# Spatially distinct FRL and Ena dependent actin networks coordinate nuclear positioning in Drosophila nurse cells

## TL;DR

This study shows how two distinct actin networks work together to position the nucleus in Drosophila nurse cells during development.

## Contribution

The paper identifies FRL as a new factor in actin bundle formation and reveals a two-tier actin network system for nuclear positioning.

## Key findings

- FRL is mainly required for cytoplasmic actin bundle formation at stage 10B.
- Ena promotes a ring canal attached actin array that persists from stage 7.
- Both FRL and Ena are needed for proper nuclear positioning during dumping.

## Abstract

Position of the nucleus is dynamically controlled to ensure a variety of cellular functions in a broad range of organisms form yeast to human. Nuclear positioning in Drosophila nurse cells is crucial during dumping when cells transfer their entire cytoplasmic content into the oocyte. An important prerequisite of effective dumping is the formation of an array of actin cables which holds the nucleus in a central position, thereby allowing transmission of the cytoplasmic cargo. Here we report the identification of FRL, a formin type of actin assembly factor, as a novel determinant of cytoplasmic actin bundle formation. We found that FRL and the formerly described Ena protein display a differential requirement. Comparison of the frl and ena loss of function situations revealed that FRL is mainly required for creation of the cytoplasmic actin subpopulation at stage 10B, while Ena mostly promotes formation of a ring canal attached actin array, already present at stage 7 and persists till dumping. Upon the concurrent absence of FRL and Ena the nuclear positioning actin cables are completely missing, strongly suggesting that nuclear positioning in the nurse cells requires the coordinated action of two spatially distinct actin networks.

Controlling the position of the nucleus in multicellular organisms is vital for life from zygote formation throughout development and the maintenance of normal homeostasis. Cells use cytoskeleton dependent forces to push or pull their nuclei into the appropriate position. In the Drosophila egg chambers, made up from 15 nurse cells and the oocyte, the nurse cell nuclei need to be kept away from the ring canals (cytoplasmic bridges between these germline cells) during the last stages of oogenesis to prevent clogging and allow the bulk transport of materials (dumping) towards the oocyte. This was thought to be achieved by cytoplasmic actin cables spanning from the cortical membranes to the nucleus. Whereas we present further evidence in support of the importance of these actin arrays, we found that the prominent cytoplasmic actin bundles represent only one part of a two-tier mechanism, critically relying on the coordinated action of two distinct actin networks that differ in both the temporal and spatial regulation of their initiation, and their contribution to dumping efficiency. We hope that clarification of an important mechanistic aspect will be imperative to gain novel insights into the general means of nuclear positioning in fruit flies, and potentially, in other organisms.