Hereditary spastic paraplegia (HSP) comprises a heterogeneous group of neurological disorders. So far several loci, known as SPG loci, have been identified (reviewed in Casari and Rugarli 2001). The autosomal dominant form of hereditary spastic paraplegia (AD-HSP) is a neurodegenerative disorder characterized by progressive spasticity, mainly of the lower limbs. The disease in 40% of families with the pure form of AD-HSP is linked to SPG4 on chromosome 2p21-p22 due to mutations of a gene encoding spastin. This gene is expressed ubiquitously in fetal and adult human tissues (Hazan et al. 1999). The highest expression levels are found in the brain, with selective expression in the cortex and striatum. In the spinal cord, spastin is expressed exclusively in nuclei of motor neurons, suggesting that the strong neurodegenerative defects observed in patients are caused by a primary defect of spastin in neurons (Charvin et al. 2003). The human spastin gene encodes a predicted 616-amino-acid long protein and is a member of the large family of proteins with an AAA domain (ATPases Associated with diverse cellular Activities). When human spastin was identified, no clear invertebrate homologs were known, not even in the complete genome sequence of Caenorhabditis elegans; indeed there are a confusing number of similar sequences in humans, C. elegans, yeast and even prokaryotes.

With the availability of the first draft of the genome sequence of Drosophila (Adams et al. 2000) a predicted gene product, CG5977, with much higher sequence similarity to the human spastin than any other AAA domain protein, became evident. Our analysis revealed that the initial prediction, based on the genomic sequence contained errors, that a first version based on a partial cDNA clone (LD23843) was lacking the N-terminus, and that EST clones existed which appeared to extend the similarity with human spastin from the N- to the C-terminus. We sequenced one such clone, AT01057 (Fig. 1A). We also used this clone in order to prepare a probe for in situ hybridization. Subsequently AT01057 was also sequenced by the Berkeley Drosophila Genome Project. The two sequences are identical and code for a predicted protein of 758 amino acids with extended sequence similarity to human spastin, which is therefore called Drosophila spastin (spas).

Fig. 1A–C.
figure 1

Sequence analysis of Drosophila spastin. A Comparison of human and Drosophila spastin homologs reveals three regions of increased sequence conservation, the extended transmembrane domain TM+, the MIT (microtubule interacting and trafficking) and the AAA (ATPases associated with diverse cellular activities) domains, respectively. B Human, mouse and Drosophila spastin are the only proteins in current databases with precisely this domain structure. The fidgetin and katanin subfamilies belong to the AAA family, but lack transmembrane or MIT domains. The VPS4 subfamily lacks transmembrane domains. Spartin (SPG20) contains a MIT domain with 40% of sequence identity to spastin, but does not belong to the AAA family. Paraplegin (SPG7) belongs to a very different subgroup of the numerous AAA subfamilies with only 35% of sequence identity in the AAA domain with spastin and contains unrelated transmembrane domains and a peptidase domain. C A phylogenetic analysis based on the AAA domain of representative proteins reveals that spastins and fidgetins are most similar in the AAA domain, but not in the overall structure, while VPS4 proteins are less similar in the AAA domain although they contain a MIT domain and are highly conserved from yeast to humans. Spastin and paraplegin are single genes in their subfamily in Drosophila and humans, while fidgetin, katanin and VPS4 are represented by two paralogs in humans

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Closer inspection of the Drosophila sequence and of the alignment of Drosophila and human spastin reveals, besides the two known highly conserved domains [namely the AAA domain (70% identity) of 300 residues, and the MIT (microtubule interacting and trafficking molecules, Ciccarelli et al. 2003) domain (55% identity) of about 100 residues], an additional conserved region of 33 residues near the N-terminus (Fig. 1A). All three domains are also recognized with SMART domain analysis (Letunic et al. 2002) as AAA, MIT and as a putative transmembrane domain, respectively. Since the similarity in the N-terminal domain extends beyond the hydrophobic core that could be a transmembrane domain, we call this a TM+ domain. This domain appears to be specific for spastin proteins. MIT domains, and to a comparable degree AAA domains, also appear in unrelated proteins; one was discovered in spartin, the gene responsible for SPG20 (Patel et al. 2002). The association of MIT and AAA domains, however, appears to be evolutionarily old since even the highly conserved vacuolar protein sorting 4 (VPS4) subfamily shows this domain structure. The members of the AAA family most closely related to spastin, however, are not VPS4 but members of the fidgetin subfamily (Fig. 1C), which may be of neurological interest because of the shaking phenotype in the fidget mouse (Cox et al. 2000). Paraplegin, the gene responsible for SPG7, however, is only distantly related to spastin, and the paraplegin protein is localized to the mitochondria and contains a peptidase domain (Casari et al. 1998; Fig. 1B, C). Since spastin was already shown not to be targeted to the mitochondria, but appears to be localized to the nucleus (Charvin et al. 2003), it will be important to test whether the potential transmembrane domain with the highly conserved neighbouring residues could be involved in localization of spastin to the nuclear membrane system. Other studies using expression systems of wild-type and mutated spastin with reporter sequences in different cell lines have suggested an involvement of spastin in microtubule dynamics (Errico et al. 2002).

The spatial and temporal expression patterns of Drosophila spastin were analysed by in situ hybridization. For this, the EST clone AT01057 that covers spastin was used to generate digoxigenin-labeled sense and antisense RNA probes (Tautz and Pfeifle 1989). These were hybridized to wild-type whole-mount embryos. Maternally contributed spastin transcripts characterized early embryogenesis with high expression levels seen until blastoderm stage (Fig. 2A). At the cell formation stage, expression was strongest near the basal part of the cell layer underlying the surface (Fig. 2B). During germband extension and stomodeal plate formation, expression was seen in the ventral head and trunk ectoderm, as well as in cells near the cephalic furrow and in the invaginating hindgut and midgut primordia (Fig. 2C, D). After germband retraction and delamination of neuroblasts at stage 13, transcripts were observed in subsets of cells in all neuromeres of the CNS including those of the supraesophageal and subesophageal ganglia (Fig. 2E). In later embryonic stages, marked expression of spastin was observed in cell clusters throughout the supraesophageal ganglion (Fig. 2F, G, I, K, arrowheads), with pronounced expression also seen in the subesophageal ganglion (Fig. 2I, bracket). In the ventral nerve cord (VNC), transcripts were seen in two broad longitudinal stripes located laterally, and weaker expression was observed in some midline cells (Fig. 2H, L, asterisks). In addition to expression in the CNS, spastin transcripts were also observed in some sensory head organs of the peripheral nervous system (PNS), most probably the Bolwig's organs and/or the dorsal organs (Fig. 2F, G, I, K, arrows).

Fig. 2A–L.
figure 2

Embryonic expression of Drosophila spastin. Whole-mount in situ hybridization of spastin transcripts in wild-type embryos. A-F, I Lateral views; G, K dorsal views; H, L ventral views, anterior is to the left. A Preblastoderm stage, B blastoderm stage, C, D stage 9, different focal planes, E stage 13, F-L stage 16–17 embryos; stages according to Campos-Ortega and Hartenstein (1997). I, K, L Enlarged views. Arrowheads indicate the expression in cell clusters of the supraesophageal ganglion; bracket expression in the subesophageal ganglion; asterisks expression in some midline cells of the VNC; arrows expression in some sensory head organs of the PNS

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Thus, during embryogenesis, expression of Drosophila spastin is mainly restricted to the central nervous system, in contrast to the ubiquitous expression of the vertebrate spastin gene (Hazan et al. 1999).