Identification of a Neural Cell Specific Variant of Microtubule-Associated Protein 4

A conserved microtubule-binding region in Xanthomonas XopL is indispensable for induced plant cell death reactions

Pathogenic Xanthomonas bacteria cause disease on more than 400 plant species. These Gram-negative bacteria utilize the type III secretion system to inject type III effector proteins (T3Es) directly into the plant cell cytosol where they can manipulate plant pathways to promote virulence. The host range of a given Xanthomonas species is limited, and T3E repertoires are specialized during interactions with specific plant species. Some effectors, however, are retained across most strains, such as Xanthomonas Outer Protein L (XopL). As an 'ancestral' effector, XopL contributes to the virulence of multiple xanthomonads, infecting diverse plant species. XopL homologs harbor a combination of a leucine-rich-repeat (LRR) domain and an XL-box which has E3 ligase activity. Despite similar domain structure there is evidence to suggest that XopL function has diverged, exemplified by the finding that XopLs expressed in plants often display bacterial species-dependent differences in their sub-cellular localization and plant cell death reactions. We found that XopL from X. euvesicatoria (XopLXe) directly associates with plant microtubules (MTs) and causes strong cell death in agroinfection assays in N. benthamiana. Localization of XopLXe homologs from three additional Xanthomonas species, of diverse infection strategy and plant host, revealed that the distantly related X. campestris pv. campestris harbors a XopL (XopLXcc) that fails to localize to MTs and to cause plant cell death. Comparative sequence analyses of MT-binding XopLs and XopLXcc identified a proline-rich-region (PRR)/α-helical region important for MT localization. Functional analyses of XopLXe truncations and amino acid exchanges within the PRR suggest that MT-localized XopL activity is required for plant cell death reactions. This study exemplifies how the study of a T3E within the context of a genus rather than a single species can shed light on how effector localization is linked to biochemical activity.

Microtubule elongation along actin filaments induced by microtubule-associated protein 4 contributes to the formation of cellular protrusions

Actin-microtubule crosstalk is implicated in the formation of cellular protrusions, but the mechanism remains unclear. In this study, we examined the regulation of cell protrusion involving a ubiquitously expressed microtubule-associated protein (MAP) 4, and its superfamily proteins, neuronal MAP2 and tau. Fluorescence microscopy revealed that these MAPs bound to F-actin and microtubules simultaneously, and formed F-actin/microtubule hybrid bundles. The hybrid bundle-forming activity was in the order of MAP2 > MAP4 ≫ tau. Interestingly, the microtubule assembly-promoting activity of MAP4 and MAP2, but not of tau, was upregulated by their interaction with F-actin. When MAP4 was overexpressed in NG108-15 cells, the number of cell processes and maximum process length of each cell increased significantly by 28% and 30%, respectively. Super-resolution microscopy revealed that 95% of microtubules in cell processes colocalized with F-actin, and MAP4 was always found in their vicinity. These results suggest that microtubule elongation along F-actin induced by MAP4 contributes to the formation of cellular protrusions. Since MAP4, MAP2 and tau had different crosstalk activity between F-actin and microtubules, it is likely that the functional differentiation of these MAPs is a driving force for neural evolution, causing significant changes in cell morphology.

MAP4 as a New Candidate in Cardiovascular Disease

Microtubule and mitochondrial dysfunction have been implicated in the pathogenesis of cardiovascular diseases (CVDs), including cardiac hypertrophy, fibrosis, heart failure, and hypoxic/ischemic related heart dysfunction. Microtubule dynamics instability leads to disrupted cell homeostasis and cell shape, decreased cell survival, and aberrant cell division and cell cycle, while mitochondrial dysfunction contributes to abnormal metabolism and calcium flux, increased cell death, oxidative stress, and inflammation, both of which causing cell and tissue dysfunction followed by CVDs. A cytosolic skeleton protein, microtubule-associated protein 4 (MAP4), belonging to the family of microtubule-associated proteins (MAPs), is widely expressed in non-neural cells and possesses an important role in microtubule dynamics. Increased MAP4 phosphorylation results in microtubule instability. In addition, MAP4 also expresses in mitochondria and reveals a crucial role in maintaining mitochondrial homeostasis. Phosphorylated MAP4 promotes mitochondrial apoptosis, followed by cardiac injury. The aim of the present review is to highlight the novel role of MAP4 as a potential candidate in multiple cardiovascular pathologies.

Differentially expressed proteins underlying childhood cortical dysplasia with epilepsy identified by iTRAQ proteomic profiling

Cortical dysplasia accounts for at least 14% of epilepsy cases, and is mostly seen in children. However, the understanding of molecular mechanisms and pathogenesis underlying cortical dysplasia is limited. The aim of this cross-sectional study is to identify potential key molecules in the mechanisms of cortical dysplasia by screening the proteins expressed in brain tissues of childhood cortical dysplasia patients with epilepsy using isobaric tags for relative and absolute quantitation-based tandem mass spectrometry compared to controls, and several differentially expressed proteins that are not reported to be associated with cortical dysplasia previously were selected for validation using real-time polymerase chain reaction, immunoblotting and immunohistochemistry. 153 out of 3340 proteins were identified differentially expressed between childhood cortical dysplasia patients and controls. And FSCN1, CRMP1, NDRG1, DPYSL5, MAP4, and FABP3 were selected for validation and identified to be increased in childhood cortical dysplasia patients, while PRDX6 and PSAP were identified decreased. This is the first report on differentially expressed proteins in childhood cortical dysplasia. We identified differential expression of FSCN1, CRMP1, NDRG1, DPYSL5, MAP4, FABP3, PRDX6 and PSAP in childhood cortical dysplasia patients, these proteins are involved in various processes and have various function. These results may provide new directions or targets for the research of childhood cortical dysplasia, and may be helpful in revealing molecular mechanisms and pathogenesis and/or pathophysiology of childhood cortical dysplasia if further investigated.

A genome landscape of SRSF3-regulated splicing events and gene expression in human osteosarcoma U2OS cells

Alternative RNA splicing is an essential process to yield proteomic diversity in eukaryotic cells, and aberrant splicing is often associated with numerous human diseases and cancers. We recently described serine/arginine-rich splicing factor 3 (SRSF3 or SRp20) being a proto-oncogene. However, the SRSF3-regulated splicing events responsible for its oncogenic activities remain largely unknown. By global profiling of the SRSF3-regulated splicing events in human osteosarcoma U2OS cells, we found that SRSF3 regulates the expression of 60 genes including ERRFI1, ANXA1 and TGFB2, and 182 splicing events in 164 genes, including EP300, PUS3, CLINT1, PKP4, KIF23, CHK1, SMC2, CKLF, MAP4, MBNL1, MELK, DDX5, PABPC1, MAP4K4, Sp1 and SRSF1, which are primarily associated with cell proliferation or cell cycle. Two SRSF3-binding motifs, CCAGC(G)C and A(G)CAGCA, are enriched to the alternative exons. An SRSF3-binding site in the EP300 exon 14 is essential for exon 14 inclusion. We found that the expression of SRSF1 and SRSF3 are mutually dependent and coexpressed in normal and tumor tissues/cells. SRSF3 also significantly regulates the expression of at least 20 miRNAs, including a subset of oncogenic or tumor suppressive miRNAs. These data indicate that SRSF3 affects a global change of gene expression to maintain cell homeostasis.

Microtubule-associated proteins as direct crosslinkers of actin filaments and microtubules

The cytoskeletal polymers--actin, microtubules, and intermediate filaments--are interlinked by coordinated protein interactions to form a complex three-dimensional cytoskeletal network. Association of actin filaments with microtubules is important for various cellular processes such as cell division, migration, vesicle and organelle transport, and axonal growth. Several proteins including signaling molecules, motor proteins, and proteins directly or indirectly associated with microtubules and actin are involved in bridging the cytoskeletal components. Microtubule-associated proteins (MAPs) belonging to the MAP1, 2, 4 family and Tau proteins have been identified as key players that directly crosslink the two cytoskeletons. This review summarizes the current understanding of the interactions of these MAPs with actin filaments and their role in forming the actin-microtubule network and further discusses how the in vitro reconstitution assays can be used to study the dynamics of coordinated networks. Understanding the mechanisms by which actin and microtubules interact is key to decipher cancer, wound healing, and neuronal regeneration.

Microtubule-associated protein 4 binds to actin filaments and modulates their properties

We previously reported that an isoform of microtubule-associated protein 4 (MAP4) is localized to the distal area of developing neurites, where microtubules are relatively scarce, raising the possibility that MAP4 interacts with another major cytoskeletal component, actin filaments. In the present study, we examined the in vitro interaction between MAP4 and actin filaments, using bacterially expressed MAP4 and its truncated fragments. Sedimentation assays revealed that MAP4 and its microtubule-binding domain fragments bind to actin filaments under physiological conditions. The apparent dissociation constant and the binding stoichiometry of the fragments to actin were about 0.1 µm and 1 : 3 (MAP4/actin), respectively. Molecular dissection studies revealed that the actin-binding site on MAP4 is situated at the C-terminal part of the proline-rich region, where the microtubule-binding site is also located. Electron microscopy revealed that the MAP4-bound actin filaments become straighter and longer and that the number of actin bundles increases with greater concentrations of added MAP4 fragment, indicating that MAP4 binding alters the properties of the actin filaments. A multiple sequence alignment of the proline-rich regions of MAP4 and tau revealed two putative actin-binding consensus sequences.

Characterization of zebrafish Esrom (Myc-binding protein 2) RCC1-like domain splice variants

PHR protein family consists of C. elegan Rpm-1/Drosophila Highwire/Zebrafish Esrom/Mouse Phr-1/Human Pam. Esrom is required for correct neurites exiting the paused state at intermediate targets as well as pteridine synthesis. This study reports the identification and characterization of two novel Esrom splice variants, named splice variants 2 (splicing out 5' 24 bp of exon 17) and 3 (splicing out 5' 24 bp of exons 17 and 18). Polypeptides encoded by 5' 24 bp of exons 17 and 18 are part of basic amino-acid-rich region inside Esrom RCC1-like domain (RLD). These two splice variants maintain the whole protein reading frame and alternative exons usage patterns are conserved with mammal. At different developmental stages and adult zebrafish tissues, abundances of these splice variants are different. Importantly, by yeast two-hybrid screen and confocal colocalization analysis, it was found that alternative splicing of exon 18 regulates Esrom RLD interaction with kinesin family member 22 and G protein beta-subunit 1. Taken together, these results suggest that Esrom RLD functions are regulated by alternative splicing at temporal and spatial-specific manner.

Differences in the regulation of microtubule stability by the pro-rich region variants of microtubule-associated protein 4

We have recently reported a neural variant of microtubule-associated protein 4 with a short pro-rich region (MAP4-SP). Here, we show that the neural MAP4 has reduced microtubule-stabilizing activity, compared to the ubiquitous MAP4 with a long pro-rich region (MAP4-LP), both in vitro and in vivo. Fluorescence recovery after photobleaching analyses revealed that the interaction of MAP4-SP with the microtubules is very rapid, with a half-time of fluorescence recovery of 7 +/- 2.36 s, compared to 19.5 +/- 3.03 s in case of MAP4-LP. The dynamic interaction of MAP4-SP with microtubules in neural cells may contribute to the dynamic behaviors of extending neurites.