Using a Caenorhabditis elegans Parkinson's Disease Model to Assess Disease Progression and Therapy Efficiency.

Despite Parkinson's Disease (PD) being the second most common neurodegenerative disease, treatment options are limited. Consequently, there is an urgent need to identify and screen new therapeutic compounds that slow or reverse the pathology of PD. Unfortunately, few new therapeutics are being produced, partly due to the low throughput and/or poor predictability of the currently used model organisms and in vivo screening methods. Our objective was to develop a simple and affordable platform for drug screening utilizing the nematode Caenorhabditis elegans. The effect of Levodopa, the "Gold standard" of PD treatment, was explored in nematodes expressing the disease-causing α-synuclein protein. We focused on two key hallmarks of PD: plaque formation and mobility. Exposure to Levodopa ameliorated the mobility defect in C. elegans, similar to people living with PD who take the drug. Further, long-term Levodopa exposure was not detrimental to lifespan. This C. elegans-based method was used to screen a selection of small-molecule drugs for an impact on α-synuclein aggregation and mobility, identifying several promising compounds worthy of further investigation, most notably Ambroxol. The simple methodology means it can be adopted in many labs to pre-screen candidate compounds for a positive impact on disease progression.

AGC kinases and MAB4/MEL proteins maintain PIN polarity by limiting lateral diffusion in plant cells.

Polar subcellular localization of the PIN exporters of the phytohormone auxin is a key determinant of directional, intercellular auxin transport and thus a central topic of both plant cell and developmental biology. Arabidopsis mutants lacking PID, a kinase that phosphorylates PINs, or the MAB4/MEL proteins of unknown molecular function display PIN polarity defects and phenocopy pin mutants, but mechanistic insights into how these factors convey PIN polarity are missing. Here, by combining protein biochemistry with quantitative live-cell imaging, we demonstrate that PINs, MAB4/MELs, and AGC kinases interact in the same complex at the plasma membrane. MAB4/MELs are recruited to the plasma membrane by the PINs and in concert with the AGC kinases maintain PIN polarity through limiting lateral diffusion-based escape of PINs from the polar domain. The PIN-MAB4/MEL-PID protein complex has self-reinforcing properties thanks to positive feedback between AGC kinase-mediated PIN phosphorylation and MAB4/MEL recruitment. We thus uncover the molecular mechanism by which AGC kinases and MAB4/MEL proteins regulate PIN localization and plant development.

DIX Domain Polymerization Drives Assembly of Plant Cell Polarity Complexes.

Cell polarity is fundamental for tissue morphogenesis in multicellular organisms. Plants and animals evolved multicellularity independently, and it is unknown whether their polarity systems are derived from a single-celled ancestor. Planar polarity in animals is conferred by Wnt signaling, an ancient signaling pathway transduced by Dishevelled, which assembles signalosomes by dynamic head-to-tail DIX domain polymerization. In contrast, polarity-determining pathways in plants are elusive. We recently discovered Arabidopsis SOSEKI proteins, which exhibit polar localization throughout development. Here, we identify SOSEKI as ancient polar proteins across land plants. Concentration-dependent polymerization via a bona fide DIX domain allows these to recruit ANGUSTIFOLIA to polar sites, similar to the polymerization-dependent recruitment of signaling effectors by Dishevelled. Cross-kingdom domain swaps reveal functional equivalence of animal and plant DIX domains. We trace DIX domains to unicellular eukaryotes and thus show that DIX-dependent polymerization is an ancient mechanism conserved between kingdoms and central to polarity proteins.

A SOSEKI-based coordinate system interprets global polarity cues in Arabidopsis.

Multicellular development requires coordinated cell polarization relative to body axes, and translation to oriented cell division1-3. In plants, it is unknown how cell polarities are connected to organismal axes and translated to division. Here, we identify Arabidopsis SOSEKI proteins that integrate apical-basal and radial organismal axes to localize to polar cell edges. Localization does not depend on tissue context, requires cell wall integrity and is defined by a transferrable, protein-specific motif. A Domain of Unknown Function in SOSEKI proteins resembles the DIX oligomerization domain in the animal Dishevelled polarity regulator. The DIX-like domain self-interacts and is required for edge localization and for influencing division orientation, together with a second domain that defines the polar membrane domain. Our work shows that SOSEKI proteins locally interpret global polarity cues and can influence cell division orientation. Furthermore, this work reveals that, despite fundamental differences, cell polarity mechanisms in plants and animals converge on a similar protein domain.

Polarly localized kinase SGN1 is required for Casparian strip integrity and positioning.

Casparian strips are precisely localized and aligned ring-like cell wall modifications in the root of all higher plants. They set up an extracellular diffusion barrier analogous to animal tight junctions, and are crucial for maintaining the homeostatic capacity of plant roots. Casparian strips become localized because of the formation of a highly stable plasma membrane domain, consisting of a family of small transmembrane proteins called Casparian strip membrane domain proteins (CASPs). Here we report a large-scale forward genetic screen directly visualizing endodermal barrier function, which allowed us to identify factors required for the formation and integrity of Casparian strips. We present the identification and characterization of one of the mutants, schengen1 (sgn1), a receptor-like cytoplasmic kinase that we show localizes in a strictly polar fashion to the outer plasma membrane of endodermal cells and is required for the positioning and correct formation of the centrally located CASP domain.

Control of oriented cell division in the Arabidopsis embryo.

Multicellular plant development requires strict control of cell division orientation. A key unanswered question is how developmental regulators interact with the generic cell division machinery to trigger oriented divisions. We discuss the Arabidopsis embryo as a model for addressing this question. Recent progress in 3D imaging and computation now allows sketching of a framework for the developmental control of division orientation in which the signaling molecule auxin controls oriented division by preventing a geometrically defined default plane. We expect that the identification of auxin effectors, together with the identification of novel regulators of cell division will help to link developmental regulators to the division machinery.