Mechanisms of Cadmium Neurotoxicity
镉神经毒性机制

Abstract 抽象的

Cadmium is a heavy metal that increasingly contaminates food and drink products. Once ingested, cadmium exerts toxic effects that pose a significant threat to human health. The nervous system is particularly vulnerable to prolonged, low-dose cadmium exposure. This review article provides an overview of cadmium’s primary mechanisms of neurotoxicity. Cadmium gains entry into the nervous system via zinc and calcium transporters, altering the homeostasis for these metal ions. Once within the nervous system, cadmium disrupts mitochondrial respiration by decreasing ATP synthesis and increasing the production of reactive oxygen species. Cadmium also impairs normal neurotransmission by increasing neurotransmitter release asynchronicity and disrupting neurotransmitter signaling proteins. Cadmium furthermore impairs the blood–brain barrier and alters the regulation of glycogen metabolism. Together, these mechanisms represent multiple sites of biochemical perturbation that result in cumulative nervous system damage which can increase the risk for neurological and neurodegenerative disorders. Understanding the way by which cadmium exerts its effects is critical for developing effective treatment and prevention strategies against cadmium-induced neurotoxic insult.
镉是一种重金属，对食品和饮料产品的污染日益严重。一旦摄入，镉就会产生毒性作用，对人类健康构成重大威胁。神经系统特别容易受到长期、低剂量镉的影响。这篇综述文章概述了镉的主要神经毒性机制。镉通过锌和钙转运蛋白进入神经系统，改变这些金属离子的稳态。一旦进入神经系统，镉就会通过减少 ATP 合成和增加活性氧的产生来扰乱线粒体呼吸。镉还通过增加神经递质释放的异步性和破坏神经递质信号蛋白来损害正常的神经传递。镉还会损害血脑屏障并改变糖原代谢的调节。总之，这些机制代表了多个位点的生化扰动，导致累积的神经系统损伤，从而增加神经系统和神经退行性疾病的风险。了解镉发挥其作用的方式对于制定针对镉引起的神经毒性损伤的有效治疗和预防策略至关重要。

1. Introduction 一、简介

Cadmium is a highly toxic pollutant that permeates environmental, industrial, and agricultural spaces. The Agency for Toxic Substances and Disease Registry ranked cadmium as the seventh most hazardous substance to human health [1], and the Department of Health and Human Services listed cadmium as a known human carcinogen in 2021 [2]. Recent anthropogenic activities have increased human exposure to cadmium. Most commercial cadmium is a byproduct of zinc ore mining that is used in electroplating, battery production, paint pigments, and plastics [3,4]. These activities introduce cadmium to the agricultural sphere, where plants readily absorb cadmium from contaminated soil and water. Additionally, cadmium contamination of ethanol is common, with variable levels detected in wine, beer, whiskey, gin, and other alcoholic products [5]. As a result, the most common source of exposure for the general population is contaminated food and drink products [3].
镉是一种剧毒污染物，渗透到环境、工业和农业空间中。有毒物质和疾病登记局将镉列为对人类健康第七大危害物质[ 1 ]，美国卫生与公众服务部于2021年将镉列为已知的人类致癌物[ 2 ]。最近的人类活动增加了人类对镉的接触。大多数商业镉是锌矿开采的副产品，用于电镀、电池生产、油漆颜料和塑料 [ 3 , 4 ]。这些活动将镉引入农业领域，植物很容易从受污染的土壤和水中吸收镉。此外，乙醇的镉污染很常见，在葡萄酒、啤酒、威士忌、杜松子酒和其他酒精产品中检测到的镉含量各不相同[ 5 ]。因此，普通人群最常见的接触源是受污染的食品和饮料产品[ 3 ]。

Cadmium enters the human body by various routes. Uptake is facilitated by the ingestion of contaminated food and beverage products, the inhalation of aerosolized cadmium particles in cigarette smoke, and particle accumulation in the olfactory bulb following industrial fume exposure [6,7]. Due to its abiogenic nature, cadmium has no endogenous mechanism of clearance and thus exhibits a low urinary excretion rate. It accumulates in the human body with an estimated half-life of up to 23.5 years [8]. As a result of this accumulation, the estimated mass of cadmium within adults in the U.S. and Europe who have not been occupationally exposed to cadmium is between 9.5 mg and 40 mg [9]. Moreover, blood concentrations of cadmium were found to be ~0.4 µg/L [10] and cerebrospinal fluid (CSF) concentrations of cadmium were found to be 72 ng/L in humans [10,11]. Thus, CSF concentrations of cadmium in humans are only roughly five-fold lower than in blood.
镉通过多种途径进入人体。摄入受污染的食品和饮料、吸入香烟烟雾中的雾化镉颗粒以及接触工业烟雾后嗅球中的颗粒积聚都会促进镉的吸收[ 6 , 7 ]。由于其非生物性质，镉没有内源性清除机制，因此尿排泄率较低。它在人体内蓄积，估计半衰期长达 23.5 年 [ 8 ]。由于这种积累，美国和欧洲未因职业接触镉的成年人体内的镉含量估计在 9.5 毫克至 40 毫克之间[ 9 ]。此外，人类的血液镉浓度约为 0.4 µg/L [ 10 ]，脑脊液 (CSF) 镉浓度为 72 ng/L [ 10 , 11 ]。因此，人体脑脊液中的镉浓度仅比血液中的镉浓度低大约五倍。

Chronic accumulation of cadmium results in multiorgan toxicity, primarily targeting the kidney, skeleton, liver, and nervous system [12], reviewed in [9]. Among these, the nervous system is a particularly vulnerable target for cadmium toxicity. Cadmium can increase risk of peripheral neuropathy, altered equilibrium, and poor performance on visuomotor tasks [13]. Exposure to cadmium is correlated with reduced concentration, poorer cognitive function in older adults, and adverse learning outcomes in children [13,14,15,16]. Cadmium exposure has also been associated with neurodegenerative disease pathologies observed in Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS) [17,18,19]. Cadmium exerts its neurotoxic outcomes via diverse means [12,20,21] (Figure 1). Here, we review the current knowledge concerning the sites of exogenous cadmium insult that result in nervous system dysfunction.
镉的慢性蓄积会导致多器官毒性，主要针对肾脏、骨骼、肝脏和神经系统[ 12 ]，详见[ 9 ]。其中，神经系统是镉毒性特别脆弱的目标。镉会增加周围神经病变、平衡改变和视觉运动任务表现不佳的风险[ 13 ]。接触镉与老年人注意力下降、认知功能较差以及儿童学习成绩不良相关[ 13,14,15,16 ]。镉暴露还与阿尔茨海默病 (AD)、帕金森病 (PD) 和肌萎缩侧索硬化症( ALS ) 中观察到的神经退行性疾病病理相关[ 17,18,19 ]。镉通过多种方式发挥其神经毒性作用[12,20,21 ] （图1 ） 。在这里，我们回顾了有关导致神经系统功能障碍的外源性镉损伤部位的最新知识。

Figure 1. The diverse major pathways by which cadmium can increase risk for neurodegenerative disease. Cadmium accumulates in the olfactory bulb following inhalation. When either inhaled or ingested, cadmium passes into the bloodstream, which can decrease the integrity of the blood–brain barrier (BBB) via weakening of tight junctions. This allows cadmium to enter into nervous system tissue. Once within the nervous tissue, cadmium can efficiently pass the cellular membrane by co-opting transporters for other divalent cations. The primary mechanisms of neurotoxicity are disruption of glycogen metabolism, changes to neurotransmitter signaling, and mitochondrial disruption leading to oxidative stress. These perturbations together increase the risk for neurodegenerative disease.
图 1.镉增加神经退行性疾病风险的多种主要途径。镉吸入后会积聚在嗅球中。当吸入或摄入时，镉会进入血液，通过削弱紧密连接来降低血脑屏障（BBB）的完整性。这使得镉进入神经系统组织。一旦进入神经组织，镉就可以通过选择其他二价阳离子的转运蛋白来有效地穿过细胞膜。神经毒性的主要机制是糖原代谢的破坏、神经递质信号传导的改变以及导致氧化应激的线粒体破坏。这些干扰共同增加了神经退行性疾病的风险。

2. Cadmium Entry to the Nervous System
2. 镉进入神经系统

Cadmium gains entry to the nervous system primarily by oral ingestion, at which point it is absorbed into the bloodstream and can damage the blood–brain barrier (BBB) to accumulate within nervous system tissue. Cadmium inhalation provides an even more direct route to the nervous system, since the olfactory epithelium lacks protection offered by the BBB and permits cadmium uptake directly into nervous tissue [22]. Cadmium is similar to bioessential metal cations implicated in neuronal transmission, particularly calcium and zinc. Cadmium, calcium, and zinc are primarily divalent cations that possess similar chemical properties and favor the oxidation state of +2. Calcium and cadmium share similar ionic radii (0.97 Å and 0.99 Å, respectively) and charge/radius ratios (Ca²⁺ = 2.02 e/Å, Cd²⁺ = 2.06 e/Å), granting each the ability to exert similarly strong electrostatic forces on biogenic macromolecules (reviewed in [23]). Cadmium and zinc are elements in Group IIB of the periodic table with the same electron configuration, allowing similar chemical behavior within ion-protein interaction. In this way, cadmium can permeate nervous system cells and organelles by taking advantage of endogenous zinc- and calcium-specific transporters.
镉主要通过口服进入神经系统，然后被吸收到血液中，并会损害血脑屏障（BBB）并在神经系统组织内积聚。镉吸入为神经系统提供了更直接的途径，因为嗅觉上皮缺乏血脑屏障提供的保护，允许镉直接吸收到神经组织中[ 22 ]。镉与参与神经元传递的生物必需金属阳离子类似，特别是钙和锌。镉、钙和锌主要是二价阳离子，具有相似的化学性质并有利于+2氧化态。钙和镉具有相似的离子半径（分别为 0.97 Å 和 0.99 Å）和电荷/半径比（Ca ²⁺ = 2.02 e/Å，Cd ²⁺ = 2.06 e/Å），从而赋予各自施加类似强静电的能力对生物大分子的作用力（综述于[ 23 ]）。镉和锌是元素周期表第 IIB 族的元素，具有相同的电子构型，因此在离子-蛋白质相互作用中具有相似的化学行为。通过这种方式，镉可以利用内源性锌和钙特异性转运蛋白渗透神经系统细胞和细胞器。

Several studies have implicated cadmium as a competitive voltage-gated calcium channel (VGCC) inhibitor [24,25,26]. Cadmium enters the rat cerebellar granular neuron primarily through dihydropyridine-sensitive (L-type) VGCCs as it competes with Ca²⁺ for within the channel pore. Exposure to 100 µM cadmium prevented an increase in cytosolic calcium concentration after neuronal depolarization, and cadmium was able to permeate the neuron. The N-type VGCC is also implicated in cadmium-induced blockage of Ca²⁺ current in frog sympathetic neurons [27]. Cadmium completely and rapidly blocked Ca²⁺ current at voltages when Ca²⁺ channels are primarily open (0 to +30 mV), indicating that the N-type VGCC is a route of cadmium entry into sympathetic neurons. Because VGCCs are densely concentrated at the presynaptic site, the presynaptic terminal is a notable location of cadmium uptake in neuronal cells (reviewed in [28]).
多项研究表明镉是一种竞争性电压门控钙通道 (VGCC) 抑制剂 [ 24 , 25 , 26 ]。镉主要通过二氢吡啶敏感（L 型）VGCC 进入大鼠小脑颗粒神经元，因为它与 Ca ²⁺竞争通道孔内的空间。暴露于 100 µM 镉可阻止神经元去极化后胞质钙浓度的增加，并且镉能够渗透神经元。 N 型 VGCC 还与镉诱导的青蛙交感神经元 Ca ²⁺电流阻断有关 [ 27 ]。当Ca ²⁺通道主要打开时（0至+30 mV），镉在电压下完全且快速地阻断Ca ²⁺电流，表明N型VGCC是镉进入交感神经元的途径。由于 VGCC 密集地集中在突触前位点，因此突触前末梢是神经元细胞中镉摄取的一个显着位置（[ 28 ]中有综述）。

Cadmium also enters neuronal cells through zinc transporters, the most significant of which are the ZIP6 and ZnT3 transporters [28,29]. ZIP6, an importer, is localized to the plasma membrane of hippocampal pyramidal neurons while ZnT3 is an exporter plentiful on the presynaptic neuronal membrane that regulates the brain’s vesicular pool [30,31], reviewed in [22]. Mimouna et al. found that early-life cadmium exposure increased cadmium accumulation in the brain, increased ZIP6 gene expression, and decreased ZnT3 expression [29]. The simultaneous upregulation of the ZIP6 importer and downregulation of the ZnT3 exporter may lead to cadmium accumulation in these neurons. In a later study, Mimouna et al. investigated interactions between cadmium and ZnT3 in hippocampal neurons. Treatment of rat hippocampal neurons with cadmium chloride (0, 0.5, 5, 10, 25, or 50 µM) and zinc chloride (0, 10, 30, 50, 70, or 90 µM) for either 24 or 48 h downregulated ZnT3 mRNA expression, an effect attenuated by the application of zinc. Zinc supplementation at 30 µM significantly ameliorated cadmium-induced neurotoxicity in cells treated with 10 and 25 µM cadmium [32]. Presumably, the physicochemical similarities between cadmium and zinc allow cadmium to enter synaptic vesicles through ZnT3 and accumulate, ultimately resulting in cell death and disruption of neuronal plasticity.
镉还通过锌转运蛋白进入神经元细胞，其中最重要的是 ZIP6 和 ZnT3 转运蛋白 [ 28 , 29 ]。 ZIP6 是一种输入蛋白，位于海马锥体神经元的质膜上，而 ZnT3 是一种输出蛋白，大量存在于突触前神经元膜上，调节大脑的囊泡池 [ 30 , 31 ]，详见 [ 22 ]。米穆纳等人。发现生命早期接触镉会增加大脑中镉的积累，增加 ZIP6 基因表达，并降低 ZnT3 表达[ 29 ]。 ZIP6 输入蛋白的同时上调和 ZnT3 输出蛋白的下调可能导致这些神经元中镉的积累。在后来的研究中，Mimouna 等人。研究了海马神经元中镉和 ZnT3 之间的相互作用。用氯化镉（0、0.5、5、10、25 或 50 µM）和氯化锌（0、10、30、50、70 或 90 µM）处理大鼠海马神经元 24 或 48 小时，下调 ZnT3 mRNA表达，锌的应用减弱了这种效应。在用 10 µM 和 25 µM 镉处理的细胞中，补充 30 µM 锌可显着改善镉诱导的神经毒性 [ 32 ]。据推测，镉和锌之间的物理化学相似性使得镉能够通过 ZnT3 进入突触小泡并积累，最终导致细胞死亡和神经元可塑性破坏。

3. Cadmium Effects on Mitochondrial Respiration
3. 镉对线粒体呼吸的影响

Mitochondria in the nervous system perform critical roles not only in energy production [33] but also in neuronal development, function, and survival [34]. Neurons, the functional unit of the nervous system, are particularly high consumers of ATP due to their constant need to maintain the neuronal concentration gradient necessary for action potential propagation, operate the cellular machinery associated with the vesicle cycle, facilitate axonal transport, and provide energy for synaptic plasticity [33,34,35]. Thus, any disruption in mitochondrial function can result in energy deficits, significantly compromising neural activity and health.
神经系统中的线粒体不仅在能量产生中发挥着关键作用[ 33 ]，而且在神经元发育、功能和存活中也发挥着关键作用[ 34 ]。神经元是神经系统的功能单位，是 ATP 的高消耗者，因为它们不断需要维持动作电位传播所需的神经元浓度梯度、操作与囊泡循环相关的细胞机制、促进轴突运输并提供能量突触可塑性[33,34,35 ] 。因此，线粒体功能的任何破坏都可能导致能量不足，从而严重损害神经活动和健康。

Oxidative phosphorylation relies on a strong mitochondrial membrane potential (ΔΨm) in order to produce ATP via ATP synthase [35]. The electron transport chain (ETC), embedded within the inner mitochondrial matrix uses the potential energy from electron-carrying molecules in order to produce a robust ΔΨm. The four protein complexes that comprise the ETC must deftly handle redox molecules in order to appropriately produce a proton gradient, the basis of the ΔΨm. Furthermore, reactive oxygen species (ROS) are produced at low concentrations as a byproduct of the ETC. Low levels of ROS can be mitigated by antioxidant molecules within the mitochondria, such as glutathione. However, if ROS are allowed to proliferate, either via external influence or inappropriate regulation of the ETC, the resultant oxidative stress results in cellular damage.
氧化磷酸化依赖于强大的线粒体膜电位 (ΔΨm)，以便通过 ATP 合酶产生 ATP [ 35 ]。嵌入线粒体内部基质中的电子传递链 (ETC) 利用电子携带分子的势能来产生强大的 ΔΨm。组成 ETC 的四种蛋白质复合物必须巧妙地处理氧化还原分子，以便适当地产生质子梯度，这是 ΔΨm 的基础。此外，作为 ETC 的副产品，会产生低浓度的活性氧 (ROS)。线粒体内的抗氧化剂分子（例如谷胱甘肽）可以缓解低水平的 ROS。然而，如果允许 ROS 增殖，无论是通过外部影响还是 ETC 的不当调节，所产生的氧化应激都会导致细胞损伤。

This ΔΨm gradient can be regulated via mitochondrial uncoupling proteins, which can serve to respond to cellular energetic needs, maintain consistent temperature, or control osmotic swelling. However, various pathological conditions can disrupt ΔΨm, leading to impaired mitochondrial respiration. For instance, mitochondrial permeability transition pore (PTP) opening can be triggered by factors like oxidative stress that can result in ΔΨm depolarization [36]. Such depolarization can inhibit ATP synthesis and compromise overall mitochondrial function.
这种 ΔΨm 梯度可以通过线粒体解偶联蛋白进行调节，线粒体解偶联蛋白可以响应细胞的能量需求、保持一致的温度或控制渗透膨胀。然而，各种病理状况会破坏 ΔΨm，导致线粒体呼吸受损。例如，氧化应激等因素可以触发线粒体通透性转换孔（PTP）开放，从而导致 ΔΨm 去极化[ 36 ]。这种去极化会抑制 ATP 合成并损害线粒体的整体功能。

The significance of mitochondria becomes most apparent in the context of neurodegenerative diseases, including AD, PD, and ALS. These conditions are characterized by mitochondrial dysfunction [35]. Abnormalities encompass impaired energy production, heightened ROS production, and compromised calcium handling, collectively contributing to neuronal degeneration and the clinical manifestations of these diseases [36].
线粒体的重要性在神经退行性疾病（包括 AD、PD 和 ALS）的背景下变得最为明显。这些病症的特点是线粒体功能障碍[ 35 ]。异常包括能量产生受损、ROS 产生增加和钙处理受损，共同导致神经元变性和这些疾病的临床表现[ 36 ]。

3.1. Cadmium Interference with the Electron Transport Chain
3.1.镉对电子传输链的干扰

The mitochondria have emerged as primary targets in cadmium toxicity (for an excellent review focusing exclusively on this topic, see [37]). This is supported by experimental evidence in a rodent model, where cadmium exposure on isolated mitochondria from mouse livers led to extensive organelle damage [38]. One mechanism by which cadmium disrupts mitochondrial function is by interfering with specific protein complexes within the ETC such that ΔΨm is reduced and the proton-motive force that drives ATP synthesis is subsequently weakened. Cadmium interacts with Complex I of the ETC at both the Q-binding site and the NADH-binding site, decreasing the ability of Complex I to shuttle electrons and transport protons to create and maintain ΔΨm. Cadmium’s interaction with the Q_o site of Complex III redirects ROS production toward the intermembrane space, effectively bypassing the matrix antioxidant defenses [39,40]. By disrupting the normal function in Complexes I and III, the resultant decrease in ΔΨm ultimately leads to a decreased ability to efficiently synthesize ATP and increase in damaging cytosolic ROS.
线粒体已成为镉毒性的主要目标（有关专门关注该主题的优秀评论，请参阅[ 37 ]）。啮齿动物模型中的实验证据支持了这一点，其中小鼠肝脏分离线粒体的镉暴露导致广泛的细胞器损伤[ 38 ]。镉破坏线粒体功能的一种机制是干扰 ETC 内的特定蛋白质复合物，从而减少 ΔΨm，从而削弱驱动 ATP 合成的质子动力。镉与 ETC 复合物 I 在 Q 结合位点和 NADH 结合位点相互作用，降低复合物 I 穿梭电子和传输质子以产生和维持 ΔΨm 的能力。镉与复合物 III 的 Q _o位点的相互作用将 ROS 的产生重定向到膜间空间，有效地绕过基质抗氧化防御 [ 39 , 40 ]。通过破坏复合物 I 和 III 的正常功能，由此产生的 ΔΨm 减少最终导致有效合成 ATP 的能力下降，并增加破坏性胞质 ROS。

3.2. Cadmium Opens the Permeability Transition Pore
3.2.镉打开渗透性转变孔

Furthermore, cadmium induces the opening of the permeability transition pore (PTP), a dynamic protein complex residing at the interface between the inner and outer mitochondrial compartments [41]. The PTP allows for the diffusion of small molecules through the inner mitochondrial membrane, dissipating the ΔΨm and thereby halting ATP synthesis. Opening of the PTP also acts as a signal for apoptosis via release of stores of cytochrome C. The weakening of the inner mitochondrial gradient itself can trigger opening of the PTP in a feedforward mechanism that results in eventual cell death. It is not clear to what extent cadmium opens the PTP via weakening of the ΔΨm in mechanisms described above, or whether cadmium directly interacts with the PTP itself to increase the likelihood of opening, or both.
此外，镉会诱导渗透性过渡孔（PTP）的打开，这是一种位于线粒体内外隔室之间界面的动态蛋白质复合物[ 41 ]。 PTP 允许小分子通过线粒体内膜扩散，消散 ΔΨm，从而停止 ATP 合成。 PTP 的打开还通过释放细胞色素 C 储备来充当细胞凋亡的信号。线粒体内部梯度本身的减弱可以在前馈机制中触发 PTP 的打开，从而导致最终的细胞死亡。目前尚不清楚镉在上述机制中通过削弱 ΔΨm 在多大程度上打开 PTP，或者镉是否直接与 PTP 本身相互作用以增加打开的可能性，或两者兼而有之。

There is some evidence to suggest that cadmium directly interacts with the PTP to increase opening, independent of cadmium’s effects on ΔΨm. Cadmium interacts with a constituent of the PTP complex, the adenine nucleotide translocator (ANT), at the thiol groups present on the cysteine residues, potentially leading to modifications of ANT function [39]. ANT exchanges cytosolic ADP and matrix ATP, enabling cytosolic ATP export out of the mitochondria while delivering ADP to the mitochondria [39,40]. Structural studies have shown that ADP/ATP exchange of ANT proteins occurs via an “induced transition fit” model. This process begins with ADP binding at the “c-state”, where the protein is exclusively open to the intermembrane space. This binding triggers a conformational shift to “m-state”, where the protein becomes exclusively open to the mitochondrial matrix, facilitating the exchange of ADP for ATP [42]. Inhibiting ANT blocks this cadmium-induced PTP opening [43]. This cadmium-induced PTP opening can also be blocked via addition of an inhibitor of the mitochondrial calcium importer, indicating that cadmium is gaining access to the inner mitochondria via this transporter [39].
有一些证据表明镉直接与 PTP 相互作用以增加开口，与镉对 ΔΨm 的影响无关。镉与 PTP 复合物的成分、腺嘌呤核苷酸易位子 (ANT) 在半胱氨酸残基上的硫醇基团处相互作用，可能导致 ANT 功能的改变 [ 39 ]。 ANT 交换胞质 ADP 和基质 ATP，使胞质 ATP 从线粒体输出，同时将 ADP 输送到线粒体 [ 39 , 40 ]。结构研究表明，ANT 蛋白的 ADP/ATP 交换是通过“诱导过渡拟合”模型发生的。该过程始于 ADP 在“c 状态”的结合，此时蛋白质完全向膜间空间开放。这种结合触发构象转变为“m 状态”，此时蛋白质完全向线粒体基质开放，促进 ADP 与 ATP 的交换 [ 42 ]。抑制 ANT 可阻断镉诱导的 PTP 开放 [ 43 ]。这种镉诱导的 PTP 打开也可以通过添加线粒体钙输入抑制剂来阻断，这表明镉正在通过这种转运蛋白进入线粒体内部 [ 39 ]。

Furthermore, while calcium can induce PTP opening via a cyclosporin A (CsA)-dependent mechanism [43], cadmium opens the PTP independent from this calcium-CsA pathway. Because increased calcium is a potent intracellular signal within neurons for apoptosis, cadmium’s ability to bypass calcium-induced mechanisms of apoptosis represent an alternative pathway for unregulated neuronal death. The regulatory mechanisms of this CsA-independent apoptotic pathway are as yet unclear.
此外，钙可以通过环孢素 A (CsA) 依赖性机制诱导 PTP 打开 [ 43 ]，而镉则独立于钙-CsA 途径打开 PTP。由于钙的增加是神经元内细胞凋亡的有效细胞内信号，因此镉绕过钙诱导的细胞凋亡机制的能力代表了不受调节的神经元死亡的另一种途径。这种不依赖于 CsA 的凋亡途径的调节机制尚不清楚。

3.3. Recent Focuses of Mitochondrial Apoptosis/Dysfunction: ER Stress and SIRT1
3.3.线粒体凋亡/功能障碍的最新焦点：ER 应激和 SIRT1

It is increasingly clear that the relationship between mitochondrial stress and endoplasmic reticulum (ER) stress both contribute to cadmium’s toxic effects. According to multiple studies, cadmium exposure may induce crosstalk between the stress responses of the ER and mitochondria, which culminates in cell apoptosis [44,45]. One important aspect of this crosstalk is the involvement of proapoptotic proteins Bim and Bax, which are upregulated following acute cadmium exposure. Bax, in particular, translocates from the cytosol to the mitochondria causing the apoptosis of the mitochondria. Furthermore, cadmium exposure leads to the release of cytochrome c from the mitochondria to the cytoplasm. This release triggers apoptotic signaling and activates caspases, leading to cell apoptosis [44,46,47].
越来越清楚的是，线粒体应激和内质网（ER）应激之间的关系都会导致镉的毒性作用。根据多项研究，镉暴露可能会引起内质网和线粒体应激反应之间的串扰，最终导致细胞凋亡 [ 44 , 45 ]。这种串扰的一个重要方面是促凋亡蛋白 Bim 和 Bax 的参与，它们在急性镉暴露后表达上调。特别是 Bax 从细胞质转移到线粒体，导致线粒体凋亡。此外，镉暴露会导致细胞色素c从线粒体释放到细胞质。这种释放触发细胞凋亡信号并激活半胱天冬酶，导致细胞凋亡[ 44,46,47 ]。

Furthermore, exposure of cadmium to human cell lines led to an increase of intracellular ROS levels in a dose dependent manner. This generation of ROS occurred in a feed-forward fashion that ultimately induces GADD153, a marker that initiates cell death [45]. A protective measure against this process is the antioxidant resveratrol, which inhibits ER stress and GADD153 and activates sirtuin1 (SIRT1) [48].
此外，人类细胞系暴露于镉会导致细胞内ROS水平以剂量依赖性方式增加。这一代 ROS 以前馈方式发生，最终诱导 GADD153，这是一种启动细胞死亡的标记物 [ 45 ]。针对这一过程的保护措施是抗氧化剂白藜芦醇，它抑制 ER 应激和 GADD153 并激活 Sirtuin1 (SIRT1) [ 48 ]。

More recent research has also pointed towards SIRT1 as a critical regulator of the biochemical response to oxidative stress [45]. SIRT1 is a nicotinamide dinucleotide (NAD⁺)-dependent deacylases known for its ability to regulate cellular processes such as DNA repair, inflammation, fatty acid oxidation, fat differentiation, and more [48]. This suppression of SIRT1 by cadmium leads to a marked increase in oxidative stress within neuronal cells. The ensuing oxidative stress disrupts mitochondrial function, which, in turn, culminates in the death of neural cells. This phenomenon has been observed in both PC12 cells, a neuron-like cell line, and primary rat cerebral cortical neurons [45]. Activating SIRT1 prevented the buildup of ROS and cellular loss and expounds on a potential mechanism by which SIRT activators affect SIRT1 activity, particularly by deacetylating PGC-1a. This deacetylation is believed to contribute to the enhancement of oxidative metabolism, playing a crucial role in the cellular response to oxidative stress [49]. SIRT1 is structurally important for the nervous system as it promotes axonal elongation, neurite outgrowth, and dendritic branching. Furthermore, it has been found to be crucial for memory formation and its protective measures against neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and motor neuron diseases [50,51,52,53].
最近的研究还指出 SIRT1 是氧化应激生化反应的关键调节因子 [ 45 ]。 SIRT1 是一种烟酰胺二核苷酸 (NAD ⁺ ) 依赖性脱酰酶，以其调节 DNA 修复、炎症、脂肪酸氧化、脂肪分化等细胞过程的能力而闻名 [ 48 ]。镉对 SIRT1 的抑制导致神经元细胞内氧化应激显着增加。随之而来的氧化应激会破坏线粒体功能，最终导致神经细胞死亡。这种现象已在 PC12 细胞（一种神经元样细胞系）和原代大鼠大脑皮层神经元中观察到[ 45 ]。激活 SIRT1 可以防止 ROS 的积累和细胞损失，并阐述了 SIRT 激活剂影响 SIRT1 活性的潜在机制，特别是通过去乙酰化 PGC-1a。这种脱乙酰化被认为有助于增强氧化代谢，在细胞对氧化应激的反应中发挥着至关重要的作用[ 49 ]。 SIRT1 在结构上对神经系统很重要，因为它促进轴突伸长、神经突生长和树突分支。此外，它被发现对于记忆形成及其针对阿尔茨海默氏症、帕金森氏症和运动神经元疾病等神经退行性疾病的保护措施至关重要[ 50,51,52,53 ]。

The precise molecular mechanism underlying cadmium-induced neurotoxicity in the context of mitochondria-associated ER membranes (MAMs) remains unclear. MAMs consist of a diverse array of proteins, including mitofusin 2 (Mfn2), voltage-dependent anion channel (VDAC), and glucose-regulated protein 75 (Grp75). These proteins facilitate the transport of calcium ions from the ER to the mitochondria through the inositol 1,4,5-triphosphate receptors (IP3R) on the ER and the voltage-dependent anion-selective channel protein (VDAC) on the mitochondria [49]. Exposure to cadmium increased expression of Mfn2, Grp75, and VDAC1 [52]. Additionally, both PC12 cells and primary neurons exhibited a significant reduction in mitochondrial calcium uptake when Mfn2 was knocked out in response to cadmium treatment. Notably, this unveiled that the principal driver of cadmium-induced autophagy in neuronal cells may be the uptake of mitochondrial calcium facilitated by MAMs, specifically that the IP3R-Grp75-VDAC1 complex is regulated by Mfn2. The interplay between Mfn2 and the operation of the IP3R-Grp75-VDAC1 complex represents a breakthrough in the understanding of mitochondrial dysfunction following cadmium exposure [50].
线粒体相关内质网膜 (MAM) 中镉诱导的神经毒性的确切分子机制仍不清楚。 MAM 由多种蛋白质组成，包括线粒体融合蛋白 2 (Mfn2)、电压依赖性阴离子通道 (VDAC) 和葡萄糖调节蛋白 75 (Grp75)。这些蛋白质通过 ER 上的肌醇 1,4,5-三磷酸受体 (IP3R) 和线粒体上的电压依赖性阴离子选择性通道蛋白 (VDAC) 促进钙离子从 ER 转运至线粒体 [ 49 ] 。暴露于镉会增加 Mfn2、Grp75 和 VDAC1 的表达[ 52 ]。此外，当镉处理导致 Mfn2 被敲除时，PC12 细胞和原代神经元的线粒体钙吸收均显着减少。值得注意的是，这揭示了镉诱导神经元细胞自噬的主要驱动因素可能是 MAM 促进的线粒体钙的摄取，特别是 IP3R-Grp75-VDAC1 复合物受 Mfn2 调节。 Mfn2 与 IP3R-Grp75-VDAC1 复合物的运作之间的相互作用代表了对镉暴露后线粒体功能障碍的理解的突破 [ 50 ]。

3.4. Cadmium-Induced Autophagy
3.4.镉诱导的自噬

Autophagy, a regulated form of cell death, involves a series of steps directing targeted materials to the lysosome for recycling (for an extensive review focusing on autophagy in neurodegenerative diseases, see [51]). Cadmium-induced autophagy is associated with neurodegenerative disease, though the nature of the relationship remains somewhat controversial [52,53,54]. Hence, this section focuses on recently published papers on the topic of cadmium-induced autophagy.
自噬是一种受调节的细胞死亡形式，涉及一系列将目标材料引导至溶酶体进行回收的步骤（有关神经退行性疾病中自噬的广泛综述，请参阅[ 51 ]）。镉诱导的自噬与神经退行性疾病有关，尽管这种关系的性质仍然存在一些争议[ 52,53,54 ]。因此，本节重点介绍最近发表的有关镉诱导自噬主题的论文。

Since autophagy is a key process for eliminating excess protein, it is thought that disruption in autophagy process via cadmium can result in excess misfolded protein leading to neurodegenerative disease [12]. Cadmium triggers neuronal apoptosis through an increase in autophagosome formation, marked by elevated LC3-II and p62 in neuronal cells, resulting in neuronal apoptosis [55]. The drug rapamycin prevents cadmium-induced increase in LC3-II and p62. Cadmium-induced apoptosis is dependent on the overproduction of autophagosomes by preventing autophagosome–lysosome fusion [55,56,57]. However, other recent studies have shown that cadmium inhibits autophagy through calcium-dependent activation of the JNK signaling pathway in a cell culture model [58].
由于自噬是消除过量蛋白质的关键过程，因此人们认为，通过镉破坏自噬过程可能会导致过量的错误折叠蛋白质，从而导致神经退行性疾病[ 12 ]。镉通过增加自噬体形成来触发神经元凋亡，其标志是神经元细胞中 LC3-II 和 p62 升高，从而导致神经元凋亡 [ 55 ]。药物雷帕霉素可防止镉诱导的 LC3-II 和 p62 增加。镉诱导的细胞凋亡依赖于自噬体的过量产生，通过阻止自噬体-溶酶体融合[ 55,56,57 ]。然而，最近的其他研究表明，镉在细胞培养模型中通过钙依赖性激活 JNK 信号通路来抑制自噬 [ 58 ]。

Recent research has advanced the study of ameliorative strategies for preventing cadmium-induced changes to autophagic flux. Potentilla anserine, an herb native to the Qinghai–Tibet Plateau of China, is renowned for its nutrient richness and application in Chinese medicine. Emerging research highlights Potentilla anserine polysaccharide (PAP), a major bioactive component of this herb, as a candidate to prevent oxidative stress, mitochondrial cell death, and apoptosis [59,60,61,62]. PAP potentially mitigates cadmium-induced neuronal death via autophagy by suppressing the PI3K class III/Beclin-1 signaling pathway [63]. Interestingly, drugs that increase autophagy also seem to have some promise in preventing cadmium-induced neurotoxic damage. Linagliptin, an FDA-approved antidiabetic drug used to treat type 2 diabetes, also shows neuroprotective effects against cognitive decline [64,65]. Studies of linagliptin’s neuroprotective effects against cadmium exposure in rats have shown that linagliptin prevented the cognitive deficit induced by cadmium. However, linagliptin stimulated the hippocampal AMPK/mTOR pathway, which positively impacts autophagy progression. It is thought that this increase in autophagy stimulated clearance of neuronal misfolded proteins, resulting in improvement in cognitive impairment in this context [66]. Together, these results point toward the need for the further exploration of cadmium’s role in autophagic processes.
最近的研究推进了预防镉诱导的自噬通量变化的改善策略的研究。鹅委陵菜是一种原产于中国青藏高原的草本植物，以其营养丰富和在中药中的应用而闻名。新兴研究强调委陵菜多糖（PAP）是这种草药的主要生物活性成分，可作为预防氧化应激、线粒体细胞死亡和细胞凋亡的候选药物[59,60,61,62 ] 。 PAP 通过抑制 PI3K III 类/Beclin-1 信号通路，可能通过自噬减轻镉诱导的神经元死亡 [ 63 ]。有趣的是，增加自噬的药物似乎也有望预防镉引起的神经毒性损伤。利格列汀是 FDA 批准的用于治疗 2 型糖尿病的抗糖尿病药物，也显示出针对认知能力下降的神经保护作用 [ 64 , 65 ]。利格列汀对大鼠镉暴露的神经保护作用的研究表明，利格列汀可以预防镉引起的认知缺陷。然而，利格列汀刺激海马 AMPK/mTOR 通路，从而对自噬进展产生积极影响。据认为，自噬的增加刺激了神经元错误折叠蛋白的清除，从而改善了这种情况下的认知障碍[ 66 ]。总之，这些结果表明需要进一步探索镉在自噬过程中的作用。

4. The Role of Cadmium in Synaptic Transmission
4. 镉在突触传递中的作用

The synapse itself is a vulnerable target for cadmium toxicity. For the efficient transmission of a neuronal signal, biological metal cations must act in conjunction with a series of voltage-gated and ligand-gated channels. Cadmium’s physicochemical similarities to these ions, particularly calcium and zinc, permit its neurotoxicity at the synaptic level as cadmium permeates the presynaptic neuron, induces oxidative stress, and ultimately aggravates neuronal degeneration.
突触本身是镉毒性的脆弱目标。为了有效传输神经元信号，生物金属阳离子必须与一系列电压门控和配体门控通道协同作用。镉与这些离子（特别是钙和锌）的物理化学相似性，使其在突触水平具有神经毒性，因为镉渗透到突触前神经元，诱导氧化应激，并最终加剧神经元变性。

4.1. Cadmium-Induced Asynchronous Neurotransmitter Release
4.1.镉诱导的异步神经递质释放

Synchrony of neurotransmitter release is a marker of efficacious neural communication. The release of a neurotransmitter occurs within hundreds of milliseconds following the action potential to ensure precise communication between neurons [28]. Indeed, several studies have linked asynchronous release to neurodegenerative disease pathologies in AD, spinal muscular atrophy (SMA), and ALS [67,68,69,70]. Cadmium may augment asynchronous neurotransmitter release, further aggravating these neurodegenerative disease pathologies.
神经递质释放的同步是有效神经通讯的标志。神经递质的释放发生在动作电位后数百毫秒内，以确保神经元之间的精确通信[ 28 ]。事实上，一些研究已将异步释放与 AD、脊髓性肌萎缩症 (SMA) 和 ALS 中的神经退行性疾病病理联系起来 [67,68,69,70 ] 。镉可能会增加异步神经递质释放，进一步加剧这些神经退行性疾病的病理。

Cadmium application of 0.1 µM desynchronized neurotransmitter release in the distal compartment of the frog nerve terminal. This asynchrony was accompanied by a sharp increase in mitochondrial ROS production and lipid peroxidation, suggesting that cadmium-induced oxidative stress co-occurs with this desynchronization. Desynchronization was completely blocked by the administration of antioxidants and NADPH-oxidase inhibitors [55]. One possible mechanism of this asynchrony relies on cadmium’s action as a VGCC antagonist in addition to its role as initiator of oxidative stress. Extracellular cadmium likely replaced native calcium as the metal ion flowing through L-type VGCCs. This decrease of calcium inward current in the presence of cadmium leads to a blunted presynaptic spike of cytosolic calcium, which is integral for the coordination of vesicular machinery. Therefore, in the presence of cadmium, VGCCs must remain open for a longer period of time to allow sufficient calcium influx for the initiation of calcium-dependent presynaptic processes. The prolonged period in which VGCCs are open may lengthen the delay observed between the arrival of the depolarizing action potential and neurotransmitter release, accounting for the observed asynchrony.
在青蛙神经末梢的远端室中应用 0.1 µM 去同步神经递质释放的镉。这种异步性伴随着线粒体活性氧产生和脂质过氧化的急剧增加，表明镉诱导的氧化应激与这种去同步性同时发生。给予抗氧化剂和 NADPH 氧化酶抑制剂可以完全阻断去同步化[ 55 ]。这种异步性的一种可能机制依赖于镉除了作为氧化应激引发剂的作用之外还作为 VGCC 拮抗剂的作用。细胞外镉可能取代天然钙作为流经 L 型 VGCC 的金属离子。镉存在下钙内向电流的减少导致胞质钙的突触前尖峰减弱，这对于囊泡机械的协调是不可或缺的。因此，在存在镉的情况下，VGCC 必须保持较长时间的开放状态，以允许足够的钙流入，从而启动钙依赖性突触前过程。 VGCC 长时间开放可能会延长去极化动作电位到达和神经递质释放之间观察到的延迟，从而解释了观察到的异步性。

4.2. Cadmium Disruption of Neurotransmission
4.2.镉对神经传递的干扰

In addition to delaying neurotransmitter release, cadmium disrupts neurotransmitter packaging within synaptic vesicles, decreasing the amount of neurotransmitter available for each release event. There is particular evidence for this in glutamatergic neurons. Vesicular transporters rely on the proton electrochemical gradient generated by V-ATPase to package neurotransmitters into vesicles [71,72]. A volume of 50 µM of cadmium in isolated Wistar rat synaptosomes caused the dissipation of the proton gradient necessary to package glutamate into its synaptic vesicles, resulting in decreased depolarization-evoked exocytosis of glutamate and reduced extracellular glutamate concentration [73]. Although the mechanism by which the V-ATPase is disrupted was not directly observed, interaction with the thiol groups in the cysteine residues of the V-ATPase is a likely culprit.
除了延迟神经递质释放外，镉还会破坏突触小泡内的神经递质包装，减少每次释放事件可用的神经递质数量。在谷氨酸能神经元中有特别的证据表明这一点。囊泡转运蛋白依靠 V-ATP 酶产生的质子电化学梯度将神经递质包装到囊泡中 [ 71 , 72 ]。分离的 Wistar 大鼠突触体中 50 µM 的镉会导致将谷氨酸包装到突触囊泡中所需的质子梯度消散，导致去极化引起的谷氨酸胞吐作用减少，并降低细胞外谷氨酸浓度 [ 73 ]。尽管没有直接观察到 V-ATP 酶被破坏的机制，但与 V-ATP 酶半胱氨酸残基中的硫醇基团的相互作用可能是罪魁祸首。

Additionally, cadmium exposure has been observed to induce changes in cholinergic muscarinic receptors and acetylcholinesterase (AChE) variants [74]. Specifically, Cd²⁺ exposure documented an elevation of the gene expression of AChE-S (the synaptic variant) while reducing the gene expression of AChE-R (the readthrough variant). This modification in AChE variants has been linked to cell death in these neurons. Moreover, cadmium treatment disrupts muscarinic receptors, particularly the M1 and M3 receptors, which play crucial roles in the regulation of memory and learning processes. This interference with the receptors may contribute to the cognitive impairments observed following exposure to cadmium. Although the precise mechanisms by which cadmium alters muscarinic receptors and AChE variants remain incompletely elucidated, oxidative stress has been posited as a potential intermediary factor in this process.
此外，已观察到镉暴露会引起胆碱能毒蕈碱受体和乙酰胆碱酯酶（AChE）变体的变化[ 74 ]。具体而言，Cd ²⁺暴露记录了 AChE-S（突触变体）的基因表达升高，同时降低了 AChE-R（通读变体）的基因表达。 AChE 变体的这种修饰与这些神经元的细胞死亡有关。此外，镉治疗会破坏毒蕈碱受体，特别是 M1 和 M3 受体，它们在调节记忆和学习过程中发挥着至关重要的作用。这种对受体的干扰可能会导致接触镉后观察到的认知障碍。尽管镉改变毒蕈碱受体和乙酰胆碱酯酶变体的精确机制尚未完全阐明，但氧化应激已被认为是这一过程中的潜在中间因素。

5. Cadmium and Other Metals
5. 镉和其他金属

5.1. Cadmium Disruption of Zinc Signaling and Homeostasis
5.1.镉破坏锌信号传导和体内平衡

Zinc, which itself can protect against cadmium-induced hippocampal neurotoxicity [25], decreased quantal release and markedly desynchronized neurotransmitter release at a concentration of 25 µM. Zinc can function as either a prooxidant or antioxidant in cellular systems, with both excesses and deficiencies resulting in oxidative stress (reviewed in [75]). Zinc-induced oxidative stress has been connected to neurodegeneration and cell death in cultured cortical neurons [44,47] and AD [45]. The influx of cadmium through zinc transporters may disrupt this zinc allostasis, resulting in exacerbated oxidative stress. Therefore, zinc and cadmium may act synergistically to induce oxidative stress in presynaptic terminals, ultimately resulting in decreased quantal release and asynchrony that advance neurodegeneration.
锌本身可以防止镉引起的海马神经毒性[ 25 ]，在浓度为 25 µM 时，会减少量子释放并显着使神经递质释放不同步。锌可以在细胞系统中充当促氧化剂或抗氧化剂，过量和缺乏都会导致氧化应激（[ 75 ]中综述）。锌诱导的氧化应激与培养的皮质神经元 [ 44 , 47 ] 和 AD [ 45 ] 中的神经变性和细胞死亡有关。镉通过锌转运蛋白的流入可能会破坏这种锌的动态平衡，导致氧化应激加剧。因此，锌和镉可能协同作用，诱导突触前末梢氧化应激，最终导致量子释放减少和异步，从而促进神经退行性变。

The downregulation of the zinc transporter ZnT3 resulting from cadmium exposure results in downstream effects that affect critical signaling pathways in the brain. This downregulation initiates a cascade that decreases hippocampal brain-derived neurotrophic factor-tropomyosin receptor kinase B (BDNF-TrkB) and Erk1/2 signaling, intracellular messengers that play integral roles in neuronal plasticity and growth [50]. The TrkB neurotrophin receptor and subsequent BDNF activation are essential for advancing neuronal plasticity, and antidepressant binding to neurotrophin receptors, particularly TrkB, has been previously evidenced to facilitate BDNF activation and initiate neuronal plasticity [