A local store

Free Ca²⁺ in the nucleus can regulate important functions such as gene transcription. But how is the nuclear level of free Ca²⁺ controlled? In the models that have been proposed so far, Ca²⁺ simply reaches the nuclear interior from the nuclear envelope (NE) by diffusion — a mechanism that would only allow the nucleus to be regulated uniformly by Ca²⁺. However, this is not what occurs, so could the nucleus have its own local Ca²⁺ store? Nathanson and colleagues now provide the answer to this question in Nature Cell Biology.

In SKHep1 epithelial cells, the authors used the endoplasmic reticulum (ER) dye ER-Tracker to detect a nucleoplasmic reticulum — a fine, branching intranuclear network that is continuous with the ER and the NE. They confirmed the presence of this structure by showing that the ER protein calreticulin was distributed in a reticular pattern in the nucleus, as well as in the cytosol, of SKHep1 cells.

Nathanson and co-workers next showed that fluorescent Ca²⁺ dyes also labelled this intranuclear network, and they used fluorescence recovery after photobleaching to confirm that the dyes were membrane-enclosed, rather than membrane-bound. These results therefore show that there is a nuclear Ca²⁺-storing network that is continuous with the ER and the NE.

Inositol-1,4,5-trisphosphate (InsP₃) receptors mediate Ca²⁺ signalling in SKHep1 cells, and the authors found that the type II InsP₃ receptor isoform is enriched in the nucleus of these cells. This isoform is expressed, in part, along the nucleoplasmic reticulum, but what do these intranuclear InsP₃ receptors do?

The authors developed a new technique to answer this question — a technique that allowed them to photorelease intranuclear nitrophenylethyl ester (NPE)-caged InsP₃ in a highly localized fashion in individual cells using two-photon excitation. When they photoreleased InsP₃ within 1 μM of the nucleoplasmic reticulum, they detected small increases in Ca²⁺ that began at the nucleoplasmic reticulum and were greatest at the site of InsP₃ release. These data allowed them to conclude that “...the nucleoplasmic reticulum is an InsP₃-gated calcium store that can give rise to local calcium signals in the nuclear interior”.

Finally, Nathanson and colleagues monitored how the distribution of protein kinase C-γ (PKC-γ), which contains a Ca²⁺-sensitive regulatory domain, is affected by Ca²⁺. They photoreleased Ca²⁺ in either the nucleus or the cytosol and examined the effect on the distribution of green fluorescent protein (GFP)–PKC-γ. They found that nuclear Ca²⁺ signals altered the distribution of nuclear, not cytosolic, GFP–PKC-γ and vice versa, which indicates that nuclear and cytosolic Ca²⁺ signals can have effects that are independent of one another.

This work has therefore shown that “...the nucleus contains a nucleoplasmic reticulum with the capacity to regulate calcium signals in localized subnuclear regions”. This discovery potentially explains how the nucleus can regulate several, independent Ca²⁺-dependent processes simultaneously, and might have revealed a new layer of Ca²⁺ control.