Three dimensional microenvironments on multi-electrode arrays produce neuronal networks that function like the brain

• ¹ University of Melbourne, Department of Clinical Neurosciences, St Vincent's Hospital, Australia
• ² Australian Research Council Centre of Excellence for Electromaterials Science, Australia
• ³ St Vincent's Hospital (Melbourne), Department of Medicine, Australia
• ⁴ University of Wollongong, Intelligent Polymer Research Institute, Australia

Introduction: Engineered 3D constructs for cell culture mimic in vivo conditions better than traditional 2D substrates, resulting in controlled cellular growth patterns[1] and improved cell function[2-5]. More critically from a tissue engineering perspective, such 3D substrates may improve intercellular interactions and thus whole-tissue and organoid functionality. Previous studies have generated pseudo-3D neuronal networks using beads to separate neuronal positioning[6], with MEA recordings providing insight into changes in function from such substrate designs[7-8]. Functional network differences exist between these bead-based substrate networks and traditional 2D neuronal networking, although the authors point out the limited inter-layer connectivity, with 7 times more neuronal interconnectivity within layers than between due to neurites extending only on bead surfaces. We have developed a true 3D networking model based on collagen where neurite positioning and connectivity is not limited by the substrate. We used microelectrode arrays to investigate functional differences between neuronal networks formed in 2D and 3D, and determined that 3D cultures produce neuronal network function more akin to networks of the intact brain. Methods: Embryonic rat hippocampal neurons were cultured in 2D and in 3D collagen gels upon microelectrode arrays (Multichannel Systems). Cultures were maintained for 56 days, with electrophysiological recordings taken every 7 days. Neuronal cell function at each electrode and network activity between electrodes were compared between 2D and 3D cultures. Results and Discussion: Neural activation originated from spontaneously active pace-making regions within both 2D and 3D cultures. In 2D, activity flowed directly across the planar cultures resulting in a short repetitive and predictable spatial and temporal bursting pattern, with one burst at each electrode per activation of the pace-making region (Fig 1a). In contrast, the 3D constructs produced regular oscillatory activity throughout the higher order neural networks, resulting in spiking patterns of increased burst duration (Fig 1b), a pattern that more closely mimics network activity within the intact brain. Synchronous activation was observed throughout both 2D and 3D networks, indicating network connectivity throughout the cultures. Previous bead-based materials produce network functionality with both short interval regular spiking and longer duration bursts, a mix of network function observed in 2D and true 3D collagen networks observed within this study. The results observed illustrate the importance of a true 3D neuronal network microenvironment to producing network function akin to the in vivo state. Conclusions: Mimicking the natural microenvironment in cell culture applications is critical to achieving more natural single-cell function. In addition, characterisation of intercellular interactions within such tissue engineered constructs is critical to understanding whole-tissue and organoid level functionality. From a neural construct perspective, we have shown that neural network formation and function in three-dimensions mimics network function of the intact brain more accurately than conventional two-dimensional cultures. Measurement of neural network function at the whole-tissue level in three-dimensional neural cultures will optimise outcomes for network studies and allow for investigation of mechanisms behind neural network formation and neural networking disorders such as epilepsy and schizophrenia. Paired with neurons derived from induced pluripotent stem cells from patients, the findings here will facilitate a personalised medicine approach to drug efficacy studies for individuals suffering epilepsy and schizophrenia. This presentation: This presentation will concentrate on the yet to be published methods to generate such 3D networks, analysis of the networking activity, and work currently underway in applying such networks to a clinical model for drug selection for patients suffering from epilepsy. Figure 1: Typical single electrode recordings in (Ai) 2D and (bi) 3D cultures (adapted from Bourke et al [5]). Synchronicity within raster plots of spike times across multiple electrode in (Aii) 2D and (Bii) 3D cultures indicate fully connected neural networks in both culture conditions. Note the fast spiking patterns in 2D cultures and slower, highly consistent bursting patterns in 3D networks.