Understanding the pathogenesis of ARMD would be served by greater
knowledge about disease-related changes in the cells involved. It is
agreed that atrophy (i.e., the loss of a pigmented layer) is the end
stage of RPE cells in ARMD. The status of RPE cells before atrophy is
less clear, and we offer three potential pathogenetic mechanisms for
RPE atrophy: death in situ, modulation to a phenotypic unrecognizable
as RPE, or migration. The answer to this question is important, because
it provides insight into the nature of the signal received by RPE cells
during disease progression. Based on published evidence from porcine
RPE
24 25 and similar observations in human RPE (Guidry C,
unpublished data, 1998), we tested the hypothesis that RPE
cells modulate their phenotype in ARMD. Our results support this
hypothesis in the early stages of ARMD-associated RPE change: Large
hyperpigmented RPE cells preferentially expressed the cytoskeletal
protein vimentin. In contrast, grade 0 (morphologically normal) cells
were uniformly negative for vimentin content.
Other investigators using the same antibody have reported that cells
overlying choroidal melanomas also upregulate vimentin
expression.
22 34 In that study, vimentin-positive cells
were considered hyperplastic RPE cells by morphologic criteria and the
fact that other sections showed abundant cytokeratin 18–positive cells
of the same morphology.
22 The appearance of RPE vimentin
immunoreactivity also occurs in retinoblastoma
34 and
experimental retinal detachment in monkey.
35 In all these
conditions, including ARMD, the basal and basolateral aspects of RPE
are labeled.
In contrast to vimentin, αSMA immunoreactivity was detectable in only
a few cells, even at the late, atrophic stage of ARMD-associated RPE
change. Others have found that αSMA expression is undetectable in
morphologically normal or atrophic RPE overlying choroidal
tumors.
22 Further, αSMA-positive cells that are presumed
to be RPE by virtue of being labeled with a pancytokeratin antibody are
present in surgically excised CNV membranes.
20 CNV is
hypothesized to be a process with dynamic initiation,
maintenance, and involutional stages.
36 Because of
the long survival intervals between last clinical examination and donor
death
(Table 2) , CNV membranes in our donor eyes with late ARMD were at
postinvolutional stages of disease, considerably later than the
surgically excised CNV membranes examined by others.
20 Thus, it is likely that the expression of αSMA exhibits several
different phases, including transient expression during early stage of
CNV maturation. This idea is strengthened by studies showing thatα
SMA-positive cells undergo apoptosis in CNV
membranes.
37 Furthermore, these changes are not specific
to ARMD, because αSMA-positive, morphologically hyperplastic cells
also occur in subretinal membranes overlying choroidal
tumors.
22
The functional consequences of increased vimentin immunoreactivity for
RPE are currently unclear. Vimentin expression has long been associated
with the onset of mitosis
38 and may signal increased
proliferative potential. However, recent studies indicate that vimentin
expression and architecture are dynamic during other cellular
activities. For example, during active cytoskeletal remodeling
associated with cell spreading in culture, vimentin is initially found
in nonfilamentous forms that are replaced first by short fibrous
structures and then by extensive filamentous networks connected to
other cytoskeletal systems.
39 Further, a role for vimentin
in cell motility associated with wound healing is implicated by studies
in mammary epithelial cells and vimentin-deficient
fibroblasts.
40 41 Thus, it is possible that the reaction
to local perturbation reflected by increased vimentin immunoreactivity
in stage 2 RPE in ARMD includes enhanced potential for cell migration.
This notion is consistent with the position of stage 3 RPE cells within
the neurosensory retina.
Although involvement of Müller cells in ARMD-associated
fibrovascular lesions has been shown,
20 42 our observation
of increased GFAP expression by Müller cells in eyes with late
ARMD is novel and also warrants discussion. Dramatic changes in
Müller cell GFAP expression occur in response to numerous retinal
insults, including laser- or light-induced damage, diabetic
retinopathy, retinal detachment, and inherited retinal
degeneration.
31 32 43 44 45 In the case of light or laser
damage and diabetic retinopathy, increased GFAP expression may reflect
a response to generally perturbed retinal physiology.
31 46 However, in the case of retinal detachment, inherited degeneration, and
ARMD, increased GFAP expression may reflect a secondary response to a
primary insult to another cell type,
43 44 although our
results do not allow exclusion of the possibility of a primary
role for Müller cells in this process. It is interesting that
ARMD shares with retinal detachment not only upregulation of vimentin
in RPE, but also the presence of Müller cell processes separating
photoreceptors from RPE.
42 47 In ARMD the most likely
inductive cells are photoreceptors and/or RPE. These effects may be
mediated by direct contacts between Müller cells and
photoreceptors,
44 Müller cell responses to humoral
factors secreted by the RPE,
25 or both. Understanding the
relative contribution of these mechanisms to retinal degeneration
requires a more detailed analysis of photoreceptor, RPE, and
Müller cell changes at earlier stages of ARMD than was covered in
this study.