Ocular Compliance in Mice

Abstract

 

Purpose

 

Scleral stiffness may be a risk factor for glaucoma and varies significantly between people. The genes controlling this variation are unknown. Scleral stiffness can be indirectly characterized by ocular compliance (OC), the rise in ocular volume per unit rise in intraocular pressure (IOP). We measured OC in BXD mouse substrains, a powerful tool for quantitative trait locus analysis (QTLA) (Geisert. Mol Vis. 2009), to provide baseline data for future studies identifying genes determining OC/scleral properties.

 

Methods

 

Method 1: OC was measured in C57BL/6J (B6), DBA/2J (D2), and 10 BXD substrains (68 mice; 11-14 weeks old; Male & Female). Freshly enucleated eyes were cannulated and a syringe pump delivered several 500 nL PBS boluses to individual eyes (200 nL/s x 2.5 seconds) at a starting IOP of 8 mmHg while the resulting change in IOP was measured. OC was calculated as φ = ΔVolume/ΔIOP. Eye diameter was measured from photographs taken with a microscope. QTLA was used to identify probable gene loci that influence OC. Method 2: The OC-IOP relationship in B6 mice (42 mice; 10-14 weeks old; Male) was measured. Freshly enucleated eyes were cannulated and submersed in PBS. IOP was controlled using an actuated open pressure reservoir while flow rate of PBS into the eye and IOP were measured. OC was calculated based on the step response of the hydraulic-electrical analogue circuit. The OC-IOP relationship was fit to φ(P)= φ₀(P/P₀)^α, where φ₀is the OC at a reference IOP, P₀ = 8 mmHg.

 

Results

 

Method 1: OC differed between substrains (Fig 1; Kruskal-Wallis, p < 10^-6) and was independent of eye diameter (ANCOVA, p = 0.707). Preliminary QTLA identified suggestive peaks associated with OC on chromosomes 1, 7, and 11. Method 2: The average φ₀ and α for B6 mice were 106^x/1.42 nL/mmHg and 1.37 ± 0.48 (Fig. 2). Lower apparent OC values by Method 1 vs Method 2 were due to IOP elevation inherent to Method 1.

 

Conclusions

 

OC differs between BXD mouse substrains and preliminary data suggests candidate genes controlling this variation, opening the possibility of identifying genes that influence scleral mechanics in glaucoma. Analysis of the relation between OC and IOP may allow identification of additional genes.  

 

Figure 1: Median, 25^th and 75^th percentile, maximum, and minimum OC values for each substrain (n ≥ 4 mice per substrain).

 

 

Figure 2: OC vs. IOP for a B6 mouse eye with power law fit.