Ulrike C. Muller

While the accumulation of Aβ peptides, derived by proteolytic cleavage from the amyloid precursor protein APP, constitutes a hallmark of Alzheimer’s disease, the question of whether endogenous, physiological APP processing serves an essential role for normal brain function(s) has remained elusive. Here, we modified the mouse APP locus to delete the endogenous α- and β-secretase sites (APP residues S592-G621) while preserving the transmembrane topology and the APP C-terminus. These novel knockin mice (termed APPΔS622) lack APPsα, APPβ, and Aβ production and show only minimal soluble APPs shedding. These APPΔS622 mice provide a unique model to distinguish between functions mediated by unprocessed cell surface APP involved in synaptic adhesion or by proteolytic fragments. Strikingly, APPΔS622 mice exhibit pronounced impairments in both long-term potentiation (LTP) and homeostatic forms of synaptic plasticity. Extracellular field recordings revealed significant deficits in hippocampal LTP, associated with a reduced density of mature spines and reduced spine head size. LTP deficits could be restored by APPsα application, suggesting a requirement for non-amyloidogenic APP processing for LTP. In addition, APPΔS622 mice lacking endogenous Aβ displayed a lack of homeostatic synaptic scaling in response to network impairment that could be specifically rescued by exogenous Aβ. Importantly, these deficits in different forms of synaptic plasticity also led to impairments in cognitive behavior. In summary, our findings reveal an essential role of physiological APP processing products to differentially regulate complementary forms of synaptic plasticity that are critical for network stability, normal brain function and cognition.

The amyloid precursor protein (APP), a key factor in Alzheimer's disease (AD) pathology, and its two mammalian homologs, amyloid precursor-like proteins 1 and 2 (APLP1 and APLP2), are considered as members of the synaptic adhesion molecule (SAM) family. They are localized to the pre- and postsynapse, form trans-cellular dimers, and have been shown to induce presynaptic differentiation in a heterologous synapse formation assay. We demonstrate that expression of all APP family members in non-neuronal cells also promotes dendritic excitatory postsynaptic differentiation in primary mouse neurons of either sex, similar to Neurexin1β and other SAMs. Synaptogenic activity was decreased by deletion of the E1 domain and increased upon inhibition of soluble APP (sAPP) generation, reinforcing that trans-cellular interaction of APP/APLPs can induce synaptogenesis. Consistent with this, the capacity of heterologously expressed APP to induce postsynaptic specializations in contacting dendrites was reduced by the absence of APP family members at the postsynaptic site and was lost in conditional triple knock-out (cTKO) neurons. Pharmacological analyses revealed that heterologous formation of pre- and postsynapses relies on proper microtubule and actin cytoskeleton dynamics, as well as the MAP kinase pathway, similar to what has been shown for Neurexin1β and Neuroligin1. However, inhibition of the PI3K/Akt pathway selectively impaired APP-induced postsynaptic differentiation, suggesting that distinct APP signaling pathways are required for pre- and postsynaptic differentiation. Collectively, our data highlight the role of all APP family members as SAMs in trans-synaptic signaling, providing key insights into their physiological function and advancing our understanding of AD-related synaptopathies.

Amyloid precursor protein (APP) gives rise to amyloid-β, a pathological factor in Alzheimer’s disease. However, the physiological role of APP and its homolog amyloid precursor–like protein 2 (APLP2), which are also widely expressed outside the nervous system, is largely unknown. Here, we show that endothelial APP and APLP2 are required for postischemia angiogenesis after myocardial infarction (MI). We found that hypoxia induced the endothelial expression of α-secretases, resulting in nonamyloidogenic processing of APP and APLP2 into the soluble forms APPsα and APLP2sα. Loss of endothelial APP and APLP2 led to decreased neovascularization as well as increased heart failure and mortality after MI, a phenotype that could be rescued by endothelial expression of APPsα. APPsα and APLP2sα exerted their proangiogenic effect by positive allosteric modulation of the endothelial receptor tyrosine kinase KIT, which promotes postischemia neovascularization. Our data identify a function of APP and APLP2 in endothelial cells, which is required for postischemia tissue repair, and suggest approaches to improve regeneration after MI and other ischemic diseases.

Pathologic α-synuclein (α-syn) spreads from cell-to-cell, in part, through binding to the lymphocyte-activation gene 3 (Lag3). Here we report that amyloid β precursor-like protein 1 (Aplp1) interacts with Lag3 that facilitates the binding, internalization, transmission, and toxicity of pathologic α-syn. Deletion of both Aplp1 and Lag3 eliminates the loss of dopaminergic neurons and the accompanying behavioral deficits induced by α-syn preformed fibrils (PFF). Anti-Lag3 prevents the internalization of α-syn PFF by disrupting the interaction of Aplp1 and Lag3, and blocks the neurodegeneration induced by α-syn PFF in vivo. The identification of Aplp1 and the interplay with Lag3 for α-syn PFF induced pathology deepens our insight about molecular mechanisms of cell-to-cell transmission of pathologic α-syn and provides additional targets for therapeutic strategies aimed at preventing neurodegeneration in Parkinson’s disease and related α-synucleinopathies.

The neuronal cell adhesion molecule contactin-4 ( CNTN4 ) is genetically associated with autism spectrum disorder (ASD) and other psychiatric disorders. Cntn4 -deficient mouse models have previously shown that CNTN4 plays important roles in axon guidance and synaptic plasticity in the hippocampus. However, the pathogenesis and functional role of CNTN4 in the cortex has not yet been investigated. Our study found a reduction in cortical thickness in the motor cortex of Cntn4 ^−/− mice, but cortical cell migration and differentiation were unaffected. Significant morphological changes were observed in neurons in the M1 region of the motor cortex, indicating that CNTN4 is also involved in the morphology and spine density of neurons in the motor cortex. Furthermore, mass spectrometry analysis identified an interaction partner for CNTN4, confirming an interaction between CNTN4 and amyloid-precursor protein (APP). Knockout human cells for CNTN4 and/or APP revealed a relationship between CNTN4 and APP. This study demonstrates that CNTN4 contributes to cortical development and that binding and interplay with APP controls neural elongation. This is an important finding for understanding the physiological function of APP, a key protein for Alzheimer’s disease. The binding between CNTN4 and APP, which is involved in neurodevelopment, is essential for healthy nerve outgrowth.

Background: Despite the importance of the amyloid‐precursor protein (APP) to Alzheimer’s Disease pathophysiology and therapy, and notable pan‐cellular and synaptic abundance, the physiological function of APP and its homologues remains to be fully elucidated in‐vivo. Here, we interrogated how the APP family regulates neuronal cell function and long‐range neural circuit dynamics in the intact brain.Method: APP^flox/flox/APLP2^{flox /flox}/APLP1^−/− mice were crossed with NexCre mice expressing Cre prenatally from ∼E12.5 in postmitotic neuronal precursor cells of the cortex and hippocampus, producing a conditional triple knockout model (cTKO) of the APP family in excitatory forebrain neurons that is associated with pronounced behavioral deficits (Steubler et al., 2021, EMBO Journal). Neuronal function at the single cell/unit and circuit‐level was characterised using in‐vivo two‐photon and mesoscopic calcium imaging, and bilateral Neuropixels recordings in awake and anesthetised mice.Result: cTKO mice exhibited pronounced suppression in spike timing variability and dynamic excitability in visual cortex and hippocampus CA1 relative to controls, with a marked inter‐hemispheric decoherence of oscillatory activity (including low‐frequency and sleep‐associated slow waves) and dysfunctional intrahemispheric propagation of neuronal activity. Further, cTKO mice also displayed behavioural abnormalities in open field tests relative to controls which was linked to functional deficits. High‐resolution synaptic imaging revealed a significant reduction in the number of synapses containing the NMDA receptor subunit GluN1 in cTKO mice versus controls, with partial NMDA receptor agonism (d‐cycloserine, 30mg/kg, i.p.) ameliorating cellular and circuit‐level impairments as well as behavioral impairments.Conclusion: These findings provide novel in‐vivo evidence for an essential role of APP in regulating synaptic function via an effect on NMDA receptors, that elucidate the physiological role of this preeminent protein family and have important implications for disease and therapy.

The Tau protein can be phosphorylated by numerous kinases. In Alzheimer’s disease (AD) hyperphosphorylated Tau species accumulate as neurofibrillary tangles that constitute a major hallmark of AD. AD is further characterized by extracellular Aβ plaques, derived from the β-amyloid precursor protein APP. Whereas Aβ is produced by amyloidogenic APP processing, APP processing along the competing non-amyloidogenic pathway results in the secretion of neurotrophic and synaptotrophic APPsα. Recently, we demonstrated that APPsα has therapeutic effects in transgenic AD model mice and rescues Aβ-dependent impairments. Here, we examined the potential of APPsα to regulate two major Tau kinases, GSK3β and CDK5 in THY-Tau22 mice, a widely used mouse model of tauopathy. Immunohistochemistry revealed a dramatic increase in pathologically phosphorylated (AT8 and AT180) or misfolded Tau species (MC1) in the hippocampus of THY-Tau22 mice between 3 and 12 months of age. Using a highly sensitive radioactive kinase assay with recombinant human Tau as a substrate and immunoblotting, we demonstrate an increase in GSK3β and CDK5 activity in the hippocampus of THY-Tau22 mice. Interestingly, AAV-mediated intracranial expression of APPsα in THY-Tau22 mice efficiently restored normal GSK3β and CDK5 activity. Western blot analysis revealed upregulation of the CDK5 regulatory proteins p35 and p25, indicating CDK5 hyperactivation in THY-Tau22 mice. Strikingly, AAV-APPsα rescued p25 upregulation to wild-type levels even at stages of advanced Tau pathology. Sarkosyl fractionation used to study the abundance of soluble and insoluble phospho-Tau species revealed increased soluble AT8-Tau and decreased insoluble AT100-Tau species upon AAV-APPsα injection. Moreover, AAV-APPsα reduced misfolded (MC1) Tau species, particularly in somatodendritic compartments of CA1 pyramidal neurons. Finally, we show that AAV-APPsα upregulated PSD95 expression and rescued deficits in spine density of THY-Tau22 mice. Together our findings suggest that APPsα holds therapeutic potential to mitigate Tau-induced pathology.

The amyloid precursor protein APP plays a crucial role in Alzheimer pathogenesis. Its physiological functions, however, are only beginning to be unraveled. APP belongs to a small gene family, including besides APP the closely related amyloid precursor-like proteins APLP1 and APLP2, that all constitute synaptic adhesion proteins. While APP and APLP2 are ubiquitously expressed, APLP1 is specific for the nervous system. Previous genetic studies, including combined knockouts of several family members, pointed towards a unique role for APLP1, as only APP/APLP1 double knockouts were viable. We now examined brain and neuronal morphology in APLP1 single knockout (KO) animals, that have to date not been studied in detail. Here, we report that APLP1-KO mice show normal spine density in hippocampal CA1 pyramidal cells and subtle alterations in dendritic complexity. Extracellular field recordings revealed normal basal synaptic transmission and no alterations in synaptic plasticity (LTP). Further, behavioral studies revealed in APLP1-KO mice a small deficit in motor function and reduced diurnal locomotor activity, while learning and memory were not affected by the loss of APLP1. In summary, our study indicates that APP family members serve both distinct and overlapping functions that need to be considered for therapeutic treatments of Alzheimer’s disease.