Aging-dependent loss of GAP junction proteins Cx46 and Cx50 in the fiber cells of human and mouse lenses accounts for the diminished coupling conductance

Abstract

The homeostasis of the ocular lens is maintained by a microcirculation system propagated through gap junction channels. It is well established that the intercellular communications of the lens become deteriorative during aging. However, the molecular basis for this change in human lenses has not been well defined. Here, we present evidence to show that over 90% of Cx46 and Cx50 are lost in the fiber cells of normal human lenses aged 50 and above. From transparent to cataractous lenses, while Cx43 was upregulated, both Cx46 and Cx50 were significantly down-regulated in the lens epithelia. During aging of mouse lenses, Cx43 remained unchanged, but both Cx46 and Cx50 were significantly downregulated. Under oxidative stress treatment, mouse lenses develop in vitro cataractogenesis. Associated with this process, Cx43 was significantly upregulated, in contrast, Cx46 and Cx50 were sharply downregulated. Together, our results for the first time reveal that downregulation in Cx46 and Cx50 levels appears to be the major reason for the diminished coupling conductance, and the aging-dependent loss of Cx46 and Cx50 promotes senile cataractogenesis.

Connexin Gap Junctions and Hemichannels in Modulating Lens Redox Homeostasis and Oxidative Stress in Cataractogenesis

“…In comparison, Cx50 is initially synthesized in the epithelium and remains at high levels in differentiating fiber and mature fiber cells [ 13 , 14 , 15 ]. Interestingly, a recent study by Gong et al [ 16 ] reported that human and mouse lenses have different distribution patterns of connexin proteins. For example, in contrast to the mouse lens, where Cx46 is primarily expressed in the fiber cells [ 17 , 18 ], both Cx46 and Cx50 in the human lens are expressed in the epithelial cells of transparent and cataractous lenses.…”

“…For example, in contrast to the mouse lens, where Cx46 is primarily expressed in the fiber cells [ 17 , 18 ], both Cx46 and Cx50 in the human lens are expressed in the epithelial cells of transparent and cataractous lenses. Gong et al [ 16 ] further reported that the ratio between Cx43 and Cx46/Cx50 in humans is 9:1, which is higher than in mice (5:1), indicating a different expression pattern of connexin isoforms. Although transcripts for a fourth connexin, Cx23, have been detected in the zebrafish embryo and mouse lens, there is no report for its expression in the human lens [ 19 , 20 ].…”

“…Connexin-formed gap junctions and hemichannels in the lens mediate the small molecule fluxes according to their chemical concentration gradients [ 5 , 83 ]. Gong et al [ 16 ] recently found that Cx43 is significantly upregulated by almost 50% in cataractous lenses, while both Cx46 and Cx50 are downregulated. Moreover, Cx50 is downregulated more than 90%, much greater than Cx46, in the fiber cells of ≥50-year-old human lenses.…”

“…The previous study has shown that H 2 O 2 leads to a dose- and time-dependent decrease of Cx46 in differentiated chick lens cultures [ 3 ]. A recent study demonstrated that glucose oxidase-induced oxidative stress causes significant upregulation of Cx43, and downregulation of Cx46 and Cx50 [ 16 ].…”

Physiological Mechanisms Regulating Lens Transport

“…Age and oxidative stress in the lens are usually coincident and lead to changes in the lens proteome, including connexin channel density and regulation ( Gao et al, 2013 ; Gong et al, 2021 ; Quan et al, 2021 ). The lens is protected from oxidative damage by antioxidants, such as GSH, and chaperone proteins to prevent protein aggregation and dissolution from oxidative stress ( Giblin, 2000 ; Lim et al, 2020 ).…”