Lens-specific βA3/A1-conditional knockout mice: Phenotypic characteristics and calpain activation causing protein degradation and insolubilization

beta A3/A1-crystallin is a lens structural protein that plays an important role in maintaining lens transparency via interactions with other crystallins. While the function of beta A3/A1-crystallin in the retina is well studied, its functions in the lens, other than as a structural protein, remain unclear. In the current study, we generated the lens-specific beta A3/A1-crystallin conditional knockout mouse (named beta A3/A1ckO) and explored phenotypic changes and the function of the crystallin in the lens. The beta A3/A1ckO mice showed congenital cataract at birth and exhibited truncation of lens proteins. Several truncated protein fragments were recovered as a pellet during a low-speed centrifugation (800 rpm, 70 x g) followed by a relatively higher speed centrifugation (5000 rpm, 2744 x g). Mass spectrometric analysis of pellets recovered following the two centrifugations showed that among the fragments with M-r < 20 kDa, the majority of these were from beta-tubulin, and some from phakinin, alpha A-crystallin, and calpain-3. Further, we observed that in vitro activation of calpain-3 by calcium treatment of the wild-type-lens homogenate resulted in the degradation of calpain-3, alpha A-crystallin and beta-tubulin and insolubilization of these proteins. Based on these results, it was concluded that the activation of calpain 3 resulted in proteolysis of beta-tubulin, which disrupted cellular microtubular structure, and caused proteolysis of other lens proteins (alpha A-crystallin and phakinin). These proteolyzed protein fragments become insoluble, and together with the disruption of microtubular structure, and could be the causative factors in the development of congenital nuclear cataract in beta A3/A1cKO mice.

Automated summary

The beta A3/A1-crystallin protein is important in maintaining lens transparency and the loss of its function leads to congenital cataract in mice. The activation of calpain 3 results in degradation of beta-tubulin and other lens proteins, causing protein accumulation and disruption of cellular microtubular structure, which contributes to the development of cataract in beta A3/A1cKO mice.