Systemic AAV delivery of a calcium indicator in marmosets: functional validation in visual area MT

Functional optical imaging in nonhuman primates provides an important complement to electrophysiological approaches in neuroscience research, but its broader use has been limited by challenges in achieving large-scale, homogeneous expression of genetically encoded reporters, and imaging accessibility in species with gyrencephalic brains with sulci and fissures (e.g., rhesus macaques). Specifically, conventional local intracortical viral injections are invasive and often produce spatially restricted or heterogeneous expression, constraining population-level analyses. Here, we show that systemic intravenous delivery of an adeno-associated virus (AAV) capsid engineered for enhanced blood-brain barrier crossing, AAV.CAP-B10, supports robust and widespread expression of a calcium indicator CAaMP8s in the common marmoset. Intravenous delivery in two marmosets resulted in widespread cortical expression. Using a large cranial window over extrastriate visual area MT (and its satellite areas), we performed widefield single-photon imaging and two-photon cellular-resolution imaging in awake,behaving marmosets to functionally validate activity in this well studied primate visual-motion sensitive cortical area. Population level responses to visual motion and spatial organization measured with widefield imaging, as well as single-cell level motion direction tuning measured with two-photon imaging, were consistent with canonical properties of MT reported in previous electrophysiological studies. Quantitative analyses of lightsheet imaging after whole hemisphere brain clearing further confirmed the broad expression of GCaMP in both cortical and subcortical areas. Together, these results indicate that systemic delivery using AAV.CAP-B10 provides a minimally invasive approach for robust multi-scale functional optical imaging in awake, behaving marmosets.