Inhibition of TFG function causes hereditary axon degeneration by impairing endoplasmic reticulum structure

Abstract

Hereditary spastic paraplegias are a clinically and genetically heterogeneous group of gait disorders. Their pathological hallmark is a length-dependent distal axonopathy of nerve fibers in the corticospinal tract. Involvement of other neurons can cause additional neurological symptoms, which define a diverse set of complex hereditary spastic paraplegias. We present two siblings who have the unusual combination of early-onset spastic paraplegia, optic atrophy, and neuropathy. Genome-wide SNP-typing, linkage analysis, and exome sequencing revealed a homozygous c.316C>T (p.R106C) variant in the Trk-fused gene (TFG) as the only plausible mutation. Biochemical characterization of the mutant protein demonstrated a defect in its ability to self-assemble into an oligomeric complex, which is critical for normal TFG function. In cell lines, TFG inhibition slows protein secretion from the endoplasmic reticulum (ER) and alters ER morphology, disrupting organization of peripheral ER tubules and causing collapse of the ER network onto the underlying microtubule cytoskeleton. The present study provides a unique link between altered ER architecture and neurodegeneration.membrane trafficking | COPII-mediated secretion | ER exit site H ereditary spastic paraplegias (HSPs) are a diverse group of disorders characterized by spastic weakness in the lower extremities, which results from degeneration of upper motoneuron axons in the corticospinal tract (1, 2). Based on the presence or absence of other neurological abnormalities, HSPs are classified as complicated or pure, respectively. In addition to lower-limb spasticity, complicated forms of HSP may be associated with ataxia, mental retardation, dementia, extrapyramidal signs, visual dysfunction, and/or epilepsy. HSPs are typically progressive, but age of onset is highly variable. The heterogeneity in clinical presentation is accompanied by genetic heterogeneity. To date, more than 40 different genetic loci, which include nearly all modes of inheritance, have been linked to HSPs (1-5). However, in nearly half of these cases, the identities of the causative genes remain unknown. The identification of additional HSP genes and, more importantly, the functional characterization of their encoded products, will both contribute to our understanding of the pathomechanisms underlying HSPs and reveal the general requirements for lifelong axonal maintenance.More than half the HSP cases in North America and Northern Europe can be attributed to defects in organelle dynamics (6). In particular, mutations that impact the architecture of the endoplasmic reticulum (ER) are common in patients with HSP. The ER is comprised of a network of membrane tubules and sheetlike cisternae that extend throughout the cytoplasm and encase the nucleus (7-9). Several factors contribute to this architecture, including (i) membrane-bending proteins of the REEP and reticulon families; (ii) regulators of the microtubule cytoskeleton, which governs the spatial patterning of the ER network; and (iii) components of the ear...

Hereditary spastic paraplegias: identification of a novel SPG57 variant affecting TFG oligomerization and description of HSP subtypes in Sudan

“…They presented with signs of progressive pyramidal tract involvement. In contrast to the SPOAN-like HSP reported by Beetz et al, 18 patients in this family with a second novel variant (c.64C4T) had intact optic nerve and normal fundi. The electrophysiological studies confirmed variable degrees of motor neuronal demyelination in addition to motor axonal degeneration matching the previous report.…”

“…19 On the contrary, variants in the amino terminus result in AR HSP. 18 In this study, an amino terminus variant resulted in AR HSP as well. Significant clinical differences were observed between the first reported SPG57 c.316C4T variant (p.(Arg106Cys)) and the variant c.64C4T (p. (Arg22Trp)) reported here.…”

“…Structural studies indicate that full length TFG self-assembles into octameric ring-like complexes in solution. 18 Our analysis of the PB1 domain indicates that it is likely to be responsible for the octameric self-assembly, as it exhibits a similar propensity to form octamers. Thus, the coiled-coil domain may play another distinct role in promoting TFG complex assembly, potentially facilitating its ring-like conformation, which is likely under considerable strain.…”

“…Light-scattering data were collected as described previously. 18 In brief, purified proteins were applied onto a high-resolution size-exclusion chromatography column (WTC-030S5, Wyatt Technology, Santa Barbara, CA, USA), which was coupled to a three-angle light-scattering detector (miniDAWN TREOS, Wyatt Technology). Data were collected at a flow rate of 0.5 ml/min and analyzed (ASTRA software, Wyatt Technology) to determine the molecular mass of TFG proteins.…”

“…TFG is a highly conserved regulator of protein secretion and functions at the interface between the endoplasmic reticulum (ER) and ER-golgi intermediate compartments. Variants in TFG were implicated in SPOAN-like HSP (spastic paraplegia, optic atrophy, neuropathy) 18 and hereditary sensory motor neuropathy with proximal predominant involvement (HSMN-P). 19 Spastic ataxias now stand as a distinct disease category.…”