Transcripts and/or proteins of P2X receptor (P2XR) subunits have been found in virtually all mammalian tissues. Generally more than one of the seven known P2X subunits have been identified in a given cell type. Six of the seven cloned P2X subunits can efficiently form functional homotrimeric ion channels in recombinant expression systems. This is in contrast to other ligand-gated ion channel families, such as the Cys-loop or glutamate receptors, where homomeric assemblies seem to represent the exception rather than the rule. P2XR mediated responses recorded from native tissues rarely match exactly the biophysical and pharmacological properties of heterologously expressed homomeric P2XRs. Heterotrimerization of P2X subunits is likely to account for this observed diversity. While the existence of heterotrimeric P2X2/3Rs and their role in physiological processes is well established, the composition of most other P2XR heteromers and/or the interplay between distinct trimeric receptor complexes in native tissues is not clear. After a description of P2XR assembly and the structure of the intersubunit ATP-binding site, this review summarizes the distribution of P2XR subunits in selected mammalian cell types and the biochemically and/or functionally characterized heteromeric P2XRs that have been observed upon heterologous co-expression of P2XR subunits. We further provide examples where the postulated heteromeric P2XRs have been suggested to occur in native tissues and an overview of the currently available pharmacological tools that have been used to discriminate between homo- and heteromeric P2XRs.

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Overall, this extended model promises further insights into the complexity of AD and contributes to novel treatment strategies also targeting the cholinergic system.

The epsilon 4 allele of the apolipoprotein E (ApoE4) gene is the most important risk factor implicated in Alzheimer's disease (AD) etiology. ApoE4 is more susceptible to proteolysis, and ApoE fragments have been shown to promote tau hyperphosphorylation and exert neurotoxic properties. While a plethora of studies deals with the effect of ApoE and its fragments on amyloid-β peptide (Aβ) deposition and clearance, it is largely unknown whether Aβ in turn influences human or murine ApoE expression and its proteolysis. The present study is the first to show that endogenous murine ApoE becomes proteolytically processed in a way reminiscent of human ApoE fragmentation in different AD mouse models, including APP/PS1KI or 5XFAD. Murine ApoE fragments were demonstrated to accumulate mainly in synaptic fractions in AD mouse models. In vitro experiments, as well as analysis of mouse models at different time points, suggest that the amount of total ApoE is associated with extracellular Aβ while the amount of its fragments is linked to intracellular Aβ levels. Murine ApoE fragmentation is a common feature in different AD transgenic mouse models and could be directly associated with intraneuronal Aβ accumulation. Extracellular amyloid induces an elevation in full-length ApoE expression, which might present a protective mechanism toward Aβ clearance. The demonstrated fragments of murine ApoE in vitro and in vivo might therefore play a crucial role in the progression of AD pathology in murine AD models.

Familial British (FBD) and familial Danish dementia (FDD) are progressive neurodegenerative disorders characterized by cerebral deposition of the amyloidogenic peptides ABri and ADan. These amyloid peptides start with an N-terminal glutamate residue, which can be posttranslationally converted into a pyroglutamate (pGlu-) modified form, a mechanism which has been extensively described to be relevant for Aβ peptides in Alzheimer’s disease (AD). Like pGlu-Aβ peptides, pGlu-ABri peptides have an increased aggregation propensity and show higher toxicity on human neuroblastoma cells as their non-modified counterparts. We have generated novel N-terminal specific antibodies detecting the pGlu-modified forms of ABri and ADan peptides. With these antibodies we were able to identify abundant extracellular amyloid plaques, vascular and parenchymal deposits in human FBD and FDD brain tissue, as well as in a mouse model for FDD. Double-stainings using C-terminal specific antibodies in human samples revealed that highly aggregated pGlu-ABri and pGlu-ADan peptides are mainly present in plaque cores and central vascular deposits, leading to the assumption that these peptides have seeding properties. Furthermore, in an FDD-mouse model ADan peptides were detected in pre-synaptic terminals of the hippocampus where they might contribute to impaired synaptic transmission. These similarities of ABri and ADan to Aβ in AD suggest that the posttranslational pGlu-modification of amyloid peptides might represent a general pathological mechanism leading to increased aggregation and toxicity in these forms of degenerative dementias.