The glymphatic system is a brain-wide clearance pathway; its impairment contributes to the accumulation of amyloid-β. Influx of cerebrospinal fluid (CSF) depends upon the expression and perivascular localization of the astroglial water channel aquaporin-4 (AQP4). Prompted by a recent failure to find an effect of Aqp4 knock-out (KO) on CSF and interstitial fluid (ISF) tracer transport, five groups re-examined the importance of AQP4 in glymphatic transport. We concur that CSF influx is higher in wild-type mice than in four different Aqp4 KO lines and in one line that lacks perivascular AQP4 (Snta1 KO). Meta-analysis of all studies demonstrated a significant decrease in tracer transport in KO mice and rats compared to controls. Meta-regression indicated that anesthesia, age, and tracer delivery explain the opposing results. We also report that intrastriatal injections suppress glymphatic function. This validates the role of AQP4 and shows that glymphatic studies must avoid the use of invasive procedures.

A novel factor, termed Humanin (HN), antagonizes against neurotoxicity by various types of familial Alzheimer's disease (AD) genes [V642I and K595N/M596L (NL) mutants of amyloid precursor protein (APP), M146L-presenilin (PS) 1, and N141I-PS2] and by A␤1-43 with clear action specificity ineffective on neurotoxicity by polyglutamine repeat Q79 or superoxide dismutase 1 mutants. Here we report that HN can also inhibit neurotoxicity by other AD-relevant insults: other familial AD genes (A617G-APP, L648P-APP, A246E-PS1, L286V-PS1, C410Y-PS1, and H163R-PS1), APP stimulation by anti-APP antibody, and other A␤ peptides (A␤1-42 and A␤25-35). The action specificity was further indicated by the finding that HN could not suppress neurotoxicity by glutamate or prion fragment. Against the AD-relevant insults, essential roles of Cys 8 and Ser 14 were commonly indicated, and the domain from Pro 3 to Pro 19 was responsible for the rescue action of HN, in which seven residues turned out to be essential. We also compared the neuroprotective action of S14G HN (HNG) with that of activity-dependent neurotrophic factor, IGF-I, or basic FGF for the antagonism against various AD-relevant insults (V642I-APP, NL-APP, M146L-PS1, N141I-PS2, and A␤1-43). Although all of these factors could abolish neurotoxicity by A␤1-43, only HNG could abolish cytotoxicities by all of them. HN and HN derivative peptides may provide a new insight into the study of AD pathophysiology and allow new avenues for the development of therapeutic interventions for various forms of AD.

The ⑀4 genotype of apolipoprotein E (apoE4) is the most established predisposing factor in Alzheimer's disease (AD); however, it remains unclear how apoE4 contributes to the pathophysiology. Here, we report that the apoE4 protein (ApoE4) evokes apoptosis in neuronal cells through the low-density lipoprotein receptor-related protein (LRP) and heterotrimeric GTPases. We examined neuron/neuroblastoma hybrid F11 cells and found that these cells were killed by 30 g/ml ApoE4, but not by 30 g/ml ApoE3. ApoE4-induced death occurred with typical features for apoptosis in time-and dose-dependent manners, and was observed in SH-SY5Y neuroblastomas, but not in glioblastomas or non-neuronal Chinese hamster ovary cells. Activated, but not native, ␣2-macroglobulin suppressed this ApoE4 toxicity. Suppression by the antisense oligonucleotide to LRP and inhibition by low nanomolar concentrations of LRP-associated protein RAP provided evidence for the involvement of LRP. The involvement of heterotrimeric GTPases was demonstrated by the findings that (1) ApoE4-induced death was suppressed by pertussis toxin (PTX), but not by heat-inactivated PTX; and (2) transfection with PTX-resistant mutant cDNAs of G␣ i restored the toxicity of ApoE4 restricted by PTX. We thus conclude that one of the neurotoxic mechanisms triggered by ApoE4 is to activate a cell type-specific apoptogenic program involving LRP and the G i class of GTPases and that the apoE4 gene may play a direct role in the pathogenesis of AD and other forms of dementia.

Signal transduction pathways of orexin receptors were examined using a nerve-like cell line transfected with orexin receptor type-1 (OX1R) and orexin receptor type-2 (OX2R). Forskolin-stimulated cyclic adenosine 3,5-monophosphate (cAMP) accumulation in OX2R-expressing cells was inhibited by orexin in a dose-dependent manner, and the effect was abolished by pretreatment with pertussis toxin (PTX). The inhibitory effect of orexin on forskolin-stimulated cAMP accumulation was not observed in OX1R-expressing cells. Administration of orexin to these cells resulted in a transient increase of intracellular calcium concentration ([Ca(2+)](i)). Orexin-stimulated increases in [Ca(2+)](i) in OX1R- or OX2R-expressing cells were not affected by the PTX pretreatment. These observations suggest that OX1R couples exclusively to PTX-insensitive G-proteins, while OX2R couples to both PTX-sensitive and -insensitive G-proteins. To examine the relative contributions of these G-proteins in OX2R-mediated activation of neurons, we used histaminergic tuberomammillary nucleus neurons, in which OX2R is abundantly expressed. We found that a phospholipase C (PLC)-inhibitor, U73122, inhibits orexin-mediated neuronal activation, but PTX showed no effect on it. This suggests that although OX2R couples to multiple G-proteins, activation of neurons by orexins through OX2R is mediated via a PTX-insensitive, PLC dependent pathway.

Accumulation of tau has been implicated in various neurodegenerative diseases termed tauopathies. Tau is a microtubule-associated protein but is also actively released into the extracellular fluids including brain interstitial fluid and cerebrospinal fluid (CSF). However, it remains elusive whether clearance of extracellular tau impacts tau-associated neurodegeneration. Here, we show that aquaporin-4 (AQP4), a major driver of the glymphatic clearance system, facilitates the elimination of extracellular tau from the brain to CSF and subsequently to deep cervical lymph nodes. Strikingly, deletion of AQP4 not only elevated tau in CSF but also markedly exacerbated phosphorylated tau deposition and the associated neurodegeneration in the brains of transgenic mice expressing P301S mutant tau. The current study identified the clearance pathway of extracellular tau in the central nervous system, suggesting that glymphatic clearance of extracellular tau is a novel regulatory mechanism whose impairment contributes to tau aggregation and neurodegeneration.