Development of FRET-based indicators for visualizing homophilic trans interaction of a clustered protocadherin

Visualization of trans homophilic interaction of clustered protocadherin in neurons

Clustered protocadherin (Pcdh) functions as a cell recognition molecule through the homophilic interaction in the central nervous system. However, its interactions have not yet been visualized in neurons. We previously reported PcdhγB2-Förster resonance energy transfer (FRET) probes to be applicable only to cell lines. Herein, we designed γB2-FRET probes by fusing FRET donor and acceptor fluorescent proteins to a single γB2 molecule and succeeded in visualizing γB2 homophilic interaction in cultured hippocampal neurons. The γB2-FRET probe localized in the soma and neurites, and FRET signals, which were observed at contact sites between neurites, eliminated by ethylene glycol tetraacetic acid (EGTA) addition. Live imaging revealed that the FRET-negative γB2 signals rapidly moved along neurites and soma, whereas the FRET-positive signals remained in place. We observed that the γB2 proteins at synapses rarely interact homophilically. The γB2-FRET probe might allow us to elucidate the function of the homophilic interaction and the cell recognition mechanism.

Visualization of trans-interactions of a protocadherin-α between processes originating from single neurons

Clustered protocadherin (Pcdh), a cell adhesion protein, is involved in the self-recognition and non-self-discrimination of neurons by conferring diversity on the cell surface. Although the roles of Pcdh in neurons have been elucidated, it has been challenging to visualize its adhesion activity in neurons, which is a molecular function of Pcdh. Here, we present fluorescent indicators, named IPADs, which visualize the interaction of protocadherin-α4 isoform (α4). IPADs successfully visualize not only homophilic α4 trans-interactions, but also combinatorial homophilic interactions between cells. The reversible nature of IPADs overcomes a drawback of the split-GFP technique and allows for monitoring the dissociation of α4 trans-interactions. Specially designed IPADs for self-recognition are able to monitor the formation and disruption of α4 trans-interactions between processes originating from the same neurons. We expect that IPADs will be useful tools for obtaining spatiotemporal information on Pcdh interactions in neuronal self-recognition and non-self-discrimination processes.

Development of intensiometric indicators for visualizing N-cadherin interaction across cells

N-cadherin (NCad) is a classical cadherin that mediates cell–cell interactions in a Ca²⁺-dependent manner. NCad participates in various biological processes, from ontogenesis to higher brain functions, though the visualization of NCad interactions in living cells remains limited. Here, we present intensiometric NCad interaction indicators, named INCIDERs, that utilize dimerization-dependent fluorescent proteins. INCIDERs successfully visualize reversible NCad interactions across cells. Compared to FRET-based indicators, INCIDERs have a ~70-fold higher signal contrast, enabling clear identification of NCad interactions. In primary neuronal cells, NCad interactions are visualized between closely apposed processes. Furthermore, visualization of NCad interaction at cell adhesion sites in dense cell populations is achieved by two-photon microscopy. INCIDERs are useful tools in the spatiotemporal investigation of NCad interactions across cells; future research should evaluate the potential of INCIDERs in mapping complex three-dimensional architectures in multi-cellular systems.

Intensiometric N-cadherin (NCad) interaction indicators, named INCIDERs, visualize reversible NCad-mediated cell-cell interactions.