Biomimetic Chemical Sensing by Fluorescence Signals Using a Virus-like Particle-Based Platform.

The chemical receptors present in living organisms are promising tools for developing biomimetic chemical sensors. However, these receptors require lipid membranes for functioning under physiological conditions, which prevents their utilization in the production of cell-free in vitro chemical sensing systems. Here, we report the development of a cell-free biomimetic sensing platform using virus-like particles (VLPs) with intact ligand-gated Ca2+ channels and genetically encoded Ca2+ indicator (GECI). We observed that targeting GECI to the plasma membrane was essential for efficient loading GECI in the VLPs. Although the physiological Ca2+ concentration [Ca2+] maintained in the cells was low (∼10 nM), the concentration in the VLPs was high. This prevented the detection of the increase in [Ca2+] caused by binding of the ligand to the receptor. To address this problem, we employed Lyn-R-CEPIA1, which had low affinity for Ca2+, and a membrane targeting sequence. Thus, we succeeded in monitoring the activation of cyclic nucleotide-gated channels (CNG) on the VLPs by measuring the increase in fluorescence of Lyn-R-CEPIA1. Our VLP-based sensing system can act as a fundamental platform for all kinds of ligand-gated channels.

Control of Spontaneous Ca2+ Transients Is Critical for Neuronal Maturation in the Developing Neocortex.

Neural activity plays roles in the later stages of development of cortical excitatory neurons, including dendritic and axonal arborization, remodeling, and synaptogenesis. However, its role in earlier stages, such as migration and dendritogenesis, is less clear. Here we investigated roles of neural activity in the maturation of cortical neurons, using calcium imaging and expression of prokaryotic voltage-gated sodium channel, NaChBac. Calcium imaging experiments showed that postmigratory neurons in layer II/III exhibited more frequent spontaneous calcium transients than migrating neurons. To test whether such an increase of neural activity may promote neuronal maturation, we elevated the activity of migrating neurons by NaChBac expression. Elevation of neural activity impeded migration, and induced premature branching of the leading process before neurons arrived at layer II/III. Many NaChBac-expressing neurons in deep cortical layers were not attached to radial glial fibers, suggesting that these neurons had stopped migration. Morphological and immunohistochemical analyses suggested that branched leading processes of NaChBac-expressing neurons differentiated into dendrites. Our results suggest that developmental control of spontaneous calcium transients is critical for maturation of cortical excitatory neurons in vivo: keeping cellular excitability low is important for migration, and increasing spontaneous neural activity may stop migration and promote dendrite formation.

Influence of the Circulating Current on the Propagation of the Change in Membrane Potential.

The propagation of the change in potential differences across liquid membranes from the potential-sending cell to the potential-receiving cell was investigated by use of a system combined with three liquid membrane cells, which were composed of two aqueous phases and a 1,2-dichloroethane solution phase. The ionic composition of one potential-sending cell (S) was identical to that of the receiving cell (Rec), and that of another potential-sending cell (Ap) was different from that of the Rec. When the connection of cell Rec was switched from cell S to cell Ap, the change in the membrane potential was caused by the circulating current. The greater the ratio of the interfacial area of the membrane of cell Ap to that of cell Rec, the faster the change in the membrane potential propagated from cell Ap to cell Rec.