cells Review Microtubule-Associated Proteins with Regulatory Functions by Day and Pathological Potency at Night Judit Oláh, Attila Lehotzky ,Sándor Szunyogh, Tibor Szénási, Ferenc Orosz and Judit Ovádi * Institute of Enzymology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, 1117 Budapest, Hungary; [email protected] (J.O.); [email protected] (A.L.); [email protected] (S.S.); [email protected] (T.S.); [email protected] (F.O.) * Correspondence: [email protected]; Tel.: +36-1-3826-714 Received: 13 December 2019; Accepted: 2 February 2020; Published: 4 February 2020 Abstract: The sensing, integrating, and coordinating features of the eukaryotic cells are achieved by the complex ultrastructural arrays and multifarious functions of the cytoskeleton, including the microtubule network. Microtubules play crucial roles achieved by their decoration with proteins/enzymes as well as by posttranslational modifications. This review focuses on the Tubulin Polymerization Promoting Protein (TPPP/p25), a new microtubule associated protein, on its “regulatory functions by day and pathological functions at night”. Physiologically, the moonlighting TPPP/p25 modulates the dynamics and stability of the microtubule network by bundling microtubules and enhancing the tubulin acetylation due to the inhibition of tubulin deacetylases. The optimal endogenous TPPP/p25 level is crucial for its physiological functions, to the differentiation of oligodendrocytes, which are the major constituents of the myelin sheath. Pathologically, TPPP/p25 forms toxic oligomers/aggregates with α-synuclein in neurons and oligodendrocytes in Parkinson’s disease and Multiple System Atrophy, respectively; and their complex is a potential therapeutic drug target. TPPP/p25-derived microtubule hyperacetylation counteracts uncontrolled cell division. All these issues reveal the anti-mitotic and α-synuclein aggregation-promoting potency of TPPP/p25, consistent with the finding that Parkinson’s disease patients have reduced risk for certain cancers. Keywords: microtubule ultrastructure; TPPP; mitosis inhibition; tubulin acetylation; cancer; Parkinson’s disease; inclusion; drug target 1. Tubulin Polymerization Promoting Protein (TPPP/p25), A New Microtubule Regulatory Protein 1.1. Major Characteristics of the TPPP Family Microtubules are assembled from the highly conserved dimers of α- and β-tubulin and constitute the microtubule network, which is one of the major components of the eukaryotic cytoskeleton [1,2]. Microtubules are essential for cell polarity, cell shape, differentiation, and intracellular transport as well as for building of the mitotic spindles, which are the key structures in chromosome segregation during cell division. Microtubule systems can carry out a wide variety of functions at both physiological and pathological conditions that raises the following question: How can these evolutionarily-conserved proteins display multifarious functions? In living cells, microtubules interact with specific sets of microtubule-associated proteins (MAPs), including molecular motors, proteins regulating microtubule dynamics and stability and enzymes catalyzing metabolic and signaling pathways [3]. In fact, the specialized microtubule functions are determined to a large extent by the subset of partner proteins. A brain-specific protein, p25, was discovered decades ago [4,5], which was later denoted as Tubulin Polymerization Promoting Protein (TPPP/p25) and identified as a new MAP as well as a hallmark of Parkinson’s disease (PD) and other synucleinopathies [6,7]. This protein modulates the Cells 2020, 9, 357; doi:10.3390/cells9020357 www.mdpi.com/journal/cells Cells 2020, 9, 357 2 of 21 Cells 2020A brain-specific, 9, 357 protein, p25, was discovered decades ago [4,5], which was later denoted2 of as21 Tubulin Polymerization Promoting Protein (TPPP/p25) and identified as a new MAP as well as a A brain-specific protein, p25, was discovered decades ago [4,5], which was later denoted as Cellshallmark2020, 9, of 357 Parkinson’s disease (PD) and other synucleinopathies [6,7]. This protein modulates2 of the 22 Tubulindynamics Polymerization and stability of Pr theomoting microtubule Protein system, (TPPP/p25) aligns and along identified the microtubule as a new network MAP as and well forms as a hallmarkdistinct microtubule of Parkinson’s ultrastructures disease (PD) depending and other on synucleinopathie its intracellulars level [6,7]. [8] This (Figure protein 1). modulates the dynamicsdynamics andand stabilitystability ofof thethe microtubulemicrotubule system,system, alignsaligns alongalong thethe microtubulemicrotubule networknetwork andand formsforms distinctdistinct microtubulemicrotubule ultrastructuresultrastructures dependingdepending onon itsits intracellularintracellular level level [ 8[8]] (Figure (Figure1 ).1). Figure 1. Tubulin Polymerization Promoting Protein (TPPP/p25)-derived ultrastructural organization Figure 1. Tubulin Polymerization Promoting Protein (TPPP/p25)-derived ultrastructural organization of of microtubules. Images of microtubules without or with human recombinant TPPP/p25: (A) control, microtubules. Images of microtubules without or with human recombinant TPPP/p25: (A) control, (B) Figure(B) TPPP/p25-induced 1. Tubulin Polymerization microtubule Promoting bundling, Protein as visu (TPPP/alizedp25)-derived by electron ultrastructural microscopy. organization HeLa cells TPPP/p25-induced microtubule bundling, as visualized by electron microscopy. HeLa cells expressing ofexpressing microtubules. pEGFP-TPPP/p25 Images of microtubules (green): (C without) control, or ( Dwith) aggresome; human recombinant (E) perinuclear TPPP/p25: cage; (AHeLa) control, cells pEGFP-TPPP/p25 (green): (C) control, (D) aggresome; (E) perinuclear cage; HeLa cells transiently (transientlyB) TPPP/p25-induced transfected withmicrotubule EGFP-TPPP/p25 bundling, (green): as visu (F)alized different by expressionelectron microscopy. levels produce HeLa distinct cells transfected with EGFP-TPPP/p25 (green): (F) different expression levels produce distinct microtubule expressingmicrotubule pEGFP-TPPP/p25 (red) ultrastructures: (green): (1) alignment (C) control, to the (D microtubule) aggresome; network, (E) perinuclear (2) aggresome cage; HeLaformation, cells (red) ultrastructures: (1) alignment to the microtubule network, (2) aggresome formation, (3) perinuclear transiently(3) perinuclear transfected cage as with visualized EGFP-TPPP/p25 by immunofluorescence (green): (F) different microscopy expression (selected levels from produce [9] and distinct [8]). cage as visualized by immunofluorescence microscopy (selected from [9] and [8]). Scale bar: 100 nm microtubuleScale bar: 100 (red) nm (ultrastructures:A,B); 10 μm (C –(1F)). alignment to the microtubule network, (2) aggresome formation, (A,B); 10 µm (C–F). (3) perinuclear cage as visualized by immunofluorescence microscopy (selected from [9] and [8]). TPPPScaleTPPP/p25 /bar:p25 100 is is an nman intrinsically (Aintrinsically,B); 10 μm disordered ( Cdisordered–F). protein protei withoutn without a well-defined a well-defined 3D structure, 3D structure, whose middle, whose highlymiddle, flexible highly CORE flexible region CORE is region straddled is straddled by the unstructured by the unstructured N- and C-termini N- and C-termini [10]. Specific [10]. bindingSpecific TPPP/p25 is an intrinsically disordered protein without a well-defined 3D structure, whose segmentsbinding segments in the disordered in the disordered TPPP/p25 TPPP/p25 have been have identified, been identified, such as a zincsuch finger as a zinc motif, finger a consensus motif, a middle, highly flexible CORE region is straddled by the unstructured N- and C-termini [10]. Specific GTP-bindingconsensus GTP-binding domain and domain phosphorylation and phosphorylation sites ([11] and sites references ([11] and therein) references (Figure therein)2). (Figure 2). binding segments in the disordered TPPP/p25 have been identified, such as a zinc finger motif, a consensus GTP-binding domain and phosphorylation sites ([11] and references therein) (Figure 2). FigureFigure 2.2. SchemeScheme ofof thethe humanhuman TPPPTPPP/p25,/p25, TPPP3,TPPP3, andand TPPP2.TPPP2. Similar and didifferentfferent residuesresidues areare indicatedindicated byby darkdark blueblue andand red,red, respectively.respectively. TheThe zinczinc fingerfinger motif,motif, aa consensusconsensus GTP-bindingGTP-binding segmentsegment andFigureand phosphorylationphosphorylation 2. Scheme of sitesthesites human onon TPPPTPPP/p25 TPPP/p25,/p25are are also also TPPP3, indicated. indicated. and TPPP2. Similar and different residues are indicated by dark blue and red, respectively. The zinc finger motif, a consensus GTP-binding segment InandIn vitrovitro, phosphorylation, TPPP TPPP/p25/p25 is sitesis capable capable on TPPP/p25 of inducingof inducing are thealso polymerization theindicated. polymerization of tubulin of tubulin into normal, into normal, double-walled, double- andwalled, bundled and bundled microtubules microtubules [6]. At cellular [6]. At level,cellular TPPP level,/p25 TPPP/p25 co-localizes co-localizes with the microtubulewith the microtubule network innetwork variousIn vitro, in cell variousTPPP/p25 lines, andcell is itscapablelines, bundling andof inducing activityits bundling the protects polymerization activity the microtubules protects of tubulin the against microtubules into depolymerizingnormal, double-against agentswalled,depolymerizing [ 6and,8].