Distinctive Molecular and Cellular Events Are Perturbed by Active Forms of MEK1 and MKK6 MAP Kinases in the Lens

Abstract

Abstract: : Purpose: To study the role of the p38 MAP kinase pathway in the lens during development and to determine the differences between the downstream events regulated by p38 and ERK pathways and their contributions to lens cataract. Methods: The localization of the p38 members in the lens were examined by using Western blot and immunohistochemistry. A gain–of–function approach was used to perturb the p38 pathway by expressing a constitutively active form of mitogen–activated protein kinase kinase 6 (MKK6), a direct upstream activator of all p38 isoforms, under αA–crystallin promoter. Morphological and biochemical changes in the lenses of MKK6b(E) and MEK1(E) transgenic mice were examined by Western blot, immunohistochemistry, and light and electron microscopy. Results: Members of p38 pathway were found in lens epithelial cells and differentiating fibers. MKK6b(E) transgenic mice developed relatively normal lens by postnatal day 7 (P7), posterior cataract by P14, and posterior rupture around 3 weeks. No obvious alterations were observed in MKK6b(E) lens fibers at the equator. However, immunohistochemical staining showed significant increase of F–actin level and abrupt decrease of α3 and α8 connexin level in the inner maturing and newly matured fibers. In contrast, substantial spherical spaces were first observed between newly differentiated fibers around P7 in MEK1(E) transgenic lens. Conclusions: Perturbation of either p38 or ERK pathway is sufficient to cause different types of cataract in the lens. Alteration of p38 pathway specifically affects the properties of F–actin and gap junction channels in maturing and newly matured lens fibers. It suggests that the downstream molecules of the p38 pathway may be involved in regulating the maturation process of the fibers. However, alteration of ERK pathway seems to selectively disrupt the osmotic balance between newly differentiated fibers and extracellular environment. It suggests that ERK pathway in differentiating fibers may be involved in the regulation of extracellular fluids. Therefore, MKK6b(E) and MEK1(E) transgenic mice are two distinct cataract models for studying some important aspects of molecular and cellular mechanisms in the lens.

Keywords: signal transduction • cataract • transgenics/knock-outs