Self-associating split fluorescent proteins (FPs) are split FPs whose two fragments spontaneously associate to form a functional FP. They have been widely used for labeling proteins, scaffolding protein assembly and detecting cell-cell contacts. Recently developments have expanded the palette of self-associating split FPs beyond the original split GFP1-10/11. However, these new ones have suffered from suboptimal fluorescence signal after complementation. Here, by investigating the complementation process, we have demonstrated two approaches to improve split FPs: assistance through SpyTag/SpyCatcher interaction and directed evolution. The latter has yielded two split sfCherry3 variants with substantially enhanced overall brightness, facilitating the tagging of endogenous proteins by gene editing. Based on sfCherry3, we have further developed a new red-colored trans-synaptic marker called Neuroligin-1 sfCherry3 Linker Across Synaptic Partners (NLG-1 CLASP) for multiplexed visualization of neuronal synapses in living C. elegans, demonstrating its broad applications.

During neural circuit formation, most axons are guided to complex environments, coming into contact with multiple potential synaptic partners. However, it is critical that they recognize specific neurons with which to form synapses. Here, we utilize the split GFP-based marker Neuroligin-1 GFP Reconstitution Across Synaptic Partners (NLG-1 GRASP) to visualize specific synapses in live animals, and a circuit-specific behavioral assay to probe circuit function. We demonstrate that the receptor protein tyrosine phosphatase (RPTP) clr-1 is necessary for synaptic partner recognition (SPR) between the PHB sensory neurons and the AVA interneurons in C. elegans. Mutations in clr-1/RPTP result in reduced NLG-1 GRASP fluorescence and impaired behavioral output of the PHB circuit. Temperature-shift experiments demonstrate that clr-1/RPTP acts early in development, consistent with a role in SPR. Expression and cell-specific rescue experiments indicate that clr-1/RPTP functions in postsynaptic AVA neurons, and overexpression of clr-1/RPTP in AVA neurons is sufficient to direct additional PHB-AVA synaptogenesis. Genetic analysis reveals that clr-1/RPTP acts in the same pathway as the unc-6/Netrin ligand and the unc-40/DCC receptor, which act in AVA and PHB neurons, respectively. This study defines a new mechanism by which SPR is governed, and demonstrates that these three conserved families of molecules, with roles in neurological disorders and cancer, can act together to regulate communication between cells.

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SummarySleep is conserved across phyla and is shown here to be required for memory consolidation in the nematode, C. elegans. However, it is unclear how sleep collaborates with experience to change specific neurons and associated synapses to ultimately affect behavior. C. elegans neurons have defined synaptic connections and described contributions to specific behaviors. We show that spaced odor-training induces long-term memory, which transits a labile period before being stably maintained. This post-training labile period is required for long-term memory. Memory consolidation, but not acquisition, requires a single interneuron, AIY, which plays a role in odor-seeking behavior. We find that sleep and conditioning mark inhibitory synaptic connections between the butanone-sensing AWC neuron and AIY to decrease synapses and it is in the post-sleep wake phase that memory-specific synaptic changes occur. Thus, we demonstrate in the living organism how sleep initiates events lasting beyond the period of sleep to drive memory consolidation.

Olfactory dysfunction precedes dementia in several neurodegenerative disorders such as Alzheimer’s disease (AD) or Parkinson’s Disease (PD), and AD/PD are associated with progressive sleep abnormalities. However, how sleep affects cognitive performance remains unclear, perhaps due to the complexities of the human nervous system. Here we demonstrate that the transparent model organism C. elegans which has well defined neural connection sleeps after repeated odor trainings. This provides us with a platform to dissect how sleep affects memory at a synaptic resolution. We identified that sleep after training is required for the animal to retain a long-term memory of the odor. We found that if animals do not sleep in the first two hours after training, memory is not consolidated. After identifying the neurons that are required for the memory, we show that the sensory-interneuron connections within the circuit are downscaled after sleep. Therefore, we found a time-specific requirement of sleep that modulates synaptic downscaling to preserve memory. Conversely, lack of sleep post-training erases the long-term memory and destabilizes the synaptic downscaling, indicating that modulating the amount of sleep is sufficient to modulate memory. These results make C. elegans an excellent tool to ask what molecular mechanisms, cell biological processes and circuit level reorganizations are engaged during sleep to promote memory. This understanding will provide insights into the functions of sleep that affects cognitive performance in neurodegenerative diseases.