Minocycline decreases CCR2-positive monocytes in the retina and ameliorates photoreceptor degeneration in a mouse model of retinitis pigmentosa

Ryo Terauchi; Hideo Kohno; Sumiko Watanabe; Saburo Saito; Akira Watanabe; Tadashi Nakano

doi:10.1371/journal.pone.0239108

Abstract

Retinal inflammation accelerates photoreceptor cell death caused by retinal degeneration. Minocycline, a semisynthetic broad-spectrum tetracycline antibiotic, has been previously reported to rescue photoreceptor cell death in retinal degeneration. We examined the effect of minocycline on retinal photoreceptor degeneration using c-mer proto-oncogene tyrosine kinase (Mertk)^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice, which enabled the observation of CX3CR1-green fluorescent protein (GFP)- and CCR2-red fluorescent protein (RFP)-positive macrophages by fluorescence. Retinas of Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice showed photoreceptor degeneration and accumulation of GFP- and RFP-positive macrophages in the outer retina and subretinal space at 6 weeks of age. Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice were intraperitoneally administered minocycline. The number of CCR2-RFP positive cells significantly decreased after minocycline treatment. Furthermore, minocycline administration resulted in partial reversal of the thinning of the outer nuclear layer and decreased the number of apoptotic cells, as assessed by the TUNEL assay, in Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice. In conclusion, we found that minocycline ameliorated photoreceptor cell death in an inherited photoreceptor degeneration model due to Mertk gene deficiency and has an inhibitory effect on CCR2 positive macrophages, which is likely to be a neuroprotective mechanism of minocycline.

Citation: Terauchi R, Kohno H, Watanabe S, Saito S, Watanabe A, Nakano T (2021) Minocycline decreases CCR2-positive monocytes in the retina and ameliorates photoreceptor degeneration in a mouse model of retinitis pigmentosa. PLoS ONE 16(4): e0239108. https://doi.org/10.1371/journal.pone.0239108

Editor: Alfred S. Lewin, University of Florida, UNITED STATES

Received: August 26, 2020; Accepted: March 31, 2021; Published: April 22, 2021

Copyright: © 2021 Terauchi et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting information files.

Funding: Grant information: JSPS KAKENHI Grant-in-Aid for Young Scientists (Start-up) Grant Number 25893253 (for HK), Grant-in-Aid for Young Scientists (B) Grant Number 15K20288 (for HK) and Grant-in-Aid (C) for Scientific Research Grant Number 19K09960 (for HK).

Competing interests: The authors have declared that no competing interests exist.

Introduction

Inflammation in the central nervous system, as well as the retina, is considered a complicating factor in degenerative diseases [1–5]. Moreover, retinal inflammation is considered to accelerate photoreceptor cell death (PCD) in retinal degeneration (RD), including age-related macular degeneration and retinitis pigmentosa [6]. Hence, the management of inflammation is pivotal and presumably beneficial for patients with RD. The elucidation of inflammatory mechanisms for the management of RD is the major research focus.

Minocycline, a semisynthetic broad-spectrum tetracycline antibiotic, has anti-inflammatory properties [7]. Several studies, including our study, have demonstrated that minocycline can ameliorate PCD in RD [8–10]. However, the mechanism of PCD rescue by minocycline remains largely unknown. Two potential mechanisms have been suggested; the anti-apoptotic mechanism and anti-inflammatory mechanism [11]. The innate immune system, which exerts a rapid non-specific response to an antigen, has been implicated in the development of RD, including human age-related macular degeneration and retinitis pigmentosa [12]. In healthy retinas, microglia or guardians of the retina located in the outer and inner plexiform layers maintain retinal homeostasis [12]. However, in RD, microglia are activated and the activated microglia migrate to the outer retina and subretinal space, the space between the outer segments of photoreceptors and retinal pigment epithelium (RPE). Minocycline inhibits both microglial activation and migration and hence, microglial suppression by minocycline is considered the major mechanism of PCD rescue [11]. However, minocycline is not a specific drug. Furthermore, other protagonists of retinal inflammation (i.e., bone marrow-derived macrophages) invade the outer retina [10, 13, 14]. Therefore, delineating the cause and mechanism of rescue of photoreceptor cells in the degenerative stage by minocycline is important.

Recently, we generated c-mer proto-oncogene tyrosine kinase (Mertk)^{−/ −}Cx3cr1^GFP/+Ccr2^RFP/+ mice. This enabled the observation of CX3CR1-green fluorescent protein (GFP)- and CCR2-red fluorescent protein (RFP)-positive cells in inherited RD without light damage or requirement of any non-physiological procedures such as doxycycline administration (widely used for tetracycline-controlled transcriptional activation) [15]. Before RD occurs, only Cx3cr1 expression is observed, which corresponds to resting microglia [16]. In progressive RD, Ccr2 expression is markedly increased [15].

In this study, we found that minocycline administration to Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice not only ameliorated PCD but also reduced the number of CCR2-RFP-positive cells in the outer retina and subretinal space. These results indicate that Ccr2 suppression is one of the mechanisms of photoreceptor protection by minocycline.

Materials and methods

Animals

Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice were generated as previously described [15]. Mertk genotyping was performed using the following primers: wild type, forward 5ʹ-GCTTTAGCCTCCCCAGTAGC-3 ʹ and reverse 5 ʹ-GGTCACATGCAAAGCAAATG-3 ʹ; mutant, forward 5 ʹ-CGTGGAGAAGGTAGTCGTACATCT-3 ʹ and reverse 5 ʹ-TTTGCCAAGTTCTAATTCCATC-3 ʹ. Cx3Cr1 genotyping was performed using the following primers: wild-type, forward 5 ʹ-TCCACGTTCGGTCTGGTGGG-3 ʹ and reverse 5 ʹ-GGTTCCTAGTGGAGCTAGGG-3 ʹ; Cx3cr1 mutant, forward 5 ʹ-GATCACTCTCGGCATGGACG-3 ʹ and reverse 5 ʹ-GGTTCCTAGTGGAGCTAGGG-3 ʹ. Ccr2 genotyping was performed using the following primers: common, forward 5 ʹ-TAAACCTGGTCACCACATGC-3 ʹ; wild-type, reverse 5 ʹ-GGAGTAGAGTGGAGGCAGGA-3 ʹ; Ccr2 mutant, reverse 5 ʹ-CTTGATGACGTCCTCGGAG-3 ʹ.

Equal numbers of male and female mice were used in this study. All mice were housed in the animal facility at the Jikei University School of Medicine and were maintained under a 12-h light (~10 lx)/dark cycle. All animal procedures and experiments were approved by the Jikei University School of Medicine Animal Care Committee and conformed to the recommendations of both the American Veterinary Medical Association Panel on Euthanasia and the Association for Research in Vision and Ophthalmology Statement for the Use of Animals in Ophthalmic and Vision Research.

Minocycline administration

Minocycline was purchased commercially from Sigma-Aldrich (St. Louis, MO, USA) and was dissolved in phosphate-buffered saline. Thereafter, minocycline was intraperitoneally administered once daily to Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice (age: 4–6 weeks). The dose of minocycline was either 50 or 100 mg/kg. Phosphate-buffered saline was administered to the control group.

Flat-mount retina and RPE preparation

All procedures for retinal and RPE flat-mounts were performed as previously described [10]. Images of flat-mounts were captured using a confocal microscope (LSM; Carl Zeiss, Thornwood, NY, USA). Regarding retinal flat-mounts, images of the entire retina were captured at 5 μm intervals and all images were projected in one slice. Regarding RPE flat-mounts, images of the entire visible RPE were captured at 3 μm intervals and projected in one slice.

Histological analysis

All retinal sections were prepared as previously described [10, 17]. The numbers of CX3CR1-GFP- and CCR2-RFP-positive cells were counted using ImageJ (National Institutes of Health, Bethesda, MD, USA). To detect RFP, anti-RFP mouse antibody (MBL, M165-3) was used as the primary antibody, and the signals were visualized using a horseradish peroxidase-tagged anti-mouse IgG antibody (GE Healthcare, NA9310V) and a peroxidase-diaminobenzidine kit (Nacalai Tesque, Kyoto, Japan). Immunohistochemical images were captured using a confocal microscope (LSM 880; Carl Zeiss, Thornwood, NY, USA).

Apoptosis assays

Retinas were frozen, sectioned, and subjected to the TUNEL assay using an in situ apoptosis detection kit (MK500, Takara Bio, Shiga, Japan).

Flow cytometry analysis

The effect of minocycline on circulating monocytes was examined by flow cytometry analysis. Peripheral blood was collected from 16-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice and was incubated with anti-Ly6C conjugated APC-Cy7 antibody (BioLegend, 128025) on ice for 30 min. Thereafter, cells were washed and stained with propidium iodide to exclude dead cells and analyzed using a BD FACSAria III Cell Sorter (BD Biosciences, San Jose, CA, USA). Data were analyzed using FlowJo software version 10.7.1 (Tree Star, Ashland, OR, USA).

Data analysis

Continuous variables are presented as mean ± standard deviation. The Steel–Dwass test was performed for non-parametric multiple comparisons between the groups. All statistical analyses were performed using the statistical program R (version 4.0.3; R Foundation for Statistical Computing). Statistical significance was set at P < 0.05.

Results

Characterization of the Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mouse retina

In Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice, the descriptions “4-” and “6-week-old” correspond to the retinal non-degenerative and RD ongoing stage, respectively [15]. Representative CX3CR1-GFP single-positive cells observed in retinal flat-mounts of 4-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice and CX3CR1/CCR2 double-positive cells observed in RPE flat-mounts of 6-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice are presented in Fig 1A and 1B. Time series vertical sections of Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice are presented in Fig 1C–1E. In 4-week-old mice, only CX3CR1-GFP single-positive cells were visible in the inner retina (Fig 1C). Neither CX3CR1-GFP- nor CCR2-RFP-positive cells were observed in the outer retina and subretinal space. In 3-month-old mice, RD, represented by thinning of the outer nuclear layer (ONL), was observed (Fig 1D). The number of ONL nuclei decreased from approximately 12 at 4 weeks to 1–4 at 3 months. Abundant CX3CR1-GFP- and CCR2-RFP-positive cells were observed in the ONL and subretinal space (Fig 1D). Some cells were CX3CR1 and CCR2 double-positive. In 1.5-year-old mice, almost all nuclei in the ONL had disappeared, indicating severe RD (Fig 1E). The frequency of CX3CR1-GFP- and CCR2-RFP-positive cells was found to be lower compared to the ongoing degeneration stage (e.g., from 6 weeks to 3 months).

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Fig 1. Characterization of Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice.

(A) Magnified CX3CR1 single-positive cells observed in retinal flat-mounts from 4-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice and (B) CX3CR1-GFP and CCR2-RFP double-positive cells in RPE flat-mounts from 6-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice (scale bar = 20 μm). (C-E) Vertical sections from 4-week-, 3-month-, and 1.5-year-old Mertk^-/-Cx3cr1^GFP/+Ccr2^RFP/+ mice (scale bar = 50 μm). Inserts are the magnified images. GCL, ganglion cell layer; INL, inner nuclear layer; ONL, outer nuclear layer; RPE, retinal pigmented epithelium.

https://doi.org/10.1371/journal.pone.0239108.g001

Minocycline administration reduced the number of CCR2-positive cells in neural retina

The numbers of CCR2-RFP-positive cells and CX3CR1-GFP and CCR2-RFP double-positive cells were reduced in the 50 and 100 mg/kg minocycline-treated (Mino50 and Mino100) groups than in the control group (Fig 2E and 2F). In retinal flat-mounts, retinal layer boundaries are difficult to observe. However, many CCR2-RFP-positive cells were observed in the outer plexiform layer and ONL in the 3D images of the control group (Fig 2B). In the 3D image of the Mino100 group, CCR2-RFP was hardly detected (Fig 2C). On the contrary, the number of CX3CR1-GFP-positive cells was not affected by minocycline administration (Fig 2D).

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Fig 2. Minocycline administration reduced CCR2-positive cells in retinal flat-mounts.

An intraperitoneal injection of minocycline was administered daily for 14 days to Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice (age: 4–6 weeks). The minocycline group was divided into two subgroups: the Mino50 (50 mg/kg) and Mino100 (100 mg/kg) groups. Phosphate-buffered saline (PBS) was administered to the control group. (A) The retinal flat-mounts of each group were prepared after treatment at 6 weeks of age and observed by laser confocal microscopy. (B) and (C) 3D images of the control and Mino100 groups. The numbers of (F) CX3CR1-GFP positive (GFP+RFP− and GFP+RFP+) cells, (G) CCR2-RFP positive (GFP-RFP+ and GFP+RFP+) cells, and (H) CX3CR1-GFP and CCR2-RFP double-positive (GFP+RFP+) cells in each group (n ≥ 5 per group). * indicates P < 0.05. OS, outer segment. GFP, green fluorescent protein; RFP, red fluorescent protein.

https://doi.org/10.1371/journal.pone.0239108.g002

Minocycline administration reduced the number of CCR2 positive cells in the subretinal space

RPE flat-mounts were prepared to observe the apical side of the RPE, corresponding to the subretinal space (Fig 3) [10, 15, 18]. In the RPE flat-mounts from 4-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ and Mertk^+/+Cx3cr1^GFP/+Ccr2^RFP/+ mice that did not show RD, neither CX3CR1-GFP positive cells nor CCR2-RFP positive cells were observed [15]. Abundant CX3CR1-GFP positive cells were observed in the subretinal space in the control, Mino50, and Mino100 groups (Fig 3A). The number of CCR2-RFP positive cells decreased in the Mino50 and Mino100 groups compared with the control group (Fig 3E and 3F); however, the number of CX3CR1-GFP positive cells did not change in all the groups (Fig 3D), indicating that minocycline administration probably did not affect the migration of CX3CR1-GFP positive cells to the subretinal space but suppressed the migration of CCR2-positive cells.

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Fig 3. Minocycline administration reduced CCR2-positive cells in RPE flat-mount.

An intraperitoneal injection of minocycline was administered daily for 14 days to Mertk^-/-Cx3cr1^GFP/+Ccr2^RFP/+ mice (age: 4–6 weeks). (A) RPE flat-mounts from control, Mino50, and Mino100 were prepared after treatment at 6 weeks of age. (B) and (C) 3D images of the control and Mino100. The numbers of (D) CX3CR1-GFP positive (GFP+RFP− and GFP+RFP+) cells, (E) CCR2-positive (GFP−RFP+ and GFP+RFP+) cells, and (F) CX3CR1-GFP and CCR2-RFP double-positive (GFP+RFP+) cells in each group (n ≥ 5 per group). * indicates P < 0.05. GFP, green fluorescent protein; RFP, red fluorescent protein.

https://doi.org/10.1371/journal.pone.0239108.g003

Thereafter, we confirmed that the fluorescent signals in the subretinal region were not caused by auto-fluorescence by a colorimetric assay. Immunohistochemistry was performed on the frozen sections of retinas of 3-month-old Mertk^-/-Cx3cr1^GFP/+Ccr2^RFP/+ mice by the peroxidase-diaminobenzidine method. Diaminobenzidine signals were observed in the outer retina (Fig 4A), which was consistent with the fluorescence observation (Fig 1D). Diaminobenzidine staining revealed that minocycline treatment decreased the number of CCR2-RFP positive cells (Fig 4B and 4C).

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Fig 4. Diaminobenzidine staining for CCR2-RFP positive cells.

(A) and (B) Frozen sections of retinas derived from 3-month-old control and minocycline (50 and 100 mg/kg)-treated Mertk^-/-Cx3cr1^GFP/+Ccr2^RFP/+ mice were immunostained with anti-RFP antibody. Signals were visualized using the peroxidase-diaminobenzidine method. Positive signals are indicated by black arrows (scale bar = 50 μm). The number of positive cells in 1-mm wide immunostained sections of control and minocycline (50 and 100 mg/kg)-treated retinas were counted (n ≥ 5 per group) (C) * indicates P < 0.05. RFP, red fluorescent protein.

https://doi.org/10.1371/journal.pone.0239108.g004

Amelioration of PCD by minocycline administration

Finally, we evaluated the therapeutic effects of minocycline in Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice. Previously, we reported PCD amelioration by minocycline administration in a light-induced acute RD mouse model (Abca4^−/−Rdh8^−/− mice) [10]. However, the therapeutic effect of minocycline in inherited RD due to Mertk gene deficiency was unknown. First, minocycline was administered to 4-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice for 2 weeks. However, the severity of PCD did not differ between the minocycline-treated and control mice because PCD was relatively mild at the age of 6 weeks in Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice. Thereafter, minocycline (50 mg/kg) or PBS was administered daily for 2 weeks from the age of 6 weeks to Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice (Fig 5). Minocycline-treated Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mouse retinas showed fewer CCR2-RFP positive cells in the subretinal region compared with control mouse retinas (Fig 5A). The thickness of the ONL was retained in the central region but not in the peripheral region (Fig 5B and 5C), indicating amelioration of PCD by minocycline. In addition, minocycline significantly suppressed the proportion of TUNEL-positive photoreceptor cells (Fig 5D and 5E). The ONL thickness in 4-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ and wild type (B6) mice are shown as negative controls in S1 Fig.

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Fig 5. Minocycline administration ameliorates photoreceptor cell death in Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice.

Minocycline (50 mg/kg [Mino50] and 100 mg/kg [Mino100]) or phosphate-buffered saline (PBS; control) was administered once daily to Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice (age: 6–8 weeks). (A) Retinas were frozen and sectioned, and nuclei were visualized by DAPI staining. (B) Thickness and (C) the number of cells of the ONL were measured (n ≥ 5 per group). (D) Apoptotic cells in frozen sections of retinas of control and minocycline (50 mg/kg)-treated Mertk^-/-Cx3cr1^GFP/+Ccr2^RFP/+ mice were examined by the TUNEL assay. (E) Proportion of TUNEL-positive cells in the control, Mino50, and Mino100 groups are shown. (B), (C), and (E): * indicates P < 0.05. (B): ** indicates P < 0.01. Gray and black asterisks indicate P values of control vs. Mino50 and control vs. Mino100, respectively. (A) and (D): Scale bar = 50 μm. DAPI, 4’,6-diamidino-2-phenylindole; ONL, outer nuclear layer; TUNEL, TdT-mediated dUTP nick end labeling.

https://doi.org/10.1371/journal.pone.0239108.g005

Long-term minocycline administration

To examine whether minocycline has protective effects against photoreceptor degeneration in the later stage in the Mertk knockout model, Mertk ^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice were treated with minocycline from 8-week-old to 16-week-old. In the control group, the number of CCR2-RFP positive cells accumulated in the subretinal space (Fig 6A). On the other hand, fewer CCR2-RFP positive cells were observed in retinas of minocycline (50 and 100 mg/kg)-treated mice (Fig 6A and 6B). The ONL thickness in minocycline-treated retina was retained, especially in the central region of the retina (Fig 6C), and the numbers of cells in the ONL of the Mino50 and Mino100 groups were significantly higher than that of the control group (Fig 6D).

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Fig 6. Long-term minocycline administration ameliorates photoreceptor cell death in Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice.

Minocycline (50 mg/kg; Mino50), minocycline (100 mg/kg; Mino100), or phosphate-buffered saline (PBS; control) was administered once daily to Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice from 8 to 16 weeks of age. Retinas were frozen and sectioned, and nuclei were visualized by DAPI staining (A) (scale bar = 50 μm). The number of CCR2-RFP positive cells in the subretinal space was counted. Values are the average of five independent samples with standard deviations. Thickness (B) and the number of cells (C) of the ONL were measured (n ≥ 5 per group). * indicates P < 0.05. (C): the gray and black asterisks indicate P values of control vs. Mino50 and control vs. Mino100, respectively. DAPI, 4’,6-diamidino-2-phenylindole; ONL, outer nuclear layer; RFP, red fluorescent protein.

https://doi.org/10.1371/journal.pone.0239108.g006

To examine whether minocycline treatment affects circulating monocytes, we examined Ly6C positive monocytes in the peripheral blood of control, Mino50, and Mino100 treated Mertk^-/-Cx3cr1^GFP/+Ccr2^RFP/+ mice (S2A Fig). We found no significant difference in the proportion of Ly6C positive cells among control and minocycline-treated mice (S1B Fig).

Discussion

Minocycline is considered an anti-inflammatory and neuroprotective drug candidate for RD [8, 19]. In this study, we investigated the therapeutic effects of minocycline on RD induced by Mertk knockout. These mice contained GFP and RFP in the Cx3cr1 and Ccr2 loci, respectively, allowing us to monitor and evaluate microglia and macrophages during degeneration. We found that minocycline administration suppressed the number of CCR2-RFP positive cells and partially protected photoreceptor degeneration.

In Mertk knockout models, CCR2-RFP single and CCR2-RFP and Cx3cr1-GFP double-positive cells accumulated in the outer retina and subretinal space, which is consistent with the findings of our study [15]. CCR2 is regarded as an essential chemokine receptor for macrophage recruitment to inflammation sites [20] and is detectable in monocyte-derived macrophages but not in resident microglia [21]. Therefore, CCR2-RFP positive cells observed in the current study were considered macrophages. However, it should be noted that the localization of monocyte-derived macrophage in RD varies among degeneration models, that is, the retinal ischemia-reperfusion model showed the accumulation of macrophages mainly in the ganglion cell layer and inner plexiform layer [22]. Light damage induced only activated resident microglia, but not monocyte-derived macrophages, into the subretinal space [23]. These findings suggest that the effects of minocycline vary depending on the degeneration model.

Recently, it was reported that minocycline has an effect on the CCL2/CCR2 pathway and macrophage infiltration. In primary microglia (in vitro) and neuropathic pain rat models (in vivo), minocycline suppressed the upregulation of CCL2 and CCR2 [24]. In a mouse model of cerebellar hemorrhage, infiltration of monocytes and macrophages into the cerebellum was decreased by minocycline treatment [25]. In the current study, significant inhibition of the number of CCR2-RFP positive cells in both the retina and RPE was observed in flat-mounts. This finding is consistent with the findings of previous studies. However, to fully understand the neuroprotective mechanisms of minocycline, it should be clarified whether minocycline suppressed the CCL2-induced CCR2 signaling pathway and whether minocycline decreased the recruitment of CCR2-expressing monocytes or the CCR2 expression level in monocytes. Further, whether the inhibition of the CCL2/CCR2 pathway in macrophages and monocyte infiltration suppress RD remains unclear. The anti-inflammatory and neuroprotective effects of CCL2/CCR2 suppression in RD have been reported [15, 26]; however, a study reported that the elimination of the CCL2/CCR2 pathway inhibited monocyte infiltration, but did not block neurodegeneration in a light injury mouse model [13]. This issue should be resolved to elucidate the neuroprotective effects of minocycline.

We found that minocycline had a dose-dependent neuroprotective effect on photoreceptor cells (Figs 5 and 6). However, it should be noted that no significant difference was noted in the peripheral ONL thickness between control and minocycline-treated retinas. These findings suggest that minocycline was not completely protective against photoreceptor cell degeneration, which is suggested by the finding that 20%–30% of photoreceptor cells were stained with TUNEL after minocycline treatment (Fig 5D and 5E).

Because macrophages infiltrating the retina are thought to subsequently reduce Ccr2 expression [21], it is possible that the number of macrophages was misled by the altered expression level of Ccr2. To accurately verify the extent of monocyte infiltration inhibition by minocycline, an additional technique that specifically labels all retinal macrophages should be used to distinguish them from resident microglia.

In the current study, we investigated the neuroprotective effects of minocycline using a Mertk knockout mouse model. Consequently, minocycline ameliorated PCD in inherited RD due to Mertk gene deficiency and reduced the total number of CCR2-RFP positive monocyte-derived macrophages accumulating in the outer retina and subretinal space. The suppression of the chemokine receptor CCR2 in retinal macrophages might be one of the neuroprotective mechanisms of minocycline. Further studies are warranted to examine whether minocycline decreases the number of infiltrating monocytes or suppresses Ccr2 expression in monocytes.

Supporting information

S1 Fig. The outer nuclear layer thickness of 4-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ and wild type mice.

The outer nuclear layer thickness of 4-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice and wild type (B6) mice are presented as negative controls in Fig 5.

https://doi.org/10.1371/journal.pone.0239108.s001

(TIFF)

S2 Fig. The population of Ly6C positive circulating monocytes in minocycline- and control-treated Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice.

(A) Ly6C positive monocytes were detected in control Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice by flow cytometry analysis. (B) The proportion of Ly6C positive monocytes in the control, Mino50, and Mino100 groups (n = 3 per group).

https://doi.org/10.1371/journal.pone.0239108.s002

(TIFF)

References

1. Saban DR. New concepts in macrophage ontogeny in the adult neural retina. Cell Immunol. 2018;330:79–85. Epub 2018/04/29. pmid:29703455.
- View Article
- PubMed/NCBI
- Google Scholar
2. Murakami Y, Ikeda Y, Nakatake S, Fujiwara K, Tachibana T, Yoshida N, et al. C-Reactive protein and progression of vision loss in retinitis pigmentosa. Acta Ophthalmol. 2018;96(2):e174–e9. Epub 2017/06/22. pmid:28636270.
- View Article
- PubMed/NCBI
- Google Scholar
3. Guillonneau X, Eandi CM, Paques M, Sahel JA, Sapieha P, Sennlaub F. On phagocytes and macular degeneration. Prog Retin Eye Res. 2017;61:98–128. Epub 2017/06/13. pmid:28602950.
- View Article
- PubMed/NCBI
- Google Scholar
4. Karlstetter M, Scholz R, Rutar M, Wong WT, Provis JM, Langmann T. Retinal microglia: just bystander or target for therapy? Prog Retin Eye Res. 2015;45:30–57. Epub 2014/12/06. pmid:25476242.
- View Article
- PubMed/NCBI
- Google Scholar
5. Cuenca N, Fernandez-Sanchez L, Campello L, Maneu V, De la Villa P, Lax P, et al. Cellular responses following retinal injuries and therapeutic approaches for neurodegenerative diseases. Prog Retin Eye Res. 2014;43:17–75. Epub 2014/07/20. pmid:25038518.
- View Article
- PubMed/NCBI
- Google Scholar
6. Murakami Y, Ishikawa K, Nakao S, Sonoda KH. Innate immune response in retinal homeostasis and inflammatory disorders. Prog Retin Eye Res. 2020;74:100778. Epub 2019/09/11. pmid:31505218.
- View Article
- PubMed/NCBI
- Google Scholar
7. Moller T, Bard F, Bhattacharya A, Biber K, Campbell B, Dale E, et al. Critical data-based re-evaluation of minocycline as a putative specific microglia inhibitor. Glia. 2016;64(10):1788–94. Epub 2016/06/02. pmid:27246804.
- View Article
- PubMed/NCBI
- Google Scholar
8. Scholz R, Sobotka M, Caramoy A, Stempfl T, Moehle C, Langmann T. Minocycline counter-regulates pro-inflammatory microglia responses in the retina and protects from degeneration. J Neuroinflammation. 2015;12:209. Epub 2015/11/19. pmid:26576678.
- View Article
- PubMed/NCBI
- Google Scholar
9. Peng B, Xiao J, Wang K, So KF, Tipoe GL, Lin B. Suppression of microglial activation is neuroprotective in a mouse model of human retinitis pigmentosa. J Neurosci. 2014;34(24):8139–50. Epub 2014/06/13. pmid:24920619.
- View Article
- PubMed/NCBI
- Google Scholar
10. Kohno H, Chen Y, Kevany BM, Pearlman E, Miyagi M, Maeda T, et al. Photoreceptor proteins initiate microglial activation via Toll-like receptor 4 in retinal degeneration mediated by all-trans-retinal. J Biol Chem. 2013;288(21):15326–41. pmid:23572532.
- View Article
- PubMed/NCBI
- Google Scholar
11. Di Pierdomenico J, Garcia-Ayuso D, Agudo-Barriuso M, Vidal-Sanz M, Villegas-Perez MP. Role of microglial cells in photoreceptor degeneration. Neural Regen Res. 2019;14(7):1186–90. Epub 2019/02/26. pmid:30804243.
- View Article
- PubMed/NCBI
- Google Scholar
12. Fletcher EL. Contribution of microglia and monocytes to the development and progression of age related macular degeneration. Ophthalmic Physiol Opt. 2020;40(2):128–39. Epub 2020/02/06. pmid:32017190.
- View Article
- PubMed/NCBI
- Google Scholar
13. Karlen SJ, Miller EB, Wang X, Levine ES, Zawadzki RJ, Burns ME. Monocyte infiltration rather than microglia proliferation dominates the early immune response to rapid photoreceptor degeneration. J Neuroinflammation. 2018;15(1):344. Epub 2018/12/17. pmid:30553275.
- View Article
- PubMed/NCBI
- Google Scholar
14. Nussenblatt RB, Ferris F 3rd. Age-related macular degeneration and the immune response: implications for therapy. American journal of ophthalmology. 2007;144(4):618–26. pmid:17698021.
- View Article
- PubMed/NCBI
- Google Scholar
15. Kohno H, Koso H, Okano K, Sundermeier TR, Saito S, Watanabe S, et al. Expression pattern of Ccr2 and Cx3cr1 in inherited retinal degeneration. J Neuroinflammation. 2015;12:188. pmid:26458944.
- View Article
- PubMed/NCBI
- Google Scholar
16. Lee JE, Liang KJ, Fariss RN, Wong WT. Ex vivo dynamic imaging of retinal microglia using time-lapse confocal microscopy. Investigative ophthalmology & visual science. 2008;49(9):4169–76. Epub 2008/05/20. pmid:18487378.
- View Article
- PubMed/NCBI
- Google Scholar
17. Maeda A, Maeda T, Imanishi Y, Kuksa V, Alekseev A, Bronson JD, et al. Role of photoreceptor-specific retinol dehydrogenase in the retinoid cycle in vivo. The Journal of biological chemistry. 2005;280(19):18822–32. Epub 2005/03/10. pmid:15755727.
- View Article
- PubMed/NCBI
- Google Scholar
18. Kohno H, Maeda T, Perusek L, Pearlman E, Maeda A. CCL3 production by microglial cells modulates disease severity in murine models of retinal degeneration. J Immunol. 2014;192(8):3816–27. pmid:24639355.
- View Article
- PubMed/NCBI
- Google Scholar
19. Di Pierdomenico J, Scholz R, Valiente-Soriano FJ, Sánchez-Migallón MC, Vidal-Sanz M, Langmann T, et al. Neuroprotective Effects of FGF2 and Minocycline in Two Animal Models of Inherited Retinal Degeneration. Investigative ophthalmology & visual science. 2018;59(11):4392–403. Epub 2018/09/08. pmid:30193320.
- View Article
- PubMed/NCBI
- Google Scholar
20. Chu HX, Arumugam TV, Gelderblom M, Magnus T, Drummond GR, Sobey CG. Role of CCR2 in inflammatory conditions of the central nervous system. J Cereb Blood Flow Metab. 2014;34(9):1425–9. Epub 2014/07/02. pmid:24984897.
- View Article
- PubMed/NCBI
- Google Scholar
21. Ronning KE, Karlen SJ, Miller EB, Burns ME. Molecular profiling of resident and infiltrating mononuclear phagocytes during rapid adult retinal degeneration using single-cell RNA sequencing. Scientific reports. 2019;9(1):4858. Epub 2019/03/21. pmid:30890724.
- View Article
- PubMed/NCBI
- Google Scholar
22. Ahmed A, Wang LL, Abdelmaksoud S, Aboelgheit A, Saeed S, Zhang CL. Minocycline modulates microglia polarization in ischemia-reperfusion model of retinal degeneration and induces neuroprotection. Scientific reports. 2017;7(1):14065. Epub 2017/10/27. pmid:29070819.
- View Article
- PubMed/NCBI
- Google Scholar
23. O’Koren EG, Yu C, Klingeborn M, Wong AYW, Prigge CL, Mathew R, et al. Microglial Function Is Distinct in Different Anatomical Locations during Retinal Homeostasis and Degeneration. Immunity. 2019;50(3):723–37 e7. Epub 2019/03/10. pmid:30850344.
- View Article
- PubMed/NCBI
- Google Scholar
24. Piotrowska A, Kwiatkowski K, Rojewska E, Slusarczyk J, Makuch W, Basta-Kaim A, et al. Direct and indirect pharmacological modulation of CCL2/CCR2 pathway results in attenuation of neuropathic pain—In vivo and in vitro evidence. J Neuroimmunol. 2016;297:9–19. Epub 2016/07/12. pmid:27397071.
- View Article
- PubMed/NCBI
- Google Scholar
25. Xie ST, Chen AX, Song B, Fan J, Li W, Xing Z, et al. Suppression of microglial activation and monocyte infiltration ameliorates cerebellar hemorrhage induced-brain injury and ataxia. Brain Behav Immun. 2020;89:400–13. Epub 2020/07/28. pmid:32717406.
- View Article
- PubMed/NCBI
- Google Scholar
26. Sennlaub F, Auvynet C, Calippe B, Lavalette S, Poupel L, Hu SJ, et al. CCR2(+) monocytes infiltrate atrophic lesions in age-related macular disease and mediate photoreceptor degeneration in experimental subretinal inflammation in Cx3cr1 deficient mice. EMBO molecular medicine. 2013;5(11):1775–93. Epub 2013/10/22. pmid:24142887.
- View Article
- PubMed/NCBI
- Google Scholar

[ref1] 1. Saban DR. New concepts in macrophage ontogeny in the adult neural retina. Cell Immunol. 2018;330:79–85. Epub 2018/04/29. pmid:29703455.
View Article
PubMed/NCBI
Google Scholar

[2] View Article

[3] PubMed/NCBI

[4] Google Scholar

[ref2] 2. Murakami Y, Ikeda Y, Nakatake S, Fujiwara K, Tachibana T, Yoshida N, et al. C-Reactive protein and progression of vision loss in retinitis pigmentosa. Acta Ophthalmol. 2018;96(2):e174–e9. Epub 2017/06/22. pmid:28636270.
View Article
PubMed/NCBI
Google Scholar

[6] View Article

[7] PubMed/NCBI

[8] Google Scholar

[ref3] 3. Guillonneau X, Eandi CM, Paques M, Sahel JA, Sapieha P, Sennlaub F. On phagocytes and macular degeneration. Prog Retin Eye Res. 2017;61:98–128. Epub 2017/06/13. pmid:28602950.
View Article
PubMed/NCBI
Google Scholar

[10] View Article

[11] PubMed/NCBI

[12] Google Scholar

[ref4] 4. Karlstetter M, Scholz R, Rutar M, Wong WT, Provis JM, Langmann T. Retinal microglia: just bystander or target for therapy? Prog Retin Eye Res. 2015;45:30–57. Epub 2014/12/06. pmid:25476242.
View Article
PubMed/NCBI
Google Scholar

[14] View Article

[15] PubMed/NCBI

[16] Google Scholar

[ref5] 5. Cuenca N, Fernandez-Sanchez L, Campello L, Maneu V, De la Villa P, Lax P, et al. Cellular responses following retinal injuries and therapeutic approaches for neurodegenerative diseases. Prog Retin Eye Res. 2014;43:17–75. Epub 2014/07/20. pmid:25038518.
View Article
PubMed/NCBI
Google Scholar

[18] View Article

[19] PubMed/NCBI

[20] Google Scholar

[ref6] 6. Murakami Y, Ishikawa K, Nakao S, Sonoda KH. Innate immune response in retinal homeostasis and inflammatory disorders. Prog Retin Eye Res. 2020;74:100778. Epub 2019/09/11. pmid:31505218.
View Article
PubMed/NCBI
Google Scholar

[22] View Article

[23] PubMed/NCBI

[24] Google Scholar

[ref7] 7. Moller T, Bard F, Bhattacharya A, Biber K, Campbell B, Dale E, et al. Critical data-based re-evaluation of minocycline as a putative specific microglia inhibitor. Glia. 2016;64(10):1788–94. Epub 2016/06/02. pmid:27246804.
View Article
PubMed/NCBI
Google Scholar

[26] View Article

[27] PubMed/NCBI

[28] Google Scholar

[ref8] 8. Scholz R, Sobotka M, Caramoy A, Stempfl T, Moehle C, Langmann T. Minocycline counter-regulates pro-inflammatory microglia responses in the retina and protects from degeneration. J Neuroinflammation. 2015;12:209. Epub 2015/11/19. pmid:26576678.
View Article
PubMed/NCBI
Google Scholar

[30] View Article

[31] PubMed/NCBI

[32] Google Scholar

[ref9] 9. Peng B, Xiao J, Wang K, So KF, Tipoe GL, Lin B. Suppression of microglial activation is neuroprotective in a mouse model of human retinitis pigmentosa. J Neurosci. 2014;34(24):8139–50. Epub 2014/06/13. pmid:24920619.
View Article
PubMed/NCBI
Google Scholar

[34] View Article

[35] PubMed/NCBI

[36] Google Scholar

[ref10] 10. Kohno H, Chen Y, Kevany BM, Pearlman E, Miyagi M, Maeda T, et al. Photoreceptor proteins initiate microglial activation via Toll-like receptor 4 in retinal degeneration mediated by all-trans-retinal. J Biol Chem. 2013;288(21):15326–41. pmid:23572532.
View Article
PubMed/NCBI
Google Scholar

[38] View Article

[39] PubMed/NCBI

[40] Google Scholar

[ref11] 11. Di Pierdomenico J, Garcia-Ayuso D, Agudo-Barriuso M, Vidal-Sanz M, Villegas-Perez MP. Role of microglial cells in photoreceptor degeneration. Neural Regen Res. 2019;14(7):1186–90. Epub 2019/02/26. pmid:30804243.
View Article
PubMed/NCBI
Google Scholar

[42] View Article

[43] PubMed/NCBI

[44] Google Scholar

[ref12] 12. Fletcher EL. Contribution of microglia and monocytes to the development and progression of age related macular degeneration. Ophthalmic Physiol Opt. 2020;40(2):128–39. Epub 2020/02/06. pmid:32017190.
View Article
PubMed/NCBI
Google Scholar

[46] View Article

[47] PubMed/NCBI

[48] Google Scholar

[ref13] 13. Karlen SJ, Miller EB, Wang X, Levine ES, Zawadzki RJ, Burns ME. Monocyte infiltration rather than microglia proliferation dominates the early immune response to rapid photoreceptor degeneration. J Neuroinflammation. 2018;15(1):344. Epub 2018/12/17. pmid:30553275.
View Article
PubMed/NCBI
Google Scholar

[50] View Article

[51] PubMed/NCBI

[52] Google Scholar

[ref14] 14. Nussenblatt RB, Ferris F 3rd. Age-related macular degeneration and the immune response: implications for therapy. American journal of ophthalmology. 2007;144(4):618–26. pmid:17698021.
View Article
PubMed/NCBI
Google Scholar

[54] View Article

[55] PubMed/NCBI

[56] Google Scholar

[ref15] 15. Kohno H, Koso H, Okano K, Sundermeier TR, Saito S, Watanabe S, et al. Expression pattern of Ccr2 and Cx3cr1 in inherited retinal degeneration. J Neuroinflammation. 2015;12:188. pmid:26458944.
View Article
PubMed/NCBI
Google Scholar

[58] View Article

[59] PubMed/NCBI

[60] Google Scholar

[ref16] 16. Lee JE, Liang KJ, Fariss RN, Wong WT. Ex vivo dynamic imaging of retinal microglia using time-lapse confocal microscopy. Investigative ophthalmology & visual science. 2008;49(9):4169–76. Epub 2008/05/20. pmid:18487378.
View Article
PubMed/NCBI
Google Scholar

[62] View Article

[63] PubMed/NCBI

[64] Google Scholar

[ref17] 17. Maeda A, Maeda T, Imanishi Y, Kuksa V, Alekseev A, Bronson JD, et al. Role of photoreceptor-specific retinol dehydrogenase in the retinoid cycle in vivo. The Journal of biological chemistry. 2005;280(19):18822–32. Epub 2005/03/10. pmid:15755727.
View Article
PubMed/NCBI
Google Scholar

[66] View Article

[67] PubMed/NCBI

[68] Google Scholar

[ref18] 18. Kohno H, Maeda T, Perusek L, Pearlman E, Maeda A. CCL3 production by microglial cells modulates disease severity in murine models of retinal degeneration. J Immunol. 2014;192(8):3816–27. pmid:24639355.
View Article
PubMed/NCBI
Google Scholar

[70] View Article

[71] PubMed/NCBI

[72] Google Scholar

[ref19] 19. Di Pierdomenico J, Scholz R, Valiente-Soriano FJ, Sánchez-Migallón MC, Vidal-Sanz M, Langmann T, et al. Neuroprotective Effects of FGF2 and Minocycline in Two Animal Models of Inherited Retinal Degeneration. Investigative ophthalmology & visual science. 2018;59(11):4392–403. Epub 2018/09/08. pmid:30193320.
View Article
PubMed/NCBI
Google Scholar

[74] View Article

[75] PubMed/NCBI

[76] Google Scholar

[ref20] 20. Chu HX, Arumugam TV, Gelderblom M, Magnus T, Drummond GR, Sobey CG. Role of CCR2 in inflammatory conditions of the central nervous system. J Cereb Blood Flow Metab. 2014;34(9):1425–9. Epub 2014/07/02. pmid:24984897.
View Article
PubMed/NCBI
Google Scholar

[78] View Article

[79] PubMed/NCBI

[80] Google Scholar

[ref21] 21. Ronning KE, Karlen SJ, Miller EB, Burns ME. Molecular profiling of resident and infiltrating mononuclear phagocytes during rapid adult retinal degeneration using single-cell RNA sequencing. Scientific reports. 2019;9(1):4858. Epub 2019/03/21. pmid:30890724.
View Article
PubMed/NCBI
Google Scholar

[82] View Article

[83] PubMed/NCBI

[84] Google Scholar

[ref22] 22. Ahmed A, Wang LL, Abdelmaksoud S, Aboelgheit A, Saeed S, Zhang CL. Minocycline modulates microglia polarization in ischemia-reperfusion model of retinal degeneration and induces neuroprotection. Scientific reports. 2017;7(1):14065. Epub 2017/10/27. pmid:29070819.
View Article
PubMed/NCBI
Google Scholar

[86] View Article

[87] PubMed/NCBI

[88] Google Scholar

[ref23] 23. O’Koren EG, Yu C, Klingeborn M, Wong AYW, Prigge CL, Mathew R, et al. Microglial Function Is Distinct in Different Anatomical Locations during Retinal Homeostasis and Degeneration. Immunity. 2019;50(3):723–37 e7. Epub 2019/03/10. pmid:30850344.
View Article
PubMed/NCBI
Google Scholar

[90] View Article

[91] PubMed/NCBI

[92] Google Scholar

[ref24] 24. Piotrowska A, Kwiatkowski K, Rojewska E, Slusarczyk J, Makuch W, Basta-Kaim A, et al. Direct and indirect pharmacological modulation of CCL2/CCR2 pathway results in attenuation of neuropathic pain—In vivo and in vitro evidence. J Neuroimmunol. 2016;297:9–19. Epub 2016/07/12. pmid:27397071.
View Article
PubMed/NCBI
Google Scholar

[94] View Article

[95] PubMed/NCBI

[96] Google Scholar

[ref25] 25. Xie ST, Chen AX, Song B, Fan J, Li W, Xing Z, et al. Suppression of microglial activation and monocyte infiltration ameliorates cerebellar hemorrhage induced-brain injury and ataxia. Brain Behav Immun. 2020;89:400–13. Epub 2020/07/28. pmid:32717406.
View Article
PubMed/NCBI
Google Scholar

[98] View Article

[99] PubMed/NCBI

[100] Google Scholar

[ref26] 26. Sennlaub F, Auvynet C, Calippe B, Lavalette S, Poupel L, Hu SJ, et al. CCR2(+) monocytes infiltrate atrophic lesions in age-related macular disease and mediate photoreceptor degeneration in experimental subretinal inflammation in Cx3cr1 deficient mice. EMBO molecular medicine. 2013;5(11):1775–93. Epub 2013/10/22. pmid:24142887.
View Article
PubMed/NCBI
Google Scholar

[102] View Article

[103] PubMed/NCBI

[104] Google Scholar

Figures

Abstract

Introduction

Materials and methods

Animals

Minocycline administration

Flat-mount retina and RPE preparation

Histological analysis

Apoptosis assays

Flow cytometry analysis

Data analysis

Results

Characterization of the Mertk−/−Cx3cr1GFP/+Ccr2RFP/+ mouse retina

Minocycline administration reduced the number of CCR2-positive cells in neural retina

Minocycline administration reduced the number of CCR2 positive cells in the subretinal space

Amelioration of PCD by minocycline administration

Long-term minocycline administration

Discussion

Supporting information

S1 Fig. The outer nuclear layer thickness of 4-week-old Mertk−/−Cx3cr1GFP/+Ccr2RFP/+ and wild type mice.

S2 Fig. The population of Ly6C positive circulating monocytes in minocycline- and control-treated Mertk−/−Cx3cr1GFP/+Ccr2RFP/+ mice.

References

Characterization of the Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mouse retina

S1 Fig. The outer nuclear layer thickness of 4-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ and wild type mice.

S2 Fig. The population of Ly6C positive circulating monocytes in minocycline- and control-treated Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice.