Action potential-coupled Rho GTPase signaling drives presynaptic plasticity

In contrast to their postsynaptic counterparts, the contributions of activity-dependent cytoskeletal signaling to presynaptic plasticity remain controversial and poorly understood. To identify and evaluate these signaling pathways, we conducted a proteomic analysis of the presynaptic cytomatrix using in vivo biotin identification (iBioID). The resultant proteome was heavily enriched for actin cytoskeleton regulators, including Rac1, a Rho GTPase that activates the Arp2/3 complex to nucleate branched actin filaments. Strikingly, we find Rac1 and Arp2/3 are closely associated with synaptic vesicle membranes in adult mice. Using three independent approaches to alter presynaptic Rac1 activity (genetic knockout, spatially restricted inhibition, and temporal optogenetic manipulation), we discover that this pathway negatively regulates synaptic vesicle replenishment at both excitatory and inhibitory synapses, bidirectionally sculpting short-term synaptic depression. Finally, we use two-photon fluorescence lifetime imaging to show that presynaptic Rac1 activation is coupled to action potentials by voltage-gated calcium influx. Thus, this study uncovers a previously unrecognized mechanism of actin-regulated short-term presynaptic plasticity that is conserved across excitatory and inhibitory terminals. It also provides a new proteomic framework for better understanding presynaptic physiology, along with a blueprint of experimental strategies to isolate the presynaptic effects of ubiquitously expressed proteins.

Optogenetic Imaging of Protein Activity Using Two-Photon Fluorescence Lifetime Imaging Microscopy

Spatiotemporal dynamics of cellular proteins, including protein-protein interactions and conformational changes, is essential for understanding cellular functions such as synaptic plasticity, cell motility, and cell division. One of the best ways to understand the mechanisms of signal transduction is to visualize protein activity with high spatiotemporal resolution in living cells within tissues. Optogenetic probes such as fluorescent proteins, in combination with Förster Resonance Energy Transfer (FRET) techniques, enable the measurement of protein-protein interactions and conformational changes in response to signaling events in living cells. Of the various FRET detection systems, two-photon fluorescence lifetime imaging microscopy (2pFLIM) is one of the methods best suited to monitoring FRET in subcellular compartments of living cells located deep within tissues, such as brain slices. This review will introduce the principle of 2pFLIM-FRET and the use of chromoproteins for imaging intracellular protein activities and protein-protein interactions. Also, we will discuss two examples of 2pFLIM-FRET application: imaging actin polymerization in synapses of hippocampal neurons in brain sections and detecting small GTPase Cdc42 activity in astrocytes.