Clusterin expression during programmed and teratogen‐induced cell death in the postimplantation rat embryo

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Abstract
Clusterin appears to play a role in multiple cellular processes including reproductive cell function, lipid transport, complement regulation, and endocrine secretion. In addition, clusterin has been shown to be associated with both developmental and induced cell death. We have used immunohistochemistry and in situ hybridization to study the relationship between clusterin expression, normal programmed cell death (PCD) in the developing rat limb bud, and abnormal cell death induced by hyperthermia in day 11 rat embryos. Immunohistochemical localization of clusterin in day 14-16 limb buds showed that the most intense immunostaining was associated with the condensing mesenchyme of the developing digit, a tissue exhibiting low levels of PCD. Moreover, areas of digital cell death, confined to future interphalangeal spaces, were devoid of clusterin immunostaining. Clusterin immunostaining was also observed in the interdigital mesenchyme and partially overlapped the cell death that occurs in this tissue during the early development of the digits. Although clusterin immunostaining overlaps areas of interdigital cell death, most apoptotic cells in the interdigital mesenchyme and underlying the surface ectoderm were not associated with clusterin immunostaining. We also examined the expression of clusterin in day 11 rat embryos exposed to 43°C, an exposure that induces extensive cell death primarily in the developing neuroepithelium. In control embryos cultured at 37°C, clusterin mRNA and protein were expressed at high levels in the heart, a tissue that is completely resistant to the cytotoxic effects of hyperthermia. Within 2.5 hr after an exposure of 43°C, clusterin mRNA showed a dramatic induction in the prosencephalic mesenchyme and only a modest induction in the prosencephalic neuroepithelium. Although less dramatic, clusterin immunostaining was also induced in the prosencephalic mesenchyme, a tissue that is relatively resistant to the cytotoxic effects of hyperthermia. Our results suggest that clusterin is not associated with either PCD or hyperthermia-induced cell death. Nonetheless, our results suggest that clusterin plays some role in the differentiation of digital cartilage/bone and possibly in the protection of specific embryonic cell types (heart, mesenchyme) from hyperthermia-induced cell death.