LOSS OF LKB1 IN CEREBELLAR GRANULE CELLS DISRUPTS PARALLEL FIBER–PURKINJE CELL SYNAPSE REFINEMENT

The cerebellum plays a pivotal role in motor coordination and learning by integrating multiple synaptic inputs into Purkinje cells (PCs), including excitatory inputs from parallel fibers (PFs), the axon of granule cells (GCs). The serine/threonine kinase Liver Kinase B1 (LKB1) has been identified to regulate their migration and cerebellar cortical folding. However, its role in PF synapse formation and maintenance remains unclear. In this study, we generated GC-specific LKB1 conditional knockout mice (LKB1 cKO) during the postnatal period. These mice exhibited an age-dependent decline in locomotion, accompanied by progressive cerebellar atrophy that initiated in posterior lobules and progressively extended anterior lobules, along with increased microglia density.
To consider PF-PC synapse remodeling, we performed scanning electron microscopy (SEM) of whole cerebellar sagittal semithin sections, which enabled large-field and high-magnification analysis of synapses. Although the density of PF-PC synapses remained unchanged, presynaptic terminal size was increased in the LKB1 cKO mice, and the correlation between the mitochondria and presynaptic terminal area was reduced. In contrast, investigation using the 3D reconstruction of serial block-face SEM showed increased mitochondria volume within individual PF terminals despite comparable terminal volumes. Notably, whereas PF terminals in control mice typically formed a single synaptic contact with a Purkinje cell spine, a subset of PF terminals formed multiple synapses in LKB1 cKO mice. These results indicated that LKB1 in GC is essential for the PF-PC synapse refinement, and that aberrant enhancement excitatory inputs onto PCs disrupts precise cerebellar regulation underlying motor coordination.