Cellular events require the spatiotemporal interplay between actin assembly and actin disassembly. Yet, how different factors promote the integration of these two opposing processes is unclear. In particular, cellular monomeric (G)-actin is complexed with profilin, which inhibits spontaneous actin nucleation but fuels actin filament (F-actin) assembly by elongation-promoting factors (formins, Ena/VASP). In contrast, site-specific F-actin oxidation by Mical promotes F-actin disassembly and release of polymerization-impaired Mical-oxidized (Mox)-G-actin. Here we find that these two opposing processes connect with one another to orchestrate actin/cellular remodeling. Specifically, we find that profilin binds Mox-G-actin, yet these complexes do not fuel elongation factors’-mediated F-actin assembly, but instead inhibit polymerization and promote further Mox-F-actin disassembly. UsingDrosophilaas a model system, we show that similar profilin–Mical connections occur in vivo – where they underlie F-actin/cellular remodeling that accompanies Semaphorin–Plexin cellular/axon repulsion. Thus, profilin and Mical combine to impair F-actin assembly and promote F-actin disassembly, while concomitantly facilitating cellular remodeling and plasticity.

Drebrin is a key regulator of actin cytoskeleton in neuronal cells which is critical for synaptic plasticity, neuritogenesis, and neuronal migration. It is also known to orchestrate a cross-talk between actin and microtubules. Decreased level of drebrin is a hallmark of multiple neurodegenerative disorders such as Alzheimer's disease. Despite its established importance in health and disease, we still have a lot to learn about drebrin's interactome and its effects on cytoskeletal dynamics. This review aims to summarize the recently reported novel effects of drebrin on actin and its regulators. Here I will also reflect on the most recent progress made in understanding of the role of drebrin isoforms and posttranslational modifications on its functionality.

Actin cytoskeleton is critical for neuronal shape and function. Drebrin and formins are key regulators of neuronal actin networks. Neuron-specific drebrin A is highly enriched in dendritic spines (postsynaptic terminals) of mature excitatory neurons. Decreased levels of drebrin in dendritic spines is a hallmark of Alzheimer's disease, epilepsy, and other complex disorders, which calls for better understanding of its regulatory functions. Drebrin A was previously shown to inhibit actin nucleation and bundling by the diaphanous formin-2 (mDia2) - an actin nucleator that is involved in the initiation of dendritic spines. Characterization of the molecular binding interface between mDia2 and drebrin is necessary to better understand the functional consequences of this interaction and its biological relevance. Prior work suggested a multi-pronged interface between mDia2 and drebrin, which involves both N-terminal and C-terminal regions of the drebrin molecule. Here we used mass spectrometry analysis, deletion mutagenesis, and an array of synthetic peptides of neuronal drebrin A to map its formin-binding interface. The mDia2-interacting interface on drebrin was narrowed down to three highly conserved 9-16 residue sequences that were used to identify some of the key residues involved in this interaction. Deletion of the C-terminal region of drebrin greatly reduces its binding to mDia2 and the extent of its inhibition of formin-driven actin assembly. Moreover, our experiments with formins from different subfamilies showed that drebrin is a specific rather than general inhibitor of these proteins. This work contributes to a molecular level understanding of the formin-drebrin interaction and will help to unravel its biological significance.

Actin cytoskeleton is vital for neuronal function. Drebrin is a key F-actin binding protein in neurons which is linked to the filaments' stabilization. As mammalian brain develops, drebrin expression pattern switches from non-neuronal (drebrin E, Embryonic) to neuron-specific isoform (drebrin A, Adult), but the evolutionary need for such a switch is enigmatic. Prior in cellulo and in vivo work suggested a non-redundant role of drebrin isoforms in neuronal development and function, however, molecular level understanding of it is lacking. Here we used mutagenesis, bulk solution assays, and time-lapse TIRF microscopy to probe for functional differences between drebrin isoforms. We found that drebrin A and E are functionally distinct and differ in their ability to inhibit F-actin depolymerization. We showed that both isoforms act as permissive cappers of the barbed end of actin, however, drebrin A has a significantly stronger capping activity, compared to that of the non-neuronal drebrin E. Probing for the molecular level insights on the observed differences revealed that the adult-specific exon in neuronal drebrin A contains an actin binding interface which enhances its permissive capping activity. Strikingly, F-actin decoration by neuronal drebrin A confers significantly stronger resistance to cofilin-mediated severing compared to that of drebrin E. Our results provide novel molecular level insights on functional differences between drebrin isoforms, which deepen our understanding of cytoskeletal regulation in the neuronal context. Our results also helps interpreting the previously reported data related to the silencing or knockout of the neuronal drebrin isoform.

While membrane proteins such as ion channels continuously turn over and require replacement, the mechanisms of specificity of efficient channel delivery to appropriate membrane subdomains remain poorly understood. GJA1-20k is a truncated Connexin43 (Cx43) isoform arising from translation initiating at an internal start codon within the same parent GJA1 mRNA and is requisite for full-length Cx43 trafficking to cell borders. GJA1-20k does not have a full transmembrane domain, and it is not known how GJA1-20k enables forward delivery of Cx43 hemichannels. Here, we report that a RPEL-like domain at the C terminus of GJA1-20k binds directly to actin and induces an actin phenotype similar to that of an actin-capping protein. Furthermore, GJA1-20k organizes actin within the cytoplasm to physically outline a forward delivery pathway for microtubule-based trafficking of Cx43 channels to follow. In conclusion, we find that the postal address of membrane-bound Cx43 channel delivery is defined by a separate protein encoded by the same mRNA of the channel itself.