Effects Of ADAM10Deletion On Aβ Peptide Production And Cognitive Function In Adult Mouse Brain

The amyloid precursor protein (APP) was cleaved by the two proteases β-andγ-secretase to release the amyloid β peptides (Aβ). Aβ peptides are the maincomponents of senile plaques. The overproduction or aggregation of Aβ peptides isconsidered to be the main cause of Alzheimer’s disease. However, APP has anotherprocessing pathway, in which it was cleaved by α-secretase and γ-secretase. Becauseα-secretase cleaves APP within the Aβ domain, so it can preclude Aβ generation. Now,ADAM10has been confirmed to be a genuine, constitutive a-secretase of APP inneurons. However, it is unclear how ADAM10deletion influences the function ofbrain neurons, so we use the adult neuron-specific conditional ADAM10-knockoutmice (cKO) to study the effects of ADAM10deletion on APP metabolism andcognitive function of mice.ADAM10cKO mice were genotyped by PCR method. The identified ADAM10cKO homozygous mice were used in the following experiments. According to theconventional histological protocol, the brain tissues were taken out of3,6,9and12months of ADAM10cKO and wild-type C57BL/6J mice(WT), fixed, dehydrated,embedded in paraffin and. stained by HE staining.The morphology of cortex andhippocampus and the changes of neuron numbers in the cortical layer fifth andhippocampus were observed. We performed immunohistochemical staining by using7,9,11,13,15months of ADAM10cKO and ADAM10FL/FLmice as controls. Thedistribution of Aβ peptides in the cerebral cortex and hippocampus was observed, andthe relationship between Aβ deposit and age was then analyed. The cortical andhippocampal proteins were extracted from20months of ADAM10cKO andADAM10FL/FLcontrol mice. Total APP proteins and Aβ pepetides were quantitatively analyzed by Western blotting to elucidate the impact of ADAM10deletion on APPmetabolism. ADAM10cKO (n=9) and WT mice (n=9) at10months of age wereevaluated on the Morris water maze tasks. The learning and memory were assessed bydetecting the escape latency, the percentage of the target quadrant time to total timeand the times of passing through the platform during1min.HE staining showed that the morphological structures of cortex and hippocampusof3,6,9, and12months of ADAM10cKO mice were visually indistinguishable fromthose of WT controls. However, compared with the wild-type mice, the number ofneurons in the ADAM10cKO mouse cortical layer fifth reduced with age, especially,the hippocampal neurons decreased significantly. Immunohistochemical analysesindicated that there were no amyloid plaques in the brain of both ADAM10cKO andADAM10FL/FLcontrol mice, but intraneuronal soluble Aβ peptides were increased inage-dependent manner, and there was a difference in the amount of intraneuronalsoluble Aβ peptides between ADAM10cKO and control mice. Western-blot analysesshowed that compared with ADAM10FL/FLcontrol mice, the immature and mature ofthe full-length APP proteins in the hippocampus decreased by73%and62.3%,respectively（P﹤0.01）, but the full-length APP proteins in the cortex was not reducedbetween two genotypes. Moreover, the amount of total Aβpeptides in the cortex andhippocampus of ADAM10cKO mice increased by20.45%and27.21%, respectively(P﹤0.05and P﹤0.01). The amount of Aβ1-38, Aβ1-40and Aβ1-42peptides in theADAM10cKO cortex increased by32.85%,25.37%and80.95%, respectively（P﹤0.05, P﹤0.05and P<0.01）, whereas the amount of Aβ1-38, Aβ1-40and Aβ1-42peptides in the ADAM10cKO hippocampus increased by41.67%,38.36%and22.22%, respectively（P﹤0.05）, indicating a significant difference. Morris watermaze tasks showed that during the training time, the average latency of ADAM10cKO mice was longer than that of wild-type controls, and there were significantdifferences in the second and third day (P﹤0.05and P﹤0.01). During the test time,the average number of passing the platform and the percentage of the target quadranttime to total time decreased significantly in the ADAM10cKO group compared with the wild-type control group（P﹤0.05）.To sum up, ADAM10deletion leads to neuron loss in the cerebral cortex andhippocampus of adult mice. APP proteins decreased significantly in the hippocampus,but not in the cortex, which was due to hippocampal neuron loss. The intraneuronalsoluble Aβ in the cerebral cortex layer fifth and hippocampus increased in anage-dependent manner, resulting in the neurodegeneration and cognitive dysfunction.