98
M. Hild et al.
cartilage and endochondral bone when implanted in experimental animals (Wozney et al. 1988). Bmp 1 is supposed to cleave procollagen and
to facilitate collagen assembly into fibers during bone formation
(Kessler et al. 1996), although bmpl-deficient mice do not display
major skeletal abnormalities (Hogan 1996).
During early zebrafish development, tolloid is expressed on the ventral side of the midgastrula embryo. After gastrulation, strong tolloid
expression is maintained in the forming tailbud, very similar to the
expression of bmp4 (Blader et al. 1997). In vitro, wild-type Tolloid
protein leads to a specific degradation of bacculovirus-generated Chordin protein (Blader et al. 1997; Piccolo et al. 1997). As in the case of
bmp2b and bmp7, tolloid could be assigned to a dorsalized zebrafish
mutant. Work carried out in the laboratory of Mary Mullins at the
University of Pennsylvania, Philadelphia, indicates that the mild Cl
dorsalization of minifin mutants is caused by potential null mutations in
the zebrafish tolloid gene (Conners et al. 1999). Tolloid-chordino double mutants are ventralized like chordino single mutants, indicating that
chordino is epistatic to tolloid and that Tolloid does indeed function as
an inhibitor of Chordino. The relatively mild dorsalization of tolloid
mutants, however, suggests that Chordino inhibition by Tolloid is not of
central importance for dorsoventral patterning of the zebrafish embryo,
but only required in ventral-most regions, unless there are partially
redundant genes that inhibit Chordino in concert with Tolloid.
6.11 Zebrafish smadl and smad5
So far, we have dealt with the mechanisms that define positional values
along the dorsoventral axis of the pregastrula embryo by establishing
the putative Bmp2/417 gradient. However, we would also like to know
how this gradient and the local Bmp2/417 concentrations are interpreted
by the different cells along the dorsoventral axis. Bmp2 and 4 are known
to bind to transmembrane receptors of the serine-threonine kinase receptor family which, upon ligand binding, phosphorylate and activate
Smad proteins. For the mediation of BMP signaling, three different
Smad proteins have been identified: Smadl, Smad5, and more recently,
Smad8. These receptor-regulated Smad proteins usually form cytoplasmic homotrimers which, after their receptor-mediated phosphorylation,
M. Hild et al.
cartilage and endochondral bone when implanted in experimental animals (Wozney et al. 1988). Bmp 1 is supposed to cleave procollagen and
to facilitate collagen assembly into fibers during bone formation
(Kessler et al. 1996), although bmpl-deficient mice do not display
major skeletal abnormalities (Hogan 1996).
During early zebrafish development, tolloid is expressed on the ventral side of the midgastrula embryo. After gastrulation, strong tolloid
expression is maintained in the forming tailbud, very similar to the
expression of bmp4 (Blader et al. 1997). In vitro, wild-type Tolloid
protein leads to a specific degradation of bacculovirus-generated Chordin protein (Blader et al. 1997; Piccolo et al. 1997). As in the case of
bmp2b and bmp7, tolloid could be assigned to a dorsalized zebrafish
mutant. Work carried out in the laboratory of Mary Mullins at the
University of Pennsylvania, Philadelphia, indicates that the mild Cl
dorsalization of minifin mutants is caused by potential null mutations in
the zebrafish tolloid gene (Conners et al. 1999). Tolloid-chordino double mutants are ventralized like chordino single mutants, indicating that
chordino is epistatic to tolloid and that Tolloid does indeed function as
an inhibitor of Chordino. The relatively mild dorsalization of tolloid
mutants, however, suggests that Chordino inhibition by Tolloid is not of
central importance for dorsoventral patterning of the zebrafish embryo,
but only required in ventral-most regions, unless there are partially
redundant genes that inhibit Chordino in concert with Tolloid.
6.11 Zebrafish smadl and smad5
So far, we have dealt with the mechanisms that define positional values
along the dorsoventral axis of the pregastrula embryo by establishing
the putative Bmp2/417 gradient. However, we would also like to know
how this gradient and the local Bmp2/417 concentrations are interpreted
by the different cells along the dorsoventral axis. Bmp2 and 4 are known
to bind to transmembrane receptors of the serine-threonine kinase receptor family which, upon ligand binding, phosphorylate and activate
Smad proteins. For the mediation of BMP signaling, three different
Smad proteins have been identified: Smadl, Smad5, and more recently,
Smad8. These receptor-regulated Smad proteins usually form cytoplasmic homotrimers which, after their receptor-mediated phosphorylation,