Clues from Genetic Manipulation of Clusterin Expression

To date in vitro genetic manipulation of clusterin levels within brain cells has not been reported. A number of important studies utilizing nonbrain cell types suggest a potential cytoprotective role for clusterin. For example, overexpression of clusterin has been shown to protect clonal prostate cancer cells from tumor necrosis factor-(TNF-)alpha-induced cytotoxicity.68 Additionally, Sensibar determined that antisense depletion of clusterin caused an increase in cell death. Similarly, Humphreys et al69 showed that clusterin transfected L929 (murine fibrosarcoma) cells are protected from TNF-alpha-mediated cytotoxicity. Overexpressed clusterin did not protect these cells from other death-inducing agents such as colchicine, staurosporine or azide. In this system, clusterin's protective activity may be an intracellular property as protection was no observed with addition of exogenous clusterin or from secreted proteins.69 An intracellular role for clusterin has also been implicated in other cell systems where a truncated form of clusterin interacts with TGF-p receptors and may target them to the cell nucleus.5,70 Therefore, clusterin may be involved in TGF-p's neuroprotective activities.71,72 An intracellular activity for clusterin is especially pertinent for the CNS since a specific membrane receptor for clusterin, like gp330, has not been localized to parenchymal brain cells.36

Clusterin knock-out animals have been successfully made in the laboratories of Judith Harmony and Bruce Aronow. Mice without detectable clusterin survive, mature and reproduce and have no gross alterations in brain structures (personal communication Harmony and Aronow). Compensation from other molecules (e.g., apolipoprotein E?) may enable these animals to develop into mature mice under "nonstressed" conditions. However, because clusterin expression is greatly enhanced in response to most brain perturbations it will be informative to examine these knock-out mice in their response to brain lesion. We are currently utilizing two experimentally-induced brain lesions, kainic acid induced neurodegeneration and perforant path transection, to explore these questions.

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