Hyaluronan in the limbal stem cell niche: from regulation of stem cell fate to translational applications

University Of Houston, Houston TX

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Abstract

SUMMARY Limbal epithelial stem cells (LESCs) produce transient amplifying cells (TACs) that move centripetally into the cornea and differentiate into corneal epithelial cells (CECs). Thus, TACs represent a transition state between LESCs and CECs. Studies have suggested some TACs retain stem cell-like characteristics. We have recently shown that the LSCN is composed of a hyaluronan (HA)-rich matrix that is essential for maintaining LESCs, and a loss of HA triggers LESC differentiation into CECs. Our unpublished data shows that clusters of cells expressing an HA-rich matrix (HA-clusters) break away from the limbus and move into the cornea containing LESC progenies with progenitor-like properties. Aim 1 will establish whether TACs retaining progenitor-like characteristics are located within HA-clusters, and verify whether these TACs can dedifferentiate into bona fide LESCs. The mechanism by which the HA-rich LSCN maintains LESCs remains unknown. Our group recently characterized the composition of the HA-rich matrix within the LSCN, identifying that heavy chains 2 and 5 (HC2 and 5) are transferred from inter-alpha-inhibitor onto HA by TSG- 6 to form specific HA/HC and HA/HC-TSG-6 complexes, and LESCs express CD44 and RHAMM, major cell surface receptors for HA. Aim 2 will investigate whether HA/HC5(-TSG-6) complexes within the LSCN are essential for maintaining LESCs, and whether the effects of HA on LESCs are mediated through CD44 and/or RHAMM cell surface receptors. Corneal injuries that decrease the overall number of LESCs may lead to limbal stem cell deficiency (LSCD), a serious medical condition that leads to impaired vision and severe pain, and, in more severe cases, complete loss of vision. Currently, treatment options for LSCD include LESC transplantation or corneal transplantation, for which human donor corneas are required. A major challenge for treating LSCD is the efficient preservation of LESCs in human donor corneas while stored at eye banks. The composition of storage medium used by eye banks worldwide is determined by the survival of corneal endothelial cells, without taking into consideration LESCs. Studies have reported that limited LESCs are detected in human donor corneas after 4 days in storage, and we have found that there is a significant loss of HA within the LSCN that occurs concomitantly with the loss of LESCs. Aim 3 will investigate how the composition of the HA-matrix within the LSCN of human donor corneas changes over time in storage and correlate the loss of HA with a loss of LESCs, and to improve the composition of storage media to better support LESCs. Clinical impact: Corneal diseases, including LSCD, are the third leading cause of blindness worldwide. Corneal blindness is ultimately treated with corneal or LESC transplantation, for which human donor corneas with viable LESCs are essential. This proposal will establish how the LSCN maintains LESCs, unveiling fundamental biological mechanisms underlying LESC survival and LSCD pathogenesis.