Flow Shear Stress Enhances the Proliferative Potential of Cultured Radial Glial Cells Possibly Via an Activation of Mechanosensitive Calcium Channel

Flow-Induced Shear Stress Combined with Microtopography Inhibits the Differentiation of Neuro-2a Cells

Considering that neurological injuries cannot typically self-recover, there is a need to develop new methods to study neuronal outgrowth in a controllable manner in vitro. In this study, a precise flow-controlled microfluidic system featuring custom-designed chambers that integrate laser-microstructured polyethylene terephthalate (PET) substrates comprising microgrooves (MGs) was developed to investigate the combined effect of shear stress and topography on Neuro-2a (N2a) cells' behavior. The MGs were positioned parallel to the flow direction and the response of N2a cells was evaluated in terms of growth and differentiation. Our results demonstrate that flow-induced shear stress could inhibit the differentiation of N2a cells. This microfluidic system could potentially be used as a new model system to study the impact of shear stress on cell differentiation.

Calcium and Neural Stem Cell Proliferation

Intracellular calcium plays a pivotal role in central nervous system (CNS) development by regulating various processes such as cell proliferation, migration, differentiation, and maturation. However, understanding the involvement of calcium (Ca2+) in these processes during CNS development is challenging due to the dynamic nature of this cation and the evolving cell populations during development. While Ca2+ transient patterns have been observed in specific cell processes and molecules responsible for Ca2+ homeostasis have been identified in excitable and non-excitable cells, further research into Ca2+ dynamics and the underlying mechanisms in neural stem cells (NSCs) is required. This review focuses on molecules involved in Ca2+ entrance expressed in NSCs in vivo and in vitro, which are crucial for Ca2+ dynamics and signaling. It also discusses how these molecules might play a key role in balancing cell proliferation for self-renewal or promoting differentiation. These processes are finely regulated in a time-dependent manner throughout brain development, influenced by extrinsic and intrinsic factors that directly or indirectly modulate Ca2+ dynamics. Furthermore, this review addresses the potential implications of understanding Ca2+ dynamics in NSCs for treating neurological disorders. Despite significant progress in this field, unraveling the elements contributing to Ca2+ intracellular dynamics in cell proliferation remains a challenging puzzle that requires further investigation.

Neuron(s)-on-a-Chip: A Review of the Design and Use of Microfluidic Systems for Neural Tissue Culture

Neuron-on-chip (NoC) systems-microfluidic devices in which neurons are cultured-have become a promising alternative to replace or minimize the use of animal models and have greatly facilitated in vitro research. Here, we review and discuss current developments in neuron-on-chip platforms, with a particular emphasis on existing biological models, culturing techniques, biomaterials, and topologies. We also discuss how the architecture, flow, and gradients affect neuronal growth, differentiation, and development. Finally, we discuss some of the most recent applications of NoCs in fundamental research (i.e., studies on the effects of electrical, mechanical/topological, or chemical stimuli) and in disease modeling.

A controllable perfusion microfluidic chip for facilitating the development of retinal ganglion cells in human retinal organoids

Retinal organoids (ROs) derived from human pluripotent stem cells (hPSCs) have become a promising model in vitro to recapitulate human retinal development, which can be further employed to explore the mechanisms of retinal diseases. However, the current culture systems for ROs lack physiologically relevant microenvironments, such as controllable mechano-physiological cues and dynamic feedback between cells and the extracellular matrix (ECM), which limits the accurate control of RO development. Therefore, we designed a controllable perfusion microfluidic chip (CPMC) with the advantages of precisely controlling fluidic shear stress (FSS) and oxygen concentration distribution in a human embryonic stem cell (hESC)-derived RO culture system. We found that ROs cultured under this system allow for expanding the retinal progenitor cell (RPC) pool, orchestrating the retinal ganglion cell (RGC) specification, and axon growth without disturbing the spatial and temporal patterning events at the early stage of RO development. Furthermore, RNA sequencing data revealed that the activation of voltage-gated ion channels and the increased expression of ECM components synergistically improve the growth of ROs and facilitate the differentiation of RGCs. This study elaborates on the advantages of the designed CPMC to promote RO growth and provide a controllable and reliable platform for the efficient maturity of RGCs in the ROs, promising applications in modeling RGC-related disorders, drug screening, and cell transplantation.

Microfluidic Systems for Neural Cell Studies

Whereas the axons of the peripheral nervous system (PNS) spontaneously regenerate after an injury, the occurring regeneration is rarely successful because axons are usually directed by inappropriate cues. Therefore, finding successful ways to guide neurite outgrowth, in vitro, is essential for neurogenesis. Microfluidic systems reflect more appropriately the in vivo environment of cells in tissues such as the normal fluid flow within the body, consistent nutrient delivery, effective waste removal, and mechanical stimulation due to fluid shear forces. At the same time, it has been well reported that topography affects neuronal outgrowth, orientation, and differentiation. In this review, we demonstrate how topography and microfluidic flow affect neuronal behavior, either separately or in synergy, and highlight the efficacy of microfluidic systems in promoting neuronal outgrowth.

Sonomechanobiology: Vibrational stimulation of cells and its therapeutic implications

All cells possess an innate ability to respond to a range of mechanical stimuli through their complex internal machinery. This comprises various mechanosensory elements that detect these mechanical cues and diverse cytoskeletal structures that transmit the force to different parts of the cell, where they are transcribed into complex transcriptomic and signaling events that determine their response and fate. In contrast to static (or steady) mechanostimuli primarily involving constant-force loading such as compression, tension, and shear (or forces applied at very low oscillatory frequencies (≤ 1 Hz) that essentially render their effects quasi-static), dynamic mechanostimuli comprising more complex vibrational forms (e.g., time-dependent, i.e., periodic, forcing) at higher frequencies are less well understood in comparison. We review the mechanotransductive processes associated with such acoustic forcing, typically at ultrasonic frequencies (> 20 kHz), and discuss the various applications that arise from the cellular responses that are generated, particularly for regenerative therapeutics, such as exosome biogenesis, stem cell differentiation, and endothelial barrier modulation. Finally, we offer perspectives on the possible existence of a universal mechanism that is common across all forms of acoustically driven mechanostimuli that underscores the central role of the cell membrane as the key effector, and calcium as the dominant second messenger, in the mechanotransduction process.

In vivo vascularized scaffold with different shear-exposed models for lymphatic tissue regeneration

Current clinical treatments on lymphedema provide promising results, but also result in donor site morbidities. The establishment of a microenvironment optimized for lymphangiogenesis can be an alternative way to enhance lymphatic tissue formation. Hemodynamic flow stimuli have been confirmed to have an influential effect on angiogenesis in tissue engineering, but not on lymphatic vessel formation. Here, the three in vivo scaffolds generated from different blood stimuli in the subcutaneous layer, in the flow through pedicle, and in an arterio-venous (AV) loop model, were created to investigate potential of lymphangiogenesis of scaffolds containing lymphatic endothelial cells (LECs). Our results indicated that AV loop model displayed better lymphangiogenesis in comparison to the other two models with slower flow or no stimuli. Other than hemodynamic force, the supplement of LECs is required for lymphatic vessel regeneration. The in vivo scaffold generated from AV loop model provides an effective approach for engineering lymphatic tissue in the clinical treatment of lymphedema.

Biomechanical forces in the aged brain: Relationship to AD

The pathogenesis of neurodegenerative disorders, including Alzheimer's disease, has been studied with a focus on biochemical mechanisms, such as the amyloid-β plaque deposition and removal. Recently, the importance of brain microenvironmental cues, which comprise the sophisticated cellular and fluid system, has been emphasized in the aged brain or in pathological conditions. Especially, substrate rigidity and biomechanical forces of the brain microenvironment determine the function of glial cells and neurons; furthermore, these microenvironmental cues change with age. However, our understanding of role of the biomechanical cues on glial cells and neurons is relatively poor. In this review, we briefly introduce an overview of biomechanical forces that present in the aged brain and its sensations, and then examine the brain in Alzheimer's disease, which constitutes a representative neurodegenerative disorder, with regard to changes in the biomechanical forces associated with disease and aging.

A developmental pathway for epithelial-to-motoneuron transformation in C. elegans

Motoneurons and motoneuron-like pancreatic β cells arise from radial glia and ductal cells, respectively, both tube-lining progenitors that share molecular regulators. To uncover programs underlying motoneuron formation, we studied a similar, cell-division-independent transformation of the C. elegans tube-lining Y cell into the PDA motoneuron. We find that lin-12/Notch acts through ngn-1/Ngn and its regulator hlh-16/Olig to control transformation timing. lin-12 loss blocks transformation, while lin-12(gf) promotes precocious PDA formation. Early basal expression of ngn-1/Ngn and hlh-16/Olig depends on sem-4/Sall and egl-5/Hox. Later, coincident with Y cell morphological changes, ngn-1/Ngn expression is upregulated in a sem-4/Sall and egl-5/Hox-dependent but hlh-16/Olig-independent manner. Subsequently, Y cell retrograde extension forms an anchored process priming PDA axon extension. Extension requires ngn-1-dependent expression of the cytoskeleton organizers UNC-119, UNC-44/ANK, and UNC-33/CRMP, which also activate PDA terminal-gene expression. Our findings uncover cell-division-independent regulatory events leading to motoneuron generation, suggesting a conserved pathway for epithelial-to-motoneuron/motoneuron-like cell differentiation.

Rashid et al. report on a conserved epithelial-to-motoneuron transformation pathway in C. elegans requiring ngn-1/Ngn and hlh-16/Olig. lin-12/Notch regulates transformation timing through these genes, while ngn-1/Ngn and hlh-16/Olig expression levels are regulated by sem-4/Sall and egl-5/Hox. Unexpectedly, the cytoskeleton organizers UNC-119, UNC-44, and UNC-33, which are ngn-1/Ngn targets, promote motoneuron terminal identity.

Measuring the density and viscosity of culture media for optimized computational fluid dynamics analysis of in vitro devices

Culture medium is frequently modelled as water in computational fluid dynamics (CFD) analysis of in vitro culture systems involving flow, such as bioreactors and organ-on-chips. However, culture medium can be expected to have different properties to water due to its higher solute content. Furthermore, cellular activities such as metabolism and secretion of ECM proteins alter the composition of culture medium and therefore its properties during culture. As these properties directly determine the hydromechanical stimuli exerted on cells in vitro, these, along with any changes during culture must be known for CFD modelling accuracy and meaningful interpretation of cellular responses. In this study, the density and dynamic viscosity of DMEM and RPMI-1640 media supplemented with typical concentrations of foetal bovine serum (0, 5, 10 and 20% v/v) were measured to serve as a reference for computational design analysis. Any changes in the properties of medium during culture were also investigated with NCI-H460 and HN6 cell lines. The density and dynamic viscosity of the media increased proportional to the % volume of added foetal bovine serum (FBS). Importantly, the viscosity of 5% FBS-supplemented RPMI-1640 was found to increase significantly after 3 days of culture of NCI-H460 and HN6 cell lines, with distinct differences between magnitude of change for each cell line. Finally, these experimentally-derived values were applied in CFD analysis of a simple microfluidic device, which demonstrated clear differences in maximum wall shear stress and pressure between fluid models. Overall, these results highlight the importance of characterizing model-specific properties for CFD design analysis of cell culture systems.

Microphysiological models of the central nervous system with fluid flow

There are over 1,000 described neurological and neurodegenerative disorders affecting nearly 100 million Americans - roughly one third of the U.S. population. Collectively, treatment of neurological conditions is estimated to cost $800 billion every year. Lowering this societal burden will require developing better model systems in which to study these diverse disorders. Microphysiological systems are promising tools for modeling healthy and diseased neural tissues to study mechanisms and treatment of neuropathology. One major benefit of microphysiological systems is the ability to incorporate biophysical forces, namely the forces derived from biological fluid flow. Fluid flow in the central nervous system (CNS) is a complex but important element of physiology, and pathologies as diverse as traumatic or ischemic injury, cancer, neurodegenerative disease, and natural aging have all been found to alter flow pathways. In this review, we summarize recent advances in three-dimensional microphysiological systems for studying the biology and therapy of CNS disorders and highlight the ability and growing need to incorporate biological fluid flow in these miniaturized model systems.

Advances in microfluidic in vitro systems for neurological disease modeling

Neurological disorders are the leading cause of disability and the second largest cause of death worldwide. Despite significant research efforts, neurology remains one of the most failure-prone areas of drug development. The complexity of the human brain, boundaries to examining the brain directly in vivo, and the significant evolutionary gap between animal models and humans, all serve to hamper translational success. Recent advances in microfluidic in vitro models have provided new opportunities to study human cells with enhanced physiological relevance. The ability to precisely micro-engineer cell-scale architecture, tailoring form and function, has allowed for detailed dissection of cell biology using microphysiological systems (MPS) of varying complexities from single cell systems to "Organ-on-chip" models. Simplified neuronal networks have allowed for unique insights into neuronal transport and neurogenesis, while more complex 3D heterotypic cellular models such as neurovascular unit mimetics and "Organ-on-chip" systems have enabled new understanding of metabolic coupling and blood-brain barrier transport. These systems are now being developed beyond MPS toward disease specific micro-pathophysiological systems, moving from "Organ-on-chip" to "Disease-on-chip." This review gives an outline of current state of the art in microfluidic technologies for neurological disease research, discussing the challenges and limitations while highlighting the benefits and potential of integrating technologies. We provide examples of where such toolsets have enabled novel insights and how these technologies may empower future investigation into neurological diseases.

RNA-binding proteins balance brain function in health and disease

Posttranscriptional gene expression including splicing, RNA transport, translation, and RNA decay provides an important regulatory layer in many if not all molecular pathways. Research in the last decades has positioned RNA-binding proteins (RBPs) right in the center of posttranscriptional gene regulation. Here, we propose interdependent networks of RBPs to regulate complex pathways within the central nervous system (CNS). These are involved in multiple aspects of neuronal development and functioning, including higher cognition. Therefore, it is not sufficient to unravel the individual contribution of a single RBP and its consequences but rather to study and understand the tight interplay between different RBPs. In this review, we summarize recent findings in the field of RBP biology and discuss the complex interplay between different RBPs. Second, we emphasize the underlying dynamics within an RBP network and how this might regulate key processes such as neurogenesis, synaptic transmission, and synaptic plasticity. Importantly, we envision that dysfunction of specific RBPs could lead to perturbation within the RBP network. This would have direct and indirect (compensatory) effects in mRNA binding and translational control leading to global changes in cellular expression programs in general and in synaptic plasticity in particular. Therefore, we focus on RBP dysfunction and how this might cause neuropsychiatric and neurodegenerative disorders. Based on recent findings, we propose that alterations in the entire regulatory RBP network might account for phenotypic dysfunctions observed in complex diseases including neurodegeneration, epilepsy, and autism spectrum disorders.

Near-Infrared Light Triggered-Release in Deep Brain Regions Using Ultra-photosensitive Nanovesicles

Remote and minimally-invasive modulation of biological systems with light has transformed modern biology and neuroscience. However, light absorption and scattering significantly prevents penetration to deep brain regions. Herein, we describe the use of gold-coated mechanoresponsive nanovesicles, which consist of liposomes made from the artificial phospholipid Rad-PC-Rad as a tool for the delivery of bioactive molecules into brain tissue. Near-infrared picosecond laser pulses activated the gold-coating on the surface of nanovesicles, creating nanomechanical stress and leading to near-complete vesicle cargo release in sub-seconds. Compared to natural phospholipid liposomes, the photo-release was possible at 40 times lower laser energy. This high photosensitivity enables photorelease of molecules down to a depth of 4 mm in mouse brain. This promising tool provides a versatile platform to optically release functional molecules to modulate brain circuits.

The role of extracellular matrix in biomechanics and its impact on bioengineering of cells and 3D tissues

The cells and tissues of the human body are constantly exposed to exogenous and endogenous forces that are referred to as biomechanical cues. They guide and impact cellular processes and cell fate decisions on the nano-, micro- and macro-scale, and are therefore critical for normal tissue development and maintaining tissue homeostasis. Alterations in the extracellular matrix composition of a tissue combined with abnormal mechanosensing and mechanotransduction can aberrantly activate signaling pathways that promote disease development. Such processes are therefore highly relevant for disease modelling or when aiming for the development of novel therapies. In this mini review, we describe the main biomechanical cues that impact cellular fates. We highlight their role during development, homeostasis and in disease. We also discuss current techniques and tools that allow us to study the impact of biomechanical cues on cell and tissue development under physiological conditions, and we point out directions, in which in vitro biomechanics can be of use in the future.

Influence of mechanical fluid shear stress on the osteogenic differentiation protocols for Equine adipose tissue-derived mesenchymal stem cells

Cell-based therapies have become a promising approach to promote tissue regeneration and the treatment of musculoskeletal disorders. Bone regeneration maintains bone homeostasis, mechanical stability and physical performance. Mechanical stimulation showed to induce stem cell differentiation into the osteogenic fate. However, the effect of various osteogenic protocols on the osteogenic commitment of equine adipose-derived stem cells is not fully elucidated. Here we examined the influence of fluid-based shear stress (FSS) via mechanical rocking to assess whether mechanical stimulation promotes osteogenic differentiation of equine adipose-derived stem cells (ASCs). ASCs were induced into osteogenic fate using osteogenic differentiation medium (ODM) protocol or additional supplementation of 5 mM CaCl₂ and 7.5 mM CaCl₂ protocol compared to cells cultivated in basal medium (BM) up to 21 day. The ASCs proliferation pattern was evaluated using the sulforhodamine B (SRB) protein assay. Osteogenic differentiation examined via semi-quantification of alizarin red staining (ARS) and alkaline phosphatase activity (ALP) as well as, via quantification of osteocalcin (OC), alkaline phosphatase (ALP), osteopontin (OP), and collagen type-1 (COL1) gene expression using RT-qPCR. We show that mechanical FSS increased the proliferation pattern of ASCs compared to the static conditions. Mechanical FSS together with 5 mM CaCl₂ and 7.5 mM CaCl₂ promoted osteogenic nodule formation and increased ARS intensity compared to the standard osteogenic protocols. We observed that combined mechanical FSS with ODM protocol increase ALP activity compared to static culture conditions. We report that ALP and OC osteogenic markers expression were upregulated under mechanical FSS culture condition particularly with the ODM protocol. Taken together, it can be assumed that mechanical stress using FSS promotes the efficiency of the osteogenic differentiation protocols of ASCs through independent mechanisms.