Download New research illuminates mystery of cone death in retinitis

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Bio-Ophthalmology
New research illuminates mystery of
cone death in retinitis pigmentosa
N
ew research conducted at Harvard
Medical School has provided
a fresh perspective on why
healthy cells die in patients with retinitis
pigmentosa (RP).
While RP is a disorder arising mainly
from genetic mutations within rod
photoreceptors, it has been widely
observed that rod cell death is invariably
followed by cone cell death. For decades,
however, researchers have wondered why.
In nearly all cases, the cones are
completely healthy and it is their loss,
rather than the loss of rods, that causes
most of the devastation in patients with RP.
As cones are responsible for our fine and
colour vision, their degeneration inevitably
leads to a significant alteration in the quality
of life.
Further compounding the mystery
of healthy cone cell death has been the
equally puzzling observation that the
reverse situation does not apply: cone cell
death does not appear to be followed by
widespread rod cell death.
Intrigued by these observations, Drs
Claudio Punzo, Karl Kornacker, and
Constance Cepko set about on a series of
experiments to explore how this apparent
riddle might be resolved. Their findings
appeared in a recent edition of Nature
Neuroscience (2008, 11:44-52).
On a clinical level, a significant
proportion of sufferers present with
symptoms in their late teens to early adult
years. Initial symptoms include loss of
peripheral vision and loss of night vision.
This phase is followed by continual loss
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of rod photoreceptors and diminishing
low light visual ability. Eventually, a similar
demise of the cone cells is observed, and,
in many cases, complete or near-complete
blindness is the outcome.
While modern life and technology
provides the capacity to avoid dim lighting
conditions, the loss of rod photoreceptor
cells may not appear as a major
devastation. However, the inevitable loss
of the cone cells following rod cell death
is where the true impact of the disease
is realised. The Harvard researchers
wanted to understand what was causing
this cone cell death and, by finding the
cause, potentially explore if any therapeutic
intervention might be possible to extend
the life span of cone photoreceptor cells.
The researchers examined four mouse
models of RP. In each of the models, cone
cell death always started at the end of
the rod cell death phase and appeared
to spread from the central retina to
the periphery. Although the timing of
the phases was different in the different
models, there were sufficient common
features to suggest that an underlying
common mechanism might explain the
kinetics of cell death.
To see if such a common mechanism
could be found, the research team decided
to analyse global gene expression in the
rod and cone photoreceptors. Using
sophisticated gene chip technologies,
researchers could take a snapshot of both
rod and cone cell populations at various
time points in the demise of the retina
and get a ring-side view of the activity of
nearly 200 genes during various stages of
photoreceptor cell death.
When the data was crunched, almost
35 per cent of the “hits” showed activity
in genes involved with cellular metabolism.
One hit in particular – the insulin/mTOR
signalling pathway – clearly stood out.
The insulin/mTOR signalling pathway
is known to be a critical pathway in
regulating a number of aspects of cellular
metabolism. Its identification in a global
gene expression assay of dying rod and
cone photoreceptors suggested that there
may be a link between the pathway and cell
death. Under normal conditions, the mTOR
protein interacts with several cell proteins
to facilitate high-energy processes such as
protein translation.
However, under conditions of stress
– such as nutrient deprivation – mTOR
has the opposite effect. Dr Cepko and
colleagues observed that the active mTOR
was progressively reduced in the retinas
of the four RP animal models and that its
depletion coincided with cone cell death.
This certainly appeared to be a smoking
gun but a clearer understanding of the
mechanism would be required before all
the dots could be joined together.
The observations around mTOR activity
suggested that a nutritional imbalance,
possibly caused by reduced glucose
levels, was occurring in cones during
degeneration. In support of this model
additional assays showed the transcription
factor HIF-1α/β (hypoxia inducible factor
1) and its target, GLUT-1 (glucose
transporter 1) were up-regulated in the
cone photoreceptors of all models, which
is what one would expect in cells trying to
overcome nutrient deprivation.
A further consequence of such nutrient
deprivation would be the activation of
“autophagy” in which cells re-absorb
proteins and organelles in an effort to
retrieve cellular nutrients. One form of
such autophagy, “chaperone-mediated
autophagy,” or CMA, can be detected
by the expression of CMA-related genes
in dying cone cells and this is exactly
what was found in each of the RP animal
models. Now several lines of evidence
were pointing to the idea that cone cells
in the degenerating RP retina may be dying
from starvation brought about through
compromised glucose uptake and low
mTOR activity.
Once the researchers found that
something was missing they re-introduced
the putative missing part to see what would
happen. When one of the animal models
of RP was administered with insulin over
a four-week period, cone cell survival
improved. It appeared that facilitating
glucose uptake ameliorated cone cell death.
Taken together, these observations provide
an entirely new mechanism for explaining
cone cell death in retinitis pigmentosa, not
to mention the identification of a potential
pathway to target for the development of
new therapeutics.
The model for resolving the mystery
of healthy cone cell death may also
prove to be remarkably elegant in that
it simultaneously accounts for why
cone led pathologies do not lead to rod
photoreceptor cell death. Given the role
of the retinal pigment epithelium (RPE) in
shuttling nutrients and oxygen from the
choroid to the photoreceptors the Harvard
model of cone cell demise via starvation is
entirely reasonable when oneconsiders the
number of rods to cones in the human (and
mouse) retina.
As approximately 95 per cent of human
photoreceptors are rods and approximately
20 to 30 outer segments of photoreceptors
connect in with one RPE cell, a simple
calculation shows that possibly one or
two of the RPE/outer segment contacts
are with cone cell outer segments. As the
retina degenerates, the outer nuclear layer
by Gearoid Tuohy
(ONL) breaks down and consequently the
number of RPE/cone connections becomes
less. Dr Cepko and her team suggest that
as the number of RPE/cone connections
falls below a certain threshold required
for proper flow of nutrients the reduced
supply of nutrients to the cones leads to
cell starvation. In other words, cell density
may represent a critical threshold and
it may be no coincidence that in all four
models of RP, cone cell death occurred
when there was a single layer of rods
remaining in the ONL.
The Harvard researchers
wanted to understand
what was causing this
cone cell death and,
by finding the cause,
potentially explore if any
therapeutic intervention
might be possible to
extend the life span of
cone photoreceptor cells
The mechanism of cone cell death
proposed by the Harvard researchers
also neatly explains the “reverse case”
– why the loss of cones in a cone-led
degeneration does not lead to rod
photoreceptor cell death. Cone cells
account for less than five per cent of
human photoreceptors and, even when
the majority of cones are lost, significant
cell density remains from the rod cell
population. In essence, the “critical
threshold” is never reached in coneled degenerations, and so rods never
experience a comparable starvation phase.
While the simple administration of
insulin in the RP mice can lead to cone cell
survival, it is unlikely that such a strategy
could be advised as a human therapeutic
approach. However, if the mechanism
proposed by Dr Cepko and colleagues
for cone cell death in RP is validated, then
there may be an abundance of therapeutic
targets and opportunities available for
medical intervention. Given the clinical
and genetic heterogeneity of RP, the
commonality of its final phase may provide
an attractive opportunity to treat a very
large market and extend the viability for
fine and colour vision even as the rods
are lost.