This protocol describes a microglia-neuronal co-culture established from primary neuronal cells isolated from mouse embryos at embryonic days 15-16 and primary microglia generated from the brains of neonatal mice at post-natal days 1-2.
Microglia are tissue-resident macrophages of the central nervous system (CNS), performing numerous functions that support neuronal health and CNS homeostasis. They are a major population of immune cells associated with CNS disease activity, adopting reactive phenotypes that potentially contribute to neuronal injury during chronic neurodegenerative diseases such as multiple sclerosis (MS). The distinct mechanisms by which microglia regulate neuronal function and survival during health and disease remain limited due to challenges in resolving the complex in vivo interactions between microglia, neurons, and other CNS environmental factors. Thus, the in vitro approach of co-culturing microglia and neurons remains a valuable tool for studying microglia-neuronal interactions. Here, we present a protocol to generate and co-culture primary microglia and neurons from mice. Specifically, microglia were isolated after 9-10 days in vitro from a mixed glia culture established from brain homogenates derived from neonatal mice between post-natal days 0-2. Neuronal cells were isolated from brain cortices of mouse embryos between embryonic days 16-18. After 4-5 days in vitro, neuronal cells were seeded in 96-well plates, followed by the addition of microglia to form the co-culture. Careful timing is critical for this protocol as both cell types need to reach experimental maturity to establish the co-culture. Overall, this co-culture can be useful for studying microglia-neuron interactions and can provide multiple readouts, including immunofluorescence microscopy, live imaging, as well as RNA and protein assays.
Microglia are tissue-resident macrophages that facilitate immunosurveillance and homeostasis in the central nervous system (CNS)1,2,3. They originate from yolk sac erythromyeloid progenitor cells that colonize the brain during embryonic development4,5,6 and are maintained throughout the organism's life span through self-renewal, which involves proliferation and apoptosis7. At steady-state, resting microglia have ramified morphology and engage in tissue surveillance8,9,10.
Microglia express numerous cell-surface receptors, which enables them to rapidly respond to changes in the CNS11,12 and to promote inflammatory responses in the event of infections or tissue injury12,13,14, as well as during neurodegenerative diseases9,15, such as multiple sclerosis (MS)16,17. Microglia also express receptors to various neurotransmitters and neuropeptides18,19,20, which suggests they may also respond to and regulate neuronal activity21,22. Indeed, microglia and neurons interact in various forms of bidirectional communication8,23 such as direct interactions mediated by membrane proteins or indirect interactions through soluble factors or intermediate cells23,24.
For instance, various neurotransmitters secreted by neurons can modulate the neuroprotective or inflammatory activity of microglia25,26,27. Additionally, direct interactions between neurons and microglia help to maintain microglia in a homeostatic state28. Conversely, direct interactions of microglia with neurons can shape neuronal circuitry29 and influence neuronal signaling30,31,32. As disruptions of these interactions induce hyperexcitability of neurons30 and microglia reactivity33,34, dysregulated microglia-neuronal interactions are implicated as a contributing factor to neurological diseases33,35. Indeed, psychotic23,26 and neurodegenerative diseases have been described to exhibit dysfunctional microglia-neuronal interactions33. While these observations highlight the importance of microglia-neuronal communication in the CNS, specific mechanisms of how these interactions regulate microglial and neuronal functions in health and disease are relatively unknown.
Within a complex milieu such as the CNS, multiple environmental factors can influence microglia-neuronal interactions, which limits the ability to study transient cellular interactions in vivo. Here, we present an in vitro microglia-neuronal co-culture system that can be used to study direct cellular interactions between microglia and neurons. This protocol describes the generation of primary microglia and neurons from the cortices of neonatal mice between post-natal days 0-2 and embryonic mice days 16-18, respectively. Neurons and microglia are then co-cultured in 96-well plates for downstream high-throughput experiments. We previously used this approach to demonstrate that microglia phagocytosis protects neurons from oxidized phosphatidylcholine mediated cell death37, suggesting that this method can help to understand the roles of microglia in the context of neurodegeneration and MS. Similarly, microglia-neuronal co-cultures may also be useful for investigating the impact of microglia-neuronal crosstalk in other contexts such as viral infections38 or neuronal injury and repair39. Overall, in vitro microglia-neuronal co-culture systems enable researchers to study microglia-neuronal interactions in a manipulatable and controlled environment, which complements in vivo models.
All animals used in this study were housed and handled with approval from the University Animal Care Committee (UACC) of the University of Saskatchewan and the Canadian Council on Animal Care (CCAC). Post-natal days 0-2 CD1 male and female mice and embryonic days 16-18 (E16-18) embryos from pregnant CD1 mice were used for this study. The details of the reagents and the equipment used are listed in the Table of Materials.
1. Primary microglia culture
NOTE: It is crucial to time the mixed glia and neuronal cultures so that microglia are mature and ready for harvest within 2 days after neurons are seeded into a 96-well plate.
2. Primary neuron culture
3. Co-culture of primary neurons and microglia
NOTE: All subsequent steps are to be done in a sterile biosafety cabinet.
A flowchart showing the key steps of the mixed glia culture for microglia is shown in Figure 1A. Overall, sparse cells and excessive cellular debris are expected on day 1 (Figure 1B). By day 4, increased cell number should be observed, especially with the generation of adherent astrocytes, as indicated by their elongated morphology (Figure 1C). A few microglia may be observed on top of the astrocytes or as small round cells floating in media. By day 8, a confluent layer of astrocytes will form the adherent layers of cells, whereas microglia will appear as round and bright cells that are semi-attached to the astrocytes or floating in media (Figure 1D). If microglial density is low by day 8, the culture may be incubated for additional days before harvest.
A flow chart showing the key steps of the neuronal culture is shown in Figure 2A. In a healthy culture, neurons will attach and form neurite processes after 1-3 days in vitro (Figure 2B). Similarly, upon seeding neurons into a 96-well plate, they will appear round initially, but by day 3, neurons should become adherent and form neurite processes that connect with neighboring neurons (Figure 2C). Conversely, unhealthy neuron cultures will contain cell clumps and cell debris, which can result from deficient cell dissociation or mishandling of neurons (Figure 2D).
A flow chart showing the key steps of the microglia-neuronal co-culture protocol is shown in Figure 3A. After adding 2.5 x 104 microglia to 7.5 x 104 neurons, round and large microglia should appear on top or between the neurons and neurite processes (Figure 3B). To further visualize the microglia-neuronal co-culture, microglia and neurons alone or in co-culture were labeled with ionized calcium-binding adaptor molecule 1 (IBA-1, red) and tubulin βIII (TUJ1, green) antibodies37 for immunofluorescence microscopy (Figure 3C–F).
Figure 1: Overview of primary microglia culture. (A) Flow chart of the key steps in generating a mixed glia culture. (B–D) Representative 10x bright-field microscope images of mixed glia culture on day 1 (B), day 4 (C), and day 8 (D). Scale bars: 100 µm. Please click here to view a larger version of this figure.
Figure 2: Overview of primary neuron culture. (A) Flow chart of the key steps in generating neurons. (B) Representative 10x bright field microscope images showing neurons cultured in T-25 flasks. (C) Representative 10x bright field microscope of neurons cultured in a 96-well plate. (D) Representative 10x bright field microscope of an unhealthy neuronal cluster in a 96-well plate. Scale bars: 100 µm. Please click here to view a larger version of this figure.
Figure 3: Overview of microglia-neuron co-culture. (A) Flow chart of the key steps for the microglia-neuronal co-culture. (B) Representative 10x bright-field microscope images of microglia-neuronal co-culture in 96-well plates. Scale bars: 100 µm. (C,D) Representative immunofluorescence microscopy images of neurons alone (C), microglia alone (D), day 2 neuron and microglia co-culture (E), or 12-day neuron and microglia co-culture (F) in a 96-well plate. Microglia and neurons are labeled with IBA-1 (red) and tubulin βIII (green), respectively. Nuclei are stained with DAPI (blue). Immunofluorescence images were acquired using a 25x, 0.8 NA, water immersion objective with a widefield fluorescence microscope. Scale bars: 50 µm. Please click here to view a larger version of this figure.
This article describes a protocol for isolating and culturing mouse primary neurons and primary microglia, which are subsequently used to establish a microglia-neuronal co-culture that can be used to study how microglia and neuron interactions regulate their cellular health and function. This relatively simple and accessible approach can provide critical insights into the mechanisms and functional outcomes of microglia neuron interactions in the CNS.
To achieve an optimal co-culture, several critical aspects in this protocol require extra care. During brain isolations (steps 1.2.6 and 2.2.7), meninges attached to the surface of both the embryonic and post-natal brains should be removed completely before collecting the tissues. Like other neuroglia protocols, incomplete removal of the meninges may result in cell culture becoming contaminated with endothelial cells or blood-derived cells40,41,42. Additionally, when culturing primary neurons (steps 2.3.7 and 3.1.4), it is crucial to generate a single-cell suspension and avoid cell clumps before seeding neurons into flasks or 96-well plates. Neuronal aggregates will not properly adhere in culture and will lead to excessive neuronal stress and cell death. In addition, extended treatment of neurons with trypsin when attempting to dissociate them from T-25 flasks may cause unwanted cell death (step 3.1.2). To mitigate this, neurons may be directly seeded in 96-well plates at the desired cell density following dissociation from embryonic brains (step 2.3.7). Moreover, in this protocol, a 3:1 neuron: microglia ratio was selected to partially represent an elevated amount of microglia that may be present in the CNS microenvironment during neuroinflammation. Alternatively, the ratio of cells in the co-culture may be adjusted per experimental need. In addition, the timepoint selected for co-culture neurons may be extended if one aims to assess more mature neuronal states.
Notably, when dissociating neurons from T-25 flasks to 96-well plates, using trypsin alone may result in incomplete dissociation and the formation of cell clumps instead of a monolayer culture in 96-well plates (Figure 2D). To address this issue, neurons can be dissociated from the T-25 flask using 1x Versene solution containing 1 mg/mL DNase I. Other factors contributing to neuronal aggregation and reduced viability include incomplete trituration of the cell suspension, inadequate neutralization of trypsin with heat-inactivated FBS, excessive centrifugation, or prolonged cell pellet formation. Therefore, ensuring proper tissue dissociation and the generation of a single-cell suspension are critical for establishing a healthy microglia and neuron co-culture.
This protocol aims to recapitulate microglia-neuronal interactions in vitro to better understand the functional importance of their interactions in the CNS. While other types of co-culture systems have been developed, such as culturing individual cell populations on different surfaces to allow for independent treatment of two cell populations43,44, this mixed culture system enables direct and close contact of microglia and neurons. In addition, when maintained properly through half media changes every 3 days, this mixed culture system can remain healthy from 12 days (Figure 3F) to potentially 3 weeks, allowing for long-term studies of neuronal-microglia interactions45.
While the relative simplicity of the co-culture system is advantageous for accessibility, a key limitation of this approach is that the two-dimensional cell culture does not fully represent the CNS tissue environment. Indeed, a tri-culture of microglia, neurons, and astrocytes is shown to mimic in vivo neuroinflammatory response more reliably than microglia or astrocyte-neuronal co-culture46,47. Furthermore, recent developments in brain organoid systems with microglia and other glial cells, including astrocytes and oligodendrocytes, are more suitable for studying three-dimensional (3D) tissue architecture and interactions of the CNS48. Similarly, a human 3D triculture system comprising neurons, microglia, and astrocytes is shown to be effective in modeling Alzheimer's disease condition49. Other human microglia-neuron co-cultures may be more physiologically relevant to neurodegenerative disease studies. For example, the co-culture of microglia and motor neurons derived from human induced pluripotent stem cells50 may be useful for studying diseases such as amyotrophic lateral sclerosis51. Nevertheless, given the relative abundance and low cost of generating primary neurons and microglia from mice, assessing microglia-neuronal co-culture in 96-well plates can be a high throughput approach to studying the mechanisms and functional implications of microglia-neuronal interactions. Downstream assays may include using live cell imaging to determine cellular functional changes in real-time, RNA sequencing to determine cellular transcriptional changes, immunofluorescence microscopy to determine cellular phenotypic changes, mass spectrometry to determine cellular and proteomic changes, or enzyme-linked immunosorbent assay and western blot to measure protein secreted into the co-culture supernatant.
As microglia are important mediators of CNS health and diseases, this co-culture can also be used to investigate the effect of disease-related molecules and compounds on microglia-neuronal bi-directional communication. In the context of MS, activated microglia may take on a destructive role, contributing to the worsening of pathology52,53. For example, they may secrete pro-inflammatory cytokines and reactive oxygen species, as well as promote astrocyte activation45. This protocol may help to further interrogate the harmful role of microglia by treating microglia in the co-culture with inflammatory cytokines found elevated in MS and assessing the outcomes of neuron health. Alternatively, this microglia-neuronal co-culture has been used to show the protective effect of microglia on neurons in response to oxidized phosphatidylcholine, an oxidative product discovered in MS54. Moreover, neuronal apoptosis remodels microglia gene expression during development by promoting microglia survival and phagocytosis55. Therefore, this protocol may also help to determine the impact of neuronal death on microglia phenotype and function.
In summary, this protocol describes the generation and co-culturing of primary mouse-derived neurons and microglia in vitro. The microglia-neuronal co-culture set up in 96-well plates can be a high throughput approach to interrogate the mechanisms and function of microglia interactions with neurons in the context of CNS health and disease, including MS.
The authors have nothing to disclose.
JP acknowledges funding support from the Natural Sciences and Engineering Research Council of Canada and the University of Saskatchewan College of Medicine. YD acknowledges funding support from the University of Saskatchewan College of Medicine Startup Fund, the Natural Sciences and Engineering Research Council of Canada Discovery Grant (RGPIN-2023-03659), MS Canada Catalyst Grant (1019973), Saskatchewan Health Research Foundation Establishment Grant (6368), and Brain Canada Foundation Future Leaders in Canadian Brain Research Grant. Figure 1A, Figure 2A, and Figure 3A were created with BioRender.com.
10 cm Petri dish | Fisher | 07-202-011 | Sterile |
1x Versene | Gibco | 15040-066 | |
B-27 Plus Neuronal Culture System | Gibco | A3653401 | |
Dissection microscope | VWR | ||
DNase I | Roche | 11284932001 | |
Dulbecco’s Modified Eagle Medium (DMEM) | Gibco | 11960-044 | |
Fetal Bovine Serum | ThermoFisher Sci | 12483-020 | |
HBSS (10x) | Gibco | 14065-056 | |
Hemacytometer | Hausser Scientific | 1475 | |
HEPES | ThermoFisher Sci | 15630080 | |
Leibovitz’s L-15 Medium (1x) | Fisher Scientific | 21083027 | |
Macrophage colony stimulating factor | Peprotech | 315-02 | |
Micro-Forceps | RWD | F11020-11 | Autoclaved/Sterile |
Non-essential amino acids | Cytiva | SH3023801 | |
PBS (10x) | ThermoFisher Sci | AM9625 | |
Penicillin Streptomycin Glutamine (100x) | Gibco | 103780-16 | |
Poly-L-ornithine hydrobromide | Sigma | P3655-100MG | |
Sodium pyruvate (100 mM) | Gibco | 11360-070 | |
Spring scissors | RWD | S11008-42 | Autoclaved/Sterile |
Surgical blade | Feather | 08-916-5D | Sterile |
T-25 flasks | Fisher | 10-126-9 | |
T-75 flasks | Fisher | 13-680-65 | |
Tissue forceps | Codman | 30-4218 | Autoclaved/Sterile |
Tissue scissors | RWD | S12052-10 | Autoclaved/Sterile |
Trypan Blue | Thermofisher Sci | 15250-061 | |
Trypsin (2.5%) | ThermoFisher Sci | 15090046 | |
Widefield Immunofluorescence Microscope | Zeiss |
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