Modulation of intrinsic excitability is an alternative to classical synaptic plasticity for implementing activity-dependent changes in neuronal networks. In this issue of Neuron, Nelson et al. reveal a new form of plasticity of intrinsic excitability that can be triggered rapidly when synaptic inhibition reduces spontaneous firing, resulting in persistent enhancement of firing rate and neuronal gain.
Local field potentials (LFPs) are of growing importance in neurophysiological investigations. LFPs supplement action potential recordings by indexing activity relevant to EEG, magnetoencephalographic, and hemodynamic (fMRI) signals. Recent reports suggest that LFPs reflect activity within very small domains of several hundred micrometers. We examined this conclusion by comparing LFP, current source density (CSD), and multiunit activity (MUA) signals in macaque auditory cortex. Estimated by frequency tuning bandwidths, these signals' "listening areas" differ systematically with an order of MUA < CSD < LFP. Computational analyses confirm that observed LFPs receive local contributions. Direct measurements indicate passive spread of LFPs to sites more than a centimeter from their origins. These findings appear to be independent of the frequency content of the LFP. Our results challenge the idea that LFP recordings typically integrate over extremely circumscribed local domains. Rather, LFPs appear as a mixture of local potentials with "volume conducted" potentials from distant sites.
Guidelines for submitting commentsPolicy: Comments that contribute to the discussion of the article will be posted within approximately three business days. We do not accept anonymous comments. Please include your email address; the address will not be displayed in the posted comment. Cell Press Editors will screen the comments to ensure that they are relevant and appropriate but comments will not be edited. The ultimate decision on publication of an online comment is at the Editors' discretion. Formatting: Please include a title for the comment and your affiliation. Note that symbols (e.g. Greek letters) may not transmit properly in this form due to potential software compatibility issues. Please spell out the words in place of the symbols (e.g. replace “α” with “alpha”). Comments should be no more than 8,000 characters (including spaces ) in length. References may be included when necessary but should be kept to a minimum. Be careful if copying and pasting from a Word document. Smart quotes can cause problems in the form. If you experience difficulties, please convert to a plain text file and then copy and paste into the form.
Controlling complex movements is a profoundly challenging problem for the nervous system, and working out how neurons in the motor system solve this problem is an equally challenging problem for neuroscience. The difficulty is that even such apparently simple acts as reaching to a nearby object involve the coordinated action of the muscles controlling the position of several joints. This poses a question: does the activity of neurons represent low-level dynamic aspects of movement such as movement forces and muscle activations, or does it reflect high-level kinematic parameters such as the direction and velocity of hand movements? Signals in spinal motoneurons, plainly, correspond to single-muscle actions; neurons in premotor areas seem to specify movement goals rather than details (Pesaran et al., 2006). But at the level of primary motor cortex (M1), the question of “muscles or movements” remains open and is the subject of active study.
Symmetrically dividing neuroepithelial cells may produce two daughters that are both proliferating or both postmitotic, as highlighted by Zigman et al. in this issue of Neuron and Sanada and Tsai in a recent issue of Cell. Here, I will attempt to offer a simple explanation why these results may be so different.
In this issue of Neuron, Fischer and Ullsperger (2013) demonstrate that EEG signatures of real and fictive learning differ early in processing, but the latent information in each event converges at the temporospatial nexus commonly associated with the P3b component.
In the developing cerebellar cortex, granule neuron precursors (GNPs) proliferate and commence differentiation in a superficial zone, the external granule layer (EGL). The molecular basis of the transition from proliferating precursors to immature differentiating neurons remains unknown. Notch signaling is an evolutionarily conserved pathway regulating the differentiation of precursor cells of many lineages. Notch2 is specifically expressed in proliferating GNPs in the EGL. Treatment of GNPs with soluble Notch ligand Jagged1, or overexpression of activated Notch2 or its downstream target HES1, maintains precursor proliferation. The addition of GNP mitogens Jagged1 or Sonic Hedgehog (Shh) upregulates the expression of HES1, suggesting a role for HES1 in maintaining precursor proliferation.
Naturally occurring cell death (NOCD) is a prominent feature of the developing nervous system. During this process, neurons express bcl-2, a major regulator of cell death whose expression may determine whether a neuron dies or survives. To gain insight into the possible role of bcl-2 during NOCD in vivo, we generated lines of transgenic mice in which neurons overexpress the human BCL-2 protein under the control of the neuron-specific enolase (NSE) or phosphoglycerate kinase (PGK) promoters. BCL-2 overexpression reduced neuronal loss during the NOCD period, which led to hypertrophy of the nervous system. For instance, the facial nucleus and the ganglion cell layer of the retina had, respectively, 40% and 50% more neurons than normal. Consistent with this finding, more axons than normal were found in the facial and optic nerves. We also tested whether neurons overexpressing BCL-2 were more resistant to permanent ischemia induced by middle cerebral artery occlusion; in transgenic mice, the volume of the brain infarction was reduced by 50% as compared with wild-type mice. These animals represent an invaluable tool for studying the effects of increased neuronal numbers on brain function as well as the mechanisms that control the survival of neurons during development and adulthood.
L-type Ca2+ channels are characterized by their unique sensitivity to organic Ca2+ channel modulators like the 1,4-dihydropyridines (DHPs). To identify molecular motifs mediating DHP sensitivity, we transferred this sensitivity from L-type Ca2+ channels to the DHP-insensitive class A brain Ca2+ channel, BI-2. Expression of chimeras revealed minimum sequence stretches conferring DHP sensitivity including segments IIIS5, IIIS6, and the connecting linker, as well as the IVS5-IVS6 linker plus segment IVS6. DHP agonist and antagonist effects are determined by different regions within the repeat IV motif. Sequence regions responsible for DHP sensitivity comprise only 9.4% of the overall primary structure of a DHP-sensitive alpha 1A/alpha 1S construct. This chimera fully exhibits the DHP sensitivity of channels formed by L-type alpha 1 subunits. In addition, it displays the electrophysiological properties of alpha 1A, as well as its sensitivity toward the peptide toxins omega-agatoxin IVA and omega-conotoxin MVIIC.
P400, inositol 1,4,5-trisphosphate receptor (InsP3-R), is a key protein to understanding the mechanisms of inositol 1,4,5-trisphosphate (InsP3)-mediated Ca2+ mobilization. We obtained the cerebellar-type P400/InsP3-R cDNA and generated an L cell transfectant (L15) that produces cDNA-derived P400/InsP3-R. In membranes, this protein displays high affinity, specificity, and capacity for InsP3, as does the cerebellar P400/InsP3-R. InsP3 can also induce greater 45Ca2+ release from the membrane vesicles of L15 cells than from those of control L cells. These results provide direct evidence that the cDNA-derived P400/InsP3-R protein is actually involved in physiological Ca2+ mobilization, through binding to InsP3 molecules in the same manner as the cerebellar P400/InsP3-R.
The inositol 1,4,5-trisphosphate receptor (IP3R) is expressed at very high levels in cerebellar Purkinje cells. Within these neurons, it has a widespread distribution throughout the endoplasmic reticulum (ER) and is present at particularly high concentrations at sites of membrane appositions within peculiar stacks of ER cisternae. Here we report that stacks of ER cisternae, reminiscent of those observed in Purkinje cells, can be induced by overexpression of full-length IP3R, but not of mutant forms of the protein in COS cells. Within these stacks the IP3R forms a crystalline array at apposed cisternal faces. Additionally, we show that Purkinje cell stacks are not permanent structures. Our findings suggest that massive stack formation in purkinje cells represents an adaptive response of the ER to hypoxic conditions and is due to the presence of the high concentration of IP3R in its membranes.
The inositol 1,4,5-trisphosphate (IP3) receptor, a Ca(2+)-permeable channel, plays a key role in intracellular Ca2+ signaling. The effects of ATP on the IP3 receptor at the single-channel level were characterized after channel incorporation into planar lipid bilayers. ATP alone was not sufficient to open the IP3-gated channel, but addition of ATP or nonhydrolyzable ATP analogs in the presence of IP3 increased the frequency of channel openings 4.8-fold and increased the average duration of channel openings 2.5-fold; channel conductance was unchanged. High concentrations of ATP (> 4 mM) decreased channel activity most probably by competing with IP3-binding site. Allosteric modulation of IP3-induced Ca2+ release by ATP may contribute to the maintenance of cell viability during periods of energy starvation.
Inositol 1,4,5-trisphosphate (InsP3) mobilizes internal Ca2+ in cells by binding to a receptor protein, which has recently been purified and molecularly cloned. To clarify those neuronal functions that are regulated by InsP3, we have localized this InsP3 receptor protein immunocytochemically in the retina, a neural tissue of well-defined structure and function. Positive staining in neurons is confined almost exclusively to the synaptic layers. Using dissociated retinal neurons, we have further localized the receptor to presynaptic terminals of photoreceptors and bipolar cells, as well as the synaptic processes of amacrine cells. The specific association of InsP3 receptors with synaptic terminals suggests a role for InsP3 in synaptic modulation, especially with respect to transmitter release.
The presence and distribution of calsequestrin (CS), Ca2+ pump, and inositol 1,4,5-trisphosphate (IP3) receptor were investigated biochemically and immunologically in microsomal (P3) fractions isolated from chicken cerebrum and cerebellum. Two different batches of polyclonal antibodies specific for chicken skeletal muscle CS identified a Ca2+ binding, CS-like protein that was extremely enriched in cerebellum P3 fractions and absent from all cerebrum fractions. The cerebellum CS-like protein was deemed authentic CS because the N-terminal amino acid domain and peptide mapping were identical to those of skeletal muscle CS in the same species. CS was detected in striated muscles and cerebellum only. Cerebellum P3 fractions were also found to be considerably enriched in Ca2+ pump and IP3 receptor compared with the homologous cerebrum fractions, as judged by measurements of Ca2+ uptake, Ca2(+)-ATPase activity, IP3-induced Ca2+ release, and [3H]IP3 binding, respectively. Cerebellum microsomal fractions therefore appear to contain membrane fragments endowed with Ca2+ pump, IP3 receptor, and CS, i.e., three key components of a Ca2+ storage organelle.
The dependency of purified mouse cerebellar type 1 inositol 1,4,5-trisphosphate receptor (IP3R1)/Ca2+ channel function on cytoplasmic Ca2+ was examined. In contrast to the channels in crude systems, the purified IP3R1 reconstituted into planar lipid bilayers did not show the bell-shaped dependence on Ca2+. It was activated with increasing Ca2+ sublinearly without inhibition even up to 200 microM. The addition of calmodulin to the cytoplasmic side inhibited the channel at high Ca2+ concentrations. Calmodulin antagonists reversed the Ca2+-dependent inactivation of the native channels in cerebellar microsomes. These results indicate that the bell-shaped dependence on cytoplasmic Ca2+ is not an intrinsic property of the IP3R1, and the Ca2+-dependent inactivation is directly mediated by calmodulin.
Huntington's disease (HD) is caused by polyglutamine expansion (exp) in huntingtin (Htt). The type 1 inositol (1,4,5)-triphosphate receptor (InsP3R1) is an intracellular calcium (Ca2+) release channel that plays an important role in neuronal function. In a yeast two-hybrid screen with the InsP3R1 carboxy terminus, we isolated Htt-associated protein-1A (HAP1A). We show that an InsP3R1-HAP1A-Htt ternary complex is formed in vitro and in vivo. In planar lipid bilayer reconstitution experiments, InsP3R1 activation by InsP3 is sensitized by Httexp, but not by normal Htt. Transfection of full-length Httexp or caspase-resistant Httexp, but not normal Htt, into medium spiny striatal neurons faciliates Ca2+ release in response to threshold concentrations of the selective mGluR1/5 agonist 3,5-DHPG. Our findings identify a novel molecular link between Htt and InsP3R1-mediated neuronal Ca2+ signaling and provide an explanation for the derangement of cytosolic Ca2+ signaling in HD patients and mouse models.
Inositol 1,4,5-trisphosphate (IP3) selectively evokes an inward (excitatory) current in cultured lobster olfactory receptor neurons (ORNs) and directly activates two types of channels in cell-free patches of plasma membrane from the ORNs. The IP3-activated channels have kinetic properties of odor-activated channels in the ORNs and pharmacological properties of intracellular IP3-activated channels in other systems. An antibody directed against an intracellular, cerebellar IP3 receptor recognizes a protein with a molecular weight similar to the mammalian receptor in the ORNs. The antibody selectively increases odor-evoked inward currents and IP3-activated unitary currents in the ORNs. The data provide further evidence for IP3 as an olfactory second messenger and implicate at least one and possibly two novel plasma membrane IP3 receptors in olfactory transduction.
Xestospongins (Xe's) A, C, D, araguspongine B, and demethylxestospongin B, a group of macrocyclic bis-1-oxaquinolizidines isolated from the Australian sponge, Xestospongia species, are shown to be potent blockers of IP3-mediated Ca2+ release from endoplasmic reticulum vesicles of rabbit cerebellum. XeC blocks IP3-induced Ca2+ release (IC50 = 358 nM) without interacting with the IP3-binding site, suggesting a mechanism that is independent of the IP3 effector site. Analysis of Pheochromocytoma cells and primary astrocytes loaded with Ca2+-sensitive dye reveals that XeC selectively blocks bradykinin- and carbamylcholine-induced Ca2+ efflux from endoplasmic reticulum stores. Xe's represent a new class of potent, membrane permeable IP3 receptor blockers exhibiting a high selectivity over ryanodine receptors. Xe's are a valuable tool for investigating the structure and function of IP3 receptors and Ca2+ signaling in neuronal and nonneuronal cells.
IP(s)3, a metabolically stable analog of 1,4,5-inositol trisphosphate (IP3), inhibited action potential firing when injected into hippocampal pyramidal cells. This effect was associated with decreased input resistance, a more negative resting potential, outward rectification at depolarized potentials, and an afterhyperpolarization. The response to IP(s)3 was unaffected by antagonists of Na+, Ca2+, and Cl- conductances, but was sensitive to changes in extracellular K+ concentration. The IP(s)3-induced conductance was voltage-dependent, was activated in 10 ms with depolarization, and was blocked by extracellular Ba2+ or intracellular Ca2+ chelation. It was not suppressed by other K+ conductance antagonists. Thus, IP(s)3 may activate a novel K+ conductance in CA1 pyramidal cells. IP3 itself did not elicit this conductance, suggesting it may be rapidly metabolized in these cells.
Neurexins, a family of cell surface proteins specific to brain, are transcribed from two promoters in three genes, resulting in three alpha- and three beta-neurexins. In situ hybridization revealed differential but overlapping distributions of neurexin isoforms in different classes of neurons. PCRs demonstrated that alpha-neurexins are alternatively spliced at five canonical positions, and beta-neurexins at two. Characterization of many independent bovine neurexin I alpha cDNAs suggests that different splice sites are used independently. This creates the potential to express more than 1000 distinct neurexin proteins in brain. The splicing pattern is conserved in rat and cow. Thus, in addition to somatic gene rearrangement (immunoglobulins and T cell receptors) and large gene families (odorant receptors), alternative splicing potentially represents a third mechanism for creating a large number of cell surface receptors that are expressed by specific subsets of cells.
Tau from Alzheimer's disease (AD) paired helical filaments (PHF-tau) is phosphorylated at sites not found in autopsy-derived adult tau from normal human brains, and this suggested that PHF-tau is abnormally phosphorylated. To explore this hypothesis, we examined human adult tau from brain biopsies and demonstrated that biopsy-derived tau is phosphorylated at most sites thought to be abnormally phosphorylated in PHF-tau. These sites also were phosphorylated in autopsy-derived human fetal tau and rapidly processed rat tau. The hypophosphorylation of autopsy-derived adult human tau is due to rapid dephosphorylation postmortem, and protein phosphatases 2A (PP2A) and 2B (PP2B) in human brain biopsies dephosphorylate tau in a site-specific manner. The down-regulation of phosphatases (i.e., PP2A and PP2B) in the AD brain could lead to the generation of maximally phosphorylated PHF-tau that does not bind microtubules and aggregates as PHFs in neurofibrillary tangles and dystrophic neurites.
Guidelines for submitting commentsPolicy: Comments that contribute to the discussion of the article will be posted within approximately three business days. We do not accept anonymous comments. Please include your email address; the address will not be displayed in the posted comment. Cell Press Editors will screen the comments to ensure that they are relevant and appropriate but comments will not be edited. The ultimate decision on publication of an online comment is at the Editors' discretion. Formatting: Please include a title for the comment and your affiliation. Note that symbols (e.g. Greek letters) may not transmit properly in this form due to potential software compatibility issues. Please spell out the words in place of the symbols (e.g. replace “α” with “alpha”). Comments should be no more than 8,000 characters (including spaces ) in length. References may be included when necessary but should be kept to a minimum. Be careful if copying and pasting from a Word document. Smart quotes can cause problems in the form. If you experience difficulties, please convert to a plain text file and then copy and paste into the form.
Odorant receptors and signaling proteins are localized to sensory cilia on olfactory dendrites. Using a GFP-tagged odorant receptor protein, Caenorhabditis elegans ODR-10, we characterized protein sorting and transport in olfactory neurons in vivo. ODR-10 is transported in rapidly moving dendritic vesicles that shuttle between the cell body and the cilia. Anterograde and retrograde vesicles move at different speeds, suggesting that dendrites have polarized transport mechanisms. Residues immediately after the seventh membrane-spanning domain of ODR-10 are required for localization; these residues are conserved in many G protein-coupled receptors. UNC-101 encodes a mu1 subunit of the AP-1 clathrin adaptor complex. In unc-101 mutants, dendritic vesicles are absent, ODR-10 receptor is evenly distributed over the plasma membrane, and other cilia membrane proteins are also mislocalized, implicating AP-1 in protein sorting to olfactory cilia.
Neurotrophins influence growth and survival of specific populations of neurons through activation of Trks, members of the receptor tyrosine kinase (RTK) family. In this report, we describe the identification and characterization of two substrates of Trk kinases, rAPS and SH2-B, which are closely related Src homolog 2 (SH2) domain-containing signaling molecules. rAPS and SH2-B are substrates of TrkB and TrkC in cortical neurons and SH2-B is a substrate of TrkA in sympathetic neurons. Moreover, rAPS and SH2-B bind to Grb2, and both are sufficient to mediate NGF induction of Ras, MAP kinase (MAPK), and morphological differentiation of PC12 cells. Lastly, antibody perturbation and transient transfection experiments indicate that SH2-B, or a closely related molecule, is necessary for NGF-dependent signaling in neonatal sympathetic neurons. Together, these observations indicate that rAPS and SH2-B mediate Trk signaling in developing neurons.
Cerebellar granule cells are the most abundant type of neuron in the brain, but the molecular mechanisms that control their generation are incompletely understood. We show that Sonic hedgehog (Shh), which is made by Purkinje cells, regulates the division of granule cell precursors (GCPs). Treatment of GCPs with Shh prevents differentiation and induces a potent, long-lasting proliferative response. This response can be inhibited by basic fibroblast growth factor or by activation of protein kinase A. Blocking Shh function in vivo dramatically reduces GCP proliferation. These findings provide insight into the mechanisms of normal growth and tumorigenesis in the cerebellum.
