Conditional expression of fascin increases tumor progression in a mouse model of intestinal cancer

While absent from normal epithelia, an actin bundling protein, fascin, becomes expressed in invasive carcinoma of different origins. It is highly enriched at the tumors' invasive front suggesting that it could play a role in cancer invasion. Multiple studies have shown that fascin, through its role in formation of cellular protrusions such as filopodia and invadopodia, enhances cancer cell migration and invasion in vitro. However, the role of fascin in vivo remains unknown. We have generated a compound transgenic mouse model that allows expression of fascin in the intestinal epithelium in the Apc-mutated background. Conditional expression of fascin led to decrease in mice survival and increase in tumor burden compared to control animals. Induction of fascin expression in adult tumor-bearing animals accelerated tumor progression and led to formation of invasive adenocarcinoma. Altogether, our study shows that fascin can promote tumor progression in vivo, but also unravels an unexpected role of fascin in tumor initiation.

A new role for the architecture of microvillar actin bundles in apical retention of membrane proteins

The bundled architecture of actin filaments is not needed for intestinal microvillar morphogenesis, as shown in knockout mice devoid of microvillar actin-bundling proteins. This architecture is essential for the apical anchorage of digestive proteins, probably via the recruitment of key players in apical retention, such as myosin-1a, and, as a result, for intestinal physiology.

Actin-bundling proteins are identified as key players in the morphogenesis of thin membrane protrusions. Until now, functional redundancy among the actin-bundling proteins villin, espin, and plastin-1 has prevented definitive conclusions regarding their role in intestinal microvilli. We report that triple knockout mice lacking these microvillar actin-bundling proteins suffer from growth delay but surprisingly still develop microvilli. However, the microvillar actin filaments are sparse and lack the characteristic organization of bundles. This correlates with a highly inefficient apical retention of enzymes and transporters that accumulate in subapical endocytic compartments. Myosin-1a, a motor involved in the anchorage of membrane proteins in microvilli, is also mislocalized. These findings illustrate, in vivo, a precise role for local actin filament architecture in the stabilization of apical cargoes into microvilli. Hence, the function of actin-bundling proteins is not to enable microvillar protrusion, as has been assumed, but to confer the appropriate actin organization for the apical retention of proteins essential for normal intestinal physiology.