http://www.physiology.ucla.edu/images/lecture/NeherFlyer.pdf

Multiple roles of calcium ions in the regulation of neurotransmitter release.

The Life Cycle of Ca2+ Ions in Dendritic Spines

The physical distance between presynaptic Ca2+ channels and the Ca2+ sensors that trigger exocytosis of neurotransmitter-containing vesicles is a key determinant of the signalling properties of synapses in the nervous system. Recent functional analysis indicates that in some fast central synapses, transmitter release is triggered by a small number of Ca2+ channels that are coupled to Ca2+ sensors at the nanometre scale. Molecular analysis suggests that this tight coupling is generated by protein–protein interactions involving Ca2+ channels, Ca2+ sensors and various other synaptic proteins. Nanodomain coupling has several functional advantages, as it increases the efficacy, speed and energy efficiency of synaptic transmission.

/pdf/nanodomain-coupling-between-ca-2-channels-and-sensors-of-4hy9re7nen.pdf

Nanodomain coupling between Ca 2+ channels and sensors of exocytosis at fast mammalian synapses

Different types of synapses are specialized to interpret spike trains in their own way by virtue of the complement of short-term synaptic plasticity mechanisms they possess. Numerous types of short-term, use-dependent synaptic plasticity regulate neurotransmitter release. Short-term depression is prominent after a single conditioning stimulus and recovers in seconds. Sustained presynaptic activation can result in more profound depression that recovers more slowly. An enhancement of release known as facilitation is prominent after single conditioning stimuli and lasts for hundreds of milliseconds. Finally, tetanic activation can enhance synaptic strength for tens of seconds to minutes through processes known as augmentation and posttetantic potentiation. Progress in clarifying the properties, mechanisms, and functional roles of these forms of short-term plasticity is reviewed here.

/pdf/short-term-presynaptic-plasticity-3rt7ht4e90.pdf

Short-Term Presynaptic Plasticity

Calretinin (CR), calbindin D-28k (CB) and parvalbumin (PV) belong to the large family of EF-hand calcium-binding proteins, which comprises more than 200 members in man. Structurally these proteins are characterized by the presence of a variable number of evolutionary well-conserved helix-loop-helix motives, which bind Ca2+ ions with high affinity. Functionally, they fall into two groups: by interaction with target proteins, calcium sensors translate calcium concentrations into signaling cascades, whereas calcium buffers are thought to modify the spatiotemporal aspects of calcium transients. Although CR, CB and PV are currently being considered calcium buffers, this may change as we learn more about their biology. Remarkable differences in their biophysical properties have led to the distinction of fast and slow buffers and suggested functional specificity of individual calcium buffers. Evaluation of the physiological roles of CR, CB and PV has been facilitated by the recent generation of mouse strains deficient in these proteins. Here, we review the biology of these calcium-binding proteins with distinct reference to the cerebellum, since they are particularly enriched in specific cerebellar neurons. CR is principally expressed in granule cells and their parallel fibres, while PV and CB are present throughout the axon, soma, dendrites and spines of Purkinje cells. PV is additionally found in a subpopulation of inhibitory interneurons, the stellate and basket cells. Studies on deficient mice together with in vitro work and their unique cell type-specific distribution in the cerebellum suggest that these calcium-binding proteins have evolved as functionally distinct, physiologically relevant modulators of intracellular calcium transients. Analysis of different brain regions suggests that these proteins are involved in regulating calcium pools critical for synaptic plasticity. Surprisingly, a major role of any of these three calcium-binding proteins as an endogenous neuroprotectant is not generally supported.

'New' functions for 'old' proteins: the role of the calcium-binding proteins calbindin D-28k, calretinin and parvalbumin, in cerebellar physiology. Studies with knockout mice.

1
κS1. The effect of parvalbumin (PV) on [Ca2+] transients was investigated by perfusing adrenal chromaffin cells with fura-2 and fluorescein isothiocyanate (FITC)-labelled PV. As PV diffused into cells, the decay of [Ca2+] transients was transformed from monophasic into biphasic. The proportion of the initial fast decay phase increased in parallel with the fluorescence intensity of FITC, indicating that PV is responsible for the initial fast decay phase.

2
The relationship between the fast decay phase and the [Ca2+] level was investigated using depolarizing trains of stimuli. Within a train the relative amplitude of the fast decay phase was inversely dependent on the [Ca2+] level preceding a given stimulus.

3
Based on these observations, we estimated the Ca2+ binding ratio of PV (κP), the apparent dissociation constant of PV for Ca2+ (Kdc,app), and the unbinding rate constant of Ca2+ from PV (kc-) in the cytosol of chromaffin cells. Assuming free [Mg2+] to be 0.14 mm, we obtained values of 51.4 ± 2.0 nm (n= 3) and 0.95 ± 0.026 s−1 (n= 3), for Kdc,app and kc-, respectively.

4
With the parameters obtained in the perfusion study, we simulated [Ca2+] transients, using two different Ca2+ extrusion rates (γ) – 20 and 300 s−1– which represent typical values for chromaffin cells and neuronal dendrites, respectively. The simulation indicated that Ca2+ is pumped out before it is equilibrated with PV, when γ is comparable to the equilibration rates between PV and Ca2+, resulting in the fast decay phase of a biexponential [Ca2+] transient.

5
From these results we conclude that Ca2+ buffers with slow kinetics, such as PV, may cause biexponential decays in [Ca2+] transients, thereby complicating the analysis of endogenous Ca2+ binding ratios (κS) based on time constants. Nevertheless, estimates of κS based on Ca2+ increments provide reasonable estimates for Ca2+ binding ratios before equilibration with PV.

Kinetics of Ca2+ binding to parvalbumin in bovine chromaffin cells: implications for [Ca2+] transients of neuronal dendrites.

The clearance of Ca2+ from nerve terminals is critical for determining the build-up of residual Ca2+ after repetitive presynaptic activity. We found previously that K+-dependent Na+/Ca2+ exchangers (NCKXs) show polarized distributions in axon terminals of supraoptic magnocellular neurons and play a major role in Ca2+ clearance. The role of NCKXs in presynaptic terminals, however, has not been studied. We investigated the contribution of NCKX in conjunction with other Ca2+ clearance mechanisms at the calyx of Held by analyzing the decay of Ca2+ transients evoked by depolarizing pulses. Inhibition of Na+/Ca2+ exchange by replacing external Na+ with Li+ decreased the Ca2+ decay rate by 68%. Selective inhibition of NCKX by replacing internal K+ with TEA+ (tetraethylammonium) or Li+ decreased the Ca2+ decay rate by 42%, and the additional inhibition of the K+-independent form of Na+/Ca2+ exchanger (NCX) by reducing external [Na+] caused an additional decrease by 26%. Inhibition of plasma membrane Ca2+-ATPase (PMCA) decreased the Ca2+ decay rate by 23%, whereas inhibition of SERCA (smooth endoplasmic reticulum Ca2+-ATPase) had no effect. The contribution of mitochondria was negligible for small Ca2+ transients but became apparent at [Ca2+]i > 2.5 μm, when Na+/Ca2+ exchange became saturated. Mitochondrial contribution was also observed when the duration of Ca2+ transients was prolonged by inhibiting Na+/Ca2+ exchangers or by increasing Ca2+ buffers. These results suggest that, in response to small Ca2+ transients (<2 μm), Ca2+ loads are cleared from the calyx of Held by NCKX (42%), NCX (26%), and PMCA (23%), and that mitochondria participate when the Ca2+ load is larger or prolonged.

https://www.jneurosci.org/content/jneuro/25/26/6057.full.pdf

Interplay between Na+/Ca2+ Exchangers and Mitochondria in Ca2+ Clearance at the Calyx of Held

Endogenous calcium binding ratios (κS) in dendrites of cultured hippocampal neurons were estimated according to the single compartment model for transients in intracellular Ca2+ concentration ([Ca2+]). In addition, the electrophysiological characteristics of neurons were classified by their autaptic currents and intrinsic firing patterns. These data were analysed in order to determine whether a correlation between Ca2+ buffers and electrophysiological type exists.


Ca2+ binding ratios of endogenous buffers were estimated by eliciting [Ca2+] transients with short depolarizations, while cells were loaded with fura-2. Two types of estimates could be obtained: one termed κS(τ), based on analysing time constants (τ) of [Ca2+] transients, and another termed κS(dCa), derived from an analysis of initial amplitudes of [Ca2+] transients.


Values for κS(τ) and κS(dCa) were estimated as 57 ± 10 (mean ±s.d., n = 10) and 60 ± 14 (n = 10), respectively, in excitatory neurons, and 130 ± 50 (n = 11) and 150 ± 70 (n = 11), respectively, in inhibitory neurons. The κS values of excitatory and inhibitory cells were significantly different from each other, regardless of the measurement method (Student's t test, P < 0.01). However, there was no significant difference in κS between the groups classified according to firing patterns.


Although κS(τ) values were well matched to those of κS(dCa) in most excitatory cells, the two values did not agree in three out of the fourteen inhibitory cells investigated. In these cells, the first few [Ca2+] transients after obtaining the whole cell configuration displayed a double exponential decay, suggesting that buffers with slow binding kinetics, such as parvalbumin, are involved. This hypothesis is further explored in an accompanying paper.







Spatial and temporal fine tuning of the intracellular Ca2+ level ([Ca2+]i) allows Ca2+ to serve a multitude of cellular functions as a second messenger. The spatio-temporal extent of the [Ca2+]i increase upon perturbation from its resting levels is largely determined by calcium influx channels, extrusion mechanisms, and calcium buffers. Within a given calcium flux system, calcium buffers have profound effects on the amplitude of [Ca2+] transients, the diffusional range, and the relaxation time constant of these signals. Lumped Ca2+ intracellular buffer capacity can be quantified by measuring the differential increment of Ca2+ bound buffer divided by the free calcium increase (the calcium binding ratio, κ) using the single compartment model (Neher & Augustine, 1992; Helmchen et al. 1996). Such measurements have been described for chromaffin cells (Neher & Augustine, 1992), somata of cerebellar Purkinje neurons (Fierro & Llano, 1996), dendrites of pyramidal neurons in cerebral cortex (Helmchen et al. 1996), and motoneurons (Lips & Keller, 1998). Considerable differences in endogenous calcium binding ratios (κS), as estimated in these different types of neurons, prompted us to survey κS in hippocampal neurons and to attempt a correlation between this quantity and their electrophysiological characteristics. Hippocampal neurons cultured in low density offer an excellent model system, in which inhibitory and excitatory cells can be readily identified based on the type of autaptic current (Bekkers & Stevens, 1991), thereby allowing unambiguous comparison of Ca2+ dynamics between inhibitory and excitatory cells.

Suk-Ho Lee

Papers

Kinetics of Ca2+ binding to parvalbumin in bovine chromaffin cells: implications for [Ca2+] transients of neuronal dendrites.

Interplay between Na+/Ca2+ Exchangers and Mitochondria in Ca2+ Clearance at the Calyx of Held

Differences in Ca2+ buffering properties between excitatory and inhibitory hippocampal neurons from the rat

Voltage-gated calcium channels contribute to spontaneous glutamate release directly via nanodomain coupling or indirectly via calmodulin.

Composition comprising a root extract of salvia miltiorrhiza or dimethyl lithospermate b as a sodium channel agonist