Vasodilator-stimulated phosphoprotein (VASP) is recruited into dendritic spines via G-actin-dependent mechanism and contributes to spine enlargement and stabilization.

Actin organization and dynamics are modulated by diverse actin regulators during dendritic spine development. To understand the molecular network that regulates actin organization and spine morphology, it is important to investigate dynamic redistribution of actin regulators during spine development. One of the actin regulators, vasodilator-stimulated phosphoprotein (VASP), has multiple functions in actin regulation and is known to regulate spine morphology. However, dynamics and temporal regulation of VASP during spine development have not been clarified. In this study, we performed time-lapse imaging of mouse hippocampal dissociated neurons to analyse the change in localization of VASP during spine development. We found that accumulation of VASP within spines precedes the start of persistent F-actin increase, which are temporally coupled with spine enlargement. Using domain deletion or mutation constructs of VASP, we revealed that the interaction with G-actin is important for the preceding accumulation of VASP. Furthermore, we showed that accumulation of VASP contributes to actin enrichment within spines and stabilization of spine morphology by dominant negative experiments. These data suggest that G-actin-dependent VASP recruitment has dual functions in spine development, enlargement and stabilization, through the interaction with actin and other cytoskeletal regulators.

Clathrin mediates membrane fission and budding by constricting membrane pores.

Membrane budding, which underlies fundamental processes like endocytosis, intracellular trafficking, and viral infection, is thought to involve membrane coat-forming proteins, including the most observed clathrin, to form Ω-shape profiles and helix-forming proteins like dynamin to constrict Ω-profiles' pores and thus mediate fission. Challenging this fundamental concept, we report that polymerized clathrin is required for Ω-profiles' pore closure and that clathrin around Ω-profiles' base/pore region mediates pore constriction/closure in neuroendocrine chromaffin cells. Mathematical modeling suggests that clathrin polymerization at Ω-profiles' base/pore region generates forces from its intrinsically curved shape to constrict/close the pore. This new fission function may exert broader impacts than clathrin's well-known coat-forming function during clathrin (coat)-dependent endocytosis, because it underlies not only clathrin (coat)-dependent endocytosis, but also diverse endocytic modes, including ultrafast, fast, slow, bulk, and overshoot endocytosis previously considered clathrin (coat)-independent in chromaffin cells. It mediates kiss-and-run fusion (fusion pore closure) previously considered bona fide clathrin-independent, and limits the vesicular content release rate. Furthermore, analogous to results in chromaffin cells, we found that clathrin is essential for fast and slow endocytosis at hippocampal synapses where clathrin was previously considered dispensable, suggesting clathrin in mediating synaptic vesicle endocytosis and fission. These results suggest that clathrin and likely other intrinsically curved coat proteins are a new class of fission proteins underlying vesicle budding and fusion. The half-a-century concept and studies that attribute vesicle-coat contents' function to Ω-profile formation and classify budding as coat-protein (e.g., clathrin)-dependent or -independent may need to be re-defined and re-examined by considering clathrin's pivotal role in pore constriction/closure.

Regulation of mitochondrial dynamics and distribution by synapse position and neuronal activity in the axon.

Proper distribution of axonal mitochondria is critical for multiple neuronal functions. To understand the underlying mechanisms for population behavior, quantitative characterisation of elemental dynamics on multiple time scales is required. Here we investigated the stability and transport of axonal mitochondria using live-cell imaging of cultured mouse hippocampal neurons. We first characterised the long-term stability of stationary mitochondria. At a given moment, about 10% of the mitochondria were in a state of transport and the remaining 90% were stationary. Among these stationary mitochondria, 40% of them remained in the same position over several days. The rest of the mitochondria transited to mobile state stochastically and this process could be detected and quantitatively analysed by time-lapse imaging with intervals of 30 min. The stability of axonal mitochondria increased from 2 to 3 weeks in culture, was decreased by tetrodotoxin treatment, and was higher near synapses. Stationary mitochondria should be generated by pause of moving mitochondria and subsequent stabilisation. Therefore, we next analysed pause events of moving mitochondria by repetitive imaging at 0.3 Hz. We found that the probability of transient pause increased with field stimulation, decreased with tetrodotoxin treatment, and was higher near synapses. Finally, by combining parameters obtained from time-lapse imaging with different time scales, we could estimate transition rates between different mitochondrial states. The analyses suggested specific developmental regulation in the probability of paused mitochondria to transit into stationary state. These findings indicate that multiple mitochondrial behaviors, especially those regulated by neuronal activity and synapse location, determine their distribution in the axon.

A conformational switch in clathrin light chain regulates lattice structure and endocytosis at the plasma membrane of mammalian cells.

Conformational changes in endocytic proteins are regulators of clathrin-mediated endocytosis. Three clathrin heavy chains associated with clathrin light chains (CLC) assemble into triskelia that link into a geometric lattice that curves to drive endocytosis. Structural changes in CLC have been shown to regulate triskelia assembly in solution, yet the nature of these changes, and their effects on lattice growth, curvature, and endocytosis in cells are unknown. Here, we develop a new correlative fluorescence resonance energy transfer (FRET) and platinum replica electron microscopy method, named FRET-CLEM. With FRET-CLEM, we measure conformational changes in clathrin at thousands of individual morphologically distinct clathrin-coated structures. We discover that the N-terminus of CLC repositions away from the plasma membrane and triskelia vertex as coats curve. Preventing this conformational switch with chemical tools increases lattice sizes and inhibits endocytosis. Thus, a specific conformational switch in the light chain regulates lattice curvature and endocytosis in mammalian cells.

The role of molecular diffusion within dendritic spines in synaptic function.

Spines are tiny nanoscale protrusions from dendrites of neurons. In the cortex and hippocampus, most of the excitatory postsynaptic sites reside in spines. The bulbous spine head is connected to the dendritic shaft by a thin membranous neck. Because the neck is narrow, spine heads are thought to function as biochemically independent signaling compartments. Thus, dynamic changes in the composition, distribution, mobility, conformations, and signaling properties of molecules contained within spines can account for much of the molecular basis of postsynaptic function and regulation. A major factor in controlling these changes is the diffusional properties of proteins within this small compartment. Advances in measurement techniques using fluorescence microscopy now make it possible to measure molecular diffusion within single dendritic spines directly. Here, we review the regulatory mechanisms of diffusion in spines by local intra-spine architecture and discuss their implications for neuronal signaling and synaptic plasticity.

Precise Temporal Regulation of Molecular Diffusion within Dendritic Spines by Actin Polymers during Structural Plasticity.

The biochemical transduction of excitatory synaptic signals occurs in the cytoplasm within dendritic spines. The associated reaction kinetics are shaped by the mobility of the signaling molecules; however, accurate monitoring of diffusional events within the femtoliter-sized spine structures has not yet been demonstrated. Here, we applied two-photon fluorescence correlation spectroscopy and raster image correlation spectroscopy to monitor protein dynamics within spines, revealing that F-actin restricts the mobility of proteins with a molecular mass of >100 kDa. This restriction is transiently removed during actin remodeling at the initial phase of spine structural plasticity. Photobleaching experiments combined with super-resolution imaging indicate that this increase in mobility facilitates molecular interactions, which may modulate the functions of key postsynaptic signaling molecules, such as Tiam1 and CaMKII. Thus, actin polymers in dendritic spines act as precise temporal regulators of molecular diffusion and modulate signal transduction during synaptic plasticity.

Computational geometry analysis of dendritic spines by structured illumination microscopy.

Dendritic spines are the postsynaptic sites that receive most of the excitatory synaptic inputs, and thus provide the structural basis for synaptic function. Here, we describe an accurate method for measurement and analysis of spine morphology based on structured illumination microscopy (SIM) and computational geometry in cultured neurons. Surface mesh data converted from SIM images were comparable to data reconstructed from electron microscopic images. Dimensional reduction and machine learning applied to large data sets enabled identification of spine phenotypes caused by genetic mutations in key signal transduction molecules. This method, combined with time-lapse live imaging and glutamate uncaging, could detect plasticity-related changes in spine head curvature. The results suggested that the concave surfaces of spines are important for the long-term structural stabilization of spines by synaptic adhesion molecules.

LIS1-dependent retrograde translocation of excitatory synapses in developing interneuron dendrites.

Synaptic remodelling coordinated with dendritic growth is essential for proper development of neural connections. After establishment of synaptic contacts, synaptic junctions are thought to become stationary and provide fixed anchoring points for further dendritic growth. However, the possibility of active translocation of synapses along dendritic protrusions, to guide the proper arrangement of synaptic distribution, has not yet been fully investigated. Here we show that immature dendrites of γ-aminobutyric acid-positive interneurons form long protrusions and that these protrusions serve as conduits for retrograde translocation of synaptic contacts to the parental dendrites. This translocation process is dependent on microtubules and the activity of LIS1, an essential regulator of dynein-mediated motility. Suppression of this retrograde translocation results in disorganized synaptic patterns on interneuron dendrites. Taken together, these findings suggest the existence of an active microtubule-dependent mechanism for synaptic translocation that helps in the establishment of proper synaptic distribution on dendrites.