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Page 577 Loh et al. Extracell Vesicles Circ Nucleic Acids 2023;4:568-87 https://dx.doi.org/10.20517/evcna.2023.34

of microtubules or enhance the retrograde transport of DCVs. To differentiate the two different
possibilities, the loss-of-function mutation of cytoplasmic dynein was introduced into the cdk5 mutant. The
double mutations blocked only the accumulation of DCVs in the dendrites where microtubule minus ends
are mixed with their plus ends regarding orientation. Based on the results, this study speculates that CDK5
enhances cytoplasmic dynein-mediated retrograde DCV transport in both axon and dendrite [Figure 6].

Dynactin, a microtubule anchor protein complex for cytoplasmic dynein and some kinesins, was reported
to mediate the bidirectional movement of DCVs in anterior pituitary cells and hippocampal neurons [78,79]
[Figure 6]. In a recent study, DCVs containing BDNF, neuropeptide Y (NPY), and tissue plasminogen
activator (tPA) showed different polarities during microtubule-based transport in the axons and dendrites
[86]
in hippocampal neurons . When the dynactin complex was disrupted by the overexpression of p50
(dynamitin) that links the p150 side arm of dynactin to its base Actin-related protein 1 (Arp1) filament, the
bidirectional movements of DCVs along the axon and dendrites were reduced. Conversely, the
overexpression of p150 coiled-coil domain 1 (CC1), where cytoplasmic dynein binds, inhibited only motor
movement processivity. Thus, it appears that the interaction of cytoplasmic dynein with dynactin may
enhance the processivity of retrograde transport of the DCVs. This result confirms that dynactin is involved
in the bidirectional movement of DCVs along microtubules in the axons and dendrites of hippocampal
neurons [Figure 6].

Myosin Va, a F-actin-based motor protein, was also implicated in the regulation of the polarity of the
bidirectional microtubule-based movement of DCVs. In cultured hippocampal neurons, the expression of
the dominant negative tail construct of myosin Va reduced the velocity of the retrograde transport of large
DCVs in the axon, while the axonal anterograde DCV transport was not affected . This suggests that
[87]
myosin Va facilitates the retrograde transport of DCVs along the microtubules.

ACTIN-MEDIATED TETHERING REGULATES TRANSPORT AND SECRETION OF DCVS AT
THE PERI-PLASMA MEMBRANE REGION
F-actin meshwork at the peri-plasma membrane region plays an important role in storing DCVs for
regulated secretion [Figure 7A]. F-actins are thought to block the uncontrolled access of DCVs to the
plasma membrane. The depolymerization of F-actins increases the secretion of RSP hormones [88,89] while
2+
their stabilization suppresses Ca -stimulated hormone secretion [88,90] . Nonetheless, for stimulated exocytosis,
DCVs should be released from the F-actin meshwork. Some actin-severing proteins are proposed to
mediate the release. Scinderin is one of the actin-severing proteins that release mucin vesicles in airway
cells and insulin vesicles in pancreatic β cells . The other F-actin-severing protein, gelsolin, also affects
[92]
[91]
[94]
[93]
amylase release in pancreatic acinar cells and insulin secretion in pancreatic β cells . In addition,
phosphatidylinositol 3 kinase (PI3K) appears to partake in the depolymerization of F-actin meshwork to
[95]
facilitate the docking of DCVs to the plasma membrane . Upon the knockdown or inhibition of p110γ, a
subunit of PI3K, the cortical F-actin meshwork was increased while the docking of DCVs to the plasma
membrane was decreased. This resulted in a reduction in Ca -stimulated insulin secretion [Figure 7B].
2+
Of note, although the major function of F-actins is to tether DCVs at the peri-plasma membrane region, the
extensive depolymerization of F-actins actually blocks the exocytosis of DCVs [96,97] . Hence, some F-actin
filaments should remain to provide DCV transport to the plasma membrane. The F-actin-based DCV
transport is mediated by the F-actin-based motor, Myosin Va. Myosin Va was found to be associated with
[101]
[100]
[98]
[99]
DCVs , melanosomes , chromaffin granules , and insulin vesicles . The interaction of Myosin Va
with DCVs appears to be mediated by Rab27a and MyRIP [Figure 7C] [102-104] . It was also found that Myosin
[105]
2+
Va is activated by increased Ca levels, apparently contributing to stimulated DCV release . Thus, the F-