TUBLIN CODE

The Tubulin Code; see reference for complete report.

“>> ABSTRACT
Microtubules are key cytoskeletal elements of all eukaryotic cells and are assembled of evolutionarily conserved α-tubulin–β-tubulin heterodimers. Despite their uniform structure, microtubules fulfill a large diversity of functions. A regulatory mechanism to control the specialization of the microtubule cytoskeleton is the ‘tubulin code’, which is generated by (i) expression of different α- and β-tubulin isotypes, and by (ii) post-translational modifications of tubulin. In this Cell Science at a Glance article and the accompanying poster, we provide a comprehensive overview of the molecular components of the tubulin code, and discuss the mechanisms by which these components contribute to the generation of functionally specialized microtubules.

Introduction
Microtubules (MTs) are the largest filamentous components of the eukaryotic cytoskeleton and are essential for every cell as they control cell shape, division, motility and differentiation. MTs fulfill many of their functions by forming specific assemblies, such as the mitotic spindle to separate the chromosomes during cell division, and the axoneme to form cilia and flagella. MTs are dynamically assembled from evolutionarily highly conserved heterodimers of α- and β-tubulin. Considering the extraordinary conservation of α- and β-tubulins, one of the key challenges is to understand how these filaments can adapt to a huge variety of functions. MTs can functionally specialize by interacting with a variety of MT-associated proteins (MAPs). These proteins can regulate MT dynamics by either stabilizing or destabilizing them, and can generate forces (motor proteins) or connect MTs to other cellular structures, such as membranes or other cytoskeletal components. Furthermore, MTs can themselves be programmed by the ‘tubulin code’ – a combination of the differential expression of α- and β-tubulin genes (tubulin isotypes) and a plethora of post-translational modifications (PTMs) – to exert specific functions as presented in the accompanying poster. Here, we review the components and mechanisms of the tubulin code, and briefly discuss their potential role in controlling MT functions.

Functions of the tubulin code in health and disease

The tubulin code is expected to adapt MTs to specialized cellular functions. So far, the incorporation of specific tubulin isotypes has been found in a few specialized MT structures and cells. In mammals, for instance, TubB1 is expressed exclusively in platelets and megakaryocytes (Wang et al., 1986), and is essential for their function (see Box 1). Apart from TubB1, TubB3 and TubB4 have also been found in specific tissues and structures – TubB3 is most prominent in neurons (Denoulet et al., 1986; Joshi and Cleveland, 1989; Lewis et al., 1985), whereas TubB4 is particularly enriched in the axonemes of cilia and flagella (Renthal et al., 1993). A striking example of isotype specialization is found in the nematode C. elegans. In this organism, MTs usually comprise 11 protofilaments, whereas in touch receptor neurons, the tubulin isotypes MEC-12 (α-tubulin) and MEC-7 (β-tubulin) assemble into 15-protofilament MTs that are essential for the function of these neurons (Fukushige et al., 1999; Lockhead et al., 2016; Savage et al., 1989).

Considering the specialized functions of certain tubulin isotypes, changes in their expression levels could influence the properties of the MTs and thus alter MT functions in cells. For instance, differential isotype expression has been observed in various cancers (reviewed in Parker et al., 2014) and could be involved in rendering these cancers more resistant to therapeutic drugs (Kamath et al., 2005; Leandro-Garcia et al., 2012; Yang et al., 2016).

Similar to changes in isotype expression, point mutations in tubulin isotypes could alter the properties and, thus, the functions of MTs. Indeed, tubulin mutations are linked to a wide spectrum of human pathologies (see Box 1).

Tubulin PTMs are differentially distributed on functionally distinct MTs and are mostly enriched on stable long-lived MTs, such as neuronal, axonemal and centriolar MTs. Furthermore, detyrosination preferentially occurs on a subset of MTs in the mitotic spindle (Geuens et al., 1986; Gundersen and Bulinski, 1986), as well as on neuronal MTs (Brown et al., 1993; Cambray-Deakin and Burgoyne, 1987; Robson and Burgoyne, 1989). Deregulation of the detyrosination–tyrosination cycle has been shown to influence tumorigenesis (Kato et al., 2004; Lafanechere et al., 1998; Mialhe et al., 2001; Souček et al., 2006), affect neuronal differentiation (Erck et al., 2005; Marcos et al., 2009) and impede proper chromosome segregation during mitosis (Barisic et al., 2015). Furthermore, detyrosination is an important regulator of cardiac muscle function (Kerr et al., 2015; Robison et al., 2016).

The specific role of K40 acetylation is not yet fully understood. Acetylation has been associated with stable MTs and thus used as a marker for MT stability. So far, it has been shown to have a role in the maturation of megakaryocytes and platelet formation (Iancu-Rubin et al., 2012; Sadoul et al., 2012) and to be essential for touch-sensing in mice (Morley et al., 2016) and C. elegans (Topalidou et al., 2012).

Glutamylation occurs on neuronal MTs during neuronal differentiation (Audebert et al., 1993, 1994). Balanced levels of polyglutamylation in neurons play an essential role in neuronal survival (Rogowski et al., 2010). Glutamylation is further enriched on mitotic spindles and midbodies (Bobinnec et al., 1998b; Lacroix et al., 2010), where it could have a role in the control of the cell cycle. Centrioles and basal bodies are hotspots of polyglutamylation (Bobinnec et al., 1998b; Geimer et al., 1997), and blocking this PTM with anti-glutamylation antibodies results in the disassembly of centrioles (Bobinnec et al., 1998a). Moreover, polyglutamylation is prominent on ciliary and flagellar axonemes (Bré et al., 1994; Fouquet et al., 1994; O’Hagan et al., 2011), where it regulates the beating behavior and integrity of these organelles (reviewed in Konno et al., 2012). In contrast to glutamylation, glycylation has so far been exclusively observed on axonemal MTs (Bré et al., 1996; Redeker et al., 1994; Rüdiger et al., 1995; Weber et al., 1996; Xia et al., 2000) and has been implicated in the mechanical stabilization of the axoneme (Pathak et al., 2011; Rogowski et al., 2009; Wloga et al., 2009).

Conclusions and perspectives

After decades of research, the mechanisms and functions of the ‘tubulin code’ have only just begun to be unraveled. Some insights at the organism level have already been gained as knockout mice for tubulin-modifying enzymes show a variety of phenotypes, and tubulin mutations (see Box 1) are found in a range of human pathologies. To determine the roles of the tubulin code at the cellular level, it will be important to establish cell biology approaches that are sensitive enough to reveal the impact of subtle alterations in MT behavior. Another great challenge in the field is the development of methods to produce tubulin ‘à la carte’ – i.e. recombinant tubulin with controlled PTMs – in order to study the mechanisms of the tubulin code in vitro. The first exciting advances in this direction have recently been made (Barisic et al., 2015; Minoura et al., 2013; Pamula et al., 2016; Sirajuddin et al., 2014; Vemu et al., 2016).

Taken together, it appears that the tubulin code controls physiological processes through a plethora of mechanisms. Disruption of these processes can lead to diseases such as ciliopathies, cancer and neurodegeneration. Thus, understanding the molecular mechanisms of the tubulin code and their impact on physiology is the key challenge for the coming years.<<" [The tubulin code at a glance by Sudarshan Gadadhar, Satish Bodakuntla, Kathiresan Natarajan, Carsten Janke J Cell Sci 2017 130: 1347-1353; doi: 10.1242/jcs.199471]

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