Large Acid-Evoked Currents, Mediated by ASIC1a, Accompany Differentiation in Human Dopaminergic Neurons

Abstract

Acid-sensing ion channels (ASICs) are proton-gated Na+ channels. They contribute to synaptic transmission, neuronal differentiation and neurodegeneration. ASICs have been mainly characterized in neurons from mice or rats and our knowledge of their properties in human neurons is scarce. Here, we functionally characterized ASICs in differentiating LUHMES cells, a human mesencephalic cell line with characteristics of dopaminergic neurons. We find that LUHMES cells express functional ASICs, predominantly homomeric ASIC1a. Expression starts early during differentiation with a striking surge in current amplitude at days 4–6 of differentiation, a time point where—based on published data—LUHMES cells start expressing synaptic markers. Peak ASIC expression therefore coincides with a critical period of LUHMES cell differentiation. It was associated with increased excitability, but not paralleled by an increase in ASIC1 mRNA or protein. In differentiating as well as in terminally differentiated LUHMES cells, ASIC activation by slight acidification elicited large currents, action potentials and a rise in cytosolic Ca2+. Applying the ASIC pore blocker diminazene during differentiation reduced the length of neurites, consistent with the hypothesis that ASICs play a critical role in LUHMES cell differentiation. In summary, our study establishes LUHMES cells as a valuable model to study the role of ASICs for neuronal differentiation and potentially also cell death in a human cell line.

Retinoic Acid-Differentiated Neuroblastoma SH-SY5Y Is an Accessible In Vitro Model to Study Native Human Acid-Sensing Ion Channels 1a (ASIC1a)

“…Moreover, the data of quantitative real-time PCR and the proton affinity parameters point to the prevalence of homomeric ASIC1a channels. Indeed, pH 50 and pHSSD 50 , equal to 6.65 and 7.19, respectively, as well as desensitization time constant (τ des ), equal to 0.49 s at the pH 5.0 stimulus, perfectly correspond to the known values for homomeric ASIC1a [12,28,30]. The established dominance of ASIC1a is consistent with previous reports of its involvement in neurodifferentiation where this channel apparently modulates maturation of dendrites and neurites and dopamine secretion in dopaminergic neurons [10][11][12].…”

“…Ligands were applied only after a stable response to pH 6.0-induced activation was achieved. Amiloride, which finds a medical application as a K + -sparing diuretic agent, is a non-specific blocker τ des of homomeric ASIC1a channels was previously reported to be in the range from 1 to 2 s [12,28,29], while τ des < 1 s was reported to be inherent to heteromeric ASIC1a/ASIC2a channels [28]. However, for heteromeric ASIC1a/ASIC2a channels, it was also shown that with greater acid stimulus τ des significantly increases and approaches a value close to 1 s. In RA-treated cells, τ des significantly decreased with stronger acid stimulus and matched perfectly to the value for homomeric ASIC1a channels [28].…”

Commentary: Large Acid-Evoked Currents, Mediated by ASIC1a, Accompany Differentiation in Human Dopaminergic Neurons

“…A third avenue to investigate is the role of ASICs in disorders involving an increase in dopamine release, such as schizophrenia. The paper published by the Grunder laboratory suggests that ASIC1a may modulate dopamine secretion from human dopaminergic neurons (Neuhof et al, 2021 ). Schizophrenia is hypothesized to occur due to an over secretion of dopamine in the mesolimbic pathway (Birtwistle and Baldwin, 1998 ; Jones et al, 2011 ; Howes et al, 2017 ).…”

“…A recent study published in Frontiers in Cellular Neuroscience from the Grunder laboratory has explored the role of ASICs in the differentiation of LUHMES cells (Neuhof et al, 2021 ). They first converted LUHMES cells into post-mitotic dopaminergic neurons and subsequently isolated ASIC mRNA.…”

“…While the functions of ASICs are often explored in murine models, few studies have investigated these channels in human cells and human cell lines (Sun et al, 2020 ; Xu et al, 2021 ). Recently, a study conducted by the Grunder laboratory has examined the function of ASICs in human dopaminergic cell lines (Neuhof et al, 2021 ). They have proposed that ASICs are responsible for the differentiation of LUHMES (Lund Human Mesencephalic) cells, a line of human cells capable of differentiating into dopaminergic-like neurons (Zuberek et al, 2018 ; Harischandra et al, 2019 ).…”