Biomedical subjects

James Q Zheng

Publications and source records attributed to James Q Zheng.

12 recordsLinked to original sources

PubMed2007

Spatial targeting of type II protein kinase A to filopodia mediates the regulation of growth cone guidance by cAMP.

The second messenger cyclic adenosine monophosphate (cAMP) plays a pivotal role in axonal growth and guidance, but its downstream mechanisms remain elusive. In this study, we report that type II protein kinase A (PKA) is highly enriched in growth cone filopodia, and this spatial localization enables the coupling of cAMP signaling to its specific effectors to regulate guidance responses. Disrupting the localization of PKA to filopodia impairs cAMP-mediated growth cone attraction and prevents the switching of repulsive responses to attraction by elevated cAMP. Our data further show that PKA targets protein phosphatase-1 (PP1) through the phosphorylation of a regulatory protein inhibitor-1 (I-1) to promote growth cone attraction. Finally, we find that I-1 and PP1 mediate growth cone repulsion induced by myelin-associated glycoprotein. These findings demonstrate that the spatial localization of type II PKA to growth cone filopodia plays an important role in the regulation of growth cone motility and guidance by cAMP.

Animals↗

PubMed2006

Acute impairment of mitochondrial trafficking by beta-amyloid peptides in hippocampal neurons.

Defects in axonal transport are often associated with a wide variety of neurological diseases including Alzheimer's disease (AD). Beta-amyloid (Abeta) is a major component of neuritic plaques associated with pathological conditions of AD brains. Here, we report that a brief exposure of cultured hippocampal neurons to Abeta molecules resulted in rapid and severe impairment of mitochondrial transport without inducing apparent cell death and significant morphological changes. Such acute inhibition of mitochondrial transport was not associated with a disruption of mitochondria potential nor involved aberrant cytoskeletal changes. Abeta also did not elicit significant Ca2+ signaling to affect mitochondrial trafficking. However, stimulation of protein kinase A (PKA) by forskolin, cAMP analogs, or neuropeptides effectively alleviated the impairment. We also show that Abeta inhibited mitochondrial transport by acting through glycogen synthase kinase 3beta (GSK3beta). Given that mitochondria are crucial organelles for many cellular functions and survival, our findings thus identify an important acute action of Abeta molecules on nerve cells that could potentially contribute to various abnormalities of neuronal functions under AD conditions. Manipulation of GSK3beta and PKA activities may represent a key approach for preventing and alleviating Abeta cytotoxicity and AD pathological conditions.

Amyloid beta-Peptides↗

PubMed2006

Activity-independent regulation of dendrite patterning by postsynaptic density protein PSD-95.

Dendritic morphology determines many aspects of neuronal function, including action potential propagation and information processing. However, the question remains as to how distinct neuronal dendrite branching patterns are established. Here, we report that postsynaptic density-95 (PSD-95), a protein involved in dendritic spine maturation and clustering of synaptic signaling proteins, plays a novel role in regulating dendrite outgrowth and branching, independent of its synaptic functions. In immature neurons, overexpression of PSD-95 decreases the proportion of primary dendrites that undergo additional branching, resulting in a marked reduction of secondary dendrite number. Conversely, knocking down PSD-95 protein in immature neurons increases secondary dendrite number. The effect of PSD-95 is activity-independent and is antagonized by cypin, a nonsynaptic protein that regulates PSD-95 localization. Binding of cypin to PSD-95 correlates with formation of stable dendrite branches. Finally, overexpression of PSD-95 in COS-7 cells disrupts microtubule organization, indicating that PSD-95 may modulate microtubules to regulate dendritic branching. Whereas many factors have been identified which regulate dendrite number, our findings provide direct evidence that proteins primarily involved in synaptic functions can also play developmental roles in shaping how a neuron patterns its dendrite branches.

Animals↗

PubMed2006

An essential role for beta-actin mRNA localization and translation in Ca2+-dependent growth cone guidance.

Axon pathfinding requires directional responses of growth cones to extracellular cues, which have been shown to involve local synthesis of protein. The identity and functions of the locally produced proteins remain, however, unclear. Here we report that Ca(2+)-dependent bidirectional turning of Xenopus laevis growth cones requires localized distribution and translation of beta-actin messenger RNA. Both beta-actin mRNA and its zipcode-binding protein, ZBP1, are localized at the growth cone and become asymmetrically distributed upon local exposure to brain-derived neurotrophic factor (BDNF). Inhibition of protein synthesis or antisense interference with beta-actin mRNA-ZBP1 binding abolishes both Ca(2+)-mediated attraction and repulsion. In addition, attraction involves a local increase in beta-actin, whereas repulsion is accompanied by a local decrease in beta-actin; thus, both produce a synthesis- and ZBP1 binding-dependent beta-actin asymmetry but with opposite polarities. Together with a similar asymmetry in Src activity during bidirectional responses, our findings indicate that Ca(2+)-dependent spatial regulation of beta-actin synthesis through Src contributes to the directional motility of growth cones during guidance.

Actins↗

PubMed2006

Directional guidance of nerve growth cones.

The intricate connections of the nervous system are established, in part, by elongating axonal fibers that are directed by complex guidance systems to home in on their specific targets. The growth cone, the major motile apparatus at the tip of axons, explores its surroundings and steers the axon along a defined path to its appropriate target. Significant progress has been made in identifying the guidance molecules and receptors that regulate growth cone pathfinding, the signaling cascades underlying distinct growth cone behaviors, and the cytoskeletal components that give rise to the directional motility of the growth cone. Recent studies have also shed light on the sophisticated mechanisms and new players utilized by the growth cone during pathfinding. It is clear that axon pathfinding requires a growth cone to sample and integrate various signals both in space and in time, and subsequently to coordinate the dynamics of its membrane, cytoskeleton and adhesion to generate specific responses.

Animals↗

PubMed2006

The molecular basis for calcium-dependent axon pathfinding.

Ca(2+) signals have profound and varied effects on growth cone motility and guidance. Modulation of Ca(2+) influx and release from stores by guidance cues shapes Ca(2+) signals, which determine the activation of downstream targets. Although the precise molecular mechanisms that underlie distinct Ca(2+)-mediated effects on growth cone behaviours remain unclear, recent studies have identified important players in both the regulation and targets of Ca(2+) signals in growth cones.

Animals↗

PubMed2004

A CaMKII/calcineurin switch controls the direction of Ca(2+)-dependent growth cone guidance.

Axon pathfinding depends on attractive and repulsive turning of growth cones to extracellular cues. Localized cytosolic Ca2+ signals are known to mediate the bidirectional responses, but downstream mechanisms remain elusive. Here, we report that calcium-calmodulin-dependent protein kinase II (CaMKII) and calcineurin (CaN) phosphatase provide a switch-like mechanism to control the direction of Ca(2+)-dependent growth cone turning. A relatively large local Ca2+ elevation preferentially activates CaMKII to induce attraction, while a modest local Ca2+ signal predominantly acts through CaN and phosphatase-1 (PP1) to produce repulsion. The resting level of intracellular Ca2+ concentrations also affects CaMKII/CaN operation: a normal baseline allows distinct turning responses to different local Ca2+ signals, while a low baseline favors CaN-PP1 activation for repulsion. Moreover, the cAMP pathway negatively regulates CaN-PP1 signaling to inhibit repulsion. Finally, CaMKII/CaN-PP1 also mediates netrin-1 guidance. Together, these findings establish a complex Ca2+ mechanism that targets the balance of CaMKII/CaN-PP1 activation to control distinct growth cone responses.

Animals↗

PubMed2004

Lipid rafts mediate chemotropic guidance of nerve growth cones.

Axon guidance requires signal transduction of extracellular cues through the plasma membrane for directional motility. Here we present evidence that cholesterol- and sphingolipid-enriched membrane microdomains (lipid rafts) mediate specific guidance responses of nerve growth cones. Disruption of lipid rafts by various approaches targeting cholesterol or gangliosides selectively abolished growth cone attraction and repulsion in BDNF and netrin-1 gradients, respectively, without affecting glutamate-induced attraction. Interestingly, local raft disruption on one side of the growth cone in bath BDNF or netrin-1 produced opposite turning responses to that induced by the gradients. Raft manipulation also blocked Semaphorin 3A-induced growth cone repulsion, inhibition, and collapse. Finally, guidance responses appeared to involve raft-dependent activation of p42/p44 MAPK and ligand-induced receptor recruitment to lipid rafts. Together with the observation of asymmetric receptor-raft associations at the growth cone in guidance gradients, our findings indicate that localized signaling through membrane rafts plays a role in mediating guidance actions of extracellular cues on developing axons.

Animals↗

PubMed2003

Frequency modulation of synchronized Ca2+ spikes in cultured hippocampal networks through G-protein-coupled receptors.

Synchronized spontaneous Ca2+ spikes in networked neurons represent periodic burst firing of action potentials, which are believed to play a major role in the development and plasticity of neuronal circuitry. How these network activities are shaped and modulated by extrinsic factors during development, however, remains to be studied. Here we report that synchronized Ca2+ spikes among cultured hippocampal neurons can be modulated by two small factors that act on G-protein-coupled receptors (GPCRs): the neuropeptide PACAP (pituitary adenylate cyclase-activating polypeptide) and the chemokine SDF-1 (stromal cell-derived factor-1). PACAP effectively increases the frequency of the synchronized Ca2+ spikes when applied acutely; the PACAP potentiation of Ca2+ spikes requires the activation of the PACAP-specific PAC1 GPCRs and is mediated by the activation of cAMP signaling pathway. SDF-1, on the other hand, significantly reduces the frequency of these Ca2+ spikes through the activation of its specific GPCR CXCR4; the inhibitory action of SDF-1 is mediated by the inhibition of cAMP pathway through the Gi component of GPCRs. Taken together, these results demonstrate that synchronized neuronal network activity can be effectively modulated by physiologically and developmentally relevant small factors that act on GPCRs to target the cAMP pathway. Such modulation of neuronal activity through GPCRs may represent a significant mechanism that underlies the neuronal plasticity during neural development and functioning.

Action Potentials↗

PubMed2003

Direct cAMP signaling through G-protein-coupled receptors mediates growth cone attraction induced by pituitary adenylate cyclase-activating polypeptide.

Developing axons are guided to their appropriate targets by environmental cues through the activation of specific receptors and intracellular signaling pathways. Here we report that gradients of pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide widely expressed in the developing nervous system, induce marked attraction of Xenopus growth cones in vitro. PACAP exerted its chemoattractive effects through PAC1, a PACAP-selective G-protein-coupled receptor (GPRC) expressed at the growth cone. Furthermore, the attraction depended on localized cAMP signaling because it was completely blocked either by global elevation of intracellular cAMP levels using forskolin or by inhibition of protein kinase A using specific inhibitors. Moreover, local direct elevation of intracellular cAMP by focal photolysis of caged cAMP compounds was sufficient to induce growth cone attraction. On the other hand, blockade of Ca2+, phospholipase C, or phosphatidyl inositol-3 kinase signaling pathways did not affect PACAP-induced growth cone attraction. Finally, PACAP-induced attraction also involved the Rho family of small GTPases and required local protein synthesis. Taken together, our results establish cAMP signaling as an independent pathway capable of mediating growth cone attraction induced by a physiologically relevant peptide acting through GPCRs. Such a direct cAMP pathway could potentially operate in other guidance systems for the accurate wiring of the nervous system.

Animals↗

PubMed2003

Cytoskeletal dynamics underlying collateral membrane protrusions induced by neurotrophins in cultured Xenopus embryonic neurons.

The establishment and refinement of neuronal connections depend on dynamic modification of the morphology and physiology of developing axons in response to extrinsic factors. In embryonic cultures of Xenopus spinal neurons, acute application of brain-derived neurotrophic factor (BDNF) induced rapid collateral protrusion of filopodium-like microspikes and lamellipodia along the neurite processes, leading to a morphologic alternation of the neuron. Both types of membrane protrusions contained high concentrations of actin filaments and depended on the polymerization of the actin cytoskeleton. Immunofluorescent staining, however, revealed the presence of microtubules (MTs) in lamellipodia induced by BDNF. These MTs appeared to have arisen from debundling of MTs in the neurite shaft at the protrusion sites, splaying and extending in the rapidly protruding lamellipodia. Inhibition of microtubule polymerization by nocodazole largely abolished the formation of lamellipodia but not of microspikes. Taken together, our results suggest that collateral sprouting of microspikes and lamellipodia involve distinctly different cytoskeletal mechanisms. Although the actin cytoskeleton is solely responsible for microspike formation, cooperative efforts by microtubules and actin filaments are essential for lamellipodial protrusion in response to extrinsic factors.

Actin Cytoskeleton↗

PubMed2002

Growth cone turning induced by direct local modification of microtubule dynamics.

Pathfinding by nerve growth cones depends on attractive and repulsive turning in response to a variety of guidance cues. Here we present direct evidence to demonstrate an essential and instructive role for microtubules (MTs) in growth cone steering. First, both growth cone attraction and repulsion induced by diffusible cues in culture can be completely blocked by low concentrations of drugs that specifically inhibit dynamic microtubule ends in the growth cone. Second, direct focal photoactivated release of the microtubule-stabilizing drug taxol on one side of the growth cone consistently induces attraction (turning toward the site of application). Using the focal pipette application method, we also show that local MT stabilization by taxol induces growth cone attraction, whereas local MT destabilization by the microtubule-disrupting drug nocodazole induces repulsion (turning away). Finally, the microtubule-initiated attractive turning requires the participation of the actin cytoskeleton: local microtubule stabilization induces preferential protrusion of lamellipodia before the attractive turning, and the attraction can be abolished by inhibition of either actin polymerization or the Rho family GTPases. Together, these results demonstrate a novel steering mechanism for growth cones in which local and selective modification of dynamic microtubules can initiate and instruct directional steering. With the subsequent concerted activity of the actin cytoskeleton, this microtubule-initiated mechanism provides the growth cone with the additional means to efficiently navigate through its environment.

Actins↗