Indeed, each PC dendrite contacts more than 100,000 parallel fibers in rodents ( Napper and Harvey, 1988). For instance, planar dendrites connected with perpendicularly oriented axonal bundles contribute to maximization of possible synaptic connections with minimal redundancy ( Cuntz, 2012 Wen and Chklovskii, 2008). The planar dendrites with a space-filling and non-overlapping arrangement are a distinctive feature of PCs, which is thought to be advantageous for efficient network formation. The dendrites are innervated by parallel fiber axons of cerebellar granule cells (GCs), which run perpendicularly across the aligned PC dendrites along the coronal axis of the cerebellum. These data indicate that βIII spectrin organizes the mouse dendritic cytoskeleton and thereby regulates the oriented growth of dendrites with respect to the afferent axons.Ĭerebellar Purkinje cells (PCs) develop highly branched dendrites in a single parasagittal plane. Furthermore, disease-associated mutations affect the ability of βIII spectrin to control dendrite orientation. βIII spectrin deficiency causes actin mislocalization and excessive microtubule invasion in dendritic protrusions, resulting in abnormally oriented branch formation. We show that βIII spectrin, a causal protein for spinocerebellar ataxia type 5, is required for the biased growth of dendrites. In the model system, PC dendrites show a preference to grow perpendicularly to aligned GC axons, which presumably contribute to the planar dendrite arborization in vivo. By using electrospun nanofiber substrates, we reproduce the perpendicular contacts between PC dendrites and GC axons in culture. The cerebellar Purkinje cell (PC) arborizes a typical planar dendrite, which forms an orthogonal network with granule cell (GC) axons. The mechanism underlying the geometrical patterning of axon and dendrite wiring remains elusive, despite its crucial importance in the formation of functional neural circuits.
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