TLR4 deficiency does not alter glaucomatous progression in a mouse model of chronic glaucoma

Abstract

Glaucoma is a leading cause of irreversible blindness worldwide. Toll-like receptor 4 (TLR4) is a pattern-recognition transmembrane receptor that induces neuroinflammatory processes in response to injury. Tlr4 is highly expressed in ocular tissues and is known to modulate inflammatory processes in both anterior and posterior segment tissues. TLR4 activation can lead to mitochondrial dysfunction and metabolic deficits in inflammatory disorders. Due to its effects on inflammation and metabolism, TLR4 is a candidate to participate in glaucoma pathogenesis. It has been suggested as a therapeutic target based on studies using acute models, such as experimentally raising IOP to ischemia-inducing levels. Nevertheless, its role in chronic glaucoma needs further evaluation. In the current study, we investigated the role of TLR4 in an inherited mouse model of chronic glaucoma, DBA/2J. To do this, we analyzed the effect of Tlr4 knockout (Tlr4 ^-/-) on glaucoma-associated phenotypes in DBA/2J mice. Our studies found no significant differences in intraocular pressure, iris disease, or glaucomatous progression in Tlr4 ^-/- compared to Tlr4 ^+/+ DBA/2J mice. These data do not identify a role for TLR4 in this chronic glaucoma, but further research is warranted to understand its role in other glaucoma models and different genetic contexts.

Figure 1.. Tlr4 deficiency does not influence the glaucoma phenotype of DBA/2J mice.

A. Representative slit-lamp images of Tlr4^+/+ (wild-type DBA/2J) and Tlr4^−/− eyes at 4, 8, and 12 months. The top row shows broad beam illumination, and the bottom row shows transillumination. The glaucoma-related changes in Tlr4^−/− mice developed at the same time as in Tlr4^+/+ mice. At 4 months of age, iris disease is not yet evident in either of the groups. Eight-month-old Tlr4^+/+ and Tlr4^−/− eyes present dispersed pigment and transillumination defects, which become more severe at 12 months of age. N > 20 mice per genotype at each age. B. IOP distributions at key ages. There was no significant effect of the genotype on IOP levels at any age (two-way ANOVA; P = 0.702). C. Representative images of nerves showing no/early (NOE), moderate (MOD), and severe (SEV) damage. Scale bar = 10 μm. D. Frequency distribution of optic nerve damage at 10.5 and 12 months of age. No significant differences were observed between the two genotypes of mice at any age (Fisher’s test at 10.5 months, P = 0.749; P = 0.169 at 12 months of age). E. RGC layer cell counts for eyes with no glaucoma (NOE, methods) or severe (SEV) glaucoma based on optic nerve damage. No significant differences in RGC layer cell numbers were observed between eyes of either genotype with severe glaucoma indicating that somal and axonal damage are not uncoupled by the Tlr4 mutation (one-way ANOVA and post hoc Tukey’s HSD test, Tlr4^+/+ SEV vs. Tlr4^−/− SEV; P = 0.384). F. Representative images of the retinas of Tlr4^+/+ and Tlr4^−/− mice with no or severe nerve damage at 40X. Scale bar = 50 μm.