Synchronization of Ca2+ oscillations: a capacitative AC electrical coupling model in neuroepithelium Masayuki Yamashita Department of Physiology I, Nara Medical University, Kashihara, Ja
Trang 1Synchronization of Ca2+ oscillations: a capacitative (AC) electrical coupling model in neuroepithelium
Masayuki Yamashita
Department of Physiology I, Nara Medical University, Kashihara, Japan
Structural organization of intracellular
of Ca2+ increase
Chemical coupling and DC electrical coupling
The lumen of the endoplasmic reticulum (ER) is
continuous with a space between the outer nuclear
membrane (ONM) and inner nuclear membrane
(INM) [1–3] Intracellular Ca2+ stores are formed within the ER lumen and the space between the ONM and the INM [1,2] In cells with a centralized nucleus surrounded by the ER (Fig 1A), intercellular commu-nication may be mediated by the release of a
transmit-Keywords
Ca2+oscillation; Ca2+store; neuronal
development; synchronization; voltage
fluctuation
Correspondence
M Yamashita, Department of Physiology I,
Nara Medical University, Shijo-cho 840,
Kashihara 634-8521, Japan
Fax: +81 744 29 0306
Tel: +81 744 29 8827
E-mail: yama@naramed-u.ac.jp
(Received 23 March 2009, revised 2
October 2009, accepted 9 October
2009)
doi:10.1111/j.1742-4658.2009.07439.x
Increases in intracellular [Ca2+] occur synchronously between cells in the neuroepithelium If neuroepithelial cells were capable of generating action potentials synchronized by gap junctions (direct current electrical coupling), the influx of Ca2+ through voltage-activated Ca2+channels would lead to
a synchronous increase in intracellular [Ca2+] However, no action poten-tial is generated in neuroepithelial cells, and the [Ca2+] increase is instead produced by the release of Ca2+ from intracellular Ca2+stores Recently, synchronous fluctuations in the membrane potential of Ca2+ stores were recorded using an organelle-specific voltage-sensitive dye On the basis of these recordings, a capacitative [alternating current (AC)] electrical cou-pling model for the synchronization of voltage fluctuations of Ca2+ store potential was proposed [Yamashita M (2006) FEBS Lett 580, 4979–4983; Yamashita M (2008) FEBS J 275, 4022–4032] Ca2+efflux from the Ca2+ store and K+counterinflux into the store cause alternating voltage changes across the store membrane, and the voltage fluctuation induces ACs In cases where the store membrane is closely apposed to the plasma mem-brane and the cells are tightly packed, which is true of neuroepithelial cells, the voltage fluctuation of the store membrane is synchronized between the cells by the AC currents through the series capacitance of these mem-branes This article provides a short review of the model and its relation-ship to the structural organization of the Ca2+store This is followed by a discussion of how the mode of synchronization of [Ca2+] increase may change during central nervous system development and new molecular insights into the synchronicity of [Ca2+] increase
Abbreviations
AC, alternating current; BK channel, big K + channel; CNS, central nervous system; DC, direct current; DiOC5(3), 3,3¢-dipentyloxacarbocyanine iodide; ER, endoplasmic reticulum; I C , capacitative current; INM, inner nuclear membrane; Ins(1,4,5)P 3 , inositol 1,4,5-trisphosphate; mAChR, muscarinic acetylcholine receptor; ONM, outer nuclear membrane; Pyk2, proline-rich tyrosine kinase 2; RGC, retinal ganglion cell.
Trang 2ter (e.g ATP) and its receptors, which stimulate the
release of Ca2+from intracellular Ca2+stores (Fig 1B
and Koizumi in this minireview series) This mode of
coupling is referred to as chemical coupling When gap
junctions are present between adjacent cells, electrical
coupling through gap junction channels may
synchro-nize plasma membrane potentials, and Ca2+ influx
through voltage-activated Ca2+ channels should lead
to a synchronous increase in intracellular [Ca2+]
(Fig 1B and Imtiaz et al in this minireview series)
This coupling mode is mediated by direct currents
(DCs) through gap junction channels, and may be
called DC electrical coupling
Alternatively, a second messenger molecule such as
inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and⁄ or Ca2+
ions may pass gap junction channels, and such
pas-sive diffusion might lead to a synchronous increase in intracellular [Ca2+] However, the results of our stud-ies on the retinal neuroepithelium contradict this dif-fusion model and provide evidence for an alternative model We have found that Ins(1,4,5)P3-mediated robust Ca2+ increases induced by a supramaximal amount of an agonist do not synchronize, despite strong gap junctional coupling in the retinal neuroepi-thelium [4,5] It has also been shown that synchro-nous Ca2+ oscillations occur in newborn retinal ganglion cells (RGCs), which lose gap junctions [5]
On the basis of these findings, an alternative model
to the passive diffusion of Ins(1,4,5)P3 or Ca2+ through gap junction channels is provided to explain the synchronization of Ca2+ oscillation between these cells
Fig 1 Structure of intracellular Ca 2+ stores and coupling modes of intracellular [Ca 2+ ] increase (A) Cells in which the nucleus is located in the center of the cell and is surrounded by ER Modified from Fig 1 in [1] with permission (B) Chemical coupling and DC electrical coupling Stored Ca 2+ ions are released by the activation of receptors by a transmitter, such as ATP (chemical coupling) Depolarization (DV) synchro-nized by gap junctional coupling activates voltage-dependent Ca2+channels to cause synchronous Ca2+influx (DC electrical coupling) The
Ca 2+ influx may cause Ca 2+ -induced Ca 2+ release to amplify the [Ca 2+ ] increase (C) Neuroepithelial cells in which the ONM is closely apposed to the plasma membrane (PM) and the cells are tightly packed in the basal layer Modified from Fig 2 in [6] (D) Capacitative (AC) electrical coupling Efflux of Ca2+from Ca2+stores and counterinflux of K+cause fluctuations in the membrane potential of the Ca2+store, inducing ACs, which can pass the membranes as capacitative currents (IC) The current loop is closed via cytoplasm and the PM or gap junc-tion (GJ), and also via the extracellular space, even in the absence of GJs NPC, nuclear pore complex; Nu, nucleoplasm.
Trang 3Capacitative [alternating current (AC)] electrical
coupling
A novel mechanism of coupling between cells that does
not depend on gap junctions or transmitters has been
proposed, on the basis of the observation that the
membrane potential of Ca2+ stores oscillates
synchro-nously between cells in the retinal neuroepithelium
[4,6] The voltage change exhibited a bistable alteration
of fast rising and fast falling, which oscillated at the
same frequency as the Ca2+ oscillations [4] The
volt-age change was recorded using an organelle-specific,
voltage-sensitive fluorescent dye,
3,3¢-dipentyloxacarbo-cyanine iodide [DiOC5(3)], and a highly sensitive video
camera, which was connected to a high-speed confocal
scanner (Nipkow disk type) [4] When the voltage
change was recorded using a photomultiplier, it was
found, surprisingly, that the bistable voltage alteration
consisted of periodic repeats of a burst of high
fre-quency (> 200 Hz) voltage fluctuations [5] The low
time resolution of the video camera (15 images per
sec-ond) did cover the high-frequency voltage fluctuation
To explain the synchronization of the store
poten-tial, a capacitative (AC) electrical coupling model has
been proposed, because the fast voltage change across
the store membrane produces ACs, which can pass the
plasma membrane capacitatively when the store
mem-brane is in close proximity to the plasma memmem-brane
The neuroepithelium consists of bipolar cells, in which
the nuclei are positioned at different levels
(pseudostr-atified columnar epithelium) In the retinal
neuroepi-thelium, the ONM is closely apposed to the plasma
membrane, and the cells are tightly packed in the basal
layer (Fig 1C) The voltage fluctuations of the Ca2+
store will induce ACs, which can pass the series
capaci-tance of the ONM and the plasma membrane as
capa-citative currents (IC in Fig 1D) The AC could
synchronize the voltage fluctuations of the Ca2+ store
between the cells by capacitative (AC) electrical
coupling [5]
The cytoplasm and the plasma membrane will make
a closed-current loop of IC (Fig 1D) Gap junctions
may also contribute to the formation of the current
loop Another path for IC is the extracellular space,
because IC can pass the plasma membrane
capacita-tively Thus, the current loop can be closed via the
extracellular space and the plasma membrane This
may allow capacitative electrical coupling between
neuroepithelial cells and newborn RGCs, which lack
gap junctions (Fig 1D) The electrical circuits of the
current loop are presented in Doc S2 of [5]
The fluctuation in the membrane potential of the
Ca2+store may be due to the movement of Ca2+ions
and the concomitant movement of other ions across the store membrane The Ca2+efflux causes a negative change in the store potential towards the equilibrium potential of Ca2+ (lumen-negative), which in turn induces a counterinflux of K+ ions to depolarize the store potential, unless a [K+] gradient is formed across the store membrane Efflux of Cl) or influx of Mg2+
may also contribute to the depolarization of the store potential The depolarization provides the driving force for Ca2+ efflux, and the Ca2+ release may also be enhanced by Ca2+-induced Ca2+release Such fluctua-tions in the membrane potential of the Ca2+ store would continue as a burst of high-frequency voltage fluctuations In fact, an increase in intracellular [Ca2+] coincides with an increase in DiOC5(3) fluorescence, which is caused by the burst of high-frequency voltage fluctuations [5]
It has been shown that voltage- and Ca2+-activated
K+ channels [big K+ channels (BK channels)] are present in the membrane of the Ca2+ store or the ONM [4,7] The store BK channels are activated by a positive voltage change on the luminal side and by an increase in the luminal [Ca2+] [4,7] Because the clos-ing of the store BK channels attenuates Ca2+ release [4], the Ca2+ efflux will decrease when the luminal
Ca2+ levels decrease to the point at which the store
BK channels close The decrease in the luminal [Ca2+] should also decrease the driving force for Ca2+efflux The closing of the store BK channels increases the time constant for the store membrane to dampen the high-frequency voltage fluctuation of the Ca2+store, which will inhibit the synchronous burst of the voltage fluctu-ations of the Ca2+ store [5] When the Ca2+ store is replenished with Ca2+ ions by Ca2+ pumps in the store membrane, and the store BK channels are reacti-vated, the voltage fluctuations of the Ca2+ store will resume
Synchronous intracellular Ca2+
increase in central nervous system (CNS) development
Figure 2 illustrates the development of neural activities relative to the cellular events that occur during the course of CNS development Neurons are born from neuroepithelial cells after they have exited the cell cycle It has been shown that the Ca2+ mobilization (Ca2+ release from Ca2+ stores) and the synchronous
Ca2+ oscillations are essential for neuroepithelial cell proliferation, for ventricular cell proliferation, and for cell cycle progression [8–17] Thus, the synchronous
Ca2+ oscillations continue during neurogenesis Cell death occurs naturally, leading to a reduction in the
Trang 4number of neurons by approximately one-half The
surviving neurons begin to generate action potentials
At this stage, the surviving neurons exhibit a
charac-teristic synchronous burst spiking, which leads to
tran-sient, synchronous increases in intracellular [Ca2+]
between the cells [18,19] Although transmitters may
play a role in modulating the bursting activity,
chemi-cal transmission is unlikely to mediate the
synchroniza-tion of spikes between the cells [18] (discussed later) It
has been proposed that the synchronous increase in
intracellular [Ca2+] is essential for the fine-tuning of
synaptic connections [18–20] Glial cells are born
following neurogenesis [21] The glial cells provide
elec-trical insulation to neurons, thereby making it possible
for individual neurons to generate action potentials
asynchronously, depending on the synaptic inputs that
they receive Thus, neural circuits are precisely formed,
and each neuron can respond to appropriate natural
stimuli
Biological significance of Ca2+
synchronicity
The above overview of the steps of CNS development
raises questions regarding the molecular events that
accompany the synchronous increase in intracellular
[Ca2+] The following sections describe a new model
and provide possible explanations regarding the
bio-logical significance of the synchronous increases in
intracellular [Ca2+] between cells
Cell cycle-dependent Ca2+mobilization and
cell–cell adhesion in the neuroepithelium
Neuroepithelial cells undergo interkinetic nuclear
migration along the apicobasal axis during cell cycle
progression [21,22] Stimulation of G-protein-coupled receptors causes the robust release of Ca2+from intra-cellular Ca2+stores in S-phase cells in the basal layer, whereas the ER and the nuclear envelope are broken down and the Ca2+ mobilization declines in M-phase cells in the apical layer [12] Spontaneous, synchronous
Ca2+ oscillations occur between S-phase neuroepit-helial cells and newborn RGCs [4,5]
The interkinetic cell shows a polarized bipolar struc-ture, whereas the M-phase cell is round Fujita and Yasuda [23] have suggested that this morphological difference is due to a change in cell–cell adhesion that
is mediated by cadherin–catenin complexes within each cell and by cadherin–cadherin interactions between the two cells The interkinetic cells adhere to each other via cadherin–catenin complexes, and these complexes are anchored to F-actin (Fig 3A) During M-phase, the cadherin–catenin complex dissociates, thereby dis-rupting cell–cell adhesion [23] As a result, M-phase cells are round (Fig 3B) These morphological and molecular changes point to a relationship between cell–cell adhesion and the synchronous Ca2+ oscilla-tions, and suggest that cadherin–catenin complexes connect interkinetic cells with each other Synchronous
Ca2+ oscillations occur in S-phase cells and newborn RGCs In contrast, in M-phase cells, the Ca2+ mobili-zation system, including the ER and the nuclear enve-lope, disappears and cadherin–catenin complexes are disassembled It is proposed that cell–cell adhesion may be regulated by the synchronous increases in intracellular [Ca2+], as described below
Synchronous increases in intracellular [Ca2+] and disassembly of cadherin–catenin complexes The cytoplasmic domain of cadherin interacts with F-actin via b-catenin and a-catenin; b-catenin binds
to cadherin and a-catenin, which in turn interacts with F-actin (Fig 3A) [24] Thus b-catenin plays a pivotal role in the regulation of cell–cell adhesion The interac-tion of b-catenin with cadherin is regulated by tyrosine phosphorylation of b-catenin [25,26], which leads to disassembly of the cadherin–catenin complex b-Cate-nin is directly tyrosine-phosphorylated by the nonre-ceptor protein tyrosine kinase proline-rich tyrosine kinase 2 (Pyk2) [26,27], or is indirectly tyrosine-phosphorylated by Src family kinase, which can be activated by Pyk2 [28] It is likely that tyrosine phos-phorylation of b-catenin is triggered by Ca2+ ions, because Pyk2 is activated by an increase in intracellu-lar [Ca2+] [29,30]
If Pyk2 is transiently activated by an increase in intracellular [Ca2+] to phosphorylate b-catenin in two
Development of neural activities
Responses to natural stimuli Spiking in
individual neurons
Synchronous Ca oscillation
Proliferation Cell death
Gliogenesis
Synapse formation
Neurogenesis
Synchronous spikes with Ca 2+ transients
Fig 2 Changes in cellular activities during CNS development.
Trang 5adherent cells, the synchronous increase in [Ca2+]
between the cells could lead to a significant change in
the homophilic binding of cadherins The coactivation
of Pyk2⁄ Src kinase between the cells would result in
the dissociation of cadherins (Fig 3A) However, if
intracellular [Ca2+] were increased in only one of the
two adherent cells, the transient activation of Pyk2
would only occur in that cell In this case, the
b-cate-nin would be rapidly dephosphorylated by a
phospha-tase in that cell without disrupting the homophilic
binding of cadherins (Fig 3C)
If the synchronous increase in intracellular [Ca2+]
were responsible for the disruption of cell–cell
adhe-sion, it would seem paradoxical that the synchronous
Ca2+oscillations would occur in S-phase cells, but not
in M-phase cells S-phase cells, however, may
gradu-ally disconnect themselves from the surrounding cells
before M-phase, at which point almost all cadherin–
catenin complexes are disassembled After mitosis, the
cells are reattached by cadherins, and the ER and the
nuclear envelope are reorganized before S-phase
Newborn RGCs are also free from surface adhesion as
they extend dendrites (Fig 3B)
In summary, a new model is put forward in which
synchronous, transient increases in intracellular [Ca2+]
between cells can facilitate the disruption of cell–cell
adhesion to destabilize cell surface contact A
reduction in the stability of cell–cell adhesion may be
an output of a coincidence detector of cellular activi-ties This decrease in cell-cell contact, in other words, the increase in freedom of cell surface, may play an essential role in the regulation of mitosis, dendrite extension, and synaptic plasticity
Capacitative (AC) electrical coupling in cortical development
Synchronous Ca2+oscillations occur in the developing cortex even before synapse formation [8,31,32] Ca2+ oscillations in the retinal ventricular zone are driven
by a muscarinic acetylcholine receptors (mAChRs), which cause the release of Ca2+ from intracellular
Ca2+stores [13,14,33] The activation of mAChRs also induces strongly synchronized electrical activities in the subplate of the cortex of newborn mice [32] The mAChR-driven electrical activity is blocked by tetro-dotoxin, suggesting that the activation of mAChRs results in the generation of action potentials [32] How-ever, it remains unknown how the activation of mAChRs induces the synchronous firing activity The capacitative (AC) coupling model may account for the generation of synchronous bursts of spikes The AC currents caused by the voltage fluctuations of the Ca2+store may pass the plasma membrane capaci-tatively (Fig 4A) This current may function as a noisy stimulus current to evoke action potentials (Fig 4B)
A
C
B
Fig 3 Hypothetical role for synchronous
[Ca 2+ ] increases in cell–cell adhesion (A)
Simultaneous increases in intracellular
[Ca2+] in two adherent cells lead to
disrup-tion of cell–cell adhesion through
disassem-bly of cadherin–catenin complexes (B)
Changes in cell shape and the plasma
mem-brane during mitosis and dendrite extension.
(C) An increase in intracellular [Ca 2+ ] in only
one of two adherent cells does not disrupt
cell–cell adhesion.
Trang 6If the voltage fluctuations of the Ca2+ store are
syn-chronous between the cells, synsyn-chronous bursts of
spikes could be generated Such capacitative coupling
may be the underlying mechanism that mediates the
synchronization of spikes during the early stages of
neurodevelopment
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0.1 s
A
B
Fig 4 Capacitative (AC) coupling model for the generation of
syn-chronous bursts of spikes (A) Fluctuation in the membrane
poten-tial of Ca2+ stores induces ACs, which can pass the plasma
membrane as capacitative currents (IC) (B) The AC current
func-tions as a noisy stimulus current to evoke bursts of spikes The
fluctuations in Ca2+ store membrane potentials were recorded
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