J Neurosci. 2025 Feb 27:e2156242025. doi: 10.1523/JNEUROSCI.2156-24.2025. Online ahead of print.
ABSTRACT
Neural circuits are refined by strengthening frequently used or advantaged synapses while eliminating redundant connections. In neonatal mice, cerebellar Purkinje cells (PCs) are initially innervated by multiple climbing fibers (CFs) of similar strength. By postnatal day 7 (P7), one CF, the “winner,” is selectively strengthened and begins dendritic translocation by P9, while both “winner” and “loser” CFs temporarily maintain somatic synapses. Although the functional differentiation of CF inputs is well understood, their structural differentiation is less clear. In this study, we examined “winner” CF synapses in dendrites and both “winner” and “loser” synapses in the soma using serial electron microscopy and immunohistochemistry in C57BL/6 mice. We found that “winner” CF synapses, both in the soma and dendrites, developed more complex pre- and postsynaptic structures than “loser” CFs, with an expanded area of postsynaptic density. Additionally, “winner” CF synapses expressed significantly higher levels of AMPA-type glutamate receptors. Notably, only dendritic “winner” synapses showed increased levels of Rab3-interacting molecule RIM, a key presynaptic regulator of neurotransmitter release. These findings reveal the molecular and structural features that enable “winner” CFs to reinforce their synaptic strength and innervation, allowing them to outcompete other inputs during early development.Significance statement The neural circuit is refined by selectively strengthening specific synapses while eliminating redundant connections. In neonates, cerebellar Purkinje cell somata are innervated by multiple climbing fibers (CFs) with similar strength. Subsequently, a single CF is strengthened as the “winner,” establishing a stable connection by translocating to dendrites. While the functional differentiation of CFs has been well-characterized, their structural differentiation remains largely unclear. Through serial electron microscopy and immunohistochemistry, we reveal that “winner” CF synapses elaborated synaptic structures with elevated AMPA receptor expression and presynaptic Rab-interacting molecule RIM. Thus, translocated “winner” CF synapses undergo molecular and anatomical strengthening, securing an irreversible competitive advantage over somatic CF synapses. The results provide a developmental basis for better understanding synaptic circuit refinement.
PMID:40015986 | DOI:10.1523/JNEUROSCI.2156-24.2025
