Akt1 deficiency in schizophrenia and impairment of hippocampal plasticity and function

Abstract

Genetic studies have associated deficient function of the serine/threonine kinase Akt1 with schizophrenia. This disorder is associated with developmental, structural, and functional abnormalities of the hippocampus that could be traced to abnormal Akt1 function. To establish a closer connection between Akt1 and hippocampal function, mice with a selective deletion of Akt1 (Akt1(-/-) mice) were examined for physiological and behavioral outcomes dependent on the hippocampus and associated with schizophrenia. Genetic deletion of Akt1 was associated with both impaired proliferative capacity of adult-born hippocampal progenitors and hippocampal long-term potentiation, indicating deficient functions of this brain region associated with neuroplasticity. Moreover, Akt1(-/-) mice demonstrated impairments in contextual fear conditioning and recall of spatial learning, behaviors known to selectively involve the hippocampus. Akt1(-/-) mice also showed reduced prepulse inhibition of the acoustic startle response, a sensorimotor gating response that is perturbed in schizophrenia. Postmortem tissue samples from patients with schizophrenia showed significant reductions of phosphorylated Akt levels in hilar neurons of the dentate gyrus, the neurogenic zone of the hippocampus. Taken together, these results implicate the Akt1 isoform in regulating hippocampal neuroplasticity and cognition and in contributing to the etiology of schizophrenia.

FIGURE 1

Akt1 regulates adult hippocampal cell proliferation, but not survival. To assess cell (a) proliferation and (b) survival, WT (black bars; n = 8) and Akt1^−/− (white bars; n = 7–8) mice were sacrificed 24 h or 14 days after the last BrdU injection, respectively. Values are expressed as the number of BrdU positive cells per 10 000 7-AAD events. Bars represent mean values + SEM. Asterisk indicates that Akt1^−/− mice differed significantly from WT (*P < 0.05).

FIGURE 2

LTP is disrupted in the Akt1^−/− mouse. LTP was assessed in area CA1 and dentate in WT (n = 4) and Akt1^−/− mice (n = 4–5). A single 1-s 100 Hz stimulus was used that reliably generated LTP in both the CA1 area (a) and dentate gyrus (b) of WT mice (gray triangles). In the CA1 (a), this tetanus generated STP in both the WT and Akt1^−/− mice (black, closed circles) during the first 5 min post-tetanus, yet failed to generate LTP in the Akt1^−/− mice. The dentate gyrus (b) appeared more severely disrupted, with the tetanus in the Akt1^−/− mice unable to generate STP. To show that basic synaptic properties in the Akt1^−/− mice remained intact, traces of the first four stimuli from representative experiments were each normalized to their largest peak response and when plotted with artifacts and offset removed (insets) illustrated similar fEPSP properties.

FIGURE 3

Akt1^−/− mice demonstrate deficits in contextual fear conditioning and recall of spatial memory. WT (black bars; n = 11) and Akt1^−/− (white bars; n = 9), mice were trained for contextual fear conditioning on day 1. (a) Mice were tested on day 2 in their respective training context, and then tested for (b) cued conditioning in a new context. Bars represent the percent time freezing ± SEM during the (a) 5-min context test and (b) the percent time freezing during the tone presentations on day 2. (c) WT (black squares; n = 9) and Akt1^−/− (white squares; n = 9) mice were trained in the MWM for 8 days with the hidden platform in the North position. On day 10, the hidden platform was moved to the South position. Values represent the mean (time in seconds taken to reach the platform) ± SEM. (d) On day 9, recall of spatial memory was assessed in the probe trial. Bars (WT, black bars; Akt1^−/−, white bars) represent the mean distance swam in each quadrant ± SEM. Asterisk indicates that Akt1^−/− mice differed significantly from WT (*P < 0.05). When calculated as the absolute time spent in the target quadrant, Akt1^−/− mice again showed a trend toward a significant reduction (WT: 32.6 ± 3.7 s, Akt1^−/−: 24.1 ± 2.7 s, P = 0.08).

FIGURE 4

Akt1^−/− mice have abnormalities in prepulse inhibition. (a) Baseline levels of PPI were compared between WT (black bars; n = 11) and Akt1^−/− mice (white bars; n = 8) across three prepulses of increasing intensities (64 dB, 68 dB, 76 dB). Bars represent the mean (% PPI) ± SEM. (b) Startle reactivity was measured across increasing pulse intensities. Bars represent the mean (startle response) ± SEM. Asterisk indicates that Akt1^−/− mice differed significantly from WT (*P < 0.05).

FIGURE 5

Reduced Akt phosphorylation at serine 473 in postmortem DG hilar neurons of patients with schizophrenia (Sz) compared with normal (N) controls. (a) The amount of pAkt (S473) reactivity in cell bodies and their processes of DG neurons from patients with Sz (black bar; n = 18) is lower than in N controls (gray bar; n = 18). The photomicrographs from a typical Sz case (b) and its matched control (c) show that moderate to high levels of pAkt (S473) occupy a lower proportion of neuronal cell bodies and their processes in the DG hilus of patients with Sz. Note that pAkt (S473) in neuronal processes (arrowed in c) is virtually absent in the patient with Sz. Scale bars = 100 μm. Asterisk indicates that Sz differed significantly from N (*P < 0.05).