Constructing cellular niche properties by localized presentation of Wnt proteins on synthetic surfaces.

Wnt signaling is crucial during embryonic development and for the maintenance of adult tissues. Depending on the tissue type, the Wnt pathway can promote stem cell self-renewal and/or direct lineage commitment. Wnt proteins are subject to lipid modification, often restricting them to act in a localized manner on responsive cells. Most methods for inducing Wnt signaling in stem cell cultures do not control the spatial presentation of the protein. To recreate the local presentation of Wnt proteins often seen in vivo, we previously developed a method to immobilize the protein onto synthetic surfaces. Here we describe a detailed protocol based on covalent binding of nucleophilic groups on Wnt proteins to activated carboxylic acid (COOH) or glutaraldehyde (COH) groups functionalized on synthetic surfaces. As an example, we describe how this method can be used to covalently immobilize Wnt3a proteins on microbeads or a glass surface. This procedure requires ∼3 h and allows for the hydrophobic protein to be stored in the absence of detergent. The immobilization efficiency of active Wnt proteins can be assessed using different T-cell factor (TCF) reporter assays as a readout for Wnt/β-catenin-dependent transcription. Immobilization efficiency can be measured 12-18 h after seeding the cells and takes 2-4 h. The covalent immobilization of Wnt proteins can also be used for single-cell analysis using Wnt-coated microbeads (12-18 h of live imaging) and to create a Wnt platform on a glass surface for stem cell maintenance and cell population analysis (3 d). The simple chemistry used for Wnt immobilization allows for adaptation to new materials and other developmental signals. Therefore, this method can also be incorporated into tissue engineering platforms in which depletion of the stem cell pool restricts the complexity and maturity of the tissue developed.