Altered action potential waveform and shorter axonal initial segment in hiPSC-derived motor neurons with mutations in VRK1

Altered NRG1/ErbB4 signaling and cholesterol metabolism dysregulation are key pathomechanisms in VRK1-related motor neuropathies and motor neuron diseases

“…In our in vitro model, the hiPSC-MNs, unlike in the PNS, are able to communicate and most likely to establish synapses between them and the increase of glutamate receptors transcripts in our patients’ hiPSC-MNs, might reflect a compensatory mechanism to reverse decreased glutamatergic signaling or fast removal of receptors from the synapse. This increase of AMPA and NMDA receptors’ transcript might be a sign of excitotoxicity (Brockington et al, 2013), but this would be contradictory to what we showed in hiPSC-MNs from our patients (Bos et al, 2022). We still need to confirm that these receptors are indeed enriched in the patient’s MNs.…”

“…In order to investigate the pathophysiological mechanisms underlying VRK1-related Motor Neuron diseases and neuropathies, we have studied the global expression of genes in hiPSC-MNs from one patient with compound heterozygous mutations in VRK1 (Patient II.2), described in (El-Bazzal et al, 2019) and one control, in duplicate, by bulk mRNA-Seq. This investigation was initiated in particular to find the missing links between the presence of mutations in VRK1 and the previously described defects in axonal growth and branching and in Action Potential initiation (Bos et al, 2022; El-Bazzal et al, 2019). The heatmap of unsupervised hierarchical clustering of the four samples showed that Patient II.2 clustered separately from the control (Figure 1A).…”

“…In the same hiPSC-MNs, we have measured the electrical activity and demonstrated a modification in the shape of the Action Potential (AP) and a depolarized AP threshold with no alteration of the proportion of firing patterns. The smaller and broader AP recorded in patients with VRK1 mutations was associated with a decrease of the peak Na + currents accompanied by a decrease in the length of the ankyrinG (AnkG)-positive Axonal Initial Segment (AIS) (Bos et al, 2022). Our results reflect a hypoexcitable state of our patient’s MNs, opposite to the common idea that hyperexcitability is an intrinsic feature of motor neurons defects, like in Amyotrophic Lateral Sclerosis (ALS) (Wainger et al, 2014) or Spinal Muscular Atrophy (SMA) (Liu et al 2015).…”

“…In order to further explore the mechanistic links between hyperexcitability and hypoexcitability and to investigate the missing cues between VRK1 loss of function and the defects of patients’ motor neurons (disassembly of CB, altered neurite growth, alterations of the AIS and abnormal AP waveform), we have performed Bulk RNA-Seq in hiPSC-derived Motor Neurons from one patient (patient II.2) published in (El-Bazzal et al, 2019) and (Bos et al, 2022) as compared to a control.…”

“…By studying patient’s cells, including induced Pluripotent Stem Cell (iPSC) derived Motor Neurons (hiPSC-MNs), we have shown that these mutations lead to reduced levels of VRK1 in the nucleus, and that this depletion alters the dynamics of coilin, a phosphorylation target of VRK1 (El-Bazzal et al, 2019). hiPSC-derived Motor Neurons (hiPSC-MN) from these patients, display Cajal Bodies (CBs) disassembly and defects in neurite outgrowth and branching, altered Action Potential (AP) waveform and decreased Axonal Initial Segment (AIS) length (Bos et al, 2022).…”