Darlene A. Dartt

Lacrimal Gland, Tear Film, and Dry Eye Syndromes 2

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Neural Regulation of Lacrimal Gland Secretory Processes: Relevance in Dry Eye Diseases

The lacrimal gland is the major contributor to the aqueous layer of the tear film which consists of water, electrolytes and proteins. The amount and composition of this layer is critical for the health, maintenance, and protection of the cells of the cornea and conjunctiva (the ocular surface). Small changes in the concentration of tear electrolytes have been correlated with dry eye syndrome. While the mechanisms of secretion of water, electrolytes and proteins from the lacrimal gland differ, all three are under tight neural control. This allows for a rapid response to meet the needs of the cells of the ocular surface in response to environmental conditions. The neural response consists of the activation of the afferent sensory nerves in the cornea and conjunctiva to stimulate efferent parasympathetic and sympathetic nerves that innervate the lacrimal gland. Neurotransmitters are released from the stimulated parasympathetic and sympathetic nerves that cause secretion of water, electrolytes, and proteins from the lacrimal gland and onto the ocular surface. This review focuses on the neural regulation of lacrimal gland secretion under normal and dry eye conditions.

Signal transduction and control of lacrimal gland protein secretion: A review

Proteins in lacrimal gland fluid are secreted primarily by the acinar cells. Secretory proteins are synthesized in the endoplasmic reticulum, modified in the Golgi apparatus, stored in secretory granules, and released upon a change in the cellular level of second messenger. The second messenger level is controlled by a process termed signal transduction. Agonists, primarily neurotransmitters in the lacrimal gland, bind to receptors in the basolateral membrane of secretory cells. This interaction activates enzymes in the membrane that cause production of second messengers. It has been hypothesized that second messengers stimulate secretion by activating specific protein kinases to phosphorylate proteins important for secretion. In the lacrimal gland, cholinergic agonists stimulate protein secretion. They act by activating phospholipase C to break down phosphatidylinositol bisphosphate into 1,4,5-inositol trisphosphate (1,4,5-IP3) and diacylglycerol (DAG). 1,4,5-IP3 causes release of Ca2+ from intracellular stores. This Ca2+, perhaps in conjunction with calmodulin, activates specific protein kinases that may be involved in secretion. DAG activates protein kinase C which stimulates protein secretion. alpha 1-Adrenergic agonists also stimulate lacrimal gland protein secretion. These agonists use a pathway that is separate from that utilized by cholinergic agonists and vasoactive intestinal peptide (VIP). The specific pathway has not been identified but may be DAG and protein kinase C. VIP, beta-adrenergic agonists, alpha-melanocyte stimulating hormone, and adrenocorticotropic hormone are lacrimal gland secretagogues. They activate adenylate cyclase to produce cAMP. cAMP stimulates protein kinase A, which perhaps causes protein secretion. Thus, three separate cellular pathways stimulate lacrimal gland protein secretion. Cholinergic agonists and VIP also stimulate lacrimal gland fluid secretion, and the same signal transduction pathways utilized by these agonists to stimulate protein secretion are most likely used for electrolyte and water secretion.

Regulation of mucin and fluid secretion by conjunctival epithelial cells.

Tears play a vital role in the health and protection of the cornea and conjunctiva. The tear film consists of multiple layers and different glands secrete each layer. Because of many and varied requirements of the ocular surface cells, the volume, composition and structure of the tear film must be exquisitely controlled. If any layer of the tear film is disrupted or altered, the entire tear film is affected, often with deleterious effects. This chapter reviews the current knowledge of the neural and growth factor regulation of electrolyte, water and protein secretion from the goblet and stratified squamous cells of the conjunctiva as well as the mechanisms used for fluid secretion. The evidence presented in this review suggests that parasympathetic nerves stimulate goblet, but not stratified squamous, cell secretion. Sympathetic nerves stimulate stratified squamous, but not goblet, cell secretion, while P2Y(2) agonists stimulate secretion from both cell types. Growth factors regulate goblet cell secretion, but their effects on stratified squamous cell secretion are unknown.

Dry eye after refractive surgery

Photorefractive keratectomy and laser in situ keratomileusis can induce or exacerbate dry eye after surgery. This manifests as an increase in degree and frequency of symptoms, corneal findings, such as superficial punctate keratopathy, and abnormal results of dry eye tests, such as the Schirmer test and tear break-up time. The cause mainly involves decreased corneal sensation, resulting in decreased feedback to the lacrimal gland and reduced tear production. Other causes may include increased evaporation, inflammation, or toxicity of medications. Dry eye may result infrequently in impaired wound healing and decreased optical quality of the cornea, but it is transient, lasting from a few weeks up to 1 year. Patients should be warned about this distressing complication. During a period of dry eye, artificial tears and punctal plugs are helpful in preventing or alleviating patient discomfort.

Bcl‐2 enhances Ca2+ signaling to support the intrinsic regenerative capacity of CNS axons

At a certain point in development, axons in the mammalian CNS undergo a profound loss of intrinsic growth capacity, which leads to poor regeneration after injury. Overexpression of Bcl‐2 prevents this loss, but the molecular basis of this effect remains unclear. Here, we report that Bcl‐2 supports axonal growth by enhancing intracellular Ca2+ signaling and activating cAMP response element binding protein (CREB) and extracellular‐regulated kinase (Erk), which stimulate the regenerative response and neuritogenesis. Expression of Bcl‐2 decreases endoplasmic reticulum (ER) Ca2+ uptake and storage, and thereby leads to a larger intracellular Ca2+ response induced by Ca2+ influx or axotomy in Bcl‐2‐expressing neurons than in control neurons. Bcl‐xL, an antiapoptotic member of the Bcl‐2 family that does not affect ER Ca2+ uptake, supports neuronal survival but cannot activate CREB and Erk or promote axon regeneration. These results suggest a novel role for ER Ca2+ in the regulation of neuronal response to injury and define a dedicated signaling event through which Bcl‐2 supports CNS regeneration.

Regulatory pathways in lacrimal gland epithelium.

Tears are a complex fluid that continuously cover the exposed surface of the eye, namely the cornea and conjunctiva. Tears are secreted in response to the multitude of environmental stresses that can harm the ocular surface such as cold, mechanical stimulation, physical injury, noxious chemicals, as well as infections from various organisms. Tears also provide nutrients and remove waste from cells of the ocular surface. Because of the varied function of tears, tears are complex and are secreted by several different tissues. Tear secretion is under tight neural control allowing tears to respond rapidly to changing environmental conditions. The lacrimal gland is the main contributor to the aqueous portion of the tear film and the regulation of secretion from this gland has been well studied. Despite multiple redundencies in pathways to stimulate secretion from the lacrimal gland, defects can occur resulting in dry eye syndromes. These diseases can have deleterious effects on vision. In this review, we summarize the latest information regarding the regulatory pathways, which control secretion from the lacrimal gland, and their roles in the pathogenesis of dry eye syndromes.

Stimulation of goblet cell mucous secretion by activation of nerves in rat conjunctiva

An epithelial debridement wound, as a stimulus to the cornea, causes conjunctival goblet cell mucous secretion in that eye. To determine if this stimulation of secretion is neurally mediated, rats were anesthetized and the local anesthetic lidocaine (1%) or buffer alone was administered topically and/or subconjunctivally for 15 min. A corneal epithelial debridement wound was made in one eye. The contralateral eye served as the control. After 5-120 min, animals were sacrificed and inferior bulbar conjunctival buttons removed. Mucus in the goblet cells was stained with Alcian blue and periodic acid-Schiffs reagent to indicate mucin-containing goblet cells. The number of mucin-containing goblet cells/0.16 mm2 was determined by light microscopy; a decrease in number indicated an increase in mucous secretion. Stimulation by corneal wounding induced goblet cell mucous secretion in that eye. Secretion was observed as rapidly as 5 min after stimulus and for as long as 120 min. Topical application of lidocaine, subconjunctival injection of lidocaine, or a combination of both inhibited wound-induced stimulation of mucous secretion. We conclude that conjunctival goblet cell mucous secretion can be neurally mediated and could serve as an immediate response to protect the ocular surface.

Regulation of tear secretion.

Although many questions remain about the regulation of secretion by the different layers of the tear film, several hypotheses can be suggested. We hypothesize first, that secretion of all layers of the tear film and all orbital glands and ocular epithelia that secrete tears is regulated; second, that neural regulation of secretion is of primary importance; and third, that the cAMP-dependent signal transduction pathway plays a pivotal role in this regulation and, except in the main lacrimal gland, Ca2+ plays a secondary role. These hypotheses are suggested as the basis for further work and not as conclusions based on current knowledge.

Conjunctival Goblet Cell Secretion Stimulated by Leukotrienes Is Reduced by Resolvins D1 and E1 To Promote Resolution of Inflammation

The conjunctiva is a mucous membrane that covers the sclera and lines the inside of the eyelids. Throughout the conjunctiva are goblet cells that secrete mucins to protect the eye. Chronic inflammatory diseases such as allergic conjunctivitis and early dry eye lead to increased goblet cell mucin secretion into tears and ocular surface disease. The purpose of this study was to determine the actions of the inflammatory mediators, the leukotrienes and the proresolution resolvins, on secretion from cultured rat and human conjunctival goblet cells. We found that both cysteinyl leukotriene (CysLT) receptors, CysLT1 and CysLT2, were present in rat conjunctiva and in rat and human cultured conjunctival goblet cells. All leukotrienes LTB4, LTC4, LTD4, and LTE4, as well as PGD2, stimulated goblet cell secretion in rat goblet cells. LTD4 and LTE4 increased the intracellular Ca2+ concentration ([Ca2+]i), and LTD4 activated ERK1/2. The CysLT1 receptor antagonist MK571 significantly decreased LTD4-stimulated rat goblet cell secretion and the increase in [Ca2+]i. Resolvins D1 (RvD1) and E1 (RvE1) completely reduced LTD4-stimulated goblet cell secretion in cultured rat goblet cells. LTD4-induced secretion from human goblet cells was blocked by RvD1. RvD1 and RvE1 prevented LTD4- and LTE4-stimulated increases in [Ca2+]i, as well as LTD4 activation of ERK1/2. We conclude that cysteinyl leukotrienes stimulate conjunctival goblet cell mucous secretion with LTD4 using the CysLT1 receptor. Stimulated secretion is terminated by preventing the increase in [Ca2+]i and activation of ERK1/2 by RvD1 and RvE1.