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.
Mechanosensation, the fundamental capacity to perceive and respond to mechanical stimuli in the environment, constitutes the core of animals’ interactions with the world. This remarkable process converts external physical forces into neural signals transmitted to the brain, forming the basis for sensory experiences like touch, pain, hearing, proprioception, and balance maintenance. These inherent mechanisms enable living organisms, including humans, to survive and interact with the ever-changing environment.
Mechanosensory neurons usually contain specialized nanoscopic compartments, known as mechanosensory organelles (MOs). Within the MOs, force-sensitive ion channels are precisely arranged to decode external stimuli. One remarkable example of mechanosensation is observed in hair cells, where the well-studied MOs called stereocilia transform physical forces, such as sound waves and head motion, into intricate electrical signals interpreted by the brain (1). However, despite the essential role of MOs in mechanosensory transduction, the mechanisms governing their organization remain not fully understood.
Cytoskeletal frameworks frequently define the organization of MOs, with examples ranging from F-actin bundles in vertebrate hair cell stereocilia (2) to microtubules (MTs) in worm touch receptors (3) and fly mechanoreceptors (4). This issue of Journal of Cell Biology presents an important advancement in our understanding of mechanosensory apparatus organization made by the research groups of Xin Liang and Landi Sun from Tsinghua University. They studied the ultrastructural organization of MOs in ciliated campaniform mechanosensory cells located on the halteres (reduced hind wings functioning as balance organs) and legs of fruit flies. Previously, the Liang group employed electron tomography (ET) and live imaging to reveal that a neuronal MT-severing enzyme, katanin p60-like 1, and an MT minus-end binding protein, Patronin, are both essential in generating short MT arrays in the MOs (5). In the article published in this issue of JCB (6), the authors, by combining ET with powerful fly genetics and high-resolution microscopy, unveil that DCX-EMAP, a member of the EMAP (echinoderm MT-associated proteins) family, is a nanoscale architect of 3D organization of the fly MOs. Their research demonstrates that DCX-EMAP serves a dual function: first, it stabilizes short MT arrays within MOs, and second, it is essential for arranging of electron-dense materials (EDMs) surrounding these MT arrays (Fig. 1). This discovery provides valuable insights into the mechanism underlying MO organization, shedding light on the intricate processes governing mechanosensory perception and signaling in fruit flies.
Prominently expressed in fly ciliated mechanoreceptors, DCX-EMAP emerges as the key component in constructing the MOs. The absence of DCX-EMAP results in complete disorganization of MOs, leading to severe locomotion defects. The DCX-EMAP protein contains an N-terminal tandem of DCX domains, which were first identified in the doublecortin (DCX) protein (Fig. 1). These DCX domains stabilize MTs by reducing the dissociation rate of tubulin dimers from both growing and shrinking MT ends, ultimately lowering the critical concentration for tubulin polymerization, increasing the MT growth rate, and decreasing the MT catastrophe frequency. In addition to the N-terminal DCX domains, DCX-EMAP contains the hydrophobic echinoderm-MT-associated-like protein (HELP) domain and multiple WD40 repeats at the C-terminus, which are conserved among all EMAP family members (Fig. 1). The HELP and WD40 domains are likely involved in protein–protein interactions, and functional analysis shows that they are responsible for MO localization and EDM organization, respectively. Therefore, DCX-EMAP functions as a dual-function architect: it not only facilitates MT assembly and increases their stability, but also aids EDM accumulation around the MTs within the MOs, illuminating its multifaceted roles in the MO organization.
The significance of DCX proteins extends far beyond the realm of flies. The doublecortin (DCX) domain-containing protein superfamily members associate with MTs and serve as protein–protein interaction platforms. They play critical roles in neuron outgrowth, differentiation, and migration. Mutations in the DCX family proteins often lead to severe developmental defects in the human nervous system, impacting cognitive and sensory functions. For instance, mutations in the DCX gene result in human X-linked lissencephaly and double cortex syndrome (7). Notably, a point mutation in DCDC2 (DCDC2a) is responsible for recessive deafness in humans. Dcdc2a contributes to the regulation of primary cilia length in sensory hair cells, likely through its involvement in MT formation and stabilization (8). Like the fly DCX-EMAP, human DCX family members, such as DCX, DCLK1/2, RP1/RP1L1, and DCDC2a, contain two MT-binding DCX domains at the N-terminus (9). Intriguingly, Song et al. showed that the fly DCX1 domain preferentially binds dynamic MTs (containing GDP-tubulin), while the DCX2 domain prefers stable MTs (containing GTP-tubulin) (6). Furthermore, the MT stabilizing effect can only be achieved with the protein containing both DCX1 and DCX2 domains as well as the linker between them (6). This discovery aligns with a prior study indicating that the C-terminal DCX domain of human doublecortin promotes MT nucleation and enhances the stability of the tubulin–tubulin interactions in the emerging MT lattice, while the N-terminal DCX domain seems to prefer the mature lattice, contributing to MT stabilization (10). Thus, the tandem domains have two distinct roles in MT stabilization. A compelling question thus arises: Could two homodimers (DCX1-DCX1 or DCX2-DXC2) promote MT stabilization in vitro and rescue the fly MO organization in vivo?
Moreover, MT-associated proteins play a crucial role in modulating the movement of MT motors. Two prominent members of the DCX family, DCX and DCLK1, have the ability to inhibit kinesin-1 activity in dendrites, while leaving kinesin-3 motility unaffected (11). This prompts an intriguing question: Could DCX-EMAP have a similar capacity to differentially regulate MT motors? The predominant localization of DCX-EMAP at the farthest reaches of mechanosensory neuron dendrites suggests a possibility that this protein may engage kinesin-3 motors to facilitate its transport from the cell body (soma) to the dendritic tip. Further investigations into these mechanisms could uncover an additional layer of complexity in the elaborate molecular choreography within sensory cells.
In summary, the authors present a remarkable advancement in unraveling the complex mechanism underlying the ultrastructural organization of fly MOs. Their findings showcase the pivotal role of a conserved MT-associated protein, DCX-EMAP, as a key molecular architect. Through its dual-domain repertoire, DCX-EMAP choreographs the precise assembly of the MT–EDM complex. The interplay allows the exquisite transduction of physical forces into intricate electrical signals, granting the little animals the remarkable ability to perceive the world with nanoscale precision. These findings open a way to deeper comprehension, holding promise for novel therapeutic avenues and transformative insights into the boundless nuances of mechanosensation.
The authors are supported by the National Institute of General Medical Sciences grant R35 GM131752.