Substrate Elasticity Exerts Functional Effects on Primary Microglia

Ependymal cells undergo astrocyte-like reactivity in response to neuroinflammation

Ependymal cells form a specialized brain-cerebrospinal fluid (CSF) interface and regulate local CSF microcirculation. It is becoming increasingly recognized that ependymal cells assume a reactive state in response to aging and disease, including conditions involving hypoxia, hydrocephalus, neurodegeneration, and neuroinflammation. Yet what transcriptional signatures govern these reactive states and whether this reactivity shares any similarities with classical descriptions of glial reactivity (i.e., in astrocytes) remain largely unexplored. Using single-cell transcriptomics, we interrogated this phenomenon by directly comparing the reactive ependymal cell transcriptome to the reactive astrocyte transcriptome using a well-established model of autoimmune-mediated neuroinflammation (MOG35-55 EAE). In doing so, we unveiled core glial reactivity-associated genes that defined the reactive ependymal cell and astrocyte response to MOG35-55 EAE. Interestingly, known reactive astrocyte genes from other CNS injury/disease contexts were also up-regulated by MOG35-55 EAE ependymal cells, suggesting that this state may be conserved in response to a variety of pathologies. We were also able to recapitulate features of the reactive ependymal cell state acutely using a classic neuroinflammatory cocktail (IFNγ/LPS) both in vitro and in vivo. Taken together, by comparing reactive ependymal cells and astrocytes, we identified a conserved signature underlying glial reactivity that was present in several neuroinflammatory contexts. Future work will explore the mechanisms driving ependymal reactivity and assess downstream functional consequences.

Interface-Mediated Neurogenic Signaling: The Impact of Surface Geometry and Chemistry on Neural Cell Behavior for Regenerative and Brain-Machine Interfacing Applications

Nanomaterial advancements have driven progress in central and peripheral nervous system applications such as tissue regeneration and brain-machine interfacing. Ideally, neural interfaces with native tissue shall seamlessly integrate, a process that is often mediated by the interfacial material properties. Surface topography and material chemistry are significant extracellular stimuli that can influence neural cell behavior to facilitate tissue integration and augment therapeutic outcomes. This review characterizes topographical modifications, including micropillars, microchannels, surface roughness, and porosity, implemented on regenerative scaffolding and brain-machine interfaces. Their impact on neural cell response is summarized through neurogenic outcome and mechanistic analysis. The effects of surface chemistry on neural cell signaling with common interfacing compounds like carbon-based nanomaterials, conductive polymers, and biologically inspired matrices are also reviewed. Finally, the impact of these extracellular mediated neural cues on intracellular signaling cascades is discussed to provide perspective on the manipulation of neuron and neuroglia cell microenvironments to drive therapeutic outcomes.

Microglial Piezo1 mechanosensitive channel as a therapeutic target in Alzheimer's disease

Microglia are the resident macrophages of the central nervous system (CNS) that control brain development, maintain neural environments, respond to injuries, and regulate neuroinflammation. Despite their significant impact on various physiological and pathological processes across mammalian biology, there remains a notable gap in our understanding of how microglia perceive and transmit mechanical signals in both normal and diseased states. Recent studies have revealed that microglia possess the ability to detect changes in the mechanical properties of their environment, such as alterations in stiffness or pressure. These changes may occur during development, aging, or in pathological conditions such as trauma or neurodegenerative diseases. This review will discuss microglial Piezo1 mechanosensitive channels as potential therapeutic targets for Alzheimer's disease (AD). The structure, function, and modulation of Piezo1 will be discussed, as well as its role in facilitating microglial clearance of misfolded amyloid-β (Aβ) proteins implicated in the pathology of AD.

Early Blood-Brain Barrier Impairment as a Pathological Hallmark in a Novel Model of Closed-Head Concussive Brain Injury (CBI) in Mice

Concussion, caused by a rotational acceleration/deceleration injury mild enough to avoid structural brain damage, is insufficiently captured in recent preclinical models, hampering the relation of pathophysiological findings on the cellular level to functional and behavioral deficits. We here describe a novel model of unrestrained, single vs. repetitive concussive brain injury (CBI) in male C56Bl/6j mice. Longitudinal behavioral assessments were conducted for up to seven days afterward, alongside the evaluation of structural cerebral integrity by in vivo magnetic resonance imaging (MRI, 9.4 T), and validated ex vivo by histology. Blood-brain barrier (BBB) integrity was analyzed by means of fluorescent dextran- as well as immunoglobulin G (IgG) extravasation, and neuroinflammatory processes were characterized both in vivo by positron emission tomography (PET) using [18F]DPA-714 and ex vivo using immunohistochemistry. While a single CBI resulted in a defined, subacute neuropsychiatric phenotype, longitudinal cognitive testing revealed a marked decrease in spatial cognition, most pronounced in mice subjected to CBI at high frequency (every 48 h). Functional deficits were correlated to a parallel disruption of the BBB, (R2 = 0.29, p < 0.01), even detectable by a significant increase in hippocampal uptake of [18F]DPA-714, which was not due to activation of microglia, as confirmed immunohistochemically. Featuring a mild but widespread disruption of the BBB without evidence of macroscopic damage, this model induces a characteristic neuro-psychiatric phenotype that correlates to the degree of BBB disruption. Based on these findings, the BBB may function as both a biomarker of CBI severity and as a potential treatment target to improve recovery from concussion.

Redox Regulation of Immunometabolism in Microglia Underpinning Diabetic Retinopathy

Diabetic retinopathy (DR) is the leading cause of visual impairment and blindness among the working-age population. Microglia, resident immune cells in the retina, are recognized as crucial drivers in the DR process. Microglia activation is a tightly regulated immunometabolic process. In the early stages of DR, the M1 phenotype commonly shifts from oxidative phosphorylation to aerobic glycolysis for energy production. Emerging evidence suggests that microglia in DR not only engage specific metabolic pathways but also rearrange their oxidation-reduction (redox) system. This redox adaptation supports metabolic reprogramming and offers potential therapeutic strategies using antioxidants. Here, we provide an overview of recent insights into the involvement of reactive oxygen species and the distinct roles played by key cellular antioxidant pathways, including the NADPH oxidase 2 system, which promotes glycolysis via enhanced glucose transporter 4 translocation to the cell membrane through the AKT/mTOR pathway, as well as the involvement of the thioredoxin and nuclear factor E2-related factor 2 antioxidant systems, which maintain microglia in an anti-inflammatory state. Therefore, we highlight the potential for targeting the modulation of microglial redox metabolism to offer new concepts for DR treatment.

Inhibition of the interaction between microglial adenosine 2A receptor and NLRP3 inflammasome attenuates neuroinflammation posttraumatic brain injury

AIMS

Adenosine 2A receptor (A2A R) is widely expressed in the brain and plays important roles in neuroinflammation, and the nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain-containing protein 3 (NLRP3) inflammasome is a crucial component of the innate immune system while the regulation of A2A R on it in the central nervous system (CNS) has not been clarified.

METHODS

The effects of microglial A2A R on NLRP3 inflammasome assembly and activation were investigated in wild-type, A2A R- or NLRP3-knockout primary microglia with pharmacological treatment. Microglial A2A R or NLRP3 conditional knockout mice were used to interrogate the effects of this regulation on neuroinflammation posttraumatic brain injury (TBI).

RESULTS

We found that A2A R directly interacted with NLRP3 and facilitated NLRP3 inflammasome assembly and activation in primary microglia while having no effects on mRNA levels of inflammasome components. Inhibition of the interaction via A2A R agonist or knockout attenuated inflammasome assembly and activation in vitro. In the TBI model, microglial A2A R and NLRP3 were co-expressed at high levels in microglia next to the peri-injured cortex, and abrogating of this interaction by microglial NLRP3 or A2A R conditional knockout attenuated the neurological deficits and neuropathology post-TBI via reducing the NLRP3 inflammasome activation.

CONCLUSION

Our results demonstrated that inhibition of the interaction between A2A R and NLRP3 in microglia could mitigate the NLRP3 inflammasome assembly and activation and ameliorate the neuroinflammation post-TBI. It provides new insights into the effects of A2A R on neuroinflammation regulation post-TBI and offers a potential target for the treatment of NLRP3 inflammasome-related CNS diseases.

Matrix stiffness-dependent microglia activation in response to inflammatory cues: in situ investigation by scanning electrochemical microscopy

Microglia play a crucial role in maintaining the homeostasis of the central nervous system (CNS) by sensing and responding to mechanical and inflammatory cues in their microenvironment. However, the interplay between mechanical and inflammatory cues in regulating microglia activation remains elusive. In this work, we constructed in vitro mechanical-inflammatory coupled microenvironment models of microglia by culturing BV2 cells (a murine microglial cell line) on polyacrylamide gels with tunable stiffness and incorporating a lipopolysaccharide (LPS) to mimic the physiological and pathological microenvironment of microglia in the hippocampus. Through characterization of activation-related proteins, cytokines, and reactive oxygen species (ROS) levels, we observed that the LPS treatment induced microglia on a stiff matrix to exhibit overexpression of NOX2, higher levels of ROS and inflammatory factors compared to those on a soft matrix. Additionally, using scanning electrochemical microscopy (SECM), we performed in situ characterization and discovered that microglia on a stiff matrix promoted extracellular ROS production, leading to a disruption in their redox balance and increased susceptibility to LPS-induced ROS production. Furthermore, the respiratory activity and migration behavior of microglia were closely associated with their activation process, with the stiff matrix-LPS-induced microglia demonstrating the most pronounced changes in respiratory activity and migration ability. This work represents the first in situ and dynamic monitoring of microglia activation state alterations under a mechanical-inflammatory coupled microenvironment using SECM. Our findings shed light on matrix stiffness-dependent activation of microglia in response to an inflammatory microenvironment, providing valuable insights into the mechanisms underlying neuroinflammatory processes in the CNS.

Mechanical properties of the brain: Focus on the essential role of Piezo1-mediated mechanotransduction in the CNS

BACKGROUND

The brain is a highly mechanosensitive organ, and changes in the mechanical properties of brain tissue influence many physiological and pathological processes. Piezo type mechanosensitive ion channel component 1 (Piezo1), a protein found in metazoans, is highly expressed in the brain and involved in sensing changes of the mechanical microenvironment. Numerous studies have shown that Piezo1-mediated mechanotransduction is closely related to glial cell activation and neuronal function. However, the precise role of Piezo1 in the brain requires further elucidation.

OBJECTIVE

This review first discusses the roles of Piezo1-mediated mechanotransduction in regulating the functions of a variety of brain cells, and then briefly assesses the impact of Piezo1-mediated mechanotransduction on the progression of brain dysfunctional disorders.

CONCLUSIONS

Mechanical signaling contributes significantly to brain function. Piezo1-mediated mechanotransduction regulates processes such as neuronal differentiation, cell migration, axon guidance, neural regeneration, and oligodendrocyte axon myelination. Additionally, Piezo1-mediated mechanotransduction plays significant roles in normal aging and brain injury, as well as the development of various brain diseases, including demyelinating diseases, Alzheimer's disease, and brain tumors. Investigating the pathophysiological mechanisms through which Piezo1-mediated mechanotransduction affects brain function will give us a novel entry point for the diagnosis and treatment of numerous brain diseases.

Modulating neuroinflammation through molecular, cellular and biomaterial-based approaches to treat spinal cord injury

The neuroinflammatory response that is elicited after spinal cord injury contributes to both tissue damage and reparative processes. The complex and dynamic cellular and molecular changes within the spinal cord microenvironment result in a functional imbalance of immune cells and their modulatory factors. To facilitate wound healing and repair, it is necessary to manipulate the immunological pathways during neuroinflammation to achieve successful therapeutic interventions. In this review, recent advancements and fresh perspectives on the consequences of neuroinflammation after SCI and modulation of the inflammatory responses through the use of molecular-, cellular-, and biomaterial-based therapies to promote tissue regeneration and functional recovery will be discussed.

Microfluidic Wound-Healing Assay for ECM and Microenvironment Properties on Microglia BV2 Cells Migration

Microglia cells, as the resident immune cells of the central nervous system (CNS), are highly motile and migratory in development and pathophysiological conditions. During their migration, microglia cells interact with their surroundings based on the various physical and chemical properties in the brain. Herein, a microfluidic wound-healing chip is developed to investigate microglial BV2 cell migration on the substrates coated with extracellular matrixes (ECMs) and substrates usually used for bio-applications on cell migration. In order to generate the cell-free space (wound), gravity was utilized as a driving force to flow the trypsin with the device. It was shown that, despite the scratch assay, the cell-free area was created without removing the extracellular matrix coating (fibronectin) using the microfluidic assay. It was found that the substrates coated with Poly-L-Lysine (PLL) and gelatin stimulated microglial BV2 migration, while collagen and fibronectin coatings had an inhibitory effect compared to the control conditions (uncoated glass substrate). In addition, the results showed that the polystyrene substrate induced higher cell migration than the PDMS and glass substrates. The microfluidic migration assay provides an in vitro microenvironment closer to in vivo conditions for further understanding the microglia migration mechanism in the brain, where the environment properties change under homeostatic and pathological conditions.

Engineered cell culture microenvironments for mechanobiology studies of brain neural cells

The biomechanical properties of the brain microenvironment, which is composed of different neural cell types, the extracellular matrix, and blood vessels, are critical for normal brain development and neural functioning. Stiffness, viscoelasticity and spatial organization of brain tissue modulate proliferation, migration, differentiation, and cell function. However, the mechanical aspects of the neural microenvironment are largely ignored in current cell culture systems. Considering the high promises of human induced pluripotent stem cell- (iPSC-) based models for disease modelling and new treatment development, and in light of the physiological relevance of neuromechanobiological features, applications of in vitro engineered neuronal microenvironments should be explored thoroughly to develop more representative in vitro brain models. In this context, recently developed biomaterials in combination with micro- and nanofabrication techniques 1) allow investigating how mechanical properties affect neural cell development and functioning; 2) enable optimal cell microenvironment engineering strategies to advance neural cell models; and 3) provide a quantitative tool to assess changes in the neuromechanobiological properties of the brain microenvironment induced by pathology. In this review, we discuss the biological and engineering aspects involved in studying neuromechanobiology within scaffold-free and scaffold-based 2D and 3D iPSC-based brain models and approaches employing primary lineages (neural/glial), cell lines and other stem cells. Finally, we discuss future experimental directions of engineered microenvironments in neuroscience.

Unraveling the Mechanobiology Underlying Traumatic Brain Injury with Advanced Technologies and Biomaterials

Traumatic brain injury (TBI) is a worldwide health and socioeconomic problem, associated with prolonged and complex neurological aftermaths, including a variety of functional deficits and neurodegenerative disorders. Research on the long-term effects has highlighted that TBI shall be regarded as a chronic health condition. The initiation and exacerbation of TBI involve a series of mechanical stimulations and perturbations, accompanied by mechanotransduction events within the brain tissues. Mechanobiology thus offers a unique perspective and likely promising approach to unravel the underlying molecular and biochemical mechanisms leading to neural cells dysfunction after TBI, which may contribute to the discovery of novel targets for future clinical treatment. This article investigates TBI and the subsequent brain dysfunction from a lens of neuromechanobiology. Following an introduction, the mechanobiological insights are examined into the molecular pathology of TBI, and then an overview is given of the latest research technologies to explore neuromechanobiology, with particular focus on microfluidics and biomaterials. Challenges and prospects in the current field are also discussed. Through this article, it is hoped that extensive technical innovation in biomedical devices and materials can be encouraged to advance the field of neuromechanobiology, paving potential ways for the research and rehabilitation of neurotrauma and neurological diseases.

Electron Microscopy of Cells Grown on Polyacrylamide Hydrogels

The composition of the cell culture environment profoundly affects cultured cells. Standard cell culture equipment such as plastic and glass provide extremely stiff surfaces compared to physiological cell environments (i.e., tissue). A growing body of evidence documents the artificial behavior and morphology of cells cultured on supraphysiologically stiff surfaces, such as glass (elastic modulus ca. 70,000 MPA) or plastic (e.g., polystyrol ca. 3300 MPA). Therefore, polymer-based hydrogels are increasingly employed as more physiologically appropriate (<100 kPA) supports for 2D or 3D culture. Since multiple properties that influence the cultured cells may be easily adjusted, hydrogels have become versatile tools for studying cells in a more native in vitro environment. Polyacrylamide-based hydrogels can be used as culture substrates for a broad variety of adherent cells and are easy to handle in most downstream biological assays, such as immunohistochemistry or molecular biology methods. We faced, however, serious difficulties with processing high stiffness polyacrylamide-based hydrogels for electron microscopy. To overcome this problem, we developed a simple protocol for embedding and processing cells grown on high stiffness polyacrylamide hydrogels that do not require modifications of routine embedding protocols. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol: Embedding of polyacrylamide-based hydrogels for transmission electron microscopy Alternate Protocol 1: Procedure for detached hydrogels Alternate Protocol 2: Procedure for attached hydrogels.

Two-Photon Polymerization of 2.5D and 3D Microstructures Fostering a Ramified Resting Phenotype in Primary Microglia

Microglia are the resident macrophages of the central nervous system and contribute to maintaining brain’s homeostasis. Current 2D “petri-dish” in vitro cell culturing platforms employed for microglia, are unrepresentative of the softness or topography of native brain tissue. This often contributes to changes in microglial morphology, exhibiting an amoeboid phenotype that considerably differs from the homeostatic ramified phenotype in healthy brain tissue. To overcome this problem, multi-scale engineered polymeric microenvironments are developed and tested for the first time with primary microglia derived from adult rhesus macaques. In particular, biomimetic 2.5D micro- and nano-pillar arrays (diameters = 0.29–1.06 µm), featuring low effective shear moduli (0.25–14.63 MPa), and 3D micro-cages (volume = 24 × 24 × 24 to 49 × 49 × 49 μm³) with and without micro- and nano-pillar decorations (pillar diameters = 0.24–1 µm) were fabricated using two-photon polymerization (2PP). Compared to microglia cultured on flat substrates, cells growing on the pillar arrays exhibit an increased expression of the ramified phenotype and a higher number of primary branches per ramified cell. The interaction between the cells and the micro-pillar-decorated cages enables a more homogenous 3D cell colonization compared to the undecorated ones. The results pave the way for the development of improved primary microglia in vitro models to study these cells in both healthy and diseased conditions.

Multiscale Mechanobiology in Brain Physiology and Diseases

Increasing evidence suggests that mechanics play a critical role in regulating brain function at different scales. Downstream integration of mechanical inputs into biochemical signals and genomic pathways causes observable and measurable effects on brain cell fate and can also lead to important pathological consequences. Despite recent advances, the mechanical forces that influence neuronal processes remain largely unexplored, and how endogenous mechanical forces are detected and transduced by brain cells into biochemical and genetic programs have received less attention. In this review, we described the composition of brain tissues and their pronounced microstructural heterogeneity. We discuss the individual role of neuronal and glial cell mechanics in brain homeostasis and diseases. We highlight how changes in the composition and mechanical properties of the extracellular matrix can modulate brain cell functions and describe key mechanisms of the mechanosensing process. We then consider the contribution of mechanobiology in the emergence of brain diseases by providing a critical review on traumatic brain injury, neurodegenerative diseases, and neuroblastoma. We show that a better understanding of the mechanobiology of brain tissues will require to manipulate the physico-chemical parameters of the cell microenvironment, and to develop three-dimensional models that can recapitulate the complexity and spatial diversity of brain tissues in a reproducible and predictable manner. Collectively, these emerging insights shed new light on the importance of mechanobiology and its implication in brain and nerve diseases.

Mechanosensing and the Hippo Pathway in Microglia: A Potential Link to Alzheimer's Disease Pathogenesis?

The activation of microglia, the inflammatory cells of the central nervous system (CNS), has been linked to the pathogenesis of Alzheimer’s disease and other neurodegenerative diseases. How microglia sense the changing brain environment, in order to respond appropriately, is still being elucidated. Microglia are able to sense and respond to the mechanical properties of their microenvironment, and the physical and molecular pathways underlying this mechanosensing/mechanotransduction in microglia have recently been investigated. The Hippo pathway functions through mechanosensing and subsequent protein kinase cascades, and is critical for neuronal development and many other cellular processes. In this review, we examine evidence for the potential involvement of Hippo pathway components specifically in microglia in the pathogenesis of Alzheimer’s disease. We suggest that the Hippo pathway is worth investigating as a mechanosensing pathway in microglia, and could be one potential therapeutic target pathway for preventing microglial-induced neurodegeneration in AD.

Patent highlights December 2020-January 2021

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.