[Neuroprotection of photoreceptor cells in rod-cone dystrophies: from cell therapy to cell signalling].

Abstract

Neuroprotection of photoreceptor cells in rod-cone dystrophies: from cell therapy to cell signalling. Neuroprotection of photoreceptor cells in rod-cone degenerations is primarily targeted at preventing the loss of function. Strategies for protecting rod cells should therefore aim not only at structural preservation but also must be assessed using functional parameters (e.g., electroretinogram). Given the number of mutations leading to an impaired visual response of rods, the preservation of cones is a realistic approach since (1) numerous mutations do not affect proteins expressed by cones; (2) the secondary degeneration of cones is the main event leading to profound visual impairment; (3) even a small proportion of functional cones is sufficient for major visual functions. Our group has (1) established and confirmed the existence of non cell autonomous mechanisms promoting cone cell viability; (2) shown that rod cell protection or replacement provides a mean to extend the survival of cones; (3) demonstrated that rod-cone trophic interactions are mediated by diffusible proteins; (4) identified by expression cloning a protein mediating such interactions: RdCVF (Rod-derived Cone Viability Factor). These studies provide clues for broad neuroprotective therapies of rod-cone dystrophies.

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Neuroprotection of photoreceptor cells in rod-cone dystrophies: from cell therapy to cell
signaling
Authors: José-Alain Sahel and Thierry Léveillard
Inserm Unit 592 : Physiopathologie Cellulaire et Moléculaire de la Rétine, Paris / Université
Pierre et Marie Curie, Paris / Institute of Ophthalmology- University College London
Departments of Ophthalmology : Centre Hospitalier National d’Ophtalmologie des Quinze-
Vingts / Fondation Ophtalmologique Rothschild
Neuroprotection of photoreceptor cells in rod-cone degenerations is primarily targeted at
preventing the loss of function. Strategies for protecting rod cells should therefore aim not
only at structural preservation but also must be assessed using functional parameters (e.g.
ERG). Given the number of mutations leading to an impaired visual response of rods, the
preservation of cones is a realistic approach since 1) numerous mutations do not affect
proteins expressed by cones; 2) the secondary degeneration of cones is the main event leading
to profound visual impairment; 3) even a small proportion of functional cones is sufficient for
major visual functions.
Our group has 1) established and confirmed the existence of non cell autonomous
mechanisms promoting cone cell viability; 2) shown that rod cell protection or replacement
provides a mean to extend the survival of cones; 3) demonstrated that rod-cone trophic
interactions are mediated by diffusible proteins; 4) identified by expression cloning a protein
mediating such interactions: RdCVF (Rod-derived Cone Viability Factor).

1) The rd1 mouse is a model of rod-cone degeneration. A recessive mutation carried by the
gene encoding the beta subunit of the rod-phophodiesterase is leading to the rapid
degeneration of rod photoreceptors through apoptosis and a secondary degeneration of cones,
while these neurons to not express the mutated gene and are not directly suffering form the
enzymatic deficit. This model does mimick the sequence of evets in patients affected with
Rod-Cone dystrophies i.e. primary loss of dark-adapted vision followed with loss of central
and light-adapted vision. By transplanting normal photoreceptors in the subretinal space of the
rd1 mouse immediately after rod death, we have demonstrated that the cones from the grafted
animal are surviving significatively longer than the shamed animals (Mohand-Saïd et al.,
2000). This confirms the existence of non cell autonomous mechanisms promoting cone
viability (Mohand-Saïd et al., 1998).

2) The most straightforward interpretation of our data is that cone degenerate when rods are
missing. We have further validated this model by showing that the trophic factor GDNF, and
the pharmacological treatment of the rd1 mouse with the calcium channel diltiazem are
generating rod protection that is translated by the persistence of cone function (Frasson et al.,
1999a; 1999b). It should be noticed that future therapeutic approaches aimed at preserving
cone function would be optimal.

3) In order to study the effect of rods on cones, we have used a co-culture system. Rod-less
retinal explants from the rd1 mouse co-cultured with rod-enriched retina show a 40% rescue
of cones. Not only this experiment demonstrated that rod are exerting a trophic activity
toward cones in vitro, but it also established that the trophic activity is diffusible (Mohand-
Saïd et al.,1998). The activity is present in the conditioned media prepared from rod-enriched
retina. Using a partial purification scheme, we have shown that the activity is carried by
molecules with the physical properties of proteins (Fintz et al., 2003).

4) We have adopted a systematic high content screening approach based of a cone-enriched
cultures system and an expression cloning protocol to identify from an expression library

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clones that encode for cone viability factor. By screening 210,000 clones, we have identified a
factor Rod-derived Cone Viability Factor (Léveillard et al., 2004). RdCVF is expressed in a
rod dependant manner and the activity of rod-enriched conditioned medium is largely
decreased by RdCVF antibodies. The current model is that cones degenerate in the rd1 mouse
by trophic support withdrawal after rod degeneration and RdCVF loss of expression. RdCVF
is a truncated thioredoxin produced by a novel gene Txnl6. Our laboratory is currently
working on RdCVF delivery and signaling.

These studies provide a rationale for protecting non functional rods and clues for broad
neuroprotective therapies of rod-cone dystrophies. Therefore, planning for clinical trials
implies the identification of genotyped patients with symmetrical progressive disease and the
implementation of outcome measurements that allow early and reproducible detection of both
rod and cone function and structure. Such facilities and programs (from fine matrix mapping,
autofluorescence confocal imaging, optical coherence tomography and adaptative optics are
currently implemented at the Clinical Investigation Center of the Centre Hospitalier National
d’Ophtalmologie des Quinze-Vingts.


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