The Journal of Neuroscience, May 15, 2002, 22(10):4045-4056
Late-Stage Immature Neocortical Neurons Reconstruct
Interhemispheric Connections and Form Synaptic Contacts with
Increased Efficiency in Adult Mouse Cortex Undergoing
Targeted Neurodegeneration
Rosemary A.
Fricker-Gates,
Jennifer
J.
Shin,
Cindy C.
Tai,
Lisa A.
Catapano, and
Jeffrey D.
Macklis
Division of Neuroscience, Children's Hospital, and Department of
Neurology and Program in Neuroscience, Harvard Medical School, Boston,
Massachusetts 02115
In the neocortex, the effectiveness of potential cellular
repopulation therapies for diseases involving neuronal loss may depend
critically on whether newly incorporated cells can differentiate appropriately into precisely the right kind of neuron, re-establish precise long-distance connections, and reconstruct complex functional circuitry. Here, we test the hypothesis that increased efficiency of
connectivity could be achieved if precursors could be more fully
differentiated toward desired phenotypes. We compared embryonic neuroblasts and immature murine neurons subregionally dissected from
either embryonic day 17 (E17) (Shin et al., 2000) or E19 primary
somatosensory (S1) cortex and postnatal day 3 (P3) purified callosal
projection neurons (CPNs) with regard to neurotransmitter and receptor
phenotype and afferent synapse formation after transplantation into
adult mouse S1 cortex undergoing targeted apoptotic degeneration of
layer II/III and V CPNs.
Two weeks after transplantation, neurons from all developmental stages
were found dispersed within layers II/III and V, many with
morphological features typical of large pyramidal neurons. Retrograde
labeling with FluoroGold revealed that 42 ± 2% of transplanted E19 immature S1 neurons formed connections with the contralateral S1
cortex by 12 weeks after transplantation, compared with 23 ± 7%
of E17 neurons. A greater percentage of E19-derived neurons received
synapses (77 ± 1%) compared with E17-derived neurons (67 ± 2%). Similar percentages of both E17 and E19 donor-derived neurons
expressed neurotransmitters and receptors [glutamate, aspartate, GABA,
GABA receptor (GABA-R), NMDA-R, AMPA-R, and kainate-R] appropriate for
endogenous adult CPNs progressively over a period of 2-12 weeks after
transplantation. Although P3 fluorescence-activated cell
sorting-purified neurons also expressed these mature phenotypic markers
after transplantation, their survival in vivo was poor.
We conclude that later-stage and region-specific immature neurons
develop a mature CPN phenotype and make appropriate connections with
recipient circuitry with increased efficiency. However, at postnatal
stages of development, limitations in survival outweigh this increased
efficiency. These results suggest that efforts to direct the
differentiation of earlier precursors precisely along specific desired
neuronal lineages could potentially make possible the highly efficient
reconstruction of complex neocortical and other CNS circuitry.
Key words:
neocortex; neural transplantation; migration; integration; connectivity; targeted degeneration; apoptosis; neuronal
degeneration; neurotransmitters; receptors; glutamate; aspartate; GABA; kainate-R; NMDA-R; AMPA-R; GABA-R
Copyright © 2002 Society for Neuroscience 0270-6474/02/22104045-12$05.00/0