Indiana University School of Optometry
Faculty Research: Joseph A. Bonanno, OD, PhD
The Corneal Endothelial Pump
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Our goal is to understand the nature of the corneal “endothelial pump” and how it is regulated. The corneal endothelium is responsible for maintaining the hydration of the cornea. This function is essential for good vision since a defective endothelium will lead to corneal edema and a reduction in corneal transparency. Devising rational medical treatments for endothelial dysfunction is needed to treat damaged or diseased corneas.

  1. We are identifying and characterizing ion transport mechanisms that are responsible for fluid secretion. We use combined physiological, molecular biology and biophysical approaches. See our latest publications for examples. Currently, our lab is focusing on the role of bicarbonate transport and carbonic anhydrase activities on lactic acid transport. The corneal epithelium and stromal cells are very glycolytic and produce lots of lactic acid. For every glucose molecule taken up by the cornea from the anterior chamber about 1.7 lactate molecules are produced. This creates an osmotic imbalance if the lactate is not efficiently removed. We have recently identified MCT 1, 2, and 4 (monocarboxylate transporters) in corneal endothelium. Lactate dependent proton fluxes are enhanced by the presence of bicarbonate and by carbonic anhydrase activity. Our hypothesis is that this facilitated lactate efflux is a significant contributor to the “endothelial pump”.
  2. Corneal endothelial cells are lost during aging and this loss is accelerated in diseases like Fuchs Dystrophy and during eye banking. Preventing this loss could delay the need for corneal transplant and extend the time that donor corneas could be used for transplantation.  Therefore, a second area of interest is protection of endothelial cells. We found that endothelial cells possess a HCO3--stimulated adenylyl cyclase called soluble AC (sAC). This enzyme appears to be responsible for basal levels of [cAMP] in the cells. The [cAMP] can also be increased by adenosine through A2b receptors. We now know that cAMP signaling is protective.
  3. Increases in cAMP can also enhance the “endothelial pump”. The mechanism is uncertain, however we do know that the increased cAMP will activate the chloride channel CFTR and also increases the total endothelial electrical resistance or barrier function due to promotion of enhanced cell to cell adhesion.
  4. Corneal endothelial cell loss during aging and in Fuchs’ Dystrophy is by apoptosis stimulated by oxidative stress. We have found that hypoxia preconditioning can protect cells from oxidative stress.  This may be useful during surgical procedures.
  5. CHED (Corneal hereditary endothelial dystrophy) and Fuchs’ Dystrophy are associated with mutations in the gene SLC4A11, which codes for a putative bicarbonate or borate transporter. The role of borate in cell biology is uncertain, but may be protective or promote cell proliferation. We are now studying these possible functions.

Current Model for Corneal Endothelial Fluid Pumping

diagram of the model

Figure 1.  Endothelial Transport Model.  At the basolateral membrane, HCO3- influx helps buffer H+ (facilitated by CAII) from lactate:H+ influx. At the apical membrane, HCO3- in the bath (aqueous humor) buffers H+ (facilitated by CAIV) from lactate:H+ efflux. Net HCO3-and lactate flux contribute to fluid transport by local osmotic coupling and possibly cotransporter coupling. HCO3- and Cl- flux is stimulated by cAMP/Protein Kinase A (e.g., activation of soluble Adenylyl cyclase; A2b receptors) by increasing CFTR permeability and NBC1 activity. CaCC, Calcium activated chloride channels; CFTR, cystic fibrosis transmembrane regulator; CAIV, Carbonic anhydrase IV attached to a Glycosophosphatidylinositol membrane stalk; MCT, monocarboxylate transporter; NHE1, Na+/H+ exchanger; AE2, anion exchanger 2; A2, adenosine receptor; AC, trans membrane bound adenylate cyclase; NBC1, 1Na+/2HCO3- cotransporter; NKCC1, Na+:K+:2Cl- cotransporter 1; CAII, carbonic anhydrase II; sAC, soluble adenylate cyclase; PKA, Protein kinase A.

Ongoing Projects

Recent Publications on Endothelial Function

  1. Yan Zhang, Jinhua Li, Qiang Xie and Joseph A. Bonanno. Molecular expression and functional involvement of the bovine calcium-activated chloride channel 1 (bCLCA1) in apical HCO3- permeability of bovine corneal endothelium Exp Eye Res. 2006 Nov; 83(5):1215-24.
  2. Cheng Q, Nguyen T, Song H, Bonanno J. Hypoxia protects human corneal endothelium from tertiary butyl hydroperoxide and paraquat-induced cell death in vitro. Exp Biol Med (Maywood). 2007 Mar; 232(3):445-53.
  3. Role of Carbonic Anhydrase IV in Corneal Endothelial HCO3- Transport. Xing Cai Sun, Jinhua Li, Miao Cui, and Joseph A. Bonanno. Invest. Ophthalmol. Vis Sci. 2008, 49: 1048-1055.
  4. Jinhua Li, KT Allen, X Sun, and JA Bonanno. Dependence of cAMP meditated increases in Cl(-) and HCO(3)(-) permeability on CFTR in bovine corneal endothelial cells. Exp Eye Res. 2008 Apr; 86(4): 684-90.
  5. Xing Cai Sun, Chang-Bin Zhai, Miao Cui, Yanqiu Chen, Lonny R. Levin, Jochen Buck, Joseph A. Bonanno. HCO3--Dependent Soluble Adenylyl Cyclase Activates the Cystic Fibrosis Transmembrane Conductance Regulator in Corneal Endothelium. Am J Physiol (Cell Physiol), 284: C1114-C1122, 2003.
  6. Jinhua Li, Xing Cai Sun, and Joseph A Bonanno. Role of NBC1 in Apical and Basolateral HCO3- Permeabilities and Transendothelial HCO3- Fluxes in Bovine Corneal Endothelium. Am J Physiol Cell Physiol. 2005 Mar; 288(3):C739-46.
  7. Kah Y. Tan-Allen, Xing Cai Sun and Joseph A. Bonanno. Characterization of Adenosine Receptors in Bovine Corneal Endothelium. Exp Eye Res. 2005 May;80(5):687-96.

Lab Personnel

Tracy Nguyen
Tracy Nguyen		 Cailing Liu
Cailing Liu		 Supriya Jalimarada
Supriya
Jalimarada

 Ken Iwadate
Ken Iwadate

Go to: Dr. Bonanno's faculty page
IU Optometry home page

URL: http://www.opt.indiana.edu/people/faculty/bonanno/iontrans.htm
Revised: July 24, 2009
IU Optometry home page: http://www.opt.indiana.edu/
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