Epithelial cell migration requires coordination of two actin modules at the leading edge: one in the lamellipodium and one in the lamella. How the two modules connect mechanistically to regulate directed edge motion is not understood. Using live-cell imaging and photoactivation approaches, we demonstrate that the actin network of the lamellipodium evolves spatio-temporally into the lamella. This occurs during the retraction phase of edge motion, when myosin II redistributes to the lamellipodial actin and condenses it into an actin arc parallel to the edge. The new actin arc moves rearward, slowing down at focal adhesions in the lamella. We propose that net edge extension occurs by nascent focal adhesions advancing the site at which new actin arcs slow down and form the base of the next protrusion event. The actin arc thereby serves as a structural element underlying the temporal and spatial connection between the lamellipodium and the lamella during directed cell motion.

The paradigm that the secretory pathway consists of a stable endoplasmic reticulum and Golgi apparatus, using discrete transport vesicles to exchange their contents, gained important support from groundbreaking biochemical and genetic studies during the 1980s. However, the subsequent development of new imaging technologies with green fluorescent protein introduced data on dynamic processes not fully accounted for by the paradigm. As a result, we may be seeing an example of how a paradigm is evolving to account for the results of new technologies and their new ways of describing cellular processes.

We describe a localization microscopy analysis method that is able to extract results in live cells using standard fluorescent proteins and xenon arc lamp illumination. Our Bayesian analysis of the blinking and bleaching (3B analysis) method models the entire dataset simultaneously as being generated by a number of fluorophores that may or may not be emitting light at any given time. The resulting technique allows many overlapping fluorophores in each frame and unifies the analysis of the localization from blinking and bleaching events. By modeling the entire dataset, we were able to use each reappearance of a fluorophore to improve the localization accuracy. The high performance of this technique allowed us to reveal the nanoscale dynamics of podosome formation and dissociation throughout an entire cell with a resolution of 50 nm on a 4-s timescale.

This study describes a unique function of taurocholate in bile canalicular formation involving signaling through a cAMP-Epac-MEK-Rap1-LKB1-AMPK pathway. In rat hepatocyte sandwich cultures, polarization was manifested by sequential progression of bile canaliculi from small structures to a fully branched network. Taurocholate accelerated canalicular network formation and concomitantly increased cAMP, which were prevented by adenyl cyclase inhibitor. The cAMP-dependent PKA inhibitor did not prevent the taurocholate effect. In contrast, activation of Epac, another cAMP downstream kinase, accelerated canalicular network formation similar to the effect of taurocholate. Inhibition of Epac downstream targets, Rap1 and MEK, blocked the taurocholate effect. Taurocholate rapidly activated MEK, LKB1, and AMPK, which were prevented by inhibition of adenyl cyclase or MEK. Our previous study showed that activated-LKB1 and AMPK participate in canalicular network formation. Linkage between bile acid synthesis, hepatocyte polarization, and regulation of energy metabolism is likely important in normal hepatocyte development and disease.

Superresolution imaging techniques based on the precise localization of single molecules, such as photoactivated localization microscopy (PALM) and stochastic optical reconstruction microscopy (STORM), achieve high resolution by fitting images of single fluorescent molecules with a theoretical Gaussian to localize them with a precision on the order of tens of nanometers. PALM/STORM rely on photoactivated proteins or photoswitching dyes, respectively, which makes them technically challenging. We present a simple and practical way of producing point localization-based superresolution images that does not require photoactivatable or photoswitching probes. Called bleaching/blinking assisted localization microscopy (BaLM), the technique relies on the intrinsic bleaching and blinking behaviors characteristic of all commonly used fluorescent probes. To detect single fluorophores, we simply acquire a stream of fluorescence images. Fluorophore bleach or blink-off events are detected by subtracting from each image of the series the subsequent image. Similarly, blink-on events are detected by subtracting from each frame the previous one. After image subtractions, fluorescence emission signals from single fluorophores are identified and the localizations are determined by fitting the fluorescence intensity distribution with a theoretical Gaussian. We also show that BaLM works with a spectrum of fluorescent molecules in the same sample. Thus, BaLM extends single molecule-based superresolution localization to samples labeled with multiple conventional fluorescent probes.

Manipulation of hematopoietic stem/progenitor cells (HSPCs) ex vivo is of clinical importance for stem cell expansion and gene therapy applications. However, most cultured HSPCs are actively cycling, and show a homing and engraftment defect compared with the predominantly quiescent noncultured HSPCs. We previously showed that HSPCs make contact with osteoblasts in vitro via a polarized membrane domain enriched in adhesion molecules such as tetraspanins. Here we show that increased cell cycling during ex vivo culture of HSPCs resulted in disruption of this membrane domain, as evidenced by disruption of polarity of the tetraspanin CD82. Chemical disruption or antibody-mediated blocking of CD82 on noncultured HSPCs resulted in decreased stromal cell adhesion, homing, and engraftment in nonobese diabetic/severe combined immunodeficiency IL-2γ(null) (NSG) mice compared with HSPCs with an intact domain. Most leukemic blasts were actively cycling and correspondingly displayed a loss of domain polarity and decreased homing in NSG mice compared with normal HSPCs. We conclude that quiescent cells, unlike actively cycling cells, display a polarized membrane domain enriched in tetraspanins that mediates homing and engraftment, providing a mechanistic explanation for the homing/engraftment defect of cycling cells and a potential new therapeutic target to enhance engraftment.

Many unexpected discoveries in developmental biology have depended on advancement of imaging technologies to visualize developmental processes as they unfold across multiple spatial and temporal scales. This essay surveys the recent advances in imaging, highlighting emerging capabilities with an eye toward those poised to have the greatest impact on developmental biology.

We recently discovered that the major mammalian bile acid, taurocholate, accelerated polarity in primary rat hepatocytes. Taurocholate increased cellular cAMP and signals through an Epac-Rap1-MEK-LKB1-AMPK pathway for its polarity effect. This review discusses possible mechanisms for how taurocholate affects different cell polarity factors, particularly AMPK, and thereby regulates events that generate polarity. These include tight junction formation, apical trafficking, recycling endosome dynamics, and cytoskeleton rearrangement. We also discuss whether the effects of taurocholate are mediated by other LKB1 downstream kinases, such as Par1 and NUAK1.

The final stage of cytokinesis is abscission, the cutting of the narrow membrane bridge connecting two daughter cells. The endosomal sorting complex required for transport (ESCRT) machinery is required for cytokinesis, and ESCRT-III has membrane scission activity in vitro, but the role of ESCRTs in abscission has been undefined. Here, we use structured illumination microscopy and time-lapse imaging to dissect the behavior of ESCRTs during abscission. Our data reveal that the ESCRT-I subunit tumor-susceptibility gene 101 (TSG101) and the ESCRT-III subunit charged multivesicular body protein 4b (CHMP4B) are sequentially recruited to the center of the intercellular bridge, forming a series of cortical rings. Late in cytokinesis, however, CHMP4B is acutely recruited to the narrow constriction site where abscission occurs. The ESCRT disassembly factor vacuolar protein sorting 4 (VPS4) follows CHMP4B to this site, and cell separation occurs immediately. That arrival of ESCRT-III and VPS4 correlates both spatially and temporally with the abscission event suggests a direct role for these proteins in cytokinetic membrane abscission.