Fluorescence resonance energy transfer (FRET) technology has been used to develop genetically encoded fluorescent indicators for various cellular functions. Although most indicators have cyan- and yellow-emitting fluorescent proteins (CFP and YFP) as FRET donor and acceptor, their poor dynamic range often prevents detection of subtle but significant signals. Here, we optimized the relative orientation of the two chromophores in the Ca ²⁺ indicator, yellow cameleon (YC), by fusing YFP at different angles. We generated circularly permuted YFPs (cpYFPs) that showed efficient maturation and acid stability. One of the cpYFPs incorporated in YC absorbs a great amount of excited energy from CFP in its Ca ²⁺ -saturated form, thereby increasing the Ca ²⁺ -dependent change in the ratio of YFP/CFP by nearly 600%. Both in cultured cells and in the nervous system of transgenic mice, the new YC enables visualization of subcellular Ca ²⁺ dynamics with better spatial and temporal resolution than before. Our study provides an important guide for the development and improvement of indicators using GFP-based FRET.

A specific neural circuit integrates temporally dispersed stimuli into a coherent memory episode.

Formation of neurofibrillary tangles (NFTs) is a common neuropathological feature found in several neurodegenerative diseases, including Alzheimer's disease. We have developed a transgenic (Tg) mouse expressing mutant human tau (V337M), derived from frontotemporal dementia parkinsonism-17. V337M Tg mice revealed tau aggregations in the hippocampus, which fulfills the histological criteria for NFTs in human neurodegenerative diseases. Concurrent with the accumulation of RNA and phosphorylated tau, neurons exhibited morphological characteristics of degenerating neurons, which include a loss of microtubules, accumulation of ribosomes, plasma and nuclear membrane ruffling, and swelling of the Golgi network. Thus, mutant tau induces neuronal degeneration associated with the accumulation of RNA and phosphorylated tau. The functional consequences of this neuronal degeneration was evidenced by the reduction of hippocampal neural activity and behavioral abnormality in Tg mice.

The entorhinal cortex provides the major cortical input to the hippocampus, and both structures have been implicated in memory processes. The dynamics of neuronal circuits in the entorhinal-hippocampal system were studied in slices by optical imaging with high spatial and temporal resolution. Reverberation of neural activity was detected in the entorhinal cortex and was more prominent when the inhibition due to γ-aminobutyric acid was slightly suppressed. Neural activity was transferred in a frequency-dependent way from the entorhinal cortex to the hippocampus. The entorhinal neuronal circuit could contribute to memory processes by holding information and selectively gating the entry of information into the hippocampus.