Controlled microfluidics to examine growth-factor induced migration of neural progenitors in the Drosophila visual system

A Micro-Optic Stalk (μOS) System to Model the Collective Migration of Retinal Neuroblasts

Contemporary regenerative therapies have introduced stem-like cells to replace damaged neurons in the visual system by recapitulating critical processes of eye development. The collective migration of neural stem cells is fundamental to retinogenesis and has been exceptionally well-studied using the fruit fly model of Drosophila Melanogaster. However, the migratory behavior of its retinal neuroblasts (RNBs) has been surprisingly understudied, despite being critical to retinal development in this invertebrate model. The current project developed a new microfluidic system to examine the collective migration of RNBs extracted from the developing visual system of Drosophila as a model for the collective motile processes of replacement neural stem cells. The system scales with the microstructure of the Drosophila optic stalk, which is a pre-cursor to the optic nerve, to produce signaling fields spatially comparable to in vivo RNB stimuli. Experiments used the micro-optic stalk system, or μOS, to demonstrate the preferred sizing and directional migration of collective, motile RNB groups in response to changes in exogenous concentrations of fibroblast growth factor (FGF), which is a key factor in development. Our data highlight the importance of cell-to-cell contacts in enabling cell cohesion during collective RNB migration and point to the unexplored synergy of invertebrate cell study and microfluidic platforms to advance regenerative strategies.

Fly-on-a-Chip: Microfluidics for Drosophila melanogaster Studies

The fruit fly or Drosophila melanogaster has been used as a promising model organism in genetics, developmental and behavioral studies as well as in the fields of neuroscience, pharmacology, and toxicology. Not only all the developmental stages of Drosophila, including embryonic, larval, and adulthood stages, have been used in experimental in vivo biology, but also the organs, tissues, and cells extracted from this model have found applications in in vitro assays. However, the manual manipulation, cellular investigation and behavioral phenotyping techniques utilized in conventional Drosophila-based in vivo and in vitro assays are mostly time-consuming, labor-intensive, and low in throughput. Moreover, stimulation of the organism with external biological, chemical, or physical signals requires precision in signal delivery, while quantification of neural and behavioral phenotypes necessitates optical and physical accessibility to Drosophila. Recently, microfluidic and lab-on-a-chip devices have emerged as powerful tools to overcome these challenges. This review paper demonstrates the role of microfluidic technology in Drosophila studies with a focus on both in vivo and in vitro investigations. The reviewed microfluidic devices are categorized based on their applications to various stages of Drosophila development. We have emphasized technologies that were utilized for tissue- and behavior-based investigations. Furthermore, the challenges and future directions in Drosophila-on-a-chip research, and its integration with other advanced technologies, will be discussed.

Invertebrate Retinal Progenitors as Regenerative Models in a Microfluidic System

Regenerative retinal therapies have introduced progenitor cells to replace dysfunctional or injured neurons and regain visual function. While contemporary cell replacement therapies have delivered retinal progenitor cells (RPCs) within customized biomaterials to promote viability and enable transplantation, outcomes have been severely limited by the misdirected and/or insufficient migration of transplanted cells. RPCs must achieve appropriate spatial and functional positioning in host retina, collectively, to restore vision, whereas movement of clustered cells differs substantially from the single cell migration studied in classical chemotaxis models. Defining how RPCs interact with each other, neighboring cell types and surrounding extracellular matrixes are critical to our understanding of retinogenesis and the development of effective, cell-based approaches to retinal replacement. The current article describes a new bio-engineering approach to investigate the migratory responses of innate collections of RPCs upon extracellular substrates by combining microfluidics with the well-established invertebrate model of Drosophila melanogaster. Experiments utilized microfluidics to investigate how the composition, size, and adhesion of RPC clusters on defined extracellular substrates affected migration to exogenous chemotactic signaling. Results demonstrated that retinal cluster size and composition influenced RPC clustering upon extracellular substrates of concanavalin (Con-A), Laminin (LM), and poly-L-lysine (PLL), and that RPC cluster size greatly altered collective migratory responses to signaling from Fibroblast Growth Factor (FGF), a primary chemotactic agent in Drosophila. These results highlight the significance of examining collective cell-biomaterial interactions on bio-substrates of emerging biomaterials to aid directional migration of transplanted cells. Our approach further introduces the benefits of pairing genetically controlled models with experimentally controlled microenvironments to advance cell replacement therapies.

Stem cells technology: a powerful tool behind new brain treatments

Stem cell research has recently become a hot research topic in biomedical research due to the foreseen unlimited potential of stem cells in tissue engineering and regenerative medicine. For many years, medicine has been facing intense challenges, such as an insufficient number of organ donations that is preventing clinicians to fulfill the increasing needs. To try and overcome this regrettable matter, research has been aiming at developing strategies to facilitate the in vitro culture and study of stem cells as a tool for tissue regeneration. Meanwhile, new developments in the microfluidics technology brought forward emerging cell culture applications that are currently allowing for a better chemical and physical control of cellular microenvironment. This review presents the latest developments in stem cell research that brought new therapies to the clinics and how the convergence of the microfluidics technology with stem cell research can have positive outcomes on the fields of regenerative medicine and high-throughput screening. These advances will bring new translational solutions for drug discovery and will upgrade in vitro cell culture to a new level of accuracy and performance. We hope this review will provide new insights into the understanding of new brain treatments from the perspective of stem cell technology especially regarding regenerative medicine and tissue engineering.

High-throughput three-dimensional chemotactic assays reveal steepness-dependent complexity in neuronal sensation to molecular gradients

Many cellular programs of neural development are under combinatorial regulation by different chemoattractive or chemorepulsive factors. Here, we describe a microfluidic platform that utilizes well-controlled three-dimensional (3D) diffusion to generate molecular gradients of varied steepness in a large array of hydrogel cylinders, allowing high-throughput 3D chemotactic assays for mechanistic dissection of steepness-dependent neuronal chemotaxis. Using this platform, we examine neuronal sensitivity to the steepness of gradient composed of netrin-1, nerve growth factor, or semaphorin3A (Sema3A) proteins, and reveal dramatic diversity and complexity in the associated chemotactic regulation of neuronal development. Particularly for Sema3A, we find that serine/threonine kinase-11 and glycogen synthase kinase-3 signaling pathways are differentially involved in steepness-dependent chemotactic regulation of coordinated neurite repellence and neuronal migration. These results provide insights to the critical role of gradient steepness in neuronal chemotaxis, and also prove the technique as an expandable platform for studying other chemoresponsive cellular systems.

Controlled microenvironments to evaluate chemotactic properties of cultured Müller glia

Emerging therapies have begun to evaluate the abilities of Müller glial cells (MGCs) to protect and/or regenerate neurons following retina injury. The migration of donor cells is central to many reparative strategies, where cells must achieve appropriate positioning to facilitate localized repair. Although chemical cues have been implicated in the MGC migratory responses of numerous retinopathies, MGC-based therapies have yet to explore the extent to which external biochemical stimuli can direct MGC behavior. The current study uses a microfluidics-based assay to evaluate the migration of cultured rMC-1 cells (as model MGC) in response to quantitatively-controlled microenvironments of signaling factors implicated in retinal regeneration: basic Fibroblast Growth factor (bFGF or FGF2); Fibroblast Growth factor 8 (FGF8); Vascular Endothelial Growth Factor (VEGF); and Epidermal Growth Factor (EGF). Findings indicate that rMC-1 cells exhibited minimal motility in response to FGF2, FGF8 and VEGF, but highly-directional migration in response to EGF. Further, the responses were blocked by inhibitors of EGF-R and of the MAPK signaling pathway. Significantly, microfluidics data demonstrate that changes in the EGF gradient (i.e. change in EGF concentration over distance) resulted in the directional chemotactic migration of the cells. By contrast, small increases in EGF concentration, alone, resulted in non-directional cell motility, or chemokinesis. This microfluidics-enhanced approach, incorporating the ability both to modulate and asses the responses of motile donor cells to a range of potential chemotactic stimuli, can be applied to potential donor cell populations obtained directly from human specimens, and readily expanded to incorporate drug-eluting biomaterials and combinations of desired ligands.