The endoplasmic reticulum (ER) is a critical organelle involved in protein synthesis, folding, and calcium homeostasis. ER stress occurs when there is an imbalance in calcium homeostasis or accumulation of misfolded proteins, leading to apoptosis. ER stress is linked to various neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's diseases. The ER stress response includes the unfolded protein response (UPR), ER-overload response (EOR), and ER-associated degradation (ERAD), which help the cell survive stress. However, prolonged ER stress can lead to cell death through pathways involving caspases and other signaling molecules. The UPR involves three main sensors: IRE1, PERK, and ATF6, which regulate protein folding and degradation. GRP78, an ER chaperone, plays a key role in the UPR by modulating these sensors. The Bcl-2 family of proteins also regulates ER and mitochondrial function, with some members promoting apoptosis and others preventing it. ER stress can also activate caspase-12, which contributes to apoptosis. The ER is involved in both the extrinsic and intrinsic apoptotic pathways, and its dysfunction can lead to cell death. Understanding the molecular mechanisms of ER stress and its role in neurodegenerative diseases is crucial for developing therapeutic strategies.The endoplasmic reticulum (ER) is a critical organelle involved in protein synthesis, folding, and calcium homeostasis. ER stress occurs when there is an imbalance in calcium homeostasis or accumulation of misfolded proteins, leading to apoptosis. ER stress is linked to various neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's diseases. The ER stress response includes the unfolded protein response (UPR), ER-overload response (EOR), and ER-associated degradation (ERAD), which help the cell survive stress. However, prolonged ER stress can lead to cell death through pathways involving caspases and other signaling molecules. The UPR involves three main sensors: IRE1, PERK, and ATF6, which regulate protein folding and degradation. GRP78, an ER chaperone, plays a key role in the UPR by modulating these sensors. The Bcl-2 family of proteins also regulates ER and mitochondrial function, with some members promoting apoptosis and others preventing it. ER stress can also activate caspase-12, which contributes to apoptosis. The ER is involved in both the extrinsic and intrinsic apoptotic pathways, and its dysfunction can lead to cell death. Understanding the molecular mechanisms of ER stress and its role in neurodegenerative diseases is crucial for developing therapeutic strategies.