β-Site amyloid precursor protein (APP) cleaving enzyme-1 (BACE1) is the β-secretase that initiates Aβ production in Alzheimer’s disease (AD). BACE1 levels are increased in AD, which could contribute to pathogenesis, yet the mechanism of BACE1 elevation is unclear. Furthermore, the normal function of BACE1 is poorly understood. We localized BACE1 in the brain at both the light and electron microscopic levels to gain insight into normal and pathophysiologic roles of BACE1 in health and AD, respectively. Our findings provide the first ultrastructural evidence that BACE1 localizes to vesicles (likely endosomes) in normal hippocampal mossy fiber terminals of both non-transgenic and APP transgenic (5XFAD) mouse brains. In some instances, BACE1-positive vesicles were located near active zones, implying a function for BACE1 at the synapse. In addition, BACE1 accumulated in swollen dystrophic autophagosome-poor presynaptic terminals surrounding amyloid plaques in 5XFAD cortex and hippocampus. Importantly, accumulations of BACE1 and APP co-localized in presynaptic dystrophies, implying increased BACE1 processing of APP in peri-plaque regions. In primary cortical neuron cultures, treatment with the lysosomal protease inhibitor leupeptin caused BACE1 levels to increase; however, exposure of neurons to the autophagy inducer trehalose did not reduce BACE1 levels. This suggests that BACE1 is degraded by lysosomes but not by autophagy. Our results imply that BACE1 elevation in AD could be linked to decreased lysosomal degradation of BACE1 within dystrophic presynaptic terminals. Elevated BACE1 and APP levels in plaque-associated presynaptic dystrophies could increase local peri-plaque Aβ generation and accelerate amyloid plaque growth in AD.Electronic supplementary materialThe online version of this article (doi:10.1007/s00401-013-1152-3) contains supplementary material, which is available to authorized users.

Alzheimer’s disease (AD) is characterized by amyloid plaques composed of the β-amyloid (Aβ) peptide surrounded by swollen presynaptic dystrophic neurites consisting of dysfunctional axons and terminals that accumulate the β-site amyloid precursor protein (APP) cleaving enzyme (BACE1) required for Aβ generation. The cellular and molecular mechanisms that govern presynaptic dystrophic neurite formation are unclear, and elucidating these processes may lead to novel AD therapeutic strategies. Previous studies suggest Aβ may disrupt microtubules, which we hypothesize have a critical role in the development of presynaptic dystrophies. To investigate this further, here we have assessed the effects of Aβ, particularly neurotoxic Aβ42, on microtubules during the formation of presynaptic dystrophic neurites in vitro and in vivo. Live-cell imaging of primary neurons revealed that exposure to Aβ42 oligomers caused varicose and beaded neurites with extensive microtubule disruption, and inhibited anterograde and retrograde trafficking. In brain sections from AD patients and the 5XFAD transgenic mouse model of amyloid pathology, dystrophic neurite halos with BACE1 elevation around amyloid plaques exhibited aberrant tubulin accumulations or voids. At the ultrastructural level, peri-plaque dystrophies were strikingly devoid of microtubules and replete with multi-lamellar vesicles resembling autophagic intermediates. Proteins of the microtubule motors, kinesin and dynein, and other neuronal proteins were aberrantly localized in peri-plaque dystrophies. Inactive pro-cathepsin D also accumulated in peri-plaque dystrophies, indicating reduced lysosomal function. Most importantly, BACE1 accumulation in peri-plaque dystrophies caused increased BACE1 cleavage of APP and Aβ generation. Our study supports the hypothesis that Aβ induces microtubule disruption in presynaptic dystrophic neurites that surround plaques, thus impairing axonal transport and leading to accumulation of BACE1 and exacerbation of amyloid pathology in AD.Electronic supplementary materialThe online version of this article (doi:10.1007/s00401-016-1558-9) contains supplementary material, which is available to authorized users.

Amyloid plaques are defining histopathologic lesions in the brains of Alzheimer's disease (AD) patients and are composed of the amyloid-beta peptide, which is widely considered to play a critical role in the pathogenesis of AD. The β-secretase, or β-site amyloid precursor protein cleaving enzyme 1 (BACE1; also called Asp2, memapsin 2), is the enzyme that initiates the generation of amyloid beta. Consequently, BACE1 is an attractive drug target for lowering cerebral levels of amyloid beta for the treatment or prevention of AD. Much has been learned about BACE1 since its discovery over 10 years ago. In the present article, we review BACE1 properties and characteristics, cell biology, in vivo validation, substrates, therapeutic potential, and inhibitor drug development. Studies relating to the physiological functions of BACE1 and the promise of BACE1 inhibition for AD will also be discussed. We conclude that therapeutic inhibition of BACE1 should be efficacious for AD, although careful titration of the drug dose may be necessary to limit mechanism-based side effects.

Substantial evidence points to a role for cerebral aggregation of amyloid beta (Ab) peptide in Alzheimer's disease (AD). Ab is derived from the sequential action of two aspartic proteases, the b-and c-secretases, on amyloid precursor protein (APP). b-Secretase initiates Ab formation by cleaving APP to generate the N-terminus of Ab (Citron et al. 1995). This cleavage produces a secreted ectodomain of APP (APPsb) and a membrane-tethered C-terminal fragment that is 99 amino acids in length (C99). Subsequently, c-secretase cleaves within the transmembrane region of C99 to release Ab that is secreted from the cell. Ab peptides may vary in length (38-42 amino acids) at the C-terminus because of the imprecise cleavage of the c-secretase. As Ab accumulation is implicated in AD pathogenesis, the identity of the b-secretase was highly sought after due to its ideal status as a drug target for lowering cerebral Ab levels. Herein, this review will discuss the identification and characterization of two aspartic proteases, beta-site APP cleaving enzyme-1 (BACE1) and beta-site APP cleaving enzyme-2 (BACE2), and provide evidence that unequivocally validates BACE1 as the b-secretase. Information regarding BACE1 physiological functions derived from deletion mutants, as well as BACE1 cell biology and substrates, will also be discussed. Identification and validation of BACE1 as the Alzheimer's b-secretaseOver a decade ago, five groups reported two unique aspartic proteases that shared 64% amino acid sequence similarity, and that served as potential b-secretase candidates: BACE1 (also termed memapsin 2 and Asp2) (Hussain et al. 1999;Sinha et al. 1999;Vassar et al. 1999;Yan et al. 1999;Lin et al. 2000), and BACE2 (also termed Asp1, memapsin 1, and Down region aspartic protease) (Saunders et al. 1999;Yan et al. 1999;Acquati et al. 2000;Bennett et al. 2000a;Lin et al. 2000;Solans et al. 2000). Prior to these reports, b-secretase properties had been well-characterized, a sequence of events that, as it turned out, was instrumental for the identification of the b-secretase. In the discussion below, we evaluate the properties of b-secretase that served as a tool to clearly validate BACE1 as the b-secretase essential for Ab formation.Although b-secretase activity is widely expressed, the highest proteolytic activity is observed in the brain (Seubert et al. 1993;Zhao et al. 1996). Consistent with this expression pattern, BACE1 is present in many tissues, but is predominantly expressed within the brain (Vassar et al. 1999;Bennett et al. 2000a;Lin et al. 2000;Marcinkiewicz and Seidah 2000). BACE2, however, is expressed at moderate to low levels across a variety of cell types, but it Received June 30, 2011; revised manuscript received September 20, 2011; accepted September 21, 2011.Address correspondence and reprint requests to Robert Vassar, PhD, Northwestern University, Feinberg School of Medicine, Department of Cell & Molecular Biology, 300 E. Superior, Tarry 8-713, Chicago, IL 60611, USA. E-mail: r-vassar@northwestern.eduAbbreviations used:...

Melatonin is released from the pineal gland into the circulatory system at night in the absence of light, acting as “hormone of darkness” to the brain and body. Melatonin also can regulate circadian phasing of the suprachiasmatic nucleus (SCN). During the day-to-night transition, melatonin exposure advances intrinsic SCN neural activity rhythms via the melatonin type-2 (MT2) receptor and downstream activation of protein kinase C (PKC). The effects of melatonin on SCN phasing have not been linked to daily changes in the expression of core genes that constitute the molecular framework of the circadian clock. Using real-time RT-PCR, we found that melatonin induces an increase in the expression of two clock genes, Period 1 (Per1) and Period 2 (Per2). This effect occurs at CT 10, when melatonin advances SCN phase, but not at CT 6, when it does not. Using anti-sense oligodeoxynucleotides (α ODNs) to Per 1 and Per 2, as well as to E-box enhancer sequences in the promoters of these genes, we show that their specific induction is necessary for the phase-altering effects of melatonin on SCN neural activity rhythms in the rat. These effects of melatonin on Per1 and Per2 were mediated by PKC. This is unlike day-active non-photic signals that reset the SCN clock by non-PCK signal transduction mechanisms and by decreasing Per1 expression. Rather, this finding extends roles for Per1 and Per2, which are critical to photic phase-resetting, to a nonphotic zeitgeber, melatonin, and suggest that the regulation of these clock gene transcripts is required for clock resetting by diverse regulatory cues.