Intraneuronal Tau Misfolding Induced by Extracellular Amyloid-ß Oligomers.

Abnormal folding and aggregation of the microtubule-associated protein, tau, is a hallmark of several neurodegenerative disorders, including Alzheimer's disease (AD). Although normal tau is an intrinsically disordered protein, it does exhibit tertiary structure whereby the N- and C-termini are often in close proximity to each other and to the contiguous microtubule-binding repeat domains that extend C-terminally from the middle of the protein. Unfolding of this paperclip-like conformation might precede formation of toxic tau oligomers and filaments, like those found in AD brain. While there are many ways to monitor tau aggregation, methods to monitor changes in tau folding are not well established. Using full length human 2N4R tau doubly labeled with the Förster resonance energy transfer (FRET) compatible fluorescent proteins, Venus and Teal, on the N- and C-termini, respectively (Venus-Tau-Teal), intensity and lifetime FRET measurements were able to distinguish folded from unfolded tau in living cells independently of tau-tau intermolecular interactions. When expression was restricted to low levels in which tau-tau aggregation was minimized, Venus-Tau-Teal was sensitive to microtubule binding, phosphorylation, and pathogenic oligomers. Of particular interest is our finding that amyloid-ß oligomers (AßOs) trigger Venus-Tau-Teal unfolding in cultured mouse neurons. We thus provide direct experimental evidence that AßOs convert normally folded tau into a conformation thought to predominate in toxic tau aggregates. This finding provides further evidence for a mechanistic connection between Aß and tau at seminal stages of AD pathogenesis.

Amyloid-ß signals through tau to drive ectopic neuronal cell cycle re-entry in Alzheimer's disease.

Normally post-mitotic neurons that aberrantly re-enter the cell cycle without dividing account for a substantial fraction of the neurons that die in Alzheimer's disease (AD). We now report that this ectopic cell cycle re-entry (CCR) requires soluble amyloid-ß (Aß) and tau, the respective building blocks of the insoluble plaques and tangles that accumulate in AD brain. Exposure of cultured wild type (WT) neurons to Aß oligomers caused CCR and activation of the non-receptor tyrosine kinase, fyn, the cAMP-regulated protein kinase A and calcium-calmodulin kinase II, which respectively phosphorylated tau on Y18, S409 and S416. In tau knockout (KO) neurons, Aß oligomers activated all three kinases, but failed to induce CCR. Expression of WT, but not Y18F, S409A or S416A tau restored CCR in tau KO neurons. Tau-dependent CCR was also observed in vivo in an AD mouse model. CCR, a seminal step in AD pathogenesis, therefore requires signaling from Aß through tau independently of their incorporation into plaques and tangles.

Alzheimer disease: a tale of two prions.

Alzheimer disease (AD) has traditionally been thought to involve the misfolding and aggregation of two different factors that contribute in parallel to pathogenesis: amyloid-ß (Aß) peptides, which represent proteolytic fragments of the transmembrane amyloid precursor protein, and tau, which normally functions as a neuronally enriched, microtubule-associated protein that predominantly accumulates in axons. Recent evidence has challenged this model, however, by revealing numerous functional interactions between Aß and tau in the context of pathogenic mechanisms for AD. Moreover, the propagation of toxic, misfolded Aß and tau bears a striking resemblance to the propagation of toxic, misfolded forms of the canonical prion protein, PrP, and misfolded Aß has been shown to induce tau misfolding in vitro through direct, intermolecular interaction. In this review we discuss evidence for the prion-like properties of both Aß and tau individually, as well as the intriguing possibility that misfolded Aß acts as a template for tau misfolding in vivo.

Flightless-I, a gelsolin family member and transcriptional regulator, preferentially binds directly to activated cytosolic CaMK-II.

In order to evaluate links between Ca2+/calmodulin (CaM)-dependent protein kinase type II (CaMK-II) and cell cycle progression, CaMK-II binding partners were sought in proliferating cells by epitope-tag tandem mass spectrometry. One protein identified was the gelsolin family member, flightless-I (Fli-I). Fli-I is not a CaMK-II substrate, but binds directly and preferentially to constitutively active (T287D) CaMK-II over inactive CaMK-II. Fli-I gradually enters the nucleus upon CaMK-II inhibition and is retained in the cytosol by T287D CaMK-II. CaMK-II inhibition and Fli-I overexpression suppress transcription of beta-catenin dependent transcriptional reporters, whereas Fli-I suppression enhances their transcription. These findings support a novel mechanism whereby cytosolic CaMK-II influences beta-catenin dependent gene expression through Fli-I.

Laminin activates CaMK-II to stabilize nascent embryonic axons.

In neurons, the interaction of laminin with its receptor, beta1 integrin, is accompanied by an increase in cytosolic Ca2+. Neuronal behavior is influenced by CaMK-II, the type II Ca2+/calmodulin-dependent protein kinase, which is enriched in axons of mouse embryonic neurons. In this study, we sought to determine whether CaMK-II is activated by laminin, and if so, how CaMK-II influences axonal growth and stability. Axons grew up to 200 microm within 1 day of plating P19 embryoid bodies on laminin-1 (EHS laminin). Activated CaMK-II was found enriched along the axon and in the growth cone as detected using a phospho-Thr(287) specific CaMK-II antibody. beta1 integrin was found in a similar pattern along the axon and in the growth cone. Direct inhibition of CaMK-II in 1-day-old neurons immediately froze growth cone dynamics, disorganized F-actin and ultimately led to axon retraction. Collapsed axonal remnants exhibited diminished phospho-CaMK-II levels. Treatment of 1-day neurons with a beta1 integrin-blocking antibody (CD29) also reduced axon length and phospho-CaMK-II levels and, like CaMK-II inhibitors, decreased CaMK-II activation. Among several CaMK-II variants detected in these cultures, the 52-kDa delta variant preferentially associated with actin and beta 3 tubulin as determined by reciprocal immunoprecipitation. Our findings indicate that persistent activation of delta CaMK-II by laminin stabilizes nascent embryonic axons through its influence on the actin cytoskeleton.