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continued to move bidirectionally. This retrograde transport bias suggests
that manipulating the fMNPs externally changes the nature of the signaling
and adaptor complexes on endosomes, biasing binding to dynein-dynactin
complexes over kinesins.
Endosome transport reversals, anterograde to retrograde, have been
reported in other systems particularly after injury, suggesting physiologically
relevant mechanisms exist for reversing the direction of transport (
Abe et al.,
2009; Bisby & Bulger, 1977; Cavalli, Kujala, Klumperman, & Goldstein,
2005; Okada, Sato-Yoshitake, & Hirokawa, 1995
). Kinesin and dynein
transport endosomal cargos along microtubules by unique mechanisms
and their stall forces, which are both in the low pN range (
Sims & Xie,
2009
) differ depending on cargo load (
Singh, Mallik, Gross, & Yu, 2005;
Toba, Watanabe, Yamaguchi-Okimoto, Toyoshima, & Higuchi, 2006
).
Complexes that can interact with dynein-dynactin and kinesin may be
present on signaling endosomes (
Schuster, Lipowsky, Assmann, Lenz, &
Steinberg, 2011
), and changes either in the internal state or in the
external adaptors may quickly transition between anterograde and
retrograde transport. Since signaling endosome directional transport is
regulated by associated signaling complexes (
Wan et al., 2008
), motor
protein adaptors (
Huang, Duan, Sun, et al., 2011
), and lipids (
Rocha
et al., 2009
), fMNPs may provide a system for studying the adaptors and
motor protein interactions regulating the localization of signaling
endosomes in the growth cone. The presence of the superparamagnetic
core in fMNP/TrkB signaling endosomes also provides a potential for
magnetic recovery under varying conditions
in vitro
or
in vivo
and from
specific regions of the cell.
4.3. Altering signaling endosome localization with
nanoparticles alters growth cone motility
Altering TrkB/fMNPs localization alters growth cone exo- and endocytosis
and adhesion. Manipulating TrkB/fMNPs altered growth cone motility
distinctly (
Fig. 2.6
). Applying a brief magnetic force was sufficient to
dramatically change both lamellar and filopodial dynamics in the peripheral
domain of the growth cone, halting all protrusive activity in the peripheral do-
main. A 15 pN force applied for as little as 1 min was sufficient to completely
immobilize both lamellar and filopodial protrusions. Interestingly, lamella and
filopodia immobilized after the magnet was removed, indicating a change
in signaling, not simply mechanical restraint. Lamella and filopodia that
turned over on average every 3-4 min before applying the magnetic field
