Mutations in the human kinesin family member 5A (KIF5A) gene were recently identified as a genetic cause of amyotrophic lateral sclerosis (ALS). Several KIF5A ALS variants cause exon 27 skipping and produce motor proteins with an altered C-terminal tail (referred to as ΔExon27). However, the underlying pathogenic mechanism is still unknown. In this study, we performed a comprehensive analysis of ΔExon27 at the single-molecule, cellular, and organism levels. Our results show that ΔExon27 is prone to form cytoplasmic aggregates and is neurotoxic. The mutation relieves motor autoinhibition and increases motor self-association, leading to drastically enhanced processivity on microtubules. Finally, ectopic expression of ΔExon27 in Drosophila melanogaster causes wing defects, motor impairment, paralysis and premature death. Our results suggest gain of function as an underlying disease mechanism in KIF5A-associated ALS.

ALS-linked KIF5A ΔExon27 mutant causes neuronal toxicity through gain of function

KIF5A is a kinesin superfamily motor protein that transports various cargos in neurons. Mutations in Kif5a cause familial amyotrophic lateral sclerosis (ALS). These ALS mutations are in the intron of Kif5a and induce mis-splicing of KIF5A mRNA, leading to splicing out of exon 27, which in human KIF5A encodes the cargo-binding tail domain of KIF5A. Therefore, it has been suggested that ALS is caused by loss of function of KIF5A. However, the precise mechanisms regarding how mutations in KIF5A cause ALS remain unclear. Here, we show that an ALS-associated mutant of KIF5A, KIF5A(Δexon27), is predisposed to form oligomers and aggregates in cultured mouse cell lines. Interestingly, purified KIF5A(Δexon27) oligomers showed more active movement on microtubules than wild type KIF5A in vitro. Purified KIF5A(Δexon27) was prone to form aggregates in vitro. Moreover, KIF5A(Δexon27)-expressing Caenorhabditis elegans neurons showed morphological defects. These data collectively suggest that ALS-associated mutations of KIF5A are toxic gain-of-function mutations rather than simple loss-of-function mutations.

/pdf/an-als-associated-kif5a-mutant-forms-oligomers-and-ezzgvlhy.pdf

An ALS‐associated KIF5A mutant forms oligomers and aggregates and induces neuronal toxicity

Despite continuing progress in kinesin enzyme mechanochemistry and emerging understanding of the cargo recognition machinery, it is not known how these functions are coupled and controlled by the α-helical coiled coils encoded by a large component of kinesin protein sequences. Here, we combine computational structure prediction with single-particle negative-stain electron microscopy to reveal the coiled-coil architecture of heterotetrameric kinesin-1 in its compact state. An unusual flexion in the scaffold enables folding of the complex, bringing the kinesin heavy chain-light chain interface into close apposition with a tetrameric assembly formed from the region of the molecule previously assumed to be the folding hinge. This framework for autoinhibition is required to uncover how engagement of cargo and other regulatory factors drives kinesin-1 activation. 

Molecular architecture of the autoinhibited kinesin-1 lambda particle

Despite continuing progress in kinesin enzyme mechanochemistry and emerging understanding of the cargo recognition machinery, it is not known how these functions are coupled and controlled by the alpha-helical coiled coils encoded by a large component of kinesin protein sequences. Here, we combine computational structure prediction with single-particle negative stain electron microscopy to reveal the coiled-coil architecture of heterotetrameric kinesin-1, in its compact state. An unusual flexion in the scaffold enables folding of the complex, bringing the kinesin heavy chain-light chain interface into close apposition with a tetrameric assembly formed from the region of the molecule previously assumed to be the folding hinge. This framework for autoinhibition is required to uncover how engagement of cargo and other regulatory factors drive kinesin-1 activation. Summary statement Integration of computational structure prediction with electron microscopy reveals the coiled-coil architecture of the autoinhibited compact conformer of the microtubule motor, kinesin-1.

https://doi.org/10.1101/2022.04.28.489841

Kinesin-1 undergoes autoinhibition but its functional significance has been unclear. Kinesin-1 transports multiple cargoes including cytoplasmic dynein to the microtubule plus ends. From a genetic screen for Aspergills mutants defective in dynein-mediated early endosome transport, we identified a kinesin-1 mutation kinAK895* that disrupts kinesin-1 autoinhibition. Consistent with kinAK895* making kinesin-1 constitutively active, the mutant proteins accumulate abnormally near the microtubule plus ends. Unexpectedly, our genetic data show that kinesin-1 autoinhibition is unnecessary for transporting its cargoes such as secretory vesicles. Dynein accumulates normally at the microtubule plus ends in the kinAK895* mutant. However, the frequency but not the speed of dynein-mediated early endosome transport is significantly decreased, indicating that kinesin-1 autoinhibition facilitates dynein to initiate its cargo transport. Furthermore, kinesin-1 autoinhibition promotes dynein cargo initiation in a way mechanistically distinct from LIS1-promoted dynein switching from its autoinhibited form. Thus, while dynein activation involves dynactin, cargo adapter and LIS1, this study adds kinesin-1 autoinhibition as a new regulatory factor in vivo.

/pdf/kinesin-1-autoinhibition-facilitates-the-initiation-of-1ddhxv9b.pdf

Kinesin-1 autoinhibition facilitates the initiation of dynein cargo transport

GGGGCC (G4C2) hexanucleotide repeat expansion in the C9orf72 gene is the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). The repeat is bidirectionally transcribed and confers gain of toxicity. However, the underlying toxic species is debated, and it is not clear whether antisense CCCCGG (C4G2) repeat expanded RNAs contribute to disease pathogenesis. Our study shows that C9orf72 (C4G2) antisense repeat expanded RNAs trigger the activation of the PKR/eIF2α-dependent integrated stress response independent of dipeptide repeat proteins that are produced through repeat-associated non-AUG initiated translation, leading to global translation inhibition and stress granule formation. Increased phosphorylation of PKR/eIF2α is also observed in the frontal cortex of C9orf72 FTD/ALS patients. Finally, only antisense (C4G2), but not sense (G4C2), repeat expanded RNAs can activate the PKR/eIF2α pathway. These results provide a mechanism by which antisense repeat expanded RNAs elicit neuronal toxicity in FTD/ALS caused by C9orf72 repeat expansions.

/pdf/antisense-but-not-sense-repeat-expanded-rnas-activate-pkr-1xjxv765.pdf

Ganesh Chilukuri

Papers

ALS-linked KIF5A ΔExon27 mutant causes neuronal toxicity through gain of function

Antisense, but not sense, repeat expanded RNAs activate PKR/eIF2α-dependent integrated stress response in C9orf72 FTD/ALS