Neprilysin (NEP) is the principal amyloid β (Aβ) degrading peptidase; this activity may protect against Alzheimer’s disease (AD), the most important age-related neurodegenerative process. The aim of this work was to analyze NEP mRNA expression in the frontal cortex of dogs with and without canine cognitive dysfunction syndrome (CDS), which is considered a natural model for AD. Expression of canine cerebral NEP mRNA was assessed by RT-PCR followed by qPCR in young, aged-cognitively unimpaired (CU), and aged-cognitively impaired (CI) dogs. On average, aged-CI dogs showed 80% ( ) lower expression levels of NEP mRNA than their aged-CU counterparts. Furthermore, the standard deviation of the qPCR measurements was more than 6 times higher in the cognitively healthy animals (young and aged-CU) than in the aged-CI group. Another interesting find is the determination of a positive correlation between NEP expression and the number of cholinergic neurons in basal telencephalon, indicating a probable connection between both events in these types of neurodegeneration processes. These results suggest that high expression levels of NEP might be a protective factor for canine CDS and, most likely, for other Aβ-associated neurodegenerative diseases, such as AD. 1. Introduction Amyloid β (Aβ) peptides are considered to be the main agents implicated in Alzheimer’s disease (AD), the most common dementia [1]. Currently, researchers believe that an imbalance between production and clearance of Aβ peptides (especially and ) leads to the accumulation of Aβ peptides in the brain [2, 3]. The two most important mechanisms implicated in the elimination of brain Aβ are active transport through the blood brain barrier and the enzymatic degradation of Aβ peptides by peptidases, such as neprilysin (NEP), insulin-degrading enzyme (IDE), endothelin-converting enzymes (1 and 2), or plasmin. Of these, NEP is the most relevant protease in Aβ degradation regarding either its monomeric or oligomeric Aβ conformations [4, 5]. The gene that encodes NEP (also named MME) is expressed at the highest level in the kidney and at lower levels in other tissues including the lung, adrenal glands, intestine, and central nervous system [6]. In the brain, areas with higher Aβ accumulation (hippocampus, temporal, and frontal cortex) express lower levels of NEP than areas with lower Aβ deposition (cerebellum or caudate nucleus) [7, 8]. On the other hand, some studies have found a significant age-related downregulation of NEP protein in the mouse hippocampus [9, 10] and in human frontal and temporal cortex
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