Dynamic microtubules at the vegetal cortex predict the embryonic axis in zebrafish

Abstract

SUMMARYIn zebrafish, as in many animals, maternal dorsal determinants are vegetally localized in the egg and are transported after fertilization in a microtubule-dependent manner. However, the organization of early microtubules, their dynamics and their contribution to axis formation are not fully understood. Using live imaging, we identified two populations of microtubules, perpendicular bundles and parallel arrays, which are directionally oriented and detected exclusively at the vegetal cortex before the first cell division. Perpendicular bundles emanate from the vegetal cortex, extend towards the blastoderm, and orient along the animal-vegetal axis. Parallel arrays become asymmetric on the vegetal cortex, and orient towards dorsal. We show that the orientation of microtubules at 20 minutes post-fertilization can predict where the embryonic dorsal structures in zebrafish will form. Furthermore, we find that parallel microtubule arrays colocalize with wnt8a RNA, the candidate maternal dorsal factor. Vegetal cytoplasmic granules are displaced with parallel arrays by ~20°, providing in vivo evidence of a cortical rotation-like process in zebrafish. Cortical displacement requires parallel microtubule arrays, and probably contributes to asymmetric transport of maternal determinants. Formation of parallel arrays depends on Ca 2+ signaling. Thus, microtubule polarity and organization predicts the zebrafish embryonic axis. In addition, our results suggest that cortical rotation-like processes might be more common in early development than previously thought.

Dachsous1b cadherin regulates actin and microtubule cytoskeleton during early zebrafish embryogenesis

“…wnt8a mRNA transport is mediated by microtubules, which, after egg activation, form transient parallel arrays aligning with the future dorsal side of the embryo (Lu et al, 2011;Tran et al, 2012). Therefore, we hypothesized that impaired transport of wnt8a in MZdchs1b embryos was due to microtubule abnormalities.…”

“…Embryonic patterning requires these vegetally located molecules, as their removal, either surgically (Jesuthasan and Strähle, 1997;Mizuno et al, 1999) or by maternal-effect mutations, impairs dorsal axis specification (Ge et al, 2014;Nojima et al, 2010). During early cleavages, a dynamic vegetal microtubule network mediates asymmetric transport of DDs (Lu et al, 2011;Nojima et al, 2004;Tran et al, 2012), which accumulate in a few marginal blastomeres to establish the Nieuwkoop center, a key regulator of axis determination (Gore and Sampath, 2002;Jesuthasan and Strähle, 1997;Lu et al, 2011). Disruption of these microtubule arrays impairs DD transport and axis formation (Ge et al, 2014;Gore and Sampath, 2002;Jesuthasan and Strähle, 1997;Lu et al, 2011;Nojima et al, 2004;Tran et al, 2012).…”

Kinesin-1 interacts with Bucky ball to form germ cells and is required to pattern the zebrafish body axis

“…Prior to asymmetric translocation of Sybu protein and wnt8a RNA (∼20 mpa), the vegetal MT network reorganizes to form the vegetal pMTA (Jesuthasan and Strähle, 1997), which predicts the DV axis (Tran et al, 2012). Disrupting this network, as occurs in Mgrip2a mutants (Ge et al, 2014), or by treating activated eggs with MT inhibitors, cold temperature or UV irradiation (Lu et al, 2011;Nojima et al, 2010), blocks Sybu protein and wnt8a RNA translocation.…”

“…Following fertilization, MT reorganization underlies dorsal determination (Houliston and Elinson, 1991;Jesuthasan and Strähle, 1997), as vegetal parallel microtubule arrays ( pMTAs) govern asymmetric redistribution of the vegetally localized DDs relative to the animalvegetal axis (Ge et al, 2014;Gerhart et al, 1989;Houliston and Elinson, 1991;Jesuthasan and Strähle, 1997;Tran et al, 2012). In zebrafish, pMTA formation roughly coincides with asymmetric translocation of Syntabulin protein and wnt8a and grip2a RNAs (Ge et al, 2014;Lu et al, 2011;Nojima et al, 2010;Tran et al, 2012). Although maternal grip2a contributes to alignment and bundling of the pMTA (Ge et al, 2014), and vegetal MT plus-ends are oriented dorsalward (Tran et al, 2012), it remains unclear how pMTA rearrangements are initiated and which other factors are involved.…”

“…In zebrafish, pMTA formation roughly coincides with asymmetric translocation of Syntabulin protein and wnt8a and grip2a RNAs (Ge et al, 2014;Lu et al, 2011;Nojima et al, 2010;Tran et al, 2012). Although maternal grip2a contributes to alignment and bundling of the pMTA (Ge et al, 2014), and vegetal MT plus-ends are oriented dorsalward (Tran et al, 2012), it remains unclear how pMTA rearrangements are initiated and which other factors are involved. Nevertheless, evidence from Xenopus, using antibodies and rigor mutants, implicates Kinesin-mediated transport of vegetal RNAs in oocytes (Gagnon et al, 2013;Messitt et al, 2008) and Kinesin-dependent pMTA formation (Marrari et al, 2000).…”