Physiological and pathological functions of βB2-crystallins in multiple organs: a systematic review

Crystallins, the major constituent proteins of mammalian lenses, are significant not only for the maintenance of eye lens stability, transparency, and refraction, but also fulfill various physiopathological functions in extraocular tissues. βB2-crystallin, for example, is a multifunctional protein expressed in the human retina, brain, testis, ovary, and multiple tumors. Mutations in the βB2 crystallin gene or denaturation of βB2-crystallin protein are associated with cataracts, ocular pathologies, and psychiatric disorders. A prominent role for βB2-crystallins in axonal growth and regeneration, as well as in dendritic outgrowth, has been demonstrated after optic nerve injury. Studies in βB2-crystallin-null mice revealed morphological and functional abnormalities in testis and ovaries, indicating βB2-crystallin contributes to male and female fertility in mice. Interestingly, although pathogenic significance remains obscure, several studies identified a clear correlation between βB2 crystallin expression and the prognosis of patients with breast cancer, colorectal cancer, prostate cancer, renal cell carcinoma, and glioblastoma in the African American population. This review summarizes the physiological and pathological functions of βB2-crystallin in the eye and other organs and tissues and discusses findings related to the expression and potential role of βB2-crystallin in tumors.