In 1994, myoX was first identified in a PCR screen designed to find novel myosins in the inner ear (Solc et al. 1994). The full-length cDNA sequences of myoX were determined in 2000 in mouse (Yonezawa et al. 2000), human, and cow (Berg et al. 2000), and the molecular structure and ubiquitous expression in tissues were reported (Berg et al. 2000; Yonezawa et al. 2000). The unique localization pattern of myoX in cultured cells at the tips of filopodia and in lamellipodia was identified in the same reports (Berg et al. 2000; Yonezawa et al. 2000). Subsequently, in 2001, it was shown that the head domain of myoX binds actin and hydrolyzes ATP to induce movement and force (Homma et al. 2001). In 2002, myoX was shown to participate in intrafilopodial movement and filopodia formation (Berg and Cheney 2002), and in phagocytosis (Cox et al. 2002). After these breakthroughs, studies of myoX were expanded to many different aspects by precise biochemical analyses or by identifying binding partners critical for exploring its functions in cells.
In addition to this full-length myoX, brain expresses a shorter form that lacks a motor domain (Sousa et al. 2006). This “headless” myoX might have a possible role as a natural dominant-negative regulator to suppress the function of full-length myoX in neurons.
One of the most striking properties of myoX is its forward and rearward movements within filopodia (Berg and Cheney 2002). Recently, the velocity of the forward movement was calculated as ∼600 nm/s in living cells using TIRF microscopy (Kerber et al. 2009), similar to the 340–780 nm/s reported for movement of individual myoX molecules on artificial actin bundles (Nagy et al. 2008). The rearward movement of myoX in filopodia is slow at 10–20 nm/s (Berg and Cheney 2002), presumably the rate of retrograde actin flow in filopodia. In these studies, only dimerized myoX showed intrafilopodial motility, suggesting that dimer formation of myoX is necessary for its proper movement in cells.
A forced dimer construct of tail-less myoX was first reported to preferentially select bundled actin for motility and showed poor processivity on single filaments in vitro (Nagy et al. 2008). The selectivity of actin tracks (single filaments or bundled) by myoX is controversial because a recent report shows the robust processivity of myoX on individual actin filaments (Sun et al. 2010). Sun et al. also report that myoX moves processively in a hand-over-hand manner with a left-hand helical walking path using single-molecule fluorescence techniques such as polTIRF, FIONA, and Parallax.
TIRF microscopy was used to clarify the role of the tail domain in the mechanism of myoX-induced filopodia formation (Watanabe et al. 2010). MyoX was recruited to discrete sites at the leading edge where it assembles with exponential kinetics before filopodia extension. MyoX-induced filopodia showed repeated extension–retraction cycles with each extension of 2.4 μm, which was critical to produce long filopodia. FERM domain–deleted myoX moved to the tip as in wild type, but it was transported toward the cell body during filopodia retraction, did not undergo multiple extension–retraction cycles, and failed to produce long filopodia. Deletion of the FERM domain did not change movement at the single-molecule level with the same velocity of approximately 600 nm/s as wild type, suggesting that the myoX in filopodia moves without interacting with the attached membrane via the FERM domain. These results suggest that the interaction of myoX and substrate-engaged integrin is necessary for phased elongation.
Regulating Molecules and Other Functions
Recent studies implicate myoX in a variety of cellular functions through its interactions with other molecules:
PIP3: In leukocytes, myoX localizes to phagocytic cups with phosphatidylinositol-3,4,5-triphosphate (PIP3) in a PI3K-dependent manner. Expression in macrophages of a tail domain of myoX inhibits phagocytosis (Cox et al. 2002). MyoX might provide a molecular link between PI3K and pseudopod extension during phagocytosis.
Ena/VASP: MyoX and VASP bind in vitro and in vivo, colocalize at the tips of filopodia, move together in filopodia, and there is a correlation between the length of filopodia and the concentration of VASP/myoX at the tips of filopodia (Tokuo and Ikebe 2004). These results suggest that myoX transports VASP to the tips of filopodia to support elongation by actin incorporation.
Microtubules: Through its MTH4-FERM domain, MyoX associates with microtubules. Expression of the tail domain or microinjection of anti-myoX antibodies disrupts nuclear anchoring, spindle assembly, and spindle-F-actin association in Xenopus laevis (Weber et al. 2004). These results indicate that during meiosis, myoX has a critical role in integrating the F-actin and microtubule cytoskeletons. MyoX is also essential for mitotic-spindle function (Woolner et al. 2008). Interaction with microtubules is also important in osteoclast function (McMichael et al. 2010). MyoX suppression by RNAi or overexpression of dominant-negative myoX (MTH4-FERM) leads to decreased sealing zone perimeter, motility, and resorptive capacity of osteoclasts. These results suggest that myoX plays a role in osteoclast attachment and podosome positioning by direct linkage of actin to the microtubule network.
Integrins: The FERM domain of myoX interacts with an NPXY motif within the cytoplasmic domain of β-integrin (Zhang et al. 2004). Knockdown of myoX results in decreased integrin-mediated cell adhesion to the extracellular matrix, and myoX is responsible for localization of integrins in filopodia. Localization of integrin at filopodial tips and filopodia elongation did not occur with myoX mutants deficient in integrin binding or with a β-integrin mutant deficient in myoX binding. These results suggest that binding of myoX to integrins allows tethering of the filopodial actin filaments to the extracellular matrix for stabilizing the structure of filopodia and elongation.
BMP6: In endothelial cells, myoX is a target gene of bone morphogenetic protein (BMP), and myoX colocalizes with the BMP6 receptor, ALK6, in a BMP6-dependent fashion. Other data indicate that myoX is required to guide endothelial migration toward BMP6 gradients via the regulation of filopodial function and amplification of BMP signals (Pi et al. 2007).
Netrin receptor: The FERM domain of myoX interacts with the cytoplasmic tail of the netrin receptors, DCC (deleted in colorectal cancer) and neogenin (Zhu et al. 2007). Netrins regulate axon path-finding, which is essential for proper wiring in the brain. Cortical explants derived from mouse embryos expressing dominant-negative myoX exhibit reduced neurite outgrowth in response to netrin-1. Inhibition of myoX in embryos causes impaired commissural neuronal axon projections in chicken brain. These results indicate that myoX regulates axon outgrowth and guidance in response to netrins (Zhu et al. 2007).
Using Xenopus as a model system to study the function of myoX in neurons, two reports show that knock-down of myoX expression results in retarded migration of cranial neural crest cells (Hwang et al. 2009; Nie et al. 2009). These results suggest that myoX has an essential function in neuronal development in vertebrates.
VE–cadherin: MyoX is directly associated with the VE–cadherin complex via a FERM domain. It colocalizes and moves synchronously with filopodial VE–cadherin. Expression of the FERM domain blocks the transportation of VE–cadherin along actin fibers, resulting in an almost total depletion of VE–cadherin at the cell edge. VE–cadherin trafficking along filopodia by myoX may be a pre-requisite for cell–cell junction formation in endothelial cells (Almagro et al. 2010).
MyoX functions both as a molecular transporter and a cytoskeletal regulator in cells.
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