Gerald D. Fischbach

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.

Activity-Dependent Modulation of Synaptic AMPA Receptor Accumulation

Both theoretical and experimental work have suggested that central neurons compensate for changes in excitatory synaptic input in order to maintain a relatively constant output. We report here that inhibition of excitatory synaptic transmission in cultured spinal neurons leads to an increase in mEPSC amplitudes, accompanied by an equivalent increase in the accumulation of AMPA receptors at synapses. Conversely, increasing excitatory synaptic activity leads to a decrease in synaptic AMPA receptors and a decline in mEPSC amplitude. The time course of this synaptic remodeling is slow, similar to the metabolic half-life of neuronal AMPA receptors. Moreover, inhibiting excitatory synaptic transmission significantly prolongs the half-life of the AMPA receptor subunit GluR1, suggesting that synaptic activity modulates the size of the mEPSC by regulating the turnover of postsynaptic AMPA receptors.

Neuregulin and ErbB receptor signaling pathways in the nervous system.

The neuregulins are a complex family of factors that perform many functions during neural development. Recent experiments have shown that neuregulins promote neuronal migration and differentiation, and regulate the selective expression of neurotransmitter receptors in neurons and at the neuromuscular junction. They also regulate glial commitment, proliferation, survival and differentiation. At interneuronal synapses, neuregulin ErbB receptors associate with PDZ-domain proteins at postsynaptic densities where they can modulate synaptic plasticity. How this combinatorial network - comprising many neuregulin ligands that signal through distinct combinations of dimeric ErbB receptors - elicits its multitude of biological effects is beginning to be resolved.

Glutamate receptor desensitization and its role in synaptic transmission

Responses of excitatory amino acid receptors to rapidly applied glutamate were measured in outside-out membrane patches from chick spinal neurons. The peak current varied with glutamate concentration, with a half-maximal response at 510 microM and a Hill coefficient near 2. Currents activated by 1 mM glutamate desensitized and recovered in two phases. The faster time constant was identical to the time constant of decay of synaptic currents, suggesting that glutamatergic synaptic currents are terminated, in part, by receptor desensitization. Steady-state desensitization was evident following application of only 2-3 microM glutamate, concentrations comparable to levels in the extracellular space in the intact brain. Thus, glutamate receptor desensitization can affect synaptic efficacy in two ways: at high concentrations, rapid desensitization of receptors may curtail synaptic currents; at low concentrations, there is a significant reduction in the number of activatable receptors.

Neurotransmitters decrease the calcium component of sensory neurone action potentials

RELEASE of neurotransmitters from presynaptic axon terminals requires the influx of Ca2+ ions during the nerve terminal action potential1. Action potentials recorded in some neurone cell bodies exhibit a relatively large Ca2+ component, and it has been suggested that these soma Ca2+ spikes may provide a model for Ca2+ influx across the less accessible nerve terminal membrane2. Recent data support the usefulness of this model. Serotonin (5-hydroxytryptamine, 5-HT) increases transmitter output at certain habituated sensory nerve-motoneurone synapses in the abdominal ganglion of Aplysia and it also prolongs the Ca2+ spike recorded in the sensory neurone cell body3. Enkephalin reduces the stimulated release of substance P by adult cat trigeminal neurones4 and by cultured embryonic chick dorsal root ganglion (DRG) neurones5, and it decreases the quantal content of excitatory postsynaptic potentials (e.p.s.ps, transmitter unknown) evoked in cultured rat spinal cord neurones by co-cultured DRG cells6. This peptide also decreases the duration and magnitude of the Ca2+ component of the DRG soma spike5. With the thought that modulation of Ca2+ currents may be a general correlate of presynaptic inhibition, we have studied the effect of several putative neurotransmitters on the soma spike of cultured chick sensory neurones, and report here that they decrease the calcium component of cell body action potentials.

The Simons Simplex Collection: A Resource for Identification of Autism Genetic Risk Factors

In an effort to identify de novo genetic variants that contribute to the overall risk of autism, the Simons Foundation Autism Research Initiative (SFARI) has gathered a unique sample called the Simons Simplex Collection (SSC). More than 2000 families have been evaluated to date. On average, probands in the current sample exhibit moderate to severe autistic symptoms with relatively little intellectual disability. An interactive database has been created to facilitate correlations between clinical, genetic, and neurobiological data.

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.

Deficiency of Dgcr8, a gene disrupted by the 22q11.2 microdeletion, results in altered short-term plasticity in the prefrontal cortex

Individuals with 22q11.2 microdeletions have cognitive and behavioral impairments and the highest known genetic risk for developing schizophrenia. One gene disrupted by the 22q11.2 microdeletion is DGCR8, a component of the “microprocessor” complex that is essential for microRNA production, resulting in abnormal processing of specific brain miRNAs and working memory deficits. Here, we determine the effect of Dgcr8 deficiency on the structure and function of cortical circuits by assessing their laminar organization, as well as the neuronal morphology, and intrinsic and synaptic properties of layer 5 pyramidal neurons in the prefrontal cortex of Dgcr8+/− mutant mice. We found that heterozygous Dgcr8 mutant mice have slightly fewer cortical layer 2/4 neurons and that the basal dendrites of layer 5 pyramidal neurons have slightly smaller spines. In addition to the modest structural changes, field potential and whole-cell electrophysiological recordings performed in layer 5 of the prefrontal cortex revealed greater short-term synaptic depression during brief stimulation trains applied at 50 Hz to superficial cortical layers. This finding was accompanied by a decrease in the initial phase of synaptic potentiation. Our results identify altered short-term plasticity as a neural substrate underlying the cognitive dysfunction and the increased risk for schizophrenia associated with the 22q11.2 microdeletions.

Elevated potassium induces morphological differentiation of dorsal root ganglionic neurons in dissociated cell culture.

Abstract Counts performed on dissociated cell cultures of E10 chick embryo dorsal root ganglia (DRG) maintained in control (6 m M K + ) medium showed a consistent neuron loss with time in culture, even in the presence of nerve growth factor (NGF). In medium containing elevated potassium (40 m M K + ), the neuron number did not decrease with time, but in fact increased. Over the first week in vitro this increase was about 25% in the absence and 60% in the presence of NGF. Time-lapse cinematography and sequential photography performed on sister cultures showed that the increase in neuronal counts in elevated K + media corresponded to a morphological differentiation of DRG cells previously not recognized as neurons. [ 3 H]Thymidine labeling studies in vitro were consistent with the conclusion that neurons or neuronal precursor cells did not divide during the phase of increase in the number of identifiable neurons. Dorsal root ganglion (DRG) neurons grown in 40 m M K + were more depolarized than those grown in 6 m M K + and, in response to changes in extracellular K + concentration, showed changes in membrane potential similar to those shown by neurons grown in control K + . Action potentials evoked by intracellular stimulation of DRG neurons grown in high K + lacked the plateau phase characteristic of neurons grown in control K + .

Regulation of Muscle Acetylcholine Sensitivity by Muscle Activity in Cell Culture

Muscle in tissue culture provides a good system for studying longterm changes in surface membrane acetylcholine sensitivity. Muscle fibers stimulated intermittently over prolonged periods are less sensitive to iontophoretically applied acetylcholine and bind less 125I-labeled α-bungarotoxin than inactive fibers.