Three-dimensional (3D) culture systems recapitulate interactions between cells and their surrounding extracellular matrix (ECM) in native tissues better than their two-dimensional (2D) counterparts1,2. Advancements in tissue engineering have yielded sophisticated, 3D culture platforms that enable controlled investigations into 1) how chemical and physical components of the matrix microenvironment affect cell behaviors and 2) efficacy of new therapeutic strategies for a number of diseases, including cancers2. While in vitro models cannot account for systemic factors, such as endocrine and immune signals, and thus cannot completely replace in vivo models, they provide several advantages including reproducibility, experimental control, affordability and speed. Here, we describe the use of brain-mimetic hydrogels in which 3D cultures of patient-derived brain tumor cells capture many aspects of tumor physiology, in particular, the dynamics of acquiring treatment resistance3. Compared to other in vitro methods, these cultures better represent in vivo tumor models and clinical observations3.
Glioblastoma (GBM) is the most frequent and lethal cancer originating in the brain, with a median survival of only 1-2 years4,5. In recent years, many studies have focused on the influence of tumor matrix environment in GBM6,7,8. The unique brain ECM has been reported to affect GBM cell migration, proliferation, and therapeutic resistance6,7,8,9,10,11,12. Hyaluronic acid (HA) is an abundant glycosaminoglycan (GAG) in the brain, where it interacts with other GAGs and proteoglycans to form a hydrogel-like mesh13. Many studies have reported HA overexpression in GBM tumors and its subsequent effects on cancer progression8,9,13,14,15,16,17. Other ECM components also affect GBM tumor growth and invasion6,7,15,18. For example, fibronectin and vitronectin, which are typically overexpressed in GBM, induce heterodimerization of cell surface integrin receptors through binding to the "RGD" sequence and initiate complex signaling cascades that promote tumor survival19,20,21. Besides biochemical influences, physical properties of the tissue matrix also affect GBM progression22,23.
Continual acquisition of resistance to therapies is one of the main drivers of GBM lethality4. Drugs showing promising results in 2D or gliomasphere models have failed in subsequent animal studies and clinical cases3, indicating that the effects of microenvironmental factors significantly contributed to GBM tumor response1. While animal models can provide a 3D, physiologically appropriate microenvironment to xenografted patient cells and generate clinically relevant outcomes24,25, the complexity of the brain microenvironment in vivo makes it challenging to determine which features, including cell-matrix interactions, are key to specific biological outcomes. Identification of new therapeutic targets will benefit from the use of simplified culture platforms in which biochemical and biophysical properties are defined.
Unlike previously reported biomaterial models of the GBM tumor microenvironment26,27 which have not achieved true orthogonal control over individual biochemical and physical features of the ECM, the biomaterial platform reported here enables decoupling of the contributions of multiple independent features to GBM cell phenotype. Here, we present an HA-based, orthogonally tunable, hydrogel system for 3D culture of patient-derived GBM cells. Hydrogels are formed from two polymer components: 1) biologically active HA and 2) biologically inert polyethylene glycol (PEG). PEG is a widely used biocompatible and hydrophilic material with low protein adsorption and minimal immunogenicity28. Here, approximately 5% of glucuronic acid moieties on HA chains are functionalized with thiol groups to enable crosslinking to a commercially available 4-arm-PEG terminated with maleimides via Michael-type addition. In its most common form in the body, HA exists in high molecular weight (HMW) chains. Here, a low degree of modification of HMW HA (500-750 kDa) helps to preserve native interactions of HA and its cell receptors, including CD4429. By substituting PEG-thiol for HA-thiol while maintaining a constant molar ratio of total thiols to maleimides, HA concentration can be decoupled from mechanical properties of the resulting hydrogels. Furthermore, stoichiometric controls can be used to conjugate cysteine-terminated peptides to a defined average number of maleimide-terminated arms on each 4-arm-PEG. Incorporation of ECM-derived, adhesive peptides enables interactions with integrins on cultured cells, through which biochemical and chemical signals are transduced1. Maleimide-thiol addition occurs very quickly under physiological conditions, minimizing the time required for cell encapsulation and maximizing survival of patient-derived cells. Moreover, hydrogel cultures can be treated like typical tissue specimen and are compatible with standard characterization techniques including Western blotting, flow cytometry, and immunofluorescence staining. The following protocol describes the procedures for fabricating hydrogels, establishing 3D cultures of patient-derived GBM cells and techniques for biochemical analysis.