Spatially gradated hydrogel platform as a 3D engineered tumor microenvironment

Glioblastoma mechanobiology at multiple length scales

Glioblastoma multiforme (GBM), a primary brain cancer, is one of the most aggressive forms of human cancer, with a very low patient survival rate. A characteristic feature of GBM is the diffuse infiltration of tumor cells into the surrounding brain extracellular matrix (ECM) that provide biophysical, topographical, and biochemical cues. In particular, ECM stiffness and composition is known to play a key role in controlling various GBM cell behaviors including proliferation, migration, invasion, as well as the stem-like state and response to chemotherapies. In this review, we discuss the mechanical characteristics of the GBM microenvironment at multiple length scales, and how biomaterial scaffolds such as polymeric hydrogels, and fibers, as well as microfluidic chip-based platforms have been employed as tissue mimetic models to study GBM mechanobiology. We also highlight how such tissue mimetic models can impact the field of GBM mechanobiology.

Applications of hydrogels in tissue-engineered repairing of temporomandibular joint diseases

Epidemiological studies reveal that symptoms of temporomandibular joint disorders (TMDs) occur in 60-70% of adults. The inflammatory damage caused by TMDs can easily lead to defects in the articular disc, condylar cartilage, subchondral bone and muscle of the temporomandibular joint (TMJ) and cause pain. Despite the availability of various methods for treating TMDs, few existing treatment schemes can achieve permanent recovery. This necessity drives the search for new approaches. Hydrogels, polymers with high water content, have found widespread use in tissue engineering and regeneration due to their excellent biocompatibility and mechanical properties, which resemble those of human tissues. In the context of TMD therapy, numerous experiments have demonstrated that hydrogels show favorable effects in aspects such as articular disc repair, cartilage regeneration, muscle repair, pain relief, and drug delivery. This review aims to summarize the application of hydrogels in the therapy of TMDs based on recent research findings. It also highlights deficiencies in current hydrogel research related to TMD therapy and outlines the broad potential of hydrogel applications in treating TMJ diseases in the future.

Engineering Biomaterials to Model Immune-Tumor Interactions In Vitro

Engineered biomaterial scaffolds are becoming more prominent in research laboratories to study drug efficacy for oncological applications in vitro, but do they have a place in pharmaceutical drug screening pipelines? The low efficacy of cancer drugs in phase II/III clinical trials suggests that there are critical mechanisms not properly accounted for in the pre-clinical evaluation of drug candidates. Immune cells associated with the tumor may account for some of these failures given recent successes with cancer immunotherapies; however, there are few representative platforms to study immune cells in the context of cancer as traditional 2D culture is typically monocultures and humanized animal models have a weakened immune composition. Biomaterials that replicate tumor microenvironmental cues may provide a more relevant model with greater in vitro complexity. In this review, the authors explore the pertinent microenvironmental cues that drive tumor progression in the context of the immune system, discuss how these cues can be incorporated into hydrogel design to culture immune cells, and describe progress toward precision oncological drug screening with engineered tissues.

Biomaterial-based in vitro 3D modeling of glioblastoma multiforme

Adult-onset brain cancers, such as glioblastomas, are particularly lethal. People with glioblastoma multiforme (GBM) do not anticipate living for more than 15 months if there is no cure. The results of conventional treatments over the past 20 years have been underwhelming. Tumor aggressiveness, location, and lack of systemic therapies that can penetrate the blood-brain barrier are all contributing factors. For GBM treatments that appear promising in preclinical studies, there is a considerable rate of failure in phase I and II clinical trials. Unfortunately, access becomes impossible due to the intricate architecture of tumors. In vitro, bioengineered cancer models are currently being used by researchers to study disease development, test novel therapies, and advance specialized medications. Many different techniques for creating in vitro systems have arisen over the past few decades due to developments in cellular and tissue engineering. Later-stage research may yield better results if in vitro models that resemble brain tissue and the blood-brain barrier are used. With the use of 3D preclinical models made available by biomaterials, researchers have discovered that it is possible to overcome these limitations. Innovative in vitro models for the treatment of GBM are possible using biomaterials and novel drug carriers. This review discusses the benefits and drawbacks of 3D in vitro glioblastoma modeling systems.

Screening MT1-MMP Activity and Inhibition in Three-Dimensional Tumor Spheroids

Membrane type 1 matrix metalloproteinase (MT1-MMP) has been shown to be crucial for tumor angiogenesis, invasion, and metastasis, and thus MT1-MMP is a high priority target for potential cancer therapies. To properly evaluate MT1-MMP inhibitors, a screening protocol is desired by which enzyme activity can be quantified in a tumor microenvironment-like model system. In the present study, we applied a fluorogenic, collagen model triple-helical substrate to quantify MT1-MMP activity for tumor spheroids embedded in a collagen hydrogel. The substrate was designed to be MT1-MMP selective and to possess fluorescent properties compatible with cell-based assays. The proteolysis of the substrate correlated to glioma spheroid invasion. In turn, the application of either small molecule or protein-based MMP inhibitors reduced proteolytic activity and glioma spheroid invasion. The presence of MT1-MMP in glioma spheroids was confirmed by western blotting. Thus, spheroid invasion was dependent on MT1-MMP activity, and inhibitors of MT1-MMP and invasion could be conveniently screened in a high-throughput format. The combination of the fluorogenic, triple-helical substrate, the three-dimensional tumor spheroids embedded in collagen, and Hit-Pick software resulted in an easily adaptable in vivo-like tumor microenvironment for rapidly processing inhibitor potential for anti-cancer use.

HYDRHA: Hydrogels of hyaluronic acid. New biomedical approaches in cancer, neurodegenerative diseases, and tissue engineering

In the last decade, hyaluronic acid (HA) has attracted an ever-growing interest in the biomedical engineering field as a biocompatible, biodegradable, and chemically versatile molecule. In fact, HA is a major component of the extracellular matrix (ECM) and is essential for the maintenance of cellular homeostasis and crosstalk. Innovative experimental strategies in vitro and in vivo using three-dimensional (3D) HA systems have been increasingly reported in studies of diseases, replacement of tissue and organ damage, repairing wounds, and encapsulating stem cells for tissue regeneration. The present work aims to give an overview and comparison of recent work carried out on HA systems showing advantages, limitations, and their complementarity, for a comprehensive characterization of their use. A special attention is paid to the use of HA in three important areas: cancer, diseases of the central nervous system (CNS), and tissue regeneration, discussing the most innovative experimental strategies. Finally, perspectives within and beyond these research fields are discussed.

Growing Pains: The Need for Engineered Platforms to Study Growth Plate Biology

Growth plates, or physis, are highly specialized cartilage tissues responsible for longitudinal bone growth in children and adolescents. Chondrocytes that reside in growth plates are organized into three distinct zones essential for proper function. Modeling key features of growth plates may provide an avenue to develop advanced tissue engineering strategies and perspectives for cartilage and bone regenerative medicine applications and a platform to study processes linked to disease progression. In this review, a brief introduction of the growth plates and their role in skeletal development is first provided. Injuries and diseases of the growth plates as well as physiological and pathological mechanisms associated with remodeling and disease progression are discussed. Growth plate biology, namely, its architecture and extracellular matrix organization, resident cell types, and growth factor signaling are then focused. Next, opportunities and challenges for developing 3D biomaterial models to study aspects of growth plate biology and disease in vitro are discussed. Finally, opportunities for increasingly sophisticated in vitro biomaterial models of the growth plate to study spatiotemporal aspects of growth plate remodeling, to investigate multicellular signaling underlying growth plate biology, and to develop platforms that address key roadblocks to in vivo musculoskeletal tissue engineering applications are described.

Pushing the Natural Frontier: Progress on the Integration of Biomaterial Cues toward Combinatorial Biofabrication and Tissue Engineering

The engineering of fully functional, biological-like tissues requires biomaterials to direct cellular events to a near-native, 3D niche extent. Natural biomaterials are generally seen as a safe option for cell support, but their biocompatibility and biodegradability can be just as limited as their bioactive/biomimetic performance. Furthermore, integrating different biomaterial cues and their final impact on cellular behavior is a complex equation where the outcome might be very different from the sum of individual parts. This review critically analyses recent progress on biomaterial-induced cellular responses, from simple adhesion to more complex stem cell differentiation, looking at the ever-growing possibilities of natural materials modification. Starting with a discussion on native material formulation and the inclusion of cell-instructive cues, the roles of shape and mechanical stimuli, the susceptibility to cellular remodeling, and the often-overlooked impact of cellular density and cell-cell interactions within constructs, are delved into. Along the way, synergistic and antagonistic combinations reported in vitro and in vivo are singled out, identifying needs and current lessons on the development of natural biomaterial libraries to solve the cell-material puzzle efficiently. This review brings together knowledge from different fields envisioning next-generation, combinatorial biomaterial development toward complex tissue engineering.

Investigating the Interactions of Glioma Stem Cells in the Perivascular Niche at Single-Cell Resolution using a Microfluidic Tumor Microenvironment Model

The perivascular niche (PVN) is a glioblastoma tumor microenvironment (TME) that serves as a safe haven for glioma stem cells (GSCs), and acts as a reservoir that inevitably leads to tumor recurrence. Understanding cellular interactions in the PVN that drive GSC treatment resistance and stemness is crucial to develop lasting therapies for glioblastoma. The limitations of in vivo models and in vitro assays have led to critical knowledge gaps regarding the influence of various cell types in the PVN on GSCs behavior. This study developed an organotypic triculture microfluidic model as a means to recapitulate the PVN and study its impact on GSCs. This triculture platform, comprised of endothelial cells (ECs), astrocytes, and GSCs, is used to investigate GSC invasion, proliferation and stemness. Both ECs and astrocytes significantly increased invasiveness of GSCs. This study futher identified 15 ligand-receptor pairs using single-cell RNAseq with putative chemotactic mechanisms of GSCs, where the receptor is up-regulated in GSCs and the diffusible ligand is expressed in either astrocytes or ECs. Notably, the ligand-receptor pair SAA1-FPR1 is demonstrated to be involved in chemotactic invasion of GSCs toward PVN. The novel triculture platform presented herein can be used for therapeutic development and discovery of molecular mechanisms driving GSC biology.

Molecular and Colloidal Transport in Bacterial Cellulose Hydrogels

Bacterial cellulose biofilms are complex networks of strong interwoven nanofibers that control transport and protect bacterial colonies in the film. The design of diverse applications of these bacterial cellulose films also relies on understanding and controlling transport through the fiber mesh, and transport simulations of the films are most accurate when guided by experimental characterization of the structures and the resultant diffusion inside. Diffusion through such films is a function of their key microstructural length scales, determining how molecules, as well as particles and microorganisms, permeate them. We use microscopy to study the unique bacterial cellulose film via its pore structure and quantify the mobility dynamics of various sizes of tracer particles and macromolecules. Mobility is hindered within the films, as confinement and local movement strongly depend on the void size relative to diffusing tracers. The biofilms have a naturally periodic structure of alternating dense and porous layers of nanofiber mesh, and we tune the magnitude of the spacing via fermentation conditions. Micron-sized particles can diffuse through the porous layers but cannot penetrate the dense layers. Tracer mobility in the porous layers is isotropic, indicating a largely random pore structure there. Molecular diffusion through the whole film is only slightly reduced by the structural tortuosity. Knowledge of transport variations within bacterial cellulose networks can be used to guide the design of symbiotic cultures in these structures and enhance their use in applications like biomedical implants, wound dressings, lab-grown meat, clothing textiles, and sensors.

Programming hydrogels to probe spatiotemporal cell biology

The recapitulation of complex microenvironments that regulate cell behavior during development, disease, and wound healing is key to understanding fundamental biological processes. In vitro, multicellular morphogenesis, organoid maturation, and disease modeling have traditionally been studied using either non-physiological 2D substrates or 3D biological matrices, neither of which replicate the spatiotemporal biochemical and biophysical complexity of biology. Here, we provide a guided overview of the recent advances in the programming of synthetic hydrogels that offer precise control over the spatiotemporal properties within cellular microenvironments, such as advances in the control of cell-driven remodeling, bioprinting, or user-defined manipulation of properties (e.g., via light irradiation).

Matrix Hyaluronic Acid and Hypoxia Influence a CD133+ Subset of Patient-Derived Glioblastoma Cells

Glioblastoma (GBM) displays diffusive invasion throughout the brain microenvironment, which is partially responsible for its short median survival rate (<15 months). Stem-like subpopulations (GBM stem-like cells, GSCs) are believed to play a central role in therapeutic resistance and poor patient prognosis. Given the extensive tissue remodeling and processes such as vessel co-option and regression that occur in the tumor microenvironment, it is essential to understand the role of metabolic constraint such as hypoxia on GBM cell populations. This work describes the use of a multidimensional gelatin hydrogel to culture patient-derived GBM cells, to evaluate the influence of hypoxia and the inclusion brain-mimetic hyaluronic acid on the relative activity of GSCs versus overall GBM cells. Notably, CD133+ GBM cell fraction is crucial for robust formation of tumor spheroids in multidimensional cultures. In addition, while the relative size of the CD133+ GBM subpopulation increased in response to both hypoxia and matrix-bound hyaluronan, we did not observe cell subtype-specific changes in invasion signaling pathway activation. Taken together, this study highlights the potential of biomimetic culture systems for resolving changes in the population dynamics and behavior of subsets of GBM specimens for the future development of precision medicine applications. Impact Statement This study describes a gelatin hydrogel platform to investigate the role of extracellular hyaluronic acid and hypoxia on the behavior of a CD133+ subset of cells within patient-derived glioblastoma (GBM) specimens. We report that the relative expansion of the CD133+ GBM stem cell-like population is strongly responsive to extracellular cues, highlighting the significance of biomimetic hydrogel models of the tumor microenvironment to investigate invasion and therapeutic response.

Mechanical Properties in the Glioma Microenvironment: Emerging Insights and Theranostic Opportunities

Gliomas represent the most common malignant primary brain tumors, and a high-grade subset of these tumors including glioblastoma are particularly refractory to current standard-of-care therapies including maximal surgical resection and chemoradiation. The prognosis of patients with these tumors continues to be poor with existing treatments and understanding treatment failure is required. The dynamic interplay between the tumor and its microenvironment has been increasingly recognized as a key mechanism by which cellular adaptation, tumor heterogeneity, and treatment resistance develops. Beyond ongoing lines of investigation into the peritumoral cellular milieu and microenvironmental architecture, recent studies have identified the growing role of mechanical properties of the microenvironment. Elucidating the impact of these biophysical factors on disease heterogeneity is crucial for designing durable therapies and may offer novel approaches for intervention and disease monitoring. Specifically, pharmacologic targeting of mechanical signal transduction substrates such as specific ion channels that have been implicated in glioma progression or the development of agents that alter the mechanical properties of the microenvironment to halt disease progression have the potential to be promising treatment strategies based on early studies. Similarly, the development of technology to measure mechanical properties of the microenvironment in vitro and in vivo and simulate these properties in bioengineered models may facilitate the use of mechanical properties as diagnostic or prognostic biomarkers that can guide treatment. Here, we review current perspectives on the influence of mechanical properties in glioma with a focus on biophysical features of tumor-adjacent tissue, the role of fluid mechanics, and mechanisms of mechanical signal transduction. We highlight the implications of recent discoveries for novel diagnostics, therapeutic targets, and accurate preclinical modeling of glioma.

Hydrogels Containing Gradients in Vascular Density Reveal Dose-Dependent Role of Angiocrine Cues on Stem Cell Behavior

Biomaterials that replicate patterns of microenvironmental signals from the stem cell niche offer the potential to refine platforms to regulate stem cell behavior. While significant emphasis has been placed on understanding the effects of biophysical and biochemical cues on stem cell fate, vascular-derived or angiocrine cues offer an important alternative signaling axis for biomaterial-based stem cell platforms. Elucidating dose-dependent relationships between angiocrine cues and stem cell fate are largely intractable in animal models and 2D cell cultures. In this study, microfluidic mixing devices are leveraged to generate 3D hydrogels containing lateral gradients in vascular density alongside murine hematopoietic stem cells (HSCs). Regional differences in vascular density can be generated via embossed gradients in cell, matrix, or growth factor density. HSCs co-cultured alongside vascular gradients reveal spatial patterns of HSC phenotype in response to angiocrine signals. Notably, decreased Akt signaling in high vessel density regions led to increased expansion of lineage-positive hematopoietic cells. This approach offers a combinatorial tool to rapidly screen a continuum of microenvironments with varying vascular, biophysical, and biochemical cues to reveal the influence of local angiocrine signals on HSC fate.

Engineering the Cellular Microenvironment of Post-infarct Myocardium on a Chip

Myocardial infarctions are one of the most common forms of cardiac injury and death worldwide. Infarctions cause immediate necrosis in a localized region of the myocardium, which is followed by a repair process with inflammatory, proliferative, and maturation phases. This repair process culminates in the formation of scar tissue, which often leads to heart failure in the months or years after the initial injury. In each reparative phase, the infarct microenvironment is characterized by distinct biochemical, physical, and mechanical features, such as inflammatory cytokine production, localized hypoxia, and tissue stiffening, which likely each contribute to physiological and pathological tissue remodeling by mechanisms that are incompletely understood. Traditionally, simplified two-dimensional cell culture systems or animal models have been implemented to elucidate basic pathophysiological mechanisms or predict drug responses following myocardial infarction. However, these conventional approaches offer limited spatiotemporal control over relevant features of the post-infarct cellular microenvironment. To address these gaps, Organ on a Chip models of post-infarct myocardium have recently emerged as new paradigms for dissecting the highly complex, heterogeneous, and dynamic post-infarct microenvironment. In this review, we describe recent Organ on a Chip models of post-infarct myocardium, including their limitations and future opportunities in disease modeling and drug screening.

Hyaluronic acid-based hydrogels to study cancer cell behaviors

Hyaluronic acid (HA) is a natural polysaccharide and a key component of the extracellular matrix (ECM) in many tissues. Therefore, HA-based biomaterials are extensively utilized to create three dimensional ECM mimics to study cell behaviors in vitro. Specifically, derivatives of HA have been commonly used to fabricate hydrogels with controllable properties. In this review, we discuss the various chemistries employed to fabricate HA-based hydrogels as a tunable matrix to mimic the cancer microenvironment and subsequently study cancer cell behaviors in vitro. These include Michael-addition reactions, photo-crosslinking, carbodiimide chemistry, and Diels-Alder chemistry. The utility of these HA-based hydrogels to examine cancer cell behaviors such as proliferation, migration, and invasion in vitro in various types of cancer are highlighted. Overall, such hydrogels provide a biomimetic material-based platform to probe cell-matrix interactions in cancer cells in vitro and study the mechanisms associated with cancer progression.

Patient-Specific Organoid and Organ-on-a-Chip: 3D Cell-Culture Meets 3D Printing and Numerical Simulation

The last few decades have witnessed diversified in vitro models to recapitulate the architecture and function of living organs or tissues and contribute immensely to advances in life science. Two novel 3D cell culture models: 1) Organoid, promoted mainly by the developments of stem cell biology and 2) Organ-on-a-chip, enhanced primarily due to microfluidic technology, have emerged as two promising approaches to advance the understanding of basic biological principles and clinical treatments. This review describes the comparable distinct differences between these two models and provides more insights into their complementarity and integration to recognize their merits and limitations for applicable fields. The convergence of the two approaches to produce multi-organoid-on-a-chip or human organoid-on-a-chip is emerging as a new approach for building 3D models with higher physiological relevance. Furthermore, rapid advancements in 3D printing and numerical simulations, which facilitate the design, manufacture, and results-translation of 3D cell culture models, can also serve as novel tools to promote the development and propagation of organoid and organ-on-a-chip systems. Current technological challenges and limitations, as well as expert recommendations and future solutions to address the promising combinations by incorporating organoids, organ-on-a-chip, 3D printing, and numerical simulation, are also summarized.

Gradient hydrogels for screening stiffness effects on patient-derived glioblastoma xenograft cellfates in 3D

Brain cancer is a devastating disease given its extreme invasiveness and intricate location. Glioblastoma multiforme (GBM) is one of the most common forms of brain cancer, and cancer progression is often correlated with significantly altered tissue stiffness. To elucidate the effect of matrix stiffness on GBM cell fates, previous research is largely limited to 2D studies using immortalized cell lines, which has limited physiological relevance. The objective of the study is to develop gradient hydrogels with brain-mimicking stiffness range as a 3Din vitro GBM model for screening of the effects of matrix stiffness on GBM. To increase the physiological relevance, patient-derived tumor xenograft (PDTX) GBM cells were used. Our gradient platform allows formation of cell-containing hydrogels with stiffness ranging from 40 Pa to 1,300 Pa within a few minutes. By focusing on a brain-mimicking stiffness range, this gradient hydrogel platform is designed for investigating brain cancer. Increasing stiffness led to decreased GBM proliferation and less spreading, which is accompanied by downregulation of matrix-metalloproteinases (MMPs). Using temozolomide (TMZ) as a model drug, we demonstrate that increasing stiffness led to higher drug resistance by PDTX GBM cells in 3D, suggesting matrix stiffness can directly modulate how GBM cells respond to drug treatment. While the current study focuses on stiffness gradient, the setup may also be adapted for screening other cancer niche cues such as how biochemical ligand gradient modulates brain cancer progression and drug responses using reduced materials and time.

Matrix Stiffness Modulates Patient-Derived Glioblastoma Cell Fates in Three-Dimensional Hydrogels

Cancer progression is known to be accompanied by changes in tissue stiffness. Previous studies have primarily employed immortalized cell lines and 2D hydrogel substrates, which do not recapitulate the 3D tumor niche. How matrix stiffness affects patient-derived cancer cell fate in 3D remains unclear. In this study, we report a matrix metalloproteinase-degradable poly(ethylene-glycol)-based hydrogel platform with brain-mimicking biochemical cues and tunable stiffness (40-26,600 Pa) for 3D culture of patient-derived glioblastoma xenograft (PDTX GBM) cells. Our results demonstrate that decreasing hydrogel stiffness enhanced PDTX GBM cell proliferation, and hydrogels with stiffness 240 Pa and below supported robust PDTX GBM cell spreading in 3D. PDTX GBM cells encapsulated in hydrogels demonstrated higher drug resistance than 2D control, and increasing hydrogel stiffness further enhanced drug resistance. Such 3D hydrogel platforms may provide a valuable tool for mechanistic studies of the role of niche cues in modulating cancer progression for different cancer types.

Proteinaceous Hydrogels for Bioengineering Advanced 3D Tumor Models

The establishment of tumor microenvironment using biomimetic in vitro models that recapitulate key tumor hallmarks including the tumor supporting extracellular matrix (ECM) is in high demand for accelerating the discovery and preclinical validation of more effective anticancer therapeutics. To date, ECM-mimetic hydrogels have been widely explored for 3D in vitro disease modeling owing to their bioactive properties that can be further adapted to the biochemical and biophysical properties of native tumors. Gathering on this momentum, herein the current landscape of intrinsically bioactive protein and peptide hydrogels that have been employed for 3D tumor modeling are discussed. Initially, the importance of recreating such microenvironment and the main considerations for generating ECM-mimetic 3D hydrogel in vitro tumor models are showcased. A comprehensive discussion focusing protein, peptide, or hybrid ECM-mimetic platforms employed for modeling cancer cells/stroma cross-talk and for the preclinical evaluation of candidate anticancer therapies is also provided. Further development of tumor-tunable, proteinaceous or peptide 3D microtesting platforms with microenvironment-specific biophysical and biomolecular cues will contribute to better mimic the in vivo scenario, and improve the predictability of preclinical screening of generalized or personalized therapeutics.

Biomaterials and 3D Bioprinting Strategies to Model Glioblastoma and the Blood-Brain Barrier

Glioblastoma (GBM) is the most prevalent and lethal adult primary central nervous system cancer. An immunosuppresive and highly heterogeneous tumor microenvironment, restricted delivery of chemotherapy or immunotherapy through the blood-brain barrier (BBB), together with the brain's unique biochemical and anatomical features result in its universal recurrence and poor prognosis. As conventional models fail to predict therapeutic efficacy in GBM, in vitro 3D models of GBM and BBB leveraging patient- or healthy-individual-derived cells and biomaterials through 3D bioprinting technologies potentially mimic essential physiological and pathological features of GBM and BBB. 3D-bioprinted constructs enable investigation of cellular and cell-extracellular matrix interactions in a species-matched, high-throughput, and reproducible manner, serving as screening or drug delivery platforms. Here, an overview of current 3D-bioprinted GBM and BBB models is provided, elaborating on the microenvironmental compositions of GBM and BBB, relevant biomaterials to mimic the native tissues, and bioprinting strategies to implement the model fabrication. Collectively, 3D-bioprinted GBM and BBB models are promising systems and biomimetic alternatives to traditional models for more reliable mechanistic studies and preclinical drug screenings that may eventually accelerate the drug development process for GBM.

Crosstalk between microglia and patient-derived glioblastoma cells inhibit invasion in a three-dimensional gelatin hydrogel model

BACKGROUND

Glioblastoma is the most common and deadly form of primary brain cancer, accounting for more than 13,000 new diagnoses annually in the USA alone. Microglia are the innate immune cells within the central nervous system, acting as a front-line defense against injuries and inflammation via a process that involves transformation from a quiescent to an activated phenotype. Crosstalk between GBM cells and microglia represents an important axis to consider in the development of tissue engineering platforms to examine pathophysiological processes underlying GBM progression and therapy.

METHODS

This work used a brain-mimetic hydrogel system to study patient-derived glioblastoma specimens and their interactions with microglia. Here, glioblastoma cells were either cultured alone in 3D hydrogels or in co-culture with microglia in a manner that allowed secretome-based signaling but prevented direct GBM-microglia contact. Patterns of GBM cell invasion were quantified using a three-dimensional spheroid assay. Secretome and transcriptome (via RNAseq) were used to profile the consequences of GBM-microglia interactions.

RESULTS

Microglia displayed an activated phenotype as a result of GBM crosstalk. Three-dimensional migration patterns of patient-derived glioblastoma cells showed invasion was significantly decreased in response to microglia paracrine signaling. Potential molecular mechanisms underlying with this phenotype were identified from bioinformatic analysis of secretome and RNAseq data.

CONCLUSION

The data demonstrate a tissue engineered hydrogel platform can be used to investigate crosstalk between immune cells of the tumor microenvironment related to GBM progression. Such multi-dimensional models may provide valuable insight to inform therapeutic innovations to improve GBM treatment.

Innovations in 3-Dimensional Tissue Models of Human Brain Physiology and Diseases

3-dimensional (3D) laboratory tissue cultures have emerged as an alternative to traditional 2-dimensional (2D) culture systems that do not recapitulate native cell behavior. The discrepancy between in vivo and in vitro tissue-cell-molecular responses impedes understanding of human physiology in general and creates roadblocks for the discovery of therapeutic solutions. Two parallel approaches have emerged for the design of 3D culture systems. The first is biomedical engineering methodology, including bioengineered materials, bioprinting, microfluidics and bioreactors, used alone or in combination, to mimic the microenvironments of native tissues. The second approach is organoid technology, in which stem cells are exposed to chemical and/or biological cues to activate differentiation programs that are reminiscent of human (prenatal) development. This review article describes recent technological advances in engineering 3D cultures that more closely resemble the human brain. The contributions of in vitro 3D tissue culture systems to new insights in neurophysiology, neurological diseases and regenerative medicine are highlighted. Perspectives on designing improved tissue models of the human brain are offered, focusing on an integrative approach merging biomedical engineering tools with organoid biology.

A comparative study of brain tumor cells from different age and anatomical locations using 3D biomimetic hydrogels

Brain tumors exhibit vast genotypic and phenotypic diversity depending on patient age and anatomical location. Hydrogels hold great promise as 3D in vitro models for studying brain tumor biology and drug screening, yet previous studies were limited to adult glioblastoma cells, and most studies used immortalized cell lines. Here we report a hydrogel platform that supports the proliferation and invasion of patient-derived brain tumor cell cultures (PDCs) isolated from different patient age groups and anatomical locations. Hydrogel stiffness was tuned by varying poly(ethylene-glycol) concentration. Cell adhesive peptide (CGRDS), hyaluronic acid, and MMP-cleavable crosslinkers were incorporated to facilitate cell adhesion and cell-mediated degradation. Three PDC lines were compared including adult glioblastoma cells (aGBM), pediatric glioblastoma cells (pGBM), and diffuse pontine intrinsic glioma (DIPG). A commonly used immortalized adult glioblastoma cell line U87 was included as a control. PDCs displayed stiffness-dependent behavior, with 40 Pa hydrogel promoting faster tumor proliferation and invasion. Adult GBM cells exhibited faster proliferation than pediatric GBM, and DIPG showed slowest proliferation. These results suggest both patient age and tumor location affects brain tumor behaviors. Adult GBM PDCs also exhibited very different cell proliferation and morphology from U87. The hydrogel reported here can provide a useful tool for future studies to better understand how age and anatomical locations impacts brain tumor progression using 3D in vitro models.

Converting 2D Nanofiber Membranes to 3D Hierarchical Assemblies with Structural and Compositional Gradients Regulates Cell Behavior

New methods are described for converting 2D electrospun nanofiber membranes to 3D hierarchical assemblies with structural and compositional gradients. Pore-size gradients are generated by tuning the expansion of 2D membranes in different layers with incorporation of various amounts of a surfactant during the gas-foaming process. The gradient in fiber organizations is formed by expanding 2D nanofiber membranes composed of multiple regions collected by varying rotating speeds of mandrel. A compositional gradient on 3D assemblies consisting of radially aligned nanofibers is prepared by dripping, diffusion, and crosslinking. Bone mesenchymal stem cells (BMSCs) on the 3D nanofiber assemblies with smaller pore size show significantly higher expression of hypoxia-related markers and enhanced chondrogenic differentiation compared to BMSCs cultured on the assemblies with larger pore size. The basic fibroblast growth factor gradient can accelerate fibroblast migration from the surrounding area to the center in an in vitro wound healing model. Taken together, 3D nanofiber assemblies with gradients in pore sizes, fiber organizations, and contents of signaling molecules can be used to engineer tissue constructs for tissue repair and build biomimetic disease models for studying disease biology and screening drugs, in particular, for interface tissue engineering and modeling.

Fabrication of Stiffness Gradients of GelMA Hydrogels Using a 3D Printed Micromixer

Many properties in both healthy and pathological tissues are highly influenced by the mechanical properties of the extracellular matrix. Stiffness gradient hydrogels are frequently used for exploring these complex relationships in mechanobiology. In this study, the fabrication of a simple, cost-efficient, and versatile system is reported for creation of stiffness gradients from photoactive hydrogels like gelatin-methacryloyl (GelMA). The setup includes syringe pumps for gradient generation and a 3D printed microfluidic device for homogenous mixing of GelMA precursors with different crosslinker concentration. The stiffness gradient is investigated by using rheology. A co-culture consisting of human adipose tissue-derived mesenchymal stem cells (hAD-MSCs) and human umbilical cord vein endothelial cells (HUVECs) is encapsulated in the gradient construct. It is possible to locate the stiffness ranges at which the studied cells displayed specific spreading morphology and migration rates. With the help of the described system, variable mechanical gradient constructs can be created and optimal 3D cell culture conditions can be experientially identified.

Creating Physicochemical Gradients in Modular Microporous Annealed Particle Hydrogels via a Microfluidic Method

Microporous annealed particle (MAP) hydrogels are an attractive platform for engineering biomaterials with controlled heterogeneity. Here, we introduce a microfluidic method to create physicochemical gradients within poly(ethylene glycol) based MAP hydrogels. By combining microfluidic mixing and droplet generator modules, microgels with varying properties were produced by adjusting the relative flow rates between two precursor solutions and collected layer-by-layer in a syringe. Subsequently, the microgels were injected out of the syringe and then annealed with thiol-ene click chemistry. Fluorescence intensity measurements of constructs annealed in vitro and after mock implantation into a tissue defect showed that a continuous gradient profile was achieved and maintained after injection, indicating utility for in situ hydrogel formation. The effects of physicochemical property gradients on human mesenchymal stem cells (hMSCs) were also studied. Microgel stiffness was studied first, and the hMSCs exhibited increased spreading and proliferation as stiffness increased along the gradient. Microgel degradability was also studied, revealing a critical degradability threshold above which the hMSCs spread robustly and below which they were isolated and exhibited reduced spreading. This method of generating spatial gradients in MAP hydrogels could be further used to gain new insights into cell-material interactions, which could be leveraged for tissue engineering applications. A new droplet microfluidic approach to obtain microporous annealed particle hydrogels with physicochemical gradients is presented. Gradient formation is achieved by precisely controlling the mixing of two precursor solutions, and the gradient can be maintained after injection. This approach can be leveraged to produce new materials for tissue repair and to gain unique insights on cell-material interactions.

Recent Strategies in Fabrication of Gradient Hydrogels for Tissue Engineering Applications

Hydrogels are widely used as scaffold in tissue engineering field because of their ability to mimic the cellular microenvironment. However, mimicking a completely natural cellular environment is complicated due to the differences in various physical and chemical properties of cellular environments. Recently, gradient hydrogels provide excellent heterogeneous environment to mimic the different cellular microenvironments. To create hydrogels with an anisotropic distribution, gradient hydrogels have been widely developed by adopting several gradient generation techniques. Herein, the various gradient hydrogel fabrication techniques, including dual syringe pump systems, microfluidic device, photolithography, diffusion, and bio-printing are summarized. As the effects of gradient 3D hydrogels with stems have been reviewed elsewhere, this review focuses principally on gradient hydrogel fabrication for multi-model tissue regeneration. This review provides new insights into the key points for fabrication of gradient hydrogels for multi-model tissue regeneration.

Dissecting and rebuilding the glioblastoma microenvironment with engineered materials

Glioblastoma (GBM) is the most aggressive and common form of primary brain cancer. Several decades of research have provided great insight into GBM progression; however, the prognosis remains poor with a median patient survival time of ~ 15 months. The tumour microenvironment (TME) of GBM plays a crucial role in mediating tumour progression and thus is being explored as a therapeutic target. Progress in the development of treatments targeting the TME is currently limited by a lack of model systems that can accurately recreate the distinct extracellular matrix composition and anatomic features of the brain, such as the blood-brain barrier and axonal tracts. Biomaterials can be applied to develop synthetic models of the GBM TME to mimic physiological and pathophysiological features of the brain, including cellular and ECM composition, mechanical properties, and topography. In this Review, we summarize key features of the GBM microenvironment and discuss different strategies for the engineering of GBM TME models, including 2D and 3D models featuring chemical and mechanical gradients, interfaces and fluid flow. Finally, we highlight the potential of engineered TME models as platforms for mechanistic discovery and drug screening as well as preclinical testing and precision medicine.

Perlecan domain I gradients establish stable biomimetic heparin binding growth factor gradients for cell migration in hydrogels

Growth factor gradients orchestrate many biological processes including organogenesis, wound healing, cancer invasion, and metastasis. Heparin-binding growth factor (HBGF) gradients are established in living systems by proteoglycans including the extracellular matrix heparan sulfate proteoglycan, perlecan/HSPG2. Three potential HBGF-binding glycosaminoglycan attachment sites occur in N-terminal domain I of perlecan's five domains. Our overarching goal was to form stable, biomimetic non-covalently bound HBGF gradients surrounding cells encapsulated in hyaluronate-based hydrogels by first establishing perlecan domain I (PlnD1) gradients. A versatile multichannel gradient maker device (MGMD) was designed and 3D printed, then used to create desired gradients of microparticles in hydrogels. Next, we used the device to covalently incorporate gradients of PEGylated PlnD1 in hydrogels with high-low-high or high-medium-low concentrations across the hydrogel width. Fluorescently-labeled fibroblast growth factor-2 was delivered to hydrogels in phosphate-buffered saline and allowed to electrostatically bind to the covalently pre-incorporated PlnD1, producing stable non-covalent HBGF gradients. To test cell viability after flow through the MGMD, delicate primary human salivary stem/progenitor cells were encapsulated in gradient hydrogels where they showed high viability and continued to grow. Next, to test migratory behavior in response to HBGF gradients, two cell types, preosteoblastic MC3T3-E1 cell line and breast cancer cell line MDA-MB-231 were encapsulated in or adjacent to PlnD1-modified hydrogels. Both cell lines migrated toward HBGFs bound to PlnD1. We conclude that establishing covalently-bound PlnD1 gradients in hydrogels provides a new means to establish physiologically-relevant gradients of HBGFs that are useful for a variety of applications in tissue engineering and cancer biology. STATEMENT OF SIGNIFICANCE: Gradients of heparin binding growth factors (HBGFs) direct cell behavior in living systems. HBGFs bind electrostatically to gradients of HS proteoglycans in the extracellular matrix creating HBGF gradients. We recreated HBGF gradients in physiological hyaluronate-based hydrogels using a 3D-printed multichannel gradient maker device (MGMD) that created gradients of HS proteoglycan-derived perlecan/HSPG2 domain I. We demonstrated the ability of a variety of cells, including primary salivary stem/progenitor cells, pre-osteoblastic cells and an invasive breast cancer cell line, to be co-encapsulated in gradient hydrogels by flowing them together through the MGMD. The versatile device and the ability to create HBGF gradients in hydrogels for a variety of applications is innovative and of broad utility in both cancer biology and tissue engineering applications.

Hyaluronic acid-functionalized gelatin hydrogels reveal extracellular matrix signals temper the efficacy of erlotinib against patient-derived glioblastoma specimens

Therapeutic options to treat primary glioblastoma (GBM) tumors are scarce. GBM tumors with epidermal growth factor receptor (EGFR) mutations, in particular a constitutively active EGFRvIII mutant, have extremely poor clinical outcomes. GBM tumors with concurrent EGFR amplification and active phosphatase and tensin homolog (PTEN) are sensitive to the tyrosine kinase inhibitor erlotinib, but the effect is not durable. A persistent challenge to improved treatment is the poorly understood role of cellular, metabolic, and biophysical signals from the GBM tumor microenvironment on therapeutic efficacy and acquired resistance. The intractable nature of studying GBM cell in vivo motivates tissue engineering approaches to replicate aspects of the complex GBM tumor microenvironment. Here, we profile the effect of erlotinib on two patient-derived GBM specimens: EGFR + GBM12 and EGFRvIII GBM6. We use a three-dimensional gelatin hydrogel to present brain-mimetic hyaluronic acid (HA) and evaluate the coordinated influence of extracellular matrix signals and EGFR mutation status on GBM cell migration, survival and proliferation, as well as signaling pathway activation in response to cyclic erlotinib exposure. Comparable to results observed in vivo for xenograft tumors, erlotinib exposure is not cytotoxic for GBM6 EGFRvIII specimens. We also identify a role of extracellular HA (via CD44) in altering the effect of erlotinib in GBM EGFR + cells by modifying STAT3 phosphorylation status. Taken together, we report an in vitro tissue engineered platform to monitor signaling associated with poor response to targeted inhibitors in GBM.

An omentum-inspired 3D PEG hydrogel for identifying ECM-drivers of drug resistant ovarian cancer

Ovarian cancer (OvCa) is a challenging disease to treat due to poor screening techniques and late diagnosis. There is an urgent need for additional therapy options, as patients recur in 70% of cases. The limited availability of clinical treatment options could be a result of poor predictions in early stage drug screens on standard tissue culture polystyrene (TCPS). TCPS does not capture the mechanical and biochemical cues that cells experience in vivo, which can impact how cells will respond to a drug. Therefore, an in vitro model that captures some of the microenvironment features that the cells experience in vivo could provide better insights into drug responses. In this study, we formed 3D multicellular tumor spheroids (MCTS) in microwells and encapsulated them in 3D omentum-inspired hydrogels. SKOV-3 MCTS were resistant to Paclitaxel in our 3D hydrogels compared to a monolayer on TCPS. Toward clinical application, we tested cells from patients [ovarian carcinoma ascites spheroids (OCAS)] who had been treated with Paclitaxel, and drug responses predicted by using the 3D omentum-inspired hydrogels demonstrated the lack of the Paclitaxel response of these samples. Additionally, we observed the presence of collagen production around the encapsulated SKOV-3 MCTS, but not significantly on TCPS. Our results demonstrated that our 3D omentum-inspired hydrogel is an improved in vitro drug testing platform to study the OvCa drug response for patient-derived cells and helped us identify collagen 3 as a potential driver of Paclitaxel resistance in 3D.

The study of establishment of an in vivo tumor model by three-dimensional cells culture systems methods and evaluation of antitumor effect of biotin-conjugated pullulan acetate nanoparticles

In this study, three-dimensional (3D) hydrogels were used for human hepatocellular carcinoma (HepG2) cells culture systems in vitro and establishment of an in vivo xenografted tumor model. Based on our previous work on the biotin-conjugated pullulan acetate nanoparticles (Bio-PA NPs) as anticancer drug carriers, we further studied the anti-tumor effect of the NPs in two-dimensional (2D) and 3D cell culture system. When embedded in 3D hydrogels, HepG2 cells formed tumor spheroids and the cytoplasmic actin microfilamentrates were rearranged over a period of 7 days. In vitro cytotoxicity results indicated that HepG2 cells in 3D hydrogels were more resistant to Bio-PA NPs treatments compared to the 2D system. The tumor formation rate of in vivo xenografted tumor model using 3D culture systems method was 98.2%, which was significantly higher than that using of 2D cultured cells (76.4%). Then we injected the 3D HepG2 cells systems in the right anterior axillary of female Balb/c nude mice, and evaluate the in vivo anti-tumor efficacy of Bio-PA NPs. In summary, these results suggested that HepG2 cells in 3D hydrogel system has shown the potential to provide an in vitro and in vivo model and for the evaluation of Bio-PA NPs.

Perivascular signals alter global gene expression profile of glioblastoma and response to temozolomide in a gelatin hydrogel

Glioblastoma (GBM) is the most common primary malignant brain tumor, with patients exhibiting poor survival (median survival time: 15 months). Difficulties in treating GBM include not only the inability to resect the diffusively-invading tumor cells, but also therapeutic resistance. The perivascular niche (PVN) within the GBM tumor microenvironment contributes significantly to tumor cell invasion, cancer stem cell maintenance, and has been shown to protect tumor cells from radiation and chemotherapy. In this study, we examine how the inclusion of non-tumor cells in culture with tumor cells within a hydrogel impacts the overall gene expression profile of an in vitro artificial perivascular niche (PVN) comprised of endothelial and stromal cells directly cultured with GBM tumor cells within a methacrylamide-functionalized gelatin hydrogel. Using RNA-seq, we demonstrate that genes related to angiogenesis and extracellular matrix remodeling are upregulated in the PVN model compared to hydrogels containing only tumor or perivascular niche cells, while downregulated genes are related to cell cycle and DNA damage repair. Signaling pathways and genes commonly implicated in GBM malignancy, such as MGMT, EGFR, PI3K-Akt signaling, and Ras/MAPK signaling are also upregulated in the PVN model. We describe the kinetics of gene expression within the PVN hydrogels over a course of 14 days, observing the patterns associated with tumor cell-mediated endothelial network co-option and regression. We finally examine the effect of temozolomide, a frontline chemotherapy used clinically against GBM, on the PVN culture. Notably, the PVN model is less responsive to TMZ compared to hydrogels containing only tumor cells. Overall, these results demonstrate that inclusion of cellular and matrix-associated elements of the PVN within an in vitro model of GBM allows for the development of gene expression patterns and therapeutic response relevant to GBM.

A bioprinted human-glioblastoma-on-a-chip for the identification of patient-specific responses to chemoradiotherapy

Patient-specific ex vivo models of human tumours that recapitulate the pathological characteristics and complex ecology of native tumours could help determine the most appropriate cancer treatment for individual patients. Here, we show that bioprinted reconstituted glioblastoma tumours consisting of patient-derived tumour cells, vascular endothelial cells and decellularized extracellular matrix from brain tissue in a compartmentalized cancer-stroma concentric-ring structure that sustains a radial oxygen gradient, recapitulate the structural, biochemical and biophysical properties of the native tumours. We also show that the glioblastoma-on-a-chip reproduces clinically observed patient-specific resistances to treatment with concurrent chemoradiation and temozolomide, and that the model can be used to determine drug combinations associated with superior tumour killing. The patient-specific tumour-on-a-chip model might be useful for the identification of effective treatments for glioblastoma patients resistant to the standard first-line treatment.

Glioblastoma's Next Top Model: Novel Culture Systems for Brain Cancer Radiotherapy Research

Glioblastoma (GBM), the most common and aggressive primary brain tumor in adults, remains one of the least treatable cancers. Current standard of care—combining surgical resection, radiation, and alkylating chemotherapy—results in a median survival of only 15 months. Despite decades of investment and research into the development of new therapies, most candidate anti-glioma compounds fail to translate into effective treatments in clinical trials. One key issue underlying this failure of therapies that work in pre-clinical models to generate meaningful improvement in human patients is the profound mismatch between drug discovery systems—cell cultures and mouse models—and the actual tumors they are supposed to imitate. Indeed, current strategies that evaluate the effects of novel treatments on GBM cells in vitro fail to account for a wide range of factors known to influence tumor growth. These include secreted factors, the brain’s unique extracellular matrix, circulatory structures, the presence of non-tumor brain cells, and nutrient sources available for tumor metabolism. While mouse models provide a more realistic testing ground for potential therapies, they still fail to account for the full complexity of tumor-microenvironment interactions, as well as the role of the immune system. Based on the limitations of current models, researchers have begun to develop and implement novel culture systems that better recapitulate the complex reality of brain tumors growing in situ. A rise in the use of patient derived cells, creative combinations of added growth factors and supplements, may provide a more effective proving ground for the development of novel therapies. This review will summarize and analyze these exciting developments in 3D culturing systems. Special attention will be paid to how they enhance the design and identification of compounds that increase the efficacy of radiotherapy, a bedrock of GBM treatment.

Perivascular signals alter global gene expression profile of glioblastoma and response to temozolomide in a gelatin hydrogel

Glioblastoma (GBM) is the most common primary malignant brain tumor, with patients exhibiting poor survival (median survival time: 15 months). Difficulties in treating GBM include not only the inability to resect the diffusively-invading tumor cells, but also therapeutic resistance. The perivascular niche (PVN) within the GBM tumor microenvironment contributes significantly to tumor cell invasion, cancer stem cell maintenance, and has been shown to protect tumor cells from radiation and chemotherapy. In this study, we examine how the inclusion of non-tumor cells in culture with tumor cells within a hydrogel impacts the overall gene expression profile of an in vitro artificial perivascular niche (PVN) comprised of endothelial and stromal cells directly cultured with GBM tumor cells within a methacrylamide-functionalized gelatin hydrogel. Using RNA-seq, we demonstrate that genes related to angiogenesis and extracellular matrix remodeling are upregulated in the PVN model compared to hydrogels containing only tumor or perivascular niche cells, while downregulated genes are related to cell cycle and DNA damage repair. Signaling pathways and genes commonly implicated in GBM malignancy, such as MGMT, EGFR, PI3K-Akt signaling, and Ras/MAPK signaling are also upregulated in the PVN model. We describe the kinetics of gene expression within the PVN hydrogels over a course of 14 days, observing the patterns associated with tumor cell-mediated endothelial network co-option and regression. We finally examine the effect of temozolomide, a frontline chemotherapy used clinically against GBM, on the PVN culture. Notably, the PVN model is less responsive to TMZ compared to hydrogels containing only tumor cells. Overall, these results demonstrate that inclusion of cellular and matrix-associated elements of the PVN within an in vitro model of GBM allows for the development of gene expression patterns and therapeutic response relevant to GBM.

The Feasibility of Encapsulated Embryonic Medullary Reticular Cells to Grow and Differentiate Into Neurons in Functionalized Gelatin-Based Hydrogels

The study of the behavior of embryonic neurons in controlled in vitro conditions require methodologies that take advantage of advanced tissue engineering approaches to replicate elements of the developing brain extracellular matrix. We report here a series of experiments that explore the potential of photo-polymerized gelatin hydrogels to culture primary embryonic neurons. We employed large medullary reticular neurons whose activity is essential for brain arousal as well as a library of gelatin hydrogels that span a range of mechanical properties, inclusion of brain-mimetic hyaluronic acid, and adhesion peptides. These hydrogel platforms showed inherent capabilities to sustain neuronal viability and were permissive for neuronal differentiation, resulting in the development of neurite outgrowth under specific conditions. The maturation of embryonic medullary reticular cells took place in the absence of growth factors or other exogenous bioactive molecules. Immunocytochemistry labeling of neuron-specific tubulin confirmed the initiation of neural differentiation. Thus, this methodology provides an important validation for future studies of nerve cell growth and maintenance.

Influence of Hyaluronic Acid Transitions in Tumor Microenvironment on Glioblastoma Malignancy and Invasive Behavior

The extracellular matrix (ECM) is critical in tumor growth and invasive potential of cancer cells. In glioblastoma tumors, some components of the native brain ECM such as hyaluronic acid (HA) have been suggested as key regulators of processes associated with poor patient outlook such as invasion and therapeutic resistance. Given the importance of cell-mediated remodeling during invasion, it is likely that the molecular weight of available HA polymer may strongly influence GBM progression. Biomaterial platforms therefore provide a unique opportunity to systematically examine the influence of the molecular weight distribution of HA on GBM cell activity. Here we report the relationship between the molecular weight of matrix-bound HA within a methacrylamidefunctionalized gelatin (GelMA) hydrogel, the invasive phenotype of a patient-derived xenograft GBM population that exhibits significant in vivo invasivity, and the local production of soluble HA during GBM cell invasion. Hyaluronic acid of different molecular weights spanning a range associated with cell-mediated remodeling (10, 60, and 500 kDa) was photopolymerized into GelMA hydrogels, with cell activity compared to GelMA only conditions (-HA). Polymerization conditions were tuned to create a homologous series of GelMA hydrogels with conserved poroelastic properties (i.e., shear modulus, Poisson's ratio, and diffusivity). GBM migration was strongly influenced by HA molecular weight; while markers associated with active remodeling of HA (hyaluronan synthase and hyaluronidase) were found to be uninfluenced. These results provide new information regarding the importance of local hyaluronic acid content on the invasive phenotype of GBM.

Perspective: The role of mechanobiology in the etiology of brain metastasis

Tumor latency and dormancy are obstacles to effective cancer treatment. In brain metastases, emergence of a lesion can occur at varying intervals from diagnosis and in some cases following successful treatment of the primary tumor. Genetic factors that drive brain metastases have been identified, such as those involved in cell adhesion, signaling, extravasation, and metabolism. From this wealth of knowledge, vexing questions still remain; why is there a difference in strategy to facilitate outgrowth and why is there a difference in latency? One missing link may be the role of tissue biophysics of the brain microenvironment in infiltrating cells. Here, I discuss the mechanical cues that may influence disseminated tumor cells in the brain, as a function of age and disease. I further discuss in vitro and in vivo preclinical models such as 3D culture systems and zebrafish to study the role of the mechanical environment in brain metastasis in an effort of providing novel targeted therapeutics.

Perspective on Translating Biomaterials Into Glioma Therapy: Lessons From in vitro Models

Glioblastoma (GBM) is the most common and malignant form of brain cancer. Even with aggressive standard of care, GBM almost always recurs because its diffuse, infiltrative nature makes these tumors difficult to treat. The use of biomaterials is one strategy that has been, and is being, employed to study and overcome recurrence. Biomaterials have been used in GBM in two ways: in vitro as mediums in which to model the tumor microenvironment, and in vivo to sustain release of cytotoxic therapeutics. In vitro systems are a useful platform for studying the effects of drugs and tissue-level effectors on tumor cells in a physiologically relevant context. These systems have aided examination of how glioma cells respond to a variety of natural, synthetic, and semi-synthetic biomaterials with varying substrate properties, biochemical factor presentations, and non-malignant parenchymal cell compositions in both 2D and 3D environments. The current in vivo paradigm is completely different, however. Polymeric implants are simply used to line the post-surgical resection cavities and deliver secondary therapies, offering moderate impacts on survival. Instead, perhaps we can use the data generated from in vitro systems to design novel biomaterial-based treatments for GBM akin to a tissue engineering approach. Here we offer our perspective on the topic, summarizing how biomaterials have been used to identify facets of glioma biology in vitro and discussing the elements that show promise for translating these systems in vivo as new therapies for GBM.

Hypoxia activates enhanced invasive potential and endogenous hyaluronic acid production by glioblastoma cells

Glioblastoma (GBM) is the most common, aggressive, and deadly form of adult brain cancer, and is associated with a short survival rate (median 12-15 months, 5+ year less than 5%). The complex tumor microenvironment includes matrix transitions at the tumor margin, such as gradations in hyaluronic acid (HA). In addition, metabolic stress induced by decreased oxygen content across the tumor may contribute to tumor progression. However, cross-talk between matrix composition and metabolic stress remains unclear. In this study, we fabricated an in vitro brain memetic HA-decorated gelatin hydrogel platform incorporating variable oxygen concentrations to mimic intra-tumoral hypoxia. We observed that EGFR status (wildtype vs. a constitutively active EGFRvIII mutant) of U87 GBM cells affected proliferation and metabolic activity in response to hypoxia and matrix-bound HA. The use of an invasion assay revealed that invasion was significantly enhanced in both cell types under hypoxia. Moreover, we observed compensatory secretion of soluble HA in cases of enhanced GBM cell invasion, consistent with our previous findings using other GBM cell lines. Interestingly, U87 GBM cells adapted to hypoxia by shifting toward a more anaerobic metabolic state, a mechanism that may contribute to GBM cell invasion. Collectively, these data demonstrate that the use of a three-dimensional hydrogel provides a robust method to study the impact of matrix composition and metabolic challenges on GBM cell invasion, a key factor contributing to the most common, aggressive, and deadly form of adult brain cancer.

Recent advances in brain tumor therapy: application of electrospun nanofibers

Despite much effort to treat glioblastoma multiforme (GBM), the median survival of patients has not significantly improved. The high rate of tumor recurrence after tumor resection and the blood-brain barrier (BBB) decrease the treatment efficacy. Local drug delivery at the surgical resection site via implantable electrospun nanofibers not only circumvents the BBB, but can also reduce the rate of tumor recurrence. Nanofibers can provide a sustained release and a high concentration of chemotherapeutics at the tumor vicinity, while decreasing their systemic exposure and toxicity. In another scenario, aligned nanofibers can mimic the topographical features of the brain extracellular matrix (ECM), which can be utilized for in vitro studies on GBM cell migration. This strategy is beneficial to investigate the interactions of tumor cells with the microenvironment which has a dominant role in regulating tumor formation, progression, and metastasis.

Role of Microenvironment in Glioma Invasion: What We Learned from In Vitro Models

The invasion properties of glioblastoma hamper a radical surgery and are responsible for its recurrence. Understanding the invasion mechanisms is thus critical to devise new therapeutic strategies. Therefore, the creation of in vitro models that enable these mechanisms to be studied represents a crucial step. Since in vitro models represent an over-simplification of the in vivo system, in these years it has been attempted to increase the level of complexity of in vitro assays to create models that could better mimic the behaviour of the cells in vivo. These levels of complexity involved: 1. The dimension of the system, moving from two-dimensional to three-dimensional models; 2. The use of microfluidic systems; 3. The use of mixed cultures of tumour cells and cells of the tumour micro-environment in order to mimic the complex cross-talk between tumour cells and their micro-environment; 4. And the source of cells used in an attempt to move from commercial lines to patient-based models. In this review, we will summarize the evidence obtained exploring these different levels of complexity and highlighting advantages and limitations of each system used.