In addition to binding with MT, MAP1A-HC interacts with the membrane-associated guanylate kinases (MAGUKs) through a C-terminal consensus domain (Brenman et al., 1998; Reese et al., 2007). Here we report that MAP1A mutation causes ataxia, tremors, and late-onset degeneration of cerebellar Purkinje cells, which are preceded by structural abnormalities in Purkinje cell dendrites and the axon initial segment (AIS). encodes a precursor polypeptide that is proteolytically cleaved to produce a MAP1A heavy chain (MAP1A-HC) and a light chain (LC2; Langkopf et al., 1992). These proteins can bind to MTs independently or as a complex that can include LC1, a proteolytic cleavage product from MAP1B precursor protein (Hammarback et al., 1991), and LC3, an independently encoded autophagosomal protein (Vallee and Davis, 1983; Mann and Hammarback, 1994; Kabeya et al., 2000). In addition to binding with MT, MAP1A-HC interacts with the membrane-associated guanylate kinases (MAGUKs) through a C-terminal consensus domain (Brenman et al., 1998; Reese et al., 2007). Here we report that MAP1A mutation causes ataxia, tremors, and late-onset degeneration of cerebellar Purkinje cells, which are preceded by structural abnormalities in Purkinje cell dendrites and the axon initial segment (AIS). We demonstrate that MT networks are altered in mutant Purkinje cells and that both the heavy and light chain of MAP1B is abnormally distributed in soma and dendrites of these neurons before structural defects. Furthermore, MAP1A deficiency results in decreased PSD-93 (also known as Chapsyn-110 or Dlg2) in Purkinje cells, suggesting that MAP1A is required to maintain normal levels CVT 6883 of this MAGUK protein. Together, our results demonstrate the importance of MAP1A in neuronal MT organization, synaptic protein modulation, and neuronal survival in the adult CNS. Materials and Methods Mice. All animal protocols were approved by the Animal Care and Use Committee of The Jackson Laboratory. The mouse stain was maintained on the C57BLKS/J CVT 6883 background. Tg-Map1a mice were a kind gift from Dr. Akihiro Ikeda at the University of Wisconsin-Madison, and this strain was maintained on the C57BL/6J background (Ikeda et al., 2002). For transgenic rescue experiments, Tg-Map1a mice were crossed with knock-out ES cells (C57BL/6NJ-cassettes (genomic sequence encoding the light chain (2766C3014 aa), and this sequence was inserted downstream of the neuron-specific enolase (NSE) promoter (Twyman and Jones, 1997). This construct (pNSE-LC2C3Myc) was injected into the pronucleus of allele was differentiated from the wild-type (WT) allele by PCR using the Map1a-F (5-GCTGAGTCGCCAGTTGGCTT-3) and Map1a-R (5AGTCATCTCAGGTGGGGATG-3) primers; the amplicon is made up of 92 bp and WT amplicon is made up of 99 bp. Tg-Map1a transgenic mice were identified with the TgMap1a-F (5-TCTGGGACCTCACTCCTCTG-3) and TgMap1a-R (5-TCTTGGTGAGTTCCCCTGAG-3) PCR primers. The transgene, derived from 129P2/OlaHsd sequence, generated a 228 bp amplicon, while C57BLKS/J or C57BL/6J alleles generated a 150 bp amplicon due to a polymorphic microsatellite. To distinguish Tg-Map1a; allele, and the PCR products were sequenced to distinguish the transgenic versus the endogenous WT allele. The cassettes) was genotyped with the primer pair RAF5 (5-CACACCTCCCCCTGAACCTGAAAC-3) and Map1a-in5DR (5-CCCACTTTCCTGATATACTCAC-3). The cassettes) was identified with Map1a-in5UF (5-CCCCAATGATTTGATCAGCTTC-3) LW-1 antibody and Map1a-in5DR primers. The Tg-pNSE-LC2C3Myc allele was genotyped with primer pair Map1a-lastXnF (5-GTGACTCTGATTCCCACTCATG-3) and 3T4AR (5-GTGGTACACTTACCTGGTACC-3). All PCR conditions were as follows: 35 cycles at 94C for 30 s, 58C for 30 s, and 72C for 30 s. Both male and female mice were used in our studies and no CVT 6883 sex-related differences were observed. At least three mice were used for each genotype at each age analyzed. Genomic mapping. Homozygous mice were crossed to C3HeB/FeJ mice, and F1 heterozygotes were intercrossed to generate F2 mice. Genome scans were performed with polymorphic microsatellite markers (MIT markers) using genomic DNA collected from 15 affected and 15 unaffected F2 mice. For fine mapping, 1233 F2 mice were analyzed using MIT markers. Immunohistochemistry. Mice were transcardially perfused with 10% neutral buffered formalin.