Krzysztof Palczewski, Ph.D.
John H. Hord Professor and Chair
Department of Pharmacology
Senior Fellow of the American Asthma Foundation
School of Medicine, W319
2109 Adelbert Rd.
Case Western Reserve University
10900 Euclid Ave.
Cleveland, Ohio 44106-4965
The light-sensing apparatus of the eye is found within the rods and cones—two
types of specialized cells located in the posterior of the retina. Of the two types
of receptors, rod cells exhibit greater light sensitivity (lower threshold) and
a slower reaction time. Cone cells, on the other hand, respond rapidly, and provide
greater discrimination of temporal, spatial, and spectral detail. The light signal
captured by photoreceptor cells triggers a cascade of chemical reactions, called
phototransduction, which ultimately generates a neuronal signal.
Like the rod cell, cone cell activation involves the photoisomerization of the 11-cis-retinal
chromophore bound to an opsin-like transmembrane protein. In the case of the cone
cell, however, there are three variants of the transmembrane protein. When bound
to the chromophore, each of the resulting visual receptor pigments exhibit a characteristic
red, blue, or green absorption maxima which leads ultimately to color vision. Recent
work indicates that the differences in absorption maxima are a function of differences
in amino acid sequences within each pigment. Similar analyses of structure, reactivity
and function will have to be performed for all the critical receptors, catalysts
(G-proteins, kinases, phosphoesterases, retinal dehydrogenases), and reaction terminators
(arrestins, recoverins, guanylate cyclase activating proteins) within the cone cells
Light-triggered events initiated in rod and cone outer segments were the subject
of numerous investigations during the last two decades, most notably using molecular
approaches and electrophysiological measurements of the isolated retina or photoreceptor
cells. The light events are intimately intertwined with the regeneration reactions
that involve two cell systems. Every photon of light that triggers photoisomerization
is counterbalanced by regeneration of rhodopsin with newly synthesized 11-cis-retinal.
Contributions from numerous investigators have provided substantial advances in
our fundamental knowledge of phototransduction and the regeneration of rhodopsin.
These have included the identification of phototransduction and retinoid processing
enzymes, cation channels, and retinoid-binding proteins in the retina-RPE system,
and determination of the mechanisms of action of these proteins. Furthermore, within
the past decade there has been substantial new information regarding the links between
specific retinal diseases and identified abnormalities of the retinoid cycle.
Many unresolved issues relevant to phototransduction, light- and dark-adaptation,
and the chemical processing of retinoid cycle intermediates remain unanswered, including
the enzymology of the retinoid cycle, the mechanisms by which these intermediates
diffuse within and between the photoreceptors and the RPE, and the dependence of
phototransduction reactions on the operation of the cycle. These important questions
pose exciting challenges for future research on the visual cycle, and are certain
to continue as the subject of intense interest for Professor Palczewski’s
The goal of Professor Palczewski’s laboratory is to:
- Understand the biochemical basis underlying the mechanism of rhodopsin inactivation
and restoration of the cGMP level.
- Delineate the biochemical basis underpinning the similarities and differences between
rod and cone cell phototransduction.
- Understand the enzymology of the isomerization of all-trans-retinol to 11-cis-retinol
in the retina.
Knowledge about phototransduction in the retina, a system with great experimental
advantages, will improve further understanding of similar events in hormonal signaling,
cellular communication and immune regulation, and provide baseline information for
further studies of retinal disease processes.