Injured astrocytes are repaired by Synaptotagmin XI-regulated lysosome exocytosis.

Astrocytes are known to facilitate repair following brain injury; however, little is known about how injured astrocytes repair themselves. Repair of cell membrane injury requires Ca(2+)-triggered vesicle exocytosis. In astrocytes, lysosomes are the main Ca(2+)-regulated exocytic vesicles. Here we show that astrocyte cell membrane injury results in a large and rapid calcium increase. This triggers robust lysosome exocytosis where the fusing lysosomes release all luminal contents and merge fully with the plasma membrane. In contrast to this, receptor stimulation produces a small sustained calcium increase, which is associated with partial release of the lysosomal luminal content, and the lysosome membrane does not merge into the plasma membrane. In most cells, lysosomes express the synaptotagmin (Syt) isoform Syt VII; however, this isoform is not present on astrocyte lysosomes and exogenous expression of Syt VII on lysosome inhibits their exocytosis. Deletion of one of the most abundant Syt isoform in astrocyte--Syt XI--suppresses astrocyte lysosome exocytosis. This identifies lysosome as Syt XI-regulated exocytic vesicle in astrocytes. Further, inhibition of lysosome exocytosis (by Syt XI depletion or Syt VII expression) prevents repair of injured astrocytes. These results identify the lysosomes and Syt XI as the sub-cellular and molecular regulators, respectively of astrocyte cell membrane repair.

Imaging cell membrane injury and subcellular processes involved in repair.

The ability of injured cells to heal is a fundamental cellular process, but cellular and molecular mechanisms involved in healing injured cells are poorly understood. Here assays are described to monitor the ability and kinetics of healing of cultured cells following localized injury. The first protocol describes an end point based approach to simultaneously assess cell membrane repair ability of hundreds of cells. The second protocol describes a real time imaging approach to monitor the kinetics of cell membrane repair in individual cells following localized injury with a pulsed laser. As healing injured cells involves trafficking of specific proteins and subcellular compartments to the site of injury, the third protocol describes the use of above end point based approach to assess one such trafficking event (lysosomal exocytosis) in hundreds of cells injured simultaneously and the last protocol describes the use of pulsed laser injury together with TIRF microscopy to monitor the dynamics of individual subcellular compartments in injured cells at high spatial and temporal resolution. While the protocols here describe the use of these approaches to study the link between cell membrane repair and lysosomal exocytosis in cultured muscle cells, they can be applied as such for any other adherent cultured cell and subcellular compartment of choice.

Profilin-2 increased expression and its altered interaction with ß-actin in the striatum of 3-nitropropionic acid-induced Huntington's disease in rats.

Subacute systemic treatment with 3-nitropropionic acid (3-NP) causes specific lesions in the cortex and the striatum, and Huntington's disease behavioral phenotypes in rats. We investigated differentially expressed genes in the striatum, and examined status of a highly expressed huntingtin interacting protein, profilin 2 (Pfn2) in relation to 3-NP-induced striatal neurodegeneration, employing both in vivo animal model and in vitro primary striatal neuronal cultures. Golgi staining of 3-NP-treated rat brain revealed significantly altered dendritic spine morphology and decreased spine density in the cortex and the striatum, as compared to the control. We employed suppression subtractive hybridization (SSH) method to screen differentially expressed genes during striatal neurodegeneration in these animals. Forward and reverse SSH provided a library of 188 clones, which were used for reverse northern dot blot analysis to identify greatly altered striatal-specific genes. Sequence analysis of the clones identified 23 genes, expressions of which were ⩾1.5-fold changed (16 up-regulated) in the striatum of 3-NP-treated rats. Immunoprecipitation assay showed decreased binding of Pfn2 with ß-actin, the level of which remained unaffected in the striata and cortices of 3-NP-treated rats. Primary cultures of striatal glutamic acid decarboxylase-65/67 immunopositive GABAergic neurons revealed loss of co-existence of Pfn2 and ß-actin in fluorescence imaging studies following 3-NP treatment for 24h. Since Pfn2 is known to regulate dendritic spine dynamics by interacting with ß-actin, the reduction in its binding affinity to Pfn2 following 3-NP neurotoxic insult, and the accompanying aberrations of the dendritic spine structure and loss of spine density in striatal neurons suggest that Pfn2 may be involved in neurodegeneration in 3-NP-treated rat model of HD.