Neural networks evolve their functional features over time. At present, no techniques allow detailed neuron recording over repeated experimental session: the only method to repeatedly recording single cell activity in vivo is by means of chronically implanted electrodes. Unfortunately, with electrophysiology, cell death and gliosis give some uncertainty in monitoring the same neuron over months, while imaging techniques may solve such problem allowing unequivocal identification of the neural cell. Fluorescent contrast labels suffer, during longer recording, from some leakage and to load new indicator is quite difficult. Reporter genes, as spotlighted by the Brainbow mouse, are an appealing approach even if quite immature. In fact, at present, genetically encoded sensors for brain imaging are almost represented by fluorescent proteins. Those GFP-like reporters still suffer from narrow linearity and relatively poor brightness, hampering enough space resolution to track in living animals followed over time, the connectomic studies precognized by the Brainbow's dead brain slices.
Marko Mank and colleagues started filling this gap by means of mutagenesis on the TnC calcium biosensor. These efforts increased overall signal strength and sensitivity in the regime of physiologically relevant calcium concentrations leading to a new biosensor, the TN-XXL, that is functional in vivo in flies and mice and, according to the authors, can be used to obtain tuning curves of neurons in visual cortex using in vivo two-photon imaging. As a perspective, the new reporter will be useful to get more insigths about calcium role into plasticity and degeneration.
Marco Mank, Alexandre Ferrão Santos, Stephan Direnberger, Thomas D Mrsic-Flogel, Sonja B Hofer, Valentin Stein, Thomas Hendel, Dierk F Reiff, Christiaan Levelt, Alexander Borst, Tobias Bonhoeffer, Mark Hübener, Oliver Griesbeck (2008). A genetically encoded calcium indicator for chronic in vivo two-photon imaging Nature Methods, 5 (9), 805-811 DOI: 10.1038/NMETH.1243