Aberrant Hippocampal Calcium Waves Induced by Viral Expression of Calcium Indicators

Core Concepts

Common adeno-associated viral transduction procedures for expressing genetically encoded calcium indicators in the hippocampus can induce aberrant, spatially confined calcium micro-waves that slowly propagate through the CA1 and CA3 regions.

Abstract

The authors report that common adeno-associated viral (AAV) transduction procedures for expressing genetically encoded calcium indicators (GECIs) like GCaMP6, GCaMP7, or R-CaMP1.07 under the synapsin promoter can induce aberrant, spatially confined calcium micro-waves in the hippocampus. These calcium waves were typically first observed 4 weeks after viral injection, persisted up to at least 8 weeks, and were confined to the CA1 and CA3 regions, but not the dentate gyrus or neocortex. The occurrence of these calcium waves was robust, observed across multiple laboratories using standard viral transduction and two-photon imaging protocols. The calcium waves were not restricted to a single GECI variant and were also observed with R-CaMP1.07 expression. Reducing the viral transduction titer decreased but did not prevent the occurrence of the calcium waves. In contrast, an alternative viral transduction approach using Cre-dependent sparse GCaMP6 expression in principal neurons avoided the aberrant calcium waves. Transgenic GCaMP mouse lines also did not exhibit these calcium waves. The authors suggest that the underlying mechanisms likely involve the expression level and density of the calcium indicators, as well as the targeted brain region, with the CA1 and CA3 regions being particularly prone to generating these artefactual calcium waves under certain conditions. They aim to raise awareness in the field about this transduction-induced artefact and provide potential solutions to avoid it when performing in vivo calcium imaging of the hippocampus.

Stats

Hippocampal CA1 pyramidal layer thickness: 49 ± 12.5 µm ipsilateral, 50.3 ± 11.1 µm contralateral Hippocampal CA1 thickness: 553.3 ± 14 µm ipsilateral, 555.8 ± 62 µm contralateral

Quotes

"Aberrant hippocampal Ca2+ micro-waves following synapsin-dependent adenoviral expression of Ca2+ indicators" "Common AAV transduction procedures induce artefactual spatially confined Ca2+ waves in the hippocampus."

Key Insights Distilled From

Aberrant hippocampal Ca2+ micro-waves following synapsin-dependent adeno-associated viral expression of Ca2+ indicators

by Masala,N., M... at www.biorxiv.org 11-11-2023

https://www.biorxiv.org/content/10.1101/2023.11.08.566169v2

Deeper Inquiries

What are the potential cellular and network mechanisms underlying the generation of these aberrant calcium waves in the hippocampus following viral GECI expression

The generation of aberrant calcium waves in the hippocampus following viral GECI expression could be attributed to several potential cellular and network mechanisms. One possible mechanism is the impact of exogenous calcium indicators on intracellular calcium dynamics. The introduction of high levels of GECIs, such as GCaMP variants, may alter the normal calcium buffering capacity within neurons, leading to disruptions in calcium signaling. This alteration in calcium dynamics could trigger abnormal calcium waves that propagate through the hippocampal circuitry. Another potential mechanism could involve the density and distribution of GECI expression in the hippocampal regions. The specific promoter used for GECI expression, in this case, the synapsin promoter, may lead to high levels of GECI expression in certain neuronal populations, affecting their excitability and calcium handling. This localized overexpression of GECIs could create conditions conducive to the generation of aberrant calcium waves. Furthermore, the interaction between GECIs and neuronal activity could play a role in the generation of these abnormal calcium waves. The presence of GECIs may modulate neuronal firing patterns, synaptic transmission, and network activity, leading to the emergence of synchronized calcium events that manifest as the observed micro-scale calcium waves in the hippocampus.

How generalizable is this phenomenon across different animal models and brain regions beyond the hippocampus

The phenomenon of aberrant calcium waves following viral GECI expression in the hippocampus appears to be generalizable across different animal models and brain regions beyond the hippocampus. The study demonstrated the occurrence of these abnormal calcium waves not only in wild-type mice but also in genetic mouse models of disease, such as the Scn2aA263V model of genetic epilepsy and a mouse model of Alzheimer's disease. This suggests that the generation of aberrant calcium waves is not limited to a specific genetic background but can be observed in various experimental contexts. Moreover, while the aberrant calcium waves were predominantly observed in the CA1 and CA3 regions of the hippocampus, they were notably absent in the dentate gyrus and neocortex following GECI expression. This regional specificity indicates that the phenomenon may be influenced by the unique circuitry and cellular composition of different brain regions. The robustness of the aberrant calcium waves across different laboratories and experimental conditions further supports the generalizability of this phenomenon.

Could these aberrant calcium waves have functional consequences for neuronal activity and information processing in the hippocampus, and if so, how could they be mitigated

The aberrant calcium waves induced by viral GECI expression in the hippocampus could potentially have functional consequences for neuronal activity and information processing in this brain region. These abnormal calcium events may disrupt the normal patterns of neuronal signaling, leading to altered synaptic transmission, network synchronization, and information encoding. The propagation of calcium waves through the hippocampal circuitry could interfere with the precise timing of neuronal firing, affecting the generation of place cell activity, memory formation, and cognitive processes reliant on hippocampal function. To mitigate the potential functional consequences of these aberrant calcium waves, alternative viral transduction methods could be employed to avoid the artifact. For example, the use of conditional GECI expression in a sparse population of principal cells under the CaMKII promoter or the utilization of transgenic mouse lines expressing GECIs without inducing the aberrant calcium waves could be effective strategies. By carefully selecting the viral transduction approach, researchers can minimize the risk of disrupting normal neuronal activity and ensure the reliability of calcium imaging experiments in the hippocampus.