Substrate-bound protein gradients for cell culture fabricated by microfluidic networks and microcontact printing

Recent progresses in novel in vitro models of primary neurons: A biomaterial perspective

Central nervous system (CNS) diseases have been a growing threat to the health of humanity, emphasizing the urgent need of exploring the pathogenesis and therapeutic approaches of various CNS diseases. Primary neurons are directly obtained from animals or humans, which have wide applications including disease modeling, mechanism exploration and drug development. However, traditional two-dimensional (2D) monoculture cannot resemble the native microenvironment of CNS. With the increasing understanding of the complexity of the CNS and the remarkable development of novel biomaterials, in vitro models have experienced great innovation from 2D monoculture toward three-dimensional (3D) multicellular culture. The scope of this review includes the progress of various in vitro models of primary neurons in recent years to provide a holistic view of the modalities and applications of primary neuron models and how they have been connected with the revolution of biofabrication techniques. Special attention has been paid to the interaction between primary neurons and biomaterials. First, a brief introduction on the history of CNS modeling and primary neuron culture was conducted. Next, detailed progress in novel in vitro models were discussed ranging from 2D culture, ex vivo model, spheroid, scaffold-based model, 3D bioprinting model, and microfluidic chip. Modalities, applications, advantages, and limitations of the aforementioned models were described separately. Finally, we explored future prospects, providing new insights into how basic science research methodologies have advanced our understanding of the CNS, and highlighted some future directions of primary neuron culture in the next few decades.

Grayscale surface patterning using electrophoretic motion through a heterogeneous hydrogel material

Chemical surface patterning can be an incredibly powerful tool in a variety of applications, as it enables precise spatial control over surface properties. But the equipment required to create functional surface patterns-especially "grayscale" patterns where independent control over species placement and density are needed-is often expensive and inaccessible. In this work, we leveraged equipment and methods readily available to many research labs, namely 3D printing and electroblotting, to generate controlled grayscale surface patterns. Three-dimensional-printed molds were used to cast polyacrylamide hydrogels with regions of variable polymer density; regions of low polymer density within the hydrogels served as reservoirs for proteins that were later driven onto a target surface using electrophoresis. This mechanism was used to deposit grayscale patterns of fluorescently labeled proteins, and the fluorescent intensity of these patterns was measured and compared to a theoretical analysis of the deposition mechanism.

Ovarian and Breast Cancer Migration Dynamics on Laminin and Fibronectin Bidirectional Gradient Fibers Fabricated via Multiphoton Excited Photochemistry

Introduction

Migration mis-regulation is a hallmark of cancer, and remains an important problem in cancer biology. We postulate the needs for better in vitro models to understand the details of cell-matrix interactions. Here, we utilized multiphoton excited (MPE) photochemistry to fabricate models to systematically study migration dynamics operative in breast and ovarian cancer. Gradients are a convenient means to modulate concentration and also have been implicated in metastases.

Methods

We specifically pattern sub-micron structured gradients from laminin and fibronectin whose up-regulation is associated with increased metastasis and poor prognosis. We developed a new continuous linear bi-directional gradient design, permitting exploration of the underlying cell-matrix interactions of migration, including speed, directness, and f-actin cytoskeleton alignment as a function of concentration. These new models provide both contact guidance and ECM binding cues, and provide a more relevant environment than possible with existing technologies such as flow chambers or 2D printed surfaces.

Results

We found an overall increase in these processes with increasing concentration on both laminin and fibronectin gradients for a series of ovarian and breast cancer lines. Moreover, directness was higher for more metastatic cells, indicating that epithelial or mesenchymal state of the cell type governs the dynamics. However, the specifics of the speed and directedness depend on both the cell type and protein, thus we found that we must consider these processes collectively to obtain a self-consistent picture of the migration. For this purpose, we performed a linear discriminate analysis (LDA) and successfully classified the different cell types on the two protein gradients without molecular biology analysis.

Conclusions

The bi-gradient structures are versatile tools to performing detailed studies of cell migration, specifically haptotxis. We further suggest the can be used in assessing efficacy of drug treatments targeted at specific matrix components.

Stripe Assay to Study the Attractive or Repulsive Activity of a Protein Substrate Using Dissociated Hippocampal Neurons

Growing axons develop a highly motile structure at their tip, termed the growth cone. The growth cone contacts extracellular environmental cues to navigate axonal growth. Netrin, slit, semaphorin, and ephrins are known guidance molecules that can attract or repel axons upon binding to receptors and co-receptors on the axon. The activated receptors initiate various signaling molecules in the growth cone that alter the structure and movement of the neuron. Here, we describe the detailed protocol for a stripe assay to assess the ability of a guidance molecule to attract or repel neurons. In this method, dissociated hippocampal neurons from E15.5 mice are cultured on laminin-coated dishes processed with alternating stripes of ectodomain of fibronectin and leucine-rich transmembrane protein-2 (FLRT2) and control immunoglobulin G (IgG) fragment crystallizable region (Fc) protein. Both axons and cell bodies were strongly repelled from the FLRT2-coated stripe regions after 24 h of culture. Immunostaining with tau1 showed that ~90% of the neurons were distributed on the Fc-coated stripes compared to the FLRT2-Fc-coated stripes (~10%). This result indicates that FLRT2 has a strong repulsive effect on these neurons. This powerful method is applicable not only for primary cultured neurons but also for a variety of other cells, such as neuroblasts.

Micropatterned substrates coated with neuronal adhesion molecules for high-content study of synapse formation

Studying the roles of different proteins and the mechanisms involved in synaptogenesis is hindered by the complexity and heterogeneity of synapse types, and by the spatial and temporal unpredictability of spontaneous synapse formation. Here we demonstrate a robust and high-content method to induce selectively presynaptic or postsynaptic structures at controlled locations. Neurons are cultured on micropatterned substrates comprising arrays of micron-scale dots coated with various synaptogenic adhesion molecules. When plated on neurexin-1β-coated micropatterns, neurons expressing neuroligin-1 exhibit specific dendritic organization and selective recruitment of the postsynaptic scaffolding molecule PSD-95. Furthermore, functional AMPA receptors are trapped at neurexin-1β dots, as revealed by live-imaging experiments. In contrast, neurons plated on SynCAM1-coated substrates exhibit strongly patterned axons and selectively assemble functional presynapses. N-cadherin coating, however, is not able to elicit synapses, indicating the specificity of our system. This method opens the way to both fundamental and therapeutic studies of various synaptic systems.

Label-free characterization of emerging human neuronal networks

The emergent self-organization of a neuronal network in a developing nervous system is the result of a remarkably orchestrated process involving a multitude of chemical, mechanical and electrical signals. Little is known about the dynamic behavior of a developing network (especially in a human model) primarily due to a lack of practical and non-invasive methods to measure and quantify the process. Here we demonstrate that by using a novel optical interferometric technique, we can non-invasively measure several fundamental properties of neural networks from the sub-cellular to the cell population level. We applied this method to quantify network formation in human stem cell derived neurons and show for the first time, correlations between trends in the growth, transport, and spatial organization of such a system. Quantifying the fundamental behavior of such cell lines without compromising their viability may provide an important new tool in future longitudinal studies.

Investigating axonal guidance with microdevice-based approaches

The precise wiring of the nervous system relies on processes by which axons navigate in a complex environment and are guided by a concerted action of attractive and repulsive factors to reach their target. Investigating these guidance processes depends critically on our ability to control in space and time the microenvironment of neurons. The implementation of microfabrication techniques in cell biology now enables a precise control of the extracellular physical and chemical environment of cultured cells. However, microtechnology is only beginning to be applied in the field of axon guidance due to specific requirements of neuronal cultures. Here we review microdevices specifically designed to study axonal guidance and compare them with the conventional assays used to probe gradient sensing in cell biology. We also discuss how innovative microdevice-based approaches will enable the investigation of important systems-level questions on the gradient sensing properties of nerve cells, such as the sensitivity and robustness in the detection of directional signals or the combinatorial response to multiple cues.

New perspectives on neuronal development via microfluidic environments

Understanding the signals that guide neuronal development and direct formation of axons, dendrites, and synapses during wiring of the brain is a fundamental challenge in developmental neuroscience. Discovery of how local signals shape developing neurons has been impeded by the inability of conventional culture methods to interrogate microenvironments of complex neuronal cytoarchitectures, where different subdomains encounter distinct chemical, physical, and fluidic features. Microfabrication techniques are facilitating the creation of microenvironments tailored to neuronal structures and subdomains with unprecedented access and control. The design, fabrication, and properties of microfluidic devices offer significant advantages for addressing unresolved issues of neuronal development. These high-resolution approaches are poised to contribute new insights into mechanisms for restoring neuronal function and connectivity compromised by injury, stress, and neurodegeneration.

Balancing of ephrin/Eph forward and reverse signaling as the driving force of adaptive topographic mapping

The retinotectal projection, which topographically maps retinal axons onto the tectum of the midbrain, is an ideal model system with which to investigate the molecular genetics of embryonic brain wiring. Corroborating Sperry's seminal hypothesis, ephrin/Eph counter-gradients on both retina and tectum were found to represent matching chemospecificity markers. Intriguingly, however, it has never been possible to reconstitute topographically appropriate fiber growth in vitro with these cues. Moreover, experimentally derived molecular mechanisms have failed to provide explanations as to why the mapping adapts to grossly diverse targets in some experiments, while displaying strict point-to-point specificity in others. In vitro, ephrin-A/EphA forward, as well as reverse, signaling mediate differential repulsion to retinal fibers, instead of providing topographic guidance. We argue that those responses are indicative of ephrin-A and EphA being members of a guidance system that requires two counteracting cues per axis. Experimentally, we demonstrate by introducing novel double-cue stripe assays that the simultaneous presence of both cues indeed suffices to elicit topographically appropriate guidance. The peculiar mechanism, which uses forward and reverse signaling through a single receptor/ligand combination, entails fiber/fiber interactions. We therefore propose to extend Sperry's model to include ephrin-A/EphA-based fiber/fiber chemospecificity, eventually out-competing fiber/target interactions. By computational simulation, we show that our model is consistent with stripe assay results. More importantly, however, it not only accounts for classical in vivo evidence of point-to-point and adaptive topographic mapping, but also for the map duplication found in retinal EphA knock-in mice. Nonetheless, it is based on a single constraint of topographic growth cone navigation: the balancing of ephrin-A/EphA forward and reverse signaling.

High-throughput methods to define complex stem cell niches

The potential of stem cells in clinics and as a diagnostic tool is still largely unmet, partially due to a lack of in vitro models that efficiently mimic the in vivo stem cell microenvironment-or niche-and thus would allow reproducible propagation of stem cells or their controlled differentiation in vitro. The current methodological challenges in studying and manipulating stem cells have spurred intense development and application of microfabrication and micropatterning technologies in stem cell biology. These approaches can be readily used to dissect the complex molecular interplay of stem cells and their niche and study single-cell behavior in high-throughput. Increased merging of microfabrication with advanced biomaterials technologies may ultimately result in functional artificial niches capable of recapitulating extrinsic stem cell regulation in vitro and on a single-cell level.

Label-free biological and chemical sensors

Highly sensitive, label-free biodetection methods have applications in both the fundamental research and healthcare diagnostics arenas. Therefore, the development of new transduction methods and the improvement of the existing methods will significantly impact these areas. A brief overview of the different types of biosensors and the critical parameters governing their performance will be given. Additionally, a more in-depth discussion of optical devices, surface functionalization methods to increase device specificity, and fluidic techniques to improve sample delivery will be reviewed.