Biomedical subjects

V Knezevic

Publications and source records attributed to V Knezevic.

7 recordsLinked to original sources

PubMed1999

Do 5'Hoxd genes play a role in initiating or maintaining A-P polarizing signals in the limb?

The analysis of multiple null mutants generated through targeted disruption indicates that the 5' members of the Hoxd and Hoxa clusters determine the skeletal pattern in the limb by regulating the formation and growth of the different chondrogenic precursors for the skeletal elements. While these studies have established that together these genes are the major players in regulating formation of the limb skeleton, the roles of individual members have often been difficult to evaluate fully due to extensive functional overlap between various 5'Hoxd and 5'Hoxa genes. The analysis of gain-of-function mutants provides a complementary approach to elucidate gene function in the presence of multiple redundancies. This approach has recently revealed that Hoxd-12 can induce Sonic hedgehog and suggests a new role for certain 5'Hoxd genes in the initiation of Sonic hedgehog expression and its maintenance through feedback regulation. Thus, some 5'Hoxd genes may be a part of the regulatory network that positions and reinforces polarizing signals in the posterior-distal limb bud.

Body Patterning↗

PubMed1998

Continuing organizer function during chick tail development.

Development of the posterior body (lumbosacral region and tail) in vertebrates is delayed relative to gastrulation. In amniotes, it proceeds with the replacement of the regressed node and primitive streak by a caudal blastema-like mass of mesenchyme known as the tail bud. Despite apparent morphological dissimilarities, recent results suggest that tail development in amniotes is in essence a continuation of gastrulation, as is the case in Xenopus. However, this has been inferred primarily from the outcome of fate mapping studies demonstrating discrete, regionalized cell populations in the tail bud, like those present at gastrulation. Our analysis of the tail bud distribution of several molecular markers that are expressed in specific spatial domains during chick gastrulation confirms these results. Furthermore, we present evidence that gastrulation-like ingression movements from the surface continue in the early chick tail bud and that the established tail bud retains organizer activity. This 'tail organizer' has the expected properties of being able to recruit uncommitted host cells into a new embryonic axis and induce host neural tissue with posteriorly regionalized gene expression when grafted to competent host cells that are otherwise destined to form only extra-embryonic tissue. Together, these results indicate that chick tail development is mechanistically continuous with gastrulation and that the developing tail in chick may serve as a useful experimental adjunct to investigate the molecular basis of inductive interactions operating during gastrulation, considering that residual tail organizing activity is still present at a surprisingly late stage.

Animals↗

PubMed1997

Two novel chick T-box genes related to mouse Brachyury are expressed in different, non-overlapping mesodermal domains during gastrulation.

The mouse Brachyury (T) gene plays critical roles in the genesis of normal mesoderm during gastrulation and in the maintenance of a functioning notochord. Abrogation of Brachyury (T) expression within the chordamesoderm of homozygous null mutants nevertheless spares anterior axis formation. An intriguing possibility to explain the preservation of anterior axis formation in these mutants would be the existence of other genes compensating for the loss of Brachyury. This compensation and the recent demonstration that Brachyury is the prototype for an evolutionarily conserved family, prompted a search for other T-box genes participating in axis formation. The chick Brachyury orthologue and two related chick T-box genes that are expressed at the onset of gastrulation have been isolated. One of these novel genes (Ch-TbxT) becomes restricted to the axial mesoderm lineage and is a potential candidate for complementing or extending Brachyury function in the anterior axis (formation of the head process, prechordal plate). The other gene (Ch-Tbx6L), together with chick T, appears to mark primitive streak progenitors before gastrulation. As cells leave the primitive streak, Ch-Tbx6L becomes restricted to the early paraxial mesoderm lineage and could play a role in regulating somitogenesis.

Amino Acid Sequence↗

PubMed1997

Hoxd-12 differentially affects preaxial and postaxial chondrogenic branches in the limb and regulates Sonic hedgehog in a positive feedback loop.

Several 5' members of the Hoxd cluster are expressed in nested posterior-distal domains of the limb bud suggesting a role in regulating anteroposterior pattern of skeletal elements. While loss-of-function mutants have demonstrated a regulatory role for these genes in the developing limb, extensive functional overlaps between various different Hox genes has hampered elucidation of the roles played by individual members. In particular, the function of Hoxd-12 in the limb remains obscure. Using a gain-of-function approach, we find that Hoxd-12 misexpression in transgenic mice produces apparent transformations of anterior digits to posterior morphology and digit duplications, while associated tibial hemimelia and other changes indicate that formation/growth of certain skeletal elements is selectively inhibited. If the digital arch represents an anterior bending of the main limb axis, then the results are all reconcilable with a model in which Hoxd-12 promotes formation of postaxial chondrogenic condensations branching from this main axis (including the anteriormost digit) and selectively antagonizes formation of 'true' preaxial condensations that branch from this main axis (such as the tibia). Hoxd-12 misexpression can also induce ectopic Sonic hedgehog (Shh) expression, resulting in mirror-image polydactyly in the limb. Misexpression of Hoxd-12 in other lateral plate derivatives (sternum, pelvis) likewise phenocopies several luxoid/luxate class mouse mutants that all share ectopic Shh signalling. This suggests that feedback activation of Shh expression may be a major function of Hoxd-12. Hoxd-12 can bind to and transactivate the Shh promoter in vitro. Furthermore, expression of either exogenous Hoxd-11 or Hoxd-12 in cultured limb bud cells, together with FGF, induces expression of the endogenous Shh gene. Together these results suggest that certain 5' Hoxd genes directly amplify the posterior Shh polarizing signal in a reinforcing positive feedback loop during limb bud outgrowth.

Animals↗

PubMed1996

Differentiation potential of rat amnion.

The differentiation potential of rat amnion was investigated by explantation to different extrauterine sites and by culturing in vitro. Amnion differentiated into full skin by a process that is morphologically indistinguishable from normal skin development (interactions of the surface embryonic ectoderm and underlying mesenchyme). This process is not dependent on the age of the amnion but is dependent on the culture conditions used. Possible implications of these findings are discussed.

Amnion↗

PubMed1995

The organizer-associated chick homeobox gene, Gnot1, is expressed before gastrulation and regulated synergistically by activin and retinoic acid.

Gnot1, a Not (for notochord) family homeobox gene, is expressed in the chick pregastrulation blastoderm. Gnot1 expression in the epiblast is upregulated as the posteriorly derived hypoblast moves forward anteriorly to form a layer beneath it, which is of particular interest considering the known inductive role of the hypoblast in axis formation in the chick. Both activin and retinoic acid are able to activate Gnot1 expression in cultured blastodermal cells and show a strong synergistic effect when applied in combination. Strong superinduction of Gnot1 transcripts in the presence of cycloheximide also indicates the presence of a potent and labile intracellular inhibitor capable of modulating Gnot1 expression. During gastrulation, Gnot1 transcripts become localized specifically to tissues associated with "organizer" function (Hensen's node, head process, notochord). The expression data and the response to mesoderm inducing factors and axial "caudalizing" signals suggest that Gnot1 may be involved in specification of the embryonic body axis and could play a part in regulating features of the trunk/tail organizer in the chick embryo.

Activins↗

PubMed1994

p185neu is expressed in yolk sac during rat postimplantation development.

We have shown that the neu oncogene product (p185neu) is not present in the rat embryo before organogenesis. However, coincident with the onset of organogenesis, p185neu was detected in neural and connective tissue as well as in the secretory epithelium as was described by Kokai et al. (1987). In addition, p185neu is also expressed in the rat visceral yolk sac (VYS) endodermal cells but not in the mesenchymal and mesothelial layers of the same structure nor in the amnion. The first detectable sign of p185neu expression in VYS was found at d 11 of gestation and the levels of protein increased towards the end of pregnancy. In the yolk sac carcinoma (YSC), which is considered to be the malignant counterpart of the rat yolk sac, p185neu was observed only within columnar epithelial cells (the visceral component of the neoplasm) while parietal endoderm-like cells were devoid of detectable protein. From d 9 of pregnancy up to delivery some of the trophoblastic giant cells also showed a faint to moderate immunoreactivity. Results are presented which would indicate a possible role of p185neu in rat embryogenesis.

Animals↗