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Intrinsic optical properties of biological tissue can be modulated with specific genetic alterations, and used as a
phenotypic response to probe specific signaling pathways. Utilizing this approach, we used optical scatter imaging to
probe the effect of BCL-xL on subcellular particle size distribution within monolayers of living CSM14.1 and iBMK
cells. Expression of YFP-Bcl-xL shifted the center of the subcellular particle diameter distribution from 1.5μm to 2μm
in CSM 14.1 cells and from 1.5 to 1.8 μm in iBMK cells. This shift was also observed in cells expressing YFP-TM, in
which YFP is directly fused to the C-terminal transmembrane (TM) domain of BCL-xL, but not in cells expressing YFP
or YFP-BCL-xL-ΔTM, which lack the TM domain. YFP and YFP-BCL-xL-ΔTM were diffusely distributed in the
cytoplasm, while YFP-TM and YFP-BCL-xL were localized on the mitochondria. The measured particle sizes,
combined with the localization of the TM domain to the mitochondria, suggest that morphological disturbances of the
mitochondrial membrane effected by the TM domain of Bcl-xL, underlie the measured optical scatter changes. We have
also found that expression of BCL-2, another anti-apoptotic protein, in iBMK cells, results in a subcellular particle
diameter increase similar to that induced by BCL-xL. However, BCl-xL-ΔTM induced as much apoptosis resistance as
BCL-xL. Thus, mitochondrial morphology changes are not required for apoptosis resistance. Nonetheless, expression of
YFP-TM also conferred a moderate level of apoptosis resistance, while apoptosis resistant iBMK cells lacking Bax and
Bak, showed an increase in the light scattering contribution of particles less than 1.5 μm in diameter. Our results suggest
a possible secondary role of the BCL-xL TM domain in apoptosis resistance. However, the functional relationship
between mitochondrial morphology and apoptosis resistance remains to be fully elucidated.
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