Characterization of the spontaneous ataxia phenotype in the HCN-ex3/-ex3 mouse line

Abstract

Transgenic mouse lines were generated by pronucleus injection of a mini-gene. Upon aging one founder line developed an ataxia phenotype although the transgene expression system was inactive. Therefore, the question was whether this phenotype was initiated by transgene insertion mutagenesis. Using BAC and PCR cloning it was discovered that insertion of the transgene caused deletion of exon 3 of the HCN1 (hyperpolarization-activated, cyclic nucleotide-gated channel 1) gene thereby establishing a novel knock-out mouse model (HCN1-ex3/-ex3). HCN1 channels are responsible for "pacemaker" currents important for rhythmic firing in both heart and brain. Analysis of motor behavior revealed that HCN1-ex3/-ex3 animals have an impaired performance in rotarod, beam walking and string suspension tests. These motor deficits are detectable at six weeks of age and progress with time. Comparable results were obtained with a previous HCN1 deficient mouse line targeting exon 4 (HCN1-ex4/-ex4). Histological analysis of the cerebelli of both mutants revealed a reduction in Purkinje cell number and dentritic tree complexity. Purkinje cells are essential components of the cerebellar circuit for performing coordinated movements suggesting that ataxia phenotype of HCN1 mutants depends on changes in Purkinje cell number. The findings of this study propose a novel function of HCN1 dependent currents in the regulation of Purkinje cell homoeostasis upon aging. Skeletal muscle tissue of HCN1-ex3/-ex3 mice showed myopathic signs (internalized myonuclei and atrophic fibers). In contrast, HCN1-ex4/-ex4 mouse model did not. Morphometric analysis showed that the size of muscle fibers in HCN1-ex3/-ex3 is smaller compared to wild type mice. Most of the atrophic fibers correspond to fiber type I (strong NADH-staining). Since HCN1 protein was not detected in skeletal muscle of wild types, the observed muscle phenotype of HCN1-ex3/-ex3 mice is probably induced by the lack of HCN1 expression.