Genetics of Apoptosis

of BAD, and Ca2+-dependent activation of the transcription factor MEF2 leads to
upregulation of Nur77/TR3 (Youn et al., 1999), which can bind to mitochondria
and induce Cyt.c release (Li et al., 2000). In contrast, privileged transport of Ca2+
between the ER and mitochondria may sensitize mitochondria to the effects of
proapoptotic Bcl-2 family members (Hajnoczky et al., 2000).

3.2

Ca2+ signaling between the ER and mitochondria

Mitochondria have the capacity to accumulate high Ca2+ loads, and Ca2+ shuttling
between the ER and mitochondria regulates normal physiologic processes, including
oxidative respiration (Rizzuto et al., 2000). Ca2+ can freely diffuse across the OMM,
and transport across the impermeable inner mitochondrial membrane (IMM) is
facilitated by a mitochondrial membrane potential (∆m)-driven, low-affinity
uniporter that is activated by elevated [Ca2+]c (see Bernardi, 1999; Crompton, 1999;
Duchen, 2000 for reviews). Calcium efflux from the mitochondria occurs through a
Na+-Ca2+ exchanger and also by a Na+-independent mechanism (probably a Na+-Ca^2

• exchanger). Simultaneous measurement of [Ca2+]m and [Ca2+]c has revealed that

after activation of the IP 3 R or RyR, cytosolic Ca2+ spikes are simultaneously
accompanied by parallel spikes in [Ca2+]m, highlighting the close coupling of ER Ca^2

• release with mitochondrial uptake (Rizzuto et al., 1998). However, IP 3 R and RyR

channels only raise global [Ca2+]c from -100 nm to ~1 μM, an increase in [Ca2+]c
insufficient to activate the mitochondrial Ca2+ uniporter (Hajnoczky et a/., 2000b).
This issue has been resolved by a model accounting for privileged mitochondrial
uptake sites that sense high local [Ca2+]c in the proximity of ER Ca2+ release sites.
Close contacts between the ER and mitochondria are clearly visible by thin-section
electron microscopy, and in living cells mitochondria exist as a highly interconnected
tubular network that is intimately associated with the ER network. Using live
visualization of ER and mitochondria in HeLa cells transfected with appropriately
targeted GFP constructs, Rizzuto et al. (1998) estimated that 5-20% of mitochondria
are in close apposition with the ER. Furthermore, IP 3 Rs and RyRs are not
homogeneously distributed throughout the ER but concentrated within subdomains
highly active in Ca2+ release, which are often in close apposition and facing
mitochondrial surfaces (Mignery et al., 1989; Satoh et al., 1990; Takei et a/., 1992;
Ramesh et al., 1998; Sharma et al., 2000). It has been estimated that in some cell
types, coordinated Ca2+ release by clusters of IP 3 Rs or RyRs may elevate local [Ca2+]

c at ER-mitochondrial junctions as high as 20-50 μM (> 20-fold higher than global
[Ca2+]c rises), facilitating rapid uptake by the mitochondrial uniporter (Hajnoczky et
al., 2000b).

108 GENETICS OF APOPTOSIS