Inflammatory Ocular Diseases and Sphingolipid Signaling

Abstract

Inflammation is a powerful immune counter-measure to tissue damage and infection. The inflammatory response is complex and requires the involvement of myriad signaling pathways and metabolic processes, all governed by a multitude of regulatory systems. Although inflammation is a vital defense against tissue injury and a necessary step in tissue healing, the mechanisms which modulate the initiation, intensity, and duration of this innate immune response can malfunction and result in inappropriate or out-of-control inflammation, even in the absence of an appropriate stimulus. Though the human eye exists in an immune-privileged microenvironment, it is not spared from this. The eye is neither devoid of immune cells nor is it fully sequestered from systemic immune responses, and is therefore fully capable of ruining itself through localized inflammatory dysfunction and systemic inflammatory disease (Taylor AW, Front Immunol 7:37, 2016; Zhou R, Caspi RR, Biol Rep 2, 2010). In fact, a wide range of ocular inflammatory diseases exist and are major causes of blindness in humans. Advances in the understanding of inflammatory processes have revealed new key pathways and molecular factors involved in the mechanisms of inflammation. Lipids and sphingolipids are increasingly being recognized as having important signaling roles in the pathophysiology of ocular inflammatory diseases. What follows below is a discussion of fundamental inflammatory processes, the place of sphingolipids as mediators of said processes, brief descriptions of major inflammatory ocular diseases, and new findings implicating sphingolipids in their pathogenesis.

8.1. Inflammation and Ocular Immunity

The innate immune system is developed to defend against a diverse array of threats. The ability to detect tissue damage and pathogen invasion is provided by ‘professional immune cells’ such as circulating monocytes and neutrophils, resident and recruited macrophages, dendritic cells, other specialized cells which either reside within tissue or circulate throughout the body, and a variety of ‘non-professional’ cells (Newton and Dixit 2012). Identification of threats is dependent upon intracellular and surface-bound Pattern Recognition Receptors (PRRs), which detect Pathogen-Associated Molecular Patterns (PAMPs) and Damage-Associated Molecular Patterns (DAMPs) (Akira 2009; Lampron et al. 2013). PAMPs include nucleic acids, lipoproteins, carbohydrates, and other molecules originating from foreign organisms, while DAMPs are endogenous molecules released by stressed and dying cells. PAMPs and DAMPs associate with and activate PRRs, which then initiate signaling cascades leading to recruitment of leukocytes and the initiation of inflammatory responses. The different families of PRRs are expressed constitutively in macrophages, dendritic cells, and even epithelial and endothelial cells, and are responsible for detecting and initiating responses to different types of PAMPs and DAMPs. These include transmembrane Toll-like receptors (TLRs) and C-type lectin receptors (CLRs), as well as cytoplasmic Retinoic acid-inducible gene (RIG)-I-like receptors (RLRs) and NOD-like receptors (NLRs). Activation of TLRs, CLRs, RLRs, and certain NLRs initiates signaling cascades involving transcription factors such as Nuclear factor-κB (NF-κB), Interferon regulatory factors (IRFs), and activation of Mitogen-activated protein kinase (MAPK) pathways to upregulate transcription of genes involved in inflammation. The pro-inflammatory genes activated by PRRs code for chemokines, pro-inflammatory cytokines, and proteins which also modulate PRR signaling (Kawai and Akira 2010). Assembly and activation of inflammasomes, which induce and regulate pro-inflammatory cytokine generation, is a critical role of PRRs. Recent work has shown that activation of inflammasomes and induction of the general inflammatory response are mediated in part by the complement system (Arbore and Kemper 2016). Furthermore, complement has been shown to have a potential role in the resolution phase of inflammation, for example via the actions of C5a and C3a, which stimulate vascular endothelial growth factor (VEGF) expression in post-injury angiogenesis and may be involved in choroidal neovascularization and the development of neovascular age-related macular degeneration (AMD) (Nozaki et al. 2006). The five classic symptoms of inflammation, i.e. erythema, edema, heat, pain, and loss of function, are products of the actions of the pro-inflammatory cytokines produced in response to mechanisms involving PRR signaling. Pro-inflammatory cytokines modify vascular endothelium permeability through the up-regulation of vascular cell adhesion molecule 1 (VCAM-1) and untercellular adhesion molecule 1 (ICAM-1), which stimulate selectin and integrin ligands on endothelial cells, leading to loosening of endothelial tight junctions (Ransohoff et al. 2003). This ultimately has the effect of increasing local blood flow and facilitating the movement of plasma components and immune cells through inflamed tissue, resulting in the redness and swelling characteristic of inflammation. Furthermore, increasing endothelial permeability can also facilitate leukocyte passage across the blood-brain barrier and other obstacles (Takeuchi and Akira 2010).

Ocular tissue exists in an immune-privileged environment which provides a buffer against the free movement of cells and some larger molecules between the eyes and systemic circulation. Immune privilege status in the eye is thought to be conferred by a combination of physical barriers such as the blood-retinal barrier, an immunosuppressive ocular microenvironment created by cell-bound and soluble inhibitory factors within the eye (Caspi et al. 1987; Stein-Streilein 2008), and the lack of direct lymphatic drainage pathways (Stein-Streilein 2008; Streilein 2003; Zhou and Caspi 2010). Certain ocular tissues have been shown to have direct immunosuppressive roles. For instance, retinal pigmented epithelial (RPE) cells have been shown to suppress T cell cytokine production via production of PD-L1 (Sugita et al. 2009b) and stimulate CD4⁺ T cells to convert to T regulatory cells through constitutive production of the Cathepsin L inhibitor CTLA-2α (Sugita et al. 2008, 2009a). Nevertheless, several inflammatory diseases commonly manifest themselves in ocular tissue and are major causes of blindness and disability worldwide. Increasing emphasis on research regarding the roles of lipids, and specifically sphingolipids, in inflammatory mechanisms has led to their recognition as potential key elements in the pathogenesis of multiple ocular inflammatory diseases. As well as being major structural elements of eukaryotic cells, certain species of sphingolipids, such as ceramide (Cer), ceramide 1-phosphate (C1P), and sphingosine 1-phosphate (S1P) have been shown to act as signaling molecules with regulatory roles in various physiologic and pathogenic processes (Dressler et al. 1992; Hannun and Obeid 2008; Porter et al. 2018; Qi et al. 2017). They have also been tied to wound healing, notably in the cornea (Nicholas et al. 2017). Sphingolipid metabolic diseases are often tied to visual dysfunction. Notorious among these are lysosomal storage diseases, which arise from genetic defects resulting in partial or total loss of lysosomal enzymes that degrade sphingolipids, causing harmful accumulation of precursor molecules. These include GM1 gangliosidosis, Tay Sachs disease, Sandhoff disease, Gaucher disease, etc. These diseases tend to lead to various ocular abnormalities such as the development of “cherry-red macula” (Chen et al. 2014). Other ocular diseases, discussed below, have clear inflammatory components and have been linked to sphingolipid signaling and/or metabolic abnormalities.

8.2. Uveitis

Uveitis is a condition characterized by inflammation of the uvea, which is the pigmented middle layer of the eye containing the anterior uvea (consisting of the iris and ciliary body), and posterior uvea (choroid). Uveitis is responsible for causing blindness in roughly 30,000 people annually in the United States (Acharya et al. 2013). Anterior uveitis is commonly associated with pain, erythema, and photophobia, while intermediate and posterior uveitis can present with “floaters” and visual deficits. Inflammation of the uvea is capable of causing severe damage to the retina, optic nerve, and vitreous, often leading to complications such as macular edema, development of cataracts, and glaucoma (Ness et al. 2017). Uveitis may result from a number of diseases. The etiology of uveitis may be idiopathic, infectious, or noninfectious, with causes ranging from localized viral infection to direct ocular trauma and systemic disease (Rathinam and Babu 2013). Uveitic diseases may be either localized to the eye or may be a manifestation of diseases affecting multiple organ systems, such as systemic sarcoidosis (Caspi 2010), where anterior granulomatous uveitis manifests in up to 70% of cases with ocular involvement (Herbort et al. 2009). Analyses of lipidomics data from aqueous and retinal tissue from an endotoxin-induced uveitis rat model showed significant increases in total sphingolipid levels during the acute inflammatory stage of induced uveitis. Notably, the levels of C₁₂ C1P and multiple species of Cer and SM were significantly elevated, suggesting an important role played by sphingolipids in uveitis (Wang et al. 2018). Analyses from Gaucher disease patients have shown increases in glucosylceramide (GlcCer), causing vitreous opacity and subsequent infiltration of macrophages, which may also suggest the involvement of Cer and Cer metabolites in certain forms of uveitis, though uveitis pathogenesis is highly complex and insufficient evidence exists to firmly establish a sphingolipid-mediated mechanism (Astudillo et al. 2016; Fujiwaki et al. 2004). However, the effectiveness of FTY720, an FDA-approved Sphingosine (Sph) analog used to treat Multiple Sclerosis (MS), in treating uveitis and other autoimmune ocular diseases supports a causal relationship of sphingolipid involvement in uveitis and other such diseases (Chen et al. 2014). FTY720 has been shown to suppress experimental autoimmune uveitis in mouse models by substantially reducing peripheral lymphocyte accumulation (Commodaro et al. 2010; Kurose et al. 2000). FTY720 has also been shown to delay retinal degeneration in rat models (Stiles et al. 2016). The discovery that FTY720 is effective in limiting the intensity of uveitis and in delaying retinal degeneration in animal models is strongly indicative of its potential utility in treating inflammatory ocular diseases and highlights the possibility of finding molecular targets within the sphingolipid metabolic pathways to utilize for drug development.

8.3. Diabetic Retinopathy

Diabetic retinopathy (DR) is a common complication of both type 1 and type 2 diabetes mellitus (DM) and involves chronic low-grade inflammation and degenerative neurovascular changes throughout the retina (Abcouwer 2013). It is a leading cause of vision loss and can be expected to develop within 20 years of DM diagnosis (Klein 1987). Left untreated, DR can lead to destruction and detachment of the retina as well as neovascularization of the retina and vitreous through chronic ischemia (Wang et al. 2013). Nonproliferative DR (NPDR) is the early, often asymptomatic form of DR which is characterized by various microvascular abnormalities such as vessel occlusion and microaneurysms. NPDR can exist asymptomatically for years, however, it can cause vision loss through macular edema and is capable of rapid progression to more debilitating forms of the disease, i.e. proliferative diabetic retinopathy (PDR) (Aiello 2003). PDR is an advanced stage of DR, which is characterized by marked proliferation of blood vessels into the retina with increased risk of preretinal and vitreous hemorrhage (Vingolo et al. 2017). VEGF, which promotes vascular permeability and disassembly of endothelial cell junctions, has been shown to be significantly upregulated in the vitreous of DR patients (Aiello et al. 1994). Elevation of retinal glial cell (RGC) and RPE cell cytokine expression also contributes to retinal vascular inflammation and endothelial damage (Abcouwer 2013; Busik et al. 2008; Mohr 2004). It has also been suggested in animal models that leukostasis of the retinal vasculature is a potentially important contributor to ischemia and endothelial damage, and has a possible role in the mechanisms of ocular inflammation seen in DR (Kim et al. 2005). Retinal neovascularization requires the presence of a collagenous scaffold in the form of the vitreoretinal interface. Disruption of this interface as seen in posterior vitreous detachment (PVD) has been shown to be associated with protection from PDR and has been suggested as a potential surgical treatment for PDR (de Smet et al. 2013). Currently though, no fully effective treatment for DR exists. In humans and animal DR models, SMase increases due to TNF-α and IL-1β were shown to increase Cer generation and subsequently stimulate cytokine-mediated inflammation and regulate retinal microangiopathy (Kim et al. 1991; Mathias et al. 1991; Opreanu et al. 2010, 2011). Cer then activates NF-κB, a ubiquitously expressed proinflammatory transcription factor which stimulates transcription of the cytokines IL-1β, IL-6, and IL-8, monocyte chemoattractant protein 1 (MCP1), and cyclooxygenase 2 (COX2) (Schutze et al. 1992; Xiao et al. 2005). Cer is also capable of COX2 induction in macrophages through stimulation by lipopolysaccharide (LPS) and cEBP activation (Cho et al. 2003). Furthermore, very long chain (VLC) Cer species have been shown to mediate vascular permeability and are decreased in DM animal models (Kady et al. 2018; Tikhonenko et al. 2010). GlcCer may play a role in neuroinflammation and retinal cell death in DR, as abnormal GlcCer accumulation can cause mitochondrial, endoplasmic reticular, and endolysosomal dysfunctions (Astudillo et al. 2016; Fujiwaki et al. 2004). GlcCer has also been shown to increase in retinal neurodegeneration and in hyperglycemic retinal neurons in in vitro experiments (Busik et al. 2012; Fox et al. 2006; Opreanu et al. 2011; Sugano et al. 2019). S1P receptor 2 (S1PR2) may influence neovascularization in AMD and DR, as shown by ischemia-induced retinopathy animal models which are S1PR2-deficient and do not exhibit neovascularization (Skoura et al. 2007). Lactosylceramide (LacCer) may also play a role in DR, as evidenced by its apparent involvement in inflammation (Pannu et al. 2005), VEGF-mediated angiogenesis (Kolmakova et al. 2009), and its observed changes in human diabetic retina and vitreous samples (Wilmott et al. 2019). Macrophage influx and choroidal/subretinal neovascularization was also significantly inhibited via blockage of S1P with Sonepcizumab, further demonstrating the extent of sphingolipid involvement in DR (Xie et al. 2009).

8.4. Glaucoma

Glaucoma refers to a family of eye diseases which have been traditionally defined by optic nerve damage from elevated intraocular pressure (IOP). It is one of the world’s leading causes of blindness, affecting roughly 80 million people worldwide (Cook and Foster 2012; Quigley and Broman 2006). There are multiple types of glaucoma, all of which may be characterized by progressive optic nerve head degeneration and RGC death, but not all are associated with IOP elevation. Angle-closure glaucoma is characterized by narrowing or complete closure of the anterior chamber angle, and may be due to anatomical predispositions such as defects in the iris or lens (as in primary angle-closure glaucoma) or due to a secondary process such as neovascularization or inflammation (as in secondary angle-closure glaucoma) (Weinreb et al. 2014). Closure of the anterior chamber angle prevents aqueous humor drainage from the anterior chamber, resulting in IOP elevation and optic nerve damage. Open-angle glaucoma patients have increased aqueous outflow resistance due to blockage of the trabecular meshwork, also resulting in gradual IOP elevation and subsequent optic nerve damage (Foris and Gossman 2018). Normal tension glaucoma, however, is characterized by normal or low IOP along with the RGC death, optic nerve degeneration, and visual field defects associated with other forms of glaucoma involving IOP elevation (Gramer et al. 1986; Quigley 2011). Clearly, there are likely to be other factors at play in the pathogenesis of glaucoma, which are independent of IOP elevation. Reports have strongly suggested the involvement of an inflammatory response in conjunction with glaucomatous neurodegeneration (Luo et al. 2010). Specifically, astrocyte and microglial upregulation of TLR signaling leading to activation of proinflammatory cytokines in the optic nerve head has been identified as a probable contributor (Howell et al. 2011, 2012). Recent findings point to S1PR2 acting as a mediator of trabecular meshwork contractility, likely affecting outflow and potentially having a role in glaucoma pathogenesis (Stamer et al. 2009; Sumida and Stamer 2011). Our own long-term work has shed light on many roles of sphingolipids in the mechanisms of RGC degeneration, which may be applicable to glaucoma. Our lab and others have made novel discoveries pointing to lysosomal accumulation of Cer as being a factor in glaucomatous RGC degeneration (Fan et al. 2016; Hayreh et al. 2009a, b). Furthermore, significant reductions in plasma Sph and S1P have been shown to be associated with primary open angle glaucoma (Burgess et al. 2015). Though data pertaining to the roles of sphingolipids in glaucoma is limited, evidence is growing which suggests an important part played by sphingolipids in the disease’s pathogenesis.

8.5. Age-Related Macular Degeneration

Age-related macular degeneration is a degenerative disease of the macula which results in loss of the central visual field. AMD accounts for roughly half of all legal blindness in industrialized countries (Owen et al. 2003). Degeneration of the RPE and subsequent photoreceptor death leading to loss of central vision is the hallmark of both types of AMD. AMD etiology has two forms: dry and wet AMD. Dry AMD involves slow, progressive RPE apoptosis and has a relatively poorly-understood etiology. Dry AMD involves the formation of Drusen between the RPE and the Bruch membrane, leading to RPE and photoreceptor degeneration and progressive geographic atrophy. No effective treatment for dry AMD has yet been developed (Zajac-Pytrus et al. 2015). Wet AMD is characterized by overproduction of VEGF in the RPE, leading to breakdown of the blood-retinal barrier and choroidal/subretinal neovascularization (Nowak 2006). Weak vessel formation may lead to hemorrhage, causing macular scarring and edema, which is the major cause of vision loss in wet AMD (Campochiaro 2013). There are apparent connections between inflammatory mechanisms and AMD pathology. Subretinal Drusen contain a variety of potentially harmful constituents such as lipids, RPE-derived cellular debris, oxidation byproducts, and inflammatory factors including complement components and immunoglobulins (Anderson et al. 2002; Hageman et al. 1999, 2001; Johnson et al. 2000). Factor H (HF1), a major inhibitor of the complement pathway, which is synthesized by RPE, has been shown to accumulate within Drusen and Y402H mutation has been identified as a major risk factor for the development of AMD (Hageman et al. 2005). Further associations have been identified between AMD and several complement pathway-associated genes: complement factor H, complement factor H-related 1 and 3, complement factor B, and complement components 2 and 3 (Anderson et al. 2010). Cer-mediated inflammation and apoptotic mechanisms have been linked to RPE cell degeneration in AMD and several ocular degenerative diseases (Zhu et al. 2010). Cer synthesis and oxidative stress are responsible for contributing to mitochondrial permeabilization and caspase-3 activation, followed by apoptotic photoreceptor cell death (Barak et al. 2001; Kannan et al. 2004). Inhibition of Cer synthesis via the SMase inhibitor desipramine has been shown to have a protective effect in oxidative-stressed photoreceptors, preventing apoptotic cell death (Sanvicens and Cotter 2006). We were able to produce a similar photoreceptor protective effect by increasing degradation of Cer to Sph. We achieved this by inducing human ARPE19 cells to overexpress acid ceramidase, an enzyme which catalyzes the conversion of Cer to Sph, thereby decreasing cellular Cer levels and yielding significant protection from apoptosis (Sugano et al. 2018). The opposite effect has been observed from overproduction of Cer, which accelerated RPE cell death (Zhu et al. 2011). Cer is also implicated in AMD-related RPE degeneration, wherein activation of acid sphingomyelinase results in RPE autophagy dysfunction, complement regulatory protein recycling, endosome biogenesis, and complement activation (Kaur et al. 2018; Natoli et al. 2017; Tan et al. 2016; Toops et al. 2015).

8.6. Dry Eye Syndrome

Dysfunctional tear syndrome, also known as dry eye disease or keratoconjunctivitis sicca, is a multifactorial disorder of the tear film and ocular surface caused by tear deficiency and excessive evaporation (Bron et al. 2014). Dry eye affects roughly 17% of women and 11% of men in the United States (Moss et al. 2000) and is typically seen as a relatively minor condition, though it can cause discomfort, visual disturbance, and ocular surface damage through inflammation (Hessen and Akpek 2014). Dry eye is caused by dysfunction in the lacrimal functional unit, i.e. the synergistic unit composed of the lacrimal glands, eyelids, and ocular surface (Stern et al. 1998). Dysfunction may cause tear film hyperosmolarity and ocular surface inflammation secondary to decreased tear production and/or increased evaporative tear loss. Tear production deficiency dry eye disease can be subclassified into Sjögrens and non-Sjögrens syndrome. Sjögrens syndrome is a chronic autoimmune exocrinopathy involving inflammatory infiltration of the lacrimal glands, causing cell death and tear production deficiency (Fox 2005). Non-Sjögrens dry eye is thought to be age-related and secondary to obstruction of the lacrimal ducts leading to decreased tear output (Damato et al. 1984). Interestingly, diabetes mellitus is significantly associated with non-Sjögrens dry eye (Kaiserman et al. 2005) and growing evidence points to a chronic inflammatory component of dry eye disease, mediated by lymphocytes (Kunert et al. 2000). Conjunctival inflammation is a characteristic clinical symptom of dry eye and in this case may be dependent upon T-cell activation and upregulation of pro-inflammatory cytokines and matrix metalloproteinase (Solomon et al. 2001; Stern et al. 2002). Sphingolipids are involved in maintaining tear film lipid layer (TFLL) integrity, which is essential for proper tear film composition and thereby plays a major role in proper ocular surface lubrication and protection (Lam et al. 2011). Furthermore, SM, short chain GlcCer, and Cer levels have been shown to be elevated in dry eye patient meibomian gland samples, which are the primary source of lipids in the TFLL (Mathers and Lane 1998; Nelson et al. 2011; Nicolaides et al. 1989; Paranjpe et al. 2018; Robciuc et al. 2013; Shine and McCulley 1998). Cer, C1P, and S1P can induce inflammation by arachidonic acid release and prostaglandin formation (Hannun and Obeid 2008; Haversen et al. 2009; Jozefowski et al. 2010), which has been found to be correlated with multiple measures of tear film and meibomian gland dysfunction (Walter et al. 2016). C1P stimulates eicosanoid synthesis (Pettus et al. 2004) and activates cytosolic phospholipase A2 in prostaglandin synthesis (Pettus et al. 2005). However, in addition to inducing inflammation, C1P also has anti-inflammatory roles. For instance, C1P inhibits TNF-converting enzyme (TACE), which is a metalloproteinase that cleaves proTNF to yield its active form, TNFα (Lamour et al. 2011). TNFα activates SphK1 to produce S1P, which then binds ubiquitin E3 ligase TRAF2 and stimulates polyubiquitination of Receptor interacting protein-1 (RIP1), causing phosphorylation of IKK complex and NF-κB activation (Alvarez et al. 2010), which might have roles in various ocular inflammation processes relevant to dry eye disease. Increased Cer levels have also been shown to negatively affect TFLL stability (Arciniega et al. 2013). Altogether, these findings along with our own data strongly support sphingolipid involvement in the pathophysiology of dry eye diseases.

8.7. Conclusion

Recent advances in the understanding of sphingolipid metabolism and signaling as well as inflammatory mechanisms have identified bioactive sphingolipids as mediators in ocular disease processes. Cer, the central molecule in sphingolipid metabolism (Fig. 8.1), has received the most attention for its apparent involvement in apoptosis and stress responses (Pettus et al. 2002). However, it is important to understand that Cer, LacCer, and other sphingolipid metabolites are classes of similar lipids with distinct, species-specific effects rather than individual molecules. For example, depending on differences in chain length, degrees of unsaturation, etc. introduced by the >28 distinct enzymes which use Cer as a substrate or product, there are theoretically ~360 different possible variations of Cer. The different variations of sphingolipid species within classes apparently have distinct effects and downstream metabolites (Hannun and Obeid 2011). As if this did not complicate things enough, ocular sphingolipid research is still fairly immature and there is often insufficient foundational knowledge to confidently establish mechanisms explaining the downstream effects of known bioactive sphingolipids, despite association studies strongly supporting their involvement in biological processes. Although the exact mechanisms of sphingolipid mediation of ocular inflammatory and degenerative processes are generally incompletely-developed, there is enough evidence to link sphingolipids to several major ocular diseases (Fig. 8.2). Whether these findings implicate them as causative of pathology (in which case they may be targeted for therapeutic treatments) or show that they merely change in response to disease processes (in which case they may be used as diagnostic biomarkers) is, in many cases, also still unclear. That said, the involvement of sphingolipids in inflammation is well-established and the general trend is pointing towards sphingolipids gaining more recognition as important mediators of ocular disease. A number of inflammatory mechanisms are known to rely, at least in part, on sphingolipid signaling and metabolism. Taken into context, these novel discoveries have implications for a wide range of ocular inflammatory diseases. Though the data supporting sphingolipid involvement in the underlying mechanisms of many diseases are limited, efforts to elucidate the mechanisms of these diseases continue to turn up evidence of bioactive sphingolipid signaling and metabolism playing important roles.

Fig. 8.1.

Schematic representation of cellular sphingolipid metabolism. Ceramide (Cer) is produced primarily in the endoplasmic reticulum (ER) from serine and palmitoyl-CoA via a series of reactions in the de novo pathway. Cer is then either converted to Galactosylceramide (GalCer) by addition of a galactose or transported from the ER to the trans-Golgi, possibly via a trafficking mechanism mediated by Cer Transfer Protein (CERT). In the Golgi, Cer is either converted to Sphingomyelin (SM) by Sphingomyelin Synthase (SMS), or is glycosylated to form Glucosylceramide (GlcCer) by GlcCer Synthase (GCS). GlcCer may be converted to Lactosylceramide (LacCer) by addition of galactose with LacCer Synthase (LCS). SM from the Golgi is transported to the plasma membrane, where it may be converted by cytosolic neutral sphingomyelinase (nSMase) back to Cer, which is phosphorylated by Cer Kinase (CerK) to ceramide 1-phosphate (C1P). Alternatively, SM may be converted to Cer via secretory SMase (sSMase), which is converted to Sphingosine (Sph) by neutral ceramidase (nCDase). Sph is phosphorylated to sphingosine 1-phosphate (S1P) by Sphingosine Kinase (SphK) 1 or 2, which can signal extracellularly via membrane-bound S1P receptors (of which there are 5 known). Complex sphingolipids from the plasma membrane may enter the endolysosomal pathway and be hydrolyzed back to Cer, which is converted to Sph within the lysosome by acid ceramidase (aCDase or ASAH1, N-Acylsphingosine Amidohydrolase 1). From here, Sph is either phosphorylated to S1P or re-acylated back to Cer by Ceramide Synthase (CerS) in the salvage pathway. S1P leaves the sphingolipid metabolic pathway by conversion to ethanolamine phosphate and hexadecenal by Sphingosine 1-Phosphate Lyase (S1PL)

Fig. 8.2.

Brief summary of major sphingolipid classes and notable effects relevant to inflammatory ocular diseases. Cer ceramide, Sph sphingosine, Glc glucose, Gal galactose, P phosphate

Abbreviations

AMD

age-related macular degeneration

C1P

ceramide 1-phosphate

Cer

ceramide

CLRs

C-type lectin receptors

COX2

cyclooxygenase 2

DAMPs

Damage-Associated Molecular Patterns

DM

diabetes mellitus

DR

diabetic retinopathy

GlcCer

glucosylceramide

ICAM-1

Intercellular adhesion molecule 1

IOP

intraocular pressure

IRFs

Interferon regulatory factors

LacCer

lactosylceramide

LPS

lipopolysaccharide

MAPK

Mitogen-activated protein kinase

MCP1

monocyte chemoattractant protein 1

MS

Multiple Sclerosis

NLR

NOD-like receptors

NPDR

Nonproliferative diabetic retinopathy

PAMPs

Pathogen-Associated Molecular Patterns

PDR

proliferative diabetic retinopathy

PRR

Pattern Recognition Receptors

PVD

posterior vitreous detachment

RGC

retinal glial cell

RIP1

Receptor interacting protein-1

RLRs

Retinoic acid-inducible gene (RIG)-I-like receptors

RPE

retinal pigmented epithelial

S1P

sphingosine 1-phosphate

S1PR2

S1P receptor 2

NF-κB

Nuclear factor-κB

Sph

Sphingosine

TFLL

tear film lipid layer

TLRs

Toll-like receptors

VCAM-1

Vascular cell adhesion molecule 1

VEGF

Vascular endothelial growth factor

VLC

very long chain