PIP₃ regulates spinule formation in dendritic spines during structural long-term potentiation

Dendritic spines are small, highly motile structures on dendritic shafts that provide flexibility to neuronal networks. Spinules are small protrusions that project from spines. The number and the length of spinules increase in response to activity including theta burst stimulation and glutamate application. However, what function spinules exert and how their formation is regulated still remains unclear. Phosphatidylinositol-3,4,5-trisphosphate (PIP₃) plays important roles in cell motility such as filopodia and lamellipodia by recruiting downstream proteins such as Akt and WAVE to the membrane, respectively. Here we reveal that PIP₃ regulates spinule formation during structural long-term potentiation (sLTP) of single spines in CA1 pyramidal neurons of hippocampal slices from rats. Since the local distribution of PIP₃ is important to exert its functions, the subcellular distribution of PIP₃ was investigated using a fluorescence lifetime-based PIP₃ probe. PIP₃ accumulates to a greater extent in spines than in dendritic shafts, which is regulated by the subcellular activity pattern of proteins that produce and degrade PIP₃. Subspine imaging revealed that when sLTP was induced in a single spine, PIP₃ accumulates in the spinule whereas PIP₃ concentration in the spine decreased.