Biology subjects

Hill, R. B.

Publications and source records attributed to Hill, R. B..

7 recordsLinked to original sources

bioRxiv2026

Pathogenic DRP1 variants reveal a role for biomolecular condensation in mitochondrial fission

Fission is essential for proper mitochondrial function and for cellular homeostasis. Dysfunction in mitochondrial fission is associated with several neurological disorders, including the rare and lethal encephalopathy EMPF1, which is caused by de novo heterozygous DNM1L variants. DNM1L encodes the mitochondrial fission mechanoenzyme DRP1, which can intrinsically self-assemble and induce membrane scission. Wild-type DRP1 puncta that appear throughout the cytoplasm are thought to be pre-scission complexes of well-ordered oligomeric assemblies. Immunofluorescence imaging of patient-derived EMPF1 fibroblasts carrying assembly-deficient DNM1L variants reveals elongated mitochondrial networks consistent with impaired fission. Despite this loss-of-function phenotype, these cells retain essentially wild-type numbers of DRP1 puncta. We confirmed the previously reported inability of purified pathogenic DRP1 variants p.Gly363Asp and p.Gly401Ser to assemble under conditions in which WT DRP1 forms helical polymers. Under macromolecular crowding conditions, however, both wild-type and mutant DRP1 access condensed states whose formation depends on protein concentration and solution conditions. Acute treatment of EMPF1 fibroblasts with 1,6-hexanediol preferentially alters DRP1 puncta fluorescence intensity and distribution in mutant cells relative to wild type, indicating genotype-dependent differences in puncta material properties. Together, these findings support a model in which DRP1 puncta occupy a continuum of condensed states, only a subset of which mature into fission-competent assemblies, revealing biomolecular condensation as a previously unrecognized layer of DRP1 regulation. Biasing DRP1 along this continuum may provide a mechanistic basis for impaired fission in EMPF1 and suggest opportunities to restore productive assembly in select pathogenic contexts. Significance StatementDRP1 puncta associated with mitochondrial fission are thought to be well-ordered oligomeric assemblies that precede membrane scission. Yet their dynamic behavior within cells has remained difficult to reconcile as well-ordered assembly. Under prevailing models, cells bearing pathogenic DNM1L variants impaired in assembly would be expected to lack puncta, but we show these cells retain wild-type puncta levels. We demonstrate that both wild-type and pathogenic mutant DRP1 populate multiple condensed states in vitro, and that disease variants are biased toward more fluid, chemically sensitive assemblies. These findings identify biomolecular condensation as a regulatory layer of DRP1 organization and suggest that shifting DRP1 along this assembly continuum may restore productive fission in select pathogenic contexts.

biophysics↗

bioRxiv2025

Drp1 Proteins Released from Hydrolysis-driven Scaffold Disassembly Trigger Nucleotide-dependent Membrane Remodeling to Promote Scission

Dynamin-related protein (Drp1) drives mitochondrial fission, dysregulation of which leads to neurodegenerative, metabolic, and apoptotic disorders. The precise mechanism of fission completion is unclear. One prevailing model is based on GTP-driven assembly of Drp1 helices that increase confinement via force generation. However, constriction to nanoscopic tubule radii appears necessary but not sufficient for scission. The other is based on GTP-driven disassembly of a constricting Drp1 scaffold that drives a membrane disturbance, but the relation of disassembly to scission and GTP hydrolysis remain uncertain. Elucidation of mitochondrial fission is complicated by the multiple time-involved in the dynamics of mechanoenzyme activity, oligomer disassembly, and membrane remodeling. Using machine learning, synchrotron x-ray scattering, and a theoretical model, our data support a model where progressive GTP hydrolysis enable free Drp1s to increase their capacity for inducing membrane negative Gaussian curvature (NGC). Furthermore, we identify and Drp1 variants that diminish this progressive capacity. Machine learning reveals that predicted NGC-generating sequences of the Drp1 oligomer are not in contact with the confined lipid tube, that scission-enhancing membrane remodeling is triggered by free Drp1 released upon disassembly. TeaserFree Drp1 released from disassembly of oligomeric Drp1 constrictase promotes GTP-hydrolysis dependent mitochondrial scission.

biophysics↗

bioRxiv2024

Dynamins combine mechano-constriction and membrane remodeling to enable two-step mitochondrial fission via a snap-through instability.

Mitochondrial fission is controlled by dynamin proteins, the dysregulation of which is correlated with diverse diseases. Fission dynamins are GTP hydrolysis-driven mechanoenzymes that self-oligomerize into helical structures that constrict membrane to achieve fission, but details are not well understood. However, dynamins can also remodel membranes by inducing negative Gaussian curvature, the type of curvature required for completion of fission. Here, we examine how these drastically different mechanisms synergistically exert their influences on a membrane, via a mechanical model calibrated with small-angle X-ray scattering structural data. We find that free dynamin can trigger a "snap-through instability" that enforces a shape transition from an oligomer-confined cylindrical membrane to a drastically narrower catenoid-shaped neck within the spontaneous hemi-fission regime, in a manner that depends critically on the length of the confined tube. These results indicate how the combination of dynamin assembly, and paradoxically disassembly, can lead to diverse pathways to scission. TeaserDynamin mechano-constriction by assembly and curvature-driven instability by free monomers synergistically drive mitochondrial fission.

biophysics↗

bioRxiv2023

The variable domain from the mitochondrial fission mechanoenzyme Drp1 promotes liquid-liquid phase separation

Dynamins are an essential superfamily of mechanoenzymes that remodel membranes and often contain a "variable domain" (VD) important for regulation. For the mitochondrial fission dynamin, Drp1, a regulatory role for the VD is demonstrated by mutations that can elongate, or fragment, mitochondria. How the VD encodes inhibitory and stimulatory activity is unclear. Here, isolated VD is shown to be intrinsically disordered (ID) yet undergoes a cooperative transition in the stabilizing osmolyte TMAO. However, the TMAO stabilized state is not folded and surprisingly appears as a condensed state. Other co-solutes including known molecular crowder Ficoll PM 70, also induce a condensed state. Fluorescence recovery after photobleaching experiments reveal this state to be liquid-like indicating the VD undergoes a liquid-liquid phase separation under crowding conditions. These crowding conditions also enhance binding to cardiolipin, a mitochondrial lipid, raising the possibility that phase separation may enable rapid tuning of Drp1 assembly necessary for fission.

biophysics↗

bioRxiv2023

Human Fis1 directly interacts with Drp1 in an evolutionarily conserved manner to promote mitochondrial fission

Mitochondrial Fission Protein 1 (Fis1) and Dynamin Related Protein 1 (Drp1) are the only two proteins evolutionarily conserved for mitochondrial fission, and directly interact in S. cerevisiae to facilitate membrane scission. However, it remains unclear if a direct interaction is conserved in higher eukaryotes as other Drp1 recruiters, not present in yeast, are known. Using NMR, differential scanning fluorimetry, and microscale thermophoresis, we determined that human Fis1 directly interacts with human Drp1 (KD = 12-68 {micro}M), and appears to prevent Drp1 assembly, but not GTP hydrolysis. Similar to yeast, the Fis1-Drp1 interaction appears governed by two structural features of Fis1: its N-terminal arm and a conserved surface. Alanine scanning mutagenesis of the arm identified both loss- and gain-of-function alleles with mitochondrial morphologies ranging from highly elongated (N6A) to highly fragmented (E7A) demonstrating a profound ability of Fis1 to govern morphology in human cells. An integrated analysis identified a conserved Fis1 residue, Y76, that upon substitution to alanine, but not phenylalanine, also caused highly fragmented mitochondria. The similar phenotypic effects of the E7A and Y76A substitutions, along with NMR data, support that intramolecular interactions occur between the arm and a conserved surface on Fis1 to promote Drp1-mediated fission as in S. cerevisiae. These findings indicate that some aspects of Drp1-mediated fission in humans derive from direct Fis1-Drp1 interactions that are conserved across eukaryotes.

biochemistry↗

bioRxiv2023

A conserved, non-canonical insert in mitochondrial fission protein 1 (FIS1) is required for DRP1 and TBC1D15 recruitment and fission

Mitochondrial fission protein 1 (FIS1) is conserved in all eukaryotes yet its activity in metazoans is thought divergent from lower eukaryotes like fungi. To address this discrepancy, structure-based sequence alignments revealed a conserved but non-canonical, three-residue insert in a turn of FIS1, suggesting conserved activity. In vertebrate FIS1 this insert is serine (S45), lysine (K46), and tyrosine (Y47). To determine the biological role of this "SKY insert", three variants were evaluated for their fold, and tested in HCT116 cells for altered mitochondrial morphology and recruitment of effectors, DRP1 and TBC1D15. Substitution of the SKY insert with three alanine residues (AAA), or deletion of the insert ({Delta}SKY), did not substantially alter the fold or thermal stability of the protein. Replacing SKY with a canonical turn ({Delta}SKYD49G) introduced significant conformational heterogeneity by NMR that was removed upon deletion of a known regulatory region, the FIS1 arm. Expression of AAA fragmented mitochondria into perinuclear clumps associated with increased mitochondrial DRP1 similar to the wild-type protein. In contrast, expression of {Delta}SKY variants elongated mitochondrial networks and reduced mitochondrial DRP1. Co-expression of YFP-TBC1D15 partially rescued mitochondrial morphology and DRP1 recruitment for {Delta}SKY variants, although {Delta}SKY variants were markedly unable to support TBC1D15 assembly into punctate structures found upon co-expression with wildtype FIS1 or the AAA variant. Collectively these results show that FIS1 activity can be modulated by conserved residues supporting a generalized model whereby FIS1 is governed by intramolecular interactions between the regulatory FIS1 arm and SKY insert that may be conserved across species.

cell biology↗

bioRxiv2020

Inhibition of Fis1 by Novel Peptide Pep213 or Molecular Suppression Reverses Diabetic- and High Glucose-Induced Endothelial Dysfunction in Human Resistance Arteries

Mitochondrial dysfunction is one of several factors that drive development of vascular endothelial dysfunction in type 2 diabetes (T2DM). In endothelial cells from T2DM patients, mitochondrial networks are highly fragmentated with increased expression of mitochondrial fission protein 1 (Fis1). However, whether manipulation of Fis1 expression and activity in endothelial vessels from T2DM patients alters endothelial function remains unknown. Here, molecular suppression of Fis1 reversed impaired endothelium-dependent vasodilation of vessels from T2DM patients, as well as healthy human vessels exposed to high (33 mM) or low (2.5 mM) glucose, while preserving NO bioavailability and improving endothelial cell layer integrity. Conversely, overexpression of Fis1 in healthy vessels impaired vasodilation and increased mitochondrial superoxide, suggesting a causative role. Application of a novel and specific Fis1 inhibitor, pep213, improved endothelium-dependent vasodilation of vessels from T2DM patients, as well as healthy vessels exposed to high glucose or Fis1 overexpression, by improving NO bioavailability and decreasing excess mitochondrial ROS generation. The specificity of pep213 was determined through multiple biophysical techniques and a 1.85 [A] crystal structure of pep213 in complex with Fis1. These data support that excessive mitochondrial fragmentation drives endothelial vessel dysfunction and supports a potential novel therapeutic route for treating diabetic microvascular disease through pharmacological inhibition of Fis1.

physiology↗