Biology subjects

Holmgren, M.

Publications and source records attributed to Holmgren, M..

8 recordsLinked to original sources

bioRxiv2026

Direct detection of electrogenic polyamine transport

Polyamines are cationic metabolites that support critical cellular processes including transcription, translation, and cell proliferation. Although polyamine synthesis and transport are emerging therapeutic targets in human disease, the polyamine transport system remains poorly understood. ATP13A2-4 were recently identified as endolysosomal polyamine exporters, transporting putrescine (PUT), spermidine (SPD), and spermine (SPM) from the vesicular lumen to the cytosol. Despite interest in pharmacologically targeting these transporters, existing assays of transporter function rely on indirect or endpoint measures that cannot resolve transport as it occurs. Here, we report the first voltage-clamp recordings of polyamine transporter activity, directly detecting ATP13A2- and ATP13A3-mediated charge movement across cell membranes. When expressed in Xenopus oocytes, these transporters localized to the plasma membrane, enabling electrophysiological measurements. Polyamines induced inward currents dependent on transporter catalytic activity, and their dose-dependence correlated with ATPase assays. ATP13A2 currents were largest in response to SPM, whereas ATP13A3 currents were largest with PUT, consistent with their established substrate preferences. Polyamine-induced currents were blocked by the polyamine transport inhibitor AMXT-1501. These findings establish ATP13A2 and ATP13A3 as electrogenic polyamine pumps and demonstrate the utility of two-electrode voltage clamp as a label-free, transporter-resolved assay to complement other approaches in polyamine transporter-targeted drug discovery.

biophysics↗

bioRxiv2025

Anatomical and Molecular Characterization of the Zebrafish Meninges

The meninges are a set of connective tissue layers that surround the central nervous system, protecting the brain from mechanical shock, supporting its buoyancy, guarding it from infection and injury, and maintaining brain homeostasis. Despite their critical role, the molecular identity, developmental origins, and functional properties of the cell types populating the meninges remain poorly characterized. This is in large part due to lack of cell type specific markers and difficulty in visualizing and studying these structures through the thick mammalian skull. Here, we show that the zebrafish, a genetically and experimentally accessible vertebrate, possesses an easily imaged mammalian-like meninges. Anatomical and cellular characterization of its composition via histology, electron microscopy, and confocal imaging shows that the adult zebrafish possesses complex multilayered meninges with double-layered dura mater and intricate leptomeningeal layers. Using single cell transcriptomics, we define the molecular identities of meningeal cell populations, including a unique ependymin (epd)-expressing cell population that constitutes the major cellular component of the leptomeningeal barrier and is essential for brain development and survival. These findings support the use of zebrafish as a useful comparative model for studying the meninges, provide a foundational description for future zebrafish meningeal research, and identify a new Leptomeningeal Barrier Cell that serves as the primary epithelial cell component of the leptomeninges.

developmental biology↗

bioRxiv2023

ATP1A1-linked diseases require a malfunctioning protein product from one allele

Heterozygous germline variants in ATP1A1, the gene encoding the 1 subunit of the Na+/K+-ATPase (NKA), have been linked to diseases including primary hyperaldosteronism and the peripheral neuropathy Charcot-Marie-Tooth disease (CMT). ATP1A1 variants that cause CMT induce loss-of-function of NKA. This heterodimeric ({beta}) enzyme hydrolyzes ATP to establish transmembrane electrochemical gradients of Na+ and K+ that are essential for electrical signaling and cell survival. Of the 4 catalytic subunit isoforms, 1 is ubiquitously expressed and is the predominant paralog in peripheral axons. Human population sequencing datasets indicate strong negative selection against both missense and protein-null ATP1A1 variants. To test whether haploinsufficiency generated by heterozygous protein-null alleles are sufficient to cause disease, we tested the neuromuscular characteristics of heterozygous Atp1a1+/- knockout mice and their wildtype littermates, while also evaluating if exercise increased CMT penetrance. We found that Atp1a1+/- mice were phenotypically normal up to 18 months of age. Consistent with the observations in mice, we report clinical phenotyping of a healthy adult human who lacks any clinical features of known ATP1A1-related diseases despite carrying a protein-null early truncation variant, p.Y148*. Taken together, these results suggest that a malfunctioning gene product is required for disease induction by ATP1A1 variants and that if any pathology is associated with protein-null variants, they may display low penetrance or high age of onset.

physiology↗

bioRxiv2022

Structural bases for blockade and activation of BK channels by Ba2+ ions

Ba2+ increases open probability and blocks the permeation pathway of BK channels. Here we discuss Cryo-EM structures of BK channels with Ba2+. They reveal that Ba2+ occupies site S3 in the selectivity filter, leading to blockade. Ba2+ are also detected at all high affinity Ca2+ binding sites of the gating ring. However, the architectures of this region in these structures represent intermediate transitions between the extreme close and open configurations.

biophysics↗

bioRxiv2021

Influence of Mpv17 on hair-cell mitochondrial homeostasis, synapse integrity, and vulnerability to damage in the zebrafish lateral line

Noise exposure is particularly stressful to hair-cell mitochondria, which must produce enough energy to meet high metabolic demands as well as regulate local intracellular Ca2+ concentrations. Mitochondrial Inner Membrane Protein 17 (Mpv17) functions as a non-selective channel and plays a role in maintaining mitochondrial homeostasis. In zebrafish, hair cells in mpv17a9/a9 mutants displayed elevated levels of reactive oxygen species (ROS), elevated mitochondrial calcium, hyperpolarized transmembrane potential, and greater vulnerability to neomycin, indicating impaired mitochondrial function. Using a strong water current to overstimulate hair cells in the zebrafish lateral line, we observed mpv17a9/a9 mutant hair cells were more vulnerable to morphological disruption and hair-cell loss than wild type siblings simultaneously exposed to the same stimulus. To determine the role of mitochondrial homeostasis on hair-cell synapse integrity, we surveyed synapse number in mpv17a9/a9 mutants and wild type siblings as well as the sizes of presynaptic dense bodies (ribbons) and postsynaptic densities immediately following stimulus exposure. We observed mechanically injured mpv17a9/a9 neuromasts, while they lost a greater number of hair cells, lost a similar number of synapses per hair cell relative to wild type. Additionally, we quantified the size of hair cell pre- and postsynaptic structures and observed significantly enlarged wild type postsynaptic densities, yet relatively little change in the size of mpv17a9/a9 postsynaptic densities following stimulation. These results suggest impaired hair-cell mitochondrial activity influences synaptic morphology and hair-cell survival but does not exacerbate synapse loss following mechanical injury.

neuroscience↗

bioRxiv2020

Analysis of SARS-CoV-2 ORF3a structure reveals chloride binding sites

SARS-CoV-2 ORF3a is believed to form ion channels, which may be involved in the modulation of virus release, and has been implicated in various cellular processes like the up-regulation of fibrinogen expression in lung epithelial cells, downregulation of type 1 interferon receptor, caspase-dependent apoptosis, and increasing IFNAR1 ubiquitination. ORF3a assemblies as homotetramers, which are stabilized by residue C133. A recent cryoEM structure of a homodimeric complex of ORF3a has been released. A lower-resolution cryoEM map of the tetramer suggests two dimers form it, arranged side by side. The dimers cryoEM structure revealed that each protomer contains three transmembrane helices arranged in a clockwise configuration forming a six helices transmembrane domain. This domains potential permeation pathway has six constrictions narrowing to about 1 [A] in radius, suggesting the structure solved is in a closed or inactivated state. At the cytosol end, the permeation pathway encounters a large and polar cavity formed by multiple beta strands from both protomers, which opens to the cytosolic milieu. We modeled the tetramer following the arrangement suggested by the low-resolution tetramer cryoEM map. Molecular dynamics simulations of the tetramer embedded in a membrane and solvated with 0.5 M of KCl were performed. Our simulations show the cytosolic cavity is quickly populated by both K+ and Cl-, yet with different dynamics. K+ ions moved relatively free inside the cavity without forming proper coordination sites. In contrast, Cl- ions enter the cavity, and three of them can become stably coordinated near the intracellular entrance of the potential permeation pathway by an inter-subunit network of positively charged amino acids. Consequently, the central cavitys electrostatic potential changed from being entirely positive at the beginning of the simulation to more electronegative at the end.

biophysics↗

bioRxiv2020

Mechanical overstimulation causes acute injury followed by fast recovery in lateral-line neuromasts of larval zebrafish

Excess noise damages sensory hair cells, resulting in loss of synaptic connections with auditory nerves and, in some cases, hair-cell death. The cellular mechanisms underlying mechanically induced hair-cell damage and subsequent repair are not completely understood. Hair cells in neuromasts of larval zebrafish are structurally and functionally comparable to mammalian hair cells but undergo robust regeneration following ototoxic damage. We therefore developed a model for mechanically induced hair-cell damage in this highly tractable system. Free swimming larvae exposed to strong water wave stimulus for 2 hours displayed mechanical injury to neuromasts, including afferent neurite retraction, damaged hair bundles, and reduced mechanotransduction. Synapse loss was observed in apparently intact exposed neuromasts, and this loss was exacerbated by inhibiting glutamate uptake. Mechanical damage also elicited an inflammatory response and macrophage recruitment. Remarkably, neuromast hair-cell morphology and mechanotransduction recovered within hours following exposure, suggesting severely damaged neuromasts undergo repair. Our results indicate functional changes and synapse loss in mechanically damaged lateral-line neuromasts that share key features of damage observed in noise-exposed mammalian ear. Yet, unlike the mammalian ear, mechanical damage to neuromasts is rapidly reversible.

neuroscience↗

bioRxiv2018

The BK channel gating ring is strongly coupled to the voltage sensor

The open probability of large conductance voltage- and calcium-dependent potassium (BK) channels is regulated allosterically by changes in the transmembrane voltage and intracellular concentration of divalent ions (Ca2+ and Mg2+). The divalent cation sensors reside within the gating ring formed by eight Regulator of Conductance of Potassium (RCK) domains, two from each of the four channel subunits. Overall, the gating ring contains 12 sites that can bind Ca2+ with different affinities. Using patch-clamp fluorometry, we have shown robust changes in FRET signals within the gating ring in response to divalent ions and voltage, which do not directly track open probability. Only the conformational changes triggered through the RCK1 binding site are voltage-dependent in presence of Ca2+. Because the gating ring is outside the electric field, it must gain voltage sensitivity from coupling to the voltage-dependent channel opening, the voltage sensor or both. Here we demonstrate that alterations of voltage sensor dynamics known to shift gating currents produce a cognate shift in the gating ring voltage dependence, whereas changing BK channels relative probability of opening had little effect. These results strongly suggest that the conformational changes of the RCK1 domain of the gating ring are tightly coupled to the voltage sensor function, and this interaction is central to the allosteric modulation of BK channels.

biophysics↗