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Cytokine transcripts expressed by microglia in vitro are not expressed by ameboid microglia of the developing rat
S D Hurley1, S A Walter, S L Semple-Rowland
1University of Florida Brain Institute, University of Florida, Gainesville, USA.
Abstract:
Because of morphological similarities between ameboid microglia in the developing central nervous system (CNS), brain macrophages in the injured CNS, and cultured microglia in vitro, it is thought that these cell types are functionally equivalent. To investigate the validity of this assumption, we have compared mRNA levels of interleukin-1alpha and -1beta (IL-1alpha and IL-1beta), tumor necrosis factor-alpha and -beta (TNF-alpha and TNF-beta), transforming growth factor-beta1 (TGF-beta1), and macrophage colony-stimulating factor (M-CSF) in the postnatal day 4 (P4) supraventricular corpus callosum (SVCC) with those in unstimulated cultured microglia. Control tissues included spleen, cortex, hippocampus, and cerebellum. Our analyses have shown that while IL-1alpha, IL-1beta, TNF-alpha, TNF-beta, and TGF-beta1 transcripts are abundantly expressed by cultured microglia, they are very low to virtually undetectable in the SVCC. These data strongly suggest that ameboid microglia, which are concentrated in the SVCC, are unlikely to be a significant source of these cytokines. Our study, which shows clear differences in the functional status of cultured microglia vs. ameboid microglia in vivo, stresses the importance of using caution when interpreting in vitro findings in terms of the in vivo functions of microglia.
Insights
Ameboid microglia in the developing brain (SVCC) show low cytokine mRNA levels, unlike cultured microglia. This suggests caution is needed when extrapolating in vitro findings to in vivo microglial functions.
Area of Science:
- Neuroscience
- Immunology
- Cell Biology
Background:
- Microglia, the resident immune cells of the central nervous system (CNS), exist in various forms, including ameboid microglia in the developing CNS and cultured microglia.
- Morphological similarities between these cell types have led to the assumption of functional equivalence.
Purpose of the Study:
- To investigate the functional equivalence of ameboid microglia in vivo and cultured microglia in vitro.
- To compare the expression of key cytokine messenger RNAs (mRNAs) in ameboid microglia within the postnatal day 4 (P4) supraventricular corpus callosum (SVCC) and in unstimulated cultured microglia.
Main Methods:
- Quantitative analysis of mRNA levels for interleukin-1alpha/beta (IL-1alpha/beta), tumor necrosis factor-alpha/beta (TNF-alpha/beta), transforming growth factor-beta1 (TGF-beta1), and macrophage colony-stimulating factor (M-CSF).
- Comparison of mRNA expression in the P4 SVCC, spleen, cortex, hippocampus, and cerebellum with unstimulated cultured microglia.
Main Results:
- Cultured microglia abundantly expressed IL-1alpha, IL-1beta, TNF-alpha, TNF-beta, and TGF-beta1 transcripts.
- These cytokine transcripts were found at very low or undetectable levels in the P4 SVCC, where ameboid microglia are concentrated.
Conclusions:
- Ameboid microglia in the developing CNS (SVCC) are unlikely to be a significant source of the studied cytokines.
- Significant differences exist in the functional status of cultured microglia versus ameboid microglia in vivo.
- Caution is essential when interpreting in vitro findings regarding the in vivo functions of microglia.