SPAST and hereditary spastic paraplegia: We examined whether spastin depletion affected the polarisation of BMP receptors, as this pathway has been implicated in HSP; cellular or in vivo animal models of multiple HSP subtypes have demonstrated upregulated BMP signalling, and axonal abnormalities in vertebrate and invertebrate HSP models have been rescued by inhibiting this upregulated signalling [22, 45–50].