Kwok On Lai

Cdk5 regulates EphA4-mediated dendritic spine retraction through an ephexin1-dependent mechanism.

The development of dendritic spines is thought to be crucial for synaptic plasticity. Dendritic spines are retracted upon Eph receptor A4 (EphA4) activation, but the mechanisms that control this process are not well understood. Here we report an important function of cyclin-dependent kinase 5 (Cdk5) in EphA4-dependent spine retraction in mice. We found that blocking Cdk5 activity inhibits ephrin-A1–triggered spine retraction and reduction of mEPSC frequency at hippocampal synapses. The activation of EphA4 resulted in the recruitment of Cdk5 to EphA4, leading to the tyrosine phosphorylation and activation of Cdk5. EphA4 and Cdk5 then enhanced the activation of ephexin1, a guanine-nucleotide exchange factor that regulates activation of the small Rho GTPase RhoA. The association between EphA4 and ephexin1 was significantly reduced in Cdk5−/− brains and Cdk5-dependent phosphorylation of ephexin1 was required for the ephrin-A1–mediated regulation of spine density. These findings suggest that ephrin-A1 promotes EphA4-dependent spine retraction through the activation of Cdk5 and ephexin1, which in turn modulates actin cytoskeletal dynamics.

Synapse development and plasticity: roles of ephrin/Eph receptor signaling

The receptor tyrosine kinase Eph and its membrane-bound ligand ephrin are emerging key players in synapse formation and plasticity in the central nervous system. Understanding how ephrin/Eph regulate synapse formation and functions is often complicated by the fact that both ligands and receptors are expressed in the pre-synaptic and post-synaptic neurons and upon their interaction, bi-directional signaling cascades can be triggered. By elucidating the respective downstream targets and generating signaling-deficient mutants, the specific roles of forward (Eph receptor) and reverse (ephrin) signaling are beginning to be unraveled. In this review, we summarize recent advances in our understanding of how ephrin and Eph differentially participate in specific aspects of synapse formation in developing neurons, and activity-dependent plasticity in the adult brain.

Cloning and expression of a novel neurotrophin, NT-7, from carp.

Neurotrophins have been demonstrated to play important roles in the development and functioning of the nervous system. This family of proteins consists of four homologous members in mammals: NGF, BDNF, NT-3, and NT-4/5. A new member, called NT-6, was recently cloned from the platyfish Xiphophorus maculatus. This protein shares closer structural relationship to NGF than the other neurotrophins, but contains a characteristic insertion of 22 amino acids that constituted the heparin-binding domain. Here we report the cloning of a novel neurotrophin from the fish Cyprinus carpio (carp), which shared about 66% amino acid identity to Xiphophorus NGF and NT-6. The neurotrophin, designated NT-7, possesses structural characteristics common to all known neurotrophins, such as the presence of six conserved cysteine residues and the flanking conserved sequences. In addition, there is an insertion of 15 amino acids at the position corresponding to that observed for NT-6. The neurotrophic activity of NT-7 was demonstrated by its ability to promote neurite outgrowth and neuronal survival of chick dorsal root ganglia. Phosphorylation assay of various Trk receptors overexpressed in fibroblasts suggested that NT-7 could activate TrkA but not TrkB or TrkC. Northern blot analysis revealed that NT-7 was predominantly expressed in peripheral tissues, though weak expression was also detected in the brain. Like NT-6, this novel neurotrophin might represent yet another NGF-like neurotrophin in lower vertebrates.

APCCdh1 mediates EphA4-dependent downregulation of AMPA receptors in homeostatic plasticity

Homeostatic plasticity is crucial for maintaining neuronal output by counteracting unrestrained changes in synaptic strength. Chronic elevation of synaptic activity by bicuculline reduces the amplitude of miniature excitatory postsynaptic currents (mEPSCs), but the underlying mechanisms of this effect remain unclear. We found that activation of EphA4 resulted in a decrease in synaptic and surface GluR1 and attenuated mEPSC amplitude through a degradation pathway that requires the ubiquitin proteasome system (UPS). Elevated synaptic activity resulted in increased tyrosine phosphorylation of EphA4, which associated with the ubiquitin ligase anaphase-promoting complex (APC) and its activator Cdh1 in neurons in a ligand-dependent manner. APCCdh1 interacted with and targeted GluR1 for proteasomal degradation in vitro, whereas depletion of Cdh1 in neurons abolished the EphA4-dependent downregulation of GluR1. Knockdown of EphA4 or Cdh1 prevented the reduction in mEPSC amplitude in neurons that was a result of chronic elevated activity. Our results define a mechanism by which EphA4 regulates homeostatic plasticity through an APCCdh1-dependent degradation pathway.

TrkB phosphorylation by Cdk5 is required for activity-dependent structural plasticity and spatial memory

The neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor TrkB participate in diverse neuronal functions, including activity-dependent synaptic plasticity that is crucial for learning and memory. On binding to BDNF, TrkB is not only autophosphorylated at tyrosine residues but also undergoes serine phosphorylation at S478 by the serine/threonine kinase cyclin-dependent kinase 5 (Cdk5). However, the in vivo function of this serine phosphorylation remains unknown. We generated knock-in mice lacking this serine phosphorylation (TrkbS478A/S478A mice) and found that the TrkB phosphorylation–deficient mice displayed impaired spatial memory and compromised hippocampal long-term potentiation (LTP). S478 phosphorylation of TrkB regulates its interaction with the Rac1-specific guanine nucleotide exchange factor TIAM1, leading to activation of Rac1 and phosphorylation of S6 ribosomal protein during activity-dependent dendritic spine remodeling. These findings reveal the importance of Cdk5-mediated S478 phosphorylation of TrkB in activity-dependent structural plasticity, which is crucial for LTP and spatial memory formation.

Importin-mediated retrograde transport of CREB2 from distal processes to the nucleus in neurons

Signals received at distal synapses of neurons must be conveyed to the nucleus to initiate the changes in transcription that underlie long-lasting synaptic plasticity. The presence of importin nuclear transporters and of select transcription factors at synapses raises the possibility that importins directly transport transcription factors from synapse to nucleus to modulate gene expression. Here, we show that cyclic AMP response element binding protein 2 (CREB2)/activating transcription factor 4 (ATF4), a transcriptional repressor that modulates long-term synaptic plasticity and memory, localizes to distal dendrites of rodent hippocampal neurons and neurites of Aplysia sensory neurons (SNs) and binds to specific importin α isoforms. Binding of CREB2 to importin α is required for its transport from distal dendrites to the soma and for its translocation into the nucleus. CREB2 accumulates in the nucleus during long-term depression (LTD) but not long-term potentiation of rodent hippocampal synapses, and during LTD but not long-term facilitation (LTF) of Aplysia sensory-motor synapses. Time-lapse microscopy of CREB2 tagged with a photoconvertible fluorescent protein further reveals retrograde transport of CREB2 from distal neurites to the nucleus of Aplysia SN during phenylalanine-methionine-arginine-phenylalanine-amide (FMRFamide)-induced LTD. Together, our findings indicate that CREB2 is a novel cargo of importin α that translocates from distal synaptic sites to the nucleus after stimuli that induce LTD of neuronal synapses.

Recent advances in understanding the roles of Cdk5 in synaptic plasticity

The molecular composition of the postsynaptic density is modified during synaptic plasticity, which forms the molecular basis of learning and memory. Such changes in synaptic composition depends in part on the intricate regulation of phosphorylation of specific proteins via different protein kinases, including a serine/threonine kinase, cyclin-dependent kinase 5 (Cdk5). However, the mechanisms underlying the involvement of Cdk5 in neural plasticity remain elusive. Recently, the identification of a number of synaptic proteins as substrates or interacting proteins with Cdk5 provides important clues on how this kinase modulates the efficacy of synaptic transmission. In this review, we summarize the recent findings to illustrate the multi-faceted roles of Cdk5 in synaptic plasticity through affecting dendritic spine formation, ion channel conductance, protein expression, and transcription in the postsynaptic neurons. Importantly, dysregulation of Cdk5 has been linked to Alzheimers disease, which involves perturbations in synaptic functions and memory formation. Understanding the mechanisms by which Cdk5 regulates synaptic plasticity may therefore provide important insights in the design of novel therapeutic strategies for neurodegenerative diseases.

Structural plasticity of dendritic spines: The underlying mechanisms and its dysregulation in brain disorders

Dendritic spines are specialized structures on neuronal processes where the majority of excitatory synapses are localized. Spines are highly dynamic, and their stabilization and morphology are influenced by synaptic activity. This extrinsic regulation of spine morphogenesis underlies experience-dependent brain development and information storage within the brain circuitry. In this review, we summarize recent findings that demonstrate the phenomenon of activity-dependent structural plasticity and the molecular mechanisms by which synaptic activity sculpt neuronal connections. Impaired structural plasticity is associated with perturbed brain function in neurodevelopmental disorders such as autism. Information from the mechanistic studies therefore provides important insights into the design of therapeutic strategies for these brain disorders.

Tyk2/STAT3 Signaling Mediates β-Amyloid-Induced Neuronal Cell Death: Implications in Alzheimer's Disease

One of the pathological hallmarks of Alzheimers disease (AD) is deposition of extracellular amyloid-β (Aβ) peptide, which is generated from the cleavage of amyloid precursor protein (APP). Accumulation of Aβ is thought to associate with the progressive neuronal death observed in AD. However, the precise signaling mechanisms underlying the action of Aβ in AD pathophysiology are not completely understood. Here, we report the involvement of the transcription factor signal transducer and activator of transcription 3 (STAT3) in mediating Aβ-induced neuronal death. We find that tyrosine phosphorylation of STAT3 is elevated in the cortex and hippocampus of APP/PS1 transgenic mice. Treatment of cultured rat neurons with Aβ or intrahippocampal injection of mice with Aβ both induces tyrosine phosphorylation of STAT3 in neurons. Importantly, reduction of either the expression or activation of STAT3 markedly attenuates Aβ-induced neuronal apoptosis, suggesting that STAT3 activation contributes to neuronal death after Aβ exposure. We further identify Tyk2 as the tyrosine kinase that acts upstream of STAT3, as Aβ-induced activation of STAT3 and caspase-3-dependent neuronal death can be inhibited in tyk2−/− neurons. Finally, increased tyrosine phosphorylation of STAT3 is also observed in postmortem brains of AD patients. Our observations collectively reveal a novel role of STAT3 in Aβ-induced neuronal death and suggest the potential involvement of Tyk2/STAT3 signaling in AD pathophysiology.

Expression of Eph receptors in skeletal muscle and their localization at the neuromuscular junction

The participation of ephrins and Eph receptors in guiding motor axons during muscle innervation has been well documented, but little is known about their expression and functional significance in muscle at later developmental stages. Our present study investigates the expression and localization of Eph receptors and ephrins in skeletal muscle. Prominent expression of EphA4, EphA7, and ephrin-A ligands was detected in muscle during embryonic development. More importantly, both EphA4 and EphA7, as well as ephrin-A2, were localized at the neuromuscular junction (NMJ) of adult muscle. Despite their relative abundance, they were not localized at the synapses during embryonic stages. The concentration of EphA4, EphA7, and ephrin-A2 at the NMJ was observed at postnatal stages and the synaptic localization became prominent at later developmental stages. In addition, expression of Eph receptors was increased by neuregulin and after nerve injury. Furthermore, we demonstrated that overexpression of EphA4 led to tyrosine phosphorylation of the actin-binding protein cortactin and that EphA4 was coimmunoprecipitated with cortactin in muscle. Taken together, our findings indicate that EphA4 is associated with the actin cytoskeleton. Since actin cytoskeleton is critical to the formation and stability of NMJ, the present findings raise the intriguing possibility that Eph receptors may have a novel role in NMJ formation and/or maintenance.