Cdc42- and IRSp53-dependent contractile filopodia tether presumptive lens and retina to coordinate epithelial invagination

Abstract

The vertebrate lens provides an excellent model with which to study the mechanisms required for epithelial invagination. In the mouse, the lens forms from the head surface ectoderm. A domain of ectoderm first thickens to form the lens placode and then invaginates to form the lens pit. The epithelium of the lens placode remains in close apposition to the epithelium of the presumptive retina as these structures undergo a coordinated invagination. Here, we show that F-actin-rich basal filopodia that link adjacent presumptive lens and retinal epithelia function as physical tethers that coordinate invagination. The filopodia, most of which originate in the presumptive lens, form at E9.5 when presumptive lens and retinal epithelia first come into close contact, and have retracted by E11.5 when invagination is complete. At E10.5 -the lens pit stage -there is approximately one filopodium per epithelial cell. Formation of filopodia is dependent on the Rho family GTPase Cdc42 and the Cdc42 effector IRSp53 (Baiap2). Loss of filopodia results in reduced lens pit invagination. Pharmacological manipulation of the actin-myosin contraction pathway showed that the filopodia can respond rapidly in length to change inter-epithelial distance. These data suggest that the lens-retina interepithelial filopodia are a fine-tuning mechanism to assist in lens pit invagination by transmitting the forces between presumptive lens and retina. Although invagination of the archenteron in sea urchins and dorsal closure in Drosophila are known to be partly dependent on filopodia, this mechanism of morphogenesis has not previously been identified in vertebrates.

In vitro organogenesis in three dimensions: self-organising stem cells

“…In addition to these biochemical interactions, mechanical links may play a role in the coordinated eye development. Another recent report has shown an intriguing connection between lens and neural retinal epithelium, which invaginate at the same time (Chauhan et al, 2009). Close observations of this process reveal that there is a gap between the basement membranes of the two tissues, refuting the idea of mechanical pushing between them.…”

Apical constriction: themes and variations on a cellular mechanism driving morphogenesis

“…A different type of constraint may be at work in the vertebrate eye, where two epithelia -the neural retina and the overlying lens placode -invaginate in close apposition to form a cup. In this case, thin actin-myosin-containing cellular protrusions connect the two epithelia during invagination, and disruption of these protrusions results in a reduction in the depth of lens cell invagination (Chauhan et al, 2009). In this case, the out-of-plane bending of one tissue could be mechanically coupled to that of another.…”

Cdc42 is required for cytoskeletal support of endothelial cell adhesion during blood vessel formation

“…Because filopodia are thought to be important for angiogenesis (Gerhardt et al, 2003) and Cdc42 has been shown to be necessary for filopodial development (Chauhan et al, 2009;Mattila and Lappalainen, 2008), we assessed sprouting EC filopodia in Cdc42…”