Alzheimer's disease(AD) is the most common form of dementia, divided into familial and sporadic forms. The former is caused by genetic factors, while the latter is influenced by a combination of genetic and environmental factors. The main pathological features of AD include the formation of senile plaques in brain tissue, composed of β-amyloid protein deposits, and neurofibrillary tangles caused by abnormal phosphorylation of tau protein[20]. Tau protein is a microtubule-associated protein, and its normal phosphorylation is a key process in regulating tau function in the brain, playing a crucial role in stabilizing microtubules. Various kinases, such as glycogen synthase kinase-3 (GSK-3) and cyclin-dependent kinase 5 (CDK5), are involved in tau phosphorylation[21, 22, 23]. However, in neurodegenerative diseases (including AD), tau protein undergoes hyperphosphorylation, leading to a decrease in its ability to bind to microtubule proteins. Consequently, the protein separates from axonal microtubules, followed by misfolding of tau protein, aggregation, and ultimately the formation of neurofibrillary tangles observed in AD patients[24, 25]. Understanding the intricate balance of kinases and phosphatases in the process of tau phosphorylation is crucial for developing therapeutic strategies for neurodegenerative diseases[26]. Actively researching how to regulate tau phosphorylation may serve as a potential target for treating AD and other tauopathies.
In this study, we initially obtained the gene expression profile of the gene dataset GSE5747 (Alzheimer's disease: neurofibrillary tangles) from the GEO database. Subsequently, we identified 33 DEGs associated with ER stress. Our findings suggest that ER stress is likely involved in the progression of AD. To further investigate this, we conducted GO and KEGG pathway analyses, revealing several biological processes primarily involved in ear development, positive regulation of ossification, and mesenchyme morphogenesis. As well as pathways such as the basal cell carcinoma signaling pathway, breast cancer, and pertussis signaling pathway.
The ER is the primary site within eukaryotic cells for protein synthesis, lipid production, and calcium ion storage. Proper protein folding within the ER is crucial for maintaining ER homeostasis[27]. Tau protein plays a key role in the pathological processes of AD and is associated with ER stress. Disruption of the folding and processing of tau protein leads to the accumulation of misfolded or unfolded proteins, triggering ER stress. This, in turn, activates the UPR through three pathways: the IRE1α-XBP1 pathway, the PERK pathway, and the ATF6 pathway, to alleviate ER stress and restore ER homeostasis[5, 28].In the PERK pathway, activation of PERK leads to phosphorylation of eIF2α, resulting in reduced protein translation. This helps decrease the protein load entering the ER, allowing existing proteins to fold correctly.In the ATF6 pathway, ATF6 is transported to the Golgi apparatus, where it is cleaved to release its cytoplasmic domain. Subsequently, this domain enters the nucleus and activates the transcription of genes involved in protein folding and degradation within the ER.In the IRE1 pathway, IRE1 undergoes oligomerization and autophosphorylation, leading to its activation as an endoribonuclease. This results in unconventional splicing of XBP1 mRNA, producing an active transcription factor that induces the expression of UPR target genes involved in protein folding, degradation, and ER-associated degradation (ERAD)[28, 29, 30].
While initially protective, prolonged or severe ER stress can trigger apoptotic pathways, leading to cell death. This helps eliminate cells that cannot recover from stress and prevents the accumulation of damaged proteins that could harm the organism[31].
GO analysis reveals that both the gene dataset GSE4757 and endoplasmic reticulum stress are associated with the same biological process: mesenchyme morphogenesis. Mesenchyme stem cells(MSCs) originate from the mesoderm during embryonic development, and mesenchyme morphogenesis refers to the spatial rearrangement and differentiation of mesenchymal stem cells (a loose network of embryonic connective tissue) into various tissues and organs during embryonic development[32, 43, 44]. Mesenchyme plays a crucial role in shaping the overall body plan and contributes to the formation of many different structures, including bone, cartilage, muscle, blood vessels, and connective tissue[33]. Dysregulation of mesenchyme morphogenesis can lead to developmental defects and has also been implicated in the research of diseases such as AD, systemic chronic inflammation, myocardial infarction/heart failure, with some progress already achieved[34, 35, 36].
The connection between mesenchyme morphogenesis and AD lies in the potential therapeutic applications of MSCs in treating AD. MSCs have the ability to differentiate into different cell types, including neurons and glial cells, and possess immunomodulatory and anti-inflammatory properties[37, 38]. Some studies suggest that MSCs have potential in AD treatment, primarily focusing on their ability to promote neurogenesis, regulate neuroinflammation to improve cognitive function, and secrete various neuroprotective and growth factors to protect neurons from cell death[38, 39, 40]. MSCs have been shown to engulf and clear the characteristic pathological hallmark of AD, amyloid-βplaques. By promoting the clearance of amyloid beta, MSCs may help reduce the burden of toxic protein aggregates in the brain[41], indicating promising therapeutic potential in AD treatment. Despite the effectiveness of therapies based on MSCs in preclinical studies, their clinical application has faced significant challenges, and there are still some hurdles in this direction[42].
We also constructed a protein-protein interaction network of DEGs related to ER stress using STRING and identified four hub genes associated with ER stress in AD through Cytoscape: BMP2, DLX5, vWF, and EDN1.
Bone Morphogenetic Protein-2 (BMP2), also known as bone formation protein, is a member of the transforming growth factor-β (TGF-β) superfamily of proteins and is a highly conserved functional protein with similar structures. BMP2 has diverse biological functions, manifested in promoting fat deposition and inducing the directional differentiation and proliferation of undifferentiated MSCs into chondrocytes and osteoblasts[45]. BMP2 also influences organ formation during embryonic development, playing a decisive role in determining whether embryonic development is abnormal and regulating the maintenance of homeostasis in adult tissues[46]. The Smad-dependent signaling pathway is the classic pathway of BMP2, crucial for regulating the differentiation of osteoblasts and osteoclasts. When the TGF-β family ligand binds to the type II receptor, it activates the type II receptor kinase, which then phosphorylates the GS domain in the type I receptor. The phosphorylated GS domain recruits SMADs (R-SMADs) regulated by the receptor, the phosphorylated R-Smads dissociate from the type I receptor, and then combine with Smad 4 to form a complex[47, 48, 50]. Subsequently, the complex is transported into the cell nucleus, where it binds to target genes, regulating their positive or negative transcription[47]. In addition to the SMAD-mediated signaling pathway, BMP2 can also activate non-SMAD signaling pathways, such as the MAPK/ERK pathway and PI3K/Akt pathway, further promoting its biological effects[47, 49, 51].
Distal-less homeobox 5 (DLX5) plays a crucial role in embryonic development, particularly in the formation of various tissues and organs, including the skeleton, teeth, and central nervous system. Specifically, DLX5 is involved in the development of skeletal elements (such as bone and cartilage) and regulates the differentiation of mesenchymal stem cells into osteoblasts (bone-forming cells) and chondrocytes (cartilage-forming cells)[52, 53, 54]. Mutations or dysregulation of DLX5 can lead to skeletal abnormalities and developmental disorders[53]. Von Willebrand Factor (VWF) is a glycoprotein involved in blood clotting, playing a crucial role in hemostasis by mediating platelet adhesion at sites of vascular injury and stabilizing factor VIII. VWF is synthesized by endothelial cells (vascular endothelial cells) and megakaryocytes (cells that produce platelets) and stored in endothelial cells and platelet granules, released into the blood during vascular injury[55]. Endothelin-1 (EDN1) is a peptide substance that plays a crucial role in regulating vascular constriction and dilation, and its signaling imbalance is associated with various cardiovascular diseases, including hypertension, heart failure, and atherosclerosis[56].
ER stress and the UPR play crucial roles in the occurrence and development of AD.In this study, we screened DEGs related to AD and ER stress, analyzed their hub gene functions, and the involved signaling pathways, but further validation is required through cellular experiments and clinical samples. Targeting the UPR pathway may maintain global protein homeostasis, improve neuroinflammation, and hold promise in treating a range of neurodegenerative diseases such as AD. A thorough understanding of the signaling pathways and physiological functions of UPR-related molecules will facilitate the development of novel therapies.The unknown mechanisms between ER stress and AD still need further elucidation, which will provide new directions for the treatment of AD.