Aberrant Cortical Activity In Multiple GCaMP6-Expressing Transgenic Mouse Lines

Abstract

Transgenic mouse lines are invaluable tools for neuroscience but as with any technique, care must be taken to ensure that the tool itself does not unduly affect the system under study. Here we report aberrant electrical activity, similar to interictal spikes, and accompanying fluorescence events in some genotypes of transgenic mice expressing GCaMP6 genetically-encoded calcium sensors. These epileptiform events have been observed particularly, but not exclusively, in mice with Emx1-Cre and Ai93 transgenes, across multiple laboratories. The events occur at >0.1 Hz, are very large in amplitude (>1.0 mV local field potentials, >10% df/f widefield imaging signals), and typically cover large regions of cortex. Many properties of neuronal responses and behavior seem normal despite these events, though rare subjects exhibit overt generalized seizures. The underlying mechanisms of this phenomenon remain unclear, but we speculate about possible causes on the basis of diverse observations. We encourage researchers to be aware of these activity patterns while interpreting neuronal recordings from affected mouse lines and when considering which lines to study.

Entrainment of Arteriole Vasomotor Fluctuations by Neural Activity Is a Basis of Blood-Oxygenation-Level-Dependent “Resting-State” Connectivity

“…We choose to use the LFP as an aggregate measure of neuronal activity, as opposed to intracellular calcium signals (Du et al, 2014; Ma et al, 2016; Vanni et al, 2017), as a means to identify different frequency bands of neuronal activation and to connect our work with past neurovascular studies (Keller et al, 2013; Lachaux et al, 2007; Niessing et al, 2005; Nir et al, 2007; Nir et al, 2008; Thompson et al, 2013). We also were cautious of potential ictal events in mice bred to express intracellular calcium reporters (Steinmetz et al, 2017). Lastly, we center the window across the vibrissa area of parietal cortex, as confirmed by mapping the amplitude of the LFP upon stimulation of different vibrissae (O’Connor et al, 2002).…”

Presynaptic GCaMP expression decreases vesicle release probability at the calyx of Held

“…GECIs such as GCaMPs are widely employed for calcium imaging as they have distinct advantages over traditional calcium dyes such as ease of use, relatively consistent expression, fluorescence stability, and the ability to tag specific neuronal populations and subcellular domains (de Juan‐Sanz et al, ; T. W. Chen et al, ; Xing & Wu, ; Zhang & Ding, ). However, expression at high levels may result in exogenous calcium chelation, affecting synaptic transmission in undesirable ways (Steinmetz et al, ). In the present study, we expressed GCaMP6m specifically in the presynaptic terminal, and we investigated its effect on synaptic transmission.…”

“…Unfortunately, GCaMP6m augments endogenous cytosolic Ca 2+ buffering and alters the transmission profile of infected synapses. Several transgenic mouse lines expressing GCaMP6m have been shown to generate abnormal neuronal spiking in the cortex region; however, the authors used a combination of transgenic mice, and the aberrations were more prominent in the transgenic GCaMP6m mice expressing Cre recombinase, not ruling out the Cre toxicity as a causative factor (Steinmetz et al, ). Here, we expressed GCaMP6m at the calyx of Held presynapse using viral‐mediated transduction of GCaMP6m expression, which was more tightly controlled; we may have expressed the probe at lower levels and for a shorter time period (less than three weeks).…”

“…Similarly, the buffering capacity depends on the level of GCaMP expression, which is generally difficult to control in transgenic experimental models. Synaptic physiology and subsequent neuronal activity of cells may be affected by the inherent calcium buffering activity of GCaMP (Steinmetz et al, ). However, the extent to which expression of GCAMP indicators alters synaptic transmission and presynaptic short‐term plasticity has not been examined directly.…”

Utilizing Miniature Fluorescence Microscopy to Image Hippocampal Place Cell Ensemble Function in Thy1.GCaMP6f Transgenic Mice

“…Such changes can become a potential confounding factor when comparing calcium imaging data taken weeks apart within an animal. Mice in which calcium sensors are transgenically expressed have the advantage of stable sensor expression levels over time (Chen et al., ); however, several lines of these mice have been reported to exhibit aberrant neuronal activity (Steinmetz et al., ).…”