Bernhard Trinczek

MARK/PAR1 kinase is a regulator of microtubule-dependent transport in axons

Microtubule-dependent transport of vesicles and organelles appears saltatory because particles switch between periods of rest, random Brownian motion, and active transport. The transport can be regulated through motor proteins, cargo adaptors, or microtubule tracks. We report here a mechanism whereby microtubule associated proteins (MAPs) represent obstacles to motors which can be regulated by microtubule affinity regulating kinase (MARK)/Par-1, a family of kinases that is known for its involvement in establishing cell polarity and in phosphorylating tau protein during Alzheimer neurodegeneration. Expression of MARK causes the phosphorylation of MAPs at their KXGS motifs, thereby detaching MAPs from the microtubules and thus facilitating the transport of particles. This occurs without impairing the intrinsic activity of motors because the velocity during active movement remains unchanged. In primary retinal ganglion cells, transfection with tau leads to the inhibition of axonal transport of mitochondria, APP vesicles, and other cell components which leads to starvation of axons and vulnerability against stress. This transport inhibition can be rescued by phosphorylating tau with MARK.

The Akt activation inhibitor TCN-P inhibits Akt phosphorylation by binding to the PH domain of Akt and blocking its recruitment to the plasma membrane

Persistently hyperphosphorylated Akt contributes to human oncogenesis and resistance to therapy. Triciribine (TCN) phosphate (TCN-P), the active metabolite of the Akt phosphorylation inhibitor TCN, is in clinical trials, but the mechanism by which TCN-P inhibits Akt phosphorylation is unknown. Here we show that in vitro, TCN-P inhibits neither Akt activity nor the phosphorylation of Akt S473 and T308 by mammalian target of rapamycin or phosphoinositide-dependent kinase 1. However, in intact cells, TCN inhibits EGF-stimulated Akt recruitment to the plasma membrane and phosphorylation of Akt. Surface plasmon resonance shows that TCN, but not TCN, binds Akt-derived pleckstrin homology (PH) domain (KD: 690 nM). Furthermore, nuclear magnetic resonance spectroscopy shows that TCN-P, but not TCN, binds to the PH domain in the vicinity of the PIP3-binding pocket. Finally, constitutively active Akt mutants, Akt1-T308D/S473D and myr-Akt1, but not the transforming mutant Akt1-E17K, are resistant to TCN and rescue from its inhibition of proliferation and induction of apoptosis. Thus, the results of our studies indicate that TCN-P binds to the PH domain of Akt and blocks its recruitment to the membrane, and that the subsequent inhibition of Akt phosphorylation contributes to TCN-P antiproliferative and proapoptotic activities, suggesting that this drug may be beneficial to patients whose tumors express persistently phosphorylated Akt.

The dual role of tau in cell polarisation and organelle trafficking

Tau is one of the microtubule-associated proteins (MAPs) which copurify with microtubules through cycles of assembly and disassembly [1, 2]. One of its functions is to stabilize micro-tubules [3], although this function can also be assumed by other MAPs so that transgenic mice lacking tau develop almost normally [4]. Other functions include the induction of microtubule bundles during cell process formation and neuritogenesis [5–9], anchoring of cellular enzymes such as kinases, phosphatases, or lipases [10–13], interactions between the cytoskeleton and the plasma membrane [14], and the regulation of intracellular traffic [15, 16]. In Alzheimer’s disease (AD), tau aggregates into paired helical filaments (PHFs) which coalesce into neurofibrillary tangles and related deposits. The progression of tau aggregates correlates with the stages of AD and is an early sign of cellular degeneration [17–20]. Similar features occur in other tauopathies, notably FTDP-17 associated with mutations in the tau gene (reviews [21, 22]). Finally, tau is elevated in the cerebrospinal fluid of AD patients which makes it a potential tool for early diagnosis [23].

Tau Protein and Alzheimer Paired Helical Filament Assembly: Interacting Domains and Control Residues

Tau protein can interact with tubulin, the subunits of microtubules, and it can interact with itself in the paired helical filaments of Alzheimer’s disease. Both interactions are thought to be caused by post-translational modifications of the protein. We have analyzed the domain structure of tau, the effect of the domains on tau-tubulin or tau-tau interactions, and the residues that are critically involved. Two key residues appear to be particularly important in switching between different states, Ser-262 and Cys-322. Ser-262 can be phosphorylated by the kinase pllO(MARK), which abolishes tau’s binding to microtubules. Cys-322 can be oxidized to disulfide bridges, which leads to the formation of antiparallel tau dimers and paired helical filaments. The results argue that the tau-microtubule interactions and the tau-tau interactions lead ing to PHFs are controlled by different chemical modifications.