Abstract

Dynamic Ca²⁺ signals reflect acute changes in membrane excitability, and also mediate signaling cascades in chronic processes. In both cases, chronic Ca²⁺ imaging is often desired, but challenged by the cytotoxicity intrinsic to calmodulin (CaM)-based GCaMP, a series of genetically-encoded Ca²⁺ indicators that have been widely applied. Here, we demonstrate the performance of GCaMP-X in chronic Ca²⁺ 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 Ca²⁺ fluctuations, in comparison with GCaMP. Chronic Ca²⁺ imaging of one month or longer was conducted for cultured cortical neurons expressing GCaMP-X, unveiling that spontaneous/local Ca²⁺ transients progressively developed into autonomous/global Ca²⁺ oscillations. Along with the morphological indices of neurite length and soma size, the major metrics of oscillatory Ca²⁺, including rate, amplitude and synchrony were also examined. Dysregulations of both neuritogenesis and Ca²⁺ oscillations became discernible around 2–3 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 Ca²⁺ to neural development and neuronal health. In summary, GCaMP-X provides a viable solution for Ca²⁺ imaging applications involving long-time and/or high-level expression of Ca²⁺ probes.