Gabriel Corfas

ARIA, a protein that stimulates acetylcholine receptor synthesis, is a member of the neu ligand family

Motor neurons stimulate their postsynaptic muscle targets to synthesize neurotransmitter receptors. Polypeptide signaling molecules may mediate this inductive interaction. Here we report the purification of ARIA, a protein that stimulates the synthesis of muscle acetylcholine receptors, and the isolation of ARIA cDNA. Recombinant ARIA increases acetylcholine receptor synthesis greater than 3-fold, and it induces tyrosine phosphorylation of a 185 kd muscle protein. The ARIA cDNA hybridizes with mRNAs that are expressed in the spinal cord from E4, a time prior to the onset of neuromuscular synapse formation, through adulthood. By E7, hybridizing mRNAs are concentrated in motor neurons. Chicken ARIA is homologous to the rat Neu differentiation factor and human here-gulin, ligands for the receptor tyrosine kinase encoded by the neu (c-erbB2, HER2) proto-oncogene. Our data suggest that members of the ARIA protein family promote the formation and maintenance of chemical synapses and, furthermore, that receptor tyrosine kinases play important roles in this process.

EphA Receptors Regulate Growth Cone Dynamics through the Novel Guanine Nucleotide Exchange Factor Ephexin

Eph receptors transduce short-range repulsive signals for axon guidance by modulating actin dynamics within growth cones. We report the cloning and characterization of ephexin, a novel Eph receptor-interacting protein that is a member of the Dbl family of guanine nucleotide exchange factors (GEFs) for Rho GTPases. Ephrin-A stimulation of EphA receptors modulates the activity of ephexin leading to RhoA activation, Cdc42 and Rac1 inhibition, and cell morphology changes. In addition, expression of a mutant form of ephexin in primary neurons interferes with ephrin-A-induced growth cone collapse. The association of ephexin with Eph receptors constitutes a molecular link between Eph receptors and the actin cytoskeleton and provides a novel mechanism for achieving highly localized regulation of growth cone motility.

Distinct roles of matrix metalloproteases in the early- and late-phase development of neuropathic pain.

Treatment of neuropathic pain, triggered by multiple insults to the nervous system, is a clinical challenge because the underlying mechanisms of neuropathic pain development remain poorly understood. Most treatments do not differentiate between different phases of neuropathic pain pathophysiology and simply focus on blocking neurotransmission, producing transient pain relief. Here, we report that early- and late-phase neuropathic pain development in rats and mice after nerve injury require different matrix metalloproteinases (MMPs). After spinal nerve ligation, MMP-9 shows a rapid and transient upregulation in injured dorsal root ganglion (DRG) primary sensory neurons consistent with an early phase of neuropathic pain, whereas MMP-2 shows a delayed response in DRG satellite cells and spinal astrocytes consistent with a late phase of neuropathic pain. Local inhibition of MMP-9 by an intrathecal route inhibits the early phase of neuropathic pain, whereas inhibition of MMP-2 suppresses the late phase of neuropathic pain. Further, intrathecal administration of MMP-9 or MMP-2 is sufficient to produce neuropathic pain symptoms. After nerve injury, MMP-9 induces neuropathic pain through interleukin-1β cleavage and microglial activation at early times, whereas MMP-2 maintains neuropathic pain through interleukin-1β cleavage and astrocyte activation at later times. Inhibition of MMP may provide a novel therapeutic approach for the treatment of neuropathic pain at different phases.

Neuregulin and erbB Receptors Play a Critical Role in Neuronal Migration

The migration of neuronal precursors along radial glial fibers is a critical step in the formation of the nervous system. In this report, we show that neuregulin-erbB receptor signaling plays a crucial role in the migration of cerebellar granule cells along radial glial fibers. Granule cells express neuregulin (NRG), and radial glia cells express erbB4 in the developing cerebellum and in vitro. When the glial erbB receptors are blocked, neurons fail to induce radial glia formation, and their migration along radial glial fibers is impaired. Moreover, soluble NRG is as effective as neuron-glia contact in the induction of radial glia formation. These results suggest that the activation of glial erbB4 by NRG is an early critical step in the neuronal migration program.

Neuregulin 1-erbB signaling and the molecular/cellular basis of schizophrenia

Schizophrenia is a devastating psychiatric disease that affects 0.5–1% of the worlds adult population. The hypothesis that this disease is a developmental disorder of the nervous system with late onset of its characteristic symptoms has been gaining acceptance in past years. However, the anatomical, cellular and molecular bases of schizophrenia remain unclear. Numerous studies point to alterations in different aspects of brain development as possible causes of schizophrenia, including defects in neuronal migration, neurotransmitter receptor expression and myelination. Recently, the gene that encodes neuregulin-1 (NRG1) has been identified as a potential susceptibility gene for schizophrenia, and defects in the expression of erbB3, one of the NRG1 receptors, have been shown to occur in the prefrontal cortex of schizophrenic patients, suggesting that NRG1-erbB signaling is involved in the pathogenesis of schizophrenia. These findings open new approaches to defining the molecular and cellular basis of schizophrenia in more mechanistic terms.

A Critical Period for Social Experience–Dependent Oligodendrocyte Maturation and Myelination

A Critical Period for Glia The brain develops in fits and starts—while one system is completed, another system may still be under construction. Such transient states are known as critical periods, and during these specific aspects of brain development may become more sensitive to outside agents than they would be later. Makinodan et al. (p. 1357) observed the effect of environmental conditions on the brains of mice bioengineered to develop fluorescent oligodendrocytes. The mice were exposed to a variety of social conditions during rearing, ranging from isolation to a normal laboratory cage setting, or to settings enriched with extra buddies and a steady rotation of new play toys. The results show that social isolation leaves a developmental trace that persists into adulthood. Specifically, they found that oligodendrocytes, which produce the myelin that insulates neurons, were underdeveloped, suggesting that there may be a critical period that governs development of these glial oligodendrocyte cells. In mice, early social experience regulates prefrontal cortex myelination that is essential for normal cognitive development. Early social isolation results in adult behavioral and cognitive dysfunction that correlates with white matter alterations. However, how social deprivation influences myelination and the significance of these myelin defects in the adult remained undefined. We show that mice isolated for 2 weeks immediately after weaning have alterations in prefrontal cortex function and myelination that do not recover with reintroduction into a social environment. These alterations, which occur only during this critical period, are phenocopied by loss of oligodendrocyte ErbB3 receptors, and social isolation leads to reduced expression of the ErbB3 ligand neuregulin-1. These findings indicate that social experience regulates prefrontal cortex myelination through neuregulin-1/ErbB3 signaling and that this is essential for normal cognitive function, thus providing a cellular and molecular context to understand the consequences of social isolation.

Targeting angiogenesis with a conjugate of HPMA copolymer and TNP-470.

Angiogenesis is crucial for tumor growth. Angiogenesis inhibitors, such as O-(chloracetyl-carbamoyl) fumagillol (TNP-470), are thus emerging as a new class of anticancer drugs. In clinical trials, TNP-470 slowed tumor growth in patients with metastatic cancer. However, at higher doses necessary for tumor regression, many patients experienced neurotoxicity. We therefore synthesized and characterized a water-soluble conjugate of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer, Gly-Phe-Leu-Gly linker and TNP-470. This conjugate accumulated selectively in tumor vessels because of the enhanced permeability and retention (EPR) effect. HPMA copolymer–TNP-470 substantially enhanced and prolonged the activity of TNP-470 in vivo in tumor and hepatectomy models. Polymer conjugation prevented TNP-470 from crossing the blood-brain barrier (BBB) and decreased its accumulation in normal organs, thereby avoiding drug-related toxicities. Treatment with TNP-470 caused weight loss and neurotoxic effects in mice, whereas treatment with the conjugate did not. This new approach for targeting angiogenesis inhibitors specifically to the tumor vasculature may provide a new strategy for the rational design of cancer therapies.

Presenilin-Dependent ErbB4 Nuclear Signaling Regulates the Timing of Astrogenesis in the Developing Brain

Embryonic multipotent neural precursors are exposed to extracellular signals instructing them to adopt different fates, neuronal or glial. However, the mechanisms by which precursors integrate these signals to make timely fate choices remained undefined. Here we show that direct nuclear signaling by a receptor tyrosine kinase inhibits the responses of precursors to astrocyte differentiation factors while maintaining their neurogenic potential. Upon neuregulin-induced activation and presenilin-dependent cleavage of ErbB4, the receptors intracellular domain forms a complex with TAB2 and the corepressor N-CoR. This complex undergoes nuclear translocation and binds promoters of astrocytic genes, repressing their expression. Consistent with this observation, astrogenesis occurs precociously in ErbB4 knockout mice. Our studies define how presenilin-dependent nuclear signaling by a receptor tyrosine kinase directly regulates gene transcription and cell fate. This pathway could be of importance for neural stem cell biology and for understanding the pathogenesis of Alzheimers disease.

Loss of erbB signaling in oligodendrocytes alters myelin and dopaminergic function, a potential mechanism for neuropsychiatric disorders

Several psychiatric disorders are associated with white matter defects, suggesting that oligodendrocyte (OL) abnormalities underlie some aspects of these diseases. Neuregulin 1 (NRG1) and its receptor, erbB4, are genetically linked with susceptibility to schizophrenia and bipolar disorder. In vitro studies suggest that NRG1-erbB signaling is important for OL development. To test whether erbB signaling contributes to psychiatric disorders by regulating the structure or function of OLs, we analyzed transgenic mice in which erbB signaling is blocked in OLs in vivo. Here we show that loss of erbB signaling leads to changes in OL number and morphology, reduced myelin thickness, and slower conduction velocity in CNS axons. Furthermore, these transgenic mice have increased levels of dopamine receptors and transporters and behavioral alterations consistent with neuropsychiatric disorders. These results indicate that defects in white matter can cause alterations in dopaminergic function and behavior relevant to neuropsychiatric disorders.

Differential expression of ARIA isoforms in the rat brain

ARIA, heregulin, neu differentiation factor, and glial growth factor are members of a new family of growth and differentiation factors whose effects have been assayed on Schwann cells, skeletal muscle cells, and mammary tumor cell lines. To gain insight into their roles in the CNS, we studied the expression of ARIA in the rat brain. We found ARIA mRNA in all cholinergic neurons throughout the CNS, including motor neurons and cells of the medial septal nucleus and the nucleus basalis of Meynert. We also found that ARIA induces tyrosine phosphorylation of a 185 kDa protein in central and peripheral targets of these cholinergic neurons. ARIA mRNA, however, is not restricted to cholinergic neurons, suggesting that it may also play a role at other types of synapses. Its distribution in germinal layers of the telencephalon and cerebellum suggests that it may also play a role in the proliferation and/or migration of neuronal and glial precursor cells.