Characterization of chick Frizzled-10 in the development of the chick neural tube

Roeben Munji
San Francisco State University

Wnt signaling is a key intercellular signaling pathway in embryonic development. It is involved in processes as diverse as axis induction, asymmetric cell division, segmentation and patterning of the central nervous system. Our particular interest is in the role of Wnt signaling in the development of the neural tube, the rudimentary structure that matures into the nervous system. Two members of the Wnt family of secreted signaling factors, Wnt-1 and Wnt-3a, have been implicated in the control of cell proliferation and fate specification in neural tube development. Although these functional roles for Wnt-1 and Wnt-3a have been identified, the mechanisms by which Wnt-1 and Wnt-3a activate different cellular responses have not been defined. It has been suggested that the Wnt receptor, Frizzled, may confer the specificity of the Wnt signal for a particular cellular response. To date, the specific Frizzled receptor(s) that mediate Wnt-1/3a signaling are unknown. However, recent analysis of the expression patterns of frizzled homologues in human (Koike et al, 1999), chick (Kawakami et al, 2000; Stark et al, 2000), zebrafish (Nasevicius et al, 2000) and Xenopus (Moriwaki et al, 2000) has identified Frizzled-10 (Fz10) as a candidate Wnt-1/3a receptor. To further analyze the expression pattern of chick Fz10, I conducted wholemount in situ hybridization, which confirmed that chick fz10 is expressed in the dorsal neural tube, overlapping Wnt-1 and Wnt-3a expression patterns. These findings have led us to hypothesize that chick Fz10 may mediate Wnt-1/3a signaling in the dorsal neural tube. We have set out to determine whether chick Fz10 mediates and confers specificity for Wnt1/3a signaling by testing the sufficiency and requirement of chick Fz10 for regulating cell proliferation and fate specification. I am utilizing in ovo electroporations of chick embryos to 1) ectopically express Fz10 2) inhibit translation of endogenous Fz10 with antisense morpholino and 3) inhibit Wnt/frizzled interactions with a secreted form of Fz10. Ectopic expression of Fz10 revealed that it alone is not sufficient to modulate proliferation and the fate of specific cell lineages in the dorsal neural tube. To further test for sufficiency, I am currently analyzing embryos co-electroporated with Fz10 and Wnt-3a to determine if ectopic Fz10 and Wnt-3a are together sufficient to modulate proliferation and/or lineage specification. Analysis of embryos electroporated with either morpholino or secreted Fz10 indicated that inhibition of endogenous Fz10 activity results in a reduction of proliferation in the dorsal neural tube. However, these loss-of-function analyses show little or no effect on lineage specification. Consistent with known roles of Wnt-1 and Wnt-3a, our results suggest that Frizzled-10 regulates proliferation.