Drosophila spichthyin inhibits BMP signaling and regulates synaptic growth and axonal microtubules

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To understand the functions of NIPA1, mutated in the neurodegenerative disease hereditary spastic paraplegia, and of ichthyin, mutated in autosomal recessive congenital ichthyosis, we have studied their Drosophila melanogaster ortholog, spichthyin (Spict). Spict is found on early endosomes. Loss of Spict leads to upregulation of bone morphogenetic protein (BMP) signaling and expansion of the neuromuscular junction. BMP signaling is also necessary for a normal microtubule cytoskeleton and axonal transport; analysis of loss- and gain-of-function phenotypes indicate that Spict may antagonize this function of BMP signaling. Spict interacts with BMP receptors and promotes their internalization from the plasma membrane, implying that it inhibits BMP signaling by regulating BMP receptor traffic. This is the first demonstration of a role for a hereditary spastic paraplegia protein or ichthyin family member in a specific signaling pathway, and implies disease mechanisms for hereditary spastic paraplegia that involve dependence of the microtubule cytoskeleton on BMP signaling.

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Figure 1: NIPA1 homologs and Drosophila spict gene.
Figure 2: Drosophila spict expression and localization.
Figure 3: spict null mutants cause BMP-dependent NMJ overgrowth.
Figure 4: Spict regulates microtubules by inhibiting BMP signaling.
Figure 5: Spict overexpression and tkv mutants impair fast axonal transport.
Figure 6: Spict overexpression and reduction of BMP signaling impairs axonal transport by disrupting the microtubule cytoskeleton.
Figure 7: Involvement of Spict in BMP signaling.

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We thank H. Aberle, D. Coulson, J. Drummond, V. Korolchuk, S. Sweeney and the Bloomington and Szeged Drosophila Stock Centers for fly stocks, S. Eaton, G. Marqués, M. O'Connor, J. Rocha, K. Wang and F. Wirtz-Peitz for DNA constructs, M. González-Gaitán, H. Krämer, T. Littleton, S. Sweeney, P. ten Dijke, A. Tolkovsky and the Developmental Studies Hybridoma Bank for antibodies, and L. Masuda-Nakagawa, D. Rubinsztein and O'Kane lab members for helpful discussions. X.W. was supported by scholarships from Cambridge Overseas Trust and a UK Government Overseas Research Studentship award. H.T.H.T. was supported by Tom Wahlig Stiftung, Croucher Foundation, Cambridge Overseas Trust and a UK Government Overseas Research Studentship award. E.R. is a Wellcome Trust Advanced Clinical Fellow. C.J.O'K. was supported by a Research Development Fellowship from the Biotechnology and Biological Sciences Research Council.

Author information

X.W. performed most experiments, analyzed data and wrote the manuscript; W.R.S. performed crosses to generate a null mutant; H.T.H.T. designed S2 redistribution experiments; E.R. designed S2 redistribution experiments and supervised the project; C.J.O'K. analyzed data, wrote the manuscript and supervised the project.

Correspondence to Cahir J O'Kane.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Fig. 1

Localization of tagged Spict. (PDF 393 kb)

Supplementary Fig. 2

Topology of Spict in S2 cells. (PDF 394 kb)

Supplementary Fig. 3

spict null mutants cause synaptic overgrowth at the NMJ. (PDF 563 kb)

Supplementary Fig. 4

BMP signaling is required for synaptic overgrowth in spictmut, and for microtubule stability and axonal transport. (PDF 409 kb)

Supplementary Fig. 5

Involvement of Spict in BMP signaling. (PDF 523 kb)

Supplementary Video 1

Time-lapse analysis of Syt-eGFP vesicles in a segmental nerve of a wild type larva carrying OK6-GAL4. Movies are played back in real time (2 frames per second). Scale bar is 10 μm. Anterograde movement is to the right, retrograde to the left. Both anterograde and retrograde movements can be observed. (MOV 84 kb)

Supplementary Video 2

Time-lapse analysis of Syt-eGFP puncta in a segmental nerve of a tkv (tkv7/tkv16713) larva carrying OK6-GAL4. Movies are played back in real time (2 frames per second). Scale bar is 10 μm. Anterograde movement is to the right, retrograde to the left. Large nonmotile bright aggregates are observed within the nerve. (MOV 82 kb)

Supplementary Video 3

Time-lapse analysis of Syt-eGFP vesicles in a segmental nerve of a spictmut larva carrying OK6-GAL4. Movies are played back in real time (2 frames per second). Scale bar is 10 μm. Anterograde movement is to the right, retrograde to the left. Both anterograde and retrograde movements can be observed. (MOV 132 kb)

Supplementary Video 4

Time-lapse analysis of Syt-eGFP puncta in a segmental nerve of a larva overexpressing Spict under control of OK6-GAL4. Movies are played back in real time (2 frames per second). Scale bar is 10 μm. Anterograde movement is to the right, retrograde to the left. Large nonmotile bright aggregates are observed within the nerve. (MOV 221 kb)

Supplementary Methods (PDF 101 kb)

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