Sequential expression and differential localization of I-, L-, and T-fimbrin during differentiation of the mouse intestine and yolk sac

During the differentiation of the intestine epithelium, three cytoskeletal proteins, villin, fimbrin, and myosin I, are sequentially expressed and localized to the apical membrane. Recently, we found that in the adult mouse and human, three fimbrin isoforms are expressed in a cell specific manner. I-fimbrin is expressed by intestine and kidney epithelial cells, L-fimbrin is expressed by leukocytes and many tumors, while T-fimbrin is expressed by various cells and tissues. Because non-intestinal isoforms of fimbrin could be expressed early in development, the expression of fimbrin isoforms during days 10.5 to 16.5 of intestine development was investigated. By immunofluorescence microscopy, T-fimbrin was detected in the early stages of intestinal epithelial cell differentiation until day 14.5 and was localized predominantly at the apical surface. L-fimbrin was also detected during this period but it was localized to the basal surface of the epithelium instead of the apical surface. By day 16.5 no L or T-fimbrin was detected in the epithelium. I-fimbrin was first detected at day 14.5 and a brush border-like apical localization pattern was seen by day 16.5. Unlike the intestinal cells, the visceral endoderm expressed I, L, and T-fimbrin throughout the period examined, with the level of I-fimbrin increasing as time progresses. L-fimbrin was more evident at the earlier stage than at the later stage of the development. Collectively, these results suggest that three fimbrin isoforms play different roles during epithelial cell differentiation. T- and I-fimbrin expression could be critical for the formation and extension of the microvilli whereas L-fimbrin may play a role in controlling cell adhesion.

Expression and localization of villin, fimbrin, and myosin I in differentiating mouse F9 teratocarcinoma cells

F9 embryonic carcinoma cells are a multipotent cell line which can be induced to differentiate into cells resembling the visceral endoderm, an extraembryonic absorptive epithelium characterized by apical microvilli. We have examined the role of villin, fimbrin, and myosin I, the major actin-binding proteins in the intestinal and visceral yolk sac microvilli, in the development of epithelial polarity and the assembly of the microvillus cytoskeleton in differentiating F9 cells. By immunoblot analysis villin was first detected at 4 days of differentiation. Confocal microscopy localized villin at Day 4 to the apical surface and by Day 6 to the basolateral surfaces as well. In comparison, fimbrin and myosin I were both present in undifferentiated F9 cells and became associated with the apical surface after villin during differentiation to visceral endoderm. The accumulation of villin, fimbrin, and myosin I at the apical surface in differentiating F9 cells correlated with the appearance of microvilli containing organized actin filament bundles. Two mouse villin cDNAs were isolated and characterized to examine villin expression during F9 differentiation. Mouse villin was encoded by two transcripts (3.8 and 3.4 kb) which differ in their 3'-noncoding region. Both villin mRNAs were first detected by Day 4 of differentiation and their appearance coincided with expression of the visceral endoderm marker alpha-fetoprotein. The pattern of expression and order of accumulation of villin, fimbrin, and myosin I in differentiating F9 cells are common to developing gut and yolk sac epithelium. This suggests that microvillus assembly is directed by a sequence of temporally and spatially regulated localizations of these actin-binding proteins.

Differential localization of villin and fimbrin during development of the mouse visceral endoderm and intestinal epithelium

The apical surface of transporting epithelia is specially modified to absorb nutrients efficiently by amplifying its surface area as microvilli. Each microvillus is supported by an underlying core of bundled actin filaments. Villin and fimbrin are two actin-binding proteins that bundle actin filaments in the intestine and kidney brush border epithelium. To better understand their function in the assembly of the cytoskeleton during epithelial differentiation, we examined the pattern of villin and fimbrin expression in the developing mouse using immunofluorescence and immunoelectron microscopy. Villin is first detected at day 5 in the primitive endoderm of the postimplantation embryo and is later restricted to the visceral endoderm. By day 8.5, villin becomes redistributed to the apical surface in the visceral endoderm, appearing in the gut at day 10 and concentrating in the apical cytoplasm of the differentiating intestinal epithelium 2–3 days later. In contrast, fimbrin is found in the oocyte and in all tissues of the early embryo. In both the visceral endoderm and gut epithelium, fimbrin concentrates at the apical surface 2–3 days after villin; this redistribution occurs when the visceral endoderm microvilli first contain organized microfilament bundles and when microvilli first begin to appear in the gut. These results suggest a common mechanism of assembly of the absorptive surface of two different tissues in the embryo and identify villin as a useful marker for the visceral endoderm.