A role for the EGFR and its relevant ligands in normal lens
development has not been established with certainty. Available evidence
indicates that embryonic lens development does not involve the EGFR,
whereas postnatal and adult lens growth does seem to be influenced by
the EGFR in several species including humans.
The EGFR could not be detected using in situ hybridization in embryonic
lenses from mice engineered to express altered components of this
signaling system.
24 25 With knockouts of receptor
function, lens abnormalities can most easily be attributed to
mechanical trauma resulting from the thin fibrotic corneas that develop
and open eyelids at birth.
24 26 This leads to the prolapse
and adherence of the lens to the cornea. Mouse mutants with ablated
tyrosine kinase activity of the EGFR or engineered for TGF-α
deficiency also show gross lens abnormalities or no lens at
all.
27 28 In these cases, it is fairly certain that the
abnormalities do not arise as a differentiation defect but rather are
due to failure of the lens to separate from the cornea, failure of an
anterior chamber to form, or extrusion of the lens through the
underdeveloped cornea. With lenticular TGF-α
overexpressors,
25 29 the perioptic mesenchyme proliferates
and migrates abnormally to surround the lens, most likely in response
to the high levels of TGF-α being released locally into the eye
globe. Until this point, the lens develops normally. It is felt that
the abnormal presence of the perioptic mesenchyme effectively deprives
the developing lens of important developmental factors originating in
the surrounding ocular tissues.
30 31
Although clearly not implicated during embryonic lens development, the
EGFR may have additional significant roles during postnatal and adult
life, the time when most lens growth occurs. Similar to the mouse, the
embryonic chicken lens does not apparently express the EGFR as assessed
by reverse transcription–polymerase chain reaction
(RT–PCR).
32 However, the current data and additional
studies with cultured cells and freshly isolated tissues from postnatal
and adult lenses provide strong evidence for an important role for the
EGFR in maintaining lens growth patterns. The presence of EGFRs in
cultured lens epithelial cells from several species was indicated
through the use of conventional receptor binding
assays.
33 34 More recently, RT–PCR has been used to
amplify the message for the EGFR in cultures of rabbit and human
epithelial cells and in freshly isolated rabbit epithelial
cells.
35 The effects of receptor occupancy on lens
epithelial cell behavior have been dependent on experimental conditions
and the lineage of the cell type examined. In general, EGF has been
shown to be a potent mitogen for normally amitotic central epithelial
cells in organ culture or in cultures derived from central epithelial
cells.
17 33 Similar proliferative responses to TGF-α
have also been noted in cultured central epithelial
cells.
36 However, cultures initiated from more peripheral
regions of the anterior epithelium (which may be more annular pad–like
with regard to fiber cell commitment) did not proliferate in response
to EGF.
34 This indicates that central and the most
peripheral lens epithelial cells may respond differentially to EGF or
TGF-α treatment. In preliminary experiments, we have not observed any
significant proliferative response during the time course of our
experiments in response to either EGF or TGF-α (data not shown). To
date, a role for EGF/TGF-α in regulating the cell division
responsible for continuous lens growth remains to be determined. EGF
has also been implicated, along with several other growth factors, in
promoting the appearance of lens fiber–like structures called lentoids
in cultured human epithelial cells.
15 16 However in these
studies, lentoid formation occurred only after vigorous proliferation
and therefore may be a secondary response to cellular crowding. Our
results clearly show that EGFR occupancy may directly and significantly
increase the accumulation of differentiation-specific cytoskeletal
proteins in short-term cell culture in the absence of any significant
cell proliferation.