Background: ALS is characterized by progressive motor neuron degeneration which typically leads to death within 3 to 5 years. We previously developed an animal model for sporadic ALS (sALS), the predominant subtype affecting 90% of ALS patients (Wong et al., 2022). Intrathecal delivery of cerebrospinal fluid (CSF) from sALS patients, but not familial ALS patients, induced motor disability, upper and lower motor neuron degeneration, and TDP-43 mislocalization in adult wildtype mice. Furthermore, we identified apolipoprotein B-100 (ApoB) as the neurotoxic factor in sALS CSF responsible for inducing disease pathology. Low-density lipoprotein receptor (LDLR) and sortilin are known ApoB receptors, which have previously been linked to cell death. However, whether LDLR and sortilin are critical for sALS CSF/ApoB-induced motor neuron death is unknown.
Objectives: To determine whether sALS CSF/ApoB-induced neurotoxicity is LDLR and/or sortilin-mediated.
Methods: To identify which receptors mediate ApoB-induced neurotoxicity, human iPSC-derived motor neurons (HMNs) were cultured for 8 days then treated for 24 hours with: 1) ApoB, 2) sortilin antibody + ApoB, or 3) LDLR antibody + ApoB. HMNs were fixed in 4% paraformaldehyde for ChAT immunocytochemistry.
To assess whether LDLR is necessary for ApoB-induced neurotoxicity in vivo, adult C57BL/6J and LDLR KO mice underwent laminectomies at cervical levels 4 and 5, then received intrathecal injections into the subarachnoid space delivering 3µl of: 1) saline, 2) sALS CSF, or 3) human ApoB protein. Forelimb motor function was assessed at 1 day post injection and mice were perfused for ChAT and TDP-43 immunostaining.
Results: ApoB-treated HMNs and sortilin antibody + ApoB-treated HMNs were significantly smaller than untreated HMNs, whereas HMNs treated with LDLR antibody + ApoB had similar cluster sizes as untreated HMNs, indicating that blocking LDLR prevents ApoB-induced cell death. Unlike wildtype mice, LDLR KO mice injected with sALS CSF or ApoB did not develop motor disability, motor neuron loss or TDP-43 mislocalization.
Conclusions: Our studies suggest that ApoB binding to LDLR is a critical step for inducing motor neuron degeneration in sALS. Future studies will elucidate signaling cascades downstream of LDLR that lead to motor neuron death.