Although retinal degenerative disorders have similar clinical manifestations—namely, the degeneration of rod photoreceptors, which eventually leads to cone degeneration and blindness—the underlying genetic abnormalities can be very different.
1 2 Given the genetic heterogeneity of retinitis pigmentosa, one practical treatment strategy would be to promote photoreceptor survival through a common mechanism, rather than to attempt to target a large number of specific mutations. Evidence is accumulating in this regard that activation of a gp130-dependent mechanism in Müller cells by neurotrophic factors, such as members of the IL-6 family of cytokines, represents a viable approach to treatment without identifying the specific gene mutation. Photoreceptor rescue by neurotrophic factors first came to prominence when it was demonstrated in the RCS rat that injection of bFGF protects photoreceptors.
5 Subsequently, a screen of a panel of neurotrophic factors in a second model, the light-damage model of photoreceptor degeneration in the albino rat, revealed the photoreceptor protective properties of CNTF.
6 Still later, studies showed that whereas some factors, such as bFGF, rescue photoreceptors in only one or a few animal models, CNTF is protective over a broad range of models and species.
6 7 8 The breadth of protection afforded by CNTF leads immediately to the hypothesis that a common mechanism exists in mammalian retina through which CNTF or CNTF-like molecules can promote photoreceptor survival. That mechanism probably involves a gp130-dependent signaling pathway. A recent report that cardiotrophin-like cytokine (CLC) protects photoreceptors is consistent with this hypothesis (Wen R, et al.
IOVS 2001;42:ARVO Abstract 3380), as is the present work, which provides clear evidence that CT-1 also protects photoreceptors. Because all three are members of the IL-6 family of cytokines, it is very probable that they work by the same mechanism.
CT-1, originally identified in a screening conducted to find factors that induce a hypertrophic response in neonatal cardiac muscle cells,
9 is a polypeptide of approximately 200 amino acid (aa) residues (human CT-1: 201 aa, mouse CT-1: 203 aa) with 80% amino acid identity between mouse and human. Similar to CNTF, the N-terminal of CT-1 does not have a conventional hydrophobic secretion signal sequence
9 17 although CT-1 is secreted nonetheless.
11 Being a member of the IL-6 family of cytokines, CT-1 has 24% amino acid identity to LIF and 19% identity to CNTF. Analysis of the secondary structure predicted for CT-1 also indicates similarity with other members of the family. More important, CT-1 binds to a receptor complex containing LIFRβ and gp130, commonly shared by other members of the family.
12 Functionally, CT-1 not only induces a hypertrophic response in cardiac myocytes,
9 17 but also has cytoprotective effect on them.
10 Expression of CT-1 mRNA is found to be high in heart and skeleton muscles, suggesting that CT-1 is also a candidate motor neuron survival factor.
9 17 Indeed, CT-1 supports long-term survival of spinal motor neurons.
11
CT-1 activates several signal transduction pathways. The cytoprotective effect of CT-1 on myocardial cells is clearly mediated through the Erk-dependent and the PI3 kinase/Akt pathways,
10 13 14 whereas the STAT3-dependent pathway is responsible for the hypertrophic response of cardiac muscle cells.
10 14 In the present work, intravitreal injection of CT-1 activated STAT1, STAT3, and Erks, entirely consistent with previous findings by Peterson et al.
18 that a CNTF mutein axokine also stimulates phosphorylation of STAT1, STAT3, and Erks. The localization of CT-1–induced phospho-STAT3 in Müller cells in the present work is consistent with the finding that axokine-induced STAT3 and Erk phosphorylation occurs in Müller cells.
18 These findings are consistent with the hypothesis that CT-1 acts through signaling pathways similar to those of CNTF in the retina and that Müller cells are the major target cells.
An important finding of the present work concerns dose and regimen. Long-term protection of photoreceptors can be achieved by repeated injection of CT-1. For the potential clinical use of any neurotrophic factor, such as CNTF or CT-1, a crucial issue is whether the factor in question offers long-term protection, and if so, how best to achieve it. In the present work, we used an animal model with rapid photoreceptor degeneration. Photoreceptor death in these animals was evident as early as PD8 and by PD20, only 1 to 2 rows of photoreceptor nuclei remain in the ONL from the original 11 to 12 rows. The peak of degeneration occurs between PD10 and PD12 during which approximately 50% of photoreceptors disappeared.
15 Considering the rapidity and severity of the degeneration, it is surprising to see that nearly 50% of photoreceptors remained at PD30 (the arbitrary end point of these experiments) with repeated CT-1 injections at 3-day intervals. Of specific note, relatively few photoreceptors died between PD20 and PD30 with repeated CT-1 injections (every 3 days), indicating that such treatment effectively prevented the degenerative process. It is also worth noting that less of a protective effect was observed when the intervals were extended to 4 or 5 days and that the protective effect was roughly inversely proportional to the length of the interval
(Fig. 3) . Two important points can be deduced from these observations: that the protein available to the responsive cells rapidly diminishes after a bolus injection and that the memory of the responsive cells is short. It is therefore reasonable to presume that more frequent treatment would result in enhanced protection. More to the point, maximum efficacy is likely to require sustained delivery.
In summary, the present work demonstrates the photoreceptor protective properties of CT-1 and highlights the role of a gp130-dependent mechanism in Müller cells in photoreceptor protection. It also points out the potential importance of sustained delivery of neurotrophic factors such as CT-1 for long-term rescue of photoreceptors.
The authors thank Yun Liu and Xinyu Zhao for excellent technical assistance.