Beta-amyloid secretases and beta-amloid degrading enzyme expression in lens.

PURPOSE: Beta- and gamma-secretases are proteases involved in the processing of the Alzheimer precursor protein (AbetaPP) that releases the transmembrane beta-amyloid fragment (Abeta), associated with age-dependent disease in lens and brain. Gamma-secretase is a protein complex containing Presenilin and Nicastrin proteins, which also processes Notch and other receptors involved in the eye and lens development. Neprilysin (NEP), a major protease involved in degrading Abeta, acts with beta- and gamma-secretases to regulate steady-state levels of Abeta. Previously, we demonstrated AbetaPP and Presenilin expression and processing in the lens and demonstrated cell degeneration in classic Alzheimer disease (AD) transgenic and systemic oxidative stress animal models, suggesting that additional AbetaPP processing proteins are also present in the lens. Here we investigate lens expression of beta-secretases, nicastrin and NEP proteins, and compare their protein distribution to Notch and Presenilin in lens.
METHODS: RT-PCR was used to analyze mRNA transcripts. Immunoblots and immunohistochemistry were used to examine the protein expression and distribution of secretase and Abeta degrading proteins, as well as Presenilin and Notch proteins in mouse lenses.
RESULTS: Beta-acting cleaving enzymes, BACE (BACE1) and BACE2, Nicastrin, Presenilins, Notch and NEP are expressed in the lens. In situ examination of protein distribution in lens indicates expression of each of these proteins is upregulated in peripheral elongating fiber cells at the lens equatorial margin and overlaps with Notch and Presenilin proteins, and also with the distribution of AbetaPP and Abeta proteins demonstrated in a previous study. Neprilysin exon 1-4 splicing, previously described as diagnostic for neuronal expression, also occurs in lens.
CONCLUSIONS: BACE, BACE2, Nicastrin and NEP are expressed primarily in elongating peripheral fiber cells, overlapping with Notch, Presenilin, and AbetaPP protein distribution in lens, consistent with their role in regulating Notch and AbetaPP ectodomain shedding in lens. Lens expression of beta- and gamma-secretases together with NEP suggests these proteins may also regulate Abeta turnover in the lens. The presence of Abeta processing and degrading proteases in lens provides further evidence that Alzheimer-related cell biology is fundamentally involved in lens development, and provides additional evidence that mechanisms of Alzheimer pathophysiology can contribute to lens degeneration, suggesting further that therapeutics targeting Abeta proteases may be applicable to lens degenerative disease.