PURPOSE. The family of tubby-like proteins (TULPs), consisting of four fami
ly members, ale all expressed in the retina. at varying levels. Mutations w
ithin two members, tub and TULP1, are known to lead to retinal degeneration
in mouse and humans, respectively, suggesting the functional importance of
this family of proteins in the retina. Despite :I high degree of conservat
ion in the carboxy-teminal region (eg, putative functional domain of the ge
nes) among family members, they are unable to compensate for one another. T
he purpose of this study was to provide a rationale for this lack of compen
sation by investigating the spatial distribution of tubby gene family membe
rs in the retina and to investigate the mechanism of photoreceptor cell dea
th in tubby mice.
METHODS. In situ hybridization using riboprobes specific fur each tubby gen
e family member and immunohistochemistry for TUB and TULP1 were performed t
o determine their expression patterns in the retina of tubby and wild-type
control mice. The terminal dUTP nick-end labeling (TUNEL) assay was perform
ed to detect apoptotic cells in the retina of tubby and wild-type control m
ice.
RESULTS. tub mRNA was found to be expressed throughout the retina, with hig
hest expression in the ganglion cell layer (GCL) and photoreceptor cells. i
n contrast, Tulp1 expression was observed only in photoreceptor cells and T
ulp3 mRNA was expressed at a moderate level only in the inner nuclear layer
(INL) and GCL. The results of the immunohistochemical analysis paralleled
those observed in the in situ studies. TUB immunoreactivity was most highly
concentrated in the GCL, in the inner and outermost regions of the INL, in
the outer plexiform layer (OPL), and in the inner segments of photorecepto
r cells. Similarly, TULP1 immunoreactivity was observed in the OPL and inne
r segments of the photoreceptor cells. No differences ill expression at the
mRNA or protein level were observed for any of the molecules tested in tub
by or wild-type mice. TUNEL-positive cells were detected in the ONL of tubb
y mice, whereas very few were seen in the same layer of age-matched control
mice.
CONCLUSIONS. Although all tubby gene family members are expressed in the re
tina, they each have different cell-specific expression patterns, which may
account in part for their inability to compensate for the loss of one fami
ly member. The photoreceptor cell death in tubby mice occurs through an apo
ptotic mechanism, which is known to be the common final outcome of other fo
rms of retinal degeneration.