Critical role of soluble amyloid-β for early hippocampal hyperactivity in a mouse model of Alzheimer's disease

Abstract

Alzheimer's disease (AD) is characterized by a progressive dysfunction of central neurons. Recent experimental evidence indicates that in the cortex, in addition to the silencing of a fraction of neurons, other neurons are hyperactive in amyloid-β (Aβ) plaque-enriched regions. However, it has remained unknown what comes first, neuronal silencing or hyperactivity, and what mechanisms might underlie the primary neuronal dysfunction. Here we examine the activity patterns of hippocampal CA1 neurons in a mouse model of AD in vivo using two-photon Ca(2+) imaging. We found that neuronal activity in the plaque-bearing CA1 region of older mice is profoundly altered. There was a marked increase in the fractions of both silent and hyperactive neurons, as previously also found in the cortex. Remarkably, in the hippocampus of young mice, we observed a selective increase in hyperactive neurons already before the formation of plaques, suggesting that soluble species of Aβ may underlie this impairment. Indeed, we found that acute treatment with the γ-secretase inhibitor LY-411575 reduces soluble Aβ levels and rescues the neuronal dysfunction. Furthermore, we demonstrate that direct application of soluble Aβ can induce neuronal hyperactivity in wild-type mice. Thus, our study identifies hippocampal hyperactivity as a very early functional impairment in AD transgenic mice and provides direct evidence that soluble Aβ is crucial for hippocampal hyperactivity.

Fig. 1.

Altered neuronal activity in hippocampus of plaque-bearing transgenic mice. (A) Schematic of the experimental preparation for in vivo imaging. (B) (Left) Detailed view of the boxed region in A. (Right) In vivo image of a CA1 pyramidal cell layer. (C) (Left) CA1 neurons imaged in vivo in a wild-type mouse. (Right) Spontaneous Ca²⁺ transients of the corresponding neurons marked (Left). (D) (Left) CA1 neurons imaged in vivo in a transgenic (tg) mouse with thioflavin-S–positive plaques (light blue). (Right) Spontaneous Ca²⁺ transients of the corresponding neurons marked (Left). Traces are color-coded to mark neurons that were either silent (blue) or hyperactive (red) during the recording period. (E) Histograms showing the frequency distribution of Ca²⁺ transients in wild-type (Upper; n = 312 cells in five mice) and tg (Lower; n = 349 cells in five mice) mice. Note increased fractions of silent and hyperactive neurons in tg mice. Pie charts show the relative proportions of silent, normal, and hyperactive neurons.

Fig. 2.

Abnormal hyperactivity of hippocampal neurons in predepositing transgenic mice. (A and B) (Left) CA1 neurons imaged in vivo in a wild-type and a transgenic mouse, respectively. (Center) Activity maps, in which hue is determined by the frequency of spontaneous Ca²⁺ transients, overlaid with the anatomical image (Left). (Right) Spontaneous Ca²⁺ transients of the corresponding neurons marked (Left). (C) The frequency distribution of spontaneous Ca²⁺ transients was shifted toward higher frequencies in tg (Lower; n = 818 cells in seven mice) compared with wild-type mice (Upper; n = 693 cells in six mice). Pie charts show the relative proportions of silent, normal, and hyperactive neurons.

Fig. 3.

Cellular mechanisms of spontaneous Ca²⁺ transients in wild-type and transgenic mice. (A) Simultaneous in vivo recordings of spontaneous Ca²⁺ transients (black trace) and underlying action potential (AP) firing (red trace; number of APs indicated) in a cell-attached configuration from a CA1 neuron in a wild-type mouse. (B) Examples of spontaneous Ca²⁺ transients (black trace) evoked from three consecutive single APs (red trace) in a WT mouse. (C) Fractions of single APs and trains of APs optically detected in CA1 neurons of WT mice (n = 15 cells in three mice). (D) Spontaneous Ca²⁺ transients in normal (black traces) and hyperactive (red traces) CA1 neurons of a transgenic mouse before, during, and after local application of CNQX and APV. (E) Simultaneous in vivo recordings of spontaneous Ca²⁺ transients (black trace) and underlying action potential firing (red trace) in a cell-attached configuration from a CA1 neuron in a tg mouse. (F) Number of spontaneous Ca²⁺ transients triggered (AP) and not triggered (non-AP) by action potential firing in tg mice (n = 13 cells in three mice).

Fig. 4.

A single dose of γ-secretase inhibitor reduces soluble Aβ levels and rescues neuronal dysfunction in transgenic mice. (A and B) Acute LY-411575 treatment significantly reduced TBS- and SDS-soluble Aβ-40 and Aβ-42 hippocampal levels in transgenic mice (n = 4–5 mice per group; *P < 0.001 for Aβ-40 and *P < 0.05 for Aβ-42, Student’s t test). (C–E) (Upper) CA1 neurons imaged in vivo in a WT, a tg, and an LY-411575–treated tg mouse, respectively. (Lower) Activity maps, in which hue is determined by the frequency of spontaneous Ca²⁺ transients, overlaid with the anatomical image (Upper). (F) The fraction of hyperactive neurons was significantly smaller in LY-411575–treated tg (5.13 ± 1.47%, n = 5 mice) than in untreated tg mice (25.85 ± 3.59%, n = 7 mice; *P < 0.001, Student’s t test) and not significantly different from untreated WT mice (1.85 ± 0.86%, n = 6 mice; P > 0.05, Student’s t test). (G) Cumulative distributions of spontaneous Ca²⁺ transients in LY-411575–treated tg mice (n = 709 cells) were not significantly different from untreated WT (n = 693 cells) and LY-411575–treated WT controls (n = 841 cells; P > 0.05, Kolmogorov–Smirnov test), but they were significantly different from vehicle-treated tg (n = 484 cells) and untreated tg mice (n = 818 cells; *P < 0.001, Kolmogorov–Smirnov test). All error bars denote SEM.

Fig. 5.

Soluble Aβ induces hyperactivity in hippocampal neurons of wild-type mice. (A) Ca²⁺ transients in CA1 neurons of a wild-type mouse before, during, and after local application of synthetic AβS26C dimer solution (30 s, 100 nM in the application pipette). (B) Rates of Ca²⁺ transients in all recorded neurons from the experiment in A before (black) and during (red) dimer application. (C) Summary graph from the experiment in A showing the effect of dimers on the frequency of Ca²⁺ transients (normalized to control, n = 24 cells; *P < 0.001, Student’s t test). (D) Summary graph from all experiments illustrating the effect of dimers on the frequency of Ca²⁺ transients (normalized to control, n = 83 cells in three mice; *P < 0.001, Student’s t test). (E) Summary graph showing that heat-denatured dimers have no significant effect on the frequency of Ca²⁺ transients (normalized to control, n = 158 cells in three mice; P > 0.05, Student’s t test). All error bars denote SEM.