The use of "primitive" neuroglial cell lines that preserve their mitotic potential in vitro but will differentiate in vivo was proposed for a long time to be a feasible alternative to primary cell cultures. Whereas the PC12 line is well-established as the workhorse of a plethora of in vitro experiments studying neurotoxic or neurotrophic mechanisms in the CNS, its capacity to differentiate and integrate in vivo in the host brain is limited at best. Because the usefulness of this cell line in humans may be seriously hampered by its uncontrolled capacity to proliferate in a nondifferentiated state, significant efforts were made to identify new human neuronal cell lines that will overcome this obstacle. One of the candidates that, under certain conditions, can differentiate into a "mature neuronal phenotype" is the human cortical neuronal cell line HCN-1, which has many of the characteristics of immature neuroepithelial cells but, interestingly, does not respond to traditional growth factors for CNS cells, like bFGF. Another example is the human neuroblastoma cell line SH-SY5Y, which can synthesize dopamine in vitro and differentiate in vivo. Still, a puzzling observation seen, as discussed earlier, with primary cell grafts too is that in parkin-sonian animal models the functional benefit postgraft-ing may not be due to dopamine release by the implanted cells. The use of immortalized primary cell cultures has become a popular approach. Experiments with SV40 tumor antigen immortalized dopaminergic cells showed that they can be used for in vitro analysis (e.g., the SN4741 line) of the effects of BDNF on mesencephalic cultures. In vivo, similar immortalized dopaminergic cells (the 1RB3 AN27 clone) survived for more than 1 year in grafts and were associated with clinical improvement but, again, did not show significant differentiation and integration in the host brain.
One of the most studied neuronal cells is the NT2 line derived from a human teratocarcinoma that can differentiate in vitro, following treatment with retinoic acid, into cells with a mature neuronal phenotype known as "hNT" (or the older designation "NT2N") cells. In vivo, grafted hNT cells aquired a mature phenotype (including growth of neuritic processes and synaptic contacts) and could survive for more than a year in the mouse CNS. Interestingly, later studies showed that in vitro treatments of NT2 cells are not critical to their in vivo differentiation. One of the most convincing applications of using these cells in transplantation comes from experiments using a rat CNS ischemic model, in which hNT cells induced a more robust recovery than fetal rat striatal grafts. In the rat 6-OHDA parkinsonian model, hNT cells grafted into the striatum and substantia nigra were shown to survive in the host parenchyma and generate TH immunoreactivity when pretreated with LiCl. When made for human use, these cells are called LBS-neurons.
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