Effects of Swelling on Glial Cell Function

Three-dimensional confocal morphometry - a new approach for studying dynamic changes in cell morphology in brain slices

Pathological states in the central nervous system lead to dramatic changes in the activity of neuroactive substances in the extracellular space, to changes in ionic homeostasis and often to cell swelling. To quantify changes in cell morphology over a certain period of time, we employed a new technique, three-dimensional confocal morphometry. In our experiments, performed on enhanced green fluorescent protein/glial fibrillary acidic protein astrocytes in brain slices in situ and thus preserving the extracellular microenvironment, confocal morphometry revealed that the application of hypotonic solution evoked two types of volume change. In one population of astrocytes, hypotonic stress evoked small cell volume changes followed by a regulatory volume decrease, while in the second population volume changes were significantly larger without subsequent volume regulation. Three-dimensional cell reconstruction revealed that even though the total astrocyte volume increased during hypotonic stress, the morphological changes in various cell compartments and processes were more complex than have been previously shown, including swelling, shrinking and structural rearrangement. Our data show that astrocytes in brain slices in situ during hypotonic stress display complex behaviour. One population of astrocytes is highly capable of cell volume regulation, while the second population is characterized by prominent cell swelling, accompanied by plastic changes in morphology. It is possible to speculate that these two astrocyte populations play different roles during physiological and pathological states.

Three-dimensional confocal morphometry reveals structural changes in astrocyte morphology in situ

Neuronal activity and many pathological states in the CNS are accompanied by transient astrocytic swelling, which affects excitability, extrasynaptic transmission, and neuron-glia interactions. By using three-dimensional confocal morphometry (3DCM), we quantified the morphometric parameters of astrocytes in intact tissue. In experiments performed in brain cortex slices from transgenic GFAP/EGFP mice, we applied 3DCM to study the dynamic changes in astrocyte morphology during hypotonic stress. Our morphometric analysis showed that the effect of a 10-min application of hypotonic solution (200 mmol/kg) on the swelling of different cell compartments was dependent on the extent of the swelling of the total astrocyte volume. If the swelling of the whole cell, i.e., soma and processes, was less than approximately 10%, there were no differences between the swelling of the soma and the processes. However, if the swelling of the total cell volume was greater than 10%, the swelling of the processes was greater than the swelling of the soma. Analyzing the effect of hypotonic solution on the morphology of these astrocytes revealed that the total cell volume increased; however, certain cell compartments were distinguished in which the volume increased, whereas in other compartments cell volume decreased or apparently did not change, and the structure of some compartments was altered. Our data show that astrocytes in brain slices undergoing hypotonic stress display cell volume regulation as well as transient changes in morphology.

Changes in organic solutes, volume, energy state, and metabolism associated with osmotic stress in a glial cell line: a multinuclear NMR study

Diffusion-weighted in vivo 1H-NMR spectroscopy of F98 glioma cells embedded in basement membrane gel threads showed that the initial cell swelling to about 180% of the original volume induced under hypotonic stress was followed by a regulatory volume decrease to nearly 100% of the control volume in Dulbecco's modified Eagle's medium (DMEM) but only to 130% in Krebs-Henseleit buffer (KHB, containing only glucose as a substrate) after 7 h. The initial cell shrinkage to approx. 70% induced by the hypertonic stress was compensated by a regulatory volume increase which after 7 h reached almost 100% of the control value in KHB and 75% in DMEM. 1H-, 13C- and 31P-NMR spectroscopy of perchloric acid extracts showed that these volume regulatory processes were accompanied by pronounced changes in the content of organic osmolytes. Adaptation of intra- to extracellular osmolarity was preferentially mediated by a decrease in the cytosolic taurine level under hypotonic stress and by an intracellular accumulation of amino acids under hypertonic stress. If these solutes were not available in sufficient quantities (as in KHB), the osmolarity of the cytosol was increasingly modified by biosynthesis of products and intermediates of essential metabolic pathways, such as alanine, glutamate and glycerophosphocholine in addition to ethanolamine. The cellular nucleoside triphosphate level measured by in vivo 31P-NMR spectroscopy indicated that the energy state of the cells was more easily sustained under hypotonic than hypertonic conditions.