The endoplasmic reticulum is an intracellular organelle with multiple functions, including lipid synthesis, calcium storage, and protein processing. It regulates protein folding to ensure proper functionality, facilitates protein transportation, and detects misfolded proteins, which are then retained in the ER for degradation [
238,
239]. Various conditions such as hypoxia, oxidative stress, viral infections, nutrient depletion, protein mutations, impaired glycosylation, or disrupted disulfide bond formation can interfere with the ER’s physiological functions. These disturbances can lead to ER saturation and the accumulation of misfolded proteins in the ER lumen, resulting in endoplasmic reticulum stress [
240,
241,
242]. ER stress activates multiple signaling pathways aimed at restoring cellular homeostasis [
238,
241]. The unfolded protein response (UPR) is triggered in response to ER stress and encompasses a range of signaling mechanisms designed to reduce protein synthesis, enhance protein folding, and increase protein degradation [
243]. Additionally, the ER-associated degradation (ERAD) system is responsible for retro-transporting misfolded proteins from the ER lumen to the cytosol for clearance through the ubiquitin-proteasome system [
241,
244]. UPR and ERAD are two independent quality-control mechanisms that can interact to eliminate misfolded proteins and maintain protein folding homeostasis [
244]. UPR can lead to either cell survival or apoptosis [
242]. Prolonged activation of UPR pathways can shift the balance toward cell death [
241]. Three main UPR regulators have been extensively described: activating transcription factor (ATF)-6, inositol-requiring protein 1 (IRE1), and protein kinase RNA-like endoplasmic reticulum kinase (PERK). These transmembrane proteins with ER lumen domains serve as sensors of ER stress [
242]. ATF-6 activates cascades to enhance ER folding capacity, promote clearance of misfolded proteins through ERAD, and may also have a proapoptotic effect [
238,
242,
245]. IRE1 initiates a pathway involving spliced X-box binding protein-1 (sXBP1), which promotes ERAD components, ER folding proteins, and autophagy [
238]. Furthermore, IRE1 activates JNK through tumor necrosis factor receptor 2 (TNRF2) and ASK1, leading to apoptosis [
242], as well as inflammation via NF-kB induction [
246]. PERK activates eukaryotic initiation factor 2α (eIF2α), which downregulates overall protein translation, indirectly protecting the cell from protein misfolding [
242]. PERK also upregulates activating transcription factor 4 (ATF-4), which triggers CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP), exerting a proapoptotic function [
268]. In this regard, Han et al. demonstrated that a sustained activation of ATF-4 and CHOP increases protein synthesis and leads to cell death through oxidative stress and ATP depletion [
243]. These findings align with previous research showing that CHOP deletion in multiple mouse models of diabetes reduces oxidative stress [
247]. A noteworthy aspect regarding the connection between ER stress and ROS generation is the ER overload response, which is activated when there is a high concentration of misfolded proteins in the ER lumen [
241]. During the ER overload response, a significant amount of calcium ions (Ca²⁺) may be released from the ER, possibly through Ca²⁺ release channels such as inositol 1,4,5-trisphosphate receptor (IP3R) or ryanodine receptor (RyR) [
238,
241]. This process can result in increased Ca²⁺ uptake from the ER to mitochondria, leading to abnormal production of H₂O₂ and disruption of the ETC, ultimately causing mitochondrial dysfunction [
238,
248]. Moreover, UPR-related signaling can activate the endoplasmic reticulum-oxidoreductin 1 (ERO1) and NOX, which are involved in oxidative protein folding under normal physiological conditions. However, in the context of ER stress, their activation can contribute to ROS production in the stressed ER [
238,
249]. Lastly, ROS generation resulting from both ER and mitochondrial dysfunction can activate NF-kB, a key transcription factor involved in inflammation and cell proliferation [
250]. The main transductions in the interconnection between ER and mitochondria are summarized in
Figure 6.
ER-related oxidative stress has been reported in both TMCs and RGCs in the context of glaucomatous optic neuropathy. Studies on TMCs have demonstrated the involvement of the PERK-eIF2α-ATF4-CHOP cascade in glaucomatous TM, both in human and murine cells, highlighting the activation of this pathway in glaucoma [
251,
252,
253,
254]. Similarly, various studies have described the implication of the PERK-eIF2α-ATF4-CHOP pathway in RGC loss [
255,
256,
257,
258]. Recent literature has highlighted the potential of targeting the PERK-eIF2α-ATF4-CHOP pathway as a therapeutic approach to prevent CHOP-related oxidative stress and apoptosis, thereby mitigating TM structure and function loss and potentially reducing IOP elevation. Notably, in a recent study conducted by Gao et al., the protective effect of valdecoxib, a selective COX-2 inhibitor commonly used in the treatment of conditions such as osteoarthritis and rheumatoid arthritis, was assessed against apoptosis induced by ER stress. The study demonstrated that valdecoxib inhibits the ATF4-CHOP pathway in “I/R-induced glaucoma-like” damaged cells, providing potential insights into its therapeutic efficacy in glaucoma management [
259].