Engineering a polarity-sensitive biosensor for time-lapse imaging of apoptotic processes and degeneration.

Apoptosis is of central importance to many areas of biological research, but there is a lack of methods that permit continuous monitoring of apoptosis or cell viability in a nontoxic and noninvasive manner. Here we report the development of a tool applicable to live-cell imaging that facilitates the visualization of real-time apoptotic changes without perturbing the cellular environment. We designed a polarity-sensitive annexin-based biosensor (pSIVA) with switchable fluorescence states, which allows detection only when bound to apoptotic cells. Using pSIVA with live-cell imaging, we observed dynamic local changes in individual rat neurons during degeneration in vitro and in vivo. Furthermore, we observed that pSIVA binding was reversible and clearly defined the critical period for neurons to be rescued. We anticipate pSIVA can be widely applied to address questions concerning spatiotemporal events in apoptotic processes, its reversibility and the general viability of cells in culture.