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Neuronal TDP-43 aggregation drives changes in microglial morphology prior to immunophenotype in amyotrophic lateral
Molly E V Swanson1,2, Miran Mrkela1,2, Clinton Turner3
1School of Biological Sciences, University of Auckland, Auckland, New Zealand.
Abstract:
Microglia are the innate immune cells of the brain with the capacity to react to damage or disease. Microglial reactions can be characterised in post-mortem tissues by assessing their pattern of protein expression, or immunophenotypes, and cell morphologies. We recently demonstrated that microglia have a phagocytic immunophenotype in early-stage ALS but transition to a dysfunctional immunophenotype by end stage, and that these states are driven by TAR DNA-binding protein 43 (TDP-43) aggregation in the human brain. However, it remains unclear how microglial morphologies are changed in ALS. Here we examine the relationship between microglial immunophenotypes and morphologies, and TDP-43 pathology in motor cortex tissue from people with ALS and from a TDP-43-driven ALS mouse model. Post-mortem human brain tissue from 10 control and 10 ALS cases was analysed alongside brain tissue from the bigenic NEFH-tTA/tetO-hTDP-43∆NLS (rNLS) mouse model of ALS at distinct disease stages. Sections were immunohistochemically labelled for microglial markers (HLA-DR, CD68, and Iba1) and phosphorylated TDP-43 (pTDP-43). Single-cell microglial HLA-DR, CD68, and Iba1 average intensities, and morphological features (cell body area, process number, total outgrowth, and branch number) were measured using custom image analysis pipelines. In human ALS motor cortex, we identified a significant change in microglial morphologies from ramified to hypertrophic, which was associated with increased Iba1 and CD68 levels. In the rNLS mouse motor cortex, the microglial morphologies changed from ramified to hypertrophic and increased Iba1 levels occurred in parallel with pTDP-43 aggregation, prior to increases in CD68 levels. Overall, the evidence presented in this study demonstrates that microglia change their morphologies prior to immunophenotype changes. These morphological changes may prime microglia near neurons with pTDP-43 aggregation for phagocytosis, in turn triggering immunophenotype changes; first, to a phagocytic state then to a dysfunctional one.
Insights
Microglia in amyotrophic lateral sclerosis (ALS) change shape before altering their protein expression. These early morphological changes in microglia may precede their transition to phagocytic and dysfunctional states, driven by TDP-43 pathology.
Area of Science:
- Neuroimmunology
- Neurodegeneration
- Cellular Biology
Background:
- Microglia, the brain's innate immune cells, respond to neurological damage and disease.
- Microglial states in amyotrophic lateral sclerosis (ALS) are linked to TAR DNA-binding protein 43 (TDP-43) aggregation, transitioning from phagocytic to dysfunctional.
- The specific changes in microglial morphology during ALS progression and their relationship with immunophenotype shifts remain unclear.
Purpose of the Study:
- To investigate the relationship between microglial immunophenotypes, morphologies, and TDP-43 pathology in human ALS and a mouse model.
- To determine the temporal sequence of microglial morphological and immunophenotypic changes in ALS.
Main Methods:
- Analysis of post-mortem human motor cortex tissue from control and ALS cases.
- Examination of brain tissue from a bigenic NEFH-tTA/tetO-hTDP-43∆NLS (rNLS) mouse model at distinct disease stages.
- Immunohistochemical labeling for microglial markers (HLA-DR, CD68, Iba1) and phosphorylated TDP-43 (pTDP-43).
- Custom image analysis to quantify microglial morphology (cell body area, process number, outgrowth, branch number) and protein expression.
Main Results:
- In human ALS, microglial morphology shifted from ramified to hypertrophic, correlating with increased Iba1 and CD68 levels.
- In the rNLS mouse model, morphological changes preceded increases in CD68 levels and occurred alongside pTDP-43 aggregation.
- Microglial morphology changes were observed prior to significant immunophenotype alterations in both human ALS and the mouse model.
Conclusions:
- Microglial morphological changes precede immunophenotype shifts in ALS.
- These early morphological alterations may prime microglia for phagocytosis in response to TDP-43 aggregation.
- The sequence of changes suggests a progression from morphological activation to phagocytic and subsequently dysfunctional microglial states in ALS.
