Biomedical subjects

Michele H Jacob

Publications and source records attributed to Michele H Jacob.

6 recordsLinked to original sources

PubMed2005

A null mutation for the alpha3 nicotinic acetylcholine (ACh) receptor gene abolishes fast synaptic activity in sympathetic ganglia and reveals that ACh output from developing preganglionic terminals is regulated in an activity-dependent retrograde manner.

In vertebrates, synaptic activity exerts an important influence on the formation of neural circuits, yet our understanding of its role in directing presynaptic and postsynaptic differentiation during synaptogenesis is incomplete. This study investigates how activity influences synaptic differentiation as synapses mature during early postnatal life. Specifically, we ask what happens to presynaptic terminals when synapses develop without functional postsynaptic receptors and without fast synaptic transmission. To address this issue, we investigated cholinergic nicotinic synapses in sympathetic ganglia of mice with a null mutation for the alpha3 nicotinic ACh receptor gene. Disrupting the alpha3 gene completely eliminates fast excitatory synaptic potentials on postganglionic sympathetic neurons, establishing a crucial role for alpha3-containing postsynaptic receptors in synaptic transmission. Interestingly, the preganglionic nerve terminals form morphologically normal synapses with sympathetic neurons, and these synapses persist without activity in postnatal animals. Surprisingly, when stimulating the preganglionic nerve at physiological rates, we discovered a significant decrease in ACh output from the presynaptic terminals in these alpha3(-/-) sympathetic ganglia. We show that this decrease in ACh output from the presynaptic terminals results, in part, from a lack of functional high-affinity choline transporters. We conclude the following: (1) fast synaptic transmission in mammalian SCG requires alpha3 expression; (2) in the absence of activity, the preganglionic nerve forms synapses that appear morphologically normal and persist for several weeks; and (3) to sustain transmitter release, developing presynaptic terminals require an activity-dependent retrograde signal.

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PubMed2004

Neuronal nicotinic synapse assembly requires the adenomatous polyposis coli tumor suppressor protein.

Normal cognitive and autonomic functions require nicotinic synaptic signaling. Despite the physiological importance of these synapses, little is known about molecular mechanisms that direct their assembly during development. We show here that the tumor-suppressor protein adenomatous polyposis coli (APC) functions in localizing alpha3-nicotinic acetylcholine receptors (nAChRs) to neuronal postsynaptic sites. Our quantitative confocal microscopy studies indicate that APC is selectively enriched at cholinergic synapses; APC surface clusters are juxtaposed to synaptic vesicle clusters and colocalize with alpha3-nAChRs but not with the neighboring synaptic glycine receptors or perisynaptic alpha7-nAChRs on chick ciliary ganglion (CG) neurons. We identify PSD (postsynaptic density)-93, beta-catenin, and microtubule end binding protein EB1 as APC binding partners. PSD-93 and beta-catenin are also enriched at alpha3-nAChR postsynaptic sites. EB1 shows close proximity to and partial overlap with alpha3-nAChR and APC surface clusters. We tested the role of APC in neuronal nicotinic synapse assembly by using retroviral-mediated in vivo overexpression of an APC dominant-negative (APC-dn) peptide to block the interaction of endogenous APC with both EB1 and PSD-93 during synapse formation in CG neurons. The overexpressed APC-dn led to dramatic decreases in alpha3-nAChR surface levels and clusters. Effects were specific to alpha3-nAChR postsynaptic sites; synaptic glycine receptor and perisynaptic alpha7-nAChR clusters were not altered. In addition, APC-dn also reduced surface membrane-associated clusters of PSD-93 and EB1. The results show that APC plays a key role in organizing excitatory cholinergic postsynaptic specializations in CG neurons. We identify APC as the first nonreceptor protein to function in localizing nAChRs to neuronal synapses in vivo.

Adenomatous Polyposis Coli Protein↗

PubMed2003

RGS3 mediates a calcium-dependent termination of G protein signaling in sensory neurons.

G proteins modulate synaptic transmission. Regulators of G protein signaling (RGS) proteins accelerate the intrinsic GTPase activity of Galpha subunits, and thus terminate G protein activation. Whether RGS proteins themselves are under cellular control is not well defined, particularly in native cells. In dorsal root ganglion neurons overexpressing RGS3, we find that G protein signaling is rapidly terminated (or "desensitized") by calcium influx through voltage-gated channels. This rapid desensitization is most likely mediated by direct binding of calcium to RGS3, as deletion of an EF-hand domain in RGS3 abolishes both the desensitization (observed physiologically) and a calcium-RGS3 interaction (observed in a gel-shift assay). A naturally occurring variant of RGS3 that lacks the EF hand neither binds calcium nor produces rapid desensitization, giving rise instead to a slower calcium-dependent desensitization that is attenuated by a calmodulin antagonist. Thus, activity-evoked calcium entry in sensory neurons may provide differential control of G protein signaling, depending on the isoform of RGS3 expressed in the cells. In complex neural circuits subjected to abundant synaptic inhibition by G proteins (as occurs in dorsal spinal cord), rapid termination of inhibition by electrical activity by EF hand-containing RGS3 may ensure the faithful transmission of information from the most active sensory inputs.

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PubMed2003

Inactivation of myelin-associated glycoprotein enhances optic nerve regeneration.

CNS regeneration in higher vertebrates is a long sought after goal in neuroscience. The lack of regeneration is attributable in part to inhibitory factors found in myelin (Caroni and Schwab, 1988a). Myelin-associated glycoprotein (MAG) is an abundant myelin protein that inhibits neurite outgrowth in vitro (McKerracher et al., 1994; Mukhopadhyay et al., 1994), but its role in regeneration remains controversial. To address this role, we performed nerve crush on embryonic day 15 chick retina-optic nerve explants and then acutely eliminated MAG function along the nerve using chromophore-assisted laser inactivation (CALI). CALI of MAG permitted significant regrowth of retinal axons past the site of lesion containing CNS myelin in contrast to various control treatments. Electron microscopy of the site of nerve crush shows abundant regenerating axons crossing the gap. When crushed optic nerve was retrogradely labeled at the nerve stump, no labeling of retinal neurons was observed. In contrast, labeling of CALI of MAG-treated crushed optic nerve showed significant retinal labeling (89 +/- 16 cells per square millimeter), a value indistinguishable from that seen with non-crushed nerve (98 +/- 13 cells per square millimeter). These findings implicate MAG as an important component of the myelin-derived inhibition of nerve regeneration. The acute loss of MAG function can promote significant axon growth across a site of CNS nerve damage.

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PubMed2002

Extrasynaptic alpha 7-nicotinic acetylcholine receptor expression in developing neurons is regulated by inputs, targets, and activity.

Alpha7-nicotinic acetylcholine receptors (nAChRs) are widely expressed in the vertebrate nervous system. alpha7-nAChR functions include postsynaptic transmission, modulating neurotransmitter release, reinforcing nicotine addiction, and a role in neurological disorders, such as schizophrenia and Alzheimer's disease. In chick parasympathetic ciliary ganglion (CG) neurons, alpha7-nAChRs are excluded from the synapse and localize perisynaptically. Despite their extrasynaptic distribution, the highly Ca2+-permeable alpha7-nAChRs have important synapse-related Ca2+-dependent signaling functions in the CG. We show here that the synaptic partners regulate alpha7-nAChR expression during synapse formation in embryonic CG neurons in situ. The absence of inputs and target tissues cause reductions in alpha7-nAChR mRNA and protein levels that primarily resemble those seen for synaptic alpha3-nAChRs. However, there is a difference in their regulation. alpha7-nAChR levels are downregulated by reduced activity, whereas alpha3-nAChR levels are not. We propose that the activity-dependent regulation of extrasynaptic alpha7-nAChR levels may be an important mechanism for postsynaptic CG neurons to detect changes in presynaptic activity levels and respond with Ca2+-dependent plasticity changes in gene expression.

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PubMed2002

Regulatory mechanisms that govern nicotinic synapse formation in neurons.

Individual cholinoceptive neurons express high levels of different neuronal nicotinic acetylcholine receptor (nAChR) subtypes, and target them to the appropriate synaptic regions for proper function. This review focuses on the intercellular and intracellular processes that regulate nAChR expression in vertebrate peripheral nervous system (PNS) and central nervous system (CNS) neurons. Specifically, we discuss the cellular and molecular mechanisms that govern the induction and maintenance of nAChR expression-innervation, target tissue interactions, soluble factors, and activity. We define the regulatory principles of interneuronal nicotinic synapse differentiation that have emerged from these studies. We also discuss the molecular players that target nAChRs to the surface membrane and the interneuronal synapse.

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