Current Biology, Vol. 12, R797–R799, December 10, 2002, ©2002 Elsevier Science Ltd. All rights reserved. PII S0960-9822(02)01310-6
Dispatch
Oocyte Patterning: Dynein and
Kinesin, Inc.
Robert S. Cohen
Recent studies show that dynein and kinesin are both
required for cargo transport to the anterior cortex
of the Drosophila oocyte. The orientation of microtubules in the oocyte suggests that kinesin mediates
anterior transport indirectly, by activating and/or
recycling dynein.
All cells must localize key molecules, organelles and
macromolecular complexes to specific subcellular
sites. In many cells, this is done by actively transporting cargoes on microtubule ‘tracks’. This is the case in
the Drosophila oocyte, where several mRNAs, and
even the nucleus, are transported to specific cortical
regions in a microtubule-dependent fashion (reviewed
in [1]). Real-time imaging indicates that RNA transport
is largely unidirectional and proceeds at speeds consistent with the involvement of known microtubulebased motor proteins, such as kinesin and dynein [2].
Recent papers, including two in this issue [3,4] and
one in a recent issue of Current Biology [5], have provided important new insights into the interconnected
roles of kinesin and dynein in cargo transport and
polarity establishment during Drosophila oogenesis.
The prototype cargoes for microtubule-dependent
transport in the Drosophila oocyte are bicoid and
oskar mRNAs, which are transported to the anterior
cortex and posterior pole, respectively, and which
define the anteroposterior axis of the egg and future
embryo [1] (Figure 1). Early clues as to which motors
transport which mRNAs came from studies in which a
reporter enzyme was linked to motor domains of
known polarity and expressed in transgenic oocytes
[6,7]. The results indicated that the oocyte’s microtubule cytoskeleton is highly polarized, with microtubule minus ends nucleated along the anterior cortex
and microtubule plus ends focused onto the posterior
pole (Figure 1). It was therefore predicted that bicoid
and oskar mRNAs are transported by minus enddirected and plus end-directed motors, respectively.
It was also predicted that a minus-end motor transports the oocyte’s nucleus to the anterior cortex
(Figure 1), the critical first step in the generation of
dorsoventral polarity [1].
Recent data indicate that the organization of the
oocyte’s microtubule cytoskeleton is not as simple as
first thought. For example, it is now known that microtubules are nucleated, not just along the anterior cortex,
but also along the lateral cortex [2]. It is clear, however,
that bicoid transcripts are modified prior to their transport into the oocyte from adjacent nurse cells, such that
they specifically associate with microtubules nucleated
Department of Molecular Biosciences, University of Kansas,
Lawrence, Kansas 66045, USA. E-mail: rcohen@ku.edu
at the oocyte’s anterior cortex [2]. Other recent data
indicate that microtubule plus ends are at least initially
pointed toward the center of the oocyte, rather than
toward the posterior pole [5]. Despite these added
complexities, there are still strong predictions that a
minus end-directed motor (dynein) directs bicoid mRNA
to the oocyte’s anterior end, and that a plus enddirected motor (kinesin) drives oskar mRNA to the posterior pole, or at least away from the anterior and lateral
cortices en route to the posterior pole.
There is now strong evidence that Khc is required
for oskar mRNA transport to the oocyte’s posterior
pole. Indeed, Khc is required for the posterior localization of all examined molecules, including dynein
heavy chain [3–5,9,10]. This was shown by analyzing
mosaic females with germ line cells homozygous for
Khc null mutations. Curiously, similar mosaic analyses
showed kinesin light chain (Klc), the cargo-binding
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Microtubule nucleated at anterior cortex
Microtubule nucleated at lateral cortex
Late stage microtubule extension
Microtubule minus end
bicoid mRNA
Microtubule plus end
gurken mRNA
Nucleus
oskar mRNA
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Figure 1. Drosophila oocyte polarity.
A mid-stage oocyte following microtubule-dependent transport
of bicoid (blue), oskar (orange) and gurken (green) mRNAs, and
the nucleus (n) to their final destinations. Anterior is to the left
and dorsal is up. As described in the text, microtubule minus
ends are nucleated along the anterior and lateral cortexes; the
two populations are somehow different, however, as endogenous bicoid mRNAs move selectively to the anterior cortex [2].
Microtubule plus ends initially point toward the cell center [8], but
may later be re-directed toward the posterior pole and facilitate
transport of oskar mRNA all the way to the pole (see [11,18]).
Microtubules also facilitate the migration of the oocyte nucleus
to a random point along the anterior cortex [19], and the subsequent transport of gurken mRNA to the region between the
nuclear and neighboring plasma membranes.
Dispatch
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K
Dy
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RNA particle
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Microtubule minus end
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Dynactin complex
Stimulation
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Kinesin–dynactin adaptor
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Kinesin
Direction of transport
Current Biology
Figure 2. A model for dynein and kinesin cooperation.
The figure illustrates the movement of mRNA particles (red)
toward the minus ends of microtubules. Cytoplasmic dynein
powers transport, while dynactin complex brings the RNA particles to the motor. The model shows two non-mutually exclusive
ways in which kinesin may enhance dynein-mediated transport
toward microtubule minus ends. The first way involves activation
of dynein motor activity (top and bottom diagrams). Because
kinesin is not known to physically interact with either dynein or
dynactin, such activation is shown to work through a yet-to-beidentified adaptor protein. As shown in the middle diagram,
kinesin could also enhance RNA transport to microtubule minus
ends by recycling dynein to the microtubule plus ends. This
would allow dynein to carry out a second round of cargo binding
and transport. As diagrammed, dynein may facilitate its own
recycling by stimulating kinesin motor activity.
domain of conventional kinesin, is not required for
posterior transport or ooplasmic streaming, another
Khc-dependent process [9]. So while Khc powers the
movement of multiple cargoes in the oocyte, it associates with them in a non-conventional way, independently of Klc.
Cytoplasmic dynein is the obvious candidate motor
for moving cargoes anteriorly, as it is the only known
minus end-directed motor in the oocyte [11]. Mutations
that block the ability of the Swallow protein to bind
dynein light chain also block anterior bicoid mRNA
localization in late stages [12]; and mutations in two
other dynein-interacting proteins, Lissencephaly 1 (Lis1) and BicD, inhibit the migration of the oocyte nucleus
to the anterior cortex [13,14]. These data are not conclusive, however, and the possibility remains that anterior transport is carried out by some other minus
end-directed motor, such as a non-conventional
member of the kinesin family. Further clouding dynein’s
possible role in anterior transport is the finding that
dynein heavy chain is concentrated at the oocyte’s
posterior pole and around its nucleus, but not at the
anterior cortex [11]. Unfortunately, direct analysis of
dynein mutants has been problematic as they arrest
oogenesis prior to oocyte determination [11].
The two papers in this issue [3,4] report compelling
evidence that cytoplasmic dynein really is the oocyte’s
anterior motor. Both groups impaired dynein function
by stage-specific overproduction of dynamitin, a conserved component of the dynactin complex through
which all known dynein functions are mediated. When
overproduced in cultured cells or transgenic mice,
dynamitin causes dynein to dissociate from dynactin,
leading to defects in dynein-mediated processes
(see [15]). By selectively driving dynamitin overexpression during middle stages of Drosophila oogenesis, both groups were able to avoid disrupting early
dynein functions which are essential for oocyte determination and for the transfer of oskar, bicoid and other
mRNAs from nurse cells into the oocyte [11]. Dynamitin overproduction during mid-oogenesis blocked
the normal posterior and nuclear localization of dynein
intermediate chain and several dynactin complex proteins, and appears to be an effective means of disrupting dynein function.
The two papers [3,4] report a number of patterning
abnormalities caused by dynamitin overproduction,
including frequent defects in bicoid mRNA localization
and less frequent defects in nuclear migration to the
anterior pole. The frequencies of these defects were
enhanced by concomitant reductions in Lis-1, Glued
(another dynactin complex protein) or dynein heavy
chain, consistent with the idea that they stem from
induced inhibition of dynein motor activity. Dynamitin
overproduction also caused unexpected defects in
gurken mRNA localization to the region above the
nucleus in the anterior-dorsal corner of the oocyte.
These defects were apparent even in oocytes whose
nuclei had faithfully migrated to the anterior cortex.
These results thus indicate that dynein is responsible
for the transport of gurken mRNA to the oocyte’s
anterior-dorsal corner and for the transport of bicoid
mRNA and the nucleus to the oocyte’s anterior cortex.
Most surprisingly, these studies [3,4] and a third [5]
show that anterior and anterior-dorsal transport also
requires kinesin. Germline clones of Khc mutant cells
were not only defective in oskar mRNA localization, as
previously reported [10], but also in gurken mRNA
localization [3–5], nuclear migration [3–5] and bicoid
mRNA localization [3]. Given that microtubule plus
ends are pointed toward the center and/or posterior
pole of the oocyte, the requirement for kinesin is likely
indirect. Kinesin might, for example, activate or
recycle dynein (see below).
Khc and dynein also appear to cooperate in ooplasmic streaming [9,11] and in the transport of cargoes to
the posterior pole, although the evidence for this last
role is somewhat controversial. Januschke et al. [3]
report a slight reduction in oskar mRNA localization
upon dynamitin overproduction, but chalk this up to a
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R799
defect in the transport of oskar mRNA into the oocyte
from nurse cells, rather than to a defect in posterior
transport per se. Duncan and Warrior [4] also report a
reduction in oskar mRNA localization, but highlight the
presence of mislocalized oskar transcripts as evidence that dynamitin overproduction interferes with
oskar mRNA transport within the oocyte. Given the
likelihood that dynamitin overproduction does not
completely inhibit dynein function, the mislocalization
of oskar mRNA reported by Duncan and Warrior [4]
might be indicative of significant cooperation between
dynein and kinesin in posterior transport.
The idea that dynein and kinesin act cooperatively
in cargo transport is not unprecedented (see [16,17]).
Real-time imaging and manipulations with optical
tweezers in Drosophila embryos have shown that
dynein/dynactin mutations block movement of individual lipid droplets, not only toward the minus ends
of microtubules, but also toward the plus ends [17].
The results suggest dynein/dynactin is required for the
activation of an unidentified plus end motor which
powers lipid droplet transport toward microtubule plus
ends [17]. One attractive model is that dynein/dynactin switches between two regulatory states, one
that directly powers transport toward microtubule
minus ends and another that indirectly powers transport toward microtubule plus ends by activation of a
plus end motor. In this way, lipid droplets and other
cargoes associated with two or more motors of
opposing polarity can avoid being tugged in two
directions, and instead move efficiently in one direction or another, depending on the regulatory states of
the opposing motors [17] (Figure 2).
Another way two motors of opposite polarity might
cooperate is by recycling one another (suggested in
[3–5,9]). For example, kinesin might transport dynein
molecules toward the plus ends of microtubules so
they can carry out a second round of cargo binding
and transport, and vice versa (Figure 2). While motor
recycling cannot explain the requirement for opposing
motors in the movement of an individual cargo, it
could play an essential role in efficient transport of a
population of cargoes, especially in cases where the
cargoes greatly outnumber the motors, or where
transported cargoes are not anchored and can diffuse
away from their target destinations. The idea that
kinesin recycles dynein in the Drosophila oocyte is
consistent with the observations cited above that
dynein is most abundant at the oocyte’s posterior pole
and that its localization there is kinesin dependent.
Precisely how dynein and kinesin cooperate warrants
further study. Do the proteins bind directly, or indirectly
through a shared cargo or regulatory complex? How do
these interactions control motor activity and are they
conserved between cell types and species? Many
important issues also remain with regard to the contribution of polarized transport in oocyte patterning. How
are RNAs and other cargoes released from their motors
on delivery to their intended destinations? What is the
nature of the asymmetry that targets gurken mRNA
to the anterodorsal corner? Is gurken mRNA transported on a subset of microtubules that terminate at
the nucleus and, if so, how are these microtubules
recognized? How are the microtubules nucleated at the
anterior cortex distinguished from those nucleated at
the lateral cortex? Clearly, the identification of the
motors involved in oocyte patterning by the papers discussed here represent a welcome first step in addressing these important questions.
References
1. Riechmann, V. and Ephrussi, A. (2001). Axis formation during
Drosophila oogenesis. Curr. Opin. Genet. Dev. 11, 374–383.
2. Cha, B.J., Koppetsch, B.S. and Theurkauf, W.E. (2001). In vivo analysis of Drosophila bicoid mRNA localization reveals a novel microtubule-dependent axis specification pathway. Cell 106, 35–46.
3. Januschke, J., Gervais, L., Dass, S., Kaltschmidt, J.A., Lopez-Schier,
H., St Johnston, D., Brand, A.H., Roth, S. and Guichet, A. (2002). Polar
transport in the Drosophila oocyte requires dynein and kinesin I
cooperation. Curr. Biol., this issue.
4. Duncan, J.E. and Warrior, R. (2002). The cytoplasmic dynein and
kinesin motors have interdependent roles in patterning the
Drosophila oocyte. Curr. Biol., this issue.
5. Brendza, R.P., Serbus, L.R., Saxton, W.M. and Duffy, J.B. (2002). Posterior Localization of dynein and dorsal-ventral axis formation depend
on kinesin in Drosophila oocytes. Curr. Biol. 12, 1541–1545.
6. Clark, I., Giniger, E., Ruohola-Baker, H., Jan, L.Y. and Jan, Y.N. (1994).
Transient posterior localization of a kinesin fusion protein reflects
anteroposterior polarity of the Drosophila oocyte. Curr. Biol. 4,
289–300.
7. Clark, I., Jan, L.Y. and Jan, Y.N. (1997). Reciprocal localization of nod
and kinesin fusion proteins indicates microtubule polarity in the
Drosophila oocyte, epithelium, neuron and muscle. Development 124,
461–470.
8. Cha, B.J., Serbus, L.R., Koppetsch, B.S. and Theurkauf, W.E. (2002).
Kinesin I- and microtubule-dependent cortical exclusion restricts pole
plasm assembly to the oocyte posterior. Nat. Cell Biol. 4, 592–598.
9. Palacios, I.M. and St Johnston, D. (2002). Kinesin light chain-independent function of kinesin heavy chain in cytoplasmic streaming,
and posterior localization in the Drosophila oocyte. Development, in
press.
10. Brendza, R.P., Serbus, L.R., Duffy, J.B. and Saxton, W.M. (2000). A
function for kinesin I in the posterior transport of oskar mRNA and
Staufen protein. Science 289, 2120–2122.
11. McGrail, M. and Hays, T.S. (1997). The microtubule motor cytoplasmic dynein is required for spindle orientation during germline cell divisions and oocyte differentiation in Drosophila. Development 124,
2409–2419.
12. Schnorrer, F., Bohmann, K. and Nusslein-Volhard, C. (2000). The molecular motor dynein is involved in targeting Swallow and bicoid RNA
to the anterior pole of Drosophila oocytes. Nat. Cell Biol. 2, 185–190.
13. Swan, A., Nguyen, T. and Suter, B. (1999). Drosophila Lissencephaly1 functions with Bic-D and dynein in oocyte determination and
nuclear positioning. Nat. Cell Biol. 1, 444–449.
14. Liu, Z., Xie, T. and Steward, R. (1999). Lis1, the Drosophila homolog
of a human lissencephaly disease gene, is required for germline cell
division and oocyte differentiation. Development 126, 4477–4488.
15. Echeverri, C.J., Paschal, B.M., Vaughan, K.T. and Vallee, R.B. (1996).
Molecular characterization of the 50-kD subunit of dynactin reveals
function for the complex in chromosome alignment and spindle organization during mitosis. J. Cell Biol. 132, 617–633.
16. Martin, M., Iyadurai, S.J., Gassman, A., Gindhart, J.G., Jr., Hays, T.S.
and Saxton, W.M. (1999). Cytoplasmic dynein, the dynactin complex,
and kinesin are interdependent and essential for fast axonal transport. Mol. Biol. Cell 10, 3717–3728.
17. Gross, S.P., Welte, M.A., Block, S.M. and Wieschaus, E.F. (2002).
Coordination of opposite-polarity microtubule motors. Cell Biol. 156,
715–724.
18. Dollar, G.L., Struckhoff, E., Michaud, J. and Cohen, R.S. (2002).
Gurken-independent polarization of the Drosophila oocyte; Rab11mediated organization of the posterior pole. Development 129,
517–526.
19. Roth, S., Jordan, P. and Karess, R. (1999). Binuclear Drosophila
oocytes: consequences and implications for dorsal-ventral patterning in oogenesis and embryogenesis. Development 126, 927–934.