Reactive gliosis of immature Bergmann glia and microglial cell activation in response to cell death of granule cell precursors induced by methylazoxymethanol treatment in developing rat cerebellum

Abstract

The morphology, organization and expression of proliferating cell nuclear antigen (PCNA) and the cytoskeletal proteins vimentin and GFAP in immature Bergmann glial cells were studied after a developmental injury induced by a single dose of the cytotoxic agent methylazoxymethanol (MAM) administered on postnatal day 5. This drug, which produces cell death of cerebellar granule cell precursors, did not induce apoptosis in Bergmann glial cells, which are in a proliferative stage. After MAM treatment, PCNA staining showed a severe depletion of PCNA-positive granule cell precursors, whereas PCNA-positive Bergmann glial nuclei in the Purkinje cell layer were preserved. Moreover, the quantitative analysis revealed an increase in the density of both Purkinje cells and PCNA-positive Bergmann glial cells per mm of Purkinje cell layer in MAM-treated rats relative to age-matched controls, but the numerical ratio between these two cell populations remains invariable after MAM treatment. Vimentin and GFAP immunocytochemistry revealed a reinforcement of the Bergmann glial palisade with overexpression of both proteins and thicker immunoreactive glial processes in MAM-treated rats. At the ultrastructural level, Bergmann glial processes closely associated with dying cells in different stages of apoptosis were observed. Frequently, these processes enclosed dying cells in extracellular compartments. Furthermore, phagosomes containing apoptotic bodies were found in Bergmann fibers of MAM-treated rats. These data indicate that the cell death of granule cell precursors triggers a reactive response in immature Bergmann glia. We suggest that this response reflects the plasticity of Bergmann glia to control the neuronal microenvironment in the maturing molecular layer, protecting healthy cells against the potentially harmful contents of dying cells. In situ labeling of cell death with the TUNEL method revealed that the cell death of granule cell precursors is of the apoptotic type. The participation of ameboid microglial cells in the phagocytosis of apoptotic cells was shown with tomato lectin histochemistry and ultrastructural analysis. Moreover, the presence of mitosis in this microglial population demonstrates its proliferative activity in regions of extensive cell death.

Pattern of Glial Fibrillary Acidic Protein Expression Following Kainate-Induced Cerebellar Lesion in Rats

“…When astrocytes create a dense plexus, gliosis is referred to as anisomorphic, whereas isomorphic gliosis denotes the response where reactive astrocytes have not formed a dense wall (29). Thus, results obtained in the present study suggest that KA in a given dose induces isomorphic gliosis along its gradient, similar to the response induced by methylazoxymethanol treatment in developing rat cerebellum (30). An increased expression of GFAP in mature astroglia is commonly associated with a reactive state, in terms of morphological plasticity and ability to proliferative (31).…”

Uteroplacental Inflammation Results in Blood Brain Barrier Breakdown, Increased Activated Caspase 3 and Lipid Peroxidation in the Late Gestation Ovine Fetal Cerebellum

“…It is possible that damage to Purkinje cells and cells of the molecular layer, as shown by the increased expression of activated caspase 3 protein, resulted in a reactive response in astrocytes and Bergmann fibers. Increased neuronal death produced by kainic acid or methylazoxymethanol significantly increased the GFAP immunoreactivity of Bergmann glia in the cerebellum of 5-day-old rats [32,36] .…”

“…Bergmann cells have soma which are spatially associated with Purkinje cells and extend radial fibers that ensheathe synapses on Purkinje cell dendrites. Bergmann fibers have a role in neuronal migration and phagocytosis during development [32][33][34][35] . It is possible that damage to Purkinje cells and cells of the molecular layer, as shown by the increased expression of activated caspase 3 protein, resulted in a reactive response in astrocytes and Bergmann fibers.…”

Postnatal Development of the Central Nervous System: Anomalies in the Formation of Cerebellum Fissures

“…Alterations of Bergmann glial radial fibers can affect granule cell migration as observed in methylazoxymethanol (MAM) (Lafarga et al, 1998)-or cisplatin-(Pisu et al, 2005 treated rats or in neonatal X-irradiated rats (Ferguson, 1996;Li et al, 2006), resulting in ectopic displacement of these neurons in the molecular layer. Because radial glial cells are the substrate for neuronal migration (for a review see Rakic and Caviness, 1995), our findings indicate that inward migration of cells from egl to the igl was affected.…”