverses impaired synaptic plasticity in AD mouse models. Thus, this study reveals a new disease-modifying therapeutic intervention for AD. Alzheimer’s disease (AD), characterized by cognitive decline, has emerged as a disease of synaptic failure. The present study reveals an unanticipated role of erythropoietin-producing hepatocellular A4 (EphA4) in mediating hippocampal synaptic dysfunctions in AD and demonstrates that blockade of the ligand-binding domain of EphA4 reverses synaptic impairment in AD mouse models. Enhanced EphA4 signaling was observed in the hippocampus of amyloid precursor protein (APP)/presenilin 1 (PS1) transgenic mouse model of AD, whereas soluble amyloid-β oligomers (Aβ), which contribute to synaptic loss in AD, induced EphA4 activation in rat hippocampal slices. EphA4 depletion in the CA1 region or interference with EphA4 function reversed the suppression of hippocampal long-term potentiation in APP/PS1 transgenic mice, suggesting that the postsynaptic EphA4 is responsible for mediating synaptic plasticity impairment in AD. Importantly, we identified a small-molecule rhynchophylline as a novel EphA4 inhibitor based on molecular docking studies. Rhynchophylline effectively blocked the EphA4-dependent signaling in hippocampal neurons, and oral administration of rhynchophylline reduced the EphA4 activity effectively in the hippocampus of APP/PS1 transgenic mice. More importantly, rhynchophylline administration restored the impaired long-term potentiation in transgenic mouse models of AD. These findings reveal a previously unidentified role of EphA4 in mediating AD-associated synaptic dysfunctions, suggesting that it is a new therapeutic target for this disease.

IL-33 ameliorates Alzheimer’s disease-like pathology and cognitive decline

Significance Dysfunction of the innate immune system is involved in the pathogenesis of Alzheimer’s disease (AD); however, the pathophysiological mechanisms underlying these dysfunctions are unclear. Here we report that stimulation of IL-33/ST2 signaling rescues memory deficits and reduces the accumulation of β-amyloid in APP/PS1 mice that exhibit select pathologies associated with AD. Although impaired IL-33/ST2 signaling is associated with early progression of AD, IL-33 injection rescues contextual memory deficits and reduces the accumulation of β-amyloid in APP/PS1 mice. IL-33 skews the microglia toward an alternative activation state with enhanced Aβ phagocytic capacity and elevated antiinflammatory gene expression, which results in a decreased proinflammatory response in the brain. Thus, this study suggests that IL-33 can be developed as a new therapeutic intervention for AD. Alzheimer’s disease (AD) is a devastating condition with no known effective treatment. AD is characterized by memory loss as well as impaired locomotor ability, reasoning, and judgment. Emerging evidence suggests that the innate immune response plays a major role in the pathogenesis of AD. In AD, the accumulation of β-amyloid (Aβ) in the brain perturbs physiological functions of the brain, including synaptic and neuronal dysfunction, microglial activation, and neuronal loss. Serum levels of soluble ST2 (sST2), a decoy receptor for interleukin (IL)-33, increase in patients with mild cognitive impairment, suggesting that impaired IL-33/ST2 signaling may contribute to the pathogenesis of AD. Therefore, we investigated the potential therapeutic role of IL-33 in AD, using transgenic mouse models. Here we report that IL-33 administration reverses synaptic plasticity impairment and memory deficits in APP/PS1 mice. IL-33 administration reduces soluble Aβ levels and amyloid plaque deposition by promoting the recruitment and Aβ phagocytic activity of microglia; this is mediated by ST2/p38 signaling activation. Furthermore, IL-33 injection modulates the innate immune response by polarizing microglia/macrophages toward an antiinflammatory phenotype and reducing the expression of proinflammatory genes, including IL-1β, IL-6, and NLRP3, in the cortices of APP/PS1 mice. Collectively, our results demonstrate a potential therapeutic role for IL-33 in AD.

Overproduction of Upper-Layer Neurons in the Neocortex Leads to Autism-like Features in Mice

The functional integrity of the neocortex depends upon proper numbers of excitatory and inhibitory neurons; however, the consequences of dysregulated neuronal production during the development of the neocortex are unclear. As excess cortical neurons are linked to the neurodevelopmental disorder autism, we investigated whether the overproduction of neurons leads to neocortical malformation and malfunction in mice. We experimentally increased the number of pyramidal neurons in the upper neocortical layers by using the small molecule XAV939 to expand the intermediate progenitor population. The resultant overpopulation of neurons perturbs development of dendrites and spines ofxa0excitatory neurons and alters the laminar distribution of interneurons. Furthermore, these phenotypic changes are accompanied by dysregulated excitatory and inhibitory synaptic connection and balance. Importantly, these mice exhibit behavioral abnormalities resembling those of human autism. Thus, our findings collectively suggest a causal relationship between neuronal overproduction and autism-like features, providing developmental insights into the etiology of autism.

Cdk5-mediated phosphorylation of RapGEF2 controls neuronal migration in the developing cerebral cortex

During cerebral cortex development, pyramidal neurons migrate through the intermediate zone and integrate into the cortical plate. These neurons undergo the multipolar–bipolar transition to initiate radial migration. While perturbation of this polarity acquisition leads to cortical malformations, how this process is initiated and regulated is largely unknown. Here we report that the specific upregulation of the Rap1 guanine nucleotide exchange factor, RapGEF2, in migrating neurons corresponds to the timing of this polarity transition. In utero electroporation and live-imaging studies reveal that RapGEF2 acts on the multipolar–bipolar transition during neuronal migration via a Rap1/N-cadherin pathway. Importantly, activation of RapGEF2 is controlled via phosphorylation by a serine/threonine kinase Cdk5, whose activity is largely restricted to the radial migration zone. Thus, the specific expression and Cdk5-dependent phosphorylation of RapGEF2 during multipolar–bipolar transition within the intermediate zone are essential for proper neuronal migration and wiring of the cerebral cortex.

Eph receptors at synapses: Implications in neurodegenerative diseases

Precise regulation of synapse formation, maintenance and plasticity is crucial for normal cognitive function, and synaptic failure has been suggested as one of the hallmarks of neurodegenerative diseases. In this review, we describe the recent progress in our understanding of how the receptor tyrosine kinase Ephs and their ligands ephrins regulate dendritic spine morphogenesis, synapse formation and maturation, as well as synaptic plasticity. In particular, we discuss the emerging evidence implicating that deregulation of Eph/ephrin signaling contributes to the aberrant synaptic functions associated with cognitive impairment in Alzheimers disease. Understanding how Eph/ephrin regulates synaptic function may therefore provide new insights into the development of therapeutic agents against neurodegenerative diseases.

CRMP2 Functional Roles in Neural Development and Therapeutic Potential in Neurological Diseases

Cytoskeletal restructuring is essential for nearly all cellular processes in the developing brain. After cell fate determination, newborn cortical neurons must migrate to their final positions while establishing proper axon-dendrite polarity. Significant progress has recently been made towards understanding the cellular and molecular mechanisms underlying neuronal polarization in vivo. Collapsin response mediator protein 2 (CRMP2) has long been identified as a microtubule-binding protein that regulates neuronal polarity in vitro. Recent studies provide new insights into the roles of CRMP2 in neuronal migration and subsequent neuronal differentiation. Both the expression and activity of CRMP2 are tightly regulated during cortex development. CRMP2 is suggested to be important in the multipolar-bipolar transition in radial migration. The increasing number of known interaction partners indicates that CRMP2 has functions beyond cytoskeletal regulation, including axonal transport, vesicle trafficking, and neurotransmitter release. This review discusses the current knowledge about CRMP2 in the context of neuronal development and highlights a recent emerging theme regarding its potential therapeutic applications.

Bidirectional signaling of ErbB and Eph receptors at synapses.

Synapse development and remodeling are regulated by a plethora of molecules such as receptor tyrosine kinases (RTKs), a family of cell surface receptors that play critical roles in neural development. Two families of RTKs implicated in synaptic functions, ErbBs and Ephs, share similar characteristics in terms of exhibiting forward and reverse signaling. In this review, we will discuss the latest advances in the functions of ErbBs and Ephs at the synapse, including dendritic spine morphogenesis, synapse formation and maturation, and synaptic transmission and plasticity. In addition to signaling at interneuronal synapses, communication between neuron and glia is increasingly implicated in the control of synaptic functions. Studies on RTKs and their cognate ligands in glial cells enhance our understanding on the nature of tripartite synapse. Implications of these signaling events in human diseases will be discussed.

Cdk5-Dependent Mst3 Phosphorylation and Activity Regulate Neuronal Migration through RhoA Inhibition

The radial migration of newborn neurons is critical for the lamination of the cerebral cortex. Proper neuronal migration requires precise and rapid reorganization of the actin and microtubule cytoskeleton. However, the underlying signaling mechanisms controlling cytoskeletal reorganization are not well understood. Here, we show that Mst3, a serine/threonine kinase highly expressed in the developing mouse brain, is essential for radial neuronal migration and final neuronal positioning in the developing mouse neocortex. Mst3 silencing by in utero electroporation perturbed the multipolar-to-bipolar transition of migrating neurons and significantly retards radial migration. Although the kinase activity of Mst3 is essential for its functions in neuronal morphogenesis and migration, it is regulated via its phosphorylation at Ser79 by a serine/threonine kinase, cyclin-dependent kinase 5 (Cdk5). Our results show that Mst3 regulates neuronal migration through modulating the activity of RhoA, a Rho-GTPase critical for actin cytoskeletal reorganization. Mst3 phosphorylates RhoA at Ser26, thereby negatively regulating the GTPase activity of RhoA. Importantly, RhoA knockdown successfully rescues neuronal migration defect in Mst3-knockdown cortices. Our findings collectively suggest that Cdk5–Mst3 signaling regulates neuronal migration via RhoA-dependent actin dynamics.

Cdk5 Regulates Activity-Dependent Gene Expression and Dendrite Development

The proper growth and arborization of dendrites in response to sensory experience are essential for neural connectivity and information processing in the brain. Although neuronal activity is important for sculpting dendrite morphology, the underlying molecular mechanisms are not well understood. Here, we report that cyclin-dependent kinase 5 (Cdk5)-mediated transcriptional regulation is a key mechanism that controls activity-dependent dendrite development in cultured rat neurons. During membrane depolarization, Cdk5 accumulates in the nucleus to regulate the expression of a subset of genes, including that of the neurotrophin brain-derived neurotrophic factor, for subsequent dendritic growth. Furthermore, Cdk5 function is mediated through the phosphorylation of methyl-CpG-binding protein 2, a key transcriptional repressor that is mutated in the mental disorder Rett syndrome. These findings collectively suggest that the nuclear import of Cdk5 is crucial for activity-dependent dendrite development by regulating neuronal gene transcription during neural development. SIGNIFICANCE STATEMENT Neural activity directs dendrite development through the regulation of gene transcription. However, how molecular signals link extracellular stimuli to the transcriptional program in the nucleus remains unclear. Here, we demonstrate that neuronal activity stimulates the translocation of the kinase Cdk5 from the cytoplasmic compartment into the nucleus; furthermore, the nuclear localization of Cdk5 is required for dendrite development in cultured neurons. Genome-wide transcriptome analysis shows that Cdk5 deficiency specifically disrupts activity-dependent gene transcription of bdnf. The action of Cdk5 is mediated through the modulation of the transcriptional repressor methyl-CpG-binding protein 2. Therefore, this study elucidates the role of nuclear Cdk5 in the regulation of activity-dependent gene transcription and dendritic growth.

Coronin 6 Regulates Acetylcholine Receptor Clustering through Modulating Receptor Anchorage to Actin Cytoskeleton

The maintenance of a high density of neurotransmitter receptors at the postsynaptic apparatus is critical for efficient neurotransmission. Acetylcholine receptors (AChRs) are neurotransmitter receptors densely packed on the postsynaptic muscle membrane at the neuromuscular junction (NMJ) via anchoring onto the actin cytoskeletal network. However, how the receptor-associated actin is coordinately regulated is not fully understood. We report here that Coronin 6, a newly identified member of the coronin family, is highly enriched at adult NMJs and regulates AChR clustering through modulating the interaction between receptors and the actin cytoskeletal network. Experiments with cultured myotubes reveal that Coronin 6 is important for both agrin- and laminin-induced AChR clustering. Furthermore, Coronin 6 forms a complex with AChRs and actin in a manner dependent on its C-terminal region and a conserved Arg29 residue at the N terminus, both of which are critical for the cytoskeletal anchorage of AChRs. Importantly, in vivo knockdown of Coronin 6 in mouse skeletal muscle fibers leads to destabilization of AChR clusters. Together, these findings demonstrate that Coronin 6 is a critical regulator of AChR clustering at the postsynaptic region of the NMJs through modulating the receptor-anchored actin cytoskeleton.