Cataract

LENS TRANSPORT

The World Health Organization has identified lens cataract as the leading cause of blindness, producing vision loss in one of six Americans over age 40, and half of all Americans older than 80. Cataract extraction is the most common Medicare surgery, adding an enormous financial burden to our healthcare system. The lens is an avascular organ which is uniquely dependent on the activity of numerous membrane channels and transporters to maintain its transparency and prevent cataract. The lens contains a single layer of epithelial cells spanning the anterior half of the lens surface, differentiating fiber cells that constitute the lens cortex, and mature fiber cells that make up the lens core. All of these cell types are coupled to neighboring cells by gap junctions. The lens depends on ion transport to create an internal circulating current, with Na+ being the primary current carrier. Na+, coupled with water, enters the lens at the anterior and posterior poles and flows inward along the extracellular spaces. Within the lens, Na+ is driven by its electrochemical gradient to move into the fiber cells, where the direction of flow is reversed and the current flows back to lens surface through gap junction channels. Gap junction coupling is concentrated at the equator in peripheral fiber cells, directing the Na+ current to the equatorial epithelium, where Na+/K+-ATPase activity pumps Na+ out of the lens to complete the circulatory loop. Fluid circulation follows the Na+ current to create a micro-circulatory system that carries nutrients into the fiber cells and allows removal of metabolic waste. The ensemble activity of the various membrane channels and transporters that drive this micro-circulatory system overcomes the lack of a lens vasculature and supports clarity, consistent with the finding that mutations in many of these channel genes have been linked to congenital cataract. Signal transduction pathways regulate lens ion transport by acting on the pumps and channels to help establish and maintain the lens circulation. Mutations in signal transduction genes, like the phosphatase and tensin homolog (PTEN) gene have also been linked to syndromic forms of hereditary cataract.

Na+ enters the lens at the poles and exits at the equator (red lines). A similar movement of water follows Na+ (blue lines). Na+ moves inward through extracellular spaces and enters the fiber cytoplasm through a leak conductance. Inward water flow enters fibers through AQP0 channels.  Na+ and water travel outward through gap junction channels to reach the epithelium, where the Na+/K+-ATPase pumps Na+ out and AQP1 transports the water out.

MAPK SIGNALING INCREASES CX50 JUNCTIONAL CONDUCTANCE

Junctional conductance measurements recorded from Xenopus oocyte pairs injected with Cx50 and constitutively active MEK1(E) alone or in combination. Cell pairs expressing water or MEK1(E) did not form functional intercellular channels. Co-expression of MEK1(E) with Cx50 significantly increased conductance. Immunoblot analysis of oocytes showed equivalent protein levels of wild-type Cx50 and total endogenous ERK. Expression of MEK1(E) stimulated phosphophorylation of ERK.

LENS STRUCTURE

DELETION OF PTEN CAUSES CATARACT

PI3K & CX50 REGULATE CELL PROLIFERATION

Control mice (left) have clear lenses, while cataracts are present in the eyes of adult lens specific PTEN knockout mice (right).

Proliferating epithelial cells in a neonatal lens are labeled with EdU (green). Deletion of either Cx50 or the catalytic subunit of PI3K (p110alpha) lead to reductions in cell proliferation in the late prenatal or early postnatal period, and smaller lenses throughout life. Deletion of Cx46 or a different catalytic subunit of PI3K (p110beta) had no effect on lens size.

A section of the lens where cell membranes are stained with wheat germ agglutinin (red). The epithelial monolayer (top) lies beneath the capsule. Lens fibers are shown in cross section and are highly coupled to each other and the epithelium by gap junctions. 

Wild-type lenses remain transparent (top). In contrast, lenses dissected from 24 week old PTEN KO mice (bottom) have ruptured, leaving behind cortical fragments and a dense nuclear cataract.

These animal models will be used to define the contribution of PI3K/Akt and PTEN signaling to the normal physiology and pathophysiology of the lens. In combination with mice lacking lens connexins, the interaction between signal transduction pathways and the channels and pumps that drive lens transport can be explored to gain insights into how transport properties can be maintained in the lens, with the ultimate goal of delaying the onset of cataract.

Related Publications:

C. Sellitto, L. Li and T.W. White (2022). Double deletion of PI3K and PTEN modifies lens postnatal growth and homeostasis. Cells. 11: 2708

A.A. Giannone, L. Li, C. Sellitto and T.W. White (2021). Physiological mechanisms regulating lens transport. Front. Physiol.  12:818649  

N.A. Delamere, M. Shahidullah, R.T. Mathias, J. Gao, X. Sun, C. Sellitto and T.W. White (2020). Signaling between TRPV1/TRPV4 and intracellular hydrostatic pressure in the mouse lens. Invest. Ophthalmol. Vis. Sci. 61:58

E.R. Muir, X. Pan, P.J. Donaldson, E. Vaghefi, Z. Jiang, C. Sellitto and T.W. White (2020). Multi-parametric MRI of the physiology and optics of the in-vivo mouse lens. Magn. Reson. Imaging 70:145-154

V. Valiunas and T.W. White (2020). Connexin43 and connexin50 channels exhibit different permeability to the second messenger inositol triphosphate. Sci. Rep. 10:8744 (full text)

Y. Chen, J. Gao, L. Li, C. Sellitto, R.T. Mathias, P.J. Donaldson and T.W. White (2019). The ciliary muscle and zonules of Zinn modulate lens intracellular hydrostatic pressure through Transient Receptor Potential Vanilloid channels.  Invest. Ophthalmol. Vis. Sci. 60:4416-4424

V. Valiunas, P.R. Brink and T.W. White (2019). Lens connexin channels have differential permeability to the second messenger cAMP.  Invest. Ophthalmol. Vis. Sci. 60:3821-3829

C. Sellitto, L. Li, E. Vaghefi, P.J. Donaldson, R.Z. Lin and T.W. White (2016). The phosphoinositide 3-kinase p110α catalytic subunit is required for normal lens growth. Invest. Ophthalmol. Vis. Sci. 57:3145-3151

J. Gao, X. Sun, T.W. White, N.A. Delamere and R.T. Mathias (2015). Feedback regulation of intracellular hydrostatic pressure in surface cells of the lens. Biophys. J. 109:1830-1839

J.M. Martinez, H.-Z. Wang, R.Z. Lin, P.R. Brink, and T.W. White (2015). Differential regulation of connexin50 and connexin46 by PI3K signaling. FEBS Letters 589:1340-1345

C. Sellitto, L. Li, J. Gao, M.L. Robinson, R.Z. Lin, R.T. Mathias and T.W. White (2013). AKT activation promotes PTEN hamartoma tumor syndrome-associated cataract development. J. Clin. Invest. 123:5401–5409

J. Gao, H. Wang, X. Sun, K. Varadaraj, L. Li, T.W White and R.T Mathias (2013).The effects of age on lens transport. Invest. Ophthalmol. Vis. Sci. 54:7174-7187

C.H. Xia, B. Chang, A.M. DeRosa, C. Cheng, T.W. White and X. Gong (2012). Cataracts and microphthalmia caused by a Gja8 mutation in extracellular loop 2. PLoS One. 7: e52894

J. Gao, X. Sun, L.C. Moore, T.W. White, P.R. Brink and R.T. Mathias (2011). Lens intracellular hydrostatic pressure is generated by the circulation of sodium and modulated by gap junction coupling. J. Gen. Physiol. 137:507-520

L. Ebihara, J.J. Tong, B. Vertel, T.W. White, and T.L. Chen (2011). Properties of connexin 46 hemichannels in dissociated lens fiber cells. Invest. Ophthalmol. Vis. Sci. 52:882-889

L. Li, C. Cheng, C.H. Xia, T.W. White, D.A. Fletcher and X. Gong (2010). Connexin mediated cataract prevention in mice. PLoS One 5:e12624

T. Shakespeare, C. Sellitto, L. Li, C. Rubinos, X. Gong, M. Srinivas, and T.W. White (2009). Interaction between connexin50 and mitogen-activated protein kinase signaling in lens homeostasis. Mol. Biol. Cell 20:2582-2592

A.M. DeRosa, G. Meşe, L. Li, C. Sellitto, P.R. Brink, X. Gong and T.W. White (2009). The cataract causing Cx50-S50P mutant inhibits Cx43 and intercellular communication in the lens epithelium. Exp. Cell Res. 315:1063-1075

T.W. White, Y. Gao, L. Li, C. Sellitto, and M. Srinivas (2007). Optimal lens epithelial cell proliferation is dependent on the connexin isoform providing gap junctional coupling. Invest. Ophthalmol. Vis. Sci. 48:5630-5637

A.M. DeRosa, C.H. Xia, X. Gong and T.W. White (2007). The cataract inducing Cx50-S50P mutation dominantly alters wild-type lens connexin channel gating. J. Cell Sci. 120:4107-4116

A.M. DeRosa, R. Mui, M. Srinivas and T.W. White (2006). Functional characterization of a naturally occurring Cx50 truncation. Invest. Ophthalmol. Vis. Sci. 47:4474-4481

C.H. Xia, D. Cheung, A.M. DeRosa, B. Chang, W.-K. Lo, T.W. White and X. Gong (2006). Knockin a3 (Cx46) connexin prevents severe cataracts caused by an a8 (Cx50)-G22R mutation J. Cell Sci. 119:2136-2144

C. Sellitto, L. Li, and T.W. White (2004). Connexin50 is required for normal postnatal lens cell proliferation. Invest. Ophthalmol. Vis. Sci. 45:3196-3202

J. Gao, X. Sun, F.J. Martinez-Wittinghan, X. Gong, T.W. White and R.T. Mathias (2004). Connections between connexins, calcium and cataracts in the lens. J. Gen. Physiol. 124:289-300

F.J. Martinez-Wittinghan, C. Sellitto, L. Li, X. Gong, P.R. Brink, R.T. Mathias and T.W. White (2003). Dominant cataracts result from incongruous mixing of wild-type lens connexins. J. Cell Biol. 161: 969-978

D.A. Gerido, C. Sellitto, L. Li and T.W. White (2003). Genetic background influences cataractogenesis, but not lens growth deficiency, in Cx50 knockout mice. Invest. Ophthalmol. Vis. Sci. 44:2669-2674

T.W. White (2002). Unique and redundant connexin contributions to lens development. Science 295:319-320