Systemic knockout of Tspo in mice does not affect retinal morphology, function and susceptibility to degeneration

Translocator protein (18 kDa) (Tspo) in the retina and implications for ocular diseases

Translocator protein (18 kDa) (Tspo), formerly known as peripheral benzodiazepine receptor is a highly conserved transmembrane protein primarily located in the outer mitochondrial membrane. In the central nervous system (CNS), especially in glia cells, Tspo is upregulated upon inflammation. Consequently, Tspo was used as a tool for diagnostic in vivo imaging of neuroinflammation in the brain and as a potential therapeutic target. Several synthetic Tspo ligands have been explored as immunomodulatory and neuroprotective therapy approaches. Although the function of Tspo and how its ligands exert these beneficial effects is not fully clear, it became a research topic of interest, especially in ocular diseases in the past few years. This review summarizes state-of-the-art knowledge of Tspo expression and its proposed functions in different cells of the retina including microglia, retinal pigment epithelium and Müller cells. Tspo is involved in cytokine signaling, oxidative stress and reactive oxygen species production, calcium signaling, neurosteroid synthesis, energy metabolism, and cholesterol efflux. We also highlight recent developments in preclinical models targeting Tspo and summarize the relevance of Tspo biology for ocular and retinal diseases. We conclude that glial upregulation of Tspo in different ocular pathologies and the use of Tspo ligands as promising therapeutic approaches in preclinical studies underline the importance of Tspo as a potential disease-modifying protein.

History of Tspo deletion and induction in vivo: Phenotypic outcomes under physiological and pathological situations

The mitochondrial translocator protein (TSPO) is an outer mitochondrial protein membrane with high affinity for cholesterol. It is expressed in most tissues but is more particularly enriched in steroidogenic tissues. TSPO is involved in various biological mechanisms and TSPO regulation has been related to several diseases. However, despite a considerable number of published studies interested in TSPO over the past forty years, the precise function of the protein remains obscure. Most of the functions attributed to TSPO have been identified using pharmacological ligands of this protein, leading to much debate about the accuracy of these findings. In addition, research on the physiological role of TSPO has been hampered by the lack of in vivo deletion models. Studies to perform genetic deletion of Tspo in animal models have long been unsuccessful, which led to the conclusions that the deletion was deleterious and the gene essential to life. During the last decades, thanks to the significant technical advances allowing genome modification, several models of animal genetically modified for TSPO have been developed. These models have modified our view regarding TSPO and profoundly improved our fundamental knowledge on this protein. However, to date, they did not allow to elucidate the precise molecular function of TSPO and numerous questions persist concerning the physiological role of TSPO and its future as a therapeutic target. This article chronologically reviews the development of deletion and induction models of TSPO.

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.

Targeting TSPO Reduces Inflammation and Apoptosis in an In Vitro Photoreceptor-Like Model of Retinal Degeneration

The 18 kDa translocator protein (TSPO) is predominantly located in the mitochondrial outer membrane, playing an important role in steroidogenesis, inflammation, survival, and cell proliferation. Its expression in the CNS, and mainly in glial cells, is upregulated in neuropathologies and brain injury. In this study, the potential of targeting TSPO for the therapeutic treatment of inflammatory-based retinal neurodegeneration was evaluated by means of an in vitro model of lipopolysaccharide (LPS)-induced degeneration in 661 W cells, a photoreceptor-like cell line. After the assessment of the expression of TSPO in 661W cells, which, to the best of our knowledge, was never investigated so far, the anti-inflammatory and cytoprotective effects of a number of known TSPO ligands, belonging to the class of N,N-dialkyl-2-arylindol-3-ylglyoxylamides (PIGAs), were evaluated, using the classic TSPO ligand PK11195 as the reference standard. All tested PIGAs showed the ability to modulate the inflammatory and apoptotic processes in 661 W photoreceptor-like cells and to reduce LPS-driven cellular cytotoxicity. The protective effect of PIGAs was, in all cases, reduced by cotreatment with the pregnenolone synthesis inhibitor SU-10603, suggesting the involvement of neurosteroids in the protective mechanism. As inflammatory processes play a crucial role in the retinal neurodegenerative disease progression toward photoreceptors' death and complete blindness, targeting TSPO might represent a successful strategy to slow down this degenerative process that may lead to the inexorable loss of vision.

Glial-mediated neuroinflammatory mechanisms in age-related macular degeneration

Age-related macular degeneration (AMD) is a neurodegenerative disorder characterized by photoreceptor and retinal pigment epithelium loss often complicated by neovascularization and is one of the leading causes of irreversible vision loss worldwide. However, the precise pathophysiology of AMD remains to date unclear, and there is a dearth of effective therapies for the early stages of the disease. A growing body of evidence has identified microglia-mediated neuroinflammation as a key driver of neuronal damage in AMD, presenting a novel avenue for the development of pharmacological agents targeting this cell population. The local microglial response interacts with other glia as well as engages in crosstalk with peripheral immunological niches. This article presents a review of the current evidence regarding the involvement of glia in the pathophysiology of AMD, an overview of the key immune circuits and effector mechanisms shown to be active in AMD, and potential therapeutic avenues targeting glial involvement.

Targeting Microglia to Treat Degenerative Eye Diseases

Microglia have been implicated in many degenerative eye disorders, including retinitis pigmentosa, age-related macular degeneration, glaucoma, diabetic retinopathy, uveitis, and retinal detachment. While the exact roles of microglia in these conditions are still being discovered, evidence from animal models suggests that they can modulate the course of disease. In this review, we highlight current strategies to target microglia in the eye and their potential as treatments for both rare and common ocular disorders. These approaches include depleting microglia with chemicals or radiation, reprogramming microglia using homeostatic signals or other small molecules, and inhibiting the downstream effects of microglia such as by blocking cytokine activity or phagocytosis. Finally, we describe areas of future research needed to fully exploit the therapeutic value of microglia in eye diseases.

Deletion of Mitochondrial Translocator Protein (TSPO) Gene Decreases Oxidative Retinal Pigment Epithelial Cell Death via Modulation of TRPM2 Channel

Simple Summary

18 kDa mitochondrial translocator protein (TSPO) is a mitochondria protein of the cellular outer membrane in the mitochondria of several cells, including ARPE19 is TSPO. Accumulating evince indicates that the presence of TSPO participated the modulations of Ca²⁺ homeostasis and mitochondrial free reactive oxygen species (fROS) generation. The deletion of TSPO gene provides to study the action of TSPO on the levels of apoptosis, ADP-ribose (ADPR), mitochondria-fROS (Mito-fROS), and apoptosis via the stimulation of Ca²⁺ permeable channels in the models of cell culture. The stimulations of oxidative stress and ADPR induce the activation of TRPM2 in the ARPE19. For clarifying the involvement of TSPO in retinal human diseases, we used the ARPE19 human cell culture model. The current results demonstrated that the deletion of TSPO induces the regulation of TRPM2 in the TSPO gene knockout ARPE19 (ARPE19-KO) In fact, the present results show that the presence of TSPO increased the upregulations of apoptosis and mitochondria oxidative cytotoxicity values via stimulation of TRPM2 in the ARPE19. Nevertheless, the blockages of PARP-1 (PJ34 and DPQ) and TRPM2 (2APB and ACA) downregulated the values of cell death and oxidative cytotoxicity in the ARPE19. In summary, present results clearly demonstrate that the deletion of TSPO decreases mitochondrial oxidative cytotoxicity-mediated cell death via the modulation of TRPM2 in the ARPE19.

Abstract

The current results indicated the possible protective actions of 18 kDa mitochondrial translocator protein (TSPO) deletion on TRPM2 stimulation, mitochondrial free ROS (Mito-fROS) and apoptotic harmful actions in the cells of adult retinal pigment epithelial19 (ARPE19). There was a direct relationship between TSPO and the disease of age-related macular degeneration. The nature of TSPO implicates upregulation of Mito-fROS and apoptosis via the activation of Ca²⁺ channels in ARPE19, although deletion of TSPO gene downregulates the activation. The decrease of oxidative cytotoxicity and apoptosis might induce in TSPO gene deleted cells by the inhibition of Mito-fROS and PARP-1 activation-induced TRPM2 cation channel activation. The ARPE19 cells were divided into two main groups as TSPO expressing (ARPE19) and non-expressing cells (ARPE19-KO). The levels of caspase -3 (Casp -3), caspase -9 (Casp -9), apoptosis, Mito-fROS, TRPM2 current and intracellular free Ca²⁺ were upregulated in the ARPE19 by the stimulations of H₂O₂ and ADP-ribose, although their levels were downregulated in the cells by the modulators of PARP-1 (DPQ and PJ34), TRPM2 (ACA and 2APB) and glutathione. However, the H₂O₂ and ADP-ribose-mediated increases were not observed in the ARPE19-KO. The expression levels of Bax, Casp -3, Casp -9 and PARP-1 were higher in the ARPE19 group as compared to the ARPE19-KO group. In summary, current results confirmed that TRPM2-mediated cell death and oxidative cytotoxicity in the ARPE19 cells were occurred by the presence of TSPO. The deletion of TSPO may be considered as a therapeutic way to TRPM2 activation-mediated retinal oxidative injury.

Lack of adrenal TSPO/PBR expression in hamsters reinforces correlation to triglyceride metabolism

Despite being a highly conserved protein, the precise role of the mitochondrial translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), remains elusive. The void created by studies that overturned a presumptive model that described TSPO/PBR as a mitochondrial cholesterol transporter for steroidogenesis has been filled with evidence that it can affect mitochondrial metabolic functions across different model systems. We previously reported that TSPO/PBR deficient steroidogenic cells upregulate mitochondrial fatty acid oxidation and presented a strong positive correlation between TSPO/PBR expression and tissues active in triglyceride metabolism or lipid storage. Nevertheless, the highlighting of inconsistencies in prior work has provoked reprisals that threaten to stifle progress. One frequent factoid presented as being supportive of a cholesterol import function is that there are no steroid-synthesizing cell types without high TSPO/PBR expression. In this study, we examine the hamster adrenal gland that is devoid of lipid droplets in the cortex and largely relies on de novo cholesterol biosynthesis and uptake for steroidogenesis. We find that Tspo expression in the hamster adrenal is imperceptible compared to the mouse. This observation is consistent with a substantially low expression of Cpt1a in the hamster adrenal, indicating minimal mitochondrial fatty acid oxidation capacity compared to the mouse. These findings provide further reinforcement that the much sought-after mechanism of TSPO/PBR function remains correlated with the extent of cellular triglyceride metabolism. Thus, TSPO/PBR could have a homeostatic function relevant only to steroidogenic systems that manage triglycerides associated with lipid droplets.

Microglia Activation and Inflammation During the Death of Mammalian Photoreceptors

Photoreceptors are highly specialized sensory neurons with unique metabolic and physiological requirements. These requirements are partially met by Müller glia and cells of the retinal pigment epithelium (RPE), which provide essential metabolites, phagocytose waste, and control the composition of the surrounding microenvironment. A third vital supporting cell type, the retinal microglia, can provide photoreceptors with neurotrophic support or exacerbate neuroinflammation and hasten neuronal cell death. Understanding the physiological requirements for photoreceptor homeostasis and the factors that drive microglia to best promote photoreceptor survival has important implications for the treatment and prevention of blinding degenerative diseases like retinitis pigmentosa and age-related macular degeneration.

The Translocator Protein (TSPO) in Mitochondrial Bioenergetics and Immune Processes

The translocator protein (TSPO) is an outer mitochondrial membrane protein that is widely used as a biomarker of neuroinflammation, being markedly upregulated in activated microglia in a range of brain pathologies. Despite its extensive use as a target in molecular imaging studies, the exact cellular functions of this protein remain in question. The long-held view that TSPO plays a fundamental role in the translocation of cholesterol through the mitochondrial membranes, and thus, steroidogenesis, has been disputed by several groups with the advent of TSPO knockout mouse models. Instead, much evidence is emerging that TSPO plays a fundamental role in cellular bioenergetics and associated mitochondrial functions, also part of a greater role in the innate immune processes of microglia. In this review, we examine the more direct experimental literature surrounding the immunomodulatory effects of TSPO. We also review studies which highlight a more central role for TSPO in mitochondrial processes, from energy metabolism, to the propagation of inflammatory responses through reactive oxygen species (ROS) modulation. In this way, we highlight a paradigm shift in approaches to TSPO functioning.