A Luminal Glycoprotein Drives Dose-Dependent Diameter Expansion of the Drosophila melanogaster Hindgut Tube

Abstract

An important step in epithelial organ development is size maturation of the organ lumen to attain correct dimensions. Here we show that the regulated expression of Tenectin (Tnc) is critical to shape the Drosophila melanogaster hindgut tube. Tnc is a secreted protein that fills the embryonic hindgut lumen during tube diameter expansion. Inside the lumen, Tnc contributes to detectable O-Glycans and forms a dense striated matrix. Loss of tnc causes a narrow hindgut tube, while Tnc over-expression drives tube dilation in a dose-dependent manner. Cellular analyses show that luminal accumulation of Tnc causes an increase in inner and outer tube diameter, and cell flattening within the tube wall, similar to the effects of a hydrostatic pressure in other systems. When Tnc expression is induced only in cells at one side of the tube wall, Tnc fills the lumen and equally affects all cells at the lumen perimeter, arguing that Tnc acts non-cell-autonomously. Moreover, when Tnc expression is directed to a segment of a tube, its luminal accumulation is restricted to this segment and affects the surrounding cells to promote a corresponding local diameter expansion. These findings suggest that deposition of Tnc into the lumen might contribute to expansion of the lumen volume, and thereby to stretching of the tube wall. Consistent with such an idea, ectopic expression of Tnc in different developing epithelial tubes is sufficient to cause dilation, while epidermal Tnc expression has no effect on morphology. Together, the results show that epithelial tube diameter can be modelled by regulating the levels and pattern of expression of a single luminal glycoprotein.

Cftr controls lumen expansion and function of Kupffer’s vesicle in zebrafish

“…In the absence of Cftr activity, the KV lumen fails to inflate, indicating that fluid acts as a force driving lumen expansion and might also promote lumen coalescence. The zebrafish gut undergoes a similar process of de novo lumen formation, beginning with inflation of multiple small lumens, followed by their coalescence into one (Bagnat et al, 2007) and a similar role in de novo lumen formation and expansion has been shown for apical membrane or secreted mucins during tubulogenesis in the vertebrate vasculature (Strilić et al, 2010) and in the ommatidium (Husain et al, 2006) and hindgut (Syed et al, 2012) in Drosophila. The role of luminal content as a driving force during tube formation is most clearly exemplified by the Drosophila tracheal system, which becomes filled with a solid chitin matrix, then liquid and finally gas during its development (Tsarouhas et al, 2007).…”

Luminal matrices: An inside view on organ morphogenesis

“…2B) [27,29]. In tnc mutants, the hindgut tube diameter remains narrow, while overexpression of Tnc leads to diametric overexpansion in a dose-dependent fashion (Fig.…”

“…Thus, a simple extrapolation of the findings from the Drosophila trachea is not possible based on molecular homologues. However, it was recently shown that the large luminal glycoprotein Tenectin (Tnc) forms a luminal matrix that acts to drive tube diameter expansion in the developing Drosophila hindgut [27]. The Tnc matrix spans the lumen and causes dose-dependent expansion, suggesting that it generates a luminal physical force during tube growth.…”

“…1B). At the cellular level, Tnc promotes an increase in apical cell surface, oriented cell elongation along the tube perimeter and cell intercalation to yield more cells along the lumen perimeter [27]. These effects are similar to those caused by hydrostatic pressure during inflation of the zebrafish brain ventricle [14], expansion of the mouse blastocyst [30], and growth of renal cysts in vitro [31,32].…”

“…Moreover, at sites of Tnc secretion, the protein forms a lumen-spanning complex with low mobility, causing local dilation. It therefore seems that Tnc can cause regional effects on diameter expansion dependent on its level of expression along the tube axis [27].…”