VDAC1 is a key protein on the mitochondrial outer membrane that serves to regulate substance and ion exchange between mitochondria and the cytoplasm upstream of cellular energy metabolism. It participates in various functions, such as energy generation, calcium ion transport and the regulation of cell apoptosis (24). Research has indicated that VDAC1 expression can vary in neurodegenerative diseases (such as Alzheimer's disease), in addition to interacting with disease-related molecules and contributing to disease progression (25). VDAC was initially purified from paramecium mitochondria in 1976(25). At present, three subtypes of VDAC family proteins have been identified in mammals, namely VDAC1, VDAC2 and VDAC3. VDAC1 is extensively expressed and exhibits notable bidirectional functionality (24,26). VDAC2 has been reported to interact with proteins containing Bcl-2 homology 3 (BH3) death domains, including truncated BH3-interacting domain death agonist, Bcl-2-like protein 11 and and Bcl-2-associated agonist of cell death, to facilitate Bcl-2 homologous antagonist/killer oligomerization, create apoptotic channels and trigger cell apoptosis (27). VDAC3, which is particularly rich in cysteine residues, is important for mitochondrial protection (28). VDAC subtypes have a tissue-specific expression pattern, where VDAC1 is highly expressed in general, especially in the cochlea. VDAC1 is the most highly expressed among the three subtypes in most organs, such as the heart, liver, brain tissue, including cochlear tissue and other mitochondrial-rich tissues. VDAC1 is considered the gatekeeper of mitochondrial and cytoplasmic material transport, and is therefore widely expressed in mitochondrial-rich tissues (29). In the cochlea (30), hair cells and spiral neurons are the concentrated areas of mitochondrial distribution, therefore, VDAC1 exhibits expression distribution in this region. In addition, in the latest research on the mechanism of hearing loss (31), it was found that VDAC1 is widely expressed in cochlear cells and increases after noise exposure.

VDAC1 consists of 19 transmembrane β chains forming a β pore, which contains a 25-residue fragment in the N-terminal domain. N-terminal domain migration serves a role in channel gating and VDAC1 dimer formation. An α-helix is located at the midpoint of the barrel hole and is horizontal, where it regulates the transport of metabolites by reducing the pore size (32). VDAC1 facilitates the transfer of a number of metabolites, such as pyruvate, succinate, malonate, nucleotides and NADH, into the mitochondria, enabling subsequent metabolism (33). In addition, VDAC1 is involved in cholesterol transport, lipid metabolism, Ca²⁺ signaling between mitochondria and the endoplasmic reticulum and redox status regulation (34). VDAC1 is also essential for mitochondria-mediated cell apoptosis. Drug-induced cell apoptosis can promote overexpression and oligomerization of VDAC1, leading to the formation of large pores on the surface of mitochondria, and promoting the transfer of AIF, cyto c and apoptotic protease-activating factor 1 (Apaf-1) to the cytoplasm, thereby activating apoptotic pathways (35-37). For example, AIF can transfer out of mitochondria and enter the nucleus to promote DNA condensation and cleavage, and promote cell apoptosis. Similarly, cyto c and Apaf-1 are also involved in related apoptotic mechanisms. Therefore, VDAC1 is considered a ‘mitochondrial gatekeeper’.

VDAC1 regulates cell fate determination through the dynamic balance of the ubiquitination/oligomerization switch. Through the classical pathway, VDAC1 serves as a substrate for the E3 ligase Parkin (38). During autophagy induction, VDAC1 can be ubiquitinated to form adapter proteins with ubiquitin-binding motifs and LC3B-binding motifs, recruiting p62/SQSTM1 or LC3B to promote autophagosome formation and induce lysosomal binding to these labelled mitochondria for selective degradation. In the traditional PINK1/Parkin mitochondrial autophagy pathway, PINK1 phosphorylates Parkin on S65 within the ubiquitin-like domain, thereby activating its E3 ubiquitin ligase activity. Notable Parkin substrates include the mitochondrial outer membrane-associated proteins mitofusin (MFN)1, MFN2, outer membrane transposase 20, mitochondrial Rho GTPase 1 and VDAC1, which clear damaged mitochondria and maintain their stability through the ubiquitination autophagy mechanism (39). In addition, the ubiquitination sites of VDAC1 include β-chain K53, K274, K12, K20, K53, K109 and K110. In a previous study on liver fibrosis, ubiquitination at the K53 site of VDAC1 was found to negatively regulate VDAC1 oligomerization and mtDNA release (40). Apart from Parkin, other E3 ubiquitin ligases, such as neural precursor cell expression and developmental downregulation 4, can connect with the ubiquitylated protein through the K48 site to negatively regulate acetaminophen-induced mitochondrial damage in liver cells (41). In Parkinson's disease, tripartite motif-containing protein 31, a triple motif protein, can promote the degradation of VDAC1 by catalyzing ubiquitination at the K48 site of VDAC1, a process which helps stabilize dopaminergic neurons (42). Therefore, investigating the ubiquitination sites and specific signaling pathways of VDAC1 in cochlear receptor cells offers a novel perspective for studying hearing loss mechanisms.

There are multiple post-translational modification modes of VDAC1, including phosphorylation, acetylation, tyrosine nitration, ubiquitination and oligomerization, but their consequences remain poorly elucidated. VDAC1 is known for its oligomerization, because it is associated with cell death. VDAC1 can form pore-like channels in the outer membrane of mitochondria, regulating the entry and exit of Ca²⁺ to maintain the balance of the outer membrane. However, under stimulation, VDAC1 will self-aggregate to form dimeric, trimeric or even oligomeric forms, expanding the pore size and disrupting the balance on the outer membrane of mitochondria. During cell apoptosis, it interacts with the apoptotic protein Bax to form an oligomeric structure that increases the central pore size of VDAC1, allowing the release of cyto c, Apaf-1, deoxyadenosine triphopshate (dATP) and AIF to promote apoptosis (35). VDAC1 oligomerization pores also facilitate mtDNA release from mitochondria into the cytoplasm, disrupting mitochondrial homeostasis and aggravating lupus erythematosus (43). In addition, VDAC1 is associated with the uptake and release of mitochondrial Ca²⁺. It can interact with the endoplasmic reticulum and become an important part of the mitochondrial Ca²⁺ connection. Excessive Ca²⁺ transport can also stimulate the regulatory effect of VDAC1(44). A previous study reported that upregulation of the mitochondrial calcium uniporter protein in a cadmium toxicity model of liver cells can trigger the oligomerization and ubiquitination of VDAC1 (45,46), where the balance between oligomerization and ubiquitination can determine the fate of cells.

Mitochondrial autophagy involves a critical step known as ubiquitination, which functions analogously to tagging substrates for subsequent lysosomal degradation. While ubiquitination marks cellular components for phagocytosis by lysosomes, the detailed mechanisms of lysosomal engulfment and degradation fall beyond the scope of this discussion (42). Previous research has elucidated the role of VDAC1 in mitochondrial ubiquitination, such as its potential as a substrate for E3 ubiquitin ligase (47). It can also regulate classic mitochondrial autophagy pathways such as the PINK1/Parkin pathway. However, the role of VADC1 in autophagy remains only partially understood, including its inhibition of the PINK1/Parkin pathway to mitigate noise-induced hearing loss (31). In addition, increased expression of VDAC1 may serve as an additional anchor site for polyubiquitination recruitment, triggering mitochondrial autophagy (48). Thus, VDAC1 has been revealed to be associated with autophagy, but further research is still required.

Given the potential role of VDAC1 in cochlear sensory cell stress, modulating VDAC1 function can enhance cochlear cell environments, mitigate oxidative stress and bolster resistance, thereby reducing cochlear cell damage. Livingston et al (56) previously showed that ischemic pretreatment of renal tubular cells activated autophagy, which prevented mitochondrial depolarization, enhanced ATP production and regulated cell apoptosis and autophagy, thereby protecting these cells from ischemic damage and offering novel insights for hearing loss treatment. It is surmised that reducing the expression of VDAC1, regulating VDAC1 ubiquitination, enhancing mitochondrial autophagy, reducing cell damage and protecting hearing are effective strategies in protecting from hearing loss. The VDAC1 oligomerization inhibitor, VBIT-4, has been previously applied to reduce mtDNA release by inhibiting VDAC1 oligomerization, which had protective significance in lupus-like diseases (57). VBIT-4 has also been reported to alleviate the neuropathological features of Alzheimer's disease mouse models by targeting VDAC1(58). In addition, VDAC1 inhibition has been achieved using CRISPR/Cas9 gene editing technology to alter its active site (59). However, this approach is currently limited to scientific research, where owing to the biological homology of VDAC1, knockout has not achieved protective results. In addition, the clinical use of the antidepressant sertraline (Sert) has been reported to act as an autophagy inducer, targeting and antagonizing VDAC1 expression and enhancing autophagy (60). Exploring the application of Sert as a protective measure against noise-induced hearing loss presents a potential intervention strategy. Consequently, targeting VDAC1 may offer a novel approach for future hearing loss treatments. It is necessary to further explore the mechanism of VDAC1 protein modification in different types of sensory cells involved in hearing loss, such as hair cells and spiral neurons, to integrate the protein modification strategies of VDAC1 and provide theoretical evidence for research on hearing protection.

In summary, there is a possible association between VDAC1 and hearing loss through mitochondrial autophagy, but there are unsolved research gaps. Further exploration and research are needed to determine which sensory cells are regulated by VDAC1 under conditions of hearing loss injury and whether this regulation can induce mitochondrial autophagy or cell death. Furthermore, little is known regarding the specific regulatory mechanism of targets in the ubiquitination process of VDAC1. The advancement in VDAC1 inhibitors for Alzheimer's and type 2 diabetes, along with the significance of ubiquitination regulation in drug development, offers a novel approach for future hearing loss treatment and prevention.