Reply to Bezprozvanny et al.: Response to Shilling et al. (10.1074/jbc.M111.300491)

This is a response to a letter by Bezprozvanny et al. (1).

Bezprozvanny et al. (1) do not agree that we presented conclusive evidence to refute the hypothesis from their laboratory that presenilins (PS) act as passive endoplasmic reticulum (ER) Ca²⁺ leak channels. However, we did not claim to have provided such evidence. On the contrary, we claimed that we could not detect evidence that supported the hypothesis. Evidence was lacking using a variety of cell systems and experimental approaches.

In response to the specific points raised, we have indeed previously described enhanced agonist-induced Ca²⁺ release from the ER in mutant PS-expressing cells (as in Ref. 2 of Bezprozvanny et al. (1)), but we have demonstrated that it is due to a gain-of-function effect of mutant PS on the activity of the inositol 1,4,5-trisphosphate receptor Ca²⁺ release channel (2, 3).

We did not observe an increased amplitude of the ionomycin response in cells expressing mutant PS, but this is consistent with our inability to observe overloaded Ca²⁺ stores. Indeed, we (3) and others (4–6) have reported that the stores can be somewhat depleted in mutant PS-expressing cells.

We specifically mentioned all of the approaches used previously.

We did not report that ryanodine receptor (RyR) expression was down-regulated, but rather that it was unchanged in PS1-M146V cortical neuron cultures. We cultured neurons isolated from embryonic day 14–16 embryos for 7–8 days in vitro (DIV), whereas previous reports of RyR up-regulation employed cells isolated from older animals and cultured for longer periods (postnatal day 0–1 pups cultured for 13 DIV) (see Ref. 4 of Bezprozvanny et al. (1)) or used brain lysates from aged animals (6 weeks to 12 months) (see Ref. 5 of Bezprozvanny et al.).

In conclusion, we could not detect evidence that supported the hypothesis that presenilins act as passive ER Ca²⁺ leak channels.