Among several small Rho GTPases observed in the chick retina, RhoB was transiently expressed during development and mainly present in glial Müller cells in the adult. The aim of this study was to compare the distribution of RhoB in the chick and mouse adult retinas and to study its potential role in the maintenance of cell morphology. The distribution of RhoB was studied in situ and pure Müller cell cultures were submitted to Clostridium difficile toxin A and lysophosphatidic acid (LPA) treatment in order to inactivate and activate Rho proteins, respectively. Cell morphology, F-actin arrangement, RhoB, and vimentin distribution were studied by immunofluorescence and confocal microscopy. The results showed that, in both species, all vimentin-containing cells also expressed RhoB in situ and in vitro. Toxin A promoted cell rounding and detachment due to actin depolymerization, changing the distribution of RhoB only in chick cells. In serum-starved cells LPA stimulated actin polymerization and cell spreading, but only in chick cells was RhoB distribution recruited to expanding cellular processes and newly formed focal adhesions. These data suggest that, although RhoB is expressed by Müller cells in chick and mouse, its role in the maintenance of cellular morphology and regulation may be different. In addition, we show that RhoB may be an interesting Müller cell marker in the adult retina.

Immunohistochemistry was used to determine the distribution of Rac1, Cdc42, RhoA and RhoB GTPases during development of the chick retina. All proteins appear as early as embryonic day 5 (E5) in cells of the vitreal margin, E7-8 in cells of the inner third of the inner nuclear layer and E9-10 in photoreceptors. From E10 until hatching, RhoA, Rac1 and Cdc42 were seen in perikarya and/or processes of amacrine, ganglion cells, and photoreceptors. Rho proteins were also observed in retinal Müller cells, with different distributions. RhoB showed a transient expression, being severely down regulated after E18. The distribution pattern of Rho proteins during the development of the chick retina suggests a concerted role in the differentiation of specific cell types, and probably during synaptogenesis.

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