Search PubMed⌕ Search

Biomedical subjects

Li-Huei Tsai

Publications and source records attributed to Li-Huei Tsai.

At least 37 records · Page 2Linked to original sources
PubMed2004

Reelin and cyclin-dependent kinase 5-dependent signals cooperate in regulating neuronal migration and synaptic transmission.

Neuronal migration and positioning in the developing brain require the coordinated interaction of multiple cellular signaling pathways. The extracellular signaling molecule Reelin and the cytoplasmic serine/threonine kinase Cdk5 (cyclin-dependent kinase 5) are both required for normal neuronal positioning, lamination of the neocortex, and foliation of the cerebellum. They also modulate synaptic transmission in the adult brain. It is not known, however, to what extent Cdk5 participates in Reelin signaling and whether both pathways interact on the genetic or biochemical level. We have used genetically altered mice to generate compound functional defects of Reelin and Cdk5 signaling. Differential neurohistochemical staging combined with the biochemical analysis of Reelin- and Cdk5-dependent signaling in primary embryonic neurons and electrophysiology in hippocampal slices reveals evidence for genetic and functional interaction between both pathways. Inhibition of Reelin or Cdk5 signaling had no discernible biochemical effect on each other. Taken together, these findings suggest that both pathways function together in a parallel, rather than a simple, linear manner to coordinate neuronal migration and neurotransmission in the developing and mature brain.

Animals↗
PubMed2004

Cyclin-dependent kinase 5 phosphorylates the N-terminal domain of the postsynaptic density protein PSD-95 in neurons.

PSD-95 (postsynaptic density 95) is a postsynaptic scaffolding protein that links NMDA receptors to the cytoskeleton and signaling molecules. The N-terminal domain of PSD-95 is involved in the synaptic targeting and clustering of PSD-95 and in the clustering of NMDA receptors at synapses. The N-terminal domain of PSD-95 contains three consensus phosphorylation sites for cyclin-dependent kinase 5 (cdk5), a proline-directed serine-threonine kinase essential for brain development and implicated in synaptic plasticity, dopamine signaling, cocaine addiction, and neurodegenerative disorders. We report that PSD-95 is phosphorylated in the N-terminal domain by cdk5 in vitro and in vivo, and that this phosphorylation is not detectable in brain lysates of cdk5-/- mice. N-terminal phosphorylated PSD-95 is found in PSD fractions together with cdk5 and its activator, p35, suggesting a role for phosphorylated PSD-95 at synapses. In heterologous cells, coexpression of active cdk5 reduces the ability of PSD-95 to multimerize and to cluster neuronal ion channels, two functions attributed to the N-terminal domain of PSD-95. Consistent with these observations, the lack of cdk5 activity in cultured neurons results in larger clusters of PSD-95. In cdk5-/- cortical neurons, more prominent PSD-95 immunostained clusters are observed than in wild-type neurons. In hippocampal neurons, the expression of DNcdk5 (inactive form of cdk5) or of the triple alanine mutant (T19A, S25A, S35A) full-length PSD-95 results in increased PSD-95 cluster size. These results identify cdk5-dependent phosphorylation of the N-terminal domain of PSD-95 as a novel mechanism for regulating the clustering of PSD-95. Moreover, these observations support the possibility that cdk5-dependent phosphorylation of PSD-95 dynamically regulates the clustering of PSD-95/NMDA receptors at synapses, thus providing a possible mechanism for rapid changes in density and/or number of receptor at synapses.

Animals↗
PubMed2004

Control of cortical neuron migration and layering: cell and non cell-autonomous effects of p35.

The migration, arrest, and ultimately positioning of cortical neurons require signaling activity from Reelin as well as from cyclin-dependent kinase 5 (Cdk5). Although both molecules control neuronal positioning, they achieve their effects by quite separate molecular pathways. Cdk5 is a serine-threonine kinase, the activity of which is dependent on its activating subunits p35 and p39. Mice deficient in Cdk5, p35, or both p35 and p39 display the hallmarks of disturbed cortical development, including cortical layer inversion, neuronal disorientation, and abnormal fiber infiltration. To distinguish between the cell- and non cell-autonomous functions of p35, we constructed p35+/+ <--> p35-/- chimeras using the lacZ gene as an independent marker for p35+/+ cells. In this shared developmental space, wild-type and mutant neurons behaved cell-autonomously with respect to layering. Wild-type cells formed a properly layered supercortex that is mirrored by an inverted mutant cortex lying underneath. However, this genotype-specific behavior was confined to the pyramidal population, and interneurons belonging to either genotype were indiscriminately distributed. However, there was also non cell-autonomous rescue of mutant neurons, and this rescue was specific only to early-born pyramidal neurons belonging to layer V. Rescued neurons reached the correct layer address and possessed appropriate neuronal morphology, orientation, and projections. Later-born neurons belonging to layers II and III were not rescued. These results demonstrate that p35 signaling can have both cell- and non cell-autonomous consequences, and their effects are not uniformly shared by cortical neurons born at different times or born at different places (projection neurons vs interneurons).

Animals↗
PubMed2004

A Jekyll and Hyde kinase: roles for Cdk5 in brain development and disease.

Cyclin-dependent kinase 5 (Cdk5) is a multi-faced kinase implicated in both development and disease of the mammalian central nervous system. These different faces of Cdk5 are preferentially regulated by the activation of Cdk5 by its different binding partners. The precise molecular and cellular mechanisms governing the role of Cdk5 in brain development and disease are unclear. Emerging evidence is now unraveling how Cdk5 normally orchestrates new signaling pathways that dictate the proper maturation and maintenance of the central nervous system. Under pathological conditions, however, Cdk5 activity goes awry and the malevolent face of Cdk5 surfaces. Recently developed animal models that display this deregulated Cdk5 activity reveal the intimate involvement of Cdk5 in tau pathology and neuronal cell death, and underscore the importance of phosphorylation in the progression of neurodegenerative diseases.

Animals↗
PubMed2004

Cdk5 phosphorylation of FAK regulates centrosome-associated miocrotubules and neuronal migration.

Cdk5 is a member of the cyclin-dependent kinase (Cdk) family. Unlike other Cdks that promote cell cycle, Cdk5 is activated in postmitotic neurons and critically regulates neuronal migration by phosphorylating its substrates during brain development. Recently, we found that Cdk5 phosphorylates focal adhesion kinase (FAK) at Serine 732 in vitro and is responsible for this phosphorylation in the developing brain. Our experiments using a phospho-specific antibody and an S732-unphosphorylatable mutant FAK suggest that S732 phosphorylation may regulate a centrosome-associated microtubule structure to promote nuclear translocation, a critical step in neuronal migration. S732 phosphorylation does not directly impact on the kinase activity of FAK, but appears to prevent the accumulation of FAK at the centrosome. Our study reveals a similarity between Cdk5 and Cdk1 in the regulation of neuronal migration and cell division, respectively. In addition, our study implicates FAK in a signaling pathway that directly regulates microtubules.

Animals↗
PubMed2003

Layering defect in p35 deficiency is linked to improper neuronal-glial interaction in radial migration.

Several genes essential for neocortical layering have been identified in recent years, but their precise roles in this process remain to be elucidated. Mice deficient in p35--an activator of cyclin-dependent kinase 5 (Cdk5)--are characterized by a neocortex that has inverted layering. To decipher the physiological mechanisms that underlie this defect, we compared time-lapse recordings between p35(-/-) and wild-type cortical slices. In the p35(-/-) neocortex, the classic modes of radial migration--somal translocation and locomotion--were largely replaced by a distinct mode of migration: branched migration. Branched migration is cell-autonomous, associated with impaired neuronal-glial interaction and rare in neurons of scrambler mice, which are deficient in Dab1. Hence, our findings suggest that inside-out layering requires distinct functions of Reelin and p35/Cdk5 signaling, with the latter being important for proper glia-guided migration.

Animals↗
PubMed2003

BACE1 suppression by RNA interference in primary cortical neurons.

Extracellular deposition of amyloid-beta (Abeta) aggregates in the brain represents one of the histopathological hallmarks of Alzheimer's disease (AD). Abeta peptides are generated from proteolysis of the amyloid precursor proteins (APPs) by beta- and gamma-secretases. Beta-secretase (BACE1) is a type I integral membrane glycoprotein that can cleave APP first to generate C-terminal 99- or 89-amino acid membrane-bound fragments containing the N terminus of Abeta peptides (betaCTF). As BACE1 cleavage is an essential step for Abeta generation, it is proposed as a key therapeutic target for treating AD. In this study, we show that small interfering RNA (siRNA) specifically targeted to BACE1 can suppress BACE1 (but not BACE2) protein expression in different cell systems. Furthermore, BACE1 siRNA reduced APP betaCTF and Abeta production in primary cortical neurons derived from both wild-type and transgenic mice harboring the Swedish APP mutant. The subcellular distribution of APP and presenilin-1 did not appear to differ in BACE1 suppressed cells. Importantly, pretreating neurons with BACE1 siRNA reduced the neurotoxicity induced by H2O2 oxidative stress. Our results indicate that BACE1 siRNA specifically impacts on beta-cleavage of APP and may be a potential therapeutic approach for treating AD.

Alzheimer Disease↗
PubMed2003

Aberrant Cdk5 activation by p25 triggers pathological events leading to neurodegeneration and neurofibrillary tangles.

Cyclin-dependent kinase 5 (Cdk5) and its regulatory subunit p35 are integral players in the proper development of the mammalian central nervous system. Proteolytic cleavage of p35 generates p25, leading to aberrant Cdk5 activation. The accumulation of p25 is implicated in several neurodegenerative diseases. In primary neurons, p25 causes apoptosis and tau hyperphosphorylation. Current mouse models expressing p25, however, fail to rigorously recapitulate these phenotypes in vivo. Here, we generated inducible transgenic mouse lines overexpressing p25 in the postnatal forebrain. Induction of p25 preferentially directed Cdk5 to pathological substrates. These animals exhibited neuronal loss in the cortex and hippocampus, accompanied by forebrain atrophy, astrogliosis, and caspase-3 activation. Endogenous tau was hyperphosphorylated at many epitopes, aggregated tau accumulated, and neurofibrillary pathology developed progressively in these animals. Our cumulative findings provide compelling evidence that in vivo deregulation of Cdk5 by p25 plays a causative role in neurodegeneration and the development of neurofibrillary pathology.

Animals↗
PubMed2003

APP processing is regulated by cytoplasmic phosphorylation.

Amyloid-beta peptide (Abeta) aggregate in senile plaque is a key characteristic of Alzheimer's disease (AD). Here, we show that phosphorylation of amyloid precursor protein (APP) on threonine 668 (P-APP) may play a role in APP metabolism. In AD brains, P-APP accumulates in large vesicular structures in afflicted hippocampal pyramidal neurons that costain with antibodies against endosome markers and the beta-secretase, BACE1. Western blot analysis reveals increased levels of T668-phosphorylated APP COOH-terminal fragments in hippocampal lysates from many AD but not control subjects. Importantly, P-APP cofractionates with endosome markers and BACE1 in an iodixanol gradient and displays extensive colocalization with BACE1 in rat primary cortical neurons. Furthermore, APP COOH-terminal fragments generated by BACE1 are preferentially phosphorylated on T668 verses those produced by alpha-secretase. The production of Abeta is significantly reduced when phosphorylation of T668 is either abolished by mutation or inhibited by T668 kinase inhibitors. Together, these results suggest that T668 phosphorylation may facilitate the BACE1 cleavage of APP to increase Abeta generation.

Alzheimer Disease↗
PubMed2003

Partial rescue of the p35-/- brain phenotype by low expression of a neuronal-specific enolase p25 transgene.

Cyclin-dependent kinase 5 (Cdk5) is activated on binding of activator proteins p35 and p39. A N-terminally truncated p35, termed p25, is generated through cleavage by the Ca(2+)-dependent protease calpain after induction of ischemia in rat brain. p25 has been shown to accumulate in brains of patients with Alzheimer's disease and may contribute to A-beta peptide-mediated toxicity. Studies from transfected neurons as well as p35 and p25 transgenic mice have indicated that Cdk5, when activated by p25, gains some toxic function compared with p35/Cdk5. It remains unclear, however, whether p25/Cdk5 signaling additionally channels into pathways usually used by p35/Cdk5 and whether p25 is associated with a loss of p35 function. To clarify these issues, we have generated p25-transgenic mice in a p35-null background. We find that low levels of p25 during development induce a partial rescue of the p35-/- phenotype in several brain regions analyzed, including a rescue of cell positioning of a subset of neurons in the neocortex. In accordance with the partial rescue of brain anatomy, phosphorylation of the Cdk5 substrate mouse disabled 1 is partially restored during development. Besides this, p25/Cdk5 fails to phosphorylate other substrates that are normally phosphorylated by p35/Cdk5. Our results show that p25 can substitute for p35/Cdk5 under certain circumstances during development. In addition, they suggest that p25 may have lost some functions of p35.

Aging↗
PubMed2003

14-3-3epsilon is important for neuronal migration by binding to NUDEL: a molecular explanation for Miller-Dieker syndrome.

Heterozygous deletions of 17p13.3 result in the human neuronal migration disorders isolated lissencephaly sequence (ILS) and the more severe Miller-Dieker syndrome (MDS). Mutations in PAFAH1B1 (the gene encoding LIS1) are responsible for ILS and contribute to MDS, but the genetic causes of the greater severity of MDS are unknown. Here, we show that the gene encoding 14-3-3epsilon (YWHAE), one of a family of ubiquitous phosphoserine/threonine-binding proteins, is always deleted in individuals with MDS. Mice deficient in Ywhae have defects in brain development and neuronal migration, similar to defects observed in mice heterozygous with respect to Pafah1b1. Mice heterozygous with respect to both genes have more severe migration defects than single heterozygotes. 14-3-3epsilon binds to CDK5/p35-phosphorylated NUDEL and this binding maintains NUDEL phosphorylation. Similar to LIS1, deficiency of 14-3-3epsilon results in mislocalization of NUDEL and LIS1, consistent with reduction of cytoplasmic dynein function. These results establish a crucial role for 14-3-3epsilon in neuronal development by sustaining the effects of CDK5 phosphorylation and provide a molecular explanation for the differences in severity of human neuronal migration defects with 17p13.3 deletions.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗
PubMed2003

Cdk5: one of the links between senile plaques and neurofibrillary tangles?

The relationship between amyloid plaques and neurofibrillary tangles, the two pathologic hallmarks of Alzheimer's disease (AD), is an unknown and controversial subject. However, emerging evidence from genetic and biochemical studies suggests that accumulation of amyloid beta peptides may play a causative role in AD pathogenesis. This led to the amyloid hypothesis, which proposes that amyloid beta peptides disrupt neuronal metabolic and ionic homeostasis and cause aberrant activation of kinases and/or inhibition of phosphatases. The resulting alteration in kinase and phosphatase activities ultimately leads to hyperphosphorylation of tau and formation of neurofibrillary tangles. Cyclin-dependent kinase 5 (Cdk5) is a tau kinase whose activity is induced by amyloid beta peptides. Its deregulation may represent one of the signal transduction pathways that connect amyloid beta toxicity to tau hyperphosphorylation. This article reviews the functions and regulation of Cdk5. Evidence that suggests deregulation of Cdk5 activity in AD by virtue of calpain cleavage of its activator p35 to p25 will be discussed.

Alzheimer Disease↗
PubMed2002

The cyclin-dependent kinase 5 activators p35 and p39 interact with the alpha-subunit of Ca2+/calmodulin-dependent protein kinase II and alpha-actinin-1 in a calcium-dependent manner.

Cyclin-dependent kinase 5 (Cdk5) is a critical regulator of neuronal migration in the developing CNS, and recent studies have revealed a role for Cdk5 in synaptogenesis and regulation of synaptic transmission. Deregulation of Cdk5 has been linked to the pathology of neurodegenerative diseases such as Alzheimer's disease. Activation of Cdk5 requires its association with a regulatory subunit, and two Cdk5 activators, p35 and p39, have been identified. To gain further insight into the functions of Cdk5, we identified proteins that interact with p39 in a yeast two-hybrid screen. In this study we report that alpha-actinin-1 and the alpha-subunit of Ca2+/calmodulin-dependent protein kinase II (CaMKIIalpha), two proteins localized at the postsynaptic density, interact with Cdk5 via their association with p35 and p39. CaMKIIalpha and alpha-actinin-1 bind to distinct regions of p35 and p39 and also can interact with each other. The association of CaMKIIalpha and alpha-actinin-1 to the Cdk5 activators, as well as to each other, is stimulated by calcium. Further, the activation of glutamate receptors increases the association of p35 and p39 with CaMKIIalpha, and the inhibition of CaMKII activation diminishes this effect. The glutamate-mediated increase in association of p35 and CaMKIIalpha is mediated in large part by NMDA receptors, suggesting that cross talk between the Cdk5 and CaMKII signal transduction pathways may be a component of the complex molecular mechanisms contributing to synaptic plasticity, memory, and learning.

Actinin↗
PubMed2002

Calpain-mediated cleavage of the cyclin-dependent kinase-5 activator p39 to p29.

The activity of cyclin-dependent kinase-5 (Cdk5) is tightly regulated by binding of its neuronal activators p35 and p39. Upon neurotoxic insults, p35 is cleaved to p25 by the Ca(2+)-dependent protease calpain. p25 is accumulated in ischemic brains and in brains of patients with Alzheimer's disease. p25 deregulates Cdk5 activity by causing prolonged activation and mislocalization of Cdk5. It is unknown whether p39, which is expressed throughout the adult rat brain, is cleaved by calpain, and whether this contributes to deregulation of Cdk5. Here, we show that calpain cleaved p39 in vitro, resulting in generation of a C-terminal p29 fragment. In vivo, p29 was generated in ischemic brain concomitant with increased calpain activity. In fresh brain lysates, generation of p29 was Ca(2+)-dependent, and calpain inhibitors abolished p29 production. The Ca(2+) ionophore ionomycin and the excitotoxin glutamate induced production of p29 in cultures of cortical neurons in a calpain-dependent manner. Like p25, p29 was more stable than p39 and caused redistribution of Cdk5 in cortical neurons. Our data suggest that neurotoxic insults lead to calpain-mediated conversion of p39 to p29, which might contribute to deregulation of Cdk5.

Animals↗
PubMed2002

Life is a journey: a genetic look at neocortical development.

Although the basic principles of neocortical development have been known for quite some time, it is only recently that our understanding of the molecular mechanisms that are involved has improved. Such understanding has been facilitated by genetic approaches that have identified key proteins involved in neocortical development, which have been placed into signalling pathways by molecular and cell-biological studies. The challenge of current research is to understand the manner in which these various signalling pathways are interconnected to gain a more comprehensive picture of the molecular intricacies that govern neocortical development.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗