Plant and mouse EB1 proteins have opposite intrinsic properties on the dynamic instability of microtubules

OBJECTIVE

Most eukaryotic cells contain microtubule filaments, which play central roles in intra-cellular organization. However, microtubule networks have a wide variety of architectures from one cell type and organism to another. Nonetheless, the sequences of tubulins, of Microtubule Associated proteins (MAPs) and the structure of microtubules are usually well conserved throughout the evolution. MAPs being known to be responsible for regulating microtubule organization and dynamics, this raises the question of the conservation of their intrinsic properties. Indeed, knowing how the intrinsic properties of individual MAPs differ between organisms might enlighten our understanding of how distinct microtubule networks are built. End-Binding protein 1 (EB1), first described as a MAP in yeast, is conserved in plants and mammals. The intrinsic properties of the mammalian and the yeast EB1 proteins have been well described in the literature but, to our knowledge, the intrinsic properties of EB1 from plant and mammals have not been compared thus far.

RESULTS

Here, using an in vitro assay, we discovered that plant and mammalian EB1 purified proteins have different intrinsic properties on microtubule dynamics. Indeed, the mammalian EB1 protein increases microtubules dynamic while the plant EB1 protein stabilizes them.

Protein tyrosine phosphatase 4A3 (PTP4A3/PRL-3) promotes the aggressiveness of human uveal melanoma through dephosphorylation of CRMP2

Uveal melanoma (UM) is an aggressive tumor in which approximately 50% of patients develop metastasis. Expression of the PTP4A3 gene, encoding a phosphatase, is predictive of poor patient survival. PTP4A3 expression in UM cells increases their migration in vitro and invasiveness in vivo. Here, we show that CRMP2 is mostly dephosphorylated on T514 in PTP4A3 expressing cells. We also demonstrate that inhibition of CRMP2 expression in UM cells expressing PTP4A3 increases their migration in vitro and invasiveness in vivo. This phenotype is accompanied by modifications of the actin microfilament network, with shortened filaments, whereas cells with a inactive mutant of the phosphatase do not show the same behavior. In addition, we showed that the cell cytoplasm becomes stiffer when CRMP2 is downregulated or PTP4A3 is expressed. Our results suggest that PTP4A3 acts upstream of CRMP2 in UM cells to enhance their migration and invasiveness and that a low level of CRMP2 in tumors is predictive of poor patient survival.

EB1 contributes to microtubule bundling and organization, along with root growth, in Arabidopsis thaliana

Microtubules are involved in plant development and adaptation to their environment, but the sustaining molecular mechanisms remain elusive. Microtubule-end-binding 1 (EB1) proteins participate in directional root growth in Arabidopsis thaliana. However, a connection to the underlying microtubule array has not been established yet. We show here that EB1 proteins contribute to the organization of cortical microtubules in growing epidermal plant cells, without significant modulation of microtubule dynamics. Using super-resolution stimulated emission depletion (STED) microscopy and an original quantification approach, we also demonstrate a significant reduction of apparent microtubule bundling in cytoplasmic-EB1-deficient plants, suggesting a function for EB1 in the interaction between adjacent microtubules. Furthermore, we observed root growth defects in EB1-deficient plants, which are not related to cell division impairment. Altogether, our results support a role for EB1 proteins in root development, in part by maintaining the organization of cortical microtubules.

This article has an associated First Person interview with the first author of the paper.

Summary: EB1 proteins affect cortical-microtubule bundling and organization in Arabidopsis thaliana, without significant modulation of microtubule dynamics. They also participate in root growth, further linking microtubules to plant development.

Native metabotropic glutamate receptor 4 depresses synaptic transmission through an unusual Gαq transduction pathway

In cerebellar cortex, mGlu₄ receptors located on parallel fibers play an essential role in normal motor function, but the molecular mechanisms involved are not yet completely understood. Using a strategy combining biochemical and electrophysiological approaches in the rodent cerebellum, we demonstrate that presynaptic mGlu₄ receptors control synaptic transmission through an atypical activation of Gα_q proteins. First, the Gα_q subunit, PLC and PKC signaling proteins present in cerebellar extracts are retained on affinity chromatography columns grafted with different sequences of the cytoplasmic domain of mGlu₄ receptor. The i2 loop and the C terminal domain were used as baits, two domains that are known to play a pivotal role in coupling selectivity and efficacy. Second, in situ proximity ligation assays show that native mGlu₄ receptors and Gα_q subunits are in close physical proximity in cerebellar cortical slices. Finally, electrophysiological experiments demonstrate that the molecular mechanisms underlying mGlu₄ receptor-mediated inhibition of transmitter release at cerebellar Parallel Fiber (PF) - Molecular Layer Interneuron (MLI) synapses involves the Gα_q-PLC signaling pathway. Taken together, our results provide compelling evidence that, in the rodent cerebellar cortex, mGlu₄ receptors act by coupling to the Gα_q protein and PLC effector system to reduce glutamate synaptic transmission.

Small and stable peptidic PEGylated quantum dots to target polyhistidine-tagged proteins with controlled stoichiometry

The use of the semiconductor quantum dots (QD) as biolabels for both ensemble and single-molecule tracking requires the development of simple and versatile methods to target individual proteins in a controlled manner, ideally in living cells. To address this challenge, we have prepared small and stable QDs (QD-ND) using a surface coating based on a peptide sequence containing a tricysteine, poly(ethylene glycol) (PEG), and an aspartic acid ligand. These QDs, with a hydrodynamic diameter of 9 +/- 1.5 nm, can selectively bind to polyhistidine-tagged (histag) proteins in vitro or in living cells. We show that the small and monodisperse size of QD-ND allows for the formation of QD-ND/histag protein complexes of well-defined stoichiometry and that the 1:1 QD/protein complex can be isolated and purified by gel electrophoresis without any destabilization in the nanomolar concentration range. We also demonstrate that QD-ND can be used to specifically label a membrane receptor with an extracellular histag expressed in living HeLa cells. Here, cytotoxicity tests reveal that cell viability remains high under the conditions required for cellular labeling with QD-ND. Finally, we apply QD-ND complexed with histag end binding protein-1 (EB1), a microtubule associated protein, to single-molecule tracking in Xenopus extracts. Specific colocalization of QD-ND/EB1 with microtubules during the mitotic spindle formation demonstrates that QD-ND and our labeling strategy provide an efficient approach to monitor the dynamic behavior of proteins involved in complex biological functions.

The evolving doublecortin (DCX) superfamily

Background

Doublecortin (DCX) domains serve as protein-interaction platforms. Mutations in members of this protein superfamily are linked to several genetic diseases. Mutations in the human DCX gene result in abnormal neuronal migration, epilepsy, and mental retardation; mutations in RP1 are associated with a form of inherited blindness, and DCDC2 has been associated with dyslectic reading disabilities.

Results

The DCX-repeat gene family is composed of eleven paralogs in human and in mouse. Its evolution was followed across vertebrates, invertebrates, and was traced to unicellular organisms, thus enabling following evolutionary additions and losses of genes or domains. The N-terminal and C-terminal DCX domains have undergone sub-specialization and divergence. Developmental in situ hybridization data for nine genes was generated. In addition, a novel co-expression analysis for most human and mouse DCX superfamily-genes was performed using high-throughput expression data extracted from Unigene. We performed an in-depth study of a complete gene superfamily using several complimentary methods.

Conclusion

This study reveals the existence and conservation of multiple members of the DCX superfamily in different species. Sequence analysis combined with expression analysis is likely to be a useful tool to predict correlations between human disease and mouse models. The sub-specialization of some members due to restricted expression patterns and sequence divergence may explain the successful addition of genes to this family throughout evolution.

The DCX Superfamily 1: Common and Divergent Roles for Members of the Mouse DCX Superfamily

The doublecortin-like (DCX) domains serve as protein-interaction platforms. DCX tandem domains appear in the product of the X-linked doublecortin (DCX) gene, in retinitis pigmentosa-1 (RP1), as well as in other gene products. Mutations in the human DCX gene are associated with abnormal neuronal migration, epilepsy, and mental retardation; mutations in RP1 are associated with a form of inherited blindness, while DCDC2 has been associated with dyslectic reading disabilities. Motivated by the possible importance of this gene family, a thorough analysis to detect all family members in the mouse was conducted. The DCX-repeat gene superfamily is composed of eleven paralogs, and we cloned the DCX domains from nine different genes. Our study questioned which functions attributed to the DCX domain, are conserved among the different members. Our results suggest that the proteins with the DCX-domain have conserved and unique roles in microtubule regulation and signal transduction. All the tested proteins stimulated microtubule assembly in vitro. Proteins with tandem repeats stabilized the microtubule cytoskeleton in transfected cells, while those with single repeats localized to actin-rich subcellular structures, or the nucleus. All tested proteins interacted with components of the JNK/MAP-kinase pathway, while only a subset interacted with Neurabin 2, and a nonoverlapping group demonstrated actin association. The sub-specialization of some members due to confined intracellular localization, and protein interactions may explain the success of this superfamily.

Doublecortin-like kinase controls neurogenesis by regulating mitotic spindles and M phase progression

The mechanisms controlling neurogenesis during brain development remain relatively unknown. Through a differential protein screen with developmental versus mature neural tissues, we identified a group of developmentally enriched microtubule-associated proteins (MAPs) including doublecortin-like kinase (DCLK), a protein that shares high homology with doublecortin (DCX). DCLK, but not DCX, is highly expressed in regions of active neurogenesis in the neocortex and cerebellum. Through a dynein-dependent mechanism, DCLK regulates the formation of bipolar mitotic spindles and the proper transition from prometaphase to metaphase during mitosis. In cultured cortical neural progenitors, DCLK RNAi Lentivirus disrupts the structure of mitotic spindles and the progression of M phase, causing an increase of cell-cycle exit index and an ectopic commitment to a neuronal fate. Furthermore, both DCLK gain and loss of function in vivo specifically promote a neuronal identity in neural progenitors. These data provide evidence that DCLK controls mitotic division by regulating spindle formation and also determines the fate of neural progenitors during cortical neurogenesis.

Missense mutations resulting in type 1 lissencephaly

Proper human brain formation is dependent upon the integrated activity of multiple genes. Malfunctioning of key proteins results in brain developmental abnormalities. Mutation(s) in the LIS1 gene or the X-linked gene doublecortin (DCX) results in a spectrum of disorders including lissencephaly, or "smooth brain", and subcortical band heterotopia, or "doublecortex". Here, we will focus on a particular subset of missense mutations in these two genes and their effect on protein structure and function.

LIS1 missense mutations: variable phenotypes result from unpredictable alterations in biochemical and cellular properties

Mutations in one allele of the human LIS1 gene cause a severe brain malformation, lissencephaly. Although most LIS1 mutations involve deletions, several point mutations with a single amino acid alteration were described. Patients carrying these mutations reveal variable phenotypic manifestations. We have analyzed the functional importance of these point mutations by examining protein stability, folding, intracellular localization, and protein-protein interactions. Our data suggest that the mutated proteins were affected at different levels, and no single assay could be used to predict the lissencephaly phenotype. Most interesting are those mutant proteins that retain partial folding and interactions. In the case of LIS1 mutated in F31S, the cellular phenotype may be modified by overexpression of specific interacting proteins. Overexpression of the PAF-AH alpha1 subunit dissolved aggregates induced by this mutant protein and increased its half-life. Overexpression of NudE or NudEL localized this mutant protein to spindle poles and kinetochores but had no effect on protein stability. Our results implicate that there are probably different biochemical and cellular mechanisms obstructed in each patient yielding the varied lissencephaly phenotypes.

LIS1, CLIP-170's key to the dynein/dynactin pathway

CLIP-170 is a plus-end tracking protein which may act as an anticatastrophe factor. It has been proposed to mediate the association of dynein/dynactin to microtubule (MT) plus ends, and it also binds to kinetochores in a dynein/dynactin-dependent fashion, both via its C-terminal domain. This domain contains two zinc finger motifs (proximal and distal), which are hypothesized to mediate protein-protein interactions. LIS1, a protein implicated in brain development, acts in several processes mediated by the dynein/dynactin pathway by interacting with dynein and other proteins. Here we demonstrate colocalization and direct interaction between CLIP-170 and LIS1. In mammalian cells, LIS1 recruitment to kinetochores is dynein/dynactin dependent, and recruitment there of CLIP-170 is dependent on its site of binding to LIS1, located in the distal zinc finger motif. Overexpression of CLIP-170 results in a zinc finger-dependent localization of a phospho-LIS1 isoform and dynactin to MT bundles, raising the possibility that CLIP-170 and LIS1 regulate dynein/dynactin binding to MTs. This work suggests that LIS1 is a regulated adapter between CLIP-170 and cytoplasmic dynein at sites involved in cargo-MT loading, and/or in the control of MT dynamics.

Interaction of Maf transcription factors with Pax-6 results in synergistic activation of the glucagon promoter

In the endocrine pancreas, alpha-cell-specific expression of the glucagon gene is mediated by DNA-binding proteins that interact with the G1 proximal promoter element. Among these proteins, the paired domain transcription factor Pax-6 has been shown to bind to G1 and to transactivate glucagon gene expression. Close to the Pax-6-binding site, we observed the presence of a binding site for a basic leucine zipper transcription factor of the Maf family. In the present study, we demonstrate the presence of Maf family members in the endocrine pancreas that bind to G1 and transactivate glucagon promoter expression. In transient transfection experiments, we found that the transactivating effect on the glucagon promoter was greatly enhanced by the simultaneous expression of Maf transcription factors and Pax-6. This enhancement on glucagon transactivation could be correlated with the ability of these proteins to interact together but does not require binding of Maf proteins to the G1 element. Furthermore, we found that Maf enhanced the Pax-6 DNA binding capacity. Our data indicate that Maf transcription factors may contribute to glucagon gene expression in the pancreas.

Specific Pax-6/Microphthalmia Transcription Factor Interactions Involve Their DNA-binding Domains and Inhibit Transcriptional Properties of Both Proteins

Pax-6 and microphthalmia transcription factor (Mitf) are required for proper eye development. Pax-6, expressed in both the neuroretina and pigmented retina, has two DNA-binding domains: the paired domain and the homeodomain. Mice homozygous for Pax-6 mutations are anophthalmic. Mitf, a basic helix-loop-helix leucine zipper (b-HLH-LZ) transcription factor associated with the onset and maintenance of pigmentation, identifies the retinal pigmented epithelium during eye development. Loss of Mitf function results in the formation of an ectopic neuroretina at the expense of the dorsal retinal pigmented epithelium. In the present study, we investigated the interaction between Pax-6 and Mitf. In transient transfection-expression experiments, we found that transactivating effects of Pax-6 and Mitf on their respective target promoters were strongly inhibited by co-transfection of both transcription factors. This repression was due to direct protein/protein interactions involving both Pax-6 DNA-binding domains and the Mitf b-HLH-LZ domain. These results suggest that Pax-6/Mitf interactions may be critical for retinal pigmented epithelium development.