Septate-Junction-Dependent Luminal Deposition of Chitin Deacetylases Restricts Tube Elongation in the Drosophila Trachea

Abstract

The function of tubular epithelial organs like the kidney and lung is critically dependent on the length and diameter of their constituting branches. Genetic analysis of tube size control during Drosophila tracheal development has revealed that epithelial septate junction (SJ) components and the dynamic chitinous luminal matrix coordinate tube growth. However, the underlying molecular mechanisms controlling tube expansion so far remained elusive. Here, we present the analysis of two luminal chitin binding proteins with predicted polysaccharide deacetylase activities (ChLDs). ChLDs are required to assemble the cable-like extracellular matrix (ECM) and restrict tracheal tube elongation. Overexpression of native, but not of mutated, ChLD versions also interferes with the structural integrity of the intraluminal ECM and causes aberrant tube elongation. Whereas ChLD mutants have normal SJ structure and function, the luminal deposition of the ChLD requires intact cellular SJs. This identifies a new molecular function for SJs in the apical secretion of ChLD and positions ChLD downstream of the SJs in tube length control. The deposition of the chitin luminal matrix first promotes and coordinates radial tube expansion. We propose that the subsequent structural modification of chitin by chitin binding deacetylases selectively instructs the termination of tube elongation to the underlying epithelium.

Tracheal branching morphogenesis inDrosophila: new insights into cell behaviour and organ architecture

“…The matrix might provide either a physical scaffold that defines the shape of the tube cells surrounding it or it could signal to tracheal cells to adjust their shape in a uniform manner. In addition, the specific chemical modification of the luminal chitin matrix via apically secreted chitin deacetylases appears to be crucial for tracheal tubes to obtain the proper length (Luschnig et al, 2006;Wang et al, 2006), in line with the observation that tube length and tube dilation are under separate genetic control (Beitel and Krasnow, 2000). It is thought that one of these putative chitin deacetylases, encoded by a gene called vermiform (verm; LCBP1 -FlyBase), is delivered to the apical lumen via a specialized secretory pathway that requires septate junctions; this is suggested by the observation that Verm is not properly secreted into the luminal space in several mutants of septate junction components, whereas Pio, for example, is normally secreted into the apical luminal space in the same mutants (Luschnig et al, 2006;Wang et al, 2006).…”

“…In addition, the specific chemical modification of the luminal chitin matrix via apically secreted chitin deacetylases appears to be crucial for tracheal tubes to obtain the proper length (Luschnig et al, 2006;Wang et al, 2006), in line with the observation that tube length and tube dilation are under separate genetic control (Beitel and Krasnow, 2000). It is thought that one of these putative chitin deacetylases, encoded by a gene called vermiform (verm; LCBP1 -FlyBase), is delivered to the apical lumen via a specialized secretory pathway that requires septate junctions; this is suggested by the observation that Verm is not properly secreted into the luminal space in several mutants of septate junction components, whereas Pio, for example, is normally secreted into the apical luminal space in the same mutants (Luschnig et al, 2006;Wang et al, 2006). The lack of Verm secretion in many septate junction mutants could indeed account for the tube-lengthexpansion defects that are observed in the absence of several septate junction components (such as Claudin, Megatrachea, Neurexin IV, Gliotactin and Neuroglian) (reviewed in Swanson and Beitel, 2006), but the exact role of septate junction components in secretion remains to be investigated.…”

“…These chitin cables are also required for the normal organization of the apical β H spectrin cytoskeleton (blue). At stage 16 of embryonic development, septate junction proteins are required for the apical secretion of Verm, which, together with Serp, is required for modifying the chitin matrix to prevent the tracheal tubes from becoming too long (Wang et al, 2006). At stage 17 of embryonic development, Wurst is essential for remodeling the chitin matrix and for clearing the tracheal airway.…”

Investigating chitin deacetylation and chitosan hydrolysis during vegetative growth in Magnaporthe oryzae

“…CDAs are broadly conserved, known to occur not only in all fungi (Ruiz‐Herrera & Ortiz‐Castellanos, 2010) but also in insects, where they have a number of developmental roles, including in tracheal development and moulting (Arakane et al . , 2009; Luschnig, Batz, Armbruster, & Krasnow, 2006; Wang, Jayaram, et al, 2006b; Xi, Pan, Ye, Yu, & Zhang, 2014). Considering the relative chemical and physical properties of chitin and chitosan, we hypothesized that chitin deacetylation could also have key roles in fungal development.…”

The Drosophila Claudin Kune-kune Is Required for Septate Junction Organization and Tracheal Tube Size Control

“…Glial cells, for example, ensheath nerve fibers and use SJs to maintain the blood-brain barrier (Auld et al 1995;Baumgartner et al 1996;Schwabe et al 2005). In the embryonic tracheal system, SJs are required for the apical secretion of the lumenal matrix modifying proteins, Vermiform (Verm) and Serpentine (Serp), which act through undefined pathways to restrict tube length (Wang et al 2006). This secretory pathway appears to be specific for Verm and Serp, since other apical proteins are secreted normally in SJ mutants.…”