A three-pool model dissecting readily releasable pool replenishment at the calyx of held - PubMed (original) (raw)

A three-pool model dissecting readily releasable pool replenishment at the calyx of held

Jun Guo et al. Sci Rep. 2015.

Erratum in

Abstract

Although vesicle replenishment is critical in maintaining exo-endocytosis recycling, the underlying mechanisms are not well understood. Previous studies have shown that both rapid and slow endocytosis recycle into a very large recycling pool instead of within the readily releasable pool (RRP), and the time course of RRP replenishment is slowed down by more intense stimulation. This finding contradicts the calcium/calmodulin-dependence of RRP replenishment. Here we address this issue and report a three-pool model for RRP replenishment at a central synapse. Both rapid and slow endocytosis provide vesicles to a large reserve pool (RP) ~42.3 times the RRP size. When moving from the RP to the RRP, vesicles entered an intermediate pool (IP) ~2.7 times the RRP size with slow RP-IP kinetics and fast IP-RRP kinetics, which was responsible for the well-established slow and rapid components of RRP replenishment. Depletion of the IP caused the slower RRP replenishment observed after intense stimulation. These results establish, for the first time, a realistic cycling model with all parameters measured, revealing the contribution of each cycling step in synaptic transmission. The results call for modification of the current view of the vesicle recycling steps and their roles.

PubMed Disclaimer

Figures

Figure 1

Figure 1. More intense stimulation slows the RRP replenishment.

(A) Left: Sampled presynaptic calcium current (ICa, upper) and membrane capacitance (Cm, lower) induced by a 20 ms depolarisation followed by a conditioning pulse of 20 ms depolarization with a 0.5 s interval. Right: Similar to Left, except that the stimulus interval is 20 s. (B) Upper: The protocol used to measure the RRP replenishment after a 20 ms depolarisation pulse. Lower: Cm induced by a 20 ms depolarisation applied at various intervals after the conditioning stimulus (n = 8). Data were normalised to the Cm induced by the conditioning pulse, and fit with a bi-exponential function (solid line) where A1 = 0.71, τ1 = 0.26 s, A2 = 0.29, τ2 = 9.5 s. (C) Similar to A, except that the conditioning stimulus was 10 pulses of 20 ms depolarisation at 10 Hz. (D) Similar to B, except that the conditioning stimulus was 10 pulses of 20 ms depolarisation at 10 Hz (n = 11). Data were normalised to the Cm induced by a 20 ms depolarisation applied at >30 s after the conditioning stimulus, and fit with a bi-exponential function (solid line) where A1 = 0.33, τ1 = 0.38 s, A2 = 0.67, τ2 = 7.8 s. The fitting curve of single pulse was also plotted for comparison (dotted line). (E) Similar to A, except that the conditioning stimulus was 10 pulses of 20 ms depolarisation at 1 Hz. (F) Similar to D, except that the conditioning stimulus was 10 pulses of 20 ms depolarisation at 1 Hz (n = 6). Data were fit with a bi-exponential function where A1 = 0.29, τ1 = 0.25 s, A2 = 0.71, τ2 = 7.9 s. (G) The plot of the normalised RRP replenishmentrapid amplitude versus calcium influx (QICa) in a 20 ms depolarisation pulse and 10 pulses of 20 ms depolarisation at 1–10 Hz (QICa: 38.9 ± 2.6 pC, n = 8, single pulse; 312 ± 34 pC, n = 11, 10 pulses at 10 Hz; 323 ± 15 pC, n = 6, 10 pulses at 1 Hz).

Figure 2

Figure 2. A three-pool model underlies rapid and slow RRP replenishment.

(A) The model-predicted RRP replenishment curves with endocytosed vesicles recycling to RP (black), IP (red) and RRP (blue) after a 20 ms depolarisation pulse. The measured data are also plotted for comparison (circle, same as Fig. 1B). The legend also applies to B and C. (B) Similar to A, but with a conditioning stimulus of 10 pulses of 20 ms depolarisation at 10 Hz. (C) Similar to A and B, but with a conditioning stimulus of 10 pulses of 20 ms depolarisation at 1 Hz. (D) The total measured and predicted ΔCm with and without endocytosis induced by 10 pulses of 20 ms depolarisation at 10 Hz (n = 11). (E) The model-predicted (with endocytosis: black curve, without endocytosis: dotted curve) and the measured (circle) ΔCm induced by each depolarising pulse (20 ms depolarisation) during a 10-pulse train at 1 Hz. (F) Similar to D, except that the conditioning stimulus was 10 pulses of 20 ms depolarisation at 1 Hz (n = 6).

Figure 3

Figure 3. Rapid and slow endocytosis do not recycle vesicles in a small recycling pool.

(A) Left: Sampled presynaptic calcium current (ICa, upper) and membrane capacitance (Cm, lower) induced by a 20 ms depolarisation applied at 0.5 s after a conditioning pulse of 20 ms depolarization with 0.3 mM GTPγS in place of GTP in the pipette solution. Right: Similar to Left, except that the stimulus interval is 20 s. (B) The model-predicted RRP replenishment curves with (scheme 1, black) and without endocytosis (scheme 2, red) after a 20 ms depolarisation pulse. Data measured with 0.3 mM GTPγS in the pipette solution are also plotted for comparison (circle). (C) Similar to A, but with a conditioning stimulus of 10 pulses of 20 ms depolarisation at 10 Hz. (D) Similar to B, except that the conditioning stimulus was 10 pulses of 20 ms depolarisation at 10 Hz.

Figure 4

Figure 4. Impact of endocytosis during high frequency stimulation.

(A) Sampled trace of EPSC recordings during a 50 Hz action potential train. Inset shows the large initial EPSC and the small stabilised EPSC for comparison. (B) The model-predicted (curves) and the measured (n = 4, circle) amplitudes of the EPSCs during a 50 Hz action potential train. Predicted traces for different depleting percentages after a single action potential are shown in different colours (yellow: 6%, red: 9%, green: 12%). Data were normalised to the first response. Each circle represents the mean amplitude from four synapses (for clarity, s.e.m. is not included). Experimental data were collected from horizontal brain slices, where a bipolar electrode was positioned in the midline of the trapezoid body to induce presynaptic action potentials and thus EPSCs. The model included endocytosis (scheme 1). (C) The model-predicted EPSC amplitude during action potential stimulation at 50 Hz with (scheme 1, colours meanings are the same as B) and without (scheme 2, black) endocytosis. Left and right panels show the same data at different scales.

Similar articles

Cited by

References

    1. Regehr W. G. Short-term presynaptic plasticity. Cold Spring Harbor perspectives in biology 4, a005702, 10.1101/cshperspect.a005702 (2012). - DOI - PMC - PubMed
    1. Abbott L. F. & Regehr W. G. Synaptic computation. Nature 431, 796–803 (2004). - PubMed
    1. Betz W. J. Depression of transmitter release at the neuromuscular junction of the frog. J. Physiol. 206, 629–644 (1970). - PMC - PubMed
    1. Koenig J. H. & Ikeda K. Synaptic vesicles have two distinct recycling pathways. J. Cell Biol. 135, 797–808 (1996). - PMC - PubMed
    1. Delgado R., Maureira C., Oliva C., Kidokoro Y. & Labarca P. Size of vesicle pools, rates of mobilization, and recycling at neuromuscular synapses of a Drosophila mutant, shibire. Neuron 28, 941–953 (2000). - PubMed

Publication types

MeSH terms

LinkOut - more resources