Mutation-induced loss of APP function causes GABAergic depletion in recessive familial Alzheimer’s disease: analysis of Osaka mutation-knockin mice

Abstract

The E693Δ (Osaka) mutation in APP is linked to familial Alzheimer’s disease. While this mutation accelerates amyloid β (Aβ) oligomerization, only patient homozygotes suffer from dementia, implying that this mutation is recessive and causes loss-of-function of amyloid precursor protein (APP). To investigate the recessive trait, we generated a new mouse model by knocking-in the Osaka mutation into endogenous mouse APP. The produced homozygous, heterozygous, and non-knockin littermates were compared for memory, neuropathology, and synaptic plasticity. Homozygotes showed memory impairment at 4 months, whereas heterozygotes did not, even at 8 months. Immunohistochemical and biochemical analyses revealed that only homozygotes displayed intraneuronal accumulation of Aβ oligomers at 8 months, followed by abnormal tau phosphorylation, synapse loss, glial activation, and neuron loss. These pathologies were not observed at younger ages, suggesting that a certain mechanism other than Aβ accumulation underlies the memory disturbance at 4 months. For the electrophysiology studies at 4 months, high-frequency stimulation evoked long-term potentiation in all mice in the presence of picrotoxin, but in the absence of picrotoxin, such potentiation was observed only in homozygotes, suggesting their GABAergic deficit. In support of this, the levels of GABA-related proteins and the number of dentate GABAergic interneurons were decreased in 4-month-old homozygotes. Since APP has been shown to play a role in dentate GABAergic synapse formation, the observed GABAergic depletion is likely associated with an impairment of the APP function presumably caused by the Osaka mutation. Oral administration of diazepam to homozygotes from 6 months improved memory at 8 months, and furthermore, prevented Aβ oligomer accumulation, indicating that GABAergic deficiency is a cause of memory impairment and also a driving force of Aβ accumulation. Our findings suggest that the Osaka mutation causes loss of APP function, leading to GABAergic depletion and memory disorder when wild-type APP is absent, providing a mechanism of the recessive heredity.

APP Osaka Mutation in Familial Alzheimer’s Disease—Its Discovery, Phenotypes, and Mechanism of Recessive Inheritance

“…To test this hypothesis, we counted the number of parvalbumin-positive GABA neurons in the dentate gyrus of OSK-KI mice. As we expected, compared with non-KI mice, only homo-KI mice showed a decrease of GABA neurons at 4 months [64] (Figure 5A). It has been shown that Aβ production depends on neuronal activity [66,67] and that the secreted Aβ, in turn, regulates synaptic activity [68].…”

“…Thus, to activate GABAergic neurotransmission, we orally administered diazepam, a positive allosteric modulator of GABA A receptor, to 6-month-old homo-KI mice for 2 months and examined their memory and Aβ oligomers at 8 months. While the number of GABA neurons in the dentate gyrus remained lower, cognitive function was recovered and Aβ oligomers were not detected in the treated mice [64]. Taken together, our findings indicate that the Osaka mutation primarily causes a loss of APP function that is essential for GABA neurons ( Figure 5B).…”

“…We compared the memory and Aβ oligomer pathology between homo-, hetero-, and non-KI mice at 4, 6, and 8 months by the Morris water maze and immunohistochemical and biochemical analyses. Only homo-KI mice displayed memory impairment and intraneuronal accumulation of Aβ oligomers followed by synapse loss, tau hyperphosphorylation, glial activation, and neuron loss [64]. This result represents the recessive inheritance of the Osaka mutation and furthermore demonstrates that this mutation promotes Aβ oligomerization even in mouse Aβ.…”

“…HFS was delivered to the molecular layer of the dentate gyrus, and fEPSP was recorded in the same region. In the presence of a GABA A receptor antagonist, picrotoxin, HFS induced LTP in all groups (homo-, hetero-, and non-KI mice) at 4 months [64]. However, at 8 months, LTP was elicited only in the hetero-and non-KI mice.…”

“…Using the homologous recombination technique, the codon CAA was deleted in endogenous mouse APP exon 17 to knock-in the Osaka mutation. Different from APP OSK mice, this knock-in mouse model, named OSK-KI mice, produces mouse Aβ under the original mouse APP promotor [64]. In 2004, homozygous KI mice were established.…”