Pathogenic TFG Mutations Underlying Hereditary Spastic Paraplegia Impair Secretory Protein Trafficking and Axon Fasciculation

Abstract

SUMMARYLength-dependent axonopathy of the corticospinal tract causes lower limb spasticity and is characteristic of several neurological disorders, including hereditary spastic paraplegia (HSP) and amyotrophic lateral sclerosis. Mutations in Trk-fused gene (TFG) have been implicated in both diseases, but the pathomechanisms by which these alterations cause neuropathy remain unclear. Here, we biochemically and genetically define the impact of a mutation within the TFG coiled-coil domain, which underlies earlyonset forms of HSP. We find that the TFG (p.R106C) mutation alters compaction of TFG ring complexes, which play a critical role in the export of cargoes from the endoplasmic reticulum (ER). Using CRISPR-mediated genome editing, we engineered human stem cells that express the mutant form of TFG at endogenous levels and identified specific defects in secretion from the ER and axon fasciculation following neuronal differentiation. Together, our data highlight a key role for TFG-mediated protein transport in the pathogenesis of HSP.

COPII‐mediated trafficking at the ER/ERGIC interface

“…Alternatively, modification of TFG could enable the recruitment of a binding partner, similar to that brought in by Sec23A or Sec24C, to further support large carrier transport. Consistent with this idea, depletion studies suggest that TFG plays a particularly critical role in the anterograde movement of large COPII cargoes …”

“…These data suggest an alternative mechanism for neurodegeneration in HSP patients harboring TFG mutations. Consistent with this idea, stem‐cell‐derived neurons harboring the TFG (p.R106C) mutation exhibit a defect in axon pathfinding, likely caused by impaired trafficking of high molecular weight adhesion molecules that normally mediate axon bundling and their maintenance . Other mutations that affect the disordered domain of TFG have been implicated in late onset sensory neuropathies .…”

“…Although deletion of TFG results in early embryo lethality, point mutations have been identified in several patients suffering from various forms of neurological disease . Analysis of these mutant alleles has underscored the importance of the TFG ring‐like architecture in regulating COPII‐mediated cargo transport.…”

“…Analysis of these mutant alleles has underscored the importance of the TFG ring‐like architecture in regulating COPII‐mediated cargo transport. Specifically, mutations in the PB1 domain or the coiled coil domain underlie early onset forms of complicated hereditary spastic paraplegia (HSP) and result in defective ring assembly without impairing the ability of TFG to homo‐oligomerize . Although mutations in other factors that form liquid droplets in neurons have been suggested to promote their aggregation leading to neuronal toxicity, localization studies indicate that amino‐terminal mutations in TFG cause it to become more diffusely distributed .…”

ER-to-Golgi Trafficking and Its Implication in Neurological Diseases

“…The conditional knockout of cTAGE5 in murine brains leads to severe developmental defects and impaired dendrite outgrowth [38]. Additionally, inactivating TFG in mice is embryonic-lethal, and TFG point mutations have been identified in several patients with neurological diseases, including HSP, Charcot-Marie-Tooth disease, and hereditary motor and sensory neuropathy [86][87][88][89][90]. Neurons derived from induced pluripotent stem cells with the TFG mutation R106C show defective axon pathfinding, decreased ER protein export, and elevated levels of ER stress [86].…”