The 440 kDa splicing variant of ankyrin B (gAnkB) is the predominantly expressed ankyrin B isoform during early brain development. It is only expressed in neurons and specifically distributed along unmyelinated axons beneath axonal plasma membranes. gAnkB is derived by acquiring a giant insertion region, which is encoded by a single giant exon. The giant insertion is over 2,000-residue-long, predicted to be intrinsically disordered and with hitherto poorly understood functions. ANK2, the encoding gene of ankyrin B, has been identified as one of the top high-risk autism-related genes. Human-patient-mimic autistic mouse model studies revealed that gAnkB deficiency leads to increased stochastic connectivity in the brain and enhanced axon branching in cultured neurons, but with an...[ Read more ]

The 440 kDa splicing variant of ankyrin B (gAnkB) is the predominantly expressed ankyrin B isoform during early brain development. It is only expressed in neurons and specifically distributed along unmyelinated axons beneath axonal plasma membranes. gAnkB is derived by acquiring a giant insertion region, which is encoded by a single giant exon. The giant insertion is over 2,000-residue-long, predicted to be intrinsically disordered and with hitherto poorly understood functions. ANK2, the encoding gene of ankyrin B, has been identified as one of the top high-risk autism-related genes. Human-patient-mimic autistic mouse model studies revealed that gAnkB deficiency leads to increased stochastic connectivity in the brain and enhanced axon branching in cultured neurons, but with an unknown underlying molecular mechanism. Here we discovered that cultured neurons from transgenic mice, with their giant exon of ANK2 specifically removed, displayed increased stochastic collateral axon branching. Also, axonal microtubule stability in these neurons decreased, with more microtubules deviating from the axonal shaft and invading into axonal filopodia. A region from the giant insertion of gAnkB containing 15-repeat was identified to be essential for direct microtubule binding and bundling both in vitro and in living cells. A gAnkB mutant, which is defective in microtubule binding and bundling due to point mutations in the 15-repeat sequence, is totally incapable of suppressing collateral axon branching in cultured gAnkB-deficient neurons. Taken together, gAnkB may play a critical role in stabilizing the physical and topological structures of neuronal axons by stabilizing microtubules via its insertion.