Background
Glaucoma is a complex multifactorial disease characterized by the progressive dysfunction and loss of retinal ganglion cells and associated visual field deficits. It remains a leading cause of vision loss affecting 70 million people worldwide [
1]. In spite of notable technical advances, the earliest neurodegenerative events that injure retinal ganglion cells during glaucoma [
1‐
5] are unclear. Degeneration of the retinal ganglion cell during glaucoma appears to be partially compartmentalized, with different mechanisms influencing the soma, axon and dendritic tree. In human glaucoma, early changes to retinal ganglion cells have been observed both in the optic nerve head, including axon transport deficits [
6‐
9], as well as in the retina with dendritic atrophy [
10‐
12]. The precise relationship of axonal damage to events at the cell soma and retinal ganglion cell synaptic connectivity has been the focus of a number of recent studies in animal models of glaucoma [
13‐
16] in which dendritic remodelling has been shown to precede gross cell loss. Recently, we demonstrated retinal ganglion cell dendritic atrophy during DBA/2J glaucoma and suggested that dendritic remodelling may be an early feature of glaucoma [
16]. However, the mechanisms by which these early retinal ganglion cell changes occur are not known.
It has been proposed that activation of immune responses may be key events that damage retinal ganglion cells during early stages of glaucoma. Using a rat model of ocular hypertension (based on the sclerosis of episcleral vessels), Johnson et al. showed gene expression changes with significant enrichment of immune response pathways and cell proliferation pathways, among others [
17]. The exploration of gene expression at the level of the optic nerve head confirmed immune cell responses and microglial activation, in the absence of significant astrocyte activation [
18]. Guo and colleagues extended this work by showing an early immune cell response, coupled with gene expression patterns, that suggested retinal ganglion cell dysfunction and dendritic remodelling [
19,
20]. Immune cell and/or microglial activation may therefore be a pertinent, early disease feature in animal models of glaucoma. Early changes to microglia [
17‐
29] and peripherally derived immune cells [
4,
30,
31] have been well documented in animal glaucoma models and in human glaucoma [
32‐
34], consistent with this scenario of early immune activation.
The DBA/2 J mouse is a widely used model of chronic glaucoma showing hallmark features of the human disease. In DBA/2 J mice, mutant alleles of two genes (
Gpnmb
R150X
and
Tyrp1
b
) cause iris pigment dispersion that results in ocular hypertension in the majority of eyes by 8 to 9 months of age. Glaucomatous damage is usually present by 12 months, signified by the loss of retinal ganglion cells and axonal degeneration within the optic nerve [
35], which appears to initiate at the optic nerve head [
2]. The interaction of multiple cell-types residing within the optic nerve head and retina are thought to be critical in glaucoma although the roles of specific cell-types are not well understood [
3,
36‐
39].
We have previously used microarray gene expression of optic nerve head and retina tissue from pre-glaucomatous (8.5 months old) and glaucomatous (10.5 months old) DBA/2 J mice [
3] in order to demonstrate that changes to inflammatory pathways, including the endothelin pathway, cell-adhesion pathways and the complement system occur early in the disease. In particular, induction of genes encoding proteins of the complement cascade was the earliest identifiable molecular change in glaucomatous DBA/2 J retinas [
3]. DBA/2 J mice deficient for
C1qa show robust protection from glaucomatous retinal ganglion cell loss and optic nerve degeneration [
3]. The complement cascade has been heavily implicated in human and animal models of glaucoma, has increased expression in the eyes of patients with end stage glaucoma, and in primate and murine glaucomatous eyes [
30,
40‐
49].
The role of the complement cascade in glaucoma is complex. In addition to its role in inflammatory signalling, complement pathways play a critical role in synaptic development and pruning [
46,
50‐
53]. During central nervous system development neurons make many immature synaptic connections, followed by the selective elimination of those that are redundant. In the retinas of
C1qa–deficient mice, retinal ganglion cells show defective synaptic elimination with limited refinement of the retinogeniculate pathway. TGF-β is thought to be important in the refinement process through activation/regulation of the complement cascade in neurons, microglia [
51] or astrocytes [
50]. The colocalization of C1q, part of the initiating complex of the classical pathway of the complement cascade, with synaptic markers such as PSD95 early in glaucomatous pathogenesis [
50] supports the concept that complement activation underpins synapse elimination in glaucoma.
Here we determine the role of C1 in early synaptic loss and dendritic atrophy during glaucoma. We quantified synaptic and dendritic atrophy during early stages of glaucoma using two models of ocular hypertension. We demonstrate that synaptic and dendritic atrophy occur prior to any detectable damage to the axon or soma in both DBA/2 J glaucoma and in a rat model where ocular hypertension (OHT) was induced using an injection of magnetic microspheres [
54]. Importantly, inhibiting C1 functions using either genetic ablation (DBA/2 J mutant for
C1qa) or pharmacological inhibition (C1 esterase inhibitor in the rat OHT model) was sufficient to preserve dendritic and synaptic architecture. Taken together with the strong protective effect of
C1qa knockout in retinal ganglion cell death and optic nerve damage, this suggests that inhibition of C1 should be considered as a therapeutic strategy for glaucoma.
Methods
Mouse strain, breeding and husbandry
Mice were housed and fed, as published [
3], in a 14 h light/10 h dark cycle with food and water available
ad libitum. All breeding and experimental procedures were undertaken in accordance with the Association for Research for Vision and Ophthalmology Statement for the Use of Animals in Ophthalmic Research. The Institutional Animal Care and Use Committee (IACUC) at The Jackson Laboratory approved this study. The DBA/2 J (D2,
n (
mice) = 24), DBA/2 J-
Gpnmb
+ (D2-
Gpnmb
+,
n = 20), and D2.
C1qa (
n = 16) strains were utilized and have been described in detail elsewhere [
3]. We used D2-
Gpnmb
+ mice as a control, non-glaucomatous substrain of DBA/2 J [
55].
Rat husbandry and IOP monitoring
Rat work was conducted in accordance with the Animals (Scientific Procedures) Act 1986. Brown Norway rats (Charles River, UK) were housed under a 12 h light/dark cycle with IOP measured at the same time of day in order to reduce variation in IOP. Breeding diet (SDS, RM3) and water were provided ad libitum.
Glaucoma induction and IOP monitoring in the rat
Baseline IOP for both eyes was established prior to glaucoma induction and IOP measured 24 h after induction and every 3 days thereafter. IOP was measured using a rebound tonometer (TonoLab, Finland) calibrated for use on the rat eye. Animals were awake and unrestrained during measurement, requiring only topical 0.4 % oxybuprocaine hydrochloride eye drops (Midoptic, UK). IOP was taken as the average of 5 repeat readings. Glaucoma was induced as described for the magnetic microbead model [
54]. Briefly, the left anterior chamber was injected with 10 μl of bead solution (sterile ~4.5 μm diameter paramagnetic microspheres, Invitrogen UK, in balanced salt solution) using a 32-gauge needle and Hamilton syringe (
n = 19). Injections were conducted under isofluorane-induced anaesthesia with topical 0.5 % chloramphenicol (Mid Optic, UK) applied to the cornea. Beads were drawn into the iridocorneal angle using a small handheld magnet in order to block trabecular meshwork outflow and induce OHT (
n = 20). The right eye remained an un-operated, normotensive (NT) control.
C1 inhibitor administration
C1 inhibitor (C1 esterase inhibitor; CINRYZE; Shire (previously ViroPharma)) or vehicle only (PBS) was administered to rats 24 h prior to induced ocular hypertension (OHT) and then once every 4 days for 16 days. Intravitreal injections of 5 μl of C1 inhibitor (100 Units/ml (n = 17)) or PBS (n = 10) were performed on the left eye under isofluorane-induced anaesthesia.
DiOlistic labelling of flat mount retinas
The protocol for retinal dissection and DiOlistic labelling was identical for both mouse and rat eyes except where stated otherwise. DiI and DiO coated tungsten bullets were prepared as described previously using 8 mg DiI and 16 mg DiO (both Life technologies, US) dissolved in 100 μl dichloromethane to 100 mg of tungsten particles (1.7 μm; Bio-Rad). Mice were killed by cervical dislocation, their eyes enucleated and retinas immediately dissected in chilled HBSS. For rats, a rising concentration of CO2 was used, with cervical dislocation confirming death. Retinas were dissected as described above. Retinas were then flat mounted ganglion cell layer (GCL) up on Millicell cell culture inserts (0.4 μm pore diameter, Life technologies, US). Retinas were then ballistically labelled with DiO/DiI bullets at a pressure of 120 psi using a Helios gene gun system (Biorad, US). 3.0 μm pore size cell culture inserts were placed between the gun and retina to prevent aggregated particles over labelling the retina. The culture inserts were then placed in Neurobasal-A media (Life technologies, US) containing 2 mM L-glutamate, 2 % B27 supplement and 1 % N2 supplement and incubated for 30 min at 37 °C, 4 % CO2 to facilitate dye diffusion. Retinas were then fixed in 4 % PFA for 30 min before mounting on glass slides with Fluoromount and coverslips.
Immunofluorescent staining of retinal sections
The protocol for immunohistochemical staining of retinal sections was identical for both mice and rats except where stated otherwise. Mice were sacrificed by cervical dislocation, their eyes enucleated and placed in 4 % PFA ON. For rats, a rising concentration of CO
2 was used, with cervical dislocation confirming death. Following this, eyes were cryoprotected in 30 % sucrose, frozen in OCT and cryosectioned at 12 μm. Eyes from the following number of animals (and central retina sections) were used: (in mice [animals (sections)]; 4mo D2
n = 6[
34], 9mo D2
n = 3[
25], 4mo D2-
Gpnmb
+
n = 9[
36], 9mo D2-
Gpnmb
+
n = 5[
28], 4mo D2.
C1qa n = 5[
29], 9mo D2.
C1qa n = 3[
30]. Rats: normotensive
n = 9[
43], normotensive plus C1 inhibitor
n = 3[
49], OHT
n = 3[
38], OHT plus C1 inhibitor
n = 3[
36]). Central retina sections were brought to RT for 1 h, permeabilized with 0.1 % Triton-X for 30mins, blocked with 5 % chick serum in PBS and stained ON at RT in primary antibody (for mice; rabbit anti-PSD-95; 51-6900, Life Technologies, USA, or goat anti-IBA1; ab5076, Abcam, UK; for rats; rabbit anti-PSD-95; ab18258, Abcam, UK). After primary antibody incubation, sections were washed 5 times in PBS, stained for 4 h at RT with secondary antibody (for mice; goat anti-rabbit AF488; Life Technologies, USA, for rats; goat anti-rabbit AF555, Abcam, UK). Slides were then washed a further 5 times with PBS, stained with DAPI (mice) or Hoechst 33342 (rats) for 15mins, mounted with fluoromount, coverslipped and sealed with nail-polish. For PSD95 quantification in DBA/2 J mice and rats, images of central retina showing the IPL and ONL were collected under identical conditions on a Leica SP8 confocal microscope and Leica DM6000B respectively (image area = 553.57 μm
2). Fluorescent intensity of antibody staining was quantified using ImageJ. Three-dimensional reconstruction of IBA1+ cells was performed using IMARIS.
Retinal ganglion cell morphological analysis
Retinas were imaged using either a Leica SP8 or Zeiss LSM 510 confocal microscope with ×20 objective (mice: 4mo D2
n = 63, 9mo D2
n = 58, 4mo D2-
Gpnmb
+
n = 27, 9mo D2-
Gpnmb
+
n = 37, 4mo D2.
C1qa n = 30, 9mo D2.
C1qa n = 52. Rats: normotensive
n = 43, normotensive plus C1 inhibitor
n = 49, OHT
n = 38, OHT plus C1 inhibitor
n = 36). Z-stack imaged (slice thickness 1 μm) were captured for mouse retinal ganglion cells and rat retinal ganglion cells ensuring that the entire dendritic tree was in frame. A number of RGC morphological features were measured using FIJI. Dendritic field area was measured by connecting the outermost dendritic tips using the convex polygon tool. The dendritic tree was traced and the mean dendritic length calculated using the Neuron J plugin. Sholl analysis was conducted using the bitmap Sholl analysis plugin for FIJI [
56]. Statistical analysis was conducted using SPSS 20 (IBM, US).
Classifying retinal ganglion cell subgroups
Retinal ganglion cells can be classified on the basis of their dendritic tree and soma morphology. To account for any bias that may arise from selective labelling of cells, we classified retinal ganglion cells from our control groups (in mice D2-
Gpnmb
+ and in rats, the normotensive group) based on classifications set out by Sun et al. [
57,
58]. There was no obvious bias in the cell types expected (in mouse; our study (expected), RGC
A 9 % (10 %), RGC
B 38 % (30 %), RGC
C 41 % (37 %), RGC
D 13 % (23 %), in rat; RGC
A 20.9 % (17.5 %), RGC
B 32.6 % (29.2 %), RGC
C 34.9 % (35.2.3 %), RGC
D 11.6 % [
18]). Once cells start to degenerate classifying based on morphology may lead to misrepresentation of cells at different stages of disease. Given that our control groups showed equal, unbiased sampling, we did not classify disease susceptible DBA/2 J or D2.
C1qa
-/- retinal ganglion cells. As lamination is unlikely to drastically change during disease, we confirmed that an even proportion of ON/OFF/ON-OFF cells were present across our samples (in mouse; ON/OFF/ON-OFF, D2-
Gpnmb
+ 87 %/4 %/9 %, D2 86 %/7 %/7 %, D2.
C1qa 86 %/6 %/8 %, in rat; normotensive 71 %/17 %/12 %, normotensive plus C1 inhibitor 74 %/12 %/14 %, OHT 82 %/7 %/10 %, OHT plus C1 inhibitor 84 %/8 %/8 %). This is further expanded on in the Discussion.
Axon labelling with PPD and grading of glaucomatous damage
The processing of optic nerves and staining with paraphenylenediamine (PPD) which darkly stains the axoplasm and myelin sheath of damaged axons has been reported previously [
59]. In brief, intracranial portions of optic nerves were fixed in 4 % PFA at RT for 48 h, processed and embedded in plastic. A segment of optic nerve from within a region up to 1 mm from the posterior surface of the sclera was sectioned (1 μm thick sections) and stained with PPD. Typically 30-50 sections are taken from each nerve. Homology between sections is considered during grading. Optic nerves were analysed and only eyes that had a corresponding nerve grade of ‘no or early damage’ (
i.e. less than 5 % axons damaged) were selected in order to evaluate early disease events prior to axonal damage.
Discussion
Multiple compartments of the retinal ganglion cell are suggested to be impacted in glaucoma [
61]. While the optic nerve head is considered a primary and critical site of damage in glaucoma [
2,
6,
8,
9,
62] early damage may also manifest at other locations including the retinal ganglion cell soma, dendrites and synapses [
3,
10‐
12,
16,
63‐
67]. In this study, we use both a genetic (DBA/2 J mice) and an inducible (rat bead) model of glaucoma to show that significant changes occur to retinal ganglion cell synapses and dendrites prior to optic nerve degeneration. This dendritic and synaptic atrophy is disease- but not age-related as there is no detectable atrophy in age and strain matched, non-glaucomatous controls (D2.
Gpnmb
+). At this stage of the disease there is no damage as detectable by PPD staining of the optic nerve or retinal ganglion cell loss, predicting that observed effects [
68] might be subtle. It cannot be ruled out that there was regional bias in the DiOlistic labelling in areas of the retina that are resistant to damage or contain retinal ganglion cells of varying morphologies. This seems unlikely given that DiOlistic labelling is diffuse (typically 5-20 cells per retina labelled evenly across the retina) and random. The preferential damage or loss of selective types of retinal ganglion cells has been investigated previously in the DBA/2 J mouse model, with neither cell type specific loss of dendritic morphology or somal shrinkage apparent [
13,
69]. In addition, we have previous shown that genetic labelling of retinal ganglion cells introduces an inherent bias that might misrepresent cells at different stages of the disease in DBA/2 J glaucoma [
16]. However, new, unbiased genetic tools that allow the visualization of specific retinal ganglion cell subtypes are being developed and provide scope for further investigation once available across mouse strains and in the rat [
68]. These early changes to retinal ganglion cell structures in the retina are consistent with the early loss of pattern electroretinogram (PERG) that occurs in DBA/2 J mice [
2,
70]. Since dendritic integrity is essential for the efficient generation of action potentials in the retinal ganglion cells, we suggest that dendritic degeneration may be an important contributor to the reduced PERG signal in early glaucoma [
71,
72].
Determining the timing of changes to retinal ganglion cell compartments in the retina in relation to axonal changes in the optic nerve head in humans and animal models will be important to fully understand the earliest drivers of glaucoma. Although early soma and axon loss have been reported in 9 months old mice in other colonies [
73], this is not the general case in our DBA/2 J colony [
35]. However, in eyes with no significant optic nerve damage (based on optic nerve damage assessment just behind the orbit) or axon transport disruption, discrete and focal axon damage is observed in the glial lamina region of the optic nerve head at this stage [
2]. The extent of the changes to retinal ganglion cell synapses and dendrites in the retina are greater than the number of focally damaged axons in the optic nerve head suggesting synaptic and dendritic atrophy are amongst the earliest pathological events in DBA/2 J glaucoma. To explore whether this is exclusive to the DBA/2 J mouse we also studied these changes in an inducible model of rat ocular hypertension, showing dendritic atrophy in eyes with consistent elevations in IOP but not in sham treated, normotensive or contralateral eyes.
Since early synaptic and dendritic atrophy were associated with an increase in complement components in the IPL [
50] the complement pathway was a promising candidate for mediating early retinal ganglion cell degeneration. The key finding in the present study is that early synaptic and dendritic atrophy were prevented through inhibition of the C1 complex, the initiating complex of the classical complement pathway. D2.
C1qa
-/- mice developed the anterior segment changes that occur in normal DBA/2 J mice with subsequent elevation in IOP [
3]. Only a small percentage of D2.
C1qa
-/- mice go on to develop optic nerve degeneration and retinal ganglion cell loss [
3]. At 9 months of age (prior to any retinal ganglion cell loss and optic nerve degeneration) we could not discern any significant reduction in retinal ganglion cell dendritic integrity or synapse number compared to age matched, control D2.
Gpnmb
+ retinas,
i.e. DBA/2 J dendritic and synaptic atrophy is prevented by
C1qa deficiency. The functional implications of this protective effect (e.g. if the PERG is restored in DBA/2 J mice deficient in
C1qa) have yet to be elucidated.
Given our findings in DBA/2 J glaucoma we sought to test whether pharmacological inhibition of C1 could be of therapeutic value in glaucoma. We demonstrate that intravitreal administration of human C1 inhibitor prior to and during the onset of OHT preserved dendritic and synaptic integrity in eyes with sustained elevations in IOP. Global genetic inhibition of C1 in the mouse protects all compartments of the retinal ganglion cell. It is important to note that, C1 inhibitor treatment in the rat did not protect the optic nerve, most likely reflecting the reduced accessibility of the drug outside of the retina (data not shown). This shows inhibiting complement in the retina is sufficient to protect dendrites and synapses even whilst axon degeneration is occurring. Our results further support compartmentalised changes in the retinal ganglion cell possibly due to independent, but not necessarily mutually exclusive, intraocular pressure-related events in the retina and optic nerve head. Studies employing the systemic administration of C1 inhibitors would clarify the protective effects of C1 inhibition in neural compartments outside the retina.
Complement activation has been shown in human and animal models of glaucoma [
3,
49,
74], as well as in other chronic neurodegenerative diseases such as Alzheimer’s disease [
75‐
82]. The classical pathway of the complement cascade is heavily implicated in glaucoma, and in DBA/2 J mice deficient in
C1qa there is a significant decrease in retinal ganglion cell loss and optic nerve degeneration compared to regular DBA/2 J mice [
3]. The complement cascade is activated by the classical, alternative or lectin pathways, converging at the cleaving of C3 into effector fragments that can modulate immune response such as chemo-attraction, phagocytosis or cell lysis. The precise mechanisms by which
C1qa or the C1 complex contribute to glaucomatous retinal ganglion cell damage is not known but the opsonisation of synapses for removal by microglia [
46,
50,
51,
53,
83], seems one likely route. C3-independent roles for the C1 complex have been also identified. For instance, C1q can bind apoptotic cells to promote their phagocytic uptake by myeloid-derived macrophages. C1q can also bind the frizzled receptor and modulate WNT signalling [
84]. Components of the C1 complex are up-regulated in multiple cell types in the retina and optic nerve head during early stages of glaucoma, particularly myeloid derived cells and retinal ganglion cells. However, the extrinsic or intrinsic factors that induce or modulate complement induction are also not known. In the brain,
C1qa expression increase in IBA1+ cells and neurons with age, and
C1qa deficiency can mitigate age-related cognitive decline [
53,
85]. Therefore, the complement cascade is playing multiple roles during different stages of glaucoma and further work is required to elucidate them.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
PAW–designed, conceived and performed mouse experiments (husbandry, DiOlistics, immunofluorescence, optic nerve assessment), wrote the manuscript; JRT–performed mouse and rat experiments (clinical examination, husbandry, DiOlistics, immunofluorescence, microscopy), wrote the manuscript; KWP–performed mouse experiments (immunofluorescence, microscopy); SDC–designed and performed rat experiments (clinical examination, husbandry, DiOlistics, immunofluorescence, microscopy); BPM–designed and conceived experiments, wrote the manuscript; JEM–designed and conceived experiments, wrote the manuscript; SWMJ–designed and conceived experiments, wrote the manuscript; GRH–designed and conceived experiments, wrote the manuscript. All Authors read and approved the final manuscript.