Background Members of the low-density lipoprotein (LDL) receptor family are involved in endocytosis and in transducing signals, but also in amyloid precursor protein (APP) processing and β-amyloid secretion. ApoER2/LRP8 is a member of this family with key roles in synaptic plasticity in the adult brain. ApoER2 is cleaved after the binding of its ligand, the reelin protein, generating an intracellular domain (ApoER2-ICD) that modulates reelin gene transcription itself. We have analyzed whether ApoER2-ICD is able to regulate the expression of other LDL receptors, and we focused on LRP3, the most unknown member of this family. We analyzed LRP3 expression in middle-aged individuals (MA) and in cases with Alzheimer’s disease (AD)-related pathology, and the relation of LRP3 with APP. Methods The effects of full-length ApoER2 and ApoER2-ICD overexpression on protein levels, in the presence of recombinant reelin or Aβ42 peptide, were evaluated by microarray, qRT-PCRs, and western blots in SH-SY5Y cells. LRP3 expression was analyzed in human frontal cortex extracts from MA subjects (mean age 51.8±4.8 years) and AD-related pathology subjects [Braak neurofibrillary tangle stages I–II, 68.4±8.8 years; III–IV, 80.4 ± 8.8 years; V–VI, 76.5±9.7 years] by qRT-PCRs and western blot; LRP3 interaction with other proteins was assessed by immunoprecipitation. In CHO cells overexpressing LRP3, protein levels of full-length APP and fragments were evaluated by western blots. Chloroquine was employed to block the lysosomal/autophagy function. Results We have identified that ApoER2 overexpression increases LRP3 expression, also after reelin stimulation of ApoER2 signaling. The same occurred following ApoER2-ICD overexpression. In extracts from subjects with AD-related pathology, the levels of LRP3 mRNA and protein were lower than those in MA subjects. Interestingly, LRP3 transfection in CHO-PS70 cells induced a decrease of full-length APP levels and APP-CTF, particularly in the membrane fraction. In cell supernatants, levels of APP fragments from the amyloidogenic (sAPPα) or non-amyloidogenic (sAPPβ) pathways, as well as Aβ peptides, were drastically reduced with respect to mock-transfected cells. The inhibitor of lysosomal/autophagy function, chloroquine, significantly increased full-length APP, APP-CTF, and sAPPα levels. Conclusions ApoER2/reelin signaling regulates LRP3 expression, whose levels are affected in AD; LRP3 is involved in the regulation of APP levels.
Background. Members of the low-density lipoprotein (LDL) receptor family are involved in endocytosis and in transducing signals, but also in APP (amyloid precursor protein) processing and β-amyloid secretion. ApoER2/LRP8 is a member of this family with key roles in synaptic plasticity in the adult brain. ApoER2 is cleaved after the binding of its ligand, the reelin protein, generating an intracellular domain (ApoER2-ICD) that modulates reelin gene transcription itself. In this work, we have analysed whether ApoER2-ICD is able to regulate the expression of other members of the LDL receptor family. We focused on LRP3, the most unknown member of the LDL receptor family, whose precise physiological role and potential participation in pathological processes such as Alzheimer’s disease (AD) are still unknown.Methods. The effects of full-length ApoER2 and ApoER2-ICD overexpression on protein levels, in presence of recombinant reelin or Ab42 peptide, were evaluated by a microarray, qRT-PCRs and western blots. The expression of LRP3 was analysed in human frontal cortex extracts from AD and non-demented subjects by qRT-PCRs and western blot; and LRP3 interaction with other proteins was assessed by immunoprecipitation. In CHO cells overexpressing LRP3, protein levels of full-length APP and fragments were evaluated by western blots. Results. We have identified that ApoER2 overexpression increases LRP3 expression. Stimulation of ApoER2 signaling by reelin increased LRP3 levels, and the same occurred following ApoER2-ICD overexpression. In human frontal cortex extracts we demonstrate that LRP3 interacts with apolipoprotein E and APP. In extracts from AD subjects, the levels of LRP3 mRNA and protein were lower than those in control subjects. Interestingly, LRP3 transfection in CHO-PS70 cells induced a decrease of full-length APP levels and APP-CTF, and in the supernatant, levels of soluble APP fragments from the amyloidogenic (sAPPa) or non-amyloidogenic (sAPPβ) pathway, as well as Aβ peptides, were drastically reduced respect to mock-transfected cells.Limitations. There is a scarce knowledge of LRP3 physiological function as a neuronal receptor.Conclusion. We describe that LRP3 expression is regulated via ApoER2/reelin signaling, and its levels are affected in AD; similarly to other LDL receptors, LRP3 is involved in APP expression.
N‐methyl‐D‐aspartate (NMDA) receptor (NMDAR) dysregulation is thought to contribute to impaired cognition and neurodegeneration in a variety of brain disorders. In a recent article, Zhong et al. proposed that deficiency of the NMDAR subunit GluN3A may be a primary pathogenic factor in sporadic Alzheimer´s disease (AD) based on evidence for degenerative excitotoxicity and cognitive impairment in aging mice lacking GluN3A. Because the result appeared to be at odds with earlier work where genetic GluN3A deletion enhanced learning in younger mice, we have now compared wild‐type and GluN3A knockout mice at later life stages using a congenic mouse strain. Rather than age‐dependent cognitive decline or neurodegeneration, we find that the enhanced performance of young adult GluN3A knockouts in memory tasks persists during aging. In sum, our analysis does not support the hypothesis that GluN3A loss underlies cognitive impairment in AD..
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