Abstract
Astrocytes in vivo extend thin processes termed peripheral astrocyte processes (PAPs), in particular around synapses where they can mediate glia-neuronal communication. The relation of PAPs to synapses is not based on coincidence, but it is not clear which stimuli and mechanisms lead to their formation and are active during process extension/ retraction in response to neuronal activity. Also, the molecular basis of the extremely fine PAP morphology (often 50 to 100 nm) is not understood. These open questions can be best investigated under in vitro conditions studying glial filopodia. We have previously analyzed filopodial mechanisms (Lavialle et al. PNAS 108:12915) applying an automated method for filopodia morphometry, which is now described in greater detail. The Filopodia Specific Shape Factor (FSSF) developed integrates number and length of filopodia. It quantifies filopodia independent of overall astrocytic shape or size, which can be intricate in itself. The algorithm supplied here permits automated image processing and measurements using ImageJ. Cells have to be sampled in higher numbers to obtain significant results. We validate the FSSF, and characterize the systematic influence of thresholding and camera pixel grid on measurements. We provide exemplary results of substance-induced filopodia dynamics (glutamate, mGluR agonists, EGF), and show that filopodia formation is highly sensitive to medium pH (CO2) and duration of cell culture. Although the FSSF was developed to study astrocyte filopodia with focus on the perisynaptic glial sheath, we expect that this parameter can also be applied to neuronal growth cones, non-neural cell types, or cell lines.
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Acknowledgements
The authors wish to thank Brigitte Rost and Torsten Schwalm for technical assistance, and Ingrid Beck for graphic artwork. This work was supported by the Deutsche Forschungsgemeinschaft (DFG 676/2-1, to AD), the Frankfurter Promotionsförderung of the Medical Faculty of the Goethe-University Frankfurt (Stipend to FK), and the Dr. Senckenbergische Stiftung Frankfurt am Main (to HWK).
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Appendix: Installation and Data Organization for the FSSF-ImageJ Macro
Appendix: Installation and Data Organization for the FSSF-ImageJ Macro
To install the analysis programme given below as a macro (not as a plugin) in Fiji (ImageJ does not contain the thresholding algorithms required), copy-paste the macro lines first into the Fiji Menu dialogue which opens under—Plugins/New/Macro. When saving it, make sure to use the file extension ‘.ijm’. Secondly, to make it easily accessible from the ImageJ menu bar, select the saved file from the file list opening under—Plugins/ Macros/Install. After that it can be readily selected from ‘Plugins/ Macros’. The macro version of this file has the advantage of easily customizing it using the Macro/Edit function. Alternatively, with an underscore (“_”) in the macro name and .txt as file extension, the macro can be moved to the Plugins folder, and thus be made available in the Plugins menu.
All images to be analysed should be in a single directory, not in subdirectories. They should be numbered at the beginning of the file name using leading zeros. An empty subdirectory named ‘FSSF’ should be contained in this directory. Open the first image of the series ‘manually’ in ImageJ/Fiji before starting the macro, the macro automatically reads out the file path for all subsequent images. All interim images and the results table (compatible with Excel) generated are automatically saved in the ‘FSSF’ subdirectory.
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Aumann, G., Friedländer, F., Thümmler, M. et al. Quantifying Filopodia in Cultured Astrocytes by an Algorithm. Neurochem Res 42, 1795–1809 (2017). https://doi.org/10.1007/s11064-017-2193-0
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DOI: https://doi.org/10.1007/s11064-017-2193-0