Based on the generally poor solubility of N
Based on the generally poor solubility of N-terminally truncated species at position 4, it is conceivable that these derivatives might contribute to the exacerbation of amyloidogenic conformations by acting as seeding elements as has been proposed for the poorly soluble N-terminally modified pE species . Along this line, it was shown that intracerebral infusion of Aβ-rich brain extracts from AD patients stimulates the premature formation of parenchymal and vascular deposits in APP Tg models, a mechanism resembling the infectivity of prion diseases and currently considered as a significant contributor to AD pathogenesis , . Whether Aβ4-x species play a role in the propagation and/or formation of pathological amyloid GSK1838705A remains to be determined although the in vitro biophysical studies presented herein, are certainly suggestive of such a likely contribution. Further studies in Tg mice of different ages, currently ongoing, will certainly indicate whether the presence of Aβ4-x precedes or is concomitant with the deposition of the full-length Aβ1–40/42 peptides. Additional experimentation in in vitro cellular culture models is also needed to further demonstrate the increased toxicity of the highly aggregation-prone Aβ4-x species. Nevertheless, it should be noted that validating the amyloidogenic potential of these truncated species, it was recently reported that Aβ4–42 was highly neurotoxic in cell culture models and its toxicity correlated with the age-dependent hippocampal neuronal loss and concomitant memory deficits displayed by Tg4–42 mouse model . Supporting the mechanistic relevance of N-terminal truncated fragments, passive immunotherapy with a monoclonal antibody recognizing both Aβ4-x and pyroglutamate-modified Aβ3-x species rescued memory deficits in the Tg4–42 model and decreased thioflavin positive plaque load in 5XFAD mice . In this sense, our novel anti-Aβ4-x and anti-Aβx-34 monoclonal and polyclonal antibodies, highly specific for the respective truncations and blind for the more abundant intact Aβ1–40 and Aβ1–42 species will provide much needed flexibility for the development of quantitative assays and attractive therapeutic approaches to further dwell into translational aspects of the disease pathogenesis. The intrinsic biophysical properties and easy extractability from the tissue deposits displayed by the C-terminal truncated peptide Aβ1–34 together with the primarily diffuse immunostaining pattern exhibited by the fragment in both AD and APP Tg models as well as the consistent presence of multiple C-terminally cleaved species in the CSF Aβ peptidome in normal and AD individuals , ,  are all suggestive of the relationship of these truncated species to brain clearance mechanisms. The findings may also indicate that Aβx-34, and perhaps additional C-terminally truncated fragments with similar solubility, may contribute to the diffuse deposits seen in AD with conventional Aβ antibodies, providing a note of caution regarding the interpretation of these lesions for the disease pathogenesis. While diffuse deposits have been consistently regarded as structures preceding the formation of amyloid plaques, based on the data presented herein, it is also plausible that they may partially reflect an opposing view, revealing the physiologic degradation mechanisms exacerbated in a futile effort to prevent excessive Aβ accumulation and preclude the formation of amyloid deposits. Further studies along these lines of investigation are certainly warranted. Increasing evidence generated over the last decade indicates that one of the main mechanisms leading to brain Aβ accumulation and likely contributing to AD is a defective clearance of the protein from the brain, which affects the delicate balance between degree of Aβ production, dynamics of aggregation and rate of brain efflux. Indeed, our recent report clearly demonstrated the detrimental role exerted by peptide oligomerization/aggregation for the brain clearance mechanisms suggestive of the potential for Aβ4-x species to be preferentially retained within the brain . Using newly developed techniques for quantifying the rates of Aβ production and clearance within the CSF in humans , it was clearly shown that sporadic AD patients do indeed exhibit significant defects in the clearance of Aβ  although the precise pathways impaired in these patients remain largely undefined. Glial phagocytosis, perivascular drainage, transport to the CSF, clearance across the BBB, and local enzymatic degradation are among the mechanisms under current investigation , , . Widespread interest in the role of Aβ enzymatic degradation and its consequences in Aβ homeostasis has increased since the early identification of neprilysin as one of the main Aβ-degrading enzymes with pathophysiological significance in Tg animal models, studies that boosted interest in a previously underappreciated aspect of Aβ catabolism. The list of enzymes capable of degrading Aβ and contribute to the overall Aβ brain homeostasis has continued to grow and some of them are being currently targeted in novel therapeutic avenues involving genetic or pharmacological approaches aimed to increase the catabolic processing of the peptide . Overall, our data highlighting the opposing properties of enzymatically generated Aβ fragments depending on the targeted peptide bond provide a note of caution to strategies aimed at indiscriminately increasing Aβ catabolism. While C-terminal truncation may lead to generation of smaller, more soluble fragments and favor Aβ brain elimination, N-terminal truncation at position 4 is likely to generate highly insoluble and aggregation prone peptides creating a potentially undesirable self-perpetuating amyloidogenic loop. Only the detailed assessment of the molecular diversity of all species composing the deposits at different stages of the disease and their relative ratios, as well as the evaluation of changes in the truncation profile with different treatment strategies will provide a comprehensive understanding of the still undefined contribution of N- and C-terminal truncations to the pathogenesis of the disease.