Regioselective synthesis of photolabile P(1,2)- and P(4,5)-(o-nitrobenzyl) esters of myo-inositol 1,2,3,4,5,6-hexakisphosphate

Synthesis and characterization of cell-permeant 6-nitrodibenzofuranyl-caged IP3

We have synthesized in a 6-nitrodibenzofuranyl (NDBF) derivative of inositol-1,4,5-trisphosphate (IP(3)) for efficient two-photon uncaging in living cells. As its hexakis acetoxymethyl ester, this caged compound may be applied at low concentration to the extracellular milieu to load the intact astrocytes in acutely isolated brain slices from the mouse cortex. Two-photon irradiation of single astrocytes evoked intracellular calcium signals that required 10% of the energy dosage compared to nitroveratyl (NV)-IP(3). Since NDBF-IP(3) has a 5-fold higher quantum yield than NV-IP(3), these data imply that photolysis of the new NDBF caged compound mobilized intracellular calcium about twice as efficiently as the NV cage.

Biologically active molecules with a "light switch"

Biologically active compounds which are light-responsive offer experimental possibilities which are otherwise very difficult to achieve. Since light can be manipulated very precisely, for example, with lasers and microscopes rapid jumps in concentration of the active form of molecules are possible with exact control of the area, time, and dosage. The development of such strategies started in the 1970s. This review summarizes new developments of the last five years and deals with "small molecules", proteins, and nucleic acids which can either be irreversibly activated with light (these compounds are referred to as "caged compounds") or reversibly switched between an active and an inactive state.

Synthesis and Two-photon Photolysis of 6-(ortho-Nitroveratryl)-Caged IP3 in Living Cells

The synthesis of a photolabile derivative of inositol-1,4,5-trisphosphate (IP3) is described. This new caged second messenger (6-ortho-nitroveratryl)-IP3 (6-NV-IP3) has an extinction coefficient of 5000 M(-1) cm(-1) at 350 nm, and a quantum yield of photolysis of 0.12. Therefore, 6-NV-IP3 is photolyzed with UV light about three times more efficiently than the widely used P(4(5))-1-(2-nitrophenyl)ethyl-caged IP3 (NPE-IP3). 6-NV-IP3 has a two-photon cross-section of about 0.035 GM at 730 nm. This absorbance is sufficiently large for effective two-photon excitation in living cells at modest power levels. Using near-IR light (5 mW, 710 nm, 80 MHz, pulse-width 70 fs), we produced focal bursts of IP3 in HeLa cells, as revealed by laser-scanning confocal imaging of intracellular Ca2+ concentrations. Therefore, 6-NV-IP3 can be used for efficient, subcellular photorelease of IP3, not only in cultured cells but also, potentially, in vivo. It is in the latter situation that two-photon photolysis should reveal its true forte.

Inositol hexakisphosphate mobilizes an endomembrane store of calcium in guard cells

myo-Inositol hexakisphosphate (InsP6) is the most abundant inositol phosphate in cells, yet it remains the most enigmatic of this class of signaling molecule. InsP6 plays a role in the processes by which the drought stress hormone abscisic acid (ABA) induces stomatal closure, conserving water and ensuring plant survival. Previous work has shown that InsP6 levels in guard cells are elevated in response to ABA, and InsP6 inactivates the plasma membrane inward K+ conductance (IK,in) in a cytosolic calcium-dependent manner. The use of laser-scanning confocal microscopy in dye-loaded patch-clamped guard cell protoplasts shows that release of InsP6 from a caged precursor mobilizes calcium. Measurement of calcium (barium) currents ICa in patch-clamped protoplasts in whole cell mode shows that InsP6 has no effect on the calcium-permeable channels in the plasma membrane activated by ABA. The InsP6-mediated inhibition of IK,in can also be observed in the absence of external calcium. Thus the InsP6-induced increase in cytoplasmic calcium does not result from calcium influx but must arise from InsP6-triggered release of calcium from endomembrane stores. Measurements of vacuolar currents in patch-clamped isolated vacuoles in whole-vacuole mode showed that InsP6 activates both the fast and slow conductances of the guard cell vacuole. These data define InsP6 as an endomembrane-acting calcium-release signal in guard cells; the vacuole may contribute to InsP6-triggered Ca2+ release, but other endomembranes may also be involved.