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Kwok-On Lai

Publications and source records attributed to Kwok-On Lai.

6 recordsLinked to original sources
PubMed2006

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.

Animals↗
PubMed2005

{alpha}-Syntrophin regulates ARMS localization at the neuromuscular junction and enhances EphA4 signaling in an ARMS-dependent manner.

EphA4 signaling has recently been implicated in the regulation of synapse formation and plasticity. In this study, we show that ankyrin repeat-rich membrane spanning (ARMS; also known as a kinase D-interacting substrate of 220 kD), a substrate for ephrin and neurotrophin receptors, was expressed in developing muscle and was concentrated at the neuromuscular junction (NMJ). Using yeast two-hybrid screening, we identified a PDZ (PSD-95, Dlg, ZO-1) domain protein, alpha-syntrophin, as an ARMS-interacting protein in muscle. Overexpression of alpha-syntrophin induced ARMS clustering in a PDZ domain-dependent manner. Coexpression of ARMS enhanced EphA4 signaling, which was further augmented by the presence of alpha-syntrophin. Moreover, the ephrin-A1-induced tyrosine phosphorylation of EphA4 was reduced in C2C12 myotubes after the blockade of ARMS and alpha-syntrophin expression by RNA interference. Finally, alpha-syntrophin-null mice exhibited a disrupted localization of ARMS and EphA4 at the NMJ and a reduced expression of ARMS in muscle. Altogether, our findings suggest that ARMS may play an important role in regulating postsynaptic signal transduction through the syntrophin-mediated localization of receptor tyrosine kinases such as EphA4.

Animals↗
PubMed2004

Identification of the Jak/Stat proteins as novel downstream targets of EphA4 signaling in muscle: implications in the regulation of acetylcholinesterase expression.

Eph receptors and their cognate ligands ephrins are important players in axon guidance and neural patterning during development of the nervous system. Much of our knowledge about the signal transduction pathways triggered by Eph receptors has been related to the modulation of actin cytoskeleton, which is fundamental in mediating the cellular responses in growth cone navigation, cell adhesion, and cell migration. In contrast, little was known about whether long term activation of Eph receptor would regulate gene expression. Here we report a novel signaling pathway of EphA4, which involves activation of the tyrosine kinase Jak2 and the transcriptional activator Stat3. Transfection of COS7 cells with EphA4, but not the kinase-dead mutant, induced tyrosine phosphorylation of Jak2, Stat1, and Stat3. Treatment of cultured C2C12 myotubes with ephrin-A1 also induced tyrosine phosphorylation of Stat3, which was abolished by the Jak2 inhibitor AG490. Moreover, Jak2 was co-immunoprecipitated with EphA4 in muscle, and both proteins were concentrated at the neuromuscular junction (NMJ) of adult muscle. By using microarray analysis, we have identified acetylcholinesterase, the critical enzyme that hydrolyzed the neurotransmitter acetylcholine at the NMJ, as a downstream target gene of the Jak/Stat pathway in muscle. More importantly, ephrin-A1 increased the expression of acetylcholinesterase protein in C2C12 myotubes, which was abolished by AG490. In contrast, ephrin-A1 reduced the expression of fibronectin mRNA in C2C12 myotubes independently of Jak2. Finally, the expression level of acetylcholinesterase in limb muscle of EphA4 null mice was significantly reduced compared with the wild-type control. Taken together, these results have identified Jak/Stat proteins as the novel downstream targets of EphA4 signaling. In addition, the present study provides the first demonstration of a potential function of Eph receptors and Jak/Stat proteins at the NMJ.

Acetylcholinesterase↗
PubMed2003

Ephrin-B1 reverse signaling activates JNK through a novel mechanism that is independent of tyrosine phosphorylation.

Eph receptors and their cognate ligand ephrins play important roles in various biological processes such as cell migration, axon guidance, and synaptic plasticity. One characteristic feature of the Eph-ephrin signal transduction is that, upon interaction with the receptor, the transmembrane B-class ephrins become tyrosine-phosphorylated and transduce intracellular signals that lead to reorganization of the cytoskeleton. Although in vitro and genetic studies have demonstrated unequivocally the significance of this reverse signaling, the underlying mechanism remains unclear. We report here that transfection of ephrin-B1 into 293 cells resulted in robust increase in JNK activity, whereas expression of truncated ephrin-B1 lacking the cytoplasmic domain had a negligible effect, indicating that the induction of JNK activity was attributed mainly to the reverse signaling. The ephrin-B1-mediated JNK activation was reduced significantly by dominant-negative TAK1, MKK4, or MKK7. Ephrin-B1 over-expressing 293 cells became rounded in morphology. Surprisingly, ephrin-B1 that lacked all six intracellular tyrosine residues still triggered JNK activation and rounding morphology of the transfected cells. Consistent with these observations, activation of JNK and the resulting morphological changes mediated by ephrin-B1 could be abolished by the JNK inhibitor SP600125 but not the Src inhibitor PP2. Taken together, our findings have identified a novel reverse signaling pathway transduced by ephrin-B1, which is independent of tyrosine phosphorylation but involves the activation of JNK through TAK1 and MKK4/MKK7 and leads to changes in cell morphology.

Animals↗
PubMedDate not supplied

Postsynaptic signaling of new players at the neuromuscular junction.

The neuromuscular junction (NMJ) represents the most well studied synapse and is widely regarded as structurally and functionally less complicated than neuronal synapses in the brain. Recent studies, however, have identified the localization and function of new signaling molecules at the NMJ. Surprisingly, many synaptic proteins previously identified in the brain are indeed also concentrated on the postsynaptic muscle side of the NMJ. These include the serine/threonine kinase Cdk5, the neurotrophin receptor TrkB, Eph receptors and ephrins, NMDA receptors and nitric oxide synthase, various PDZ-domain scaffold proteins, and beta-amyloid precursor protein. These observations indicate that the molecular composition of NMJ is much more intricate than we originally thought. The potential significance of these new signaling molecules at the NMJ will be discussed.

Animals↗