Validation of Flow Cytometry and Magnetic Bead-Based Methods to Enrich CNS Single Cell Suspensions for Quiescent Microglia

Impact of microglia isolation and culture methodology on transcriptional profile and function

BACKGROUND

Microglial isolation and culturing methods continue to be explored to maximize cellular yield, purity, responsiveness to stimulation and similarity to in vivo microglia. This study aims to evaluate five different microglia isolation methods-three variants of microglia isolation from neonatal mice and two variants of microglia isolation from adult mice-on transcriptional profile and response to HMGB1.

METHODS

Microglia from neonatal mice, age 0-3 days (P0-P3) were isolated from mixed glial cultures (MGC). We included three variations of this protocol that differed by use of GM-CSF in culture (No GM-CSF or 500 pg/mL GM-CSF), and days of culture in MGC before microglial separation (10 or 21). Protocols for studying microglia from adult mice age 6-8 weeks included isolation by adherence properties followed by 7 days of culture with 100 ng/mL GM-CSF and 100 ng/mL M-CSF (Vijaya et al. in Front Cell Neurosci 17:1082180, 2023), or acute isolation using CD11b beads (Bordt et al. in STAR Protoc 1:100035, 2020. https://doi.org/10.1016/j.xpro.2020.100035 ). Purity, yield, and RNA quality of the isolated microglia were assessed by flow cytometry, hemocytometer counting, and Bioanalyzer, respectively. Microglial responsiveness to an inflammatory stimulus, HMGB1, was evaluated by measuring TNFα, IL1β, and IFNβ concentration in supernatant by ELISA and assessing gene expression patterns using bulk mRNA sequencing.

RESULTS

All five methods demonstrated greater than 90% purity. Microglia from all cultures increased transcription and secretion of TNFα, IL1β, and IFNβ in response to HMGB1. RNA sequencing showed a larger number of differentially expressed genes in response to HMGB1 treatment in microglia cultured from neonates than from adult mice, with sparse changes among the three MGC culturing conditions. Additionally, cultured microglia derived from adult and microglia derived from MGCs from neonates display transcriptional signatures corresponding to an earlier developmental stage.

CONCLUSION

These findings suggest that while all methods provided high purity, the choice of protocol may significantly influence yield, RNA quality, baseline transcriptional profile and response to stimulation. This comparative study provides valuable insights to inform the choice of microglial isolation and culture method.

Strategies for enriching and characterizing proteins with inhibitory properties on the yeast surface

Display technologies are powerful tools for discovering binding proteins against a broad range of biological targets. However, it remains challenging to adapt display technologies for the discovery of proteins that inhibit the enzymatic activities of targets. Here, we investigate approaches for discovering and characterizing inhibitory antibodies in yeast display format using a well-defined series of constructs and the target matrix metalloproteinase-9. Three previously reported antibodies were used to create model libraries consisting of inhibitory, non-inhibitory, and non-binding constructs. Conditions that preferentially enrich for inhibitory clones were identified for both magnetic bead-based enrichments and fluorescence-activated cell sorting. Half maximal inhibitory concentration (IC50) was obtained through yeast titration assays. The IC50 of the inhibitory antibody obtained in yeast display format falls within the confidence interval of the IC50 value determined in soluble form. Overall, this study identifies strategies for the discovery and characterization of inhibitory clones directly in yeast display format.

Novel protocol for the isolation of highly purified neonatal murine microglia and astrocytes

BACKGROUND

The crosstalk and reactivity of the cell type glia, especially microglia and astrocytes, have progressively gathered research attention in understanding proper brain function regulated by the innate immune response. Therefore, methods to isolate highly viable and pure glia for the analysis on a cell-specific level are indispensable.

NEW METHOD

We modified previously established techniques: Animal numbers were reduced by multiple microglial harvests from the same mixed glial culture, thereby maximizing microglial yields following the principles of the 3Rs (replacement, reduction, and refinement). We optimized Magnetic-activated cell sorting (MACS®) of microglia and astrocytes by applying cultivated primary glial cell suspensions instead of directly sorting dissociated single cell suspension.

RESULTS

We generated highly viable and pure microglia and astrocytes derived from a single mixed culture with a purity of ~99%, as confirmed by FACS analysis. Field emission scanning electron microscopy (FESEM) demonstrated integrity of the MACS-purified glial cells. Tumor necrosis factor (TNF) and Interleukin-10 (IL-10) ELISA confirmed pro- and anti-inflammatory responses to be functional in purified glia, but significantly weakened compared to non-purified cells, further highlighting the importance of cellular crosstalk for proper immune activation.

COMPARISON WITH EXISTING METHOD(S)

Unlike previous studies that either isolated a single type of glia or displayed a substantial proportion of contamination with other cell types, we achieved isolation of both microglia and astrocytes at an increased purity (99-100%).

CONCLUSIONS

We have created an optimized protocol for the efficient purification of both primary microglia and astrocytes. Our results clearly demonstrate the importance of purity in glial cell cultivation in order to examine immune responses, which particularly holds true for astrocytes. We propose the novel protocol as a tool to investigate the cell type-specific crosstalk between microglia and astrocytes in the frame of CNS diseases.

Hippocampal microglia CD40 mediates NPSLE cognitive dysfunction in mice

Neuropsychiatric systemic lupus erythematosus (NPSLE) is the most serious and complicated clinical manifestation of lupus erythematosus. Cognitive dysfunction is the most common symptom of NPSLE. A variety of potential mechanisms or mediators related to the pathogenesis of NPSLE cognitive dysfunction have been proposed. However, the involvement of microglia CD40 has not been reported yet. This study aimed to investigate whether hippocampal microglia CD40 of MRL/MpJ-Fas^lpr (MRL/lpr) mice was involved in NPSLE cognitive dysfunction. This study found, using quantitative polymerase chain reaction, western blotting and immunohistochemistry, that hippocampal CD40 was aberrantly overexpressed in the MRL/lpr lupus mice. It also determined using flow cytometry and immunofluorescence that the aberrantly overexpressed CD40 was mainly derived from hippocampal microglia. The adeno-associated virus was used to inhibit microglia CD40 expression, and the brain damage and cognitive dysfunction of MRL/lpr mice improved. Also, imiquimod (IMQ)-induced lupus mice had the same NPSLE cognitive dysfunction, brain damage, and overexpressed hippocampal microglia CD40 as MRL/lpr mice. Therefore, IMQ-induced lupus mouse was proposed as one of the mouse models for studying NPSLE cognitive dysfunction for the first time in this study. The findings indicated that hippocampal microglia CD40 was involved in the development of NPSLE cognitive dysfunction, thus providing a novel research direction for the study of the pathogenesis of NPSLE.

Manipulation of living cells with 450 nm laser photobiomodulation

Increasing studies demonstrated that photobiomodulation (PBM) influenced specific biological effects in cells, tissues and organs, and these effects rely on the production of light irradiation. In this study, we aimed to precisely manipulate the spatial arrangement of adhesion cells in a traditional culture condition with 450 nm low intensity laser. Through 450 nm laser PBM, the adhesion of the cultured cells was significantly improved and resisted the digestion of 0.1% trypsin. Combined with a computer aided design system (CAD) and computer numerical control (CNC) system, the designed laser irradiation pattern induced the specific cell micropattern in the culture dish. RNA sequencing and biochemical experiments confirmed that the 450 nm laser prompted low-density lipoprotein (LDL) bonding to the cell surface and induced lipid peroxidation, which crosslinked and modified the protein molecules on the irradiated cell surface. In this way, the peroxidation product-modified proteins resisted trypsin proteolysis, ultimately leading to a differential detachment between the irradiated and non-irradiated cells under trypsin treatment. This convenient method did not require special biomaterial processing, has no impact on cell viability and functions, and required no changes to the conventional cell culture conditions. The photo-induced cell capturing is a great complement to existing tools by providing spatial resolution.

Isolating cells from adult murine brain for validation of cell-type specific cre-mediated deletion

BACKGROUND

TheCre/loxP system allows for the temporal and spatial investigation of the expression of a single gene in the nervous system. Current methods of validating conditional knock-out mouse models rely on heterogeneous brain tissue or primary culture. These methods may assess the extent of genetic knockdown in the brain but do not provide age-appropriate, cell-type specific information.

NEW METHOD

We isolated specific cell types from adult murine brain using FACS to assess cell type-specific gene expression in conditional mouse models.

RESULTS

We identified robust but incomplete genetic knockdown in microglia isolated from two separate microglia-specific knockout models.

COMPARISONWITH EXISTING METHODS(S)

Genetic knockdown in isolated adult microglia differed significantly from cultured primary microglia.

CONCLUSIONS

Differences observed in primary cultured microglia compared to isolated adult microglia suggest that current methods used to validate microglia-specific gene deletion over-estimate deletion efficiency. Assessment of gene expression in isolated adult microglia provides a more accurate assessment of Cre-mediated gene deletion.

Age-related changes in cerebellar and hypothalamic function accompany non-microglial immune gene expression, altered synapse organization, and excitatory amino acid neurotransmission deficits

We describe age-related molecular and neuronal changes that disrupt mobility or energy balance based on brain region and genetic background. Compared to young mice, aged C57BL/6 mice exhibit marked locomotor (but not energy balance) impairments. In contrast, aged BALB mice exhibit marked energy balance (but not locomotor) impairments. Age-related changes in cerebellar or hypothalamic gene expression accompany these phenotypes. Aging evokes upregulation of immune pattern recognition receptors and cell adhesion molecules. However, these changes do not localize to microglia, the major CNS immunocyte. Consistent with a neuronal role, there is a marked age-related increase in excitatory synapses over the cerebellum and hypothalamus. Functional imaging of these regions is consistent with age-related synaptic impairments. These studies suggest that aging reactivates a developmental program employed during embryogenesis where immune molecules guide synapse formation and pruning. Renewed activity in this program may disrupt excitatory neurotransmission, causing significant behavioral deficits.