The alpha-crystallins comprise a large fraction of the soluble protein in the vertebrate lens where they were, for many years, believed to function solely as structural proteins. This small family of crystallins is encoded by only 2 genes, the alphaA- and alphaB-crystallin genes and is collectively referred to as alpha-crystallin. They are related to the small heat shock proteins, and in vitro they exhibit molecular chaperon activity, autokinase activity, single stranded DNA binding activity, and interact with and affect the state of several cytoskeletal components. Alpha-Crystallin, especially alphaB-crystallin, has been shown to be a normal constituent of many non-lenticular tissues, and has been detected in cytoplasmic inclusion bodies found in several human pathological conditions. Toward understanding the major roles of alpha-crystallin in vivo, we are functionally deleting alpha-crystallin proteins by disrupting or knocking-out their genes in mice. We are attempting to elucidate the in vivo functions of alphaA- and alphaB-crystallin 1) in lens development and morphogenesis; 2) in maintaining a stable, transparent lens throughout the life of an organism (i.e. preventing cataract); 3) in the non-lenticular tissues where they are normally present; and 4) in non-lenticular pathological conditions. We have generated mice which lack alphaA-crystallin, mice which lack alphaB-crystallin, and mice which are deficient in both alphaA- and alphaB-crystallin. Our research has demonstrated that neither alphaA-crystallin nor alphaB-crystallin is essential for survival or reproduction of the laboratory mouse. AlphaB was thought to be an essential protein because of its high expression levels during embryogenesis and its constitutive expression in adult heart, skeletal muscle and several other organs. At a young age, mice lacking alphaA-crystallin develop cataract which eventually progresses to severe lens opacity. A key feature of this cataract is the formation within lens fiber cells of inclusion bodies containing predominantly alphaB-crystallin. In mice lacking both alphaA- and alphaB-crystallin, the rate and extent of cataract formation are greatly enhanced, but inclusion bodies are not observed, suggesting a different mechanism of cataractogenesis. These mice are microphthalmic and their severe cataract is characterized by a disorganized fiber cell compartment. In contrast, mice lacking alphaB-crystallin alone exhibit no overt cataract. Young mice lacking alphaB-crystallin exhibit no gross abnormalities when maintained under animal facility conditions, but with age begin to develop kyphosis, a curvature of the spine. Histological analysis reveals signs of skeletal muscle cell damage and regeneration in select muscle groups. Between 40 and 55 weeks of age, the mice begin to lose body fat and weight and, if not euthanized, expire shortly thereafter. Analysis of these older mice reveals severe muscle cell degeneration in areas of the tongue, in muscle cells associated with the axial skeleton, and, to a lesser extent, in select muscles of the limbs. Osteoarthritis is also evident in the vertebral column, possibly secondary to the muscle degeneration. We have found that targeted disruption of the alphaB-crystallin gene in our mice also resulted in disruption of a newly discovered, adjacent gene (HSPB2) which encodes a small heat shock protein shown to regulate Myotonic Dystrophy Protein Kinase. Although the observed muscle degenerative phenotype might be due to disruption of either (or both) of these genes, a very recent report supports the involvement of alphaB-crystallin. In this report, a point mutation in the alphaB-crystallin gene in a French family caused muscle weakness with a time course and distribution similar to that observed in our alphaB-crystallin gene knockout mice. We are currently working to elucidate the mechanisms underlying the muscular degeneration and the relative importance of alphaB-crystallin and HSPB2 (MKBP) in producing this phenotype.
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