The primary energy substrate of the lens is
glucose and uptake of
glucose from the aqueous humor is dependent on
glucose transporters. GLUT1, the facilitated
glucose transporter encoded by Slc2a1 is expressed in the epithelium of bovine, human and rat
lenses. In the current study, we examined the expression of GLUT1 in the mouse lens and determined its role in maintaining lens transparency by studying effects of postnatal deletion of Slc2a1. In situ hybridization and immunofluorescence labeling were used to determine the expression and subcellular distribution of GLUT1 in the lens. Slc2a1 was knocked out of the lens epithelium by crossing transgenic mice expressing
Cre recombinase under control of the GFAP promoter with Slc2a1loxP/loxP mice to generate Slc2a1loxP/loxP;GFAP-Cre+/0 (LensΔGlut1) mice. LensΔGlut1 mice developed visible
lens opacities by around 3 months of age, which corresponded temporally with the total loss of detectable GLUT1 expression in the lens. Spectral domain optical coherence tomography (SD-OCT) imaging was used to monitor the formation of
cataracts over time. SD-OCT imaging revealed that small nuclear
cataracts were first apparent in the
lenses of LensΔGlut1 mice beginning at about 2.7 months of age. Longitudinal SD-OCT imaging of LensΔGlut1 mice revealed disruption of mature secondary fiber cells after 3 months of age. Histological sections of eyes from LensΔGlut1 mice confirmed the disruption of the secondary fiber cells. The structural changes were most pronounced in fiber cells that had lost their organelles. In contrast, the histology of the lens epithelium in these mice appeared normal.
Lactate and
ATP were measured in
lenses from LensΔGlut1 and control mice at 2 and 3 months of age. At 2 months of age, when GLUT1 was still detectable in the lens epithelium, albeit at low levels, the amount of
lactate and
ATP were not significantly different from controls. However, in
lenses isolated from 3-month-old LensΔGlut1 mice, when GLUT1 was no longer detectable, levels of
lactate and
ATP were 50% lower than controls. Our findings demonstrate that in vivo, the transparency of mature lens fiber cells was dependent on glycolysis for
ATP and the loss of GLUT1 transporters led to
cataract formation. In contrast, lens epithelium and cortical fiber cells have mitochondria and could utilize other substrates to support their anabolic and catabolic needs.