Extracellular Calcium Controls Background Current and Neuronal Excitability via an UNC79-UNC80-NALCN Cation Channel Complex

Oscillatory activity is widespread in dynamic neuronal networks. The main paradigm for the origin of periodicity consists of specialized pacemaking elements that synchronize and drive the rest of the network; however, other models exist. Here, we studied the spontaneous emergence of synchronized periodic bursting in a network of cultured dissociated neurons from rat hippocampus and cortex. Surprisingly, about 60% of all active neurons were self-sustained oscillators when disconnected, each with its own natural frequency. The individual neuron's tendency to oscillate and the corresponding oscillation frequency are controlled by its excitability. The single neuron intrinsic oscillations were blocked by riluzole, and are thus dependent on persistent sodium leak currents. Upon a gradual retrieval of connectivity, the synchrony evolves: Loose synchrony appears already at weak connectivity, with the oscillators converging to one common oscillation frequency, yet shifted in phase across the population. Further strengthening of the connectivity causes a reduction in the mean phase shifts until zerolag is achieved, manifested by synchronous periodic network bursts. Interestingly, the frequency of network bursting matches the average of the intrinsic frequencies. Overall, the network behaves like other universal systems, where order emerges spontaneously by entrainment of independent rhythmic units. Although simplified with respect to circuitry in the brain, our results attribute a basic functional role for intrinsic single neuron excitability mechanisms in driving the network's activity and dynamics, contributing to our understanding of developing neural circuits.neuron | network | oscillator | synchrony | persistent Na current P eriodicity emerges as a key physiological characteristic at all levels of neuronal activity, from the dynamics of neurons at subthreshold potentials (1, 2), through rhythmic neuronal ensembles within local networks, and all of the way up to global oscillations measured by electroencephalography (EEG) (3). The range of observed frequencies is surprisingly wide, from the millisecond range typical for interspike intervals all of the way to several seconds in the case of slow EEG. Over the years, accumulating evidence and theory have attributed different mechanisms for the origin of each measured periodic activity. Within local networks, the role of known collective mechanisms for periodicity, such as the balance between excitatory and inhibitory neurons and recurrent network architecture (4-6), is often contrasted with single neuron contributions, for example, the role of pacemaker neurons in oscillatory network dynamics (7). Although the physiological properties of single neurons are diverse and well documented (2, 8), their role in emergent network oscillations was predicted theoretically (9), but has not been observed experimentally. This contribution may involve a subtle interplay between intrinsic excitability and network connectivity (10, 11).The connectivity and excitability together determi...

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“…The characteristic behavior of mature [14][15][16][17]] neuronal cultures in standard physiological medium (1-1.5 mM…”

Section: Resultsmentioning

confidence: 99%

“…Furthermore, calcium is important for determining the single cell excitability, for example, by regulating leak currents into the neuron via the NALCN (sodium leak channel) (14,15).…”

mentioning

confidence: 99%

Network synchronization in hippocampal neurons

Penn

Segal

Moses

2016

Proc. Natl. Acad. Sci. U.S.A.

129

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“…Two ion channels have been identified that might contribute to such responses. NALCN, a Na + leak channel (33) with minimal Ca 2+ permeability, is activated in hippocampal pyramidal cells indirectly by low [Ca 2+ ] o by a G-protein-coupled receptor (11). TRPM7 can also be activated in hippocampal neurons under conditions in which divalent cation concentrations fall, although normal intracellular Mg 2+ levels are expected to minimize such activation (34).…”

Section: Discussionmentioning

confidence: 99%

“…The bath solution contained 150 mM Na + , 5.4 mM K + , 1 mM Mg 2+ , 150 mM Cl − , 20 mM glucose, 10 mM Hepes, and 1.5 mM Ca 2+ (unless otherwise indicated), pH 7.4 adjusted by methanesulfonate, ∼330 mOsm. These solutions are similar to those used in other current-clamp studies (11,36). Data acquisition was as described above for N2A cell whole-cell voltageclamp experiments.…”

Section: Methodsmentioning

confidence: 99%

Calcium homeostasis modulator 1 (CALHM1) is the pore-forming subunit of an ion channel that mediates extracellular Ca ²⁺ regulation of neuronal excitability

Siebert²,

Cheung³

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

146

308

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Alzheimer's disease | mutagenesis | selectivity | neurodegeneration | polymorphism O riginally identified as a possible modifier of the age of onset of Alzheimer's disease (1, 2), calcium homeostasis modulator 1 (CALHM1) encodes a glycosylated membrane protein expressed throughout the brain that lacks homology to other proteins. Six human CALHM homologs have been identified, with alternatively spliced variants and different expression patterns throughout the body, and CALHM1 is conserved across >20 species. Expression of recombinant human CALHM1 in mammalian cells was found to strongly influence processing of amyloid precursor protein to amyloid beta (Aβ) under an experimental protocol that involved removal of Ca 2+ o for several minutes and its subsequent restoration to the bathing medium (1). This procedure resulted in a large rise of [Ca 2+ ] i . Accordingly, it was speculated that CALHM1 influences Aβ production by altering cellular Ca 2+ homeostasis. CALHM1 was found to homo-multimerize and it was speculated that it might function as an ion channel component or regulator of membrane ion conductances (1 Results CALHM1 Expression Induces a Voltage-Dependent Plasma MembraneConductance. Previously, outwardly rectified ion currents were observed in CALHM1-expressing Xenopus oocytes and CHO cells by a slow voltage ramp protocol (1). However, those experiments did not establish whether CALHM1 is an essential component of an underlying ion channel(s), accessory protein, or actual pore-forming subunit. Furthermore the detailed permeation and gating properties of the conductance were not determined. In addition, precautions were not taken to fully ensure lack of contribution of endogenous conductances. To distinguish whether CALHM1 is a unique ion channel or a regulator of endogenous channels, plasma membrane currents were recorded in Xenopus oocytes under conditions that minimized contributions of endogenous conductances (Fig. S1) (3, 4). Membrane depolarization in solutions containing 2 mM Ca 2+ and 1 mM Mg 2+ generated large outward currents that activated slowly (τ ∼ 3.11 ± 0.28 s at +60 mV; n = 10) and deactivated at hyperpolarized voltages (τ = 0.204 ± 0.022 s at −80 mV; n = 10) specifically in CALHM1-expressing oocytes (Fig. 1A). Expression of CALHM1-EGFP localized to the plasma membrane (Fig. 1B). Similar results were obtained in transiently transfected N2A mammalian neuroblastoma cells (Fig. 1C). Thus, expression of CALHM1 induced a voltage-dependent plasma membrane conductance.The monovalent ion permeabilities of this conductance were estimated by changing-bath [NaCl] in a nominally 0-Ca 2+ solution (free [Ca 2+ ] ∼10 μM) and measuring shifts of reversal potential, ΔE rev (Fig. 1D). Using the Goldman-Hodgkin-Katz (GHK) constant field equation, the relative permeabilities were estimated as P Na :P K :P Cl = 1:1.17:0.56. Similar results were obtained with bath Na + replaced by K + in either 0 or 2 mM Ca 2+ o .

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“…These results collectively suggest that ATP13A4 likely plays a key role in behavioral evolution during chicken domestication. Similarly, the gene UNC80, which has a function in the nervous system [23,24], also experienced positive selection and had the highest identified XP-EHH score by the sliding window analysis ( Figure 1B).…”

Section: Positively Selected Genes In Domestic Chickens Compared Withmentioning

confidence: 94%

Positive selection rather than relaxation of functional constraint drives the evolution of vision during chicken domestication

Wang

Zhang

et al. 2016

Cell Res

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As noted by Darwin, chickens have the greatest phenotypic diversity of all birds, but an interesting evolutionary difference between domestic chickens and their wild ancestor, the Red Junglefowl, is their comparatively weaker vision. Existing theories suggest that diminished visual prowess among domestic chickens reflect changes driven by the relaxation of functional constraints on vision, but the evidence identifying the underlying genetic mechanisms responsible for this change has not been definitively characterized. Here, a genome-wide analysis of the domestic chicken and Red Junglefowl genomes showed significant enrichment for positively selected genes involved in the development of vision. There were significant differences between domestic chickens and their wild ancestors regarding the level of mRNA expression for these genes in the retina. Numerous additional genes involved in the development of vision also showed significant differences in mRNA expression between domestic chickens and their wild ancestors, particularly for genes associated with phototransduction and photoreceptor development, such as RHO (rhodopsin), GUCA1A, PDE6B and NR2E3. Finally, we characterized the potential role of the VIT gene in vision, which experienced positive selection and downregulated expression in the retina of the village chicken. Overall, our results suggest that positive selection, rather than relaxation of purifying selection, contributed to the evolution of vision in domestic chickens. The progenitors of domestic chickens harboring weaker vision may have showed a reduced fear response and vigilance, making them easier to be unconsciously selected and/or domesticated.

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Extracellular Calcium Controls Background Current and Neuronal Excitability via an UNC79-UNC80-NALCN Cation Channel Complex

Cited by 188 publications

References 62 publications

Network synchronization in hippocampal neurons

Network synchronization in hippocampal neurons

Calcium homeostasis modulator 1 (CALHM1) is the pore-forming subunit of an ion channel that mediates extracellular Ca ²⁺ regulation of neuronal excitability

Positive selection rather than relaxation of functional constraint drives the evolution of vision during chicken domestication

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Extracellular Calcium Controls Background Current and Neuronal Excitability via an UNC79-UNC80-NALCN Cation Channel Complex

Cited by 188 publications

References 62 publications

Network synchronization in hippocampal neurons

Network synchronization in hippocampal neurons

Calcium homeostasis modulator 1 (CALHM1) is the pore-forming subunit of an ion channel that mediates extracellular Ca 2+ regulation of neuronal excitability

Positive selection rather than relaxation of functional constraint drives the evolution of vision during chicken domestication

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Calcium homeostasis modulator 1 (CALHM1) is the pore-forming subunit of an ion channel that mediates extracellular Ca ²⁺ regulation of neuronal excitability