Introduction

Retinas have two kinds of photoreceptors: rods and cones, named for the shape of their light sensitive compartment, the outer segment. (Figure to the left.) The human retina concentrates its red and green sensitive cones in the center of the retina; the macula (Figure to the right). These cone cells serve our need for high acuity vision in bright light. Loss of their function is the clinical problem in patients with macular degeneration, a disease often associated with aging (age related macular degeneration, ARMD). Death of these cells causes loss of central vision. As one patient put it "I can't see my granddaughter's face but I don't bump into things". This is the most common neuronal disorder of humans. Over 12 million Americans are affected, 12 times the number with Alzheimer's disease. By the year 2020, over 20 million will be affected as the number of older citizens grows. There is no current therapy and we have very limited insight into its causes.

A second form of retinal degeneration initially involves the rods. As a consequence, its victims lose peripheral vision first. Sadly, the disease usually progresses to involve the cones so that high acuity central vision is also lost. This disease is yielding to molecular biological study. Rhodopsin, the protein used in rods to capture light has been found to be mutated in over 70 different sites all of which cause loss of vision of several clinical types including retinitis pigmentosa (RP), macular degeneration, latitudinal RP, etc. A diagram of the rhodopsin molecule (see right) illustrates the many sites of mutation. A fascinating variant which caused loss of only half the retina is shown below. The importance of this case is that the rods in the top half survived for 68 years with normal function despite the presence of the mutated rhodopsin. Thus it is possible to retain cell function if only the cells can be prevented from dying. This patient teaches us that the search for a cure is not a vain exercise. Success is possible if we can only understand why rod and cone cells die and prevent it. Why should cones die if they don't express the mutant rhodopsin gene?

Transgenic Frogs: a new model for the study of apoptosis of retinal rods and cones.

Mice and rats do not have many cones; They are nocturnal. Frogs, on the other hand, have abundant cones which can be readily identified microscopically. Last October a new study reported a technique for introducing genes into the DNA of frogs (Kroll and Amaya, 1966). Its principles are diagrammed in the Figure to the left. With Kroll's help to learn her procedure and the use of a DNA construct containing the opsin promoter coupled to green fluorescent protein (GFP we have succeeded in expressing GFP in the eyes of tadpoles only 5 days after fertilization. A figure illustrating our result in an older tadpole is shown below. We will now use this approach in the next months by replacing GFP with various mutant proteins to see if they cause first, death of rods and second, death of adjacent cones in which the mutants are not expressed. If we succeed, it will be one of the first models of the human disease, macular degeneration. Progress should be rapid since we have access to these retinas only days after fertilization rather than months or years as we have experienced in mice and rats.

Photographs of a 23 day old transgenic frog tadpole. The green light emitted from the pupil arises from the rods in the back of the retina that are expressing GFP under the control of the frog opsin promoter when fluorescence is excited by bright blue light. The blue light is hidden by filters that only allow green light to pass through to the camera. The bottom image is a combination of the upper pair of images.

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