The Xenopus egg is radially symmetrical, and therefore Xwnt11 (26), and forkhead (12) in the Spemann's organizer region, Xbra in the mesoderm ring (45), and Xwnt an important developmental problem has been to under-8 in the ventral region (6,44) are examples of such stand the mechanism responsible for establishing axial genes. polarity. Immediately after fertilization, dorsal-ventral Dreyer et al. ( 13) described the developmental fate polarity is established with the rotation of the cortex ( 16).
of a number of maternal nuclear proteins in Xenopus The cortical movement leads to the formation of an early laevis that were first detected in the oocyte nucleus dorsalizing center, the so-called Nieuwkoop center, that (GV). Following oocyte maturation, these proteins are induces the overlying marginal zone cells to form mesoreleased into the cytoplasm, after which some immediderm. During gastrulation, the dorsal lip or Spemann's ately reenter the nuclei (early shifting) and others reorganizer marks the site through which the dorsal mesomain in the cytoplasm until specific stages during dederm involutes; the leading edge of the dorsal mesoderm velopment (late shifting). Several of the late-shifting marks the anterior of the embryo.
proteins reenter the nucleus at the MBT and gastrula The dorsal -ventral (D/V) and anterior -posterior (A/ stages and thus may have the potential to play im-P) patterning of the Xenopus embryo is under the conportant roles in regulating developmental processes trol of secreted growth factor-like molecules. Localized such as axial patterning. maternally expressed factors such as Vgl (10,51,54) or Xwnt11 (26) are likely candidates to initiate the process XNF7 IS REGULATED BY CYTOPLASMIC by producing a prepatterning of the embryo that is later RETENTION elaborated upon by the sequential action of other One of the late shifting proteins, xnf7 (Xenopus nugrowth factors such as FGF ( 22), activin-like molecules clear factor 7), is a maternally expressed gene whose (2,46,52), BMP (9,21,25), and Xwnt family members protein product is retained in the cytoplasm until the (6,44) [reviewed in (11,41,42)].
MBT, when it reenters the embryonic nucleus Activin, FGF, and BMP growth factors function (13,32,33,38). Xnf7 is a member of a novel family of zinc through signal transduction pathways involving recepfinger proteins, the B-box family, consisting mainly of tors and a cascade of kinase and phosphatase regulatranscription factors and protooncogenes (37,39). From tory components. These pathways undoubtably reguits domain structure, its ability to bind to DNA, and late the expression of region-specific factors such as the ability of the N-terminal acidic domain to transgoosecoid (4), Xlim (49), Xbra (45), forkhead (12,24,40), activate a reporter gene, we believe that xnf7 functions and Xnot (53), which impose a more rigid patterning as a transcription factor (30,38). In addition, members of the gastrula-stage embryo and divide it into overlapof the B-box zinc finger gene family, including xnf7 and ping yet distinct domains. A key to understanding the PML ( 14), form homodimers and heterodimers, sugprocess of pattern formation fully in vertebrates is to gesting that protein -protein interactions are imidentify the molecules that are responsive to growth portant in their functioning. factor signaling and regulate the expression of the downstream genes for region-specific factors involved XNF7 IS PHOSPHORYLATED UNDER THE DIRECTION OF SEVERAL DIFFERENT in D/V and A/P patterning. KINASES In the amphibian, early development prior to the MBT is directed strictly by maternal gene products. A Xnf7 is retained in the cytoplasm during early develseries of crucial developmental events occur at the opment by a novel anchor mechanism that depends on MBT, including the initiation of zygotic transcription, the presence of a 22-amino-acid cytoplasmic retention changes in the cell cycle, and the initiation of cellular movement (15,35,36). Genes activated at or shortly after the MBT play critical roles in morphogenesis and