1Department of Neuroscience, Tufts University, 2Neuroscience Program, Tufts Sackler School of Graduate Biomedical Sciences
This article is a part ofJoVE Neuroscience. If you think this article would be useful for your research, please recommend JoVE to your institution's librarian.Recommend JoVE to Your Librarian
Current Access Through Your IP Address
Current Access Through Your Registered Email Address
Higashimori, H., Yang, Y. Imaging Analysis of Neuron to Glia Interaction in Microfluidic Culture Platform (MCP)-based Neuronal Axon and Glia Co-culture System. J. Vis. Exp. (68), e4448, doi:10.3791/4448 (2012).
Proper neuron to glia interaction is critical to physiological function of the central nervous system (CNS). This bidirectional communication is sophisticatedly mediated by specific signaling pathways between neuron and glia1,2 . Identification and characterization of these signaling pathways is essential to the understanding of how neuron to glia interaction shapes CNS physiology. Previously, neuron and glia mixed cultures have been widely utilized for testing and characterizing signaling pathways between neuron and glia. What we have learned from these preparations and other in vivo tools, however, has suggested that mutual signaling between neuron and glia often occurred in specific compartments within neurons (i.e., axon, dendrite, or soma)3. This makes it important to develop a new culture system that allows separation of neuronal compartments and specifically examines the interaction between glia and neuronal axons/dendrites. In addition, the conventional mixed culture system is not capable of differentiating the soluble factors and direct membrane contact signals between neuron and glia. Furthermore, the large quantity of neurons and glial cells in the conventional co-culture system lacks the resolution necessary to observe the interaction between a single axon and a glial cell.
In this study, we describe a novel axon and glia co-culture system with the use of a microfluidic culture platform (MCP). In this co-culture system, neurons and glial cells are cultured in two separate chambers that are connected through multiple central channels. In this microfluidic culture platform, only neuronal processes (especially axons) can enter the glial side through the central channels. In combination with powerful fluorescent protein labeling, this system allows direct examination of signaling pathways between axonal/dendritic and glial interactions, such as axon-mediated transcriptional regulation in glia, glia-mediated receptor trafficking in neuronal terminals, and glia-mediated axon growth. The narrow diameter of the chamber also significantly prohibits the flow of the neuron-enriched medium into the glial chamber, facilitating probing of the direct membrane-protein interaction between axons/dendrites and glial surfaces.
1. Assembly of the Microfluidic Culture Chamber (MCP)
2. Preparation of Neuronal Culture and Induction of Neuronal Axons in Assembled MCP
3. Addition of Cultured Astrocytes to MCP to Establish a Compartmentalized Co-culture System
Primary astrocyte cultures were prepared from the P1-3 mouse pup brain. The brain dissociation procedure is similar to the neuronal cell isolation procedure described above. Astrocytes were first plated into 10 cm dish that were pre-coated with Polyornithine (Sigma-Aldrich, 1 mg/ml). The astrocyte culture medium (DMEM, 10% fetal bovine serum, 1% Penicillin-streptomysin) was changed every day for the next two days to remove the debris. After that, the medium was changed every three days.
Time-lapse imaging analysis of axon-induced GLT1 promoter activation in astrocytes
The compartmented neuron and astrocyte co-culture system allows only the neuronal processes, especially the axons, to selectively interact with astrocytes. Following the successful establishment of axon and astrocyte (or other glial cells) co-culture in the assembled MCP, different types of axon-glia interactions can be studied such as; axon-induced activation of astroglial gene promoter activation, astrocyte-induced axon guidance, axon-induced astroglial Ca2+ response, and axon-induced myelin synthesis in oligodendrocytes. All of these applications take advantage of the availability of powerful fluorescent reporters available for glial cells or dyes loaded to these cells.
Here we describe axon-induced transcriptional activation of astroglial glutamate transporter 1 (GLT1) promoter by using bacterial artificial chromosome (BAC) GLT1 eGFP transgenic mice and this compartmentalized co-culture system. In the BAC GLT1 eGFP transgenic mice, an eGFP fluorescence reporter was overexpressed under the control of the GLT1 genomic promoter7. The expression of the eGFP reporter correlates with endogenous GLT1 promoter activation protein expression and functional activity7 thus permitting an assessment of the GLT1 promoter activity in single astrocytes in situ. Astroglial GLT1 expression is highly dependent upon neuronal stimulation6,8 . Primary astrocytes express minimum levels of GLT1; however, GLT1 expression levels (both mRNA and protein) are strongly induced in astrocytes that are co-cultured with neurons9. The evidence is shown here by significantly increased GLT1 immunoreactivity in compartmentalized co-culture system (Figure 3). In conventional neuron and astrocyte co-cultures, high density of neurons make it less ideal to observe the effect of neuronal axons on the transcriptional activation of GLT1 promoter.
To monitor the axon-dependent GLT1 promoter activation, wild type neurons were cultured in the right side of the assembled MCP and the axons were induced following the application of the GDNF on the left side of the assembled MCP. Astrocytes derived from the BAC GLT1 eGFP mice were cultured in the left side of the assembled MCP after the axons have been induced and just entered into the left side of the assembled MCP. A total of 6 days (24 hr to 140 hr) time-lapse images were collected to monitor the eGFP fluorescence intensity change in real time and the axon growth in the MCP. As shown in Figure 4A, very minimum eGFP fluorescence expression in astrocytes was observed 24 hr following the plating of the astrocytes. Significant increase of eGFP intensity was observed 48 hr after co-culture (from 24 hr to 68 hr) when axons are well into the astrocyte side and interact directly contacting with astrocytes (Figure 4 B-C). On the other hand, eGFP fluorescence intensity was gradually decreased once axons were retracted and degenerated by kainite (200 μM) induced neuronal cell death on the neuronal (right) side of the assembled MCP (Figure 4D-F). The dynamic change of eGFP fluorescence intensity in response to the axons clearly demonstrated that astroglial GLT1 promoter activity is modulated by the axon interaction.
Figure 1. Schematic diagram of microfluidic neuron and astrocyte co-culture platform. Microfluidic culture platform (MCP) has two compartments that are connected through the central channels. Cells can be plated from the cell plating area on each side and neuronal axons are induced from the cell retention area and pass through the central channels to enter the other side (insert scale bar 50 μm).
Figure 2. Induction of neuronal axons and interaction of axons with astrocyte in MCP MCP. (A) High density of neurons in the cell retention area of the assembled MCP, revealed by βIII-tubulin immunostaining. Scale bar: 50 μm (B) Magnified view of axons crossing the central channels and entering into the other side of the MCP. Scale bar: 100 μm (C) Axon growth cone revealed by the bIII-tubulin staining when axons enter into the other side of the MCP. Scale bar: 100 μm (D) Axon bundles that grow through the central channel and enter into the other side of the MCP. (E) Astrocytes revealed by the GLT1 immunostaining in the glial side of the MCP. (F) Merged image displays contact between axons and GLT1 positive astrocytes. Scale bar: 50 μm.
Figure 3. Neuron-dependent induction of GLT1 expression in primary astrocytes. GLT1 immunoreactivity in (A) primary astrocyte cultures, Scale bar: 50 μm (B) primary neuronal cultures, and (C) primary neuron and astrocyte co-cultures. Scale bar: Neurons are shown by immunostaining of βIII-tubulin. Significant increase of GLT1 immunoreactivity was observed in neuron and astrocyte co-cultures.
Figure 4. Time-lapse images of axon-induced GLT1 promoter activation in MCP with BAC GLT1 eGFP astrocytes and wild type neurons. Dynamic changes of eGFP intensity and the growth of axons were collected through a 6d time-lapse recording following astrocyte plating. (A) 24h (B) 48h (C) 68h (D) 92h (E) 112h (F) 140h. Entry of axons (highlighted by the yellow arrows) into the astrocyte side was observed from 48h to 92h which correlate with increased eGFP fluorescence intensity. Decrease of eGFP fluorescence intensity occurred from 112h to 140h after kainite (200 μM) was applied on the neuronal side at 92h to induce neuronal cell death and axon degeneration. Scale bar: 50 μm.
The MCP based neuron and astrocytes co-culture system allows dissection of detailed neuron to astroglia signaling pathways by allowing only the axons pass the central channels and interacting with the astroglial cells. This co-culture system can be conveniently set up with conventional neuron and astrocyte culture procedures. We also described a practical application of this co-culture system by employing an eGFP based reporter for demonstrating axon-dependent GLT1 promoter activation in astrocytes.
This MCP based neuron and astroglia co-culture system offers several advantages compared to conventional co-culture methods: 1) ability to visualize and identify the explicit morphological interaction between single axons and astrocytes in real-time under high-resolution microscopy 2) a well-controlled independent microenvironment to apply pharmacological manipulation for cell-specific effect 3) analysis of promoter activation and protein expression exclusively induced/up-regulated in astrocytes that are modulated by axons contact. As the culture procedures for primary microglia10 and oligodendrocytes11 have been well established, this MCP co-culture system can be also applied to neuron to microglia and neuron to oligodendrocyte co-cultures. Meanwhile, we also recognize that this co-culture system is intended for the sensitive image analysis at the single cell level, which is complementary to the conventional approaches of analyzing large quantity of cells, such as RNA or protein analysis. In addition, our system simplifies the enormous interaction between neurons and glial cells in vivo by exemplifying interaction between single cell and axon. All of these should be considered when interpreting the results from this co-culture system. In the CNS, interactions between axons and different glial cells are widely present and are critically important for both neuron and glial functions12,13 . Development of this co-culture system will allow imaging-based dissection of these interactions.
No conflicts of interest declared.
We would like to thank Dr. Jeffrey Rothstein for providing BAC GLT1 eGFP mice and GLT1 antibody; Tufts Center for Neuroscience Research (NIH P30 NS047243; PI, Rob Jackson) for providing valuable core facilities; New faculty recruitment grant (NIH P30 5P30NS069254-02; PI, Phil Haydon) in Tufts Neuroscience Department.
|Fetal bovine serum||Hyclone||SH30070.03||for plating neuron for neuron cutlure medium|
|Fetal bovine serum||Sigma-Aldrich||F4135||for astrocyte culture medium|
|Glial derived nerve factor||R&D systems||212-GD||Apply 10-20 ng/ml to neuron side of chamber|
|Dulbecco modified eagle medium high glucose||Sigma-Aldrich||11995|
|70 mm cell strainer||BD Falcon||352350|
|Sterile glass bottom dish||MatTek Corporation|
|Microfluidic culture platforms||Xona Microfluidics LLC||SND150|
|6 wells of the culture plate||Cellstar||657 160|
Neuron culture medium
Neuron culture medium for plating cell
Astrocyte culture medium
Table 1. Materials used in the microfluidic culture platform-based neuronal axon and glia co-culture system.