Protein kinase C is a calcium sensor for presynaptic short-term plasticity

Thank you for sending your work entitled “Protein kinase C is a calcium sensor for presynaptic short-term plasticity” for consideration at eLife. Your article has been favorably evaluated by Catherine Dulac (Senior editor) and 3 reviewers, one of whom is a member of our Board of Reviewing Editors.

The following individuals responsible for the peer review of your submission have agreed to reveal their identity: Sacha Nelson (Reviewing editor); Tim Ryan (peer reviewer); and Ling-Gang Wu (peer reviewer).

The Reviewing editor and the other reviewers discussed their comments before we reached this decision, and the Reviewing editor has assembled the following comments to help you prepare a revised submission.

All three reviewers felt that the authors provide compelling and definitive evidence that for PTP in the mature Calyx of Held the key sensor is the calcium-coordinating C2 domain of PKCbeta, and that this is an important result in the field of synaptic plasticity.

Two points need addressing in the revision:

1) All three reviewers felt that more attention should be given in the Discussion to other results in the field. For example, one reviewer noted:

“My only concern is whether the finding here is of general application. Several studies have proposed that other Ca2+ binding proteins are involved in PTP, such as Munc 13 (Rhee JS, et al 2002), Calmodulin (Junge et. al, 2004). Could these proteins also be calcium sensors or the downstream targets of PKC? Alternatively, synapse specificity may explain the difference in different synapses. In addition, a PKC independent form of PTP, which involves synaptotagmin 2 as the calcium sensor, has also been reported in the calyx (He et al., Nature, 2009; Xue and Wu, J Physiol., 2010). A discussion to justify why the finding here is of more general application is therefore needed. Alternatively, a new experiment showing that the main finding here applies to other synapses would be preferred.”

The other reviewers felt that the developmental shift in mechanism was clear from other work, but that probably this needed to be discussed for the benefit of the general reader.

2) A second key issue concerns temperature. It was somewhat surprising that the experiments were not carried out at closer to physiological temperature, especially since, as one reviewer noted, “this has been a point that this lab has raised repeatedly over the years.” It is possible that this could be handled in the discussion, but obviously a single experiment showing an equal dependence on PKCβ at a more physiological temperature would be a stronger way to approach this.

1) All three reviewers felt that more attention should be given in the Discussion to other results in the field. For example, one reviewer noted:

“My only concern is whether the finding here is of general application. Several studies have proposed that other Ca2+ binding proteins are involved in PTP, such as Munc 13 (Rhee JS, et al 2002), Calmodulin (Junge et. al, 2004). Could these proteins also be calcium sensors or the downstream targets of PKC? Alternatively, synapse specificity may explain the difference in different synapses. In addition, a PKC independent form of PTP, which involves synaptotagmin 2 as the calcium sensor, has also been reported in the calyx (He et al., Nature, 2009; Xue and Wu, J Physiol., 2010). A discussion to justify why the finding here is of more general application is therefore needed. Alternatively, a new experiment showing that the main finding here applies to other synapses would be preferred.”

The other reviewers felt that the developmental shift in mechanism was clear from other work, but that probably this needed to be discussed for the benefit of the general reader.

We have revised the Discussion to address these interesting and important issues. As we now more clearly state we do not think that PKCβ is the sole calcium sensor mediating PTP at all synapses. We think it is likely that future studies will reveal additional sensors.

2) A second key issue concerns temperature. It was somewhat surprising that the experiments were not carried out at closer to physiological temperature, especially since, as one reviewer noted, “this has been a point that this lab has raised repeatedly over the years.” It is possible that this could be handled in the discussion, but obviously a single experiment showing an equal dependence on PKCβ at a more physiological temperature would be a stronger way to approach this.

Previous studies of PTP at the calyx of Held have been conducted at room temperature because of technical challenges at physiological temperatures. In the ideal world we would prefer to perform all experiments at physiological temperatures, but this is impractical. The reviewers are right that it is important to show that PTP is also observed at physiological temperatures and that it is mediated by PKC_Ca. We have been interested in this issue for some time and had begun to perform PTP experiments at near physiological temperatures. We found that higher stimulation frequencies were required to induce PTP at near physiological temperatures (Figure 1–figure supplement 1). We went on to investigate the role of PKC_Ca in this PTP. We could have used knockout mice, but our colony is currently small and breeding would have to be ramped up to obtain the necessary mice. We therefore used a PKC_Ca inhibitor that we had previously characterized using knockout mice to validate its specificity (Chu et al. 2014). We found that the PTP at 34°C is also mediated by PKC_Ca (Figure 1–figure supplement 2).