News and Publications

Cell-Cell Communication and Intracellular Signaling

H. Liu, R. Ojakian, S. Xu, T. Anderson, X. Gong

We focus mainly on pathways of (1) intercellular communication via gap junction channels and (2) intracellular signaling via distinctive MAP kinases in the lens of the eye. Using the ocular lens, one of the simplest in vivo systems; classical techniques from cell and developmental biology, electrophysiology, biochemistry, and molecular biology; and cutting-edge techniques such as gene chips and proteomics, we have learned some of the fundamental roles of gap junctions and distinctive MAP kinase pathways. Currently, we are studying novel mutant mice that have ocular and cardiovascular abnormalities.

Recently, we found that microphthalmia with small lenses and nuclear cataracts develop in mice that lack the gene for a8 connexin, a gap junction protein. In a study in which mice that lack the gene for a3 connexin were compared with mice that lack the gene for a8 connexin, we found that the protein levels of both connexins are independently regulated and that the presence of a8 connexin in the lens nucleus depends on the presence of a3 connexin, but not vice versa. More importantly, delayed denucleation occurred in the interior fibers of the lenses of mice that lacked the gene for the a8 connexin. Therefore, a8 connexin is required for maturation of fiber cells and growth control of the size of the lens.

However, mutations of the gene for a8 connexin in humans reportedly cause cataracts and have semidominant inheritance patterns. Deletion of the a8 connexin gene in mice causes ocular abnormalities with recessive inheritance. We found that an autosomal semidominant mutation associated with formation of cataracts was a missense mutation (guanine to cytosine) in the gene for a8 connexin; the glycine at codon 22 is replaced by arginine (G22R). Moreover, we showed that the a8 G22R isoform is a loss-of-function mutant for a8 and a dominant mutation for reducing the phosphorylated forms of a3 connexin in vivo.

To test the hypothesis that the alteration of endogenous a3 connexin in mice with the G22R mutation leads to severe abnormalities in the lens, we generated double-mutant mice that had the G22R mutation and lacked the gene for a3 connexin. The double-mutant mice had relatively normal cortical fibers in the lens. A functional impairment of endogenous a3 connexin, therefore, is partly responsible for the ocular abnormalities that occur in mice with the G22R mutation. These results provided novel molecular insights into cataractogenesis linked to mutations in a8 and a3 connexins in mice and humans.

PUBLICATIONS

Baldo, G.J., Gong, X., Martinez-Wittinghan, F.J., Kumar, N.M., Gilula, N.B., Mathias, R.T. Gap junctional coupling in lenses from a8 connexin knockout mice. J. Gen. Physiol. 118:447, 2001.

Chang, B., Wang, X., Hawes, N.L., Ojakian, R., Davisson, M.T., Lo, W.K., Gong, X. A Gja8 (Cx50) point mutation causes an alteration of a3 connexin (Cx46) and semi-dominant cataracts of Lop10 mice. Hum. Mol. Gen. 11:507, 2002.

Rong, P., Wang, X., Niesman, I., Wu, Y., Benedetti, L.E., Dunia, I., Levy, E., Gong, X. Disruption of Gja8 (a8 connexin) in mice leads to microphthalmia associated with retardation of lens growth and lens fiber maturation. Development 129:167, 2002.

Tian, J., Gong, X., Xie, Z. Signal-transducing function of Na⁺-K⁺-ATPase is essential for ouabain's effect on [Ca²⁺]_i in rat cardiac myocytes. Am. J. Physiol. Heart Circ. Physiol. 281:H1899, 2001.