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Electrophysiological characterisation and expression pattern of ion channels in astrocytes before and after traumatic brain injury
Electrophysiological characterisation and expression pattern of ion channels in astrocytes before and after traumatic brain injury
After traumatic brain injury (TBI) astrocytes perform various beneficial tasks like the reorganisation and support of the blood-brain barrier (BBB), the remodelling of synapses and neural circuits and controlling inflammation. They do this by becoming reactive under these conditions. Astrocyte reactivity includes, but is not limited to hypertrophy, polarization, the upregulation of glial fibrillary acidic protein (GFAP) in astrocytic processes as well as proliferation of astrocytes surrounding the lesion site. In the somatosensory cortex of mice juxtavascular astrocytes, which have their soma directly adjacent to blood vessels are more prone to proliferate in response to TBI, whereas non-juxtavascular astrocytes with the cell soma further away from the vasculature are less likely to do so. Nevertheless, the underlying mechanism of this selective proliferation is still not well understood. It is known that ion channels play an important role in cell cycle progression in different cell types and at different developmental stages. Especially potassium (K+) channels have been shown to be key players. To determine whether there are differences present between the two astrocyte subtypes per se, in this study K+ channel expression patterns and electrophysiological properties were characterized in juxtavascular astrocytes and compared with the ones of non-juxtavascular astrocytes in the unlesioned somatosensory cortex of Aldh1l1-eGFP mice. Furthermore, ion channel expression patterns were compared five days after astrocyte reactivity was induced by a stab wound lesion. Whole-cell patch-clamp recordings in somatosensory cortex slices of healthy Aldh1l1-eGFP mice revealed great heterogeneity in the resting membrane potential (Vr), the resting membrane conductance (Gr) and the input resistance (Rin) of different astrocytes. 70% of non-juxtavascular and 81% of juxtavascular control astrocytes displayed typical Ohmic passive current patterns with linear IV-curves. Blocking of inwardly rectifying Kir4.1 ion channels in electrophysiological recordings as well as immunohistochemical stainings revealed a homogeneous expression of Kir4.1 channels in astrocytes across all cortical layers. Heterogeneous expression of Kir6.2 and Kv4.3 channels in somatosensory cortex astrocytes was revealed by means of immunohistochemistry. Moreover, heterogeneous expression of hyperpolarization-activated cyclic nucleotide-gated 1 (HCN1) channels in somatosensory cortex astrocytes of the unlesioned brain was detected by immunohistochemistry, but could not be confirmed in electrophysiological recordings. All dissimilarities mentioned above could not be related to whether an astrocyte is non-juxtavascular or juxtavascular in nature. A stab wound lesion and the subsequent reactive astrogliosis triggered a downregulation of Kir4.1 in proliferating reactive astrocytes as well as the upregulation of Kv4.3 channels on polarized astrocytic processes. This was accompanied by a shift especially in juxtavascular astrocytes towards non-passive current response patterns. This proposed an important role of these two K+ channel subtypes in astrocyte proliferation suggesting that these reactive astrocytes might resemble immature astrocytes with proliferative potential. Astrocyte reactivity and proliferation had no impact on Kir6.2 and HCN1 channel expression. HCN2 channels, which were absent in astrocytes in control conditions were upregulated on processes of a subset of reactive polarizing astrocytes independently of non-juxtavascular and juxtavascular position as well as proliferative behaviour. These finding are of great interest for therapeutic approaches since there is increasing evidence that astrocyte proliferation positively affects healing processes and axon regeneration after traumatic brain injury.
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Götz, Stefanie
2019
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Götz, Stefanie (2019): Electrophysiological characterisation and expression pattern of ion channels in astrocytes before and after traumatic brain injury. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)
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Abstract

After traumatic brain injury (TBI) astrocytes perform various beneficial tasks like the reorganisation and support of the blood-brain barrier (BBB), the remodelling of synapses and neural circuits and controlling inflammation. They do this by becoming reactive under these conditions. Astrocyte reactivity includes, but is not limited to hypertrophy, polarization, the upregulation of glial fibrillary acidic protein (GFAP) in astrocytic processes as well as proliferation of astrocytes surrounding the lesion site. In the somatosensory cortex of mice juxtavascular astrocytes, which have their soma directly adjacent to blood vessels are more prone to proliferate in response to TBI, whereas non-juxtavascular astrocytes with the cell soma further away from the vasculature are less likely to do so. Nevertheless, the underlying mechanism of this selective proliferation is still not well understood. It is known that ion channels play an important role in cell cycle progression in different cell types and at different developmental stages. Especially potassium (K+) channels have been shown to be key players. To determine whether there are differences present between the two astrocyte subtypes per se, in this study K+ channel expression patterns and electrophysiological properties were characterized in juxtavascular astrocytes and compared with the ones of non-juxtavascular astrocytes in the unlesioned somatosensory cortex of Aldh1l1-eGFP mice. Furthermore, ion channel expression patterns were compared five days after astrocyte reactivity was induced by a stab wound lesion. Whole-cell patch-clamp recordings in somatosensory cortex slices of healthy Aldh1l1-eGFP mice revealed great heterogeneity in the resting membrane potential (Vr), the resting membrane conductance (Gr) and the input resistance (Rin) of different astrocytes. 70% of non-juxtavascular and 81% of juxtavascular control astrocytes displayed typical Ohmic passive current patterns with linear IV-curves. Blocking of inwardly rectifying Kir4.1 ion channels in electrophysiological recordings as well as immunohistochemical stainings revealed a homogeneous expression of Kir4.1 channels in astrocytes across all cortical layers. Heterogeneous expression of Kir6.2 and Kv4.3 channels in somatosensory cortex astrocytes was revealed by means of immunohistochemistry. Moreover, heterogeneous expression of hyperpolarization-activated cyclic nucleotide-gated 1 (HCN1) channels in somatosensory cortex astrocytes of the unlesioned brain was detected by immunohistochemistry, but could not be confirmed in electrophysiological recordings. All dissimilarities mentioned above could not be related to whether an astrocyte is non-juxtavascular or juxtavascular in nature. A stab wound lesion and the subsequent reactive astrogliosis triggered a downregulation of Kir4.1 in proliferating reactive astrocytes as well as the upregulation of Kv4.3 channels on polarized astrocytic processes. This was accompanied by a shift especially in juxtavascular astrocytes towards non-passive current response patterns. This proposed an important role of these two K+ channel subtypes in astrocyte proliferation suggesting that these reactive astrocytes might resemble immature astrocytes with proliferative potential. Astrocyte reactivity and proliferation had no impact on Kir6.2 and HCN1 channel expression. HCN2 channels, which were absent in astrocytes in control conditions were upregulated on processes of a subset of reactive polarizing astrocytes independently of non-juxtavascular and juxtavascular position as well as proliferative behaviour. These finding are of great interest for therapeutic approaches since there is increasing evidence that astrocyte proliferation positively affects healing processes and axon regeneration after traumatic brain injury.