Localization of swallow-Green Fluorescent Protein in Drosophila oogenesis and implications for the role of swallow in RNA localization

Distinct roles for hu li tai shao and swallow in cytoskeletal organization during Drosophila oogenesis

BACKGROUND

Cytoskeletal organization is essential for localization of developmentally significant molecules during Drosophila oogenesis. Swallow (Swa) and an isoform of Hu li tai shao (Ovhts-RC) have been implicated in the organization of actin filaments in developing oocytes but their precise roles have been obscured by the dependence of hts RNA localization on swa function. The functional significance of hts RNA localization in the oocyte has not been established.

RESULTS

In this study we examine Ovhts-RC distribution and cytoskeletal organization under conditions in which Swa protein and/or hts RNA localization are perturbed. We find Swa is required for overall actin organization and for the maintenance of a distinct subset of microtubules in the oocyte. hts RNA localization modulates the distribution of Ovhts-RC in the oocyte and, in turn, local actin filament proliferation.

CONCLUSIONS

Our results support separate contributions of Swa and hts RNA localization to actin organization during oogenesis. Swa is crucial for the organization of actin networks that lead to the formation of a specialized microtubule population, while Ovhts-RC acts to modulate spatially restricted actin filament growth at the oocyte cortex. This suggests RNA localization can lead to modifications of both the actin and microtubule cytoskeletons at specific subcellular locales.

The functions and regulatory principles of mRNA intracellular trafficking

The subcellular localization of RNA molecules is a key step in the control of gene expression that impacts a broad array of biological processes in different organisms and cell types. Like other aspects of posttranscriptional gene regulation discussed in this collection of reviews, the intracellular trafficking of mRNAs is modulated by a complex regulatory code implicating specific cis-regulatory elements, RNA-binding proteins, and cofactors that function combinatorially to dictate precise localization mechanisms. In this review, we first discuss the functional benefits of transcript localization, the regulatory principles involved, and specific molecular mechanisms that have been described for a few well-characterized mRNAs. We also overview some of the emerging genomic and imaging technologies that have provided significant insights into this layer of gene regulation. Finally, we highlight examples of human diseases where defective transcript localization has been documented.

Structural features of LC8-induced self-association of swallow

Cell functions depend on the collective activity of protein networks within which a few proteins, called hubs, participate in a large number of interactions. Dynein light chain LC8, first discovered as a subunit of the motor protein dynein, is considered to have a role broader than that of dynein, and its participation in diverse systems fits the description of a hub. Among its partners is Swallow with which LC8 is essential for proper localization of bicoid mRNA at the anterior cortex of Drosophila oocytes. Why LC8 is essential in this process is not clear, but emerging evidence suggests that LC8 functions by promoting self-association and/or structural organization of its diverse binding partners. This work addresses the energetics and structural features of LC8-induced Swallow self-association distant from LC8 binding. Mutational design based on a hypothetical helical wheel, intermonomer nuclear Overhauser effects assigned to residues expected at interface positions, and circular dichroism spectral characteristics indicate that the LC8-promoted dimer of Swallow is a coiled coil. Secondary chemical shifts and (15)N backbone relaxation identify the boundaries and distinguishing structural features of the coiled coil. Thermodynamic analysis of Swallow polypeptides designed to decouple self-association from LC8 binding reveals that the higher binding affinity of the engineered bivalent Swallow is of purely entropic origin and that the linker separating the coiled coil from the LC8 binding site remains disordered. We speculate that the LC8-promoted coiled coil is critical for bicoid mRNA localization because it favors structural organization of Swallow, which except for the central LC8-promoted coiled coil is primarily disordered.

Distinguishing direct from indirect roles for bicoid mRNA localization factors

Localization of bicoid mRNA to the anterior of the Drosophila oocyte is essential for patterning the anteroposterior body axis in the early embryo. bicoid mRNA localizes in a complex multistep process involving transacting factors, molecular motors and cytoskeletal components that remodel extensively during the lifetime of the mRNA. Genetic requirements for several localization factors, including Swallow and Staufen, are well established, but the precise roles of these factors and their relationship to bicoid mRNA transport particles remains unresolved. Here we use live cell imaging, super-resolution microscopy in fixed cells and immunoelectron microscopy on ultrathin frozen sections to study the distribution of Swallow, Staufen, actin and dynein relative to bicoid mRNA during late oogenesis. We show that Swallow and bicoid mRNA are transported independently and are not colocalized at their final destination. Furthermore, Swallow is not required for bicoid transport. Instead, Swallow localizes to the oocyte plasma membrane, in close proximity to actin filaments, and we present evidence that Swallow functions during the late phase of bicoid localization by regulating the actin cytoskeleton. In contrast, Staufen, dynein and bicoid mRNA form nonmembranous, electron dense particles at the oocyte anterior. Our results exclude a role for Swallow in linking bicoid mRNA to the dynein motor. Instead we propose a model for bicoid mRNA localization in which Swallow is transported independently by dynein and contributes indirectly to bicoid mRNA localization by organizing the cytoskeleton, whereas Staufen plays a direct role in dynein-dependent bicoid mRNA transport.

Localization of bicoid mRNA in late oocytes is maintained by continual active transport

Localization of bicoid mRNA to the anterior of the Drosophila oocyte is essential to produce the Bicoid protein gradient that patterns the anterior-posterior axis of the embryo. Previous studies have characterized a microtubule-dependent pathway for bicoid mRNA localization during midoogenesis, when bicoid first accumulates at the anterior. We show that the majority of bicoid is actually localized later in oogenesis, when the only known mechanism for mRNA localization is based on passive trapping. Through live imaging of fluorescently tagged endogenous bicoid mRNA, we identify a temporally distinct pathway for bicoid localization in late oocytes that utilizes a specialized subpopulation of anterior microtubules and dynein. The directional movement of bicoid RNA particles within the oocyte observed here is consistent with dynein-mediated transport. Furthermore, our results indicate that association of bicoid with the anterior oocyte cortex is dynamic and support a model for maintenance of bicoid localization by continual active transport on microtubules.

RNA localization mechanisms in oocytes

In many animals, normal development depends on the asymmetric distribution of maternal determinants, including various coding and noncoding RNAs, within the oocyte. The temporal and spatial distribution of localized RNAs is determined by intricate mechanisms that regulate their movement and anchoring. These mechanisms involve cis-acting sequences within the RNA molecules and a multitude of trans-acting factors, as well as a polarized cytoskeleton, molecular motors and specific transporting organelles. The latest studies show that the fates of localized RNAs within the oocyte cytoplasm are predetermined in the nucleus and that nuclear proteins, some of them deposited on RNAs during splicing, together with the components of the RNA-silencing pathway, dictate the proper movement, targeting, anchoring and translatability of localized RNAs.