2018 Drosophila Image Award

Cryo-EM map of the Drosophila mechanotransduction channel NOMPC

Mechanosensory transduction for senses such as touch, hearing, proprioception and balance rely on the conversion of mechanical force into nerve impulses by mechanotransduction ion channels. How force is transmitted to the pore-forming domain of these channels and switch the channels between open and closed states remains a key open question in this field, for which two major models have been proposed. One is the membrane-tension model: force applied to the membrane generates a change in membrane tension that is sufficient to gate the channel. The other is the tether model: force is transmitted via a tether to gate the channel. Drosophila NOMPC, the founding member of the TRPN sub-family of Transient Receptor Potential (TRP) channels, has been demonstrated to be a representative mechanosensititve channel gated by the tether mechanism: force transduction to the channel requires its 29 ankyrin repeats (ARs) domain tethered to the microtubule cytoskeleton. But it has remained unknown why NOMPC, particularly its ARs domain, is suitable for transducing mechanical displacements of microtubules into electric signal, and how the mechanosensing Ars domain crosstalks with the pore-forming transmembrane domain.

Here we present the near-atomic-resolution structure of Drosophila NOMPC in a membrane-like environment (side view) resolved by single particle cryo-EM. The most striking structural feature of NOMPC is that the four ARs domains of the tetrameric channel converge into a quadruple structure with each domain resembling a helical spring. Two intramolecular interaction sites were found between each adjacent ARs domains, through which the four ARs domains may coordinate to transduce mechanical force. The force is transduced from the ARs tether to the pore forming transmembrane region through a network interaction between pre-S1 linker-TRP domain-S4-S5 linker. This architecture underscores the basis of translating mechanical displacement of the ARs into opening of the pore.

Peng Jin, David Bulkley, Yanmeng Guo, Wei Zhang, Zhenhao Guo, Walter Huynh, Shenping Wu, Shan Meltzer, Tong Cheng, Lily Yeh Jan, Yuh-Nung Jan & Yifan Cheng. (2017)

Electron cryo-microscopy structure of the mechanotransduction channel NOMPC.
Nature. Jul 6;547(7661):118-122

Visualization of First- and Second-Order Neurons in the Fly Olfactory Circuits using trans-Tango

Mapping neural circuits is fundamental for understanding how the brain functions. Traditional techniques, such as electron microscopy, paired recordings and dye injection, or more recent techniques, such as the genetic application of fluorescent proteins, are powerful in mapping circuitry; however, they are either difficult to apply on a large scale or invasive. Additionally, none of these methods provides genetic access to the output neurons in a given circuit that can be further used to interrogate it by monitoring and modulating the activity of the characterized neurons in the living organism. To overcome these challenges, we have recently developed a method, termed trans-Tango, for circuit tracing, monitoring and manipulation. trans-Tango is based on interaction between an exogenous ligand-receptor pair at the synapse, and the subsequent ligand-dependent release of a tethered transcription factor in the postsynaptic neuron, allowing it to initiate the expression of a reporter gene. As a proof-of-principle, we first implemented trans-Tango in the well-studied olfactory circuits. In this video, expression of the trans-Tango ligand in Orco+ olfactory receptor neurons (green) results in postsynaptic mtdTomato expression in local interneurons and projection neurons of the olfactory system (red). Neuropil counterstain (Blue)

Mustafa Talay, Ethan B. Richman, Nathaniel J. Snell, Griffin G. Hartmann, John D. Fisher, Altar Sorkac, Juan F. Santoyo, Cambria Chou-Freed, Nived Nair, Mark Johnson, John R. Szymanski, and Gilad Barnea. (2017)

Transsynaptic Mapping of Second-Order Taste Neurons in Flies by trans-Tango
Neuron. Nov 15;96(4):783-795

Germline Invasion by Mobile Genetic Elements using Molecular Mimicry

Certain retrotransposons actively traffic to the germ plasm, a specialized structure of the Drosophila oocytes that prefigures descendants in the developing oocyte. The image shows that retrotransposon transcripts (yellow) migrate to natural and ectopic germ plasms in a genetically engineered bipolar fly oocyte, surrounded by somatic nuclei (blue). This suggests that germ plasm targeting represents a fitness strategy adopted by some retrotransposons to ensure transgenerational propagation. Shown is the oocyte from a late stage egg chamber. Accessory nurse cells are cropped from the picture.

Bhavana Tiwari, Paula Kurtz, Amanda E. Jones, Annika Wylie, James F. Amatruda, Devi Prasad Boggupalli, Graydon B. Gonsalvez, John M. Abrams. (2017)

Retrotransposons Mimic Germ Plasm Determinants to Promote Transgenerational Inheritance.
Current Biology. Oct 9;27(19):3010-3016

A testis-specific variant of Centrosomin converts mitochondria into microtubule-organizing centers

We discovered that, the Drosophila gene centrosomin (cnn) produces a short testis-specific splice variant (named CnnT), in addition to encoding the well-studied long variant that functions at the centrosome, the major microtubule-organizing center in most animal cells. However, CnnT does not localize to the centrosome, but instead, to the mitochondria. We found that on the giant mitochondria in Drosophila spermatids, CnnT forms speckles that are required to recruit gamma-tubulin ring complexes (γ-TuRCs) to organize microtubules and support spermiogenesis. We also found that this CnnT, with its Drosophila origin, is sufficient to convert mitochondria in mammalian culture cells into microtubule-organizing centers.

The time-lapse video shows a mammalian CHO cell expressing mApple-EB3 (Red) and GFP-CnnT (green). Note that CnnT expression clustered cytoplasmic mitochondria, and EB3 comets that mark microtubule growing plus ends are ejected from mitochondria where CnnT localizes, suggesting microtubules are growing from these mitochondria.

Jieyan V. Chen, Rebecca A. Buchwalter, Ling-Rong Kao, Timothy L. Megraw. (2017)

A Splice Variant of Centrosomin Converts Mitochondria to Microtubule-Organizing Centers
Current Biology. Jul 10;27(13):1928-1940

Interommatidial bristle mechanosensory neurons

The hundreds of bristles on the compound eye each contain the dendrite of a sensory neuron that also projects to the ventral brain. Shown is the expression pattern of VT17251-LexA in eye bristle mechanosensory neurons (marked by GFP in green). The the brain neuropile is stained with anti-Bruchpilot (magenta). The eye and head cuticle is shown in blue. Optogenetic activation of the eye bristle mechanosensory neurons elicits grooming of the eyes.

Stefanie Hampel, Claire E. McKellar, Julie H. Simpson, Andrew M. Seeds. (2017)
Simultaneous activation of parallel sensory pathways promotes a grooming sequence in Drosophila.
eLife Sep 9;6:e28804

One lucky germ cell

Given the ability to jump around the genome, transposon invasion often leads to animal sterility. By simply adjusting temperature that modulates the activity of P-elements (a DNA transposon), we were able to observe the initial response from germline stem cells that lead to a robust transposon-endogenizing process. Shown is the consequence of transposon invasion in germ cells (red) at 3rd instar larvae stage. Compared to modest transposon activity (left panel; low temperature), strong transposon activity (right panel; high temperature) results in severe loss of the germ cells that subsequently develops rudimentary ovary in adult stage.

Sungjin Moon, Madeline Cassani, Yu An Lin, Lu Wang, Kun Dou, ZZ Zhao Zhang (2018)

A robust transposon-endogenizing response from germline stem cells.
Developmental Cell 47(5), 660-671 (2018).

Illuminating Erk during embryogenesis

Throughout development dynamic and spatially restricted signaling patterns program the embryo.  Here we have harnessed one such signal, Erk, under optogenetic control. The word “ERK” is visualized by dynamically recruiting an activator of Erk signaling to the plasma membrane (middle) through local illuminating the embryo with blue light in the same pattern (top).  This image demonstrates the power of optogenetics to precisely spatiotemporally control signaling during embryogenesis, allowing for one to produce arbitrary developmental patterns.

Heath E. Johnson, Yogesh Goyal, Nicole L Pannucci, Trudi Schüpbach, Stanislav Y. Shvartsman, and Jared E. Toettcher. (2017)

The Spatiotemporal Limits of Developmental Erk Signaling.
Developmental Cell. Jan 23;40(2):185-192