Developmental axon regrowth and primary neuron sprouting utilize distinct actin elongation factors

Rho GTPase Signaling: A Molecular Switchboard for Regulating the Actin Cytoskeleton in Axon Guidance

Axon pathfinding is a highly dynamic process regulated by the interactions between cell-surface guidance receptors and guidance cues present in the extracellular environment. During development, precise axon pathfinding is crucial for the formation of functional neural circuits. The spatiotemporal expression of axon guidance receptors helps the navigating axon make correct decisions in a complex environment comprising both attractive and repulsive guidance cues. Axon guidance receptors initiate distinct signaling cascades that eventually influence the cytoskeleton at the growing tip of an axon, called the growth cone. The actin cytoskeleton is the primary target of these guidance signals and plays a key role in growth cone motility, exploration, and behavior. Of the many regulatory molecules that modulate the actin cytoskeleton in response to distinct guidance signals, Rho GTPases play central roles. Rho GTPases are molecular switchboards; their ON (GTP-bound) and OFF (GDP-bound) switches are controlled by their interactions with proteins that regulate the exchange of GDP for GTP or with the proteins that promote GTP hydrolysis. Various upstream signals, including axon guidance signals, regulate the activity of these Rho GTPase switch regulators. As cycling molecular switches, Rho GTPases interact with and control the activities of downstream effectors, which directly influence actin reorganization in a context-dependent manner. A deeper exploration of the spatiotemporal dynamics of Rho GTPase signaling and the molecular basis of their involvement in regulating growth cone actin cytoskeleton can unlock promising therapeutic strategies for neurodevelopmental disorders linked to dysregulated Rho GTPase signaling. This review not only provides a comprehensive overview of the field but also highlights recent discoveries that have considerably advanced our understanding of the complex regulatory roles of Rho GTPases in modulating actin cytoskeleton arrangement at the growth cone during axon guidance.

Polarized branched Actin modulates cortical mechanics to produce unequal-size daughters during asymmetric division

The control of cell shape during cytokinesis requires a precise regulation of mechanical properties of the cell cortex. Only few studies have addressed the mechanisms underlying the robust production of unequal-sized daughters during asymmetric cell division. Here we report that unequal daughter-cell sizes resulting from asymmetric sensory organ precursor divisions in Drosophila are controlled by the relative amount of cortical branched Actin between the two cell poles. We demonstrate this by mistargeting the machinery for branched Actin dynamics using nanobodies and optogenetics. We can thereby engineer the cell shape with temporal precision and thus the daughter-cell size at different stages of cytokinesis. Most strikingly, inverting cortical Actin asymmetry causes an inversion of daughter-cell sizes. Our findings uncover the physical mechanism by which the sensory organ precursor mother cell controls relative daughter-cell size: polarized cortical Actin modulates the cortical bending rigidity to set the cell surface curvature, stabilize the division and ultimately lead to unequal daughter-cell size.

The making of the Drosophila mushroom body

The mushroom body (MB) is a computational center in the Drosophila brain. The intricate neural circuits of the mushroom body enable it to store associative memories and process sensory and internal state information. The mushroom body is composed of diverse types of neurons that are precisely assembled during development. Tremendous efforts have been made to unravel the molecular and cellular mechanisms that build the mushroom body. However, we are still at the beginning of this challenging quest, with many key aspects of mushroom body assembly remaining unexplored. In this review, I provide an in-depth overview of our current understanding of mushroom body development and pertinent knowledge gaps.

The conserved Pelado/ZSWIM8 protein regulates actin dynamics by promoting linear actin filament polymerization

Actin filament polymerization can be branched or linear, which depends on the associated regulatory proteins. Competition for actin monomers occurs between proteins that induce branched or linear actin polymerization. Cell specialization requires the regulation of actin filaments to allow the formation of cell type-specific structures, like cuticular hairs in <i>Drosophila</i>, formed by linear actin filaments. Here, we report the functional analysis of CG34401/<i>pelado</i>, a gene encoding a SWIM domain-containing protein, conserved throughout the animal kingdom, called ZSWIM8 in mammals. Mutant <i>pelado</i> epithelial cells display actin hair elongation defects. This phenotype is reversed by increasing actin monomer levels or by either pushing linear actin polymerization or reducing branched actin polymerization. Similarly, in hemocytes, Pelado is essential to induce filopodia, a linear actin-based structure. We further show that this function of Pelado/ZSWIM8 is conserved in human cells, where Pelado inhibits branched actin polymerization in a cell migration context. In summary, our data indicate that the function of Pelado/ZSWIM8 in regulating actin cytoskeletal dynamics is conserved, favoring linear actin polymerization at the expense of branched filaments.

Transneuronal Dpr12/DIP-δ interactions facilitate compartmentalized dopaminergic innervation of Drosophila mushroom body axons

The mechanisms controlling wiring of neuronal networks are not completely understood. The stereotypic architecture of the Drosophila mushroom body (MB) offers a unique system to study circuit assembly. The adult medial MB γ-lobe is comprised of a long bundle of axons that wire with specific modulatory and output neurons in a tiled manner, defining five distinct zones. We found that the immunoglobulin superfamily protein Dpr12 is cell-autonomously required in γ-neurons for their developmental regrowth into the distal γ4/5 zones, where both Dpr12 and its interacting protein, DIP-δ, are enriched. DIP-δ functions in a subset of dopaminergic neurons that wire with γ-neurons within the γ4/5 zone. During metamorphosis, these dopaminergic projections arrive to the γ4/5 zone prior to γ-axons, suggesting that γ-axons extend through a prepatterned region. Thus, Dpr12/DIP-δ transneuronal interaction is required for γ4/5 zone formation. Our study sheds light onto molecular and cellular mechanisms underlying circuit formation within subcellular resolution.

Cofilin regulates axon growth and branching of Drosophila γ-neurons

The mechanisms that control intrinsic axon growth potential, and thus axon regeneration following injury, are not well understood. Developmental axon regrowth of Drosophila mushroom body γ-neurons during neuronal remodeling offers a unique opportunity to study the molecular mechanisms controlling intrinsic growth potential. Motivated by the recently uncovered developmental expression atlas of γ-neurons, we here focus on the role of the actin-severing protein cofilin during axon regrowth. We show that Twinstar (Tsr), the fly cofilin, is a crucial regulator of both axon growth and branching during developmental remodeling of γ-neurons. tsr mutant axons demonstrate growth defects both in vivo and in vitro, and also exhibit actin-rich filopodial-like structures at failed branch points in vivo Our data is inconsistent with Tsr being important for increasing G-actin availability. Furthermore, analysis of microtubule localization suggests that Tsr is required for microtubule infiltration into the axon tips and branch points. Taken together, we show that Tsr promotes axon growth and branching, likely by clearing F-actin to facilitate protrusion of microtubules.