Nan-Jie Xu

Dissociation of EphB2 Signaling Pathways Mediating Progenitor Cell Proliferation and Tumor Suppression

Signaling proteins driving the proliferation of stem and progenitor cells are often encoded by proto-oncogenes. EphB receptors represent a rare exception; they promote cell proliferation in the intestinal epithelium and function as tumor suppressors by controlling cell migration and inhibiting invasive growth. We show that cell migration and proliferation are controlled independently by the receptor EphB2. EphB2 regulated cell positioning is kinase-independent and mediated via phosphatidylinositol 3-kinase, whereas EphB2 tyrosine kinase activity regulates cell proliferation through an Abl-cyclin D1 pathway. Cyclin D1 regulation becomes uncoupled from EphB signaling during the progression from adenoma to colon carcinoma in humans, allowing continued proliferation with invasive growth. The dissociation of EphB2 signaling pathways enables the selective inhibition of the mitogenic effect without affecting the tumor suppressor function and identifies a pharmacological strategy to suppress adenoma growth.

Ephrin-B3 reverse signaling through Grb4 and cytoskeletal regulators mediates axon pruning

It has been suggested that ephrin-B proteins have receptor-like roles in the control of axon pathfinding by repulsion, although it is largely unknown how the reverse signals are coupled to downstream intracellular molecules and how they induce cytoskeletal reorganization at the axon terminal. We found that ephrin-B3 (EB3) was able to function as a repulsive guidance receptor and mediate stereotyped pruning of murine hippocampal mossy fiber axons during postnatal development. Targeted intracellular point mutants showed that axon pruning requires tyrosine phosphorylation–dependent reverse signaling and coupling to the SH2/SH3 adaptor protein Grb4 (also known as Nckβ/Nck2). Furthermore, we found that the second SH3 domain of Grb4 is required and sufficient for axon pruning/retraction by mediating interactions with Dock180 and PAK to bring about guanine nucleotide exchange and signaling downstream of Rac, respectively. Our results reveal a previously unknown pathway that controls axon pruning and elucidate the biochemical mechanism by which ephrin-B reverse signals regulate actin dynamics to bring about the retraction of growth cones.

Reduced Synaptic STIM2 Expression and Impaired Store-Operated Calcium Entry Cause Destabilization of Mature Spines in Mutant Presenilin Mice

Mushroom dendritic spine structures are essential for memory storage, and the loss of mushroom spines may explain memory defects in Alzheimers disease (AD). Here we show a significant reduction in the fraction of mushroom spines in hippocampal neurons from the presenilin-1 M146V knockin (KI) mouse model of familial AD (FAD). The stabilization of mushroom spines depends on STIM2-mediated neuronal store-operated calcium influx (nSOC) and continuous activity of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). We demonstrate that STIM2-nSOC-CaMKII pathway is compromised in KI neurons, in aging neurons, and in sporadic AD brains due to downregulation of STIM2 protein. We further establish that overexpression of STIM2 rescues synaptic nSOC, CaMKII activity, and mushroom spine loss in KI neurons. Our results identify STIM2-nSOC-CaMKII synaptic maintenance pathway as a novel potential therapeutic target for treatment of AD and age-related memory decline.

B-Ephrin Reverse Signaling Is Required for NMDA-Independent Long-Term Potentiation of Mossy Fibers in the Hippocampus

The mossy fiber to CA3 pyramidal neuron synapse in the hippocampus displays an atypical form of NMDA receptor-independent long-term potentiation (LTP). Plasticity at this synapse is expressed in the presynaptic terminal as an elevated probability of neurotransmitter release. However, evidence indicates that postsynaptic mechanisms and trans-synaptic signaling through an association between postsynaptic EphB receptors and presynaptic B-ephrins are necessary for the induction of LTP. Here we show that ephrin-B3 protein is highly expressed in mossy fiber axons and terminals. There are specific deficits in mossy fiber LTP in mice in which the cytoplasmic C-terminal signaling domain of the ephrin-B3 protein is replaced with β-galactosidase. These deficits are not observed in ephrin-B3 null mutant mice because of functional redundancy by virtue of other B-ephrins. These results indicate that B-ephrin reverse signaling into the presynaptic mossy fiber bouton is required for the induction of NMDA receptor-independent LTP at this synapse.

Reversal of morphine-induced memory impairment in mice by withdrawal in Morris water maze: possible involvement of cholinergic system

The effects of morphine and morphine withdrawal on memory performance were examined in mice by using Morris water maze task. Morphine-induced memory impairment at the doses of 5 and 10 mg/kg recovered after repeated administration. Oxotremorine, a muscarinic receptor agonist, at the dose of 0.1 mg/kg ip, and physostigmine, a cholinesterase inhibitor, at the dose of 0.1 mg/kg ip, significantly antagonized morphine (10 mg/kg sc)-induced memory impairment in mice. Furthermore, repeated naloxone (0.5 mg/kg ip) attenuated scopolamine (0.2 mg/kg ip)-induced memory impairment. By using escalating doses of morphine for 13 days, morphine-induced memory impairment was continuously maintained. When withdrawal was precipitated by naloxone (5 mg/kg ip), or administration of oxotremorine (0.1 and 0.2 mg/kg ip) or physostigmine (0.05 and 0.1 mg/kg ip), the impairment was completely reversed. These results suggest that morphine-induced memory impairment could be partially due to the inhibition of the central cholinergic activity.

Postsynaptic EphrinB3 Promotes Shaft Glutamatergic Synapse Formation

Excitatory synapses in the CNS are formed on both dendritic spines and shafts. Recent studies show that the density of shaft synapses may be independently regulated by behavioral learning and the induction of synaptic plasticity, suggesting that distinct mechanisms are involved in regulating these two types of synapses. Although the molecular mechanisms underlying spinogenesis and spine synapse formation are being delineated, those regulating shaft synapses are still unknown. Here, we show that postsynaptic ephrinB3 expression promotes the formation of glutamatergic synapses specifically on the shafts, not on spines. Reducing or increasing postsynaptic ephrinB3 expression selectively decreases or increases shaft synapse density, respectively. In the ephrinB3 knock-out mouse, although spine synapses are normal, shaft synapse formation is reduced in the hippocampus. Overexpression of glutamate receptor-interacting protein 1 (GRIP1) rescues ephrinB3 knockdown phenotype by restoring shaft synapse density. GRIP1 knockdown prevents the increase in shaft synapse density induced by ephrinB3 overexpression. Together, our results reveal a novel mechanism for independent modulation of shaft synapses through ephrinB3 reverse signaling.

Reactivation of morphine conditioned place preference by drug priming: role of environmental cues and sensitization

Rationale: Relapse is a major characteristic of drug addiction and remains the primary problem in treating drug abuse. Despite a great deal of research, the exact factors that determine renewed drug-seeking and persistent craving for them remain unclear. Objective: The present study was designed to evaluate the role of environmental cues and behavioral sensitization in reactivation of place preference following long-term extinction of morphine conditioned place preference (CPP) in rats. Methods: After being injected with morphine and saline alternately for 6 days to induce morphine CPP, the rats were subjected to extinction of conditioning for 21 days. The rats were then administered various doses of morphine, heroin, or cocaine and confined in the previous drug- or saline-paired compartment. CPP was determined. Some rats were treated with scopolamine or naloxone prior to administration of these three drugs. Results: Morphine CPP disappeared following a 21-day extinction. A single injection of morphine, heroin, or cocaine evoked place preference for the previous drug-paired side. However, place preference for the previous vehicle-paired side was induced after the animals received a single injection of morphine, heroin or cocaine and confined to the previous vehicle-paired compartment. Administration of naloxone prior to drug treatment significantly attenuated the place preference induced by morphine or heroin, but had no significant effect on the place preference elicited by cocaine. Administration of the cholinergic antagonist scopolamine before morphine, heroin and cocaine inhibited the expression of place preference. Conclusions: Environment-related cues and behavioral sensitization play critical roles in the incentive motivation underlying drug-seeking behaviors.

A dual shaping mechanism for postsynaptic ephrin-B3 as a receptor that sculpts dendrites and synapses

As the neural network becomes wired, postsynaptic signaling molecules are thought to control the growth of dendrites and synapses. However, how these molecules are coordinated to sculpt postsynaptic structures is less well understood. We find that ephrin-B3, a transmembrane ligand for Eph receptors, functions postsynaptically as a receptor to transduce reverse signals into developing dendrites of mouse hippocampal neurons. Both tyrosine phosphorylation–dependent GRB4 SH2/SH3 adaptor-mediated signals and PSD-95–discs large–zona occludens-1 (PDZ) domain–dependent signals are required for inhibition of dendrite branching, whereas only PDZ interactions are necessary for spine formation and excitatory synaptic function. PICK1 and syntenin, two PDZ domain proteins, participate with ephrin-B3 in these postsynaptic activities. PICK1 has a specific role in spine and synapse formation, and syntenin promotes both dendrite pruning and synapse formation to build postsynaptic structures that are essential for neural circuits. The study thus dissects ephrin-B reverse signaling into three distinct intracellular pathways and protein–protein interactions that mediate the maturation of postsynaptic neurons.

Ephrin reverse signaling in axon guidance and synaptogenesis

Axon-cell and axon-dendrite contact is a highly regulated process necessary for the formation of precise neural circuits and a functional neural network. Eph-ephrin interacting molecules on the membranes of axon nerve terminals and target dendrites act as bidirectional ligands/receptors to transduce signals into both the Eph-expressing and ephrin-expressing cells to regulate cytoskeletal dynamics. In particular, recent evidence indicates that ephrin reverse signal transduction events are important in controlling both axonal and dendritic elaborations of neurons in the developing nervous system. Here we review how ephrin reverse signals are transduced into neurons to control maturation of axonal pre-synaptic and dendritic post-synaptic structures.

Morphine modulates glutamate release in the hippocampal CA1 area in mice

Opiate abuse is associated with long-lasting neural adaptative changes in the brain. Increasing evidence demonstrates that opiates significantly alter the function of the glutamatergic system, while how the system is regulated still remains elusive. In the present study, we studied the effect of morphine on extracellular glutamate concentration in the hippocampus, a nucleus rich of the glutamatergic neurons. The results showed that glutamate concentration in the extracellular fluid of the hippocampus was decreased following either acute or chronic treatment of morphine. However, naloxone-induced withdrawal increased glutamate concentration significantly. These results suggest an adaptation of the glutamatergic neuronal transmission in the hippocampus after morphine treatment.