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Extracellular matrix components can influence cell behaviour by modulating a wide variety of events. In particular, the glycosaminoglycan hyaluronan is involved in many processes of the normal and pathological adult cells and it is essential for embryonic development. Two main HA receptors have been characterized in vertebrate developing embryos: CD44 and RHAMM. These receptors display completely different characteristics apart from their ability to bind hyaluronan. RHAMM is still the most mysterious hyaluronan receptor as it can act as cell surface receptor but it can also be localized in the cytoplasm or in the cell nucleus, displaying both hyaluronan dependent and independent functions. In particular, the role of RHAMM during embryogenesis is still largely unclear. We reported a detailed gene expression analysis of RHAMM during Xenopus laevis development comparing its mRNA distribution with that of the hyaluronan synthases and CD44 genes, in order to provide a first insight into the possible role of RHAMM during vertebrate embryogenesis. Our findings point out that RHAMM mRNA displays a specific distribution in proliferating regions of the developing neural tube and retina where synthesis of hyaluronan is not detected. On the contrary, RHAMM expression correlates with the expression of hyaluronan synthase-1 and hyaluronan-receptor CD44 gene expression in migrating cranial neural crest. These results suggest that during the central nervous system development RHAMM could be involved in cell proliferation and migration processes both in a hyaluronan independent and dependent manner.
Fig. 2. Spatial expression pattern of RHAMM mRNA in early Xenopus laevis embryos. (A) RHAMM maternal transcript is distributed in the animal pole blastomeres (arrow) of a blastula stage embryo (st. 6). (B) RHAMM mRNA distribution during gastrulation (st. 10.5) (arrow). (C) Specific expression of RHAMM mRNA in the neural tube (nt) and anterior neural plate of a neurula stage embryo (st. 17), frontal view. (D) Double in situ hybridization visualizing the pre-migratory cranial NCC by the gene expression of Sox9 (light blue) and RHAMM in the neural tube (dark blue). (E) Frontal view of a stage 20 embryo exhibiting RHAMM transcript localization in the eye field (e), in the neural tube (nt) and in the otic placodes (op).
Fig. 3. Spatial expression of RHAMM mRNA during cranial neural crest cells migration. (A) Lateral view of stage 23 embryo showing RHAMM gene expression in the eye field (e), in the neural tube (arrowhead) and in the cranial NCC migrating into the four branchial pouches (I, II, III, IV). (B) At stage 26 RHAMM mRNA is expressed in the eye field (e), in the neural tube (arrowhead), in the NCC migrating into the four branchial pouches (I, II, III, IV) and in the pronephric anlagen (p). (C) The expression of the cranial NCC marker Sox9 is shown for comparison in a stage 26 embryo. (D) Double in situ hybridization showing the overlapping expression of RHAMM (dark blue) with that of the NCC marker Sox9 (light blue) in a stage 26 embryo. (E) Magnification of a coronal vibratome section across the broken line in B highlighting RHAMM expression in the ventricular zone of the neural tube (arrowhead) and in the proliferating region of the optic vesicle (harrow). (F) Lateral view of a stage 32 embryo showing RHAMM transcription in the eye (e), in the neural tube (arrowhead), in the cranial NCC migrated into the four branchial pouches (I, II, III, IV) and in the pronephric anlagen (p). (F′) Vibratome horizontal section across the broken line in C showing RHAMM gene expression in the NCC component of the four branchial arches (I, II, III, IV). (G) The expression of the skeletogenic NCC marker Sox9 is shown for comparison. ect, ectodermal component of the pharyngeal pouches; mes, mesodermal component of the pharyngeal pouches; end, endodermal component of the pharyngeal pouches.
Fig. 4. Spatial expression of RHAMM mRNA in proliferating cell populations. (A) Cryostat horizontal section at the branchial level of a stage 37 embryo showing RHAMM mRNA expression in cranial NCC migrated into the branchial pouches (I, II, III, IV). (B) Adjacent section showing the proliferating cells marker cyclin D1 mRNA expression for comparison. (C) Cryostat horizontal section at eyes level, at stage 37, showing that RHAMM expression in the eye is restricted to the CMZ (arrow) and in the ventricular region of the neural tube (arrowhead). (D) Adjacent section showing the cyclin D1 mRNA expression for comparison. (C′–D′) Magnification of the embryos in (C) and (D) showing the label in the ventricular zone (arrowheads). (C′′–D′′) Magnification of the embryos in (C) and (D) showing the label in the CMZ (arrows). (E) Magnification of cryostat horizontal section at eyes level of a stage 40 embryo showing that RHAMM transcription is restricted to the CMZ (arrow) close to the lens (L). (F) Adjacent section showing the cyclin D1 mRNA expression for comparison.
hmmr ( hyaluronan-mediated motility receptor (RHAMM)) gene expression in Xenopus laevis embryos, NF stage 6, as assayed by in situ hybridization, animal up.
hmmr (hyaluronan-mediated motility receptor (RHAMM)) gene expression in Xenopus laevis embryos, NF stage 17, as assayed by in situ hybridization, anterior view, dorsal up.
hmmr (hyaluronan-mediated motility receptor (RHAMM)) gene expression in Xenopus laevis embryos, NF stage 26, as assayed by in situ hybridization, lateral view, anteriorright, dorsal up. Section position noted by dotted line.
hmmr (hyaluronan-mediated motility receptor (RHAMM)) gene expression in Xenopus laevis embryos, NF stage 32, as assayed by in situ hybridization, lateral view, anteriorright, dorsal up. Section position noted by dotted line.
