A complex regulation of molecular motors has been described
in different studies [104, 125–128], and is responsible for directed
targeting of mRNAs to their precise final destination. By following
in vivo the transport of MS2-taggedVg1RNAs localizing to the
vegetal pole ofXenopusoocytes, Mowry and coworkers observed
distinct transport kinetics and directionality in different regions of
the oocyte. While dynein is responsible for the unidirectional RNA
transport characteristics of the upper vegetal cytoplasm, kinesin-1 is
required for the bidirectional transport observed in the lower veg-
etal cytoplasm [104]. A tight temporal coordination in motor
activities is also very important for the coupling between transport
and anchoring at destination. As revealed by quantitative imaging
inDrosophilaoocytes, for example, a strong interplay between
kinesin and dynein, and between the actin and microtubule cytos-
keletons, is required for posterior accumulation of mRNA-
containing germ granules [128].
How are these molecular motors recruited to actively trans-
ported mRNAs? As shown by Bullock and coworkers, RNA binding
proteins (RBPs) associating with localizing elements play a key role
in this process. Indeed, a specific structure found in mRNAs loca-
lizing apically inDrosophilaembryos was shown to trigger RBP-
mediated recruitment of dynein and directed transport [126].
Interestingly, the recruitment of molecular motors by RBPs can
be induced by external stimuli. Following MS2-taggedcamKIIα
mRNA in cultured neurons, Bassell and coworkers were able to
show an increase in kinesin-dependent dendritic targeting of
camKIIαRNA and its associated RBP FMRP upon mGluR activa-
tion, and a concomitant increase in the association between FMRP
and Kif5 Kinesin [127].3.3.2 Visualizing
Translation in Space
and Time
Until recently, detection of proteins synthesized locally, in specific
subcellular compartment was challenging. With the advent of novel
tagging strategies, it is now possible to map mRNA translation with
a high spatiotemporal resolution in living cells or organisms. In the
TRICK method, for example, PP7 and MS2 tags recognized by
distinct fluorescent peptides are inserted respectively in the coding
region and 3^0 UTR of reporter RNAs, such that dually labeled
RNAs lose their PCP signal upon ribosomal elongation [129].
By enabling the discrimination of translated from untranslated
mRNAs, and the monitoring of the first round of translation, the
TRICK method has been particularly useful in proving thatoskar
mRNA is not translated until it reaches the posterior pole ofDro-
sophilaoocyte. The use of the alternative SunTag approach to image
translation of single mRNA molecules revealed for the first time
cell-compartment specific heterogeneities of translation [118]. Live
imaging of local translation in dendrites of primary hippocampal
neurons, for example, provided evidence for a variability in mRNA
translation rates, with translation rate higher in proximal than in the14 Caroline Medioni and Florence Besse