Decoding ischemic stroke: Perspectives on the endoplasmic reticulum, mitochondria, and their crosstalk

Abstract

Stroke is known for its high disability and mortality rates. Ischemic stroke (IS), the most prevalent form, imposes a considerable burden on affected individuals. Nevertheless, existing treatment modalities are hindered by limitations, including narrow therapeutic windows, substantial adverse effects, and suboptimal neurological recovery. Clarifying the pathological mechanism of IS is a prerequisite for developing new therapeutic strategies. In this context, the functional disruption of mitochondria, the endoplasmic reticulum (ER), and the crosstalk mechanisms between them have garnered increasing attention for their contributory roles in the progression of IS. Therefore, this review provides a comprehensive summary of the current pathomechanisms associated with the involvement of the ER and mitochondria in IS, emphasising Ca²⁺ destabilization homeostasis, ER stress, oxidative stress, disordered mitochondrial quality control, and mitochondrial transfer. Additionally, this article highlights the functional interaction between the ER and mitochondria, as well as the mitochondrial-ER contacts (MERCs) that structurally connect mitochondria and the ER, aiming to provide ideas and references for the research and treatment of IS.

Keywords: Endoplasmic reticulum; Ischemic stroke; Mitochondria; Mitochondria and endoplasmic reticulum contacts; Oxidative stress.

Fig. 1

The ER, mitochondria, and their crosstalk in ischemic stroke. ER: endoplasmic reticulum; ROS: reactive oxygen species; RyRs: ryanodine receptors; IP3Rs: inositol trisphosphate receptors; SERCA: sarco-ER Ca²⁺-ATPase; CRT: calreticulin; CNX: calnexin; UPR: unfolded protein response; GRP78: glucose-regulated protein 78; VDAC: voltage-dependent anion channel; MCU: mitochondrial calcium uniporter; ATF6: activating transcription factor 6; IRE1α: inositol-requiring enzyme 1α; PEPK: protein kinase R-like ER kinase; Sig1R: sigma 1 receptor; DJ-1: parkinson disease protein 7; VAPB: vesicle-associated membrane protein B; PTPIP51: protein tyrosine phosphatase interacting protein 51; Mfn1: mitofusin 1. This figure is created by Figdraw (http://www.figdraw.com/#/).

Fig. 2

ER is involved in ischemic stroke. During IS, dysfunction of the ER triggers a cellular Ca²⁺ imbalance and ER stress. During cerebral ischemia, hyperactivation of RyRs and IP3Rs, coupled with impaired SERCA function, leads to an imbalance characterized by excessive Ca²⁺ release and inadequate Ca²⁺ reuptake within the ER. This leads to ER Ca²⁺ depletion and aggravates Ca²⁺ overload in the cell. Simultaneously, the accumulation of unfolded or misfolded proteins within the ER lumen induces ER stress and activates the UPR. All three signalling pathways of the UPR are active in neurons after IS. In this context, unspliced XBP1 is converted to spliced XBP1 (XBP1S). ATF6 is transported to the Golgi apparatus where it is hydrolysed by resident S1P and S2P proteases, releasing the transcription factor domain (ATF6α) from the membrane. PERK activates downstream ATF4 production through eIF2α. These potent transcription factors activate downstream gene transcription through nuclear translocation, thereby enhancing protein folding capacity. In addition, by increasing the expression of the ER chaperone proteins GRP75, BiP (GRP78), and GRP94 to enhance the ability to handle misfolded proteins, the UPR strives to restore ER homeostasis and promote survival. Under conditions of sustained or intense stress, the UPR shifts towards pro-apoptotic pathways, thereby initiating inflammatory responses and pathways leading to cell death. UPR: unfolded protein response; SERCA: sarco-ER Ca²⁺-ATPase; RyRs: ryanodine receptors; IP3Rs: inositol trisphosphate receptors; CNX: calnexin; CRT: calreticulin; PERK: protein kinase R-like ER kinase; IRE1: inositol-requiring enzyme 1; ATF6: activating transcription factor 6; TRAF2: TNF receptor-associated factor 2; XBP1: X box binding protein 1; XBP1S: spliced XBP1; NF-κB: NF-κB: nuclear factor kappa-B; eIF2α: eukaryotic initiation factor 2α; ATF4: activating transcription factor 4; CHOP: C/EBP homologous protein; TXNIP: thioredoxin-interacting protein; NLRP3: NOD-like receptor protein 3; IL-1β: interleukin-1β. This figure is created by Figdraw (http://www.figdraw.com/#/).

Fig. 3

Mitochondria involved in IS. Mitochondria serve not only as crucial energy suppliers for cells but also as pivotal regulators of cell death pathways under pathological conditions. The deprivation of glucose and O₂ resulting from cerebral ischemia, along with the subsequent disruption of ion homeostasis, leads to mitochondrial dysfunction and the synthesis of ROS. Oxidative stress induces significant mitochondrial damage and disrupts the homeostasis of the mitochondrial network regulated by MQC. Notably, intercellular mitochondrial transfer has been identified in the nervous system during IS. The most typical example is that damaged mitochondria after cerebral ischemia act as “help me” messengers, which are released by neurons into astrocytes for processing and recycling. Correspondingly, astrocytes are able to transfer functional mitochondria to damaged neurons to perform neuroprotective functions. mPTP: mitochondrial permeability transition pore; ATP: adenosine triphosphate; ATP: adenosine diphosphate; NCLX: Na⁺/Ca²⁺ exchanger; SOD: superoxide dismutase; CAT: catalase; GSH-PX: glutathione peroxidase; ‌Drp1: dynamin-related protein 1; Mff: mitochondrial fission factor; Fis1: fission 1 protein; Mfn1: mitofusin 1; OPA1: optic atrophy factor 1; PGC-1α: peroxisome proliferator-activated receptor γ coactivator 1α; Nrf1: nuclear respiratory factor 1; PPARs: nuclear respiratory factor 1; ERRs: estrogen-related receptors; PINK1: PTEN-induced putative kinase 1; BNIP3: Bcl2-adenovirus E1B 19 kDa protein-interacting protein 3; NIX: NIP3-like protein X; FUNDC1: FUN14 domain containing 1; LC3II: microtubule-associated protein light chain 3II; MCU: mitochondrial calcium uniporter; VDAC: voltage-dependent anion channel; ROS: reactive oxygen species. This figure is created by Figdraw (http://www.figdraw.com/#/).

Fig. 4

The functional interplay between the ER and mitochondria. (A) Physiological Ca²⁺ transfer between the ER and mitochondria. (B) Pathological Ca²⁺ transfer between the ER and mitochondria. (C) Interplay between mitochondrial ROS and ER stress in pathological states. ER: endoplasmic reticulum; RyR: ryanodine receptor; IP3R: inositol trisphosphate receptor; SERCA: sarco-ER Ca²⁺-ATPase; VDAC: voltage-dependent anion channel; CNX: calnexin; MCU: mitochondrial calcium uniporter; ATP: adenosine triphosphate; ADP: adenosine diphosphate; ROS: reactive oxygen species; CPN1: calpain 1.

Fig. 5

IP3R-DJ-1-GRP75-VDAC complex. (A) The tetrameric complex formed by IP3R-DJ-1-GRP75-VDAC controls the transfer of Ca²⁺ from the ER to the mitochondria. (B–D) Depletion of DJ-1, GRP75, IP3R, and TG2 all affect the stability of MERCs, impairing mitochondrial-ER coupling and Ca²⁺ transfer. ER: endoplasmic reticulum; IP3Rs: inositol trisphosphate receptors; TG2: transglutaminase type 2; GRP75: glucose-regulated protein 75; VDAC: voltage-dependent anion channel; DJ-1: parkinson disease protein 7.

Fig. 6

VAPB-PTPIP51 complex. (A) Overexpression of VAPB or PTPIP51 increases the coverage of MERCs, thereby impairing autophagosome formation, whereas downregulation of these proteins reduces contacts and upregulates autophagosome formation. These effects on autophagy are linked to their role in mediating Ca²⁺ exchange between the ER and mitochondria at contact sites. (B) During the IS, a decrease in MERC coverage and an increase in organelle spacing were observed, accompanied by reduced expression of VAPB and PTPIP51, as well as increased autophagy. Knocking out either VAPB or PTPIP51 further exacerbates these pathologies. (C) Following mitochondrial damage, mitochondrial-derived ROS enhance MERC formation via VAPB-PTPIP51 tethering, thereby transferring lipid radicals from the mitochondria to the ER. Disruption of this tethering leads to the accumulation of lipid radicals in mitochondria, resulting in cell death. ROS: reactive oxygen species; VAPB: vesicle-associated membrane protein B; PTPIP51: protein tyrosine phosphatase interacting protein 51; TG2: transglutaminase type 2.

Fig. 7

Sig1R serves as a MERC tethering protein. (A) Under resting conditions, Sig1R resides with the ER-resident chaperone BiP. Upon activation, Sig1R can inhibit ER stress by promoting the degradation of misfolded proteins and alleviate oxidative stress by stabilizing the expression of antioxidant proteins. Knockout of Sig1R increases intercellular distance and inhibits its resistance to ER stress and oxidative stress. Additionally, Sig1R regulates Ca²⁺ signaling between the ER and mitochondria through interaction with IP3R3, which can promote mitochondrial ATP production. (B) Sig1R is widely involved in various pathological changes in IS. ER: endoplasmic reticulum; ROS: reactive oxygen species; Sig1R: Sigma 1 receptor; BiP: binding protein; NMDA: n-methyl-d-aspartate; GRP75: glucose-regulated protein 75; IP3R: inositol trisphosphate receptor; VDAC: voltage-dependent anion channel; DJ-1: parkinson disease protein 7; BDNF: brain-derived neurotrophic factor; GDNF: glial cell line-derived neurotrophic factor.

Fig. 8

Mfn1 and Mfn2 serve as MERC tethering proteins. (A) ER-localized Mfn2 establishes a connection between mitochondria and the ER by forming homo- or heterotypic interactions with Mfn2 or Mfn1 located in mitochondria. The knockdown of Mfn1 increases smooth ER-mitochondria contacts in cells. The elimination of Mfn2 reduces inter-organellar juxtaposition and communication and decreases mitochondrial uptake of Ca²⁺ released from the ER. However, some studies have reported opposite results. (B) REEP5 interacts with Mfn1 and Mfn2 to regulate mitochondrial localization and the accumulation of mitochondrial ROS within cells. Depletion of REEP5 results in reduced mitochondrial tethering and increased perinuclear localization. Conversely, increasing REEP5 expression promotes the distribution of mitochondria throughout the cytoplasm. REEP5: receptor expression enhancing protein 5; Mfn1: mitofusin 1; Mfn2: mitofusin 2; REEP5: receptor expression enhancing protein 5; ROS: reactive oxygen species.

Fig. 9

The potential application value of MERCs and their tethering proteins in the clinical diagnosis and treatment of IS. MERCs: mitochondria and endoplasmic reticulum contacts; ER: endoplasmic reticulum; IP3Rs: inositol trisphosphate receptors; GRP75: glucose-regulated protein 75; Sig1R: Sigma 1 receptor; VAPB: vesicle-associated membrane protein B; PTPIP51: protein tyrosine phosphatase interacting protein 51; DJ-1: parkinson disease protein 7; TG2: transglutaminase type 2; Mfn1: mitofusin 1; Mfn2: mitofusin 2; BiP: binding protein; VDAC: voltage-dependent anion channel; TUDCA: Tauroursodeoxycholic acid; REEP5: receptor expression enhancing protein 5.