Syn-jGCaMP7s AAV (Serotype 9)

GCaMP is a genetically encoded calcium indicator (GECI) that is widely used in neuroscience research. It measures intracellular Ca2+ levels through fluorescence changes because it binds directly to Ca2+. In this process, the effects of this calcium buffer on intracellular calcium signaling and cell physiology are often not taken into account. However, there is growing evidence from calcium imaging studies that GCaMP expression under certain conditions can produce aberrant activity, such as epileptic seizures. In this study, researchers examined the effects of GCaMP6 expression in the dentate gyrus (DG) on epileptogenesis. They found that viral expression of GCaMP6s but not GCaMP6f in the DG induced tonic-clonic seizures several weeks after viral injection. The cell type-specific expression of GCaMP6s suggests that granule cells (GCs) are key players in GCaMP6s-induced epilepsy. Finally, using slice electrophysiology, it was demonstrated that GCaMP6s expression increases neuronal excitability in GCs. In summary, this study highlights the ability of GCaMP6s in DG-related epileptogenesis.

To test the effect of AAV serotype on seizures, the researchers injected AAV5-Syn-GCaMP6s and AAV5-Syn-GCaMP6f in the DG (Figure 1A) and then performed longitudinal EEG experiments. Interestingly, almost no seizures were observed in both cases (Figure 1B-D). These data suggest that this serotype may play a role in seizures. After GCaMP6, newer GCaMP variants were developed, specifically jGCaMP724 and jGCaMP8. To test whether these new variants also induce seizures, the researchers injected AAV9-Syn-jGCaMP7s and AAV9-CaMKIIa-jGCaMP8s in the DG (Figure 1A). In one mouse (out of 4 mice) injected with AAV9-Syn-jGCaMP7s, a total of 11 seizure events were observed (Figure 1B-D). No seizures were detected in the other 3 mice injected with jGCaMP7s. In contrast, severe seizure events were observed in every mouse injected with AAV9-CaMKIIa-jGCaMP8s (Figure 1B-D). After week 6, two mice died. The other two mice survived but also experienced multiple seizure events (Figure 1C). On average, 114 seizure events were recorded in mice injected with AAV9-CaMKIIa-jGCaMP8s over 12 weeks (Figure 1D), which is twice the number of seizures in mice injected with AAV9-CaMKII-GCaMP6s. Notably, the intermediate variant jGCaMP7s induced few seizures, which may be due to its Syn promoter. Together, these data suggest that AAV serotype, promoter, and GCaMP variant all contribute to seizures in the DG.

Figure 1. The effect of AAV serotype and GCaMP variant on seizures. (Teng S, et al., 2024)