Volumetric and ionic regulation during the in vitro development of a corneal endothelial barrier

Abstract

Corneal endothelium is responsible for generating an ion flux between the corneal stroma and the anterior chamber of the eye that is necessary for the cornea to remain transparent. However, the ion transport regulatory mechanisms that develop during the formation of the endothelial barrier are not known. In this study, we determined the influence of cell confluence on cell volume and intracellular ionic content on the corneal endothelial cells of rabbits. Our results demonstrate that non-confluent endothelial cells display a hypertrophic volume increase, with higher intracellular contents of potassium and chlorine than those of confluent cells. In contrast, when cells reach confluence and the endothelial barrier forms, cell volume decreases and the intracellular contents of potassium and chlorine decrease. Our genetic analysis showed a higher expression of CFTR and CA2 genes in non-confluent cells, and of the gene KCNC3 in confluent cells. These results suggest that the normal ionic current that keeps the corneal stroma dehydrated and transparent is regulated by cell–cell contacts and endothelial cell confluence, and could explain why the loss of corneal endothelial cells is often associated with corneal edema and even blindness.

Introduction

Corneal endothelium is a single-cell-thick layer of tissue that forms a boundary between the corneal stroma and the anterior chamber of the eye, and is responsible for maintaining corneal transparency (Joyce, 2003, Maurice, 1972, Mergler and Pleyer, 2007). The corneal endothelium fulfills the important physiological function of regulating sodium and potassium concentrations in the anterior pole of the eye. This regulation mechanism is able to control the level of hydration of the stroma and, thus, allows the cornea to remain transparent and transmit the incoming light to the retina (Bonanno, 2003, Joyce, 2003). According to the “pump-leak hypothesis” (Bryant and McDonnell, 1998), the corneal stroma has a tendency to swell due to the presence of non-diffusible, negatively charged molecules such as glycosaminoglycans (Fischbarg and Maurice, 2004, Hedbys and Mishima, 1962, Hodson, 1997). However, the stromal hydration level is maintained by an active process of ion transport across the corneal endothelium from stroma to aqueous humor (Davies et al., 2004, Hodson and Miller, 1976, Huff and Green, 1981).

Our understanding of the function of the corneal endothelium has expanded greatly over the past years. However, the intrinsic events of ion transport that occur across the endothelial cells, and the role that several ions could play in this transport process, have not been fully explored (Green, 1991, Mergler and Pleyer, 2007). Investigation of the electrophysiology and ion channel expression of the corneal endothelium by highly sensitive measuring methods is a new field that may open new perspectives. It not only will help to better understand physiological functions of the cornea, but also may have direct clinical implications (Mergler and Pleyer, 2007).

In this context, one of the most sensitive methods that have been used to quantify the intracellular concentration of several ions is electron-probe X-ray microanalysis. By using these techniques, several authors have previously analyzed the ionic transport that takes place in renal tubular cells and other types of epithelia (Rick et al., 1983, Rick et al., 1986, Thurau et al., 1979a, Thurau et al., 1979b), whereas our research group have been able to determine the normal ionic concentrations in rabbit corneal endothelial cells (Alaminos et al., 2007).

Different types of ion channels have been described so far in corneal endothelium. Most of these channels belong to the family of voltage-gated ion channels or to the family of transient receptor potential channels. Voltage-gated ion channels are transmembrane protein pores that are permeable to ions and particularly sensitive to the change of transmembrane potential difference, and include in the cornea several sodium, potassium, chloride and calcium channels. Then, transient receptor potential channels include some store-operated calcium channels as well as a diverse group of cation channels that act as cellular sensors of diverse functions (Mergler and Pleyer, 2007).

Several studies have traditionally demonstrated that the corneal endothelium counteracts the tendency of the corneal stroma to swell by removing excess stromal fluid via the activity of Na⁺/K⁺-ATPase (Crawford et al., 1995, Lim and Fischbarg, 1981, Maurice, 1972, Tervo and Palkama, 1975) and bicarbonate-dependent Mg²⁺-ATPase (Barfort and Maurice, 1974, Hodson and Miller, 1976) ionic pumps, which are located mainly on the basolateral plasma membranes (Joyce, 2003, Tervo and Palkama, 1975). However, several reports suggest that the maintenance of stromal hydration by the corneal endothelium mainly relies on active transendothelial bicarbonate and chloride transport, with bicarbonate as the principal ionic mechanism of corneal deturgescence (Bonanno, 2003, Davies et al., 2004, Fischbarg and Lim, 1974, Hodson, 1974, Hodson and Miller, 1976). The major chloride channels which are expressed in the rabbit corneal endothelium are the chloride channels ClC-2, ClC-3, ClC-5, ClC-6 and ClC-7, the cystic fibrosis transmembrane conductance regulator (CFTR) (Sun and Bonanno, 2002, Zaidi et al., 2004) and the calcium activated chloride channel-1 (CLCA1) (Itoh et al., 2000). Finally, there is evidence which suggests that transient receptor potential (TRP) channels, as well as Ca²⁺ ionic channels could be expressed in several types of cells (Becker et al., 2005, Mergler and Pleyer, 2007). In addition, further studies elucidate the importance of exploring the function of definite ion channels, in particular, investigations of potassium and calcium channels in human corneal endothelial cells (Green et al., 1994, Mergler et al., 2003, Mergler and Pleyer, 2007, Rae and Watsky, 1996, Rae et al., 1989, Rae et al., 1990, Watsky et al., 1992). Exact regulation of all those ionic channels is crucial for a proper function of the corneal endothelium.

On the other hand it is well known that, in contrast to rabbit corneal endothelium (Hirsch et al., 1975, Mimura and Joyce, 2006, Staatz and Van Horn, 1980, Von Sallmann et al., 1961), the proliferative capacity of the human corneal endothelium is very limited. Evidence strongly suggests that cell division, if it occurs, plays only a minor role as a repair mechanism in mature corneal endothelium in vivo (Joyce, 2003). Therefore, when corneal endothelial cells are lost due to trauma or age, the remaining cells tend to migrate and hypertrophy, thereby restoring the endothelial confluent barrier but reducing the number of cells that comprise the corneal endothelium (Crawford et al., 1995, Fukami et al., 1988, Jongebloed et al., 1987, Matsubara and Tanishima, 1983, Stiemke et al., 1991). This form of repair has been designated as monolayer spreading (Joyce et al., 1990) or endothelial compensation, which is necessary for maintaining a confluent endothelial cell barrier in the cornea which in turn, is responsible for the transparency of the cornea. A number of factors can contribute to endothelial cell loss (Mergler and Pleyer, 2007), including aging, trauma, ocular surgery, Fuchs’ endothelial dystrophy, and Type 1 diabetes. This accelerated cell loss may eventually result in compromised corneal function and decompensation which will require surgical intervention. Currently, the standard treatment for endothelial decompensation is full-thickness corneal transplantation (keratoplasty), although the experimental transplantation of corneal endothelial cells is currently being researched. Consequently, a better characterization and improved knowledge of endothelial ion channels may have an impact on clinical management (Joyce, 2003).

In this context, the influence of cell confluence as a functional regulator mechanism of endothelial ionic pumps and channels has yet to be fully elucidated. By one or another mechanism, the formation of stable cell–cell contacts on confluent corneal endothelial cells has demonstrated its capability to regulate endothelial replication and maintain the corneal endothelium in a non-replicative state (Joyce, 2003). Interestingly, different authors have demonstrated that corneal endothelial cells develop mature cell–cell junctions only when the cells have spread and made direct contacts with each other, whereas the mean number of Na⁺/K⁺-ATPase pumps per cell in confluent endothelial cultures seems to vary in direct proportion to the density of cells in the culture (Crawford et al., 1995). All of these data reveal that the formation of a confluent endothelial cell barrier is able to activate different genetic pathways which, in turn, are responsible for the endothelium exerting its physiological functions in the cornea. However, the relationship between cell confluence and the activation of the endothelial ionic pumps and channels has not been established to date.

In this work, we have carried out a microanalytical and genetic study to determine the role of cell confluence on the endothelial function. First, we have quantified the intracellular content of several elements in rabbit corneal endothelial cells at different levels of cell confluence. Subsequently, we have determined the gene expression and function of different ionic pumps that are activated when the cells form a confluent endothelial barrier in culture. Although all this work has been carried out using rabbit corneal endothelial cells, which have important differences with the human, principally its high capacity for regeneration compared with the limited ability of the human corneal endothelium (Von Sallmann et al., 1961; Hirsch et al., 1975, Staatz and Van Horn, 1980, Mimura and Joyce, 2006), our results could contribute to a better understanding of the physiology of the human corneal endothelium.