Mice lacking ataxin-1 display learning deficits and decreased hippocampal paired-pulse facilitation

Abstract

Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disorder characterized by ataxia, progressive motor deterioration, and loss of cerebellar Purkinje cells. To investigate SCA1 pathogenesis and to gain insight into the function of the SCA1 gene product ataxin-1, a novel protein without homology to previously described proteins, we generated mice with a targeted deletion in the murine Sca1 gene. Mice lacking ataxin-1 are viable, fertile, and do not show any evidence of ataxia or neurodegeneration. However, Sca1 null mice demonstrate decreased exploratory behavior, pronounced deficits in the spatial version of the Morris water maze test, and impaired performance on the rotating rod apparatus. Furthermore, neurophysiological studies performed in area CA1 of the hippocampus reveal decreased paired-pulse facilitation in Sca1 null mice, whereas long-term and post-tetanic potentiations are normal. These findings demonstrate that SCA1 is not caused by loss of function of ataxin-1 and point to the possible role of ataxin-1 in learning and memory.

Fig. 1.

Generation of Sca1 null mice.A, Targeted deletion of exon 8 of theSca1 gene by homologous recombination in ES cells. The PGK-hprt expression cassette introduces a novel HindIII site into the targeted locus. Recombinant clones contain 8 or 7.5 kbHindIII fragments, detected by the 5′ and the 3′ external probes, respectively. PGK, Phosphoglycerate kinase promoter; hprt, hypoxanthine phosphoribosyl transferase; tk, thymidine kinase; H,HindIII; B, BamHI;EV, EcoRV; X,XbaI. B, Western blot analysis of brain extracts using anti-ataxin-1 antibody. Homozygous mutant mice (−/−) had no detectable ataxin-1, whereas heterozygous (+/−) and wild-type (+/+) mice from the same litter showed the expected 100 kDa protein.

Fig. 2.

Decreased exploratory behavior ofSca1 null mice in the open field test. Analysis of the total distance traveled by C57Bl/6J-129/SvEv hybrid mice (A) and pure 129/SvEv inbred mice (B) during the first 5 min interval for 6 consecutive d. Error bars indicate SEM.

Fig. 3.

Morris water maze test results for wild-type andSca1 null mice of a pure 129/SvEv genetic background. Mean escape latencies (A) and percentage of time spent in the four quadrants (B) in the hidden platform version of the Morris water maze during the last memory probe trial. C, Performance in the visible platform version of the Morris water maze test. Similar results were obtained with mice of mixed genetic background. Error bars indicate SEM.

Fig. 4.

Analysis of Sca1 null mice on the rotating rod apparatus. Performance of 5-week-old C57Bl/6J-129/SvEv mice (A), 5-week-old 129/SvEv mice (B), 6- to 9-month-old C57Bl/6J-129/SvEv mice (C), and 6-month-old pure 129/SvEv mice (D). Mice were trained four trials per day (a–d) for 4 (1–4) or 7 (1–7) d. Error bars indicate SEM.

Fig. 5.

Synaptic transmission and activity-dependent plasticity in hippocampal area CA1 of Sca1 null mice.A, LTP was induced with two 1 sec 100 Hz tetani and assayed for 60 min. Data represent the average pEPSP slope over time from seven WT animals (19 slices) and seven mutant animals (16 slices) from both mixed C57Bl/6J-129/SvEv and purebred 129/SvEv backgrounds. Error bars indicate SEM. B, PPF was elicited over a range of interpulse intervals. Data are represented as the percentage of the second EPSP slope relative to the first and are from seven WT animals (22 slices) and seven mutant animals (23 slices) from both mixed C57Bl/6J-129/SvEv and purebred 129/SvEv backgrounds. Error bars indicate SEM. Inset, Representative PPF at 50 msec interpulse interval in mutant and WT mice. Calibration: 2 mV, 9 msec.