Differential effects of Bcl-2 overexpression on fibre outgrowth and survival of embryonic dopaminergic neurons in intracerebral transplants

The causes of death of transplanted neurons are not known in detail, but ap optotic mechanisms involving caspase activation are likely to play a role. We examined whether overexpression of the anti-apoptotic protein Bcl-2 may enhance the survival of dopaminergic [tyrosine hydroxylase (TH)-immunoreact ive] grafted neurons. For this purpose, we prepared cells from embryonic da y 13 ventral mesencephalon (VM) of mice overexpressing human Bcl-2, or from their wild-type littermates. The bcl-2 transgene was strongly expressed in these cells, and resulted in protection of neuronal cultures from death tr iggered by serum deprivation or exposure to staurosporine. To model pretran splantation stress more closely in vitro, we stored dissociated embryonic m esencephalic cells for 8 h in the same type of medium used for intracerebra l transplantation. This resulted in massive cell death as quantified by lac tate dehydrogenase (LDH) release, and increased DNA fragmentation. Although this cell loss was strongly reduced by a caspase inhibitor, Bcl-2 had no s ignificant protective effect. Finally, mesencephalic cell suspensions were xenografted into the striatum of immunosuppressed hemiparkinsonian rats. Ne ither the survival of TH-immunopositive transplanted neurons nor the functi onal recovery of the rats was improved by Bcl-2, although the Bcl-2 protein was strongly expressed in transgenic grafts 5 weeks after implantation, an d dopaminergic fibre outgrowth from the grafts was significantly improved. These data suggest that cell death in neuronal transplants involves apoptot ic mechanisms that can bypass negative regulation by Bcl-2.