TrkB and TrkC signaling are required for maturation and synaptogenesis of hippocampal connections.
Recent studies have suggested a role for neurotrophins in the growth and refinement of neural connections, in dendritic growth, and in activity-dependent adult plasticity. To unravel the role of endogenous neurotrophins in the development of neural connections in the CNS, we studied the ontogeny of hippocampal afferents in trkB (-/-) and trkC (-/-) mice. Injections of lipophilic tracers in the entorhinal cortex and hippocampus of newborn mutant mice showed that the ingrowth of entorhinal and commissural/associational afferents to the hippocampus was not affected by these mutations. Similarly, injections of biocytin in postnatal mutant mice (P10-P16) did not reveal major differences in the topographic patterns of hippocampal connections. In contrast, quantification of biocytin-filled axons showed that commissural and entorhinal afferents have a reduced number of axon collaterals (21-49%) and decreased densities of axonal varicosities (8-17%) in both trkB (-/-) and trkC (-/-) mice. In addition, electron microscopic analyses showed that trkB (-/-) and trkC (-/-) mice have lower densities of synaptic contacts and important structural alterations of presynaptic boutons, such as decreased density of synaptic vesicles. Finally, immunocytochemical studies revealed a reduced expression of the synaptic-associated proteins responsible for synaptic vesicle exocytosis and neurotransmitter release (v-SNAREs and t-SNAREs), especially in trkB (-/-) mice. We conclude that neither trkB nor trkC genes are essential for the ingrowth or layer-specific targeting of hippocampal connections, although the lack of these receptors results in reduced axonal arborization and synaptic density, which indicates a role for TrkB and TrkC receptors in the developmental regulation of synaptic inputs in the CNS in vivo. The data also suggest that the genes encoding for synaptic proteins may be targets of TrkB and TrkC signaling pathways.
Martínez
A
A
Department of Animal and Plant Cell Biology, University of Barcelona, Barcelona 08028, Spain.
Alcántara
S
S
Borrell
V
V
Del Río
J A
JA
Blasi
J
J
Otal
R
R
Campos
N
N
Boronat
A
A
Barbacid
M
M
Silos-Santiago
I
I
Soriano
E
E
D000818:Animals; D000954:Antigens, Surface / Q000032:analysis; D002135:Calcium-Binding Proteins; D018728:Entorhinal Cortex / Q000737:chemistry / Q000166:cytology / Q000196:embryology; D005260:Female; D018507:Gene Expression Regulation, Developmental; D006624:Hippocampus / Q000737:chemistry / Q000166:cytology* / Q000196:embryology; D008297:Male; D008562:Membrane Glycoproteins / Q000032:analysis; D008565:Membrane Proteins; D051379:Mice; D018345:Mice, Knockout; D009419:Nerve Tissue Proteins / Q000032:analysis; D009434:Neural Pathways; D009474:Neurons / Q000737:chemistry / Q000502:physiology; D018696:Neuroprotective Agents / Q000032:analysis / Q000378:metabolism*; D009924:Organ Culture Techniques; D010641:Phenotype; D020794:Receptor Protein-Tyrosine Kinases / Q000032:analysis / Q000235:genetics*; D020801:Receptor, Ciliary Neurotrophic Factor; D020812:Receptor, trkC; D017475:Receptors, Nerve Growth Factor / Q000032:analysis / Q000235:genetics*; D015398:Signal Transduction / Q000502:physiology*; D013569:Synapses / Q000737:chemistry / Q000502:physiology*; D016708:Synaptophysin / Q000032:analysis; D050825:Synaptosomal-Associated Protein 25; D050857:Synaptotagmins; D050827:Syntaxin 1
D000954:Antigens, Surface; D002135:Calcium-Binding Proteins; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D018696:Neuroprotective Agents; D020801:Receptor, Ciliary Neurotrophic Factor; D017475:Receptors, Nerve Growth Factor; C494209:Snap25 protein, mouse; D016708:Synaptophysin; D050825:Synaptosomal-Associated Protein 25; D050827:Syntaxin 1; D050857:Synaptotagmins; D020794:Receptor Protein-Tyrosine Kinases; D020812:Receptor, trkC