The brain and eye: Treating cerebral and retinal ischemia through mitochondrial transfer

Functional role of translocator protein and its ligands in ocular diseases (Review)

The 18 kDa translocator protein (TSPO) is an essential outer mitochondrial membrane protein that is responsible for mitochondrial transport, maintenance of mitochondrial homeostasis and normal physiological cell function. The role of TSPO in the pathogenesis of ocular diseases is a growing area of interest. More notably, TSPO exerts positive effects in regulating various pathophysiological processes, such as the inflammatory response, oxidative stress, steroid synthesis and modulation of microglial function, in combination with a variety of specific ligands such as 1‑(2‑chlorophenyl‑N‑methylpropyl)‑3‑isoquinolinecarboxamide, 4'‑chlorodiazepam and XBD173. In the present review, the expression of TSPO in ocular tissues and the functional role of TSPO and its ligands in diverse ocular diseases was discussed.

Homer1 Protects against Retinal Ganglion Cell Pyroptosis by Inhibiting Endoplasmic Reticulum Stress-Associated TXNIP/NLRP3 Inflammasome Activation after Middle Cerebral Artery Occlusion-Induced Retinal Ischemia

Retinal ischemia, after cerebral ischemia, is an easily overlooked pathophysiological problem in which inflammation is considered to play an important role. Pyroptosis is a kind of cell death pattern accompanied by inflammation. Homer scaffold protein 1 (Homer1) has anti-inflammation properties and protects against ischemic injury. However, little is known about pyroptosis following middle cerebral artery occlusion (MCAO)-induced retinal ischemia and the regulatory mechanisms involved by Homer1 for the development of pyroptosis. In the present study, retinal ischemic injury was induced in mice by permanent MCAO in vivo, and retinal ganglion cells (RGCs) were subjected to Oxygen and Glucose Deprivation (OGD) to establish an in vitro model. It was shown that TXNIP/NLRP3-mediated pyroptosis was located predominantly in RGCs, which gradually increased after retinal ischemia and peaked at 24 h after retinal ischemia. Interestingly, the RGCs pyroptosis occurred not only in the cell body but also in the axon. Notably, the occurrence of pyroptosis coincided with the change of Homer1 expression in the retina after retinal ischemia and Homer1 also co-localized with RGCs. It was demonstrated that overexpression of Homer1 not only alleviated RGCs pyroptosis and inhibited the release of pro-inflammatory factors but also led to the increase in phosphorylation of AMPK, inhibition of ER stress, and preservation of visual function after retinal ischemia. In conclusion, it was suggested that Homer1 may protect against MCAO-induced retinal ischemia and RGCs pyroptosis by inhibiting endoplasmic reticulum stress-associated TXNIP/NLRP3 inflammasome activation after MCAO-induced retinal ischemia.

Evaluation of Histopathologic Findings and Safety of Intravitreal Ketamine Administration on Vitreoretinal Tissue in Rat Model: A Pilot Study

Purpose

To evaluate the safety and histological findings of intravitreal injection of ketamine in rats.

Methods

Each rat received a total volume of 0.1 ml of ketamine 0.01 mol/L (5 rats as ketamine group) or a total of 0.1 ml of normal saline 0.9% (5 rats as control group) under general anesthesia in a sterile condition. A histology assessment was performed 1 month after the intravitreal injection.

Results

Lens opacity, necrosis, and atrophy of retinal layers and optic disc were not seen in five specimens in the ketamine group and five in the normal saline group. There was no inflammation in the vitreous, retinal layers, choroid, optic disc, and optic nerve in both groups.

Conclusion

Intravitreal injection of ketamine in a special dose has no obvious adverse effect on diverse intraocular tissue.

Inhibition of mitochondrial VDAC1 oligomerization alleviates apoptosis and necroptosis of retinal neurons following OGD/R injury

Ischemia-reperfusion (I/R) injury is a common pathological mechanism in many retinal diseases, which can lead to cell death via mitochondrial dysfunction. Voltage-dependent anion channel 1 (VDAC1), which is mainly located in the outer mitochondrial membrane, is the gatekeeper of mitochondria. The permeability of mitochondrial membrane can be regulated by controlling the oligomerization of VDAC1. However, the functional mechanism of VDAC1 in retinal I/R injury was unclear. Our results demonstrate that oxygen-glucose deprivation and re-oxygenation (OGD/R) injury leads to apoptosis, necroptosis, and mitochondrial dysfunction of R28 cells. The OGD/R injury increases the levels of VDAC1 oligomerization. Inhibition of VDAC1 oligomerization by VBIT-12 rescued mitochondrial dysfunction by OGD/R and also reduced apoptosis/necroptosis of R28 cells. In vivo, the use of VBIT-12 significantly reduced aHIOP-induced neuronal death (apoptosis/necroptosis) in the rat retina. Our findings indicate that VDAC1 oligomers may open and enlarge mitochondrial membrane pores during OGD/R injury, leading to the release of death-related factors in mitochondria, resulting in apoptosis and necroptosis. This study provides a potential therapeutic strategy against ocular diseases caused by I/R injury.

Differential Effects of Cytopathic Hypoxia on Human Retinal Endothelial Cellular Behavior: Implication for Ischemic Retinopathies

Loss of barrier integrity of retinal endothelial cells (RECs) is an early feature of ischemic retinopathies (IRs), but the triggering mechanisms remain incompletely understood. Previous studies have reported mitochondrial dysfunction in several forms of IRs, which creates a cytopathic hypoxic environment where cells cannot use oxygen for energy production. Nonetheless, the contribution of cytopathic hypoxia to the REC barrier failure has not been fully explored. In this study, we dissect in-depth the role of cytopathic hypoxia in impairing the barrier function of REC. We employed the electric cell-substrate impedance sensing (ECIS) technology to monitor in real-time the impedance (Z) and hence the barrier functionality of human RECs (HRECs) under cytopathic hypoxia-inducing agent, Cobalt(II) chloride (CoCl₂). Furthermore, data were deconvoluted to test the effect of cytopathic hypoxia on the three key components of barrier integrity; R_b (paracellular resistance between HRECs), α (basolateral adhesion between HRECs and the extracellular matrix), and C_m (HREC membrane capacitance). Our results showed that CoCl₂ decreased the Z of HRECs dose-dependently. Specifically, the R_b parameter of the HREC barrier was the parameter that declined first and most significantly by the cytopathic hypoxia-inducing agent and in a dose-dependent manner. When R_b began to fall to its minimum, other parameters of the HREC barrier, including α and C_m, were unaffected. Interestingly, the compromised effect of cytopathic hypoxia on R_b was associated with mitochondrial dysfunction but not with cytotoxicity. In conclusion, our results demonstrate distinguishable dielectric properties of HRECs under cytopathic hypoxia in which the paracellular junction between adjacent HRECs is the most vulnerable target. Such selective behavior could be utilized to screen agents or genes that maintain and strengthen the assembly of HRECs tight junction complex.

The Present and Future of Mitochondrial-Based Therapeutics for Eye Disease

Translational Relevance

Mitochondria are viable therapeutic targets for a broad spectrum of ocular diseases.

Different Roles of Mitochondria in Cell Death and Inflammation: Focusing on Mitochondrial Quality Control in Ischemic Stroke and Reperfusion

Mitochondrial dysfunctions are among the main hallmarks of several brain diseases, including ischemic stroke. An insufficient supply of oxygen and glucose in brain cells, primarily neurons, triggers a cascade of events in which mitochondria are the leading characters. Mitochondrial calcium overload, reactive oxygen species (ROS) overproduction, mitochondrial permeability transition pore (mPTP) opening, and damage-associated molecular pattern (DAMP) release place mitochondria in the center of an intricate series of chance interactions. Depending on the degree to which mitochondria are affected, they promote different pathways, ranging from inflammatory response pathways to cell death pathways. In this review, we will explore the principal mitochondrial molecular mechanisms compromised during ischemic and reperfusion injury, and we will delineate potential neuroprotective strategies targeting mitochondrial dysfunction and mitochondrial homeostasis.

Eyeballing stroke: Blood flow alterations in the eye and visual impairments following transient middle cerebral artery occlusion in adult rats

Middle cerebral artery occlusion in rodents remains a widely used model of ischemic stroke. Recently, we reported the occurrence of retinal ischemia in animals subjected to middle cerebral artery occlusion, owing in part to the circulatory juxtaposition of the ophthalmic artery to the middle cerebral artery. In this study, we examined the eye hemodynamics and visual deficits in middle cerebral artery occlusion-induced stroke rats. The brain and eye were evaluated by laser Doppler at baseline (prior to middle cerebral artery occlusion), during and after middle cerebral artery occlusion. Retinal function-relevant behavioral and histological outcomes were performed at 3 and 14 days post-middle cerebral artery occlusion. Laser Doppler revealed a typical reduction of at least 80% in the ipsilateral frontoparietal cortical area of the brain during middle cerebral artery occlusion compared to baseline, which returned to near-baseline levels during reperfusion. Retinal perfusion defects closely paralleled the timing of cerebral blood flow alterations in the acute stages of middle cerebral artery occlusion in adult rats, characterized by a significant blood flow defect in the ipsilateral eye with at least 90% reduction during middle cerebral artery occlusion compared to baseline, which was restored to near-baseline levels during reperfusion. Moreover, retinal ganglion cell density and optic nerve depth were significantly decreased in the ipsilateral eye. In addition, the stroke rats displayed eye closure. Behavioral performance in a light stimulus-mediated avoidance test was significantly impaired in middle cerebral artery occlusion rats compared to control animals. In view of visual deficits in stroke patients, closely monitoring of brain and retinal perfusion via laser Doppler measurements and examination of visual impairments may facilitate the diagnosis and the treatment of stroke, including retinal ischemia.

Mitochondria: Novel Mechanisms and Therapeutic Targets for Secondary Brain Injury After Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) is a destructive form of stroke that often results in death or disability. However, the survivors usually experience sequelae of neurological impairments and psychiatric disorders, which affect their daily functionality and working capacity. The recent MISTIE III and STICH II trials have confirmed that early surgical clearance of hematomas does not improve the prognosis of survivors of ICH, so it is vital to find the intervention target of secondary brain injury (SBI) after ICH. Mitochondrial dysfunction, which may be induced by oxidative stress, neuroinflammation, and autophagy, among others, is considered to be a novel pathological mechanism of ICH. Moreover, mitochondria play an important role in promoting neuronal survival and improving neurological function after a hemorrhagic stroke. This review summarizes the mitochondrial mechanism involved in cell death, reactive oxygen species (ROS) production, inflammatory activation, blood–brain barrier (BBB) disruption, and brain edema underlying ICH. We emphasize the potential of mitochondrial protection as a potential therapeutic target for SBI after stroke and provide valuable insight into clinical strategies.

Output Regulation and Function Optimization of Mitochondria in Eukaryotes

The emergence of endosymbiosis between aerobic alpha-proteobacterium and anaerobic eukaryotic cell precursors opened the chapter of eukaryotic evolution. Multiple functions of mitochondria originated from the ancient precursors of mitochondria and underwent remodeling in eukaryotic cells. Due to the dependence on mitochondrial functions, eukaryotic cells need to constantly adjust mitochondrial output based on energy demand and cellular stress. Meanwhile, eukaryotes conduct the metabolic cooperation between different cells through the involvement of mitochondria. Under some conditions, mitochondria might also be transferred to nearby cells to provide a protective mechanism. However, the endosymbiont relationship determines the existence of various types of mitochondrial injury, such as proteotoxic stress, mutational meltdown, oxidative injure, and immune activation caused by released mitochondrial contents. Eukaryotes have a repertoire of mitochondrial optimization processes, including various mitochondrial quality-control proteins, regulation of mitochondrial dynamics and activation of mitochondrial autophagy. When these quality-control processes fail, eukaryotic cells can activate apoptosis to intercept uncontrolled cell death, thereby minimizing the damage to extracellular tissue. In this review, we describe the intracellular and extracellular context-based regulation of mitochondrial output in eukaryotic cells, and introduce new findings on multifaceted quality-control processes to deal with mitochondrial defects.

SP1-mediated upregulation of LINGO-1 promotes degeneration of retinal ganglion cells in optic nerve injury

BACKGROUNDS

Insults to the axons in the optic nerve head are the primary cause of loss of retinal ganglion cells (RGCs) in traumatic, ischemic nerve injury or degenerative ocular diseases. The central nervous system-specific leucine-rich repeat protein, LINGO-1, negatively regulates axon regeneration and neuronal survival after injury. However, the upstream molecular mechanisms that regulate LINGO-1 signaling and contribute to LINGO-1-mediated death of RGCs are unclear.

METHODS

The expression of SP1 was profiled in optic nerve crush (ONC)-injured RGCs. LINGO-1 level was examined after SP1 overexpression by qRT-PCR. Luciferase assay was used to examine the binding of SP1 to the promoter regions of LINGO-1. Primary RGCs from rat retina were isolated by immunopanning and RGCs apoptosis were determined by Tunnel. SP1 and LINGO-1 expression was investigated using immunohistochemistry and Western bolting. Neuroprotection was assessed by RGC counts, RNFL thickness, and VEP tests after inhibition of SP1 shRNA.

RESULTS

We demonstrate that SP1 was upregulated in ONC-injured RGCs. SP1 was bound to the LINGO-1 promoter, which led to increased expression of LINGO-1. Treatment with recombinant Nogo-66 or LINGO-1 promoted apoptosis of RGCs cultured under serum-deprivation conditions, while silencing of SP1 promoted the survival of RGCs. SP1 and LINGO-1 colocalized and were upregulated in ONC-injured retinas. Silencing of SP1 in vivo reduced LINGO-1 expression and protected the structure of RGCs from ONC-induced injury, but there was no sign of recovery in VEP.

CONCLUSIONS

Our findings imply that SP1 regulates LINGO-1 expression in RGCs in the injured retina and provide insight into mechanisms underlying LINGO-1-mediated RGC death in optic nerve injury.

Inter and Intracellular mitochondrial trafficking in health and disease

Neurons and glia maintain central nervous system (CNS) homeostasis through diverse mechanisms of intra- and intercellular signaling. Some of these interactions include the exchange of soluble factors between cells via direct cell-to-cell contact for both short and long-distance transfer of biological materials. Transcellular transfer of mitochondria has emerged as a key example of this communication. This transcellular transfer of mitochondria are dynamically involved in the cellular and tissue response to CNS injury and play beneficial roles in recovery. This review highlights recent research addressing the cause and effect of intra- and intercellular mitochondrial transfer with a specific focus on the future of mitochondrial transplantation therapy. We believe that mitochondrial transfer plays a crucial role during bioenergetic crisis/deficit, but the quality, quantity and mode of mitochondrial transfer determines the protective capacity for the receiving cells. Mitochondrial transplantation is a new treatment paradigm and will overcome the major bottleneck of traditional approach of correcting mitochondria-related disorders.

Stem Cells as Drug-like Biologics for Mitochondrial Repair in Stroke

Stroke is a devastating condition characterized by widespread cell death after disruption of blood flow to the brain. The poor regenerative capacity of neural cells limits substantial recovery and prolongs disruptive sequelae. Current therapeutic options are limited and do not adequately address the underlying mitochondrial dysfunction caused by the stroke. These same mitochondrial impairments that result from acute cerebral ischemia are also present in retinal ischemia. In both cases, sufficient mitochondrial activity is necessary for cell survival, and while astrocytes are able to transfer mitochondria to damaged tissues to rescue them, they do not have the capacity to completely repair damaged tissues. Therefore, it is essential to investigate this mitochondrial transfer pathway as a target of future therapeutic strategies. In this review, we examine the current literature pertinent to mitochondrial repair in stroke, with an emphasis on stem cells as a source of healthy mitochondria. Stem cells are a compelling cell type to study in this context, as their ability to mitigate stroke-induced damage through non-mitochondrial mechanisms is well established. Thus, we will focus on the latest preclinical research relevant to mitochondria-based mechanisms in the treatment of cerebral and retinal ischemia and consider which stem cells are ideally suited for this purpose.

Therapeutic use of extracellular mitochondria in CNS injury and disease

In the central nervous system (CNS), neuronal functionality is highly dependent on mitochondrial integrity and activity. In the context of a damaged or diseased brain, mitochondrial dysfunction leads to reductions in ATP levels, thus impairing ATP-dependent neural firing and neurotransmitter dynamics. Restoring mitochondrial ability to generate ATP may be a basic premise to restore neuronal functionality. Recently, emerging data in rodent and human studies suggest that mitochondria and its components are surprisingly released into extracellular space and potentially transferred between cells. Transferred mitochondria may support oxidative phosphorylation in recipient cells. In this mini-review, we (a) survey recent findings in cell to cell mitochondrial transfer and the presence of cell-free extracellular mitochondria and its components, (b) review experimental details of how to detect extracellular mitochondria and mitochondrial transfer in the CNS, (c) discuss strategies and tissue sources for mitochondria isolation, and (d) explore exogenous mitochondrial transplantation as a novel approach for CNS therapies.