1 INTRODUCTION

Vitiligo is an autoimmune disease, characterized by skin depigmentation. The autoimmune system attacks and eventually destroys epidermal melanocytes (Ezzedine et al., 2015). Around 0.5%–2% of world's population suffers from vitiligo (Bergqvist and Ezzedine, 2020) which severely affects the quality of life and mental health in patients due to the feeling of shame created by skin depigmentation (Alikhan et al., 2011; Elbuluk and Ezzedine, 2017). However, the specific pathogenesis of vitiligo still remains unknown. Not only adaptive but also innate immune system appeared to be involved in it (Picardo et al., 2015; Wang et al., 2019). Recently, interferon (IFN)-γ-induced chemokines and cytotoxic CD8+ T cells are widely accepted for their pivotal role in the autoimmune-related responses in vitiligo (Frisoli and Harris, 2017; Strassner and Harris, 2016). Pigment cells finally come to death after the attack of autoimmune system under cellular stress (Figure 1) (Rork et al., 2016). The pigment cells in vitiligo patients are more vulnerable to environmental stress under the genetic background than healthy individuals. Reactive oxygen species (ROS) are originated from mitochondria in response to oxidative stress, and they disrupt the homeostasis of organelles such as endoplasmic reticulum (ER), mitochondria, and lysosome. All the above factors could further stimulate the production and secretion of exosomes or other pro-inflammatory molecules (Bergqvist and Ezzedine, 2020; Laddha et al., 2014). Recent research has revealed that exosomes excreted by melanocytes also play a crucial role in autoimmune activation. The exosomes contain melanocyte-specific antigens, miRNAs, heat-shock proteins, and other damage-associated molecular patterns (Thulasingam et al., 2011; Wong et al., 2020; Hu et al., 2020; Henning et al., 2020), which could be discerned by antigen-presenting cells such as dendritic cells nearby and accelerate the mature of auto-reactive T cells (Al-Shobaili and Rasheed, 2014; Passeron and Ortonne, 2012). The matured CD8+ T cells can secret cytokines such as IFN-γ, whose binding of IFN-γ to its receptor could then activate the JAK-STAT pathway and generate C-X-C motif chemokine ligand (CXCL)-9 and CXCL10 secretion in the skin. These chemokines promote positive feedback in recruiting melanocyte-specific CD8+ T cells to the skin and increase inflammation. Finally, the melanocytes die by autoimmune system attack, resulting in vitiligo (Wańkowicz-Kalińska et al., 2003; Xie et al., 2016; Tokura et al., 2020; Frisoli et al., 2020). Despite the widely accepted theory outlined above, many specific mechanisms underlying vitiligo remain unknown. Thus, vitiligo has remained one of the most refractory diseases in dermatological clinic (Rodrigues et al, 2017; Bergqvist and Ezzedine, 2021).

Vitiligo pathogenesis. Melanocytes finally come to death after the attack of autoimmunity under cellular stress which could lead to the impairment of organelles such as ER stress and mitochondria dysfunction. IFN-γ-induced chemokines and cytotoxic CD8+ T cells are pivotal. All the stressors could further stimulate the production and secretion of exosomes or other pro-inflammatory molecules, which could be delivered to DCs, followed by initiation of T helper 17 cells. The CD8+ T cells secrete IFN-γ and then lead to CXCL9 and CXCL10 secretion. The CXCLs further accelerate the recruitment of melanocyte-specific CD8+ T cells. Finally, the apoptosis of melanocytes in the lesions causes vitiligo. ER, endoplasmic reticulum; IFN-γ, interferon-γ; ROS, reactive oxygen species; DCs, dendritic cells; CXCL, C-X-C motif chemokine ligand

2 UNFOLDED PROTEIN RESPONSE (UPR) SYSTEM AND MELANOCYTES SURVIVAL 2.1 The role of UPR in melanocytes survival

Unfolded protein system consists of a series of reactions that repair damaged proteins under environmental suppression, so as to maintain survival and basal function of cells. The system is critical in the pathogenesis of autoimmune diseases such as vitiligo, type 1 diabetes mellitus, Parkinson’s disease, and so forth (Shrestha et al., 2020; Maddaloni et al., 2020). The death of melanocytes attacked by immune cells under oxidative stress is the cornerstone of vitiligo pathogenesis. Because melanocytes are located in the epidermis's basal layer, they are constantly exposed to external stressors, including ultraviolet (UV), chemical toxins, heat shock, and so on (Smyrnias, 2021). Numerous organelles of melanocytes are dysfunctional in these conditions, particularly ER, whose dilation has been observed at the margins of lesions in vitiligo patients. UV-induced oxidative stress and calcium homeostasis dysregulation (chemical toxins that modulate calcium ion channels) are the primary causes of ER injury. As the main place for protein synthesis and processing, ER damage could cause accumulation of unfolded/misfolded proteins, impairing melanocytes' normal function. At this moment, the intervention UPR system holds a critical function (Li et al., 2017; Park et al., 2019; Kitamura, 2013). UPR system is one of the evolutionarily conserved survival mechanisms that enable melanocytes to recover following exposure-induced damage (Kitamura, 2013; Shpilka and Haynes, 2018; John, 2016; Ron and Walter, 2007). They primarily function via the transcription and translation of proteins. In addition, the anti-oxidative response mediated by nuclear factor erythroid 2-related factor 2 (NRF2) is also vital for coping with external stimuli such as UV damage, chemical stimulation, and so on (Merksamer and Papa, 2010; Ron and Walter, 2007). However, if UPR system fails to repair the damage caused by various unknown factors or cell defects, the damaged cells will barely survive and will induce diseases or die directly. In melanoma, UPR system support UV-damaged and mutated melanocytes (tumor cells) to survive under extremely barren micro-environment (tumor micro-environment) such as hypoxia, extracellular acid environment, lack of glucose, and lactic acid accumulation. (Jadeja et al., 2020; Cheng et al., 2013). In vitiligo, sustained stress can trigger apoptosis of melanocytes instead of mutation (Merksamer and Papa, 2010).

2.2 The pathways of UPR in melanocytes

Three trans-membrane proteins of ER: inositol-requiring enzyme 1α (IRE1α), protein kinase R-like endoplasmic reticulum kinase (PERK), and activating transcription factor 6 (ATF6) serve as cellular stress monitors and initiate UPR pathway signaling cascades (Merksamer and Papa, 2010). Without external stimuli, the three monitors are bound to chaperone: 78-kDa glucose-regulated protein (GRP78), effectively deactivating the system. On the contrary, when unfolded/misfolded proteins accumulate, GRP78 binds to the three sensors at ER membrane. Simultaneously, sensors are released, activating UPR pathways (Figure 2) (Almanza et al., 2019; Bartoszewska and Collawn, 2020).

Unfolded proteins induced UPR system under ER stress. Three trans-membrane proteins of ER: IRE1α, PERK, ATF6 are served as the monitors of cellular stress and initiate the signaling cascades of UPR pathway. In unstressed conditions, the 3 proteins are associated with GRP78 in ER. Accumulation of unfolded proteins causes GRP78 disassociation them. The activation of IRE1α splices XBP1 mRNA and further transcribes the transcription factor which regulate the expression of target genes such as IL-6 and IL-8. ATF6 is transferred to Golgi complex and then sheared by protease. The coalition of unfolded protein-GRP78 from PERK phosphorylates EIF2α, the transcription of non-essential proteins are reduced. Next, the phosphorylation of eIF2α generates ATF4 translation which is served as the role in regulating the expression of adaptive genes in protecting melanocytes from environmental stress. While ER homeostasis is restored, EIF2α is dephosphorylated through the mechanism of DNA damage-inducible protein and G1 cell arrest. IRE1α, inositol-requiring enzyme 1α; ATF6, transcription factor 6; PERK, protein kinase R-like endoplasmic reticulum kinase; GRP78, 78-kDa chaperone glucose-regulated proteins; XBP1, X-box binding protein 1; IL, interleukin; EIF2α, Then the α subunit of eukaryotic initiation factor 2

2.2.1 IRE1α pathway

IRE1α is activated and up-regulated in response to excessive accumulation of unfolded/misfolded proteins under ER stress. IRE1α is highly conserved in evolution and mediates a signal in transcriptional network, which re-establishes cellular proteostasis (Toosi et al., 2012). IRE1α activation initiates from GRP78 dissociation, which then accelerates dimerization and phosphorylation. Subsequently, IRE1α activation splices X-box binding protein 1 (XBP1) mRNA and further transcribes the transcription factor which regulates target gene expression. These downstream genes serve as a bridge between oxidative stress and pro-inflammatory cytokine production (Kitamura, 2013; Shpilka and Haynes, 2018; Siwecka et al., 2021). The expressions of cytokines such as interleukin (IL)-6 and IL-8 have been demonstrated to increase in vitiligo. They participate in the attack of autoimmune-targeted melanocytes through XBP1 pathway in vitiligo (Covino et al., 2018). Numerous genes are transcribed and translated from spliced-XBP1 mRNA. These genes are involved in lipid synthesis and metabolism, cell proliferation, apoptosis, immune and inflammatory responses, Ca2+ homeostasis, and ER-related proteostasis, all of which function to alleviate ER impairment and restore organelles homeostasis (So, 2018; Manga and Choudhury, 2020; Lin and Haynes, 2016).

2.2.2 ATF6 pathway

ATF6 and IRE1α are intimately linked. ATF6 activation could up-regulate IRE1α expression. Under ER stress, ATF6 is transferred to Golgi complex and then sheared by proteases (Park et al., 2019). The activated segment of ATF6 is generated from Golgi and then transported into the endonuclease part, where it functions as an effective transcription factor. ATF6 downstream in UPR system regulates The transcription of restoration-related genes (Rozpedek et al., 2016; Manga et al., 2010).

2.2.3 PERK pathway

Similar to IRE1α and ATF6, dissociating GRP78 from PERK activates the downstream signaling pathways in UPR (Toosi et al., 2012). Then, the alpha subunit of eukaryotic initiation factor 2 (EIF2α) is phosphorylated, and non-essential protein transcription and translation are temporarily reduced for ER restoration. Following that, EIF2α phosphorylation causes ATF4 translation, whose transcript contains regulatory sequences such as short upstream open reading frames (Cullinan and Diehl, 2006). ATF4 serves as a regulator of the expression of adaptive genes, which are intimately linked to the conservation mechanisms that protect melanocytes from environmental stress (Cullinan and Diehl, 2006; Wortel et al., 2017). While ER homeostasis is restored, EIF2α is dephosphorylated through the mechanism of DNA damage-inducible protein and G1 cell arrest (Nguyen et al., 2009). In another branch of the pathway, NRF2 could also be activated by PERK, which further regulates antioxidant response-related gene expression: heme oxygenase-1 (Elassiuty et al., 2011). The antioxidant element protects melanocytes from oxidative stress caused by UV and chemicals such as monobenzyl ether of hydroquinone, 4-tert-butylcatechol, 4-tert-amylphenol, 4-tertiary butyl phenol, and so on (Arowojolu et al., 2017; Harris, 2017). Thus, in vitiligo patients, PERK-NRF2 axis dysfunction makes melanocytes more vulnerable to oxidative irritants and challenging to repair (Bae et al., 2019). The destructed melanocytes are more sensitive to autoimmune system attacks. Interestingly, vitiligo patients have a lower incidence of melanoma than the general population. Why does it happen that vitiligo and melanoma often stand oppositely? Melanocytes with defects in UPR regulation are more susceptible to the autoimmune system rather than mutating into tumor cells, which could be one answer to the question above (Burton et al., 2020). In addition, autophagy PERK-ATF4 pathway may regulate autophagy via lysosomal associated membrane protein 3, which is also a decisive process in the fate of melanocytes (Qiao et al., 2016). However, if there are some errors in the sequence of elements along this pathway or if melanocytes are subjected to suffer from sustained external stimuli without ER restoration, the UPR system would finally shift ER restoration to cell death response. This response is mediated by over-expression of CAAT box/enhancer-binding protein/homologous protein (Bae et al., 2019).

3 DAMAGED PREMELANOSOME PROTEIN (PMEL) IN MELANOCYTES

The UPR system activated by unfolded/misfolded proteins is essential in melanocyte survival under environmental stress. As a misfolded protein, damaged PMEL is toxic to melanocytes. Therefore, damaged PMEL and accumulated toxic PMEL amyloid oligomers under ER stress contribute to vitiligo pathogenesis. Amyloids are a kind of protein aggregates that can be cytotoxic. The accumulation and deposition of superfluous amyloid fibrils are responsible for pathogenesis of many diseases, especially Parkinson’s disease (Morel and Conejero-Lara, 2019). Under physiological conditions, polymerization of prefibrillar oligomers and the final maturation of amyloids prevent the occurrence of such diseases (Jiang et al., 2020). PMEL, also known as PMEL17, gp100, ME20, gp85, and silver, is a melanosome-specific matrix protein synthesized and matured in melanocytes. As one of the amyloids, much attention has been paid to the role of PMEL in the synthesis of melanosomes and vitiligo pathogenesis (Koffie et al., 2009). Melanocytes that synthesize and process the maturation of PMEL fibrils should be investigated for their particular mechanisms to avoid potential accumulation of toxic protein aggregates (Kundu et al., 2019).

3.1 The role of PMEL in melanosome synthesis

The epidermis pigments are primarily composed of melanin granules synthesized and exocytosed from melanocytes (Kim et al., 2020). Melanin synthesis increases in response to oxidative stress, such as UV, to resist external stimuli. However, melanin is potentially toxic and acts as a “foreign object” for cells. Accordingly, melanin is insulated during its transport and storage as melanosomes (Kundu et al., 2021). As the matrix sheet of melanosomes, the correct biosynthesis and maturation of PMEL are critical. Similar to amyloid deposition in Parkinson’s disease, misfolded PMEL oligomers contribute to toxic amyloid accumulation in melanocytes. Multi-vesicular endosomes are the place for PMEL fibrils biosynthesis (Figure 3) (Rashighi and Harris, 2017). Firstly, Mα segment of PMEL is cleaved by β-APP cleaving enzyme protease (BACE) 2 and then assembled with the help of an adaptor: intraluminal vesicles (ILVs). This process requires the meditation of a cluster of CD63 and apolipoprotein E (ApoE). The remaining parts, such as C-terminal fragments, are isolated and cleaved by presenilin 2 of γ-secretase complex in lysosomes (Rashighi and Harris, 2017; Bissig et al., 2016; Watt et al., 2013). Finally, melanin deposits on matrix-forming PMEL amyloids sheets, which are described as matured melanosomes (Koffie et al., 2009; Rashighi and Harris, 2017).

PMEL amyloidogenesis in melanocytes. Mα segment of PMEL is cleaved β-APP cleaving enzyme protease 2 BACE2 and then assembled with the help of ILVs. This process needs the meditation of CD63 and ApoE. Melanin particles are deposited on the structure of PMEL amyloid sheets to form mature melanosomes. ApoE, and apolipoprotein E; BACE2, β-APP cleaving enzyme protease 2; CD, cluster of differentiation; ILVs, intraluminal vesicles

3.2 The role of PMEL in vitiligo pathogenesis

Due to the presence of melanocyte-specific antibodies in the serum of vitiligo patients, the role of humoral immunity in vitiligo pathogenesis has been described. Additionally, B-cell epitope domains on PMEL could be recognized (Niel et al., 2015). Under normal physiological conditions, PMEL does not cause pathological effects as BACE2 mediates PMEL cleavage during maturation of final melanosomes and helps eliminate unnecessary amyloid fibrils and their intermediates (Yuan et al., 2019; Kemp et al., 2001). Interestingly, some recent Parkinson’s disease therapies demonstrated depigmentation in inpatients or animals. For instance, lanabecestat, a BACE1 inhibitor, was found to induce vitiligo and post-inflammatory hypopigmentation in patients during the clinical trial of Parkinson’s disease (Wessels et al., 2020). AM-6494 and PF-06751979, acting as BACE1 inhibitors, were found to induce depigmentation in mice during experiments (Pettus et al., 2020; O'Neill et al., 2018). As these BACE1 inhibitors also serve as non-selective BACE2 inhibitors, PMEL sheet matrix formation processes are interrupted (Niel, 2016). In addition, further studies established a link between PMEL and vitiligo activity and confirmed that vitiligo is an autoimmune skin disease mainly mediated by CD8+ T cells. However, the particular mechanism of activating melanocyte antigen-specific T cells remains unclear. In the background of UPR system defects, activating autoimmunity by uncleared amyloid fibrils through a series of pathways may become one of the mechanisms (Rochin et al., 2013; Mandelcorn-Monson et al., 2003; Li et al., 2021).

4 TRANSIENT RECEPTOR POTENTIAL (TRP) CHANNELS/AUTOPHAGY AND MELANOCYTES 4.1 Ca2+ homeostasis in organelles mediated by TRP channels

When the balance of UPR system is broken, ER restoration could fail, and the cells undergo apoptosis. Additionally, autophagy is an evolutionarily conserved survival mechanism in response to cellular stress. As mentioned previously, autophagy activation enables cells to survive under oxidative stress in UPR system (Figure 4). Autophagy dysfunction is a possible mechanism in vitiligo pathogenesis by disrupting the redox balance of melanocytes. When organelles are damaged under external stress, autophagy meditates the process of engulfing and degrading cytoplasmic proteins, impaired organelles, and vesicles (Qiao et al., 2016). Protein hydrolases degrade target cargoes in autophagic lysosomes, thereby fulfilling cells' metabolic requirements and regenerating certain organelles (Park et al., 2019). In vitiligo patients, organelles could be injured by redox potential (e.g., oxidative stress induced by UV exposure) or Ca2+ homeostasis disorder (e.g., chemicals that modulate Ca2+ channels) (Kitamura, 2013; Gong et al., 2015). Studies have proved that the equilibrium of ions, especially Ca2+ in organelles, is critical in organelle homeostasis. Autophagy could be activated when Ca2+ levels become unbalanced after ROS damage in ER, mitochondria, and lysosomes (Qiao et al., 2016; Ling et al., 2017). Ca2+ influx contributes to melanocytes undergoing mitochondria-dependent apoptosis under oxidative stress (Yumnam et al., 2016; Kang et al., 2018). Recently, transient receptor potential (TRP) channels played pivotal roles in the vitiligo autophagy process (Ozdemir et al., 2016). Numerous studies have demonstrated that TRPM2 activates in response to oxidative stress, resulting in melanocyte cell apoptosis through TRPM2-mediated Ca2+ influx into mitochondria (Ozdemir et al., 2016; Xu and Ren, 2015; He et al., 2017).

The role of autophagy and UPR system through the stress-immune interface in vitiligo. The secretion of melanin in pigment cells requires energy and releases ROS as a byproduct. ROS is generated from mitochondria. The processes of melanin synthesis are in risks of generating misfolded proteins and the presence of intracellular ROS. The misfolded proteins activate UPR system, which may initiate autophagy and stimulate the production and secretion of exosomes or other pro-inflammatory cytokines. Exosomes could further activate antigen processing cells and then induce melanocyte-specific autoimmunity. Environmental exposures and genetic background modulate these pathways in vitiligo. ROS, reactive oxygen species; UPR, unfolded protein response

4.2 The imbalance of autophagy and apoptosis via TRP channels

To survive under cellular stress, autophagy is initiated to make melanocytes degrading unfolded/misfolded proteins as well as impaired organelles (Patel and Cai, 2015; Barygina et al., 2019). Lysosomal degradation plays a critical role in autophagy process. As one of the most important second messengers, Ca2+ homeostasis in lysosomes greatly maintains the normal function of lysosomal signaling and degradation (Puertollano and Kiselyov, 2009). Recently, TRP channels are the most intensively studied cation channels (mainly Ca2+) (Xie and Li, 2019; Cheng et al., 2010). TRP channels are predominantly expressed on cytomembrane. However, the members of TRPML subfamily are all expressed in the endolysosomal system (Samie et al., 2013; Medina et al., 2011). A Ca2+ concentration gradient normally exists between lysosome and cytoplasm. When TRPML1 is activated, Ca2+ flows from lysosome to cytoplasm. As a second messenger, Ca2+ influx generated by TRPML1 opening mediates several autophagic biological processes through calmodulin (CaM)/adenosine 5’-monophosphate-activated protein kinase (AMPK) and CaM/transcription factor EB (TFEB) pathways. These processes include lysosome to trans-Golgi-network trafficking, lysosome-autophagosome fusion, and lysosomal exocytosis (Medina et al., 2011; Carroll and Dunlop, 2017; Santoni et al., 2020; Zurli et al., 2020; Wen et al., 2019; González et al., 2020; Xu and Dong, 2021). As a reactive ROS sensor, in addition to autophagy regulation, TRPML1 acts as an adaptor to regulate lysosome–mitochondria communications through Ca2+ dynamics (Peng et al., 2020). Except for TRPML1, TRPML2 and TRPML3 are also under investigation. Until now, a prominent link to human diseases has remained unconfirmed. However, because these two channels share functional and structural similarities, they may also play a role in autophagy regulation (Remis et al., 2014). For example, the mutation of TRPML3 shows coat color dilution in mice with melanocyte loss induced by intracellular Ca2+ overload (Kim et al., 2019; Cecconi and Jäättelä, 2014; Hall et al., 2014; Spix et al., 2020). Studies have corroborated that high autophagic flux is essential for melanocytes to protect themselves from oxidative stress outside (Remis et al., 2014; Xu et al., 2007). Until now, TRPML3 has already been found to express in human epidermal melanocytes and participate in lysosome–autophagosome fusion (Kim et al., 2019).

Thus, the balance between apoptosis and autophagy in melanocytes may also be a factor in vitiligo pathogenesis. Ca2+ homeostasis in organelles is critical for melanocytes under environmental stress. TRP channels are particularly pivotal. In vitiligo, oxidative stress-induced mitochondrial dysfunction results in melanocyte apoptosis via TRPM2 activation. On the other hand, activating TRPML1 on lysosome enhanced autophagy, eliminating injured organelles and unfolded/misfolded proteins, protecting cells to survive from oxidative stress (Figure 5) (Xu and Ren, 2015; Xie and Li, 2019; Santoni et al., 2020; Cecconi and Jäättelä, 2014). We believe that maintaining a balance between autophagy and apoptosis via TRP channels enables melanocytes to survive under oxidative stress.

The regulation of autophagy through TRP channels. Autophagy is activated to help cells digest damaged or non-essential proteins and organelles. By releasing intraluminal Ca2+, TRPMLs have been implicated in intracellular Ca2+ signaling, endolysosome trafficking, and lysosomal functions, further regulating autophagy. As well, TRPM2-mediated Ca2+ influx contributes to mitochondria-dependent apoptosis of melanocytes under oxidative stress. Thus, the balance between apoptosis and autophagy in melanocytes may also be one of the pathogenesis of vitiligo. TRP, transient receptor potential channels

5 CONCLUSIONS

Vitiligo is an autoimmune disease characterized by skin depigmentation. The quest for etiological agents resulted in biochemical and autoimmune theories. Vitiligo is not only caused by genetic factors or melanocyte stressors such as oxidative response induced by UV and chemicals but also by autoimmune hyper-reactive T cells, which target vulnerable melanocytes (Byrne et al., 2014). Consequently, it is vital to specify the mechanisms by which melanocytes are vulnerable to autoimmune system destruction under environmental stressors (Shimshek et al., 2016). This narrative review presented a new concept of UPR/PMEL-TRP channels/autophagy axis and described step-by-step how it could be involved in vitiligo pathogenesis. The axis comprises an activated UPR system, damaged PMEL accumulation, and autophagy imbalance between mitochondrial and lysosomes via TRP channels (Figure 6).

The mechanism of damaged PMEL/UPR-TRP channels/autophagy axis system in vitiligo. PMEL is synthesized in ER and transported to the Golgi. It initiates amyloid fibril sheet formation within MVE. The amyloid sheets have been proposed to accelerate melanin polymerization, which finally lead to the mature melanosomes. Thus, PMEL must navigate the secretory pathway from the ER to endosomes in a non-amyloid form. When suffering from the stimulation of external stressors, the unfolded/misfolded PMEL caused by ER stress activate autoimmunity through autoantibodies as well as CD8+ T cells in vitiligo. It may also act through the UPR system, leading to the apoptosis of melanocytes. In addition, the abnormal process of PMEL fibril formation could induced the accumulation of toxic amyloid oligomers. Subsequently, the accumulation of toxic melanin intermediates which could not deposit on amyloid sheet could activate autoimmunity to accelerate the apoptosis of melanocytes. (1): Accumulation of toxic amyloid oligomers; (2): Accumulation of toxic melanin intermediates; PMEL, premelanosome protein 17; ROS, reactive oxygen species; ER, endoplasmic reticulum; UPR, unfolded protein response; MVE, multivesicular endosomes

The intrinsic UPR defects of melanocytes decrease their capacity to survive under environmental stimuli such as ROS production (e.g., ROS-induced heat-shock protein and oxidative potential) and impairs organelles, which gradually activates autoimmunity and finally leads to cell death (Frisoli and Harris, 2017; Theos et al., 2005; Theos et al., 2006). Vitiligo may be prevented if UPR system functions appropriately. That is to say, there is a much greater likelihood that something is wrong with whatever intrinsic or acquired defects of UPR system in melanocytes under sustained stress. The background of UPR dysfunction ultimately results in autoimmunity activation and stimulates the death of melanocytes, leading to vitiligo.

As a misfolded protein, damaged PMEL is intimately connected to UPR system. As a matrix for melanin polymerization, PMEL sheets are critical in maturation of melanosomes (Xie et al., 2016). The lack or mutation of PMEL gene is characterized by various hypopigmentation degrees (Ho et al., 2016). UPR system activation by misfolded PMEL and accumulation of toxic amyloid oligomers under ER stress contribute to vitiligo pathogenesis (Fowler et al., 2006; Schonthaler et al., 2006). Therefore, this process should be carefully regulated to prevent accumulation of toxic amyloid fibril oligomers (Periyasamy and Shinohara, 2017).

Autophagy is critical in cleaning up damaged organelles, toxic accumulation of protein fragments such as damaged PMEL. Additionally, numerous modules in UPR system exhibit a regulatory effect on autophagy through multiple pathways, as described above (Figure 2), which promotes the repair of organelles' homeostasis under external stimuli, making them less susceptible to autoimmunity attack (Patel and Cai, 2015). Ca2+ homeostasis in mitochondria and lysosomes is important in autophagy regulation (Yumnam et al., 2016; Kang et al., 2018). TRP channels have been demonstrated to participate in Ca2+ homeostasis. Under oxidative stress stimulation, TRPM2 channel expressed in mitochondria could be activated and open, leading to Ca2+ influx from cytoplasm into mitochondria. This process promotes the apoptosis of melanocytes and eventually causes depigmentation (Ozdemir et al., 2016; Xu and Ren, 2015). The opening of TRPML1 channel expressed on lysosome, which allows Ca2+ to flow into cytoplasm, promotes autophagy process, and engulfs damaged organelles (such as mitochondria), toxic cellular metabolic products, and so on (Medina et al., 2011; Carroll and Dunlop, 2017).

In conclusion, regulating UPR/PMEL-TRP channels/autophagy axis could be beneficial in rescuing melanocytes under environmental stress, providing prospects for vitiligo treatment. As a critical adaptor in UPR system, PERK mediates antioxidant responses via NRF2 pathway (Figure 2). Facilitating the activation of pathways may be beneficial in reviving dying melanocytes. Several studies found that activating melanocortin-1 receptor (MC1R) by Nle4-D-Phe7-melanocyte-stimulating hormone attenuates oxidative stress and neuronal apoptosis through NRF2 pathway in mice. The highly selective MC1R agonist pentapeptide could be employed as a skin pigmentation enhancer in mice. However, the relationship between skin pigmentation and NRF2 activation requires further validation (Fu et al., 2020; Jackson et al., 2019). Numerous NRF2 agonists demonstrated the effect of cell protection, but none of them were studied in conjunction with melanocytes and vitiligo (Wu et al., 2021; Yu et al., 2021). As GRP78 dissociation from PERK activates downstream signaling pathways in UPR, chaperone GRP78 could also be a potential therapeutic target for vitiligo. For instance, chemical chaperone-conjugated exendin-4 was demonstrated to be a cytoprotective agent for pancreatic β-cells through reducing GRP78 expression (Son et al., 2018). However, regulating GRP78 in vitiligo treatment remained scarce. In addition to UPR system, excessive accumulation of injured PMEL is a perplexing issue. Although regulating UPR system promotes injured PMEL clearance, the proper maturation of PMEL sheet is also important for melanocyte survival. It may be beneficial to treat vitiligo by regulating BACE2 activation. However, studies examining the relationship between BACE and vitiligo remained scarce. In addition to UPR system, TRP channels also play an important role in autophagy regulation. As mentioned above, TRPML1 opening in lysosomes promotes autophagy in melanocytes, whereas mitochondrial TRPM2 opening promotes melanocyte apoptosis. We thought that the balance between the two processes could be a critical factor in cell survival under external stimuli. Accordingly, the agonist of TRPML1, as well as the antagonist of TRPM2, may promote melanocyte autophagy for cell survival under external stimuli. These TRP regulators could be potential vitiligo therapeutics. For instance, TRPML1 agonist ML-SA1 was demonstrated to protect neurons from L-BMAA neurotoxicity by promoting autophagic clearance (Tedeschi et al., 2019). However, whether this could be effective in melanocytes requires further investigation. Above all, given the uncertainty surrounding all these potential drug targets regarding UPR/PMEL-TRP channels/autophagy axis, as well as the mechanism of vitiligo, much additional future research is required.

ACKNOWLEDGEMENT

We acknowledge doctor Anni Wang (Hangzhou Hospital of Traditional Chinese Medicine) for providing technical support for our drawing of figures.

CONFLICTS OF INTEREST

The authors declare that they have no conflicts of interest.

AUTHOR CONTRIBUTION

Bo Xie and Xiuzu Song drafted the manuscript. All the authors edited and approved the final version of the manuscript.