Quantitative image analysis of the spatial organization and mobility of caveolin aggregates at the plasma membrane

Caveolins are integral membrane proteins that are the principal structural component of caveolae. Newly synthesized caveolin self-associates into oligomers that further assemble into higher-order structures. Imaging fluorescently labeled caveolin at the plasma membrane with total internal reflection fluorescence (TIRF) microscopy reveals a spatially heterogeneous distribution with aggregates of various sizes. In this chapter, we present a set of image-processing tools to quantify the spatial organization and mobility of caveolin aggregates seen in TIRF images. We apply a spot detection algorithm to identify punctate features on multiple length scales, and computationally estimate the area and integrated fluorescence signal of each detected feature. We then partition the original image into two disjoint sets: one containing pixels within punctae, and the other containing pixels on the rest of the plasma membrane. From these partitions, we estimate the relative fraction of caveolin that is punctate versus diffuse. Finally, we analyze the mobility of caveolin aggregates by tracking them and classify individual trajectories as diffusive or subdiffusive using a moment scaling spectrum analysis. Together, these analyses capture multiple facets of caveolin organization and dynamics. To demonstrate their utility, we quantify the distribution of fluorescent Caveolin 1 stably transfected in HeLa cells. We analyze cells at baseline and after being exposed to the anesthetic Dibucaine that is known to scramble membrane phospholipids. Our analysis shows how this perturbation dramatically alters caveolin aggregation and mobility.