Striatal-enriched protein tyrosine phosphatase (STEP) knockout mice have enhanced hippocampal memory

Abstract

Striatal-enriched protein tyrosine phosphatase (STEP) is a brain-specific phosphatase that opposes synaptic strengthening by the regulation of key synaptic signaling proteins. Previous studies suggest a possible role for STEP in learning and memory. To demonstrate the functional importance of STEP in learning and memory, we generated STEP knockout (KO) mice and examined the effect of deletion of STEP on behavioral performance, as well as the phosphorylation and expression of its substrates. Here we report that loss of STEP leads to significantly enhanced performance in hippocampal-dependent learning and memory tasks. In addition, STEP KO mice displayed greater dominance behavior, although they were normal in their motivation, motor coordination, visual acuity and social interactions. STEP KO mice displayed enhanced tyrosine phosphorylation of extracellular-signal regulated kinase 1/2 (ERK1/2), the NR2B subunit of the N-methyl-D-aspartate receptor (NMDAR) and proline-rich tyrosine kinase (Pyk2), as well as an increased phosphorylation of ERK1/2 substrates. Concomitant with the increased phosphorylation of NR2B, synaptosomal expression of NR1/NR2B NMDARs was increased in STEP KO mice, as was the GluR1/GluR2 containing α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs), providing a potential molecular mechanism for the improved cognitive performance. The data support a role for STEP in the regulation of synaptic strengthening. The absence of STEP improves cognitive performance, and may do so by the regulation of downstream effectors necessary for synaptic transmission.

FIG. 1. Altered performance of STEP KO mice on the Morris water maze

The latency to find the platform (A) as well as the pathlength (B) were not different between WT and KO mice during all training blocks. The time spent in the four quadrants during probe trial I (Day 6) were similar between WT and STEP KO mice (C). Both groups spent significantly more time than chance in the target quadrant (p < 0.001). During probe trial II (Day 13) after reversal (D), the STEP KO mice spent significantly more time in the new target quadrant than WT mice (p < 0.05 as compared to WT). In probe trial III (Day 20), there were no significant differences between the WT and STEP KO mice (E) (n=14–16 per genotype).

FIG. 2. Differences in search strategy used by STEP KO during first and last blocks of training as well as probe trials

The search strategy (A) used by STEP KO mice did not differ from WT mice during the first training block or training blocks after reversal. The STEP KO mice used different search strategy during training blocks 5 and 7. The search strategy (B) of STEP KO mice was similar to that of WT controls during probe trials I and II. The STEP KO mice did not utilize spatial strategy during probe trial III.

FIG. 3. Enhanced performance of STEP KO mice on radial-arm water maze

The number of reference memory errors (A), latency to find the platform (B) as well as the working memory errors (C) were significantly lower in STEP KO mice as compared to WT controls during the first two days of testing. The STEP KO mice and WT littermates did not differ significantly in error rate or latency during reversal trials. However, the STEP KO mice had consistently exhibited lower error rates and latency. Perseverative errors (D) during reversal trials were similar between WT and STEP KO mice (n=15 per genotype).

FIG. 4. Comparison of Reference memory errors and turn angles between successive arm visits

The number of reference memory errors (A) during the first and last trial of each training day was similar between WT and STEP KO mice except for the first trial of the last experimental day (Day 4). The number of reference memory errors decreased over trials in both genotypes. WT mice committed significantly more reference memory errors during the first trial on day 4. The average turn angle (B) during each arm visit was comparable between WT and STEP KO mice except during the last trial of the experiment. During the last trial on day 4, STEP KO mice used a strategy that significantly reduced the turn angle.

FIG. 5. Comparison of open field activity, motor coordination and social interaction between WT controls and STEP KO mice

The total distance travelled (A), time of ambulation (C), average velocity (D), ambulatory counts (E) and ambulatory episodes (F) averaged over 3 consecutive days were not different between STEP WT and KO mice. Total distance moved per 5 min blocks (B) over 30 min showed that both WT and KO mice habituated to the environment. The latency to fall (G) from a rotarod increased significantly over 6 trials on both testing days. Total time spent in social behaviors as well as specific categories of behaviors are shown in (H). STEP KO mice showed significantly more dominance behaviors than WT controls (p < 0.03) (n=10–14 for open field and rotarod, n=5–7 for social interaction for each genotype).

FIG. 6. STEP KO mice have elevated Tyr phosphorylation of STEP substrates and increased surface expression of ionotropic glutamate receptors

Synaptosomal membrane fractions from WT and STEP KO hippocampi were probed with p-ERK1/2, ERK2, STEP, pY¹⁴⁷²-NR2B, NR2B, NR2A, GluR1, GluR2, and GluR3. Representative immunoblots are shown in (A) and histograms in (B). The STEP KO mice show no detectable STEP expression and increased Tyr phosphorylation of ERK1/2 and NR2B. The expression levels of NR1, NR2B, GluR1, and GluR2 but not NR2A and GluR3 are elevated in the STEP KO mice. The levels of phosphorylation of ERK substrates in nuclear fraction were detected by immunoblotting (C, E) and quantified (D, F). There were significant increases in activated CREB, Elk1 and synapsin I in STEP KO mice. The Tyr phosphorylation of Pyk2 was significantly enhanced in crude synaptosomal fractions from STEP KO mice (E, F) (n=5 per genotype).