Chronic Ca2+ imaging of cortical neurons with long-term expression of GCaMP-X

<p><span>Dynamic Ca²⁺ signals reflect acute changes in membrane excitability and also mediate signaling cascades in chronic processes. In both cases, chronic </span><span>Ca²⁺</span><span> imaging is often desired but challenged by the cytotoxicity intrinsic to calmodulin (CaM)-based GCaMP, a series of genetically-encoded </span><span>Ca²⁺</span><span> indicators that have been widely applied. Here, we demonstrate the performance of GCaMP-X in chronic </span><span>Ca²⁺</span><span> imaging of cortical neurons, where GCaMP-X by design is to eliminate the unwanted interactions between the conventional GCaMP and endogenous (apo)CaM-binding proteins. By expressing in adult mice at high levels over an extended time frame, GCaMP-X showed less damage and improved performance in two-photon imaging of sensory (whisker-deflection) responses or spontaneous </span><span>Ca²⁺</span><span> fluctuations, in comparison with GCaMP. Chronic </span><span>Ca²⁺</span><span> imaging of one month or longer was conducted for cultured cortical neurons expressing GCaMP-X, unveiling that spontaneous/local </span><span>Ca²⁺</span><span> transients progressively developed into autonomous/global </span><span>Ca²⁺</span><span> oscillations. Along with the morphological indices of neurite length and soma size, the major metrics of oscillatory </span><span>Ca²⁺</span><span>, including rate, amplitude and synchrony were also examined. Dysregulations of both neuritogenesis and </span><span>Ca²⁺</span><span> oscillations became discernible around 2</span><span>–</span><span>3 weeks after virus injection or drug induction to express GCaMP in newborn or mature neurons, which were exacerbated by stronger or prolonged expression of GCaMP. In contrast, neurons expressing GCaMP-X were significantly less damaged or perturbed, altogether highlighting the unique importance of oscillatory </span><span>Ca²⁺</span><span> to neural development and neuronal health. In summary, GCaMP-X provides a viable solution for </span><span>Ca²⁺</span><span> imaging applications involving long-time and/or high-level expression of </span><span>Ca²⁺</span><span> probes. </span></p>