The Role of Mitochondrial Unfolded Protein Response in Neurodegenerative Diseases

As the core hub of energy metabolism in eukaryotes, mitochondria participate in a variety of cellular activities, including metabolic regulation of the cell matrix, apoptosis, and the activation of signal transduction pathways. Their functional status is closely linked to the initiation and progression of various diseases. Neurodegenerative diseases are primarily characterized by the progressive loss and dysfunction of neurons, and mitochondrial dysfunction is considered one of the key triggers in this process. The specific mechanisms by which mitochondrial dysfunction contributes to neurodegenerative diseases have attracted widespread attention. When misfolded or unfolded proteins are detected, a process known as the mitochondrial unfolded protein response (mtUPR) is activated to promote proper protein folding or degradation, thereby restoring mitochondrial function. As a mitochondrial stress defense mechanism, mtUPR primarily regulates the expression of nuclear-encoded genes, such as chaperones and proteases, to alleviate mitochondrial stress. Studies have shown that, in addition to misfolded and unfolded proteins, other mitochondrial stresses—such as mitochondrial DNA abnormalities and reactive oxygen species (ROS)—can also induce mtUPR. The biological functions of mtUPR extend beyond mitochondria and are crucial for the health of the entire cell and even the whole organism. The mtUPR process involves communication between mitochondria and the nucleus, a phenomenon that is highly conserved and has been observed across different species. Abnormal activation or inhibition of mtUPR is closely associated with the development of various neurodegenerative diseases, such as Alzheimer’s disease and Parkinson’s disease. An in-depth exploration of the dynamic regulatory role and molecular mechanisms of mtUPR is therefore of great significance for understanding the pathogenesis of these disorders. In addition to neuron loss, neurodegenerative diseases are characterized by the accumulation of misfolded proteins in the brain, including insoluble fibrils of amyloid beta, phosphorylated tau, or α-synuclein. While the molecular pathways of mtUPR are largely conserved across different diseases, the possibility of differential regulatory factors cannot be excluded. Although mtUPR activation is predominantly recognized for its cytoprotective role, it may exert deleterious effects when overstimulated or sustained. Chronic mtUPR activity has been linked to mitochondrial dysfunction and increased neuronal vulnerability, contributing to the pathogenesis of various neurodegenerative diseases. This review summarizes the fundamental concepts, major inducers, and signaling pathways of the mtUPR. We focus on the intrinsic relationship and regulatory patterns between mtUPR and neurodegenerative diseases, providing insights that may aid the development of targeted therapies. Finally, we discuss the challenges and future directions of mtUPR research in this field, aiming to pave the way for new therapeutic breakthroughs. A major limitation arises from the experimental models currently used; most findings rely on model organisms or cultured cells, which cannot fully replicate the complexity of human neurons. Future research should therefore focus on three main directions: (1) defining the molecular switches that determine whether mtUPR acts in a protective or detrimental manner; (2) elucidating differences in mtUPR molecular pathways across various models of neurodegenerative diseases; and (3) establishing robust biomarkers for mtUPR activity.