The Biomechanical Basis of Biased Epithelial Tube Elongation in Lung and Kidney Development

Abstract

21During lung development, epithelial branches expand preferentially in longitudinal direction. 22This bias in outgrowth has been linked to a bias in cell shape and in the cell division plane. How 23 such bias arises is unknown. Here, we show that biased epithelial outgrowth occurs independent 24 of the surrounding mesenchyme. Biased outgrowth is also not the consequence of a growth 25 factor gradient, as biased outgrowth is obtained with uniform growth factor cultures, and in the 26 presence of the FGFR inhibitor SU5402. Furthermore, we note that epithelial tubes are largely 27 closed during early lung and kidney development. By simulating the reported fluid flow inside 28 segmented narrow epithelial tubes, we show that the shear stress levels on the apical surface are 29 sufficient to explain the reported bias in cell shape and outgrowth. We use a cell-based vertex 30 model to confirm that apical shear forces, unlike constricting forces, can give rise to both the 31 observed bias in cell shapes and tube elongation. We conclude that shear stress may be a more 32 general driver of biased tube elongation beyond its established role in angiogenesis. 65al., 2013), which contradicts a need for a chemoattractant gradient. In both cases, a Turing mechanism 66 may lead to the emergence of focused FGF10-FGFR2b and GDNF-RET signalling at the branch points,

Organ-Specific Branching Morphogenesis

“…The identification of mesenchymal-derived morphogens required for branching morphogenesis of the lung and the kidney epithelium has made it possible to culture branching isolated epithelia embedded in extracellular matrix (ECM) gels. While requiring the appropriate growth factors, this has demonstrated an intrinsic ability of the epithelium to form new branches (Nogawa and Ito, 1995;Bellusci et al, 1997;Qiao et al, 1999;Varner et al, 2015;Conrad et al, 2021). However, the branching pattern in these isolated cultures appears very different from the in vivo tree as the formation of new branches eventually stops, highlighting the importance of the mesenchyme for the modulation of the branching pattern and the maintenance of cellular niches.…”

“…In lungs and kidneys, FGF10 and GDNF signaling is restricted to branch tips and necessary for branch formation (Moore et al, 1996;Pichel et al, 1996;Sánchez et al, 1996;Min et al, 1998;Sekine et al, 1999;Rozen et al, 2009;Michos et al, 2010;Conrad et al, 2021). However, localized signaling is not required for biased elongation since pharmacological inhibition of FGFR signaling as well as inactivation of Fgf10 or Fgfr2 does not abrogate branch elongation in embryonic lungs (Abler et al, 2009;Conrad et al, 2021).…”

“…In lungs and kidneys, FGF10 and GDNF signaling is restricted to branch tips and necessary for branch formation (Moore et al, 1996;Pichel et al, 1996;Sánchez et al, 1996;Min et al, 1998;Sekine et al, 1999;Rozen et al, 2009;Michos et al, 2010;Conrad et al, 2021). However, localized signaling is not required for biased elongation since pharmacological inhibition of FGFR signaling as well as inactivation of Fgf10 or Fgfr2 does not abrogate branch elongation in embryonic lungs (Abler et al, 2009;Conrad et al, 2021). Also, in the kidney, branch initiation and elongation seem to be regulated separately since trunks have been observed to transiently elongate without the contribution of tip-derived cells (Shakya et al, 2005b), and deletion of Mek1/2 or pharmacological inhibition of MEK1 abrogates budding but not elongation (Fisher et al, 2001;Watanabe and Costantini, 2004;Ihermann-Hella et al, 2014).…”

“…At the same time, treatment of embryonic kidney cultures with TGF-β1 results in thicker UB stalks (Bush et al, 2004). Interestingly, inhibition of FGFR signaling or the PI3K pathway in embryonic lung cultures as well as inactivation of Fgfr2 in the kidney leads to reduced branch widths, whereas the elongation bias of the branches is not affected (Zhao et al, 2004;Wang et al, 2005;Sims-Lucas et al, 2009;Conrad et al, 2021).…”

“…Therefore, branch morphology in the lung and kidney cannot be controlled by SM. As lung and kidney epithelial branches elongate even in the absence of mesenchyme, both a force or a chemical source from the mesenchyme can be ruled out as general drivers of biased branch elongation (Nogawa and Ito, 1995;Qiao et al, 1999;Varner et al, 2015;Conrad et al, 2021). Notably, UB branches elongate and thin less in the absence of mesenchyme, suggesting that the mesenchyme affects but is not required for biased elongation (Conrad et al, 2021).…”