Bioelectric Responses of Conjunctival Goblet Cells to Dry Eye: Impact of Ion Channels on Exocytotic Function and Viability

Abstract

How ion channels impact the response of the ocular surface to dry eye is only beginning to be explored. Here, we review recent progress and provide new experimental data clarifying the exocytosis-altering actions of ion channels in conjunctival goblet cells whose release of tear-stabilizing mucin is a key adaptive response to the pre-ocular hyperosmolarity that characterizes dry eye. Patch-clamp recordings of goblet cells located in freshly excised rat conjunctiva reveal that these mucin-releasing cells respond to sustained hyperosmolarity by sequentially activating their ATP-sensitive potassium (KATP), nonspecific cation (NSC), voltage-gated calcium (VGCC), and P2X7 channels; each of which modulates exocytosis. Based on these and other new findings, we now identify four stages in the bioelectric response of conjunctival goblet cells to extracellular hyperosmolarity. To better characterize these stages, we report that high-resolution membrane capacitance (Cm) measurements of the exocytotic activity of single goblet cells demonstrate that the replenishment of mucin-filled granules after neural-evoked exocytosis is a multi-hour process, which VGCCs markedly accelerate. Yet, we also discovered that VGCC activation is high-risk since hyperosmotic-induced goblet cell death is boosted. With dry eye treatments being far from optimal, elucidating the physiologic and pathobiologic impact of the KATP/NSC/VGCC/P2X7 pathway provides a new opportunity to identify novel therapeutic strategies.

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“…As shown in Figures 2A–B , almost no stained spots were found in the cornea of control mice, whereas large number of stained spots were observed in the corneal surface of vehicle-treated mice, indicating corneal damage following CUMS. Tear production and goblet cells (responsible for the release of tear-stabilizing mucin) were assessed by cotton phenol red threads and periodic acid-Schiff (PAS) staining, respectively ( Puro, 2020 ). As shown in Figures 2C–E , CUMS resulted in reduced wetted length and goblet cells in mice.…”