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RESEARCH INTERESTS
Regulation of microtubule assembly; Xenopus development
PUBLICATIONS
Poster
XMAP215 isoforms during Xenopus development
We are trying to understand the regulation and role of the
cytoskeleton, predominantly focusing on MTs, during
oogenesis and early development in the African clawed toad,
Xenopus laevis. Like most eukaryotic cells, oocytes
and embryos of Xenopus laevis (the African clawed
toad) contain three systems of cytoplasmic filaments:
microtubules (MTs), actin filaments (F-actin), and
intermediate filaments. This "cytoskeleton" forms a
complex and dynamic network that has been postulated to
play a number of critical roles, including the establishment
and maintenance of cell and embryonic polarity, and meiotic
and mitotic cell divisions.
Interestingly, the assembly and organization of MTs in
Xenopus oocytes and eggs occurs in the absence of
a discrete MT organizing center, such as the centrosome
found in most other animal cells. Despite lacking
centrosomes, Xenopus oocytes contain an
abundance of the centrosomal protein, gamma-tubulin.
Gamma-tubulin appears to be concentrated in the oocyte
cortex, suggesting that oocyte MTs are nucleated and
organized by the oocyte cortex. This implies that the
polarity of individual MTs in the oocyte is the opposite of
that found in most somatic cells, and makes some
interesting predictions about the roles of MTs and
MT-dependent motors in the establishment of oocyte
polarity. We have recently used electron microscopy and
"hook decoration" to confirm that oocyte MTs are indeed
oriented "inside-out."
Currently, we are focusing our attention on two MT
associated proteins, XMAP215 and XMAP230, found in
Xenopus oocytes, eggs, and early embryos. These
XMAPs have very different effects on MT assembly
in vitro: XMAP230 promotes assembly by
stabilizing MTs, while XMAP215 promotes the assembly of
long, but highly dynamic, MTs. Confocal immunofluorescence
microscopy reveals that both XMAPs are associated with
spindle MTs during mitosis. XMAP 230 also associates with
interphase MTs, including the novel network of subcortical
MTs required for formation of the D-V axis in early
Xenopus embryos. Their distinct distributions and
differing effects on MT assembly in vitro suggest
that XMAPs play very different roles in regulating MT
assembly and organization in vivo.
Recent evidence indicates that XMAP215 is the Xenopus
homolog of ch-TOGp, a protein over expressed in several
human tumors. We have cloned cDNAs encoding XMAP215,
and find that two highly-related isoforms of XMAP215
protein are expressed during early development: a maternal
form (XMAP215M) present from oogenesis through
gastrulation, and a zygotic form (XMAP215Z) expressed in
the developing nervous system. We are using a number of
techniques, including confocal microscopy, immuno-
electron microscopy, in vitro MT and spindle
assembly, microinjection of specific antibodies, and
molecular techniques, to probe the function of these
XMAP215 proteins during oogenesis and development.
Additional information is available on our web site:
http://froglab.biology.utah.edu.
Selected Publications
Becker, B. and Gard, D.L. (2000). Multiple
isoforms of XMAP215 are expressed during
oogenesis and early development in Xenopus
laevis. Cell Motility and the Cytoskeleton 47:
282-295.
Cha, B.-J., L. Cassimeris and D. Gard. 1999. XMAP230 is
required for normal spindle assembly in vivo and in vitro.
J. Cell Sci. 112:4337-4346.
Pfeiffer, D. and Gard, D. 1999. Microtubules in Xenopus
oocytes are oriented with their minus-ends towards the
cortex. Cell Motil and Cytoskeleton 44:34-43.
Gard, D. 1999. Confocal microscopy and 3-D reconstruction
of the cytoskeleton of Xenopus oocytes.
Micros. Res. Tech. 44:388-414.
Cha, B.-J. and D. Gard. 1999. XMAP230 is required for the
organization of cortical microtubules and patterning of the
dorsoventral axis in fertilized Xenopus eggs.
Developmental Biology 205:275-286.
Cha, B.-J., B. Error and D. Gard. 1998. XMAP230 is required
for the assembly and organization of acetylated
microtubules in Xenopus oocytes and eggs.
J Cell Sci. 111:2315-2327.
Gard, D., B.-J. Cha and E. King. 1997. The organization and
animal-vegetal asymmetry of cytokeratin filaments in stage
VI Xenopus oocytes is dependent upon F-actin and
Microtubules. Developmental Biology 184:
95-114.
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