Diffusion in the plasma membrane of living cells is often found to display anomalous dynamics. However, the mechanism underlying this diffusion pattern remains highly controversial. Here, we study the physical mechanism underlying Kv2.1 potassium channel anomalous dynamics using single-molecule tracking. Our analysis includes both time series of individual trajectories and ensemble averages. We show that an ergodic and a nonergodic process coexist in the plasma membrane. The ergodic process resembles a fractal structure with its origin in macromolecular crowding in the cell membrane. The nonergodic process is found to be regulated by transient binding to the actin cytoskeleton and can be accurately modeled by a continuous-time random walk. When the cell is treated with drugs that inhibit actin polymerization, the diffusion pattern of Kv2.1 channels recovers ergodicity. However, the fractal structure that induces anomalous diffusion remains unaltered. These results have direct implications on the regulation of membrane receptor trafficking and signaling.anomalous subdiffusion | Brownian motion | continuous time random walk | single-particle tracking T he plasma membrane is a highly complex system with a dynamic organization required to maintain many fundamental processes that include signal transduction, receptor recognition, endocytic transport, and cell-cell adhesion. The study of the diffusion pattern of transmembrane proteins and lipids grants biophysical information on membrane organization, structure, and dynamics. In particular, single-molecule tracking provides insight into the interaction of membrane proteins with their surroundings. Stochastic molecular transport is described by the probability distribution of displacements and how it evolves over time. This distribution is called a propagator, and in the case of Brownian motion it is Gaussian. Diffusion processes that deviate from Brownian motion are considered anomalous, and they involve propagators that may or may not be Gaussian. For example, diffusion in a fractal has a stretched Gaussian propagator (1).Experimental and theoretical work suggests there is a correlation between macromolecular crowding and anomalous diffusion (2-4). However, the mechanism behind hindered diffusion in living cells remains controversial. Several models have been proposed, including membrane compartmentalization, receptor and cytoskeleton binding, and membrane heterogeneity (lipid rafts). Particle trajectories are frequently characterized by their mean square displacement (MSD) (5). A Brownian particle in a 2D space yields an MSD hΔr 2 ðtÞi ¼ 4Dt. However, in many systems the MSD scales as a sublinear power law, indicating anomalous diffusion. In general hΔr 2 ðtÞi ∝ t γ , where γ is the anomaly exponent. Anomalous subdiffusion is manifested by a characteristic exponent γ < 1, and anomalous superdiffusion by γ > 1.Two biologically relevant processes are recognized to induce anomalous subdiffusion in the plasma membrane: (i) transient immobilization and (ii) geometrical i...

Highlights d Hyperactive neurons release excess FAs in lipid particles associated with ApoE d Astrocytes endocytose neuron-derived lipid particles, delivering the FAs to LDs d Astrocytes with LDs upregulate metabolic and detoxification genes d Neural activity triggers astrocytic consumption of FAs by mitochondrial oxidation

The endoplasmic reticulum (ER) is an expansive, membrane-enclosed organelle that plays crucial roles in numerous cellular functions. We used emerging superresolution imaging technologies to clarify the morphology and dynamics of the peripheral ER, which contacts and modulates most other intracellular organelles. Peripheral components of the ER have classically been described as comprising both tubules and flat sheets. We show that this system consists almost exclusively of tubules at varying densities, including structures that we term ER matrices. Conventional optical imaging technologies had led to misidentification of these structures as sheets because of the dense clustering of tubular junctions and a previously uncharacterized rapid form of ER motion. The existence of ER matrices explains previous confounding evidence that had indicated the occurrence of ER "sheet" proliferation after overexpression of tubular junction-forming proteins.