Effects of imposed defocus of opposite sign on temporal gene expression patterns of BMP4 and BMP7 in chick RPE

Abstract

This study investigated the effects of imposed optical defocus on the expression patterns of bone morphogenetic protein 4 and 7 (BMP4, BMP7) in chick retinal pigment epithelium (RPE), as indicators of roles in postnatal eye growth regulation. BMP4 and BMP7 gene and protein expression patterns were characterized for retina, RPE and choroid tissues of young normal White-Leghorn chickens. The effects of short-term (2 and 48 h) exposure to monocular +10 and −10 diopter (D) lenses on RPE gene expression of BMP4 and BMP7 were also examined. Tissues from both treated and fellow eyes as well as from eyes of age-matched untreated birds were included in the latter experiment. Of ocular tissues comprising the posterior wall of the chick eye, RPE showed the highest expression of BMP4 and BMP7 mRNA, compared to retina and choroid. Western blots and immunohistochemisty confirmed the expression of BMP4 and BMP7 protein in all layers - retina, RPE, choroid and sclera. With imposed defocus, both BMP4 and BMP7 showed bidirectional changes in expression in RPE, however, with different temporal patterns. With +10 D lenses, BMP4 gene expression was up-regulated after both 2 and 48 h of treatment, while BMP7 expression was up-regulated only after 48 h of lens wear. With −10 D lenses, both BMP4 and BMP7 showed down-regulation of gene expression for both 2 and 48 h treatment durations. With the −10 D lens treatment applied for 48 h, gene expression for both BMP4 and BMP7 was also down-regulated in contralateral fellows of treated eyes compared to eyes of untreated chicks. The rapid changes in gene expression in chick RPE observed for both BMP4 and BMP7, up or down according to the sign of imposed optical defocus, resemble similar trends reported for BMP2. Further studies are needed to confirm the roles of BMPs as ocular growth modulators, as suggested by these data. The data also suggest a role for the RPE as a conduit for relaying growth modulatory retinal signals.

RPE and Choroid Mechanisms Underlying Ocular Growth and Myopia

“…33 Finally, recent studies have demonstrated differential expression of a number of genes in the RPE of eyes undergoing experimental manipulations perturbing normal eye growth. 34,35 …”

“…So far, there has been only limited study of the roles of RPE-derived growth factors and cytokines in postnatal eye growth regulation, 34,35 and results were not always conclusive due to the approaches used. For example, the combined tissue collection of the RPE and retina in early gene expression studies involving the chick precludes conclusion about the site of observed changes in expression.…”

“…In follow-up studies of isolated chick RPE, the gene expression of BMP -2, -4, and -7 was found to be bidirectionally regulated, with opposite signs of imposed defocus eliciting opposite responses (decreased with negative lenses, increased with positive lenses), even with exposures as short as 2 h and 2 days. 34,35 The rapidity of these responses suggests roles for these BMPs in the initiation and early phases of altered eye growth. Additional RPE gene expression studies have implicated in defocus-induced “myopic growth,” TGF-β2, which belongs to the same superfamily of growth factors as BMPs, and has been previously linked to eye growth regulation in studies involving chicks and tree shrews.…”

Differential gene expression of BMP2 and BMP receptors in chick retina & choroid induced by imposed optical defocus

“…Specifically in young chicks, all show sign-dependent differential gene expression in RPE in response to short term, imposed optical defocus (2 or 48 h). 14, 15 Differential gene expression of BMPs has also been reported in combined retina/RPE samples in another study using the form-deprivation myopia chick model. 18 These results together open the possibility that the RPE acts as a conduit for relaying growth modulatory retinal signals to the choroid and/or sclera.…”

“…14, 15 In brief, for both eyes of each bird, refractive errors (RE) were measured using retinoscopy and axial ocular dimensions measured using high-frequency A-scan ultrasonography, both at the beginning and end of the lens treatment period. All measurements were performed under isoflurane anesthesia (1% in oxygen).…”