Purpose: Alpha-crystallin is expressed at high levels in the lens in a complex of alphaA- and alphaB-crystallin subunits in 3:1 molar ratios, and is known to maintain the solubility of unpolymerized tubulin and enhance the resistance of microtubules to depolymerization, but its effect on proteins classically associated with microtubule stability (microtubule associated proteins) in the lens is unknown. In the present study we examined the expression of the brain microtubule associated protein tau in lenses of alpha-crystallin gene knockout mice.
Methods: Quantitative RT-PCR, immunoblotting, cryo-immunoelectron microscopic and immunohistochemical methods were used to characterize the expression of tau in the lenses of alphaA(-/-)-, alphaB(-/-)-, and alphaA/B(-/-)-crystallin mice.
Results: Immunoreactivity to tau, a 45-66 kDa brain microtubule associated protein that has been best characterized in neurons and neuronal pathologies, was uniquely upregulated in lens cortical fiber cells with aging and was associated with the microtubule fraction of alphaA(-/-)-, alphaB(-/-)-, and alphaA/B(-/-)-crystallin mouse lenses, but was undetectable in wild type lenses. Quantitative RT-PCR analysis further showed an upregulation of tau transcripts in alphaA(-/-)- and alphaA/B(-/-)-crystallin lenses. Brain microtubule fractions served as a positive control for tau in these experiments. An increase in phosphorylation of tau was detected in alphaA(-/-)- and alphaB(-/-)-crystallin brain proteins.
Conclusions: Although tau aggregation and alphaB-crystallin expression have been shown to increase in neurodegenerative diseases, surprisingly tau expression increases in the alpha-crystallin knockout lenses, suggesting that alphaA- and alphaB-crystallins are potentially important regulators of tau expression in lens.