PubMed1999
Reconstruction of neocortical circuitry by transplantation of neural precursors, or by manipulation of endogenous precursors, may depend critically upon both local microenvironmental control signals and the intrinsic competence of populations of precursors to appropriately respond to external molecular controls. Dependence on the developmental state of donor or endogenous precursor cells in achieving appropriate differentiation, integration, and connectivity is not clearly understood. Recent studies have demonstrated the ability to generate expandable, often clonal neural precursors at various stages of development. Transplantation of a variety of these precursors suggests that precursor differentiation and integration within the central nervous system (CNS) may depend directly on the level of cellular maturation, with less differentiated, earlier stage precursors offering more flexible but less efficient integration and more differentiated, later stage precursors offering more efficient differentiation to specific phenotypes. To further investigate this hypothesis within neocortex, we used the relatively immature HiB5 multipotent neural precursor cell line derived from embryonic day 16 hippocampus, which is less mature than precursor types that have demonstrated neuronal differentiation in adult neocortex. HiB5 cells labeled fluorescently, radioactively, and genetically were transplanted into murine neocortex under three different conditions expected to offer varying levels of instructive and permissive microenvironmental signals: (1) the developing cortex in utero; (2) regions of adult neocortex undergoing targeted pyramidal neuronal degeneration in which developmental signals are upregulated and in which later stage precursors and immature neurons undergo directed pyramidal neuron differentiation; or (3) the intact adult neocortex. Differentiation and integration of transplanted cells were examined histologically and immunocytochemically by morphology and using neuronal- and glial-specific markers. We found that these precursors underwent differentiation toward cortical neuron phenotypes with characteristic morphologies when transplanted in utero, but failed to do so under either of the adult conditions. HiB5 precursors demonstrated highly immature characteristics in vitro, consistently expressing neuroepithelial but not glial or neuronal markers. Under all conditions, donor cells survived and migrated 1-2 mm from the injection track 2 to 4 weeks after transplantation. HiB5 neural precursors transplanted into the developing cortex of embryonic mice in utero migrated within the cortex, integrated well into the host parenchyma, and differentiated toward morphologically diverse, neuronal phenotypes. HiB5 cells transplanted into the intact cortex of adult mice survived, but did not show neuronal differentiation. In contrast to slightly later stage neural precursors and embryonic neurons used in previous transplantation studies, the HiB5 cells also failed to undergo neuronal differentiation after transplantation into regions undergoing induced apoptotic neuronal degeneration in adult cortex. These results suggested that these early hippocampal-derived precursors might not be fully competent to respond to later stage differentiation and/or survival signals important in neocortex and known to be upregulated in regions undergoing targeted neuronal apoptosis, including the TrkB neurotrophin receptor ligands BDNF and NT-4/5. We investigated this hypothesis and found that undifferentiated HiB5 cells lack catalytic trkB neurotrophin receptors at the mRNA and protein levels, while confirming that they express trkC receptors under the same conditions. Taken together, these findings support a progressive sequence of neural precursor differentiation and a spectrum of competence by precursors to respond to instructive microenvironmental signals. (ABSTRACT TRUNCATED)