A chronic pro-inflammatory environment and reactive microglia are hallmarks of retinal degenerative diseases that affect vision. Our project aims at interventions overriding microglial pro-inflammatory and pro-oxidative properties to attenuate photoreceptor demise and preserve retinal integrity. The therapy concepts include ligands for the translocator protein 18 kDa (TSPO) and the cytokine interferon beta (IFN-β) in modulating microgliosis during retinal pathologies.
The retina is an extremely sophisticated and subtle structure. It is highly susceptible to a variety of noxious insults including high intensity light, hypoxia, oxidative stress and mutations in retinal genes. This necessitates constant surveillance of the retina for the detection of neuropathological signals. Microglia, the immunocompetent resident macrophages, are initially capable to fulfil this function (Langmann, 2007). They are evenly distributed in the plexiform layers and are extensively ramified during homeostatic conditions to enhance surveillance of their microenvironment (Karlstetter et al., 2015). They possess a full assortment of immune surface proteins to sense their environment for “on” and “off” signals (Karlstetter et al., 2015). Such surface proteins include receptors for complement components, cytokines, chemokines and damage-associated molecular patterns. Importantly, neuron–microglia interactions via such surface receptors contribute to the maintenance of retinal homeostasis. Examples of reciprocal signals between neurons and microglia that mediate retinal homeostasis include interactions between CX3CL1)- CX3CR1, CD200 - CD200R and Sialic acids (on neuronal glycocalyx) - Sialic acid‐binding immunoglobulin‐like lectin‐11 (SIGLEC-11) (Karlstetter et al., 2017).
In the event of an insult, such as degeneration in the retina, a local immune response involving microglia and the complement system is mounted (Karlstetter et al., 2015). Microglia respond by retracting their filopodia and upregulating cell surface molecules including cytokine and chemokine receptors. Subsequently, microglia proliferate and migrate to the damaged layers, releasing neuromodulatory factors to promote the repair of stressed cells.
If the insult persists, the initial ‘constructive’ response quickly turns destructive and is characterized by neurotoxic microglia. These overreactive cells release large amounts of pro-inflammatory and cytotoxic factors such as ROS, RNS, TNF-α and IL-1β (Scholz et al., 2015). Furthermore, overreactive microglia cause dysregulation of the complement system by up-regulating the expression of complement activators and down-regulating complement inhibitors (Madeira et al., 2018). Subsequently, microglia overactivation creates a proinflammatory environment conducive for further recruitment of microglia and exogenous infiltrating monocytes.
Translocator protein 18 kDa (TSPO), previously referred to as the peripheral benzodiazepine receptor (PBR), is a highly conserved 5α-helical transmembrane protein located on the outer mitochondrial membrane. During an active neuropathological process, a strong increase in TSPO protein that colocalizes predominantly with activated microglia is observed in the brain and retina. Simultaneously, Müller cells in the retina upregulate the expression and secretion of the endogenous TSPO ligand, Diazepam binding inhibitor (DBI) protein which is in-turn taken up by microglia. Binding of DBI or its cleavage product triakontatetraneuropeptide (TTN) to TSPO effectively limits the magnitude of microglial inflammatory responses and promotes their return to quiescence (Figure 1).
Inspired by this endogenous immunomodulatory mechanism, our laboratory is testing the ability of a synthetic and highly specific TSPO ligand, XBD173 (AC-5216, emapunil), to influence microglial reactivity in the acute white light-induced retinal degeneration mouse model (Scholz et al., 2015) and in the laser-induced choroidal neovascularization (CNV) model for neovascular age-related macular degeneration (AMD).
Interferon‐beta (IFN-β) is a type I interferon that possesses strong antiviral and immunomodulatory properties. It is an established drug for the treatment of relapsing remitting Multiple Sclerosis (MS). IFN‐β confers neuroprotection in MS by potentiating microglia-mediated phagocytosis of myelin debris with concomitant suppression of neuroinflammatory responses and disease severity. Based on this evidence, we postulated that IFN‐β may have beneficial immunomodulatory effects against chronic inflammatory responses observed in the retina. To test this, we employed the laser-induced choroidal neovascularization (CNV) mouse model and demonstrated that IFN‐β treatment strongly inhibits microgliosis and enhances the morphological transition of microglia towards a neuroprotective ramified phenotype (Lückoff et al., 2016). IFN‐β treatment also resulted in a significant reduction in vascular leakage and neoangiogenesis (Lückoff et al., 2016). Similarly, IFN‐β therapeutic effects in the retina have been corroborated in a separate study using a rabbit model, where local administration of IFN-β accelerated the repair of retinal lesions produced by laser photocoagulation. Ongoing experiments analyze the potentially beneficial effects of IFN-β in the light-damage paradigm of retinal degeneration.
The results from our studies will reveal whether TSPO ligands and IFN‐β are promising pharmacological agents to modulate microglia activation during retinal degenerative diseases.
Karlstetter, M., Scholz, R., Rutar, M., Wong, W. T., Provis, J. M., and Langmann, T. (2015). Retinal microglia: Just bystander or target for therapy? Prog. Retin. Eye Res. 45, 30–57.
Karlstetter, M., Kopatz, J., Aslanidis, A., Shahraz, A., Caramoy, A., Linnartz-Gerlach, B., et al. (2017). Polysialic acid blocks mononuclear phagocyte reactivity, inhibits complement activation, and protects from vascular damage in the retina. EMBO Mol. Med. 9, 154–166.
Langmann, T. (2007). Microglia activation in retinal degeneration. J. Leukoc. Biol. 81, 1345–1351.
Lückoff, A, Caramoy, A., Scholz, R., Prinz, M., Kalinke, U., and Langmann, T et al. (2016). Interferon‐beta signaling in retinal mononuclear phagocytes attenuates pathological neovascularization. EMBO Mol. Med. 8, 670–678.
Madeira, M. H., Rashid, K., Ambrósio, A. F., Santiago, A. R., and Langmann, T. (2018). Blockade of microglial adenosine A2A receptor impacts inflammatory mechanisms, reduces ARPE-19 cell dysfunction and prevents photoreceptor loss in vitro. Sci. Rep. 8, 2272.
Scholz, R., Caramoy, A., Bhuckory, M. B., Rashid, K., Chen, M., Xu, H., et al. (2015). Targeting translocator protein (18 kDa) (TSPO) dampens pro-inflammatory microglia reactivity in the retina and protects from degeneration. J. Neuroinflammation 12, 201.
Balser, C., Wolf, A., Herb, M., and Langmann, T. (2019). Co-inhibition of PGF and VEGF blocks their expression in mononuclear phagocytes and limits neovascularization and leakage in the murine retina. J Neuroinflammation 16, 26.
Wolf, A., and Langmann, T. (2019). Anti-VEGF-A/ANG2 combotherapy limits pathological angiogenesis in the eye: a replication study. EMBO Mol Med 11.
Dannhausen K, Mohle C, and Langmann T (2018). Immunomodulation with minocycline rescues retinal degeneration in juvenile neuronal ceroid lipofuscinosis mice highly susceptible to light damage. Dis Model Mech 11.
Rashid K, Geissl L, Wolf A, Karlstetter M, and Langmann T (2018). Transcriptional regulation of Translocator protein (18kDa) (TSPO) in microglia requires Pu.1, Ap1 and Sp factors. Biochim Biophys Acta Gene Regul Mech 1861, 1119-1133.
Akhtar-Schafer I, Wang L, Krohne TU, Xu H, and Langmann T (2018). Modulation of three key innate immune pathways for the most common retinal degenerative diseases. EMBO Mol Med 10.
Rashid K, Wolf A, and Langmann T (2018). Microglia Activation and Immunomodulatory Therapies for Retinal Degenerations. Front Cell Neurosci 12, 176.
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Dept. of Ophthalmology / RG location - Anatomy Building
Principal Investigator - B 05
Anja Volkmann (secretary)
PD Dr. Marcus Karlstetter (Postdoc)
Dr. Moran Homola (Postdoc)
Daniel Uthoff (Scientist)
Isha Akhtar-Schäfer (PhD student)
Verena Behnke (PhD student)
Amir Saeed Khan (PhD student)
Khalid Rashid (PhD student)
Mona Tabel (PhD student)
Moyinoluwa Temitope Taiwo (PhD student)
Anne Wolf (PhD student)
Carsten Balser (MD student)
Charlotte Bresgen (MD student)
Laura Hintzen (MD student)
Johanna Wiedemann (MD student)
Mathilde Verhoyen (Master student)
Claudia Bismar (technician)
Ulrike Esendink (technician)
Eva Scheiffert (technician)