Hydrogels for 3D Neural Tissue Models: Understanding Cell-Material Interactions at a Molecular Level

Abstract

The development of 3D neural tissue analogs is of great interest to a range of biomedical engineering applications including tissue engineering of neural interfaces, treatment of neurodegenerative diseases and in vitro assessment of cell-material interactions. Despite continued efforts to develop synthetic or biosynthetic hydrogels which promote the development of complex neural networks in 3D, successful long-term 3D approaches have been restricted to the use of biologically derived constructs. In this study a poly (vinyl alcohol) biosynthetic hydrogel functionalized with gelatin and sericin (PVA-SG), was used to understand the interplay between cell-cell communication and cell-material interaction. This was used to probe critical shortterm interactions that determine the success or failure of neural network growth and ultimately the development of a useful model. Complex primary ventral mesencephalic (VM) neural cells were encapsulated in PVA-SG hydrogels and critical molecular cues that demonstrate mechanosensory interaction were examined. Neuronal presence was constant over the 10 day culture, but the astrocyte population decreased in number. The lack of astrocytic support led to a reduction in neural process outgrowth from 24.0 ± 1.3 µm on Day 7 to 7.0 ± 0.1 µm on Day 10. Subsequently, purified astrocytes were studied in isolation to understand the reasons behind PVA-SG hydrogel inability to support neural network development. It was proposed that the spatially restrictive nature (or tight mesh size) of PVA-SG hydrogels limited the astrocytic actin polymerization together with a cytoplasmic-nuclear translocation of YAP over time, causing an alteration in their cell cycle. This was confirmed by the evaluation of p27 /Kip1 gene that was found to be upregulated by a twofold increase in expression at both Days 7 and 10 compared to Day 3, indicating the quiescent stage of the astrocytes in PVA-SG hydrogel. Cell migration was further studied by the quantification of MMP-2 production that was negligible compared to 2D controls, ranging from 2.7 ± 2.3% on Day 3 to 5.3 ± 2.9% on Day 10. This study demonstrates the importance of understanding astrocyte-material interactions at the molecular level, with the need to address spatial constraints in the 3D hydrogel environment. These findings will inform the design of future hydrogel constructs with greater capacity for remodeling by the cell population to create space for cell migration and neural process extension.

Biomaterials-based strategies for in vitro neural models

“…Rendering the hydrogel network density constant has enabled to test cell mobility behavior such as migration and neurite maneuvering in response to matrix tightness and was successfully used to study cell-cell, cell-material interactions. 165 Regardless of cross-linking method, one of the most significant use of hydrogels has been in vitro models of Alzheimer's disease where Aβ secreted by embedded cells is retained within the vicinity to capture toxicity mechanisms of pathophysiological conditions. 86 Perhaps one of the most popular techniques of hydrogel fabrication in recent years is the use of UV-activated moieties attached as side groups to polymer backbones, especially naturally derived ones.…”

Neuromechanobiology: An Expanding Field Driven by the Force of Greater Focus

“…Without appropriate mechanical cues, the biomaterials solution to neural tissues may not be successful, as demonstrated in a recent study of 3D neural tissue analogs where insufficient cues for astrocytic mechanosignaling through YAP/actin polymerization resulted in the inability to form appropriate neuron-astrocyte networks and growth. [264]…”

Advanced Bio-Based Polymers for Astrocyte Cell Models

“…In three-dimensional systems, more complex interactions can be modeled effectively. A critical obstacle in the development of primary three-dimensional neural tissue analogs is the need to support multiple cell types in the same environment [141]. To obtain a true three-dimensional model, the encapsulation of neural cells in hydrogels is an increasingly popular and important technique.…”