Bending over backwards: BAR proteins and the actin cytoskeleton in mammalian receptor-mediated endocytosis

The actin cytoskeleton regulates danger-associated molecular pattern signaling and PEP1 RECEPTOR1 internalization

In plants, cytoskeletal proteins assemble into dynamic polymers that play numerous roles in diverse fundamental cellular processes, including endocytosis, vesicle trafficking, and the spatial distribution of organelles and protein complexes. Plant elicitor peptides (Peps) are damage/danger-associated molecular patterns (DAMPs) that are perceived by the receptor-like kinases PEP RECEPTOR 1 (PEPR1) and PEPR2 to enhance innate immunity and inhibit root growth in Arabidopsis (Arabidopsis thaliana). To date, however, there is little evidence that the actin cytoskeleton of the host cell participates in DAMP-induced innate immunity. Here, we demonstrated that the actin cytoskeleton alters the Pep1-triggered immune response. In addition, dual-color total internal reflection fluorescence-structured illumination microscopy (TIRF-SIM) showed that PEPR1 diffusion on the plasma membrane is closely related to the actin cytoskeleton. We performed single-particle tracking to quantify individual protein particles and found that the actin cytoskeleton notably regulates PEPR1 mobility and cluster size. More importantly, we demonstrated that actin filament reconfiguration is sufficient to inhibit Pep1-induced internalization, which alters the immune response. Taken together, these findings suggest that the actin cytoskeleton functions as an integration node for Pep1 signaling and PEPR1 endocytosis.

Drivers of Morphogenesis: Curvature Sensor Self-Assembly at the Membrane

This review examines the relationships between membrane chemistry, curvature-sensing proteins, and cellular morphogenesis. Curvature-sensing proteins are often orders of magnitude smaller than the membrane curvatures they localize to. How are nanometer-scale proteins used to sense micrometer-scale membrane features? Here, we trace the journey of curvature-sensing proteins as they engage with lipid membranes through a combination of electrostatic and hydrophobic interactions. We discuss how curvature sensing hinges on membrane features like lipid charge, packing, and the directionality of membrane curvature. Once bound to the membrane, many curvature sensors undergo self-assembly (i.e., they oligomerize or form higher-order assemblies that are key for initiating and regulating cell shape transformations). Central to these discussions are the micrometer-scale curvature-sensing proteins' septins. By discussing recent literature surrounding septin membrane association, assembly, and their many functions in morphogenesis with support from other well-studied curvature sensors, we aim to synthesize possible mechanisms underlining cell shape sensing.

miR-196b-5p regulates inflammatory process and migration via targeting Nras in trabecular meshwork cells

Glaucoma, an insidious ophthalmic pathology, is typified by an aberrant surge in intraocular pressure (IOP) which culminates in the degeneration of retinal ganglion cells and optical neuropathy. The mitigation of IOP stands as the principal therapeutic strategy to forestall vision loss. The trabecular meshwork's (TM) integrity and functionality are pivotal in modulating aqueous humor egress. Despite their potential significance in glaucomatous pathophysiology, the implications of microRNAs (miRNAs) on TM functionality remain largely enigmatic. Transcriptomic sequencing was employed to delineate the miRNA expression paradigm within the limbal region of rodent glaucoma models, aiming to elucidate miRNA-mediated mechanisms within the glaucomatous milieu. Analytical scrutiny of the sequencing data disclosed 174 miRNAs with altered expression profiles, partitioned into 86 miRNAs with augmented expression and 88 with diminished expression. Notably, miRNAs such as hsa-miR-196b-5p were identified as having substantial expression discrepancies with concomitant statistical robustness, suggesting a potential contributory role in glaucomatous progression. Subsequent in vitro assays affirmed that miR-196b-5p augments the inflammatory cascade within immortalized human TM (iHTM) and glaucoma-induced human TM (GTM3) cells, concurrently attenuating cellular proliferation, motility, and cytoskeletal architecture. Additionally, miR-196b-5p implicates itself in the regulation of IOP and inflammatory processes in rodent models. At a mechanistic level, miR-196b-5p modulates its effects via the targeted repression of Nras (neuroblastoma RAS viral oncogene homolog). Collectively, these transcriptomic investigations furnish a comprehensive vista into the regulatory roles of miRNAs within the glaucomatous framework, and the identification of differentially expressed miRNAs alongside their targets could potentially illuminate novel molecular pathways implicated in glaucoma, thereby aiding in the development of innovative therapeutic avenues.

Self-assembly of CIP4 drives actin-mediated asymmetric pit-closing in clathrin-mediated endocytosis

Clathrin-mediated endocytosis is pivotal to signal transduction pathways between the extracellular environment and the intracellular space. Evidence from live-cell imaging and super-resolution microscopy of mammalian cells suggests an asymmetric distribution of actin fibres near the clathrin-coated pit, which induces asymmetric pit-closing rather than radial constriction. However, detailed molecular mechanisms of this 'asymmetricity' remain elusive. Herein, we used high-speed atomic force microscopy to demonstrate that CIP4, a multi-domain protein with a classic F-BAR domain and intrinsically disordered regions, is necessary for asymmetric pit-closing. Strong self-assembly of CIP4 via intrinsically disordered regions, together with stereospecific interactions with the curved membrane and actin-regulating proteins, generates a small actin-rich environment near the pit, which deforms the membrane and closes the pit. Our results provide mechanistic insights into how disordered and structured domain collaboration promotes spatio-temporal actin polymerisation near the plasma membrane.