Changes in Expression in BMP2 and Two Closely Related Genes in Guinea Pig Retinal Pigment Epithelium during Induction and Recovery from Myopia

**Background:** Myopia (nearsightedness) is primarily caused by excessive axial elongation of the eye and has become a major public health problem due to its rising prevalence and associated risks of retinal detachment, glaucoma, and cataracts. While animal models have shown that eye growth is visually regulated via local retinal signals relayed through the retinal pigment epithelium (RPE) to the choroid and sclera, the molecular mechanisms remain poorly understood. Bone morphogenetic protein 2 (BMP2) has been implicated in eye growth regulation, with prior studies in chicks showing bidirectional changes in Bmp2 expression in response to optical defocus. This study aimed to characterize the temporal profiles of Bmp2, Id3 (a downstream target of BMP signaling), and Nog (a BMP antagonist) gene expression in guinea pig RPE during myopia induction and recovery.

**Methods:** Two-week-old pigmented guinea pigs (n=8 for biometric measurements; additional animals for gene expression) wore monocular −10 diopter rigid gas-permeable contact lenses (CLs) for up to 7 days to induce myopia. After 7 days, CL wear was terminated and animals were followed for up to 14 days of recovery. Biometric measurements including spherical equivalent refractive error (SER), axial length (AL), choroidal thickness (ChT), and scleral thickness (ScT) were collected at baseline, 3 and 7 days of CL wear, and 3, 7, and 14 days after lens removal. RPE samples were collected from separate groups after 1 day or 1 week of CL wear, or 1 day or 2 weeks after lens removal. RNA was extracted and quantitative real-time PCR (qRT-PCR) was performed to measure Bmp2, Id3, and Nog gene expression, normalized to Gapdh. Interocular differences (treated minus control) were analyzed using repeated measures ANOVA for biometric data and Wilcoxon signed-rank test for gene expression.

**Key Results:** Significant myopia was induced after 3 and 7 days of CL wear, with mean interocular differences in AL of 0.09 ± 0.05 mm and 0.14 ± 0.04 mm, respectively (p < 0.0001 for both), and corresponding myopic shifts in SER. After lens removal, AL and SER normalized within 7 days. Choroidal thickness showed significant thinning after 7 days of CL wear (p = 0.004), followed by significant thickening at 7 days of recovery (mean interocular difference 14.2 ± 13.3 μm; p = 0.009), which normalized by day 14. No significant changes in scleral thickness were observed. Bmp2 and Id3 gene expression were significantly downregulated in the RPE of treated eyes after both 1 day (Bmp2: 78.7 ± 17.7% of control, p = 0.012; Id3: 67.3 ± 16.1%, p = 0.016) and 7 days of CL wear (Bmp2: 79.5 ± 8.3%, p = 0.002; Id3: 69.6 ± 15.4%, p = 0.005). After 1 day of recovery, Bmp2 expression was significantly upregulated (125.5 ± 12.1%, p = 0.004) and Nog expression was downregulated (64.1 ± 32.8%, p = 0.044). By 2 weeks of recovery, Bmp2 and Id3 expression had normalized, while Nog expression was significantly increased (135.6 ± 18.0%, p = 0.029).

**Clinical Implications:** This study provides strong evidence that Bmp2 in the RPE is bidirectionally regulated by optical defocus, with downregulation during myopia-inducing hyperopic defocus and upregulation during recovery (myopic defocus). The parallel changes in Id3 expression during induction support its role as a downstream target of BMP signaling, while the more complex pattern of Nog expression suggests feedback regulation. These findings validate the guinea pig RGP-CL model as an alternative to spectacle lens models for studying myopia mechanisms. The results highlight BMP2 as a potential negative growth regulator and therapeutic target for controlling myopia progression. However, further studies are needed to translate these gene expression changes into clinical therapies, including investigation of BMP2 protein levels in the choroid and potential links to retinal dopamine signaling.