11.1Introduction

The mitochondria within the retinal pigment epithelium (RPE) are an important target of oxidative stress [1]. However, endoplasmic reticulum (ER) is also affected by oxidative stress because of its anatomic proximity and associated interactive function with mitochondria [2]. The ER provides a unique, oxidizing, folding environment that favors the formation of disulÞde protein bonds, which is essential to protein maturation but sensitive to the redox status in ER [3]. Recent studies suggest that the generation of reactive oxygen species (ROS) and consequent alterations of redox status in ER are closely linked to the oxidative stress-initiated ER burden [4]. Accordingly, when ER homeostasis is disrupted and ER function is compromised by oxidative stress, a set of adaptive pathways are initiated to restore the ER homeostasis. The unfolded protein response (UPR) is an intracellular signaling pathway that tends to decrease the ER burden [5]. This activation of UPR is also operative in RPE cells on exposure to oxidative stress and serves to preserve cell function and survival [6]. However, extensive oxidative stress and protein misfolding that is not alleviated could lead to a vicious cycle, Þnally initiating apoptotic cascades [7]. It is well established that RPE cell death plays a predominant role in the pathogenesis of several human retinal diseases.

The mechanisms linking ER response to oxidative stress in RPE are still poorly characterized. Here, we attempt to summarize the critical roles of ER in RPE, the signaling pathways that mediate UPR, the mechanisms underlying interactions between ER stress and oxidative stress, and Þnally, the implications of the ER-associated processes in oxidative stress.

11.2ER Function in Normal Physiology

ER is an essential organelle in eukaryotic cells. This system of ßattened sacs and branching tubules extends throughout the cytoplasm, representing one of the largest membrane networks within cells. Two major functions of ER are essential in all eukaryotic cell types, including RPE: synthesizing and packaging proteins, and regulating metabolism and signaling pathways [8].

11.2.1 Major Roles of Rough ER (RER) and Smooth ER (SER)

The surface of the RER is embedded with ribosomes on the cytosolic surface. This type of ER is involved in the production and processing of proteins that will be exported or secreted from the cell. It is considered the Þrst compartment of the secretory pathway. The ribosomes assemble amino acids into protein units, which

are then transported into the RER for further processing. These units may be either transmembrane proteins, which become embedded in the membrane of the ER, or water-soluble proteins, which can pass completely through the membrane into the lumen. Those that reach the inside of the ER are folded into the correct threedimensional conformation. They then undergo modiÞcations, including folding of the nascent polypeptides and posttranslational modiÞcations such as glycosylation and disulÞde bond formation. The ER either transports the completed proteins to those areas of the cell where they are needed or sends them to the Golgi apparatus for further processing and modiÞcation. Moreover, RER is responsible for quality control of the protein trafÞc. Properly folded proteins are transported from ER, while misfolded proteins are directed to be rectiÞed or otherwise to undergo degradation.

Notably, RER extends continuously with the nuclear envelope, which is also studded with ribosomes. Because of their physical membranous connection, proteins synthesized for nuclei do not rely on transport vesicles. In addition, under certain conditions, the ER begins to be disturbed and unfolded proteins begin to accumulate within the organelle. When this extremely hazardous condition occurs, the organelle quickly sends a signal to the nucleus, which responds by slowing ribosomal translation through a several-step process, giving the ER extra time to restore its protein folding function and thus maintaining cellular health [9].

The smooth ER (SER) in most cells is much less extensive than the RER. Enzymes in SER are chießy involved in synthesis of lipids, metabolism of carbohydrates, and detoxiÞcation of drugs and poisons. Therefore, in some specialized cells, such as those primarily involved in lipid and carbohydrate metabolism or detoxiÞcation [10], the SER is much more extensive and is crucial to cellular function. It is noted that SER is important for the storage of calcium and regulation of calcium metabolism. For instance, in muscle cells, SER releases calcium to trigger muscle contractions; in this context, SER is also known as sarcoplasmic ER [9].

11.2.2ER and Oxidative Protein Folding

In eukaryotes, ER offers a favorable place for oxidative protein folding, a process that is responsible for the formation of disulÞde bonds between cysteine residues in proteins [11]. The driving force behind this process is a redox reaction, in which electrons are passed between several proteins and Þnally to a terminal electron acceptor. Thus, proper protein folding and disulÞde formation of proteins is considered to be dependent on the redox status within the lumen of ER. Indeed, the lumen of ER is oxidizing, with a high ratio of oxidized to reduced glutathione (GSSG/ GSH). This high ratio favors disulÞde bond formation and maturation of secreted and membrane-associated protein.

The mechanism by which ER participates in the process of oxidative protein folding has been well deÞned. Brießy, it is a complicated network, in which protein disulÞde isomerase (PDI) is responsible for the formation of the disulÞde

bonds in unfolded eukaryotic proteins. To catalyze the formation of disulÞde bonds in unfolded proteins, PDI must be reoxidized by the ER membrane-associated protein Ero1p.

11.2.3ER Resident Proteins

Many proteins synthesized, processed, and transported for use inside and outside of the cell pass transiently through the internal space of the ER lumen on their way to other locations. Other proteins, however, are targeted to constantly reside in the lumen and are well known as ER resident proteins [12]. These special proteins, e.g., glucose-regulated protein 78 (GRP78), PDI, and glucose-regulated protein 94 (GRP94), are essential for the ER to carry out its normal functions. These proteins contain a specialized retention signal consisting of Lys-Asp-Glu-Leu (KDEL) in their C-terminus that enables them to be retained by the organelle [13]. Many ER resident proteins act as molecular chaperones, helping to fold proteins. These include PDI, peptidyl prolyl cis-trans-isomerase (PPI), GRP78, GRP94, GRP170, heat shock protein 47 (HSP47), endoplasmic reticulum protein 29 (ERp29), and others [14Ð17]. Remarkably, in addition to facilitating protein folding, these ER chaperones play a role as inspectors in the ER quality control mechanism. Furthermore, when partially folded or unfolded protein aggregates in ER, those ER chaperones act to slow the protein folding reaction and prevent aberrant aggregation [17]. Thus, if these chaperones were unable to control those overloaded proteins, it could be catastrophic to the viability of the cell.

11.2.4Potential Threat to ER Function in RPE

In the eye, RPE, with its multiple complex distinct functions, depends heavily on ER to function properly. The ER supplies the proteins which are required by the RPE to dispose of metabolic waste products and transport nutrients from the blood to the photoreceptors. More importantly in the visual cycle, at least three enzymes associated with the SER convert all-trans retinol to 11-cis retinal, which is then returned to the photoreceptors [18]. In addition, a variety of growth factors that are synthesized by ER in RPE help maintain the structural integrity of choriocapillaris endothelium and photoreceptors [19].

Because all processes regulated by ER are highly sensitive to alterations in the ER luminal environment, ER functions may be compromised by alterations in the redox status, energy deprivation, or accumulation of large amounts of toxic deposits. Oxidative damage to ER is believed to be involved in neuronal cell death [20]. The RPE has a highly oxidative cellular microenvironment because of its anatomic location, its high metabolic rate, and the lipid peroxidation of phagocytosed rod outer segments. Thus, the ER in RPE could be a vulnerable target of oxidative stress, as a direct or indirect result of internal or external factors. The compromised