ERG-28 controls BK channel trafficking in the ER to regulate synaptic function and alcohol response in C. elegans

Active Zone Trafficking of CaV2/UNC-2 Channels Is Independent of β/CCB-1 and α2δ/UNC-36 Subunits

The CaV2 voltage-gated calcium channel is the major conduit of calcium ions necessary for neurotransmitter release at presynaptic active zones (AZs). The CaV2 channel is a multimeric complex that consists of a pore-forming α1 subunit and two auxiliary β and α2δ subunits. Although auxiliary subunits are critical for channel function, whether they are required for α1 trafficking is unresolved. Using endogenously fluorescent protein-tagged CaV2 channel subunits in Caenorhabditis elegans, we show that UNC-2/α1 localizes to AZs even in the absence of CCB-1/β or UNC-36/α2δ, albeit at low levels. When UNC-2 is manipulated to be trapped in the endoplasmic reticulum (ER), CCB-1 and UNC-36 fail to colocalize with UNC-2 in the ER, indicating that they do not coassemble with UNC-2 in the ER. Moreover, blocking ER-associated degradation does not further increase presynaptic UNC-2 channels in ccb-1 or unc-36 mutants, indicating that UNC-2 levels are not regulated in the ER. An unc-2 mutant lacking C-terminal AZ protein interaction sites with intact auxiliary subunit binding sites displays persistent presynaptic UNC-2 localization and a prominent increase of UNC-2 channels in nonsynaptic axonal regions, underscoring a protective role of auxiliary subunits against UNC-2 degradation. In the absence of UNC-2, presynaptic CCB-1 and UNC-36 are profoundly diminished to barely detectable levels, indicating that UNC-2 is required for the presynaptic localization of CCB-1 and UNC-36. Together, our findings demonstrate that although the pore-forming subunit does not require auxiliary subunits for its trafficking and transport to AZs, it recruits auxiliary subunits to stabilize and expand calcium channel signalosomes.SIGNIFICANCE STATEMENT Synaptic transmission in the neuron hinges on the coupling of synaptic vesicle exocytosis with calcium influx. This calcium influx is mediated by CaV2 voltage-gated calcium channels. These channels consist of one pore-forming α1 subunit and two auxiliary β and α2δ subunits. The auxiliary subunits enhance channel function and regulate the overall level of channels at presynaptic terminals. However, it is not settled how these auxiliary subunits regulate the overall channel level. Our study in C. elegans finds that although the auxiliary subunits do not coassemble with α1 and aid trafficking, they are recruited to α1 and stabilize the channel complex at presynaptic terminals. Our study suggests that drugs that target the auxiliary subunits can directly destabilize and have an impact on CaV2 channels.

De novo phytosterol synthesis in animals

Sterols are vital for nearly all eukaryotes. Their distribution differs in plants and animals, with phytosterols commonly found in plants whereas most animals are dominated by cholesterol. We show that sitosterol, a common sterol of plants, is the most abundant sterol in gutless marine annelids. Using multiomics, metabolite imaging, heterologous gene expression, and enzyme assays, we show that these animals synthesize sitosterol de novo using a noncanonical C-24 sterol methyltransferase (C₂₄-SMT). This enzyme is essential for sitosterol synthesis in plants, but not known from most bilaterian animals. Our phylogenetic analyses revealed that C₂₄-SMTs are present in representatives of at least five animal phyla, indicating that the synthesis of sterols common to plants is more widespread in animals than currently known.

Regulation of Neurotransmitter Release by K+ Channels

K+ channels play potent roles in the process of neurotransmitter release by influencing the action potential waveform and modulating neuronal excitability and release probability. These diverse effects of K+ channel activation are ensured by the wide variety of K+ channel genes and their differential expression in different cell types. Accordingly, a variety of K+ channels have been implicated in regulating neurotransmitter release, including the Ca2+- and voltage-gated K+ channel Slo1 (also known as BK channel), voltage-gated K+ channels of the Kv3 (Shaw-type), Kv1 (Shaker-type), and Kv7 (KCNQ) families, G-protein-gated inwardly rectifying K+ (GIRK) channels, and SLO-2 (a Ca2+-. Cl-, and voltage-gated K+ channel in C. elegans). These channels vary in their expression patterns, subcellular localization, and biophysical properties. Their roles in neurotransmitter release may also vary depending on the synapse and physiological or experimental conditions. This chapter summarizes key findings about the roles of K+ channels in regulating neurotransmitter release.

Regulation of Ryanodine Receptor-Dependent Neurotransmitter Release by AIP, Calstabins, and Presenilins

Ryanodine receptors (RyRs) are Ca2+ release channels located in the endoplasmic reticulum membrane. Presynaptic RyRs play important roles in neurotransmitter release and synaptic plasticity. Recent studies suggest that the proper function of presynaptic RyRs relies on several regulatory proteins, including aryl hydrocarbon receptor-interacting protein, calstabins, and presenilins. Dysfunctions of these regulatory proteins can greatly impact neurotransmitter release and synaptic plasticity by altering the function or expression of RyRs. This chapter aims to describe the interaction between these proteins and RyRs, elucidating their crucial role in regulating synaptic function.

The Non Catalytic Protein ERG28 has a Functional Role in Cholesterol Synthesis and is Coregulated Transcriptionally

The enzymatic pathway of cholesterol biosynthesis has been well characterized. However, there remain several potential interacting proteins that may play ancillary roles in the regulation of cholesterol production. Here, we identified ERG28 (chromosome 14 open reading frame 1 [C14orf1]), a homologue of the yeast protein Erg28p, as a player in mammalian cholesterol synthesis. ERG28 is conserved from yeast to humans but has been largely overlooked in mammals. Using quantitative RT-PCR, luciferase assays, and publicly available chromatin immunoprecipitation sequencing data, we found that transcription of this gene is driven by the transcription factor SREBP-2, akin to most cholesterol synthesis enzymes, as well as identifying sterol-responsive elements and cofactor binding sites in its proximal promoter. Based on a split luciferase system, ERG28 interacted with itself and two enzymes of cholesterol synthesis (NSDHL and SC4MOL). Huh7 ERG28-KO cell lines were generated, revealing reduced total cholesterol levels in sterol-depleted environments. In addition, radiolabeled metabolic flux assays showed a 60-75% reduction in the rate of cholesterol synthesis in the KO versus wild-type cells, which could be rescued by expression of ectopic ERG28. Unexpectedly, KO of ERG28 also impaired the activation of SREBP-2 under sterol-replete conditions, by a yet-to-be defined mechanism. These results indicate that ERG28 is clearly involved in cholesterol synthesis, although the precise role this noncatalytic protein plays in this complex metabolic pathway remains to be fully elucidated. A deeper understanding of ERG28, and other ancillary proteins of cholesterol synthesis, may help inform therapeutic strategies for diseases associated with aberrant cholesterol metabolism.

NGLY1: Insights from C. elegans

Peptide:N-glycanase is an evolutionarily conserved deglycosylating enzyme that catalyzes the removal of N-linked glycans from cytosolic glycoproteins. Recessive mutations that inactivate this enzyme cause NGLY1 deficiency, a multisystemic disorder with symptoms including developmental delay and defects in cognition and motor control. Developing treatments for NGLY1 deficiency will require an understanding of how failure to deglycosylate NGLY1 substrates perturbs cellular and organismal function. In this review, I highlight insights into peptide:N-glycanase biology gained by studies in the highly tractable genetic model animal C. elegans. I focus on the recent discovery of SKN-1A/Nrf1, an N-glycosylated transcription factor, as a peptide:N-glycanase substrate critical for regulation of the proteasome. I describe the elaborate post-translational mechanism that culminates in activation of SKN-1A/Nrf1 via NGLY1-dependent 'sequence editing' and discuss the implications of these findings for our understanding of NGLY1 deficiency.

UNC-2 CaV2 Channel Localization at Presynaptic Active Zones Depends on UNC-10/RIM and SYD-2/Liprin-α in Caenorhabditis elegans

Presynaptic active zone proteins couple calcium influx with synaptic vesicle exocytosis. However, the control of presynaptic calcium channel localization by active zone proteins is not completely understood. In a Caenorhabditis elegans (C. elegans) forward genetic screen, we find that UNC-10/RIM (Rab3-interacting molecule) and SYD-2/Liprin-α regulate presynaptic localization of UNC-2, the CaV2 channel ortholog. We further quantitatively analyzed live animals using endogenously GFP-tagged UNC-2 and active zone components. Consistent with the interaction between RIM and CaV2 in mammals, the intensity and number of UNC-2 channel puncta at presynaptic terminals were greatly reduced in unc-10 mutant animals. To understand how SYD-2 regulates presynaptic UNC-2 channel localization, we analyzed presynaptic localization of endogenous SYD-2, UNC-10, RIMB-1/RIM-BP (RIM binding protein), and ELKS-1. Our analysis revealed that although SYD-2 is the most critical for active zone assembly, loss of SYD-2 function does not completely abolish presynaptic localization of UNC-10, RIMB-1, and ELKS-1, suggesting an existence of SYD-2-independent active zone assembly. UNC-2 localization analysis in double and triple mutants of active zone components show that SYD-2 promotes UNC-2 localization by partially controlling UNC-10 localization, and ELKS-1 and RIMB-1 also contribute to UNC-2 channel localization. In addition, we find that core active zone proteins are unequal in their abundance. Although the abundance of UNC-10 at the active zone is comparable to UNC-2, SYD-2 and ELKS-1 are twice more and RIMB-1 four times more abundant than UNC-2. Together our data show that UNC-10, SYD-2, RIMB-1, and ELKS-1 control presynaptic UNC-2 channel localization in redundant yet distinct manners.SIGNIFICANCE STATEMENT Precise control of neurotransmission is dependent on the tight coupling of the calcium influx through voltage-gated calcium channels (VGCCs) to the exocytosis machinery at the presynaptic active zones. However, how these VGCCs are tethered to the active zone is incompletely understood. To understand the mechanism of presynaptic VGCC localization, we performed a C. elegans forward genetic screen and quantitatively analyzed endogenous active zones and presynaptic VGCCs. In addition to RIM, our study finds that SYD-2/Liprin-α is critical for presynaptic localization of VGCCs. Yet, the loss of SYD-2, a core active zone scaffolding protein, does not completely abolish the presynaptic localization of the VGCC, showing that the active zone is a resilient structure assembled by redundant mechanisms.

Transcriptional analysis of the response of C. elegans to ethanol exposure

Ethanol-induced transcriptional changes underlie important physiological responses to ethanol that are likely to contribute to the addictive properties of the drug. We examined the transcriptional responses of Caenorhabditis elegans across a timecourse of ethanol exposure, between 30 min and 8 h, to determine what genes and genetic pathways are regulated in response to ethanol in this model. We found that short exposures to ethanol (up to 2 h) induced expression of metabolic enzymes involved in metabolizing ethanol and retinol, while longer exposure (8 h) had much more profound effects on the transcriptome. Several genes that are known to be involved in the physiological response to ethanol, including direct ethanol targets, were regulated at 8 h of exposure. This longer exposure to ethanol also resulted in the regulation of genes involved in cilia function, which is consistent with an important role for the effects of ethanol on cilia in the deleterious effects of chronic ethanol consumption in humans. Finally, we found that food deprivation for an 8-h period induced gene expression changes that were somewhat ameliorated by the presence of ethanol, supporting previous observations that worms can use ethanol as a calorie source.

Sterol Evolution: Cholesterol Synthesis in Animals Is Less a Required Trait Than an Acquired Taste

Although cholesterol is considered a vital lipid for animals, not all animals can make cholesterol. A new study employing that celebrated elegant worm, Caenorhabditis elegans, investigates mechanisms that evolved in a sterol auxotroph to enable survival without sterol synthesis.

BK channel density is regulated by endoplasmic reticulum associated degradation and influenced by the SKN-1A/NRF1 transcription factor

Ion channels are present at specific levels within subcellular compartments of excitable cells. The regulation of ion channel trafficking and targeting is an effective way to control cell excitability. The BK channel is a calcium-activated potassium channel that serves as a negative feedback mechanism at presynaptic axon terminals and sites of muscle excitation. The C. elegans BK channel ortholog, SLO-1, requires an endoplasmic reticulum (ER) membrane protein for efficient anterograde transport to these locations. Here, we found that, in the absence of this ER membrane protein, SLO-1 channels that are seemingly normally folded and expressed at physiological levels undergo SEL-11/HRD1-mediated ER-associated degradation (ERAD). This SLO-1 degradation is also indirectly regulated by a SKN-1A/NRF1-mediated transcriptional mechanism that controls proteasome levels. Therefore, our data indicate that SLO-1 channel density is regulated by the competitive balance between the efficiency of ER trafficking machinery and the capacity of ERAD.

Excitable cells, such as neurons and muscles, are essential for the movement and behavior of animals. These cells express a set of specific types of ion channels that allow the selective passage of ions across the plasma membrane. The alteration in the levels of these ion channels influences cell excitability and the function of excitable cells. The regulation of ion channel trafficking and targeting is an effective way to control the function of excitable cells. The BK SLO-1 channel is a calcium-activated potassium channel that reduces excitability at presynaptic axon terminals and sites of muscle excitation. In a C. elegans genetic study, authors found that the delayed exit of SLO-1 channels from the ER causes their degradation by a mechanism called ER-associated degradation (ERAD). Interestingly, the same components that directly mediate SLO-1 ERAD also process a key transcriptional factor that maintains proteasome levels, thus indirectly influencing SLO-1 degradation. These data show that the levels of SLO-1 channels are regulated by the competitive balance between the efficiency of ER trafficking machinery and the capacity of ERAD.

Single Amino Acid Changes in the Ryanodine Receptor in the Human Population Have Effects In Vivo on Caenorhabditis elegans Neuro-Muscular Function

The ryanodine receptor mediates intracellular calcium ion release with excitation of nerve and muscle cells. Ryanodine receptor missense variants cause a number of myopathologies, such as malignant hyperthermia, and have been linked with various neuropathologies, including Alzheimer's disease. We characterized the consequences of ryanodine receptor variants in vivo. Eight Caenorhabditis elegans strains, with ryanodine receptor modifications equivalent to human myopathic RYR1 variants, were generated by genome editing. In humans, these variants are rare and confer sensitivity to the inhalational anaesthetic halothane when heterozygous. Increased sensitivity to halothane was found in both homozygous and heterozygous C. elegans. Close analysis revealed distinct subtle locomotion defects, due to the different single amino acid residue changes, even in the absence of the external triggering agent. Distinct pre- and postsynaptic consequences of the variants were characterized through the responses to cholinergic pharmacological agents. The range of phenotypes reflects the complexity of the regulatory inputs to the ryanodine receptor and the criticality of the calcium ion channel opening properties, in different cell types and with age. Ryanodine receptors with these single amino acid residue changes still function as calcium ion channels, but with altered properties which are likely to have subtle consequences for human carriers of such variants. The long-term consequences of subtly altered calcium ion signalling could be cumulative and may be focussed in the smaller nerve cells rather than the more robust muscle cells. It was important to assess phenotypes in vivo to properly appreciate consequences for a whole organism.

BK channel clustering is required for normal behavioral alcohol sensitivity in C. elegans

The large conductance, calcium- and voltage-activated potassium channel, known as the BK channel, is one of the central proteins that mediate alcohol intoxication and tolerance across species. Although ethanol targets BK channels through direct interaction, how ethanol-mediated BK channel activation causes behavioral intoxication is poorly understood. In. C. elegans, loss of function in SLO-1, the BK channel ortholog, confers profound ethanol resistance in movement and egg-laying behaviors. Here, we show that depletion of SLO-1 channels clustered at the active zones with no change in the overall channel expression level results in locomotory resistance to the intoxicating effect of ethanol, equivalent to that of slo-1 loss-of-function mutants. Likewise, depletion of clustered SLO-1 channels in the sarcolemma and neurons leads to ethanol-resistant egg-laying behavior. By contrast, reduction in the overall SLO-1 channel level by over 70% causes only moderate ethanol resistance in movement, and minimal, if any, resistance in egg laying. Our findings strongly suggest that behavioral ethanol sensitivity is conferred by local, but not global, depression of excitability via clustered BK channels. Given that clustered BK channels are functionally coupled to, and localize near, calcium channels, ethanol may mediate its behavioral effects by targeting BK channels and their coupled calcium channels.

Calnexin revealed as an ether-a-go-go chaperone by getting mutant worms up and going

Pierce examines new work revealing that calnexin controls the biogenesis of ERG-type K⁺ channels in Caenorhabditis elegans.

The role of ion channels in cell excitability was first revealed in a series of voltage clamp experiments by Hodgkin and Huxley in the 1950s. However, it was not until the 1970s that patch-clamp recording ushered in a revolution that allowed physiologists to witness how ion channels flicker open and closed at angstrom scale and with microsecond resolution. The unexpectedly tight seal made by the patch pipette in the whole-cell configuration later allowed molecular biologists to suck up the insides of identified cells to unveil their unique molecular contents. By refining these techniques, researchers have scrutinized the surface and contents of excitable cells in detail over the past few decades. However, these powerful approaches do not discern which molecules are responsible for the dynamic control of the genesis, abundance, and subcellular localization of ion channels. In this dark territory, teams of unknown and poorly understood molecules guide specific ion channels through translation, folding, and modification, and then they shuttle them toward and away from distinct membrane domains via different subcellular routes. A central challenge in understanding these processes is the likelihood that these diverse regulatory molecules may be specific to ion channel subtypes, cell types, and circumstance. In work described in this issue, Bai et al. (2018. J. Gen. Physiol.https://doi.org/10.1085/jgp.201812025) begin to shed light on the biogenesis of UNC-103, a K⁺ channel found in Caenorhabditis elegans.

Identification of a consensus motif in Erg28p required for C-4 demethylation in yeast ergosterol biosynthesis based on mutation analysis

The Erg28p protein is localized to the endoplasmic reticulum, where it acts as a scaffold to tether the C-4 demethylase complex involved in the sterol biosynthesis pathway of Saccharomyces cerevisiae. However, due to the challenges involved in characterizing the interactions of membrane proteins, the precise region of Erg28p that is responsible for the assembly of this enzyme complex remains unknown. To address this question, mutants with serial truncations in the C-terminus of Erg28p were constructed based on a topology prediction of its transmembrane domain. Sterol profiles demonstrated that intermediates involved in the stepwise removal of the two C-4 methyl groups from the tetracyclic sterol ring were accumulated in the ERG28Δ135-447 strain. Homologous alignment of Erg28p further identified a highly conserved 10-amino acid sequence (63LS/QARTFGT/LWT72) within the truncated region of ERG28Δ136-273. Complementation of the BY4741/erg28 strain with the ScERG28Δ175-204 plasmid resulted both in a significant growth inhibition and a reduction of ergosterol biosynthesis compared with the plasmid without the Δ175-204 truncation. Furthermore, homology modeling of the Erg28p mutant indicated that the deletion of residues 63-72 significantly disrupted the 3D structure of the four parallel helices in Erg28p. Taken together, the data indicate that the region spanning amino acids 63-72 constitutes a key consensus motif within Erg28p that is required for sterol C-4 demethylation during ergosterol biosynthesis in S. cerevisiae.

Alcohol and the Brain: Neuronal Molecular Targets, Synapses, and Circuits

Ethanol is one of the most commonly abused drugs. Although environmental and genetic factors contribute to the etiology of alcohol use disorders, it is ethanol's actions in the brain that explain (1) acute ethanol-related behavioral changes, such as stimulant followed by depressant effects, and (2) chronic changes in behavior, including escalated use, tolerance, compulsive seeking, and dependence. Our knowledge of ethanol use and abuse thus relies on understanding its effects on the brain. Scientists have employed both bottom-up and top-down approaches, building from molecular targets to behavioral analyses and vice versa, respectively. This review highlights current progress in the field, focusing on recent and emerging molecular, cellular, and circuit effects of the drug that impact ethanol-related behaviors. The focus of the field is now on pinpointing which molecular effects in specific neurons within a brain region contribute to behavioral changes across the course of acute and chronic ethanol exposure.

Aldicarb-induced Paralysis Assay to Determine Defects in Synaptic Transmission in Caenorhabditis elegans

Aldicarb treatment causes an accumulation of acetylcholine in the synaptic cleft of the neuromuscular junction, resulting in sustained muscle activation and eventually paralysis. Aldicarb-induced paralysis assay is an easy and fast method to determine whether synaptic transmission of a C. elegans mutant of interest is altered. This assay is based on the correlation of the rate of neurotransmitter release with the rate of paralysis. In this protocol, we describe a method for simultaneously assessing the aldicarb sensitivity of animals with different genotypes.