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A Kv1.5 to Kv1.3 switch in endogenous hippocampal microglia and a role in proliferation
1Department of Physiology, University of Toronto, Toronto, Ontario, Canada M5S 1A1.
Abstract:
The proliferation of microglia is a normal process in CNS development and in the defense against pathological insults, although, paradoxically, it contributes to several brain diseases. We have examined the types of voltage-activated K(+) currents (Kv) and their roles in microglial proliferation. Microglia were tissue-printed directly from the hippocampal region using brain slices from 5- to 14-d-old rats. Immediately after tissue prints were prepared, unipolar and bipolar microglia expressed a large Kv current, and the cells were not proliferating. Surprisingly, this current was biophysically and pharmacologically distinct from Kv1.3, which has been found in dissociated, cultured microglia, but it was very similar to Kv1.5. After several days in culture the microglia became highly proliferative, and although the Kv prevalence and current density decreased, many cells exhibited a prominent Kv that was indistinguishable from Kv1.3. The Kv1.5-like current was present in nonproliferating cells, whereas proliferating cells expressed the Kv1.3-like current. Immunocytochemical staining showed a dramatic shift in expression and localization of Kv1.3 and Kv1.5 proteins in microglia: Kv1.5 moving away from the surface and Kv1.3 moving to the surface as the cells were cultured. K(+) channel blockers inhibited proliferation, and the pharmacology of this inhibition correlated with the type of Kv current expressed. Our study, which introduces a method for the physiological examination of microglia from identified brain regions, demonstrates the differential expression of two functional Kv subunits and shows that a functional delayed rectifier current is necessary for microglia proliferation.
Insights
Microglia proliferation is linked to specific voltage-activated potassium (Kv) currents. Non-proliferating cells show Kv1.5 currents, while proliferating cells express Kv1.3, essential for their growth.
Area of Science:
- Neuroscience
- Cell Biology
- Ion Channel Physiology
Background:
- Microglial proliferation is crucial for CNS development and disease.
- Voltage-activated potassium (Kv) currents play roles in microglial function.
- Previous studies identified Kv1.3 in cultured microglia, but its role in native cells is unclear.
Purpose of the Study:
- To investigate the types of Kv currents in microglia and their role in proliferation.
- To differentiate Kv currents in non-proliferating versus proliferating microglia.
- To establish a method for studying microglia physiology in their native brain environment.
Main Methods:
- Tissue-printing of microglia directly from rat hippocampal slices.
- Electrophysiological recordings to characterize Kv currents.
- Immunocytochemistry to assess Kv1.3 and Kv1.5 protein expression and localization.
- Pharmacological inhibition of Kv channels.
Main Results:
- Unipolar and bipolar microglia in tissue prints expressed a Kv1.5-like current and were non-proliferative.
- Cultured microglia became proliferative and expressed a Kv1.3-like current.
- Kv1.5 protein decreased at the surface, while Kv1.3 increased during culture.
- Kv channel blockers inhibited proliferation, with sensitivity correlating to expressed Kv type.
Conclusions:
- Microglial proliferation is dependent on a functional delayed rectifier current.
- Differential expression of Kv1.5 and Kv1.3 channels correlates with microglial proliferation state.
- A novel method allows physiological study of microglia in identified brain regions.