Neocortical neurons lacking the protein-tyrosine kinase B receptor displayabnormal differentiation and process elongation in vitro and in vivo

The spatial and temporal expression of the protein-tyrosine kinase B (TrkB) receptor and its ligands has been correlated with the development of the n eocortex. Activation of the receptor has been associated with neocortical n euronal survival, differentiation, connectivity and neurotransmitter releas e. Although such findings suggest an important role for TrkB signaling in c orticogenesis, conclusive evidence from targeted gene deletion ("knockout"; TrkB -/-) mice has been limited, due in part to the neonatal lethality of most of these mutant mice and the confounding variables associated with the poor health of those few surviving slightly longer postnatally. In the pre sent study, the effects of TrkB signaling on the survival, differentiation and integration of neocortical neurons was directly investigated in vitro a nd in vivo. First, we conducted a neuron-specific immunocytochemical analys is of TrkB -/- mice to determine whether early cortical structure and patte rns of histogenesis were normal or perturbed. We then employed in vitro and in vivo approaches to extend the life of TrkB -/- neocortical neurons beyo nd the period possible in TrkB -/- mutant mice themselves: (i) dissociated cell culture to directly compare the developmental potential of TrkB -/-, /- and +/+ neurons; and (ii) neural transplantation into homochronic wild-t ype recipients to investigate the cell-autonomous effects of the receptor k nockout on the differentiation, growth and integration of neocortical neuro ns. These latter experiments allowed, for the first time, study of the surv ival and differentiation potential of TrkB -/- neocortical neurons beyond t he initial stages of corticogenesis. Direct comparison of brains of TrkB -/ -, +/- and +/+ littermates immunocytochemically labeled with antibodies to microtubule-associated protein-2, neurofilament and beta-tubulin III reveal ed subtle anatomical anomalies in the mutant mice. These anomalies include abnormally diffuse microtubule-associated protein-2 positive neurons just d orsal to the corpus callosum, and heterotopic aggregations of postmitotic n eurons in the subventricular zones of the ganglionic eminences, both sugges ting delayed neuronal migration and differentiation. Cell culture experimen ts revealed substantially reduced survival by TrkB -/- neocortical neurons, and a significant reduction in neurite outgrowth by surviving TrkB -/- neu rons. In experiments where prelabeled embryonic or neonatal TrkB -/- neocor tical neurons were transplanted into the cerebral cortices of neonatal wild -type recipients, a similar quantitatively significant defect in the format ion of dendrites, as well as reduced integration of TrkB -/- neocortical ne urons, was also evident. These findings demonstrate cell-autonomous abnormalities in the development of neocortical neurons from TrkB -/- mice, and the subtle, but potentially critical, role of protein-tyrosine kinase B signaling in neocortical neuro nal survival, differentiation and connectivity. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.