Fast calcium transients in dendritic spines driven by extreme statistics
Authors: Nathalie Rouach
Eduard Korkotian
Kanishka Basnayake
Alexis Bemelmans
David Holcman
David Mazaud
Journal: PLoS biology (2019)
Discipline: BIOLOGY & BIOCHEMISTRY
Keywords: Fast calcium transients; cellular microdomains; trafficking; local ATP production; endoplasmic reticulum-mitochondria complex
Affiliations: Computational Biology and Applied Mathematics, Institut de Biologie de l'École Normale Supérieure, Paris, France., Center for Interdisciplinary Research in Biology, Collège de France, Centre National de la Recherche Scientifique UMR 7241, Institut National de la Santé et de la Recherche Médicale U1050, Labex Memolife, Paris Sciences et Lettres Research University, Paris, France., Commissariat à l'Energie Atomique et aux Energies Alternatives, Département de la Recherche Fondamentale, Institut de biologie François Jacob, Molecular Imaging Research Center and Centre National de la Recherche Scientifique UMR9199, Université Paris-Sud, Neurodegenerative Diseases Laboratory, Fontenay-aux-Roses, France., Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel., Faculty of Biology, Perm State University, Perm, Russia., Department of Applied Mathematics and Theoretical Physics, Churchill College, University of Cambridge, Cambridge, United Kingdom.

Abstract:  Fast calcium transients (&lt;10 ms) remain difficult to analyse in cellular microdomains, yet they can modulate key cellular events such as trafficking, local ATP production by endoplasmic reticulum-mitochondria complex (ER-mitochondria complex), or spontaneous activity in astrocytes. In dendritic spines receiving synaptic inputs, we show here that in the presence of a spine apparatus (SA), which is an extension of the smooth ER, a calcium-induced calcium release (CICR) is triggered at the base of the spine by the fastest calcium ions arriving at a Ryanodyne receptor (RyR). The mechanism relies on the asymmetric distributions of RyRs and sarco/ER calcium-ATPase (SERCA) pumps that we predict using a computational model and further confirm experimentally in culture and slice hippocampal neurons. The present mechanism for which the statistics of the fastest particles arriving at a small target, followed by an amplification, is likely to be generic in molecular transduction across cellular microcompartments, such as thin neuronal processes, astrocytes, endfeets, or protrusions. 

Retracted: No

PubPeer comments:
 This paper investigates calcium-induced calcium release in dendritic spines. It compares calcium dynamics in spines containing a spine apparatus and those without. It is reported that uncaging of calcium in the spine head can rapidly trigger calcium-induced calcium release at the base of the neck of spines with a spine apparatus, but not in spines without. Immunochemical labelling suggests that the ryanodine receptors are expressed at the base of the spine, posing an apparent paradox: how can calcium in the head trigger calcium-induced release rapidly at the base of the neck? Much of the paper is given over to diffusion modelling, which suggests that *if the ryanodine receptors are sufficiently sensitive to calcium*, time is only required for a few calcium ions to arrive at the neck base to trigger calcium release (this is not implausible given the well known properties of diffusion). ![image] What is the experimental evidence for the existence of the calcium-induced calcium release being modelled? The relevant experiments are those in Fig. 1A-C (panels D and E are modelling); the whole figure is reproduced here. Panels B and C show fluorescence signals reporting intracellular calcium concentration as calcium is uncaged in the head or dendrite of a spine with a spine apparatus (SP+) or without (SP-), as determined by subsequent correlative immunochemistry (top part of A). The key difference is the response in the base (red) when calcium is uncaged in the spine head (left column of B and C): there is a prominent calcium rise in the base of the SP+ spine but not in the SP- spine. There are a number of deficiencies in the presentation of these data. Although the difference between the neck-base calcium responses appears dramatic in the examples shown for SP+ and SP- spines, no average time courses are shown. Nor are any statistics given for any of the measurements or fits. The numbers of spines analysed are quite unclear. The legend indicates n = 19 SP+ and 27 SP- spines. But, as just mentioned, no group data or statistics are given. Fig. S4 seems to analyse the same data, where we find a statement:  Here, the number of spines is N = 3 for SP+ and 7 for SP-. It is unclear to which panel(s) this applies and, if not to all, what the sample sizes are for the others. It is also unclear how the discrepancy between these small numbers and those in the legend of Fig. 1 (19 and 27) arises. Setting aside the absent statistics and group data, there is no experimental proof that the observation of Fig. 1 is due to calcium-induced calcium release. Indeed, the authors own modelling in Fig. 1D,E shows that quite a significant neck-base calcium response is expected in the absence of any calcium-induced calcium release. Compare the red trace relative to the blue trace in Fig. 1E with the same ratio in Fig. 1B left. Are the two panels so qualitatively different that no quantification or further evidence is required? There might be slightly more than a two-fold difference in amplitude ratio, while the overall time course is actually quite similar. Variations by a factor of two could plausibly arise from uncertainties in model parameters. Indeed, the modelling and specimen results imply the existence of extensive variability, since the model predicts a neck-base response in the absence of calcium-induced calcium release, yet the specimen SP- trace shows none. Equivalent variation in the opposite direction would probably enable the model to predict the SP+ response without any need to invoke calcium-induced calcium release. So the discrepancy driving the rest of the study does not seem to be firmly established. In this respect, it is worth noting that the model assumes that the dendrite absorbs all calcium ions reaching it (Methods:  Ions arriving at the dendritic shaft located at the base are considered to be lost and do not return to the spine during the time scale of the simulations (absorbing boundary) ). This will cause the model to underestimate the calcium concentration at the base of the neck, further reducing the need to invoke calcium-induced calcium release. An alternative explanation for the SP+ SP- differences in the specimen data could be that they arose from variations of spine morphology, positioning of the uncaging spot or positioning of the measurement spot. For instance, maybe the SP+ spines had shorter and wider necks? This would allow quicker diffusion of calcium from the spine head to the base of the neck and therefore cause a faster and greater rise of calcium there. Indeed, this explanation would fit with the faster decay time constant reported for the example SP+ spine shown in Fig. 1 (tau = 5.3ms for SP+ and tau = 7.0ms for SP-). More extensive analysis of spine and dendritic morphology, of the stimulation positions, of response profiles, of time courses and correlation with the post-experiment immunolabelling would be required to eliminate such confounding explanations, but none is provided. Further experimental evidence for calcium-induced calcium release might have been obtained pharmacologically or by demonstrating the regenerative nature of the putative calcium-induced calcium release by varying uncaging power. It should also be possible to trigger the postulated release directly through uncaging of a very low concentration of calcium at the base of the spine. If something seems unclear in the paper, why not examine the raw data? PLoS have an excellent data-availability policy requiring raw data to be published at the same time as the paper. https: journals.plos.org plosone s data-availability Unfortunately, that excellent policy has not been applied here. Although the paper is accompanied by some data, it turns out that none whatsoever is supplied in relation to Fig. 1 or S4. The  Data availability statement in the paper offers little in the way of explanation for this absence:  All relevant data are within the paper and its Supporting Information files. The authors do provide a slightly more in-depth analysis of responses to glutamate uncaging, to which the population responses of SP+ and SP- spines do appear to differ, but, here too, these different responses could easily arise under the influence of a number of other confounding factors. For instance, maybe SP+ spines have more glutamate receptors and wider necks. Finally, regarding the diffusion modelling, it seems strange that no endogenous calcium buffers were included in the model, as they might be thought to have some influence on intracellular calcium diffusion and concentrations. There is also no justification advanced for their absence from the model. In conclusion, although the diffusion modelling demonstrates the possibility of the remote calcium-induced calcium release mechanism (subject to the usual caveats about uncertainty of model parameters and the specific issue of the omission of endogenous calcium buffers), it is simply not possible to evaluate the strength of the experimental evidence for calcium-induced calcium release, because the analysis is incomplete and superficial. The authors have not complied with the PLoS data-availability policy and thus the reader is not in a position to replicate the study s findings. ,