Genetics of Apoptosis

3.

Regulation of mitochondrial apoptosis by ER Ca2+ signals

3.1

ER Ca2+ release

The ER, or its equivalent in muscle cells, the sacroplasmic reticulum (SR), represents
the cell’s largest calcium store. SERCA pumps located in the ER membrane maintain
the [Ca2+]ER up to three orders of magnitude higher than the [Ca2+]c (Pozzan et al.,
1994). The release of Ca2+ from the ER is primarily achieved by two well-characterized
types of channels, the inositol 1, 4, 5-triphosphate receptor (IP 3 R) and the ryanodine
receptor (RyR) families (Pozzan et al., 1994; Berridge et al., 2000). Both channels
are stimulated by second messengers generated by receptor stimulation at the plasma
membrane (IP 3 for IP 3 R, and cyclic ADP ribose for RyR). The IP 3 R and RyR are
also activated by cytosolic Ca2+ in a bell-shape fashion, with elevations in [Ca2+]c
being stimulatory up to a certain point and inhibitory at higher concentrations
(Pozzan et al., 1994; Berridge et al., 2000). This feedbackregulation by [Ca2+]c permits
communication between ER receptors generating coordinated bursts of Ca2+ release
that can be organized into propagating waves varying in speed, amplitude, frequency,
and spatiotemporal pattern (Berridge et al., 1998). By modulating these factors, the
ER can generate a wide range of different Ca2+-dependent cellular responses,
including proliferation, fertilization, differentiation, and apoptosis (Berridge et al.,
2000). For an in-depth understanding of ER Ca2+ signaling pathways, we refer the
reader to recent reviews (Clapham, 1995; Berridge et al., 1998,2000; Sorrentino and
Rizzuto, 2001).
Release of calcium from intracellular stores has been observed in many forms of
apoptosis and controlled oscillations of Ca2+ by IP 3 R-mediated spikes may directly
regulate the cell-death machinery. Jurkat cells deficient in type 1 IP 3 R do not increase
[Ca2+]c in response to dexamethasone, ionizing radiation, and TCR or Fas
stimulation, and resist subsequent apoptosis (Jayaraman and Marks, 1997).
Moreover, targeted disruption of all three IP 3 R isoforms in the chick DT40 B-cell
line blocked Ca2+ mobilization and apoptosis by B-cell receptor cross-linking, and
the degree of resistance increased with the number of IP 3 Rs knocked out, suggesting
that all three isoforms can contribute to apoptotic signaling (Sugawara et al., 1997).
The way in which Ca2+ signals engage cell-death pathways may largely be decided
by the spatiotemporal pattern and intensity of ER Ca2+ release. IP3R/RyR-dependent
global increases in [Ca2+]c can influence apoptosis by several mechanisms. For
example, Ca2+-dependent activation of calpains has been implicated in activating
caspases in several forms of apoptosis (Wang, 2000), including ER stress-(Nakagawa
and Yuan, 2000), B-cell receptor- (Ruiz-Vela et al., 1999; 2001), and radiation-
induced apoptosis (Waterhouse et al., 1998). In addition, calpain cleavage of BAX
has been reported to increase its proapoptotic activity (Wood et al., 1998), and in
some cases calpain may cooperate with caspases in the execution phase of apoptosis
(Wood and Newcomb, 1999). As discussed above, Ca2+ signals can lead to activation

THE ROLE OF THE ENDOPLASMIC RETICULUM IN APOPTOSIS 107