Mini Review

TorsinA, microtubules and cell polarity

Giulia Ferrari ToninelliPierFranco SpanoMaurizio Memo

Department of Biomedical Sciences and Biotechnolo-gies, University of Brescia Medical School, Brescia, Italy

Reprint requests to: Prof. M. Memo,Dept of Biomedical Sciences and Biotechnologies,University of Brescia Medical School,Via Europa 11, 25123 Brescia, ItalyE-mail: memo@med.unibs.it

Summary

Early-onset primary dystonia is an inherited disordercharacterized by involuntary twisting, repetitive move-ments and abnormal postures. It has recently beendemonstrated that the DYT1 gene is the most relevantgene associated with primary generalized dystonia.The DYT1 gene product is a 332-aminoacid long pro-tein, termed TorsinA, whose function is still not clear.Based on the results obtained in other species, weproposed that TorsinA, similarly to OOC-5 in nema-todes, directs and/or stabilizes the subcellular localiza-tion of specific kinases, which may in turn phosphory-late microtubule associated proteins, such as tau. Inthis way, TorsinA may contribute to maintaining theappropriate site-directed polarization and control neu-rite outgrowth.

KEY WORDS: Drosophila, dystonia, nematodes, neuron, Tau pro -

teins.

Introduction

Dystonia is a neurological movement disorder charac-terized by involuntary muscle contractions, which forcecertain parts of the body into abnormal, sometimespainful, movements or postures. Dystonia can affectany part of the body including the arms and legs, trunk,neck, eyelids, face, or vocal cords. Features such ascognition, strength, and the senses, including visionand hearing are normal. While dystonia is not fatal, it isa chronic disorder and prognosis is difficult to predict. Itis the third most common movement disorder afterParkinson’s disease and tremor, affecting more than300,000 people in North America. Dystonia does notdiscriminate, but affects all races and ethnic groups. Early onset primary torsion dystonia (PTD) is an inherit-

ed disorder whose features are involuntary twisting,repetitive movements and abnormal postures. It onsetsduring childhood and adolescence, a life period charac-terized by elevated motor learning and synaptic plastici-ty: it usually starts monolaterally in an arm or leg andprogresses by spreading to other limbs and the trunk,but only occasionally to cranial muscles. Progression ofsymptoms continues over about a five-year period fromonset and then stabilizes.Little is known about the pathophysiology of this inherit-ed disorder, however knowledge of secondary dysto-nias and some recent studies on PTD patients havemade it possible to clarify at least some aspects of thedisease. Brain imaging studies revealed metabolic ab-normalities in the basal ganglia circuitry (1), probablyinvolving dopaminergic transmission. Moreover, a bio-chemical and autoradiography study showed a trend to-wards a reduction in D1 and D2 receptor binding inpostmortem striatum of dystonic brains (2).More progress has been made at molecular and cellu-lar level, mainly after identification and cloning of theDYT1 gene, the most relevant gene associated with pri-mary generalized dystonia (3). DYT1 gene product is a332-aminoacid long protein, termed TorsinA, which be-longs to the AAA+ superfamily of ATPases. The dis-ease is associated with a GAG deletion within the cod-ing region of the DYT1 gene, which results in the lossof one of a pair of glutamic acid residues in the C-termi-nus of the protein. The AAA+ superfamily of ATPases,which includes TorsinA and various additional proteinssuch as N-ethylmaleimide-sensitive factor, cytoplasmicdynein and heat shock proteins, represents a noveltype of molecular chaperone, which regulates the cor-rect folding and assembling of proteins, as well as theirdissassembling and degradation (4).

From humans to nematodes: OOC-5, the TorsinA homologous for C. elegans

Very little is known about the contribution of both wild-type and mutated TorsinA in functioning neurons. Aclue comes from identification, by cloning and databasesearch, of a number of TorsinA related proteins (torps)in various species (5). An interesting member of the“torp family” is the C. elegans homologue termed OOC-5. Like TorsinA, OOC-5 is endowed with ATPase activi-ty and belongs to the AAA+ superfamily. OOC-5 is es-sential for the establishment of cell polarity in the earlystages of cell division in the nematode embryo (6). Inparticular, this protein is critical for rotation of the nu-clear-centrosome complex and regulates the asymmet-ric distribution of a family of proteins termed PARs (par-titioning defective). These proteins are involved in di-recting the asymmetric cell division during embryogene-

Functional Neurology 2003; 18(1): 7-10 7

sis. In fact, mutations in the OOC-5 gene lead to alteredlocalization of PAR proteins and cause disruption ofoocyte formation (7).One member of this protein family, termed PAR-1, is aserine/threonine kinase highly conserved from C. ele -gans to Drosophila and mammals, that must be local-ized asymmetrically in the cells on order to carry out thephosphorylation of specific proteins necessary for thegeneration of cell polarity (8). In nematodes, PAR-1 islocalized at the posterior pole of the zygote and con-trols the partitioning of cell fate determinants (9). InDrosophila, par-1 is responsible for the oocyte differen-tiation and, later on, localizes to basolateral membranein the follicle cells and regulates the axis formation (10).In both these systems, the regulation of cell polarity byPAR-1 and its homologues, seems to exert its functionsthrough the regulation of microtubule dynamics or sta-bility (11,12). The mammalian homologues of PAR-1are a family of proteins named MARK.

From nematodes to humans: involvement ofMARKs and MAPs

Mammalian neurons, too, are highly polarized cells andthis peculiar feature is essential for all their functions,from the acquisition of information to the processingand release of neurotransmitters (13). The mechanismsby which neurons establish and maintain their polarityare not yet well understood. Nevertheless, a growingbody of evidence indicates that cytoskeletal compo-nents and organization play an important role in theseprocesses.It is notable that establishment of polarity in the mam-malian neuron, such as in nematodes, Drosophila andepithelial cells, requires an oriented distribution of thedifferent cytoskeleton regulating proteins. For instance,microtubule-associated proteins (MAPs) are differential-ly distributed in specific compartments of the neuronwith tau present only in the axon and MAP2 selectivelylocalized in dendrites (14,15). Furthermore, in neuronsas in the other organisms, these regulating proteinsare, in turn, triggered by specific kinases. A particulargroup of serine/threonine kinases, the MARK (MAP/mi-crotubule affinity regulating kinases) proteins, canphosphorylate the microtubule associated proteins tau,MAP4 and MAP2c on their microtubule binding domainin vitro (16) and can induce the detachment of this pro-tein by microtubules, leading to destabilization. A directaction of MARKs on neuronal polarity has been demon-strated; in fact, outgrowth of cell processes is blockedwhen these kinases are inactivated (17). Intriguingly,the MARK proteins are the mammalian homologues ofPAR-1. Therefore MARK kinases can regulate neuronalpolarity, just as PAR-1 can orient the embryo axis in ne-matodes and in D r o s o p h i l a, by an evolutionary con-served regulation of microtubule dynamics.

Is Tau the ultimate target of TorsinA activity inmammalian neurons?

Several data indicate the role of tau protein in axonalgrowth and in the establishment of neuronal polarity. In

mature neurons, the protein is specifically located in theaxon whereas it is absent in dendritic arbor (14,15) andinhibition of tau expression by means of an antisensetechnique revealed that this protein is essential for axongrowth (18). Tau function is regulated by phosphoryla-tion and this phosphorylation seems to be strongly as-sociated with cell polarity. In fact, during the period ofaxonogenesis in the nascent neuron, the protein ismore highly phosphorylated in the soma than in the ax-on. This proximo-distal gradient is dynamic and poten-tially regulated by upstream signals (19). Indeed, duringthe development of the rat brain there is a temporallydefined switch in the expression of tau isoforms, fol-lowed by a reduction in tau phosphorylation, suggestinga progressive reduction in neuronal plasticity and a re-inforcement of mature neuronal cytoarchitecture(20,21).Furthermore, although no neuropathological changeswere found in the brains of patients with generalizedDYT1 dystonia, some cases of primary focal diseasedisplay neuronal loss and the presence of neurofibril-lary tangles in the locus coeruleus and in the substantianigra pars compacta (22). In dystonia muscolorum (dt),a hereditary sensory neuropathy of the mouse charac-terized by progressive loss of limb coordination, Dolpe

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G. Ferrari Toninelli et al.

Fig. 1 - A) Expression of TorsinA in SH-SY5Y human neuronalcells. TorsinA is distributed, with a punctate pattern, in the cy-tosol and along the neurite axis (arrows). B) Detail of TorsinAlocalization in a neurite. Cells were cultured and differentiatedas previously described (27). Arrows indicate the discontinuouslocalization of the protein.

et al. (23) observed a marked reduction of microtubuleassociated proteins with accumulation of neurofila-ments in axonal swelling. Dystonia is also a commonmanifestation of at least two tauopathies, in which taupositive neurofibrillary pathology is the most predomi-nant neuropathological feature (24): progressivesupranuclear palsy (PSP) and cortical basal degenera-tion (CBD). In fact, unilateral limb dystonia and arm lev-itation have been reported in some cases of PSP (25),while dystonia, often accompanied by painful rigidityand fixed contractures, is a common symptom in theinitial phase of CBD (26).An intriguing possibility is that in neurons TorsinA, simi-larly to OOC-5 in nematodes, directs and/or stabilizesthe subcellular localization of MARKs/PAR1 kinases,which may in turn phosphorylate microtubule associat-ed proteins, such as tau. In this way, TorsinA may con-tribute to maintaining the appropriate site-directed po-larization and control neurite outgrowth.In conclusion, TorsinA appears to be involved in a cel-lular mechanism responsible for the management ofproper addressing and folding of specific proteins anddisappearance and/or alteration of these mechanismsmay result in protein aggregation and cell dysfunction.Thus, functional TorsinA seems to have a protective ca-pacity within cells. Solving the mystery of TorsinA’s cellular function mayhave important implications in dystonia as well asParkinson’s disease and other neurological disorders.

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TorsinA, microtubules and cell polarity

Fig. 2 - Hypothesis on possible functional role of TorsinA in theneuron. By a mechanism highly conserved from nematodes tohumans, AAA+ ATPases (OOC-5 and TorsinA) could direct thesubcellular localization of PAR-1/MARK kinases to phosphory-late target proteins involved in the dynamics and stability of mi-crotubules. In this way TorsinA, similarly to OOC-5 in C. ele -

g a n s, could participate in establishing and maintaining neu-ronal polarity.

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