CPOS Seminar: "Engineering a genetically encoded tissue-penetrant integral calcium sensor"

Speaker: Kaamini M. Dhanabalan, Postdoctoral Fellow, Department of Chemical Engineering, UC Santa Barbara, Mukherjee Group

Calcium is a key signaling molecule in diverse biological processes, including neural activity,synaptic transmission, immune signaling, and cardiac function. While optical reporters such as GCaMP enable high-resolution calcium imaging, they are limited by shallow tissue penetrationand a restricted field of view. Magnetic resonance imaging (MRI), on the other hand, enables whole-organ imaging but lacks genetic tools to record calcium dynamics. To address this challenge, we engineered a genetically encoded, MRI-compatible calcium recorder, CaSpR (Calcium Spike Recorder), based on water channels (Aquaporins) whose diffusion properties can be visualized by diffusion-weighted MRI.
   CaSpR links intracellular calcium fluctuations to aquaporin stability through a synthetic gene circuit. Specifically, we modulated the metabolic stability of destabilized aquaporins using a bioorthogonal split protease whose activity is gated by calcium. Calcium-binding protein motifs were incorporated into a split protease construct, ensuring reconstitution and activation only in he presence of calcium. When calcium levels rise, the active protease stabilizes aquaporins, producing an MRI-on state. In the absence of calcium, the protease remains inactive, leading to aquaporin degradation (MRI-off state).
   We engineered and validated CaSpR in multiple mammalian cell lines, confirming MRI-based recording of calcium activity in response to pharmacological and chemogenetic stimuli. Ongoing work extends this system to the mouse brain for noninvasive tracking of neuromodulation. CaSpR represents one of the first genetic tools capable of whole-brain calcium mapping in awake animals, with broad potential for studying neural, immune, and developmental processes where calcium dynamics define cellular function.