Disturbed Presynaptic Ca2+ Signaling in Photoreceptors in the EAE Mouse Model of Multiple Sclerosis

Abstract

Summary Multiple sclerosis (MS) is a demyelinating disease caused by an auto-reactive immune system. Recent studies also demonstrated synapse dysfunctions in MS patients and MS mouse models. We previously observed decreased synaptic vesicle exocytosis in photoreceptor synapses in the EAE mouse model of MS at an early, preclinical stage. In the present study, we analyzed whether synaptic defects are associated with altered presynaptic Ca 2+ signaling. Using high-resolution immunolabeling, we found a reduced signal intensity of Cav-channels and RIM2 at active zones in early, preclinical EAE. In line with these morphological alterations, depolarization-evoked increases of presynaptic Ca 2+ were significantly smaller. In contrast, basal presynaptic Ca 2+ was elevated. We observed a decreased expression of Na + /K + -ATPase and plasma membrane Ca 2+ ATPase 2 (PMCA2), but not PMCA1, in photoreceptor terminals of EAE mice that could contribute to elevated basal Ca 2+ . Thus, complex Ca 2+ signaling alterations contribute to synaptic dysfunctions in photoreceptors in early EAE.

Early Changes in Exo- and Endocytosis in the EAE Mouse Model of Multiple Sclerosis Correlate with Decreased Synaptic Ribbon Size and Reduced Ribbon-Associated Vesicle Pools in Rod Photoreceptor Synapses

“…For this purpose, we analyzed the length of individual synaptic ribbons of rod photoreceptor synapses in early EAE mice (vs. the control group) by 3D super-resolution structured illumination microscopy (3D SR-SIM) ( Figure 3 ). 3D SR-SIM is particularly useful to examine the contour length ( Figure 1 (A2)) of synaptic ribbons [ 24 , 25 ]. Using 3D SR-SIM, we found that the contour length of photoreceptor synaptic ribbons was significantly smaller in MOG/CFA-injected mice in comparison to CFA-injected control mice on day 9 after injection ( Figure 3 (A1–B2); for quantification, Figure 3 (C1,C2)).…”

“…Thus, the published functional changes observed on day 9 after injection [ 24 ] correlate with the morphological changes described in the present study. Since a recent study [ 25 ] found alterations of presynaptic Ca 2+ signaling even at earlier time points, i.e., on day 7 and day 8 after injection, we also analyzed mice obtained on day 8 and day 7 after injection for possible defects in synaptic vesicle exocytosis. For this purpose, we monitored synaptic vesicle exocytosis by optical recording of SypHy fluorescence from our transgenic SypHy reporter mice, exactly as previously described [ 24 ], but now on retinal slices obtained from mice on day 8 and day 7 after injection of MOG/CFA (EAE group) or CFA (control group) ( Figure 7 ).…”

“…Of note, retinal synapses were morphologically and functionally compromised in the experimental auto-immune encephalomyelitis (EAE) mouse model of MS [ 24 , 25 ]. Synapse alterations occurred in the pre-clinical phase even before the onset of optic nerve demyelination both in MS patients as well as in the EAE mouse model [ 7 , 24 , 25 ]. In the retina, photoreceptor synapses were particularly strongly affected by EAE [ 24 , 25 ].…”

“…Synapse alterations occurred in the pre-clinical phase even before the onset of optic nerve demyelination both in MS patients as well as in the EAE mouse model [ 7 , 24 , 25 ]. In the retina, photoreceptor synapses were particularly strongly affected by EAE [ 24 , 25 ]. Photoreceptor synapses are continuously active ribbon synapses that contain large presynaptic specializations, the synaptic ribbons [ 26 , 27 , 28 ].…”

“…These data emphasize the role of the synaptic ribbon for depolarization-evoked exocytic and endocytic vesicle trafficking in early EAE. In contrast to the decreased evoked responses, basal vesicle cycling under resting conditions was elevated in early EAE photoreceptors, most likely resulting from elevated basal presynaptic Ca 2+ levels that were recently identified [ 25 ].…”