PAK5 kinase is an inhibitor of MARK/Par-1, which leads to stable microtubules and dynamic actin.

MARK/Par-1 is a kinase involved in development of embryonic polarity. In neurons, MARK phosphorylates tau protein and causes its detachment from microtubules, the tracks of axonal transport. Because the target sites of MARK on tau occur at an early stage of Alzheimer neurodegeneration, we searched for interaction partners of MARK. Here we report that MARK2 is negatively regulated by PAK5, a neuronal member of the p21-activated kinase family. PAK5 suppresses the activity of MARK2 toward its target, tau protein. The inhibition requires the binding between the PAK5 and MARK2 catalytic domains, but does not require phosphorylation. In transfected Chinese hamster ovary (CHO) cells both kinases show a vesicular distribution with partial colocalization on endosomes containing AP-1/2. Although MARK2 transfected alone destabilizes microtubules and stabilizes actin stress fibers, PAK5 keeps microtubules stable through the down-regulation of MARK2 but destabilizes the F-actin network so that stress fibers and focal adhesions disappear and cells develop filopodia. The results point to an inverse relationship between actin- and microtubule-related signaling by the PAK5 and MARK2 pathways that affect both cytoskeletal networks.

An mtDNA mutation accelerates liver aging by interfering with the ROS response and mitochondrial life cycle.

Mitochondrial dysfunction affects liver metabolism, but it remains unclear whether this interferes with normal liver aging. We investigated several mitochondrial pathways in hepatocytes and liver tissue from a conplastic mouse strain compared with the control C57BL/6NTac strain over 18 months of life. The C57BL/6NTac-mtNODLtJ mice differed from C57BL/6NTac mice by a point mutation in mitochondrial-encoded subunit 3 of cytochrome c oxidase. Young C57BL/6NTac-mtNODLtJ mice showed reduced mitochondrial metabolism but similar reactive oxygen species (ROS) production to C57BL/6NTac mice. Whereas ROS increased almost equally up to 9 months in both strains, different mitochondrial adaptation strategies resulted in decreasing ROS in advanced age in C57BL/6NTac mice, but persistent ROS production in C57BL/6NTac-mtNODLtJ mice. Only the conplastic strain developed elongated mitochondrial networks with artificial loop structures, depressed autophagy, high mitochondrial respiration and up-regulated antioxidative response. Our results indicate that mtDNA mutations accelerate liver ballooning degeneration and carry a serious risk of premature organ aging.

Identification of Tcf-4 as a transcriptional target of p53 signalling.

T-cell factor (Tcf)-4 is a main transcription factor to pass on Wnt/beta-catenin signalling. The tumour suppressor protein p53 contributes as a transcription factor to cell-cycle arrest and apoptosis induction. Mutations of components in p53 and Wnt/beta-catenin signalling networks play a part in tumour formation. Here, we identify the Tcf-4 gene as a downstream effector of p53. Induction of wild-type p53 in a tet-off regulated human colon cell system leads to the reduction of Tcf-4 mRNA and protein levels. Also, mRNA of the Tcf-4 target gene uPAR is downregulated after p53 induction. Expression of a luciferase reporter controlled by the Tcf-4 promoter is repressed by wild-type p53, but not by a p53 mutant deficient in DNA binding. Such a regulation is seen in cell lines of different origin. These findings directly link Wnt/beta-catenin signalling and p53 tumour suppressor function and may provide a mechanism by which loss of p53 function contributes to progression in the adenoma/carcinoma sequence in colon tumours. Furthermore, since Tcf-4 is expressed in many tissues and downregulation of Tcf-4 by p53 is seen in several different cell types, this regulation likely plays a role in proliferation control of all tissues that can express p53 and Tcf-4.

Cyclin B1 transcription is enhanced by the p300 coactivator and regulated during the cell cycle by a CHR-dependent repression mechanism.

Cyclin B is a central regulator of transition from the G(2) phase of the cell cycle to mitosis. In mammalian cells two B-type cyclins have been characterised, cyclin B1 and B2. Both are expressed with a maximum in G(2) and their synthesis is mainly regulated on the transcriptional level. We show that a single cell cycle genes homology region, lacking a functional cell cycle-dependent element in tandem with it, contributes most of the cell cycle-dependent transcription from the cyclin B1 promoter. The coactivator p300 binds to the cyclin B1 promoter and synergises with the transcription factor NF-Y in activating transcription of cyclin B1.

Spred1 and TESK1--two new interaction partners of the kinase MARKK/TAO1 that link the microtubule and actin cytoskeleton.

The signaling from MARKK/TAO1 to the MAP/microtubule affinity-regulating kinase MARK/Par1 to phosphorylated microtubule associated proteins (MAPs) renders microtubules dynamic and plays a role in neurite outgrowth or polarity development. Because hyperphosphorylation of Tau at MARK target sites is a hallmark of Alzheimer neurodegeneration, we searched for upstream regulators by the yeast two-hybrid approach and identified two new interaction partners of MARKK, the regulatory Sprouty-related protein with EVH-1 domain1 (Spred1) and the testis-specific protein kinase (TESK1). Spred1-MARKK binding has no effect on the activity of MARKK; therefore, it does not change microtubule (MT) stability. Spred1-TESK1 binding causes inhibition of TESK1. Because TESK1 can phosphorylate cofilin and thus stabilizes F-actin stress fibers, the inhibition of TESK1 by Spred1 makes F-actin fibers dynamic. A third element in this interaction triangle is that TESK1 binds to and inhibits MARKK. Thus, in Chinese hamster ovary (CHO) cells the elevation of MARKK results in MT disruption (via activation of MARK/Par1 and phosphorylation of MAPs), but this can be blocked by TESK1. Similarly, enhanced TESK1 activity results in increased stress fibers (via phospho-cofilin), but this can be blocked by elevating Spred1. Thus, the three-way interaction between Spred1, MARKK, and TESK1 represents a pathway that links regulation of both the microtubule- and F-actin cytoskeleton.