• Lysyl oxidate-like 1 knockout (Loxl1^-/-) mice have decreased vision without elevated intraocular pressure. • Loxl1^-/- mice exhibit biometric changes of the anterior segment of the eye. • Loxl1^-/- mice have altered elastin and collagen structure in peripapillary sclera. • Structural alternations of peripapillary sclera correlate with its increased stiffness in Loxl1^-/- mice.

Marfan syndrome is an inherited connective tissue disorder that affects the cardiovascular, musculoskeletal, and ocular systems. It is caused by pathogenic variants in the fibrillin-1 gene (FBN1). Fibrillin is a primary component of microfibrils, which are found throughout the extracellular matrix (ECM) and provide elasticity and resilience to connective tissue. Microfibrils also play a role in signaling by sequestering growth factors and interacting with cell surface receptors. In many tissues, microfibrils are interwoven with elastin, collagens, and other elements of the ECM. However, uniquely in the ciliary zonule of the eye, microfibrils exist in cell-free bundles largely devoid of other components. This structure offers a rare opportunity to study a pure population of fibrillin microfibrils in a relatively native state. Here, we briefly review the organization of the zonule and describe recent experiments in which we measure zonular biomechanics, providing insights into microfibril dynamics that would be challenging to obtain in other contexts.

Exfoliation syndrome (XFS) is a systemic connective tissue disorder with elusive pathophysiology. We hypothesize that a mouse model with elastic fiber defects caused by lack of lysyl oxidase like 1 (LOXL1 encoded by Loxl1), combined with microfibril deficiency due to Fbn1 mutation (encoding fibrillin-1, Fbn1^C1041G/+) will display ocular and systemic phenotypes of XFS. Loxl1^-/- was crossed with Fbn1^C1041G/+ to create double mutant (dbm) mice. Intraocular pressure (IOP), visual acuity (VA), electroretinogram (ERG) and biometry were characterized in 4 genotypes (wt, Fbn1^C1041G/+, Loxl1^-/-, dbm) at 16 weeks old. Optic nerve area was measured by ImageJ and axon counting was achieved by AxonJ. Deep whole-body phenotyping was performed in wt and dbm mice. Two-tailed Student's t-test was used for statistical analysis. There was no difference in IOP between the 4 genotypes. VA was significantly reduced only in dbm mice. The majority of biometric parameters showed significant differences in all 3 mutant genotypes compared to wt, and dbm had exacerbated anomalies compared to single mutants. Dbm mice showed reduced retinal function and significantly enlarged ON area when compared with wt. Dbm mice exhibited severe systemic phenotypes related to abnormal elastic fibers, such as pelvic organ prolapse, cardiovascular and pulmonary abnormalities. Ocular and systemic findings in dbm mice support functional overlap between fibrillin-1 and LOXL1, two prominent components of exfoliation material. Although no elevated IOP or reduction of axon numbers was detected in dbm mice at 16-week-old, their reduced retinal function and enlarged ON area indicate early retinal ganglion cell dysfunction. Dbm mice also provide insight on the link between XFS and systemic diseases in humans.

Two major constituents of exfoliation material, fibrillin-1 and lysyl oxidase-like 1 (encoded by FBN1 and LOXL1), are implicated in exfoliation glaucoma, yet their individual contributions to ocular phenotype are minor. To test the hypothesis that a combination of FBN1 mutation and LOXL1 deficiency exacerbates ocular phenotypes, the pan-lysyl oxidase inhibitor B-aminopropionitrile (BAPN) was used to treat adult wild -type (WT) mice and mice heterozygous for a missense mutation in Fbn1 (Fbn1C1041G/& thorn;) for 8 weeks and their eyes were examined. Although intraocular pressure did not change and exfoliation material was not detected in the eyes, BAPN treatment worsened optic nerve and axon expansion in Fbn1C1041G/& thorn; mice, an early sign of axonal damage in rodent models of glaucoma. Disruption of elastic fibers was detected only in Fbn1C1041G/& thorn; mice, which increased with BAPN treatment, as shown by histologic and immunohistochemical staining of the optic nerve pia mater. Transmission electron microscopy showed that Fbn1C1041G/& thorn; mice had fewer microfibrils, smaller elastin cores, and a lower density of elastic fibers compared with WT mice in control groups. BAPN treatment led to elastin core expansion in both WT and Fbn1C1041G/& thorn; mice, but an increase in the density of elastic fiber was confined to Fbn1C1041G/& thorn; mice. LOX inhibition had a stronger effect on optic nerve and elastic fiber parameters in the context of Fbn1 mutation, indicating the Marfan mouse model with LOX inhibition warrants further investigation for exfoliation glaucoma pathogenesis. (Am J Pathol 2024, 194: 1317-1328; https://doi.org/10.1016/j.ajpath.2024.03.002)

We previously reported ocular phenotypes of 1-year-old 129S1/SvlmJ lysyl oxidase-like 1 null (Loxl1^−/−) mice. Here we sought to characterize age-dependent changes in C57BL/6J Loxl1^−/− mice in a longitudinal fashion. Retinal ganglion cell (RGC) function was assessed by electroretinography (ERG), and optic nerves were evaluated by histological analysis. Ocular biometric measurements were obtained by optical coherence tomography (OCT). We detected reduced RGC function, revealed by decreased amplitude and increased latency of ERG positive scotopic threshold responses (pSTRs) in Loxl1^−/− mice compared to age-matched wt mice. In addition, there is significant inter-eye asymmetry of RGC function, as well as age-related RGC function loss observed only in Loxl1^−/− mice. Histologically, we observed enlarged optic nerve areas in Loxl1^−/− mice compared to wt mice. Significant ocular biometric differences between two groups were detected, most notably, age-related axial elongation of the globe, accompanied by deepening of anterior chamber depth (ACD). Though eyes elongate with age in both groups, this is more pronounced in Loxl1^−/− mice, and the elongation of the globe correlated with decreased RGC function. The correlation of age-related reduction in RGC function with globe axial elongation may have implications for the association of axial myopia with glaucoma and aging in humans.