Molecular Engineering Applied to Cell Biology and Neurobiology 
biochemical signal is merely coincidental or is 
necessary or sufficient for various cell responses. 
One problem is that the photochemistry 
currently used is irreversible, so that once each 
molecule has been photolyzed and has released 
or absorbed its cargo, it cannot be recycled back 
to its initial state. Therefore the experiment is 
soon over, because the molecules have been used 
up. The next challenge is to find reversible photo- 
chemistry in which light of one wavelength 
causes messenger release, and light of a different 
wavelength triggers reuptake. By alternating 
these two colors of illumination, one would then 
be able to generate artificial oscillations in mes- 
senger level whose effects could be compared 
with those of the many known natural oscilla- 
tions. Furthermore, steady illumination with ad- 
jacent spots of the two wavelengths should create 
a standing spatial gradient of messenger level, 
which would help test whether analogous natural 
gradients are important in controlling the direc- 
tion of cell movement and outgrowth, as often 
hypothesized. 
Eventually we hope to extend optical methods 
to detect macromolecular biochemical signals 
such as protein phosphorylation or gene tran- 
scription. These events currently are assayed by 
grinding millions of cells, so that time resolution 
is limited and differences between individual 
cells or subregions are impossible to discern. Our 
experience with imaging ionic messengers and 
cAMP suggests that cells have considerable indi- 
viduality and complex behavior patterns. These 
were missed with destructive population assays, 
which might be somewhat analogous to studying 
human psychology on the basis only of anony- 
mous nationwide averages in which the respon- 
dents are executed after each poll. We therefore 
seek continuous, nondestructive readout from 
single cells. Possible approaches include mi- 
croinjection of peptides whose fluorescence is 
altered by phosphorylation, or development of 
membrane-permeant fluorogenic substrates for 
reporter enzymes whose nucleotide sequences 
can be fused to genes or promoter sequences of 
interest. 
Our projects encompass a wide range of disci- 
plines, including organic synthesis, theoretical 
and experimental optical spectroscopy and pho- 
tochemistry, protein chemistry, computerized 
microscopy and image processing, cell biology, 
and neurobiology. 
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