Calcium signaling in lacrimal glands

Abstract

Lacrimal glands provide the important function of lubricating and protecting the ocular surface. Failure of proper lacrimal gland function results in a number of debilitating dry eye diseases. Lacrimal glands secrete lipids, mucins, proteins, salts and water and these secretions are at least partially regulated by neurotransmitter-mediated cell signaling. The predominant signaling mechanism for lacrimal secretion involves activation of phospholipase C, generation of the Ca(2+)-mobilizing messenger, IP3, and release of Ca(2+) stored in the endoplasmic reticulum. The loss of Ca(2+) from the endoplasmic reticulum then triggers a process known as store-operated Ca(2+) entry, involving a Ca(2+) sensor in the endoplasmic reticulum, STIM1, which activates plasma membrane store-operated channels comprised of Orai subunits. Recent studies with deletions of the channel subunit, Orai1, confirm the important role of SOCE in both fluid and protein secretion in lacrimal glands, both in vivo and in vitro.

Keywords: Calcium entry; Calcium oscillations; Calcium release; Calcium signaling; Fluid secretion; IP3; Lacrimal; Orai; Protein secretion; STIM.

Figure 1. Two phases of Ca²⁺ signaling in in vitro lacrimal gland preparations

Top: Changes in intracellular Ca²⁺ in slices of rat lacrimal gland are inferred from the efflux rate of ⁸⁶Rb⁺. Redrawn from data originally presented in [11]. Bottom: Changes in intracellular Ca²⁺ in mouse lacrimal acinar cells are measured with the Ca²⁺ indicator, Fura-2. In both cases, in the absence of extracellular Ca²⁺, the response is transient, and subsequently restored by addition of Ca²⁺.

Figure 2. Muscarinic receptor-induced sinusoidal Ca²⁺ oscillations in mouse lacrimal cells

Top: A single, fura-2-loaded mouse lacrimal acinar cell was exposed to 0.5 μM methacholine (MeCh) inducing sinusoidal Ca²⁺ oscillations on an elevated basal level of Ca²⁺. MeCh-induced oscillations occur over a narrow concentration range, and with frequencies that are relatively constant at different agonist concentrations (Bottom figure). These findings were originally published in [46].

Figure 3. Evidence for the independence of Ca²⁺ influx from receptor activation

In both experiments, Ca²⁺ stores were discharged by addition of epinephrine in the absence of extracellular Ca²⁺, the α-adrenergic receptors were then blocked by phentolamine (Phentol.), and the status of Ca²⁺ stores assessed by addition of carbachol (Carb.). In the experiment with open circles, Ca²⁺ was added before phentolamine, so Ca²⁺ could flow into the cell through presumed receptor activated channels. In the experiment with closed circles, phentolamine was added before Ca²⁺ such that receptor-activated channels would presumably be closed. Nonetheless, the stores were refilled with similar efficiency in both cases, indicating that it is not receptor activation per se that is responsible for Ca²⁺ entry and refilling of intracellular stores. Redrawn from data originally presented in [25].

Figure 4. Appearance of eyes from wild type (WT), heterozygous (Het) and homozygous Orai1 knockout (KO) mice

The eyes from wild type and heterozygous mice appeared normal, while in many (but not all) cases the eyes from knockout animals showed signs of inflammation.