The relationship between opsin levels and the amount of
11-
cis retinal in the retina was examined at P15, an age
when OS has been elaborated and degeneration is minimal. In
Bouse A mice, for which the overexpression of opsin was on
average 23%, the amount of 11-
cis retinal per unit density
of ONL nuclei exceeded the average for nontransgenic mice by 26%
(
Table 1 , right column). Relative amounts of opsin were normalized to
the amount of peripherin/rds in the analyzed sample because its levels
at P10 were comparable in both
Bouse transgenic and
nontransgenic retinas as determined by western analyses (data not
shown). Because at P15 retinal degeneration in
Bouse C is
already under way (data not shown), peripherin/rds was used to account
for the loss of OS assuming that opsin and peripherin/rds disappear at
the same rate with the reduction in the OS. The values for relative
opsin level are effectively scaled to the number of photoreceptors
present, and the level of opsin overexpression may be compared with the
observed increase in 11-
cis retinal in
Bouse A
mice. The results suggest that in
Bouse A opsin
overexpression leads to increase of 11-
cis retinal
sufficient to match the chromophore requirement of the 23% opsin
overexpression. There would appear, however, to be a limit to such an
induced increase in 11-
cis retinal level. In the higher
expressing
Bouse B and C, a substantial percentage of opsin
molecules have no 11-
cis retinal chromophore. It has been
suggested that free opsin arising in vitamin A deprivation, as well as
some mutant forms of opsin that cannot bind 11-
cis retinal,
can excite the phototransduction cascade.
53 It would be of
interest in future studies to examine the effect in
Bouse B
and
Bouse C mice of systemic vitamin A administration, a
treatment that has been found in other studies of transgenic
mice
54 and human trials
55 to promote recovery
of visual sensitivity.