DCX-EMAP is a core organizer for the ultrastructure of Drosophila mechanosensory organelles

Doublecortin-like kinase is required for cnidocyte development in Nematostella vectensis

The complex morphology of neurons requires precise control of their microtubule cytoskeleton. This is achieved by microtubule-associated proteins (MAPs) that regulate the assembly and stability of microtubules, and transport of molecules and vesicles along them. While many of these MAPs function in all cells, some are specifically or predominantly involved in regulating microtubules in neurons. Here we use the sea anemone Nematostella vectensis as a model organism to provide new insights into the early evolution of neural microtubule regulation. As a cnidarian, Nematostella belongs to an outgroup to all bilaterians and thus occupies an informative phylogenetic position for reconstructing the evolution of nervous system development. We identified an ortholog of the microtubule-binding protein doublecortin-like kinase (NvDclk1) as a gene that is predominantly expressed in neurons and cnidocytes (stinging cells), two classes of cells belonging to the neural lineage in cnidarians. A transgenic NvDclk1 reporter line revealed an elaborate network of neurite-like processes emerging from cnidocytes in the tentacles and the body column. A transgene expressing NvDclk1 under the control of the NvDclk1 promoter suggests that NvDclk1 localizes to microtubules and therefore likely functions as a microtubule-binding protein. Further, we generated a mutant for NvDclk1 using CRISPR/Cas9 and show that the mutants fail to generate mature cnidocytes. Our results support the hypothesis that the elaboration of programs for microtubule regulation occurred early in the evolution of nervous systems.

Resolving the In Situ Three-Dimensional Structure of Fly Mechanosensory Organelles Using Serial Section Electron Tomography

Mechanosensory organelles (MOs) are specialized subcellular entities where force-sensitive channels and supporting structures (e.g., microtubule cytoskeleton) are organized in an orderly manner. The delicate structure of MOs needs to be resolved to understand the mechanisms by which they detect forces and how they are formed. Here, we describe a protocol that allows obtaining detailed information about the nanoscopic ultrastructure of fly MOs by using serial section electron tomography (SS-ET). To preserve fine structural details, the tissues are cryo-immobilized using a high-pressure freezer followed by freeze-substitution at low temperature and embedding in resin at room temperature. Then, sample sections are prepared and used to acquire the dual-axis tilt series images, which are further processed for tomographic reconstruction. Finally, tomograms of consecutive sections are combined into a single larger volume using microtubules as fiducial markers. Using this protocol, we managed to reconstruct the sensory organelles, which provide novel molecular insights as to how fly mechanosensory organelles work and are formed. Based on our experience, we think that, with minimal modifications, this protocol can be adapted to a wide range of applications using different cell and tissue samples. Key features • Resolving the high-resolution 3D ultrastructure of subcellular organelles using serial section electron tomography (SS-ET). • Compared with single-axis tilt series, dual-axis tilt series provides a much wider coverage of Fourier space, improving resolution and features in the reconstructed tomograms. • The use of high-pressure freezing and freeze-substitution maximally preserves the fine structural details.

Nanoscale architect: Illuminating the key organizer of the fruit fly's sensory world

Mechanosensory neurons utilize specialized compartments called mechanosensory organelles (MOs) to process external forces, yet the MO organization mechanisms remained unclear. In this issue, Song et al. (2023. J. Cell Biol.https://doi.org/10.1083/jcb.202209116) discovered that a microtubule-binding protein, DCX-EMAP, is the key organizer of fly MOs.