The release of surface-anchored α-tectorin, an apical extracellular matrix protein, mediates tectorial membrane organization

Abstract

The tectorial membrane (TM) is an apical extracellular matrix (ECM) that hovers over the cochlear sensory epithelium and plays an essential role in auditory transduction. The TM forms facing the luminal endolymph-filled space and exhibits complex ultrastructure. Contrary to the current extracellular assembly model, which posits that secreted collagen fibrils and ECM components self-arrange in the extracellular space, we show that surface tethering of α-tectorin (TECTA) via a glycosylphosphatidylinositol anchor is essential to prevent diffusion of secreted TM components. In the absence of surface-tethered TECTA, collagen fibrils aggregate randomly and fail to recruit TM glycoproteins. Conversely, conversion of TECTA into a transmembrane form results in a layer of collagens on the epithelial surface that fails to form a multilayered structure. We propose a three-dimensional printing model for TM morphogenesis: A new layer of ECM is printed on the cell surface concomitant with the release of a preestablished layer to generate the multilayered TM.

The cryo-EM structure of the human uromodulin filament core reveals a unique assembly mechanism

“…Recently, Kim and colleagues proposed the "3D printing model" for surface-tethered, TECTA-mediated ECM organization (Kim et al, 2019), which is in agreement with our proposed model of UMOD polymerization at the surface of TAL cells in the kidney tubule.…”

“…For instance, another human ZP protein, alpha-tectorin (TECTA), forms filaments creating the basis for the apical extracellular matrix (ECM) on the cochlear supporting cells and shares important features with UMOD: a highly similar linker region and a conserved, C-terminal protease cleavage site ( Figure 4—figure supplement 2 ). Recently, Kim and colleagues proposed the ‘3D printing model’ for surface-tethered, TECTA-mediated ECM organization ( Kim et al, 2019 ), which is in agreement with our proposed model of UMOD polymerization at the surface of TAL cells in the kidney tubule. Thus, the cryo-EM structure of the UMOD filament core might be representative for the core structure of multiple proteins with a C-terminal ZP module that become functional after polymerization.…”

“…GP2 has been shown to convert from a longer, membrane-bound glycoprotein to a soluble glycoprotein with lower apparent molecular mass ( Havinga et al, 1985 ), and from its similarity with UMOD we predict it also forms homopolymers of a similar type. In the extracellular matrix (ECM) that covers the cochlear epithelium, alpha-tectorin (TECTA) builds up via its ZP domains and must be tethered to the cell surface for the ECM to form correctly ( Kim et al, 2019 ). In contrast, the membrane glycoprotein endoglin (EGLN) also contains a ZP module but is known to not form fibers.…”

Cryo-EM Structure of Native Human Uromodulin, a Zona Pellucida Module Polymer

“…By postulating that the protofilament remains membrane anchored, it also rationalizes the observation that activated UMOD subunits do not incorporate into polymers growing on the surface of nearby cells ( Figure S5G). Importantly, this provides a solution to the physical problem of assembling the long UMOD polymer in an extracellular environment that is constantly subjected to high flow; clarifies why the growing mammalian ZP thickens from the inside, a process that also depends on membrane anchoring of ZP2 and ZP3 (Jovine et al, 2002;Qi et al, 2002) ; and is compatible with the recent hypothesis that membrane tethering of ZP module protein α-tectorin is essential for generating layers of extracellular matrix whose progressive release generates the tectorial membrane (Kim et al, 2019a) .…”

Computational and experimental characterization of the novel ECM glycoprotein SNED1 and prediction of its interactome

“…An interesting feature of SNED1 is the presence of a NIDO domain (SMART: SM00539) in its Nterminal region (amino acids 103-260). This domain is only found in 4 other human or rodent proteins in addition to SNED1: the basement membrane proteins nidogen-1 and nidogen-2 (24,25), mucin-4, and alpha-tectorin, a component of the tectorial membrane, the apical ECM of the inner ear (26)(27)(28). Identity between the NIDO domain of human SNED1 and that of other vertebrate SNED1 orthologues ranges between 73% and 92% (19).…”