BDNF-altering cell death mechanisms of brain disorders: pyroptosis and/or ferroptosis? A systematic review

Introduction

Cell death, survival, proliferation, and differentiation are all common processes in our bodies, and each one is critical to our overall health. These processes are also affected by increasing age [1], [2], [3]. Cell death is crucial for maintaining homeostasis in the body by removing damaged cells [4]. The first three categories used to categorize cell death were Type I (apoptosis), Type II (autophagy), and Type III (necrosis) [5]. In recent years, numerous novel cell death modalities, such as ferroptosis and pyroptosis, have been discovered and described in terms of corresponding stimuli, molecular mechanisms, and morphologies. Some of these modalities share signal paths that are overlapping yet not identical. Ferroptosis was first proposed in 2012 as a non-apoptotic, iron-dependent method of cell death characterized by the accumulation of lipid reactive oxygen species (ROS) [6]. Recent studies have demonstrated that ferroptosis plays a crucial regulatory role in the onset and progression of many diseases, and it has since become the focus and a hotspot of research on the treatment and prognosis improvement of related disorders [7]. In addition to ferroptosis, pyroptosis has recently gained an increasing amount of interest due to its link to innate immunity and illness. In 2001, coined the term pyroptosis, combining two Greek words, pyro (fire/fever) and ptosis (to-sis, falling), to describe pro-inflammatory cell death [8]. This process distinguished pyroptosis from other types of cell death, such as apoptosis (non-inflammatory program cell death). Pyroptosis research grew in popularity after the Gasdermins family was discovered. Previous research has linked the mechanisms of pyroptosis and ferroptosis to lower levels of brain-derived neurotrophic factor (BDNF) [9, 10]. This reduction in BDNF adversely affects brain health, contributing to cognitive impairment, depression, and anxiety, as BDNF is a crucial growth factor essential for neurogenesis, neuronal survival, and maintenance [11]. However, previous studies did not explain how BDNF is involved in the two mechanisms. Additionally, crosstalk between ferroptosis and pyroptosis has been reported in cases of COVID-19 [12] and autoimmune diseases [13], but no studies have examined this interaction in the context of brain health. Therefore, the purpose of this review is to elucidate the role of BDNF in the mechanisms of ferroptosis and pyroptosis and to explore the relationship between these mechanisms and brain health.

Materials and methods

This systematic review was conducted according to Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines [14]. This review summarizes pre-clinical studies that evaluated ferroptosis and pyroptosis mechanisms in various conditions that affect brain health, especially changes in the level of brain-derived neurotrophic factor (BDNF) level.

Risk of bias and calculations

The Systematic Review Centre for Laboratory Animal Experimentation’s (SYRCLE) 10-item bias risk assessment checklist was used to examine each of the articles that were included. With the use of SYRCLE’s Risk of Bias Tool-a screening tool created for risk of bias evaluation in animal research [27], potential biases impacting the chosen publications were assessed. A “yes” (green) score means a low potential for bias, a “no” (red) score means a high chance of bias, and an “unclear” (yellow) score means an unknown risk of bias. Two reviewers (AH and FW) independently assessed the quality of the included studies. Any discrepancies were resolved by discussion or by consulting a third reviewer (NFJ). A risk-of-bias summary table, generated automatically in Review Manager version 5.3, is included as (Figure 1). This Review Manager tool encompasses five domains: 1) randomization process, 2) deviations from intended interventions, 3) missing outcome data, 4) measurement of the outcome data, and 5) selection of the reported result. These domains are utilized to evaluate the risk of bias in the articles included in this review.

Figure 1:

Risk of bias in individual studies. The (+) indicates “low risk” of bias and the (!) indicates “some concerns” of bias. This risk of bias summary table was created automatically by review manager, version 5.3. This risk of bias summary table consists of 5 domains; 1) randomization process (D1), 2) deviations from intended interventions (D2), 3) missing outcome data (D3), 4) measurement of the outcome data (D4), and 5) selection of the reported result (D5).

Results

Study selection and data extraction

The electronic search performed yielded 95 articles in total with 18 articles from PubMed, 60 articles from Scopus, and 17 from Web of Science. After removing 33 duplicates, 62 articles were screened based on title and abstract. Of the 62 articles, 29 were screened further because 32 were unrelated articles and 1 was not an English article. The full text of 29 articles was screened and 15 articles were excluded for the following reasons: Twelve review articles, one chapter in a book, and two editorial articles. Finally, only 14 articles were included in this systematic review. The study selection process is summarized using a flowchart as shown in (Figure 2). Included study characteristics are summarized in (Table 1).

Figure 2:

The study selection process is summarized using the PRISMA flowchart.

Discussion

Stress [16], sleep deprivation [10, 17], and diabetes [15] all had different effects on ferroptosis mechanisms. All of these effects can lead to neurodegeneration caused by ferroptosis and have an impact on brain health. Hepcidin levels in type 2 diabetes mellitus (T2DM) decreased, causing changes in ferroportin regulation, which resulted in iron accumulation in the brain [15]. Iron accumulation can also be caused by the activation of iron regulatory proteins (IRPs) [10, 15, 17]. The binding of iron regulatory proteins (IRPs) with iron-responsive elements (IREs) results in the activation of transferrin receptor 1 (TfR1) and reduced iron storage which is ferritin, this process will lead to iron accumulation [28].

In some cases, ferritin levels are high to indicate excess iron in the intracellular [16]. Both conditions; 1) decreased clearance of iron due to reduced ferritin and 2) increased ferritin levels, indicate excess iron levels in the intracellular and lead to neuron damage [29]. Iron accumulation is more than just a process that contributes to ferroptosis mechanisms. In ferroptosis, an increase in reactive oxygen species (ROS) also contributes to the neurodegenerative process.

In ferroptosis mechanisms, reactive oxygen species (ROS) increase. This is because iron accumulation in ferroptosis mechanisms initiates the Fenton reaction, which increases ROS [10, 15–17]. The Fenton reaction is a reaction that occurs between iron and hydrogen peroxide. This reaction generates hydroxide and hydroxyl radicals leading to an increase in ROS [30]. Increased ROS levels will cause lipid degradation via the lipid peroxidation process [31]. This process contributes to neuronal loss in ferroptosis mechanisms by increasing malondialdehyde (MDA) levels [10, 15–17], a known marker of lipid peroxidation. This process is referred to as non-enzymatic lipid peroxidation [32].

Additionally, there is an enzymatic lipid peroxidation mechanism involving high expression of acyl-coenzyme A (CoA) synthetase long-chain family member 4 (ASCL4) [15, 17, 32]. Polyunsaturated fatty acids (PUFAs) are more prone to oxidation, so increased PUFA synthesis resulted in increased susceptibility to lipid peroxidation [33]. ASCL4 was involved in the PUFA synthesis mechanism by adding coenzyme A (CoA) to PUFAs. Then, lysophosphatidylcholine acyltransferase 3 (LPCAT3) transfers PUFAs-CoA into phospholipids, which are then oxidized by arachidonate lipoxygenase (ALOX) enzyme resulting in increased MDA lipid peroxidation in ferroptosis [34, 35]. MDA lipid peroxidation is a hallmark of ferroptosis mechanisms, and it can alter the structure and function of cell membranes, resulting in cell dysfunction and extensive tissue damage [36].

Lipid peroxidation worsens when the system x_c ⁻ cysteine/glutamate antiporter and Sirt1/Nrf2 signaling pathway do not function properly in the ferroptosis mechanism [10, 15–17] XCT/SLC7A11 is a light-chain subunit and CD98hc/SLC3A2 is a heavy-chain subunit, both composed of system x_c ⁻ cysteine/glutamate antiporter [37]. This antiporter is responsible for the uptake of extracellular cysteine and the transport of intracellular glutamate as an exchange. Cysteine is required for the synthesis of glutathione (GSH) and glutathione peroxidase 4 (GPX4), both of which function to reduce ROS and lipid peroxidation [38]. Sirt1/Nrf2 signaling can also reduce ROS. Sirtuin 1 (Sirt1) can regulate the transcription of nuclear erythroid factor 2-related factor 2 (Nrf2), which leads to the activation of antioxidant genes such as superoxide dismutase (SOD) and the reduction of ROS [39].

A decrease in brain-derived neurotrophic factors (BDNF) levels, in addition to iron accumulation, increased ROS levels, and lipid peroxidation, may also contribute to neurodegenerations caused by ferroptosis mechanisms. Reduced BDNF levels resulted in synapse impairment [15–17] since these studies report on the reduced protein levels of synaptophysin (SYN) and postsynaptic density protein 95 (PSD95). SYN is a glycoprotein that is present in the membrane of neuron synaptic vesicles [40] and PSD95 is a protein complex that is attached to the postsynaptic membrane [41], so the decreased level of both proteins indicated that synapse signaling is affected. Other than that, reduced BDNF protein levels also caused reduced in neurogenesis [10], as this study reports the doublecortin (DCX) expression and protein level decreased. DCX is a marker for young neurons [42].

Other than ferroptosis, pyroptosis is also a mode of cell death that can cause neurodegenerations. There are various implications of pyroptosis mechanism activation such as depression [18], stress [19], brain ischemia [21, 25, 26], sleep deprivation [24], perioperative neurocognitive disorders [23], Alzheimer’s disease [9], chemo fog [20], and cardiac arrest [22]. Based on the findings from all these studies, there was a significantly increased level of NLRP3 inflammasomes which affect brain health. Several mechanisms contribute to NLRP3 inflammasome overactivation-induced pyroptosis. The P2X7 receptor, a subgroup of the P2X family found on cell membranes, is abundant in many neurons and immune cells [43], while nuclear factor kappa B (NF/kB) is a key transcriptional activator of the NLRP3 inflammasomes [19, 20]. ATP activation of the P2X7 receptor results in the activation of nuclear factor kappa B (NF-B), which results in the activation of NLRP3 inflammasome-induced pyroptosis mechanisms [18].

Aside from that, oxidative stress can activate NLRP3 inflammasomes [21, 22]. The activation of the NLRP3 inflammasome may be upstream of the pyroptosis mechanism, but gasdermin-D (GSDMD) may be the true executor of pyroptosis-induced neuronal loss. The gasdermins, a class of pore-forming proteins, have recently been linked to the immune response [44]. Gasdermin-D (GSDMD) particularly, is one of the six identified paralogous genes of the gasdermin family [18, 44]. The activation of NLRP3 inflammasomes will result in the activation of caspase-1, which will then activate GSDMD-induced pore formation in neuron cells, resulting in neurodegeneration [18, 21, 22, 25]. Because of this function, GSDMD is known as the pyroptosis executor.

Besides activation of NLRP3 inflammasomes and GSDMD, BDNF regulation may also be linked to pyroptosis mechanisms. This is because all the studies included in this review reported that BDNF levels were found to be decreased as NLRP3 inflammasome expression increased. Other than that, Tropomyosin receptor kinase B (TrkB) protein level also decreased as reported by [20]. TrkB is a receptor for BDNF and its signaling maintains neuronal survival, synapse plasticity, and neuronal growth [45]. Reduced BDNF caused reduced synaptophysin (SYN) and postsynaptic density protein 95 (PSD 95) protein levels, indicating synapse impairment [9, 26]. Reduced synaptophysin (SYN) also leads to a reduction in the secretion of neurotransmitters protein levels such as serotonin and dopamine [20].

Furthermore, the synapsin-1 protein level also decreased as reported by [19], synapsin-1 also important for synapse maintenance since this protein is associated with the cytoplasmic surface of synaptic vesicles [46]. Based on these findings, the signaling of BDNF and TrkB receptors may be involved in the pyroptosis mechanism pathway by causing synapse impairment and leading to neurodegenerations.

Ferroptosis and pyroptosis are two distinct modes of cell death. Based on the articles included in this review, ferroptosis is an iron-dependent form of cell death initiated by iron accumulation in the brain. This process leads to oxidative stress and increased lipid peroxidation at the cell membrane, ultimately resulting in cell death. In contrast, pyroptosis is a pro-inflammatory form of cell death that begins with activating the inflammasome, subsequently activating caspase-1. Activated caspase-1 then activates pro-inflammatory cytokines (interleukin-1β and interleukin-18), and the cleavage of gasdermin-D (GSDMD). The activation of GSDMD causes pore formation in the cell membrane, allowing the release of activated pro-inflammatory cytokines through these pores, ultimately leading to cell death.

Although ferroptosis and pyroptosis are different types of cell death, studies included in this review suggest the possibility that ferroptosis and pyroptosis may occur simultaneously or sequentially, contributing to brain disorders, such as in cases of COVID-19 [12] and autoimmune diseases [13]. There is a possibility that both mechanisms are involved together in some of the studies included in this review, particularly in conditions such as stress and sleep deprivation. Stress has been shown to cause depression-like symptoms, as reported by two studies [16, 19]. These studies indicate the involvement of different mechanisms: one study reports the involvement of ferroptosis [16], while the other implicates pyroptosis [19]. Sleep deprivation has been linked to cognitive impairment through both ferroptosis mechanisms [10, 17] and pyroptosis mechanisms [24]. However, none of the studies included in this review have investigated the concurrent occurrence of both ferroptosis and pyroptosis and their combined impact on brain health.

Additionally, both ferroptosis and pyroptosis lead to a decrease in BDNF levels, resulting in neurodegeneration, particularly in the hippocampus. This observation strengthens the suggestion that ferroptosis and pyroptosis may occur simultaneously or sequentially, contributing to brain disorders. Ferroptosis, an iron-dependent cell death mechanism, increases ROS in the intracellular environment (oxidative stress), which leads to increased MDA lipid peroxidation. Increased ROS can also stimulate the activation of pyroptosis mechanisms [20, 22], suggesting a potential interaction between these pathways. The hippocampus, which is crucial for learning and memory, is particularly vulnerable to such oxidative stress and inflammation. The reduction in BDNF, a key neurotrophic factor in the hippocampus, exacerbates synaptic dysfunction and neuronal loss. However, more research is needed to elucidate the specific pathways through which ferroptosis and pyroptosis lower BDNF levels and contribute to hippocampal neurodegeneration. All the possible pathways are summarized and visualized in Figure 3.

Figure 3:

A diagram that summarizes the level of BDNF in ferroptosis and pyroptosis mechanisms. Image created with BioRender.com. This Figure illustrates the relationship between ferroptosis and pyroptosis mechanisms and their impact on various brain health conditions. The conditions related to brain health include depression, stress, sleep deprivation, Alzheimer’s disease, chemo fog, brain ischemia, cardiac arrest, and perioperative neurocognitive disorders (PDN). Both ferroptosis and pyroptosis contribute to synapse impairment and reduced neurogenesis in the hippocampus, ultimately leading to neurodegeneration.

Conclusions

In conclusion, BDNF may play a role in ferroptosis and pyroptosis mechanisms, but more research is needed to determine its precise role. It should be noted that BDNF itself has different forms; proBDNF and mature BDNF, and each one activates distinct cell signaling pathways, resulting in two different fates for neurons; death or survival. The findings of the selected articles in this review only reported that BDNF levels were decreased in these two modes of cell death mechanisms; however, it is still unclear how BDNF is involved in mediating these mechanisms. Both mechanisms are thought to be linked and could regulate neurodegeneration, whether simultaneously or sequentially. Further investigation into the relationship between ferroptosis and pyroptosis is highly anticipated.