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bioRxiv2023

ATP-induced crosslinking of a biomolecular condensate

DEAD-box helicases are important regulators of biomolecular condensates. However, the mechanisms through which these enzymes affect the dynamics of biomolecular condensates have not been systematically explored. Here, we demonstrate the mechanism by which mutation of a DEAD-box helicases catalytic core alters ribonucleoprotein condensate dynamics in the presence of ATP. Through altering RNA length within the system, we are able to attribute the altered biomolecular dynamics and material properties to physical crosslinking of RNA facilitated by the mutant helicase. These results suggest the mutant condensates approach a gel transition when RNA length is increased to lengths comparable to eukaryotic mRNA. Lastly, we show that this crosslinking effect is tunable with ATP concentration, uncovering a system whose RNA mobility and material properties vary with enzyme activity. More generally, these findings point to a fundamental mechanism for modulating condensate dynamics and emergent material properties through nonequilibrium, molecular-scale interactions. SignificanceBiomolecular condensates are membraneless organelles which organize cellular biochemistry. These structures have a diversity of material properties and dynamics which are crucial to their function. How condensate properties are determined by biomolecular interactions and enzyme activity remain open questions. DEAD-box helicases have been identified as central regulators of many protein-RNA condensates, though their specific mechanistic roles are ill-defined. In this work, we demonstrate that a DEAD-box helicase mutation crosslinks condensate RNA in an ATP-dependent fashion via protein-RNA clamping. Protein and RNA diffusion can be tuned with ATP concentration, corresponding to an order of magnitude change in condensate viscosity. These findings expand our understanding of control points for cellular biomolecular condensates that have implications for medicine and bioengineering.

biophysics↗

bioRxiv2023

A Key Piece of the Puzzle: The central tetramer of the Saccharomyces cerevisiae septin protofilament and Its Implications for Self-Assembly

Septins, often described as the fourth component of the cytoskeleton, are structural proteins found in a vast variety of living beings. They are related to small GTPases and thus, generally, present GTPase activity which may play an important (although incompletely understood) role in their organization and function. Septins polymerase into long non-polar filaments, in which each subunit interacts with two others by alternating interfaces, NC and G. In Saccharomyces cerevisiae four septins are organized in the following manner, [Cdc11-Cdc12-Cdc3-Cdc10- Cdc10-Cdc3-Cdc12-Cdc11]n in order to form filaments. Although septins were originally discovered in yeast and much is known regarding their biochemistry and function, only limited structural information about them is currently available. Here we present crystal structures of Cdc3/Cdc10 which provide the first view of the physiological interfaces formed by yeast septins. The G-interface has properties which place it in between that formed by SEPT2/SEPT6 and SEPT7/SEPT3 in human filaments. Switch I from Cdc10 contributes significantly to the interface, whereas in Cdc3 it is largely disorded. However, the significant negative charge density of the latter suggests it may have a unique role. At the NC-interface, we describe an elegant means by which the sidechain of a glutamine from helix 0 imitates a peptide group in order to retain hydrogen-bond continuity at the kink between helices 5 and 6 in the neighbouring subunit, thereby justifying the conservation of the helical distortion. Its absence from Cdc11, along with this structures other unusual features are critically discussed by comparison with Cdc3 and Cdc10. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=61 SRC="FIGDIR/small/537027v1_ufig1.gif" ALT="Figure 1"> View larger version (17K): org.highwire.dtl.DTLVardef@10c1eccorg.highwire.dtl.DTLVardef@ba0dacorg.highwire.dtl.DTLVardef@d7b866org.highwire.dtl.DTLVardef@8b6d7e_HPS_FORMAT_FIGEXP M_FIG C_FIG HIGHLIGHTSO_LIThe first crystal structure of a yeast septin heterodimer (Cdc3-Cdc10) provides important insights into their structural biology. C_LIO_LIIdentification of common features and differences between yeast and human septins, sheds light on the unique characteristics of yeast septin filaments. C_LIO_LIThe Cdc3G-Cdc10{Delta}1-10 crystal structure could be a crucial piece of the puzzle towards obtaining a high-resolution cryo-EM structure of the yeast septin octamer. C_LI

molecular biology↗

bioRxiv2023

Puromycin reveals a distinct conformation of neuronal ribosomes

Puromycin is covalently added to the nascent chain of proteins by the peptidyl transferase activity of the ribosome and the dissociation of the puromycylated peptide typically follows this event. It was postulated that blocking the translocation of the ribosome with emetine could retain the puromycylated peptide on the ribosome, but evidence against this has recently been published (Hobson et al., 2020 https://doi.org/10.7554/eLife.60048; Enam et al., 2020 https://doi.org/10.7554/eLife.60303). In neurons, puromycylated nascent chains remain in the ribosome even in the absence of emetine, yet direct evidence for this has been lacking. Using biochemistry and cryo-electron microscopy, we show that the puromycylated peptides remain in the ribosome exit channel in the large subunit in a subset of neuronal ribosomes stalled in the hybrid state. These results validate previous experiments to localize stalled polysomes in neurons and provide insight into how neuronal ribosomes are stalled. Moreover, in these hybrid-state neuronal ribosomes, anisomycin, which usually blocks puromycylation, competes poorly with puromycin in the puromycylation reaction, allowing a simple assay to determine the proportion of nascent chains that are stalled in this state. In early hippocampal neuronal cultures, over 50% of all nascent peptides are found in these stalled polysomes. These results provide new insights into the stalling mechanisms of neuronal ribosomes and suggest that puromycylated peptides can be used to reveal subcellular sites of hybrid-state stalled ribosomes in neurons. Significance StatementPuromycin can be covalently linked to the nascent polypeptide chain on ribosomes, followed by dissociation of the puromycylated polypeptide. Here, we conclusively show that in stalled ribosomes isolated from neuronal RNA granules, the puromycylated peptide remains in the polypeptide exit tunnel of the ribosome. This validates previous data using this technique to localize stalled ribosomes in neurons and suggests a unique ribosomal conformation in these cells. Further evidence for the unique state of these ribosomes is the resistance of puromycylation to the inhibitor anisomycin, which prevents puromycylation in all other cellular contexts. These results provide insight into the mechanism underlying neuronal ribosome stalling and resolve a controversy about using puromycin to localize ribosome stalling in neurons.

neuroscience↗

bioRxiv2023

O antigen biogenesis sensitises Escherichia coli K-12 to bile salts, a likely cause for how it lost its O antigen

Escherichia coli K-12 is a model organism for bacteriology and has served as a workhorse for molecular biology and biochemistry for over a century since its first isolation in 1922. However, Escherichia coli K-12 strains are phenotypically devoid of an O antigen (OAg) since early reports in the scientific literature. Recent studies reported the presence of independent mutations that abolish OAg biogenesis in E. coli K-12 strains from the same original source, suggesting unknown evolutionary forces have selected for loss of OAg during the early propagation of K-12. Here, we show for the first time that restoration of OAg in E. coli K-12 strain MG1655 synergistically sensitises bacteria to vancomycin with bile salts (VBS). Suppressor mutants surviving lethal doses of VBS mostly contained disruptions in OAg biogenesis. We present data supporting a model where the transient presence and accumulation of lipid-carried OAg intermediates in the bacterial periplasm interfere with peptidoglycan synthesis, causing growth defects that are synergistically enhanced by bile salts. Lastly, we demonstrate that continuous bile salt exposure of OAg-producing MG1655 in the laboratory, can recreate a scenario where OAg disruption is selected for. Hence our work provides a likely explanation for the long-held mystery of how E. coli K-12 lost its OAg production and opens new avenues for exploring long-standing questions on the intricate network coordinating the synthesis of different cell envelope components in Gram-negative bacteria. Significance statementEscherichia coli K-12 is the most studied microorganism, widely used in laboratories for studying bacteriology and as a tool for molecular biology. The reason why it is devoid of O antigen remains a long-standing question. Our work has uncovered a previously unknown selection pressure of bile salts on bacterial O antigen biogenesis, which provides a plausible scenario for how the early propagation of E. coli K-12 strains in bile salt containing media could have led to loss of O antigen in K-12. Our results also suggest that the accumulation of O antigen intermediates in the bacterial periplasm may interfere with bacterial cell wall synthesis, which paves a new research direction into the interplay of different cell envelope component synthesis pathways.

microbiology↗

bioRxiv2023

Inhibition of LRRK2 kinase activity rescues deficits in striatal dopamine dynamics in VPS35 p.D620N knock-in mice

Dysregulation of dopamine neurotransmission profoundly affects motor, motivation and learning behaviors, and is often observed during the prodromal phase of Parkinsons disease (PD). However, the mechanism underlying these pathophysiological changes remains to be elucidated. Mutations in vacuolar protein sorting 35 (VPS35) and leucine-rich repeat kinase 2 (LRRK2) both lead to autosomal dominant PD, and VPS35 and LRRK2 may physically interact to govern the trafficking of synaptic cargos within the endo-lysosomal network in a kinase-dependent manner. To better understand the functional role of VPS35 and LRRK2 on dopamine physiology, we examined Vps35 haploinsufficient (Haplo) and Vps35 p.D620N knock-in (VKI) mice and how their behavior, dopamine kinetics and biochemistry are influenced by LRRK2 kinase inhibitors. We found Vps35 p.D620N significantly elevates LRRK2-mediated phosphorylation of Rab10, Rab12 and Rab29. In contrast, Vps35 haploinsufficiency reduces phosphorylation of Rab12. While striatal dopamine transporter (DAT) expression and function is similarly impaired in both VKI and Haplo mice, that physiology is normalized in VKI by treatment with the LRRK2 kinase inhibitor, MLi-2. As a corollary, VKI animals show a significant increase in amphetamine induced hyperlocomotion, compared to Haplo mice, that is also abolished by MLi-2. Taken together, these data show Vps35 p.D620N confers a gain-of-function with respect to LRRK2 kinase activation, and VPS35 and LRRK2 functionally interact to regulate DAT trafficking and striatal dopamine neurotransmission.

neuroscience↗

bioRxiv2023

Formation of a stable RNase Y-RicT (YaaT) complex requires RicA (YmcA) and RicF (YlbF)

In Bacillus subtilis, the RicT (YaaT), RicA (YmcA) and RicF (YlbF) proteins, which form a stable ternary complex, are needed together with RNase Y (Rny), to cleave and thereby stabilize several key transcripts encoding enzymes of intermediary metabolism. We show here that RicT, but not RicA or RicF, forms a stable complex with Rny, and that this association requires the presence of RicA and RicF. We propose that RicT is handed off from the ternary complex to Rny. We show further that the two iron-sulfur clusters carried by the ternary Ric complex are required for the formation of the stable RicT-Rny complex. We demonstrate that proteins of the degradosome-like network of B. subtilis, which also interact with Rny, are dispensable for processing of the gapA operon. Thus, Rny participates in distinct RNA-related processes, determined by its binding partners, and a RicT-Rny complex is likely the functional entity for gapA mRNA maturation. IMPORTANCEThe action of nucleases on RNA is universal and essential for all forms of life and includes processing steps that lead to the mature and functional forms of certain transcripts. In B. subtilis it has been shown that key transcripts for energy producing steps of glycolysis, for nitrogen assimilation and for oxidative phosphorylation, all of them crucial processes of intermediary metabolism, are cleaved at specific locations, resulting in mRNA stabilization. The proteins required for these cleavages in B. subtilis (Rny (RNase Y), RicA (YmcA), RicF (YlbF) and RicT (YaaT)) are broadly conserved among the firmicutes, including in several important pathogens, hinting that regulatory mechanisms they control may also be conserved. Several aspects of these regulatory events have been explored: phenotypes associated with the absence of these proteins have been described, the impact of these absences on the transcriptome has been documented, and there has been significant exploration of the biochemistry and structural biology of Rny and the Ric proteins. The present study further advances our understanding of the association of Ric proteins and Rny and shows that a complex of Rny with RicT is probably the entity that carries out mRNA maturation.

microbiology↗

bioRxiv2023

Modelling the effect of matrix metalloproteinases in dermal wound healing

With over 2 million people in the UK suffering from chronic wounds, understanding the biochemistry and pharmacology that underpins these wounds and wound healing is of high importance. Chronic wounds are characterised by high levels of matrix metalloproteinases (MMPs), which are necessary for the modification of healthy tissue in the healing process. Overexposure of MMPs, however, adversely affects healing of the wound by causing further destruction of the surrounding extracellular matrix. In this work, we propose a mathematical model that focuses on the interaction of MMPs with dermal cells using a system of partial differential equations. Using biologically realistic parameter values, this model gives rise to travelling waves corresponding to a front of healthy cells invading a wound. From the arising travelling wave analysis, we observe that deregulated apoptosis results in the emergence of chronic wounds, characterised by elevated MMP concentrations. We also observe hysteresis effects when both the apoptotic rate and MMP production rate are varied, providing further insight into the management (and potential reversal) of chronic wounds.

biophysics↗

bioRxiv2023

Four classic de novo genes all have plausible homologs and likely evolved from retro-duplicated or pseudogenic sequences

Despite being previously regarded as extremely unlikely, the idea that entirely novel protein-coding genes can emerge from non-coding sequences has gradually become accepted over the past two decades. Examples of "de novo origination", resulting in lineage-specific "orphan" genes, lacking orthologs, are now produced every year. However, many are likely cases of duplicates that are difficult to recognize. Here, I re-examine the claims and show that four very well-known examples of genes alleged to have emerged de novo "from scratch" - namely FLJ33706 in humans, Goddard in fruit flies, BSC4 in bakers yeast and AFGP2 in codfish - all have plausible evolutionary ancestors in pre-existing genes. In the case of the first two, highly diverged retrogenes that code for regulatory proteins may have been misidentified as being orphans. The antifreeze glycoproteins in cod, moreover, are shown to have likely not evolved from repetitive non-genic sequences but, as in other related cases, from an apolipoprotein that may well have been pseudogenized before later being reactivated. These findings detract from various claims made about de novo gene birth and show there has been a tendency not to invest the necessary effort in searching for homologs outside of a very limited syntenic or phylostratigraphic methodology. An approach used here for improving homology detection draws upon similarities, not just in terms of statistical sequence analysis, but also with biochemistry and function, in order to obviate failure.

evolutionary biology↗

bioRxiv2023

Labelizer: systematic selection of protein residues for covalent fluorophore labeling

An essential requirement for the use of fluorescent dyes in biomedicine, molecular biology, biochemistry, biophysics and optical imaging is their (covalent) attachment to biomolecules. There is, however, no systematic and automated approach for the selection of suitable labeling sites in macromolecules, which is particular problematic for proteins. Here, we present a general and quantitative strategy to identify optimal residues for protein labeling using a naive Bayes classifier. Based on a literature search and bioinformatics analysis of >100 proteins with ~400 successfully labeled residues, we identified four parameters, which we combined into a labeling score to rank residues for their suitability as a label-site. The utility of our approach for the systematic selection of single residues and of residue pairs for FRET experiments is supported by data from the literature and by new experiments on different proteins. To make the method available to a large community of researchers, we developed a python package called "labelizer", that performs an analysis of a pdb-structure (or structural models), label score calculation, and FRET assay scoring. We further provide a webserver (https://labelizer.bio.lmu.de/) to conveniently apply our approach and to build up a central open-access database of (non-)successfully labeled protein residues to continuously improve and refine the labelizer approach.

biophysics↗

bioRxiv2023

Sequence features of transcriptional activation domains are consistent with the surfactant mechanism of gene activation

Transcriptional activation domains (ADs) of gene activators remain enigmatic for decades as they are short, extremely variable in sequence, structurally disordered, and interact fuzzily to a spectrum of targets. We showed that the single required characteristic of the most common acidic ADs is an amphiphilic aromatic-acidic surfactant-like property which is the key for the local gene-promoter chromatin phase transition and the formation of "transcription factory" condensates. We demonstrate that the presence of tryptophan and aspartic acid residues in the AD sequence is sufficient for in vivo functionality, even when present only as a single pair of residues within a 20-amino-acid sequence containing only 18 additional glycine residues. We demonstrate that breaking the amphipathic -helix in AD by prolines increases AD functionality. The proposed mechanism is paradigm-shifting for gene activation area and generally for biochemistry as it relies on near-stochastic allosteric interactions critical for the key biological function.

molecular biology↗

bioRxiv2023

The mechanism for polar localization of the type IVa pilus machine

Type IVa pili (T4aP) are important for bacterial motility, adhesion, biofilm formation and virulence. This versatility is based on their cycles of extension, adhesion, and retraction. The conserved T4aP machine (T4aPM) drives these cycles, however the piliation pattern varies between species. To understand how these patterns are established, we focused on the T4aPM in Myxococcus xanthus that assembles following an outside-in pathway, starting with the polar incorporation of the PilQ secretin forming a multimeric T4aP conduit in the outer membrane. We demonstrate that PilQ recruitment to the nascent poles initiates during cytokinesis, but most is recruited to the new poles in the daughters after completion of cytokinesis. This recruitment depends on the peptidoglycan-binding AMIN domains in PilQ. Moreover, the pilotin Tgl stimulates PilQ multimerization in the outer membrane, is transiently recruited to the nascent and new poles in a PilQ-dependent manner, and dissociates after completion of secretin assembly. Altogether, our data support a model whereby PilQ polar recruitment and multimerization occur in two steps: The PilQ AMIN domains bind septal and polar peptidoglycan, thereby enabling polar Tgl localization, which then stimulates secretin multimerization in the outer membrane. Using computational analyses, we provide evidence for a conserved mechanism of T4aPM pilotins whereby the pilotin transiently interacts with the unfolded {beta}-lip, i.e. the region that eventually inserts into the outer membrane, of the secretin monomer. Finally, we suggest that the presence/absence of AMIN domain(s) in T4aPM secretins determines the different T4aPM localization patterns across bacteria. ImportanceType IVa pili (T4aP) are widespread bacterial cell surface structures with important functions in motility, surface adhesion, biofilm formation and virulence. Different bacteria have adapted different piliation patterns. To address how these patterns are established, we focused on the bipolar localization of the T4aP machine in the model organism M. xanthus by studying the localization of the PilQ secretin, the first component of this machine that assembles at the poles. Based on experiments using a combination of fluorescence microscopy, biochemistry and computational structural analysis, we propose that PilQ, and specifically its AMIN domains, binds septal and polar peptidoglycan, thereby enabling polar Tgl localization, which then stimulates PilQ multimerization in the outer membrane. We also propose that the presence and absence of AMIN domains in T4aP secretins determine the different piliation patterns across bacteria.

microbiology↗

bioRxiv2023

Mechanism of branching morphogenesis inspired by diatom silica formation

The silica-based cell walls of diatoms are prime examples of genetically controlled, species-specific mineral architectures. The physical principles underlying morphogenesis of their hierarchically structured silica patterns are not understood, yet such insight could indicate novel routes towards synthesizing functional inorganic materials. Recent advances in imaging nascent diatom silica allow rationalizing possible mechanisms of their pattern formation. Here, we combine theory and experiments on the model diatom Thalassiosira pseudonana to put forward a minimal model of branched rib patterns - a fundamental feature of the silica cell wall. We quantitatively recapitulate the time-course of rib pattern morphogenesis by accounting for silica biochemistry with autocatalytic formation of diffusible silica precursors followed by conversion into solid silica. We propose that silica deposition releases an inhibitor that slows down up-stream precursor conversion, thereby implementing a self-replicating reaction-diffusion system featuring a non-classical Turing mechanism. The proposed mechanism highlights the role of geometrical cues for guided self-organization, rationalizing the instructive role for the single initial pattern seed known as primary silicification site. The mechanism of branching morphogenesis that we characterize here is possibly generic and may apply also in other biological systems. Significance statementThe formation of minerals by living organisms is a widespread biological phenomenon occurring throughout the evolutionary tree-of-life. The silica-based cell walls of diatom microalgae are impressive examples featuring intricate architectures and outstanding materials properties that still defy their reconstitution in vitro. Here, we developed a minimal mathematical model that explains the formation of branched patterns of silica ribs, providing unprecedented understanding of basic physico-chemical processes capable of guiding silica morphogenesis in diatoms. The generic mechanism of branching morphogenesis identified here could provide recipes for bottom-up synthesis of mineral-nanowire networks for technological applications. Moreover, similar mechanisms may apply in the biological morphogenesis of other branched structures, like corals, bacterial colonies, tracheal networks, fungal plexuses, or the vascular system.

biophysics↗

bioRxiv2023

Diffusiophoresis promotes phase separation and transport of biomolecular condensates

The internal microenvironment of a living cell is heterogeneous and comprises a multitude of organelles with distinct biochemistry. Amongst them are biomolecular condensates, which are membrane-less, phase-separated compartments enriched in system-specific proteins and nucleic acids. The heterogeneity of the cell engenders the presence of multiple spatiotemporal gradients in chemistry, charge, concentration, temperature, and pressure. Such thermodynamic gradients can lead to non-equilibrium driving forces for the formation and transport of biomolecular condensates. Here, we report how ion gradients impact the transport processes of biomolecular condensates on the mesoscale and biomolecules on the microscale. Utilizing a microfluidic platform, we demonstrate that the presence of ion concentration gradients can accelerate the transport of biomolecules, including nucleic acids and proteins, via diffusiophoresis. This hydrodynamic transport process allows localized enrichment of biomolecules, thereby promoting the location-specific formation of biomolecular condensates via phase separation. The ion gradients further impart active motility of condensates, allowing them to exhibit enhanced diffusion along the gradient. Coupled with a reentrant phase behavior, the gradient-induced active motility leads to a dynamical redistribution of condensates that ultimately extends their lifetime. Together, our results demonstrate diffusiophoresis as a non-equilibrium thermodynamic force that governs the formation and transport of biomolecular condensates.

biophysics↗

bioRxiv2023

The protein interactome of the citrus Huanglongbing pathogen Candidatus Liberibacter asiaticus

Candidatus Liberibacter asiaticus (CLas) is the causal agent of the devastating citrus Huanglongbing (HLB) disease. Our understanding of the pathogenicity mechanism and biology of CLas remain limited because CLas has not been cultured in artificial media. CLas encodes 1136 proteins of which 415 have unknown functions. Since genetic studies of CLas genes with unknown functions are impossible, we utilized genome-wide protein-protein interactions (PPIs) yeast-two-hybrid (Y2H) assays to help solve the mystery. PPIs are fundamental to all cellular processes and machinery and instrumental in investigating uncharacterized proteins and inferring biological pathways. In total, 916 bait and 936 prey proteins were included in the three-phase screening, which identified 4245 interactions. The false positive rate of the Y2H assay was estimated to be 3.1%. Pull-down assays confirmed the robustness of our Y2H. The average interactions per node for CLas Y2H interactome were approximately 15.6, significantly higher than free-living bacteria, indicating genome reduction has led to a multi-function of proteins. PPIs provide clues for functions of 371 uncharacterized proteins of CLas. Forty HUB node proteins were identified which might play critical roles in CLas, including a quinone oxidoreductase and LysR that are known to protect bacteria against oxidative stress. This explains why CLas survives well in the phloem even though it triggers immune-mediated disease, systemic and chronic production of reactive oxygen species, and phloem cell death. This PPI database facilitates the investigation of CLas cellular biochemistry and physiology, functions of uncharacterized proteins, and pathogenicity mechanisms of the pathogen.

systems biology↗

bioRxiv2023

Molecular signatures of hyperexcitability and lithium responsiveness in bipolar disorder patient neurons provide alternative therapeutic strategies.

Bipolar disorder (BD) is a multifactorial psychiatric illness affecting about 1% of the world population. The first line treatment, lithium (Li), is effective in only a subset of patients and its mechanism of action remains largely elusive. In the present study, we used iPSC-derived neurons from BD patients responsive (LR) or not (LNR) to lithium and combined electrophysiology, calcium imaging, biochemistry, transcriptomics, and phosphoproteomics to report mechanistic insights into neuronal hyperactivity in BD, and Lis mode of action. We show a selective rescue of neuronal hyperactivity by Li in BD LR neurons through changes in Na+ currents. The whole transcriptome sequencing revealed altered gene expression in BD neurons in pathways related to glutamatergic transmission, and Li selectively altered those involved in cell signaling and ion transport/channel activity. We found the therapeutic effect of Li in BD LR patients was associated with Akt signaling and confirmed that an Akt activator mimics Li effect in BD LR neurons. Further, we showed that AMP-activated protein kinase (AMPK) reduces neural network activity and sodium currents in BD LNR patients. These findings suggest the potential for novel treatment strategies in BD, such as Akt activators in BD LR cases, and the use of AMPK activators for BD LNR patients.

neuroscience↗

bioRxiv2023

Nuclear Factor Kappa B Over-Activation in the Intervertebral Disc Leads to Macrophage Recruitment and Severe Disc Degeneration

ObjectiveLow back pain (LBP) is the leading cause of global disability and is thought to be driven primarily by intervertebral disc (IVD) degeneration (DD). Persistent upregulation of catabolic enzymes and inflammatory mediators have been associated with severe cases of DD. Nuclear factor kappa B (NF-{kappa}B) is a master transcription regulator of immune responses and is over expressed during inflammatory-driven musculoskeletal diseases, including DD. However, its role in triggering DD is unknown. Therefore, this study investigated the effect of NF-{kappa}B pathway over-activation on IVD integrity and DD pathology. MethodsUsing skeletally mature mouse model, we genetically targeted IVD cells for canonical NF-{kappa}B pathway activation via expression of a constitutively active form of inhibitor of {kappa}B kinase B (IKK{beta}), and assessed changes in IVD cellularity, structural integrity including histology, disc height, and extracellular matrix (ECM) biochemistry, biomechanics, expression of inflammatory, catabolic, and neurotropic mediators, and changes in macrophage subsets, longitudinally up to 6-months post activation. ResultsProlonged NF-{kappa}B activation led to severe structural degeneration, with a loss of glycosaminoglycan (GAG) content and complete loss of nucleus pulposus (NP) cellularity. Structural and compositional changes decreased IVD height and compressive mechanical properties with prolonged NF-{kappa}B activation. These alterations were accompanied by increases in gene expression of inflammatory molecules (Il1b, Il6, Nos2), chemokines (Mcp1, Mif), catabolic enzymes (Mmp3, Mmp9, Adamts4), and neurotrophic factors (Bdnf, Ngf) within IVD tissue. Increased recruitment of activated F4/80+ macrophages exhibited a greater abundance of pro-inflammatory (CD38+) over inflammatory-resolving (CD206+) macrophage subsets in the IVD, with temporal changes in the relative abundance of macrophage subsets over time, providing evidence for temporal regulation of macrophage polarization in DD in vivo, where macrophages participate in resolving the inflammatory cascade but promote fibrotic transformation of the IVD matrix. We further show that NF-{kappa}B driven secretory factors from IVD cells increase macrophage migration and inflammatory activation, and that the secretome of inflammatory-resolving macrophages mitigates effects of NF-{kappa}B overactivation. ConclusionOverall the observed results suggest prolonged NF-{kappa}B activation can induce severe DD, acting through increases in inflammatory cytokines, chemotactic proteins, catabolic enzymes, and the recruitment and inflammatory activation of a macrophage cell populations, that can be mitigated with inflammatory-resolving macrophage secretome.

bioengineering↗

bioRxiv2023

Mitochondrial protein FKBP8 captures PDZD8 to form mitochondria-ER contacts

Mitochondria-ER membrane contact sites (MERCS) represent a fundamental ultrastructural feature underlying unique biochemistry and physiology in eukaryotic cells. The ER protein PDZD8 is required for the formation of MERCS in many cell types, however, its tethering partner on the outer mitochondrial membrane (OMM) is currently unknown. Here we identified the OMM protein FKBP8 as the tethering partner of PDZD8 using a combination of unbiased proximity proteomics, CRISPR-Cas9 endogenous protein tagging, Cryo-Electron Microscopy (Cryo-EM) tomography, and correlative light-EM (CLEM). Single molecule tracking revealed highly dynamic diffusion properties of PDZD8 along the ER membrane with significant pauses and capture at MERCS. Overexpression of FKBP8 was sufficient to narrow the ER-OMM distance, whereas independent versus combined deletions of these two proteins demonstrated their interdependence for MERCS formation. Furthermore, PDZD8 enhances mitochondrial complexity in a FKBP8-dependent manner. Our results identify a novel ER-mitochondria tethering complex that regulates mitochondrial morphology in mammalian cells.

cell biology↗

bioRxiv2023

Conserved nucleocytoplasmic density homeostasis drives cellular organization across eukaryotes

The packing and confinement of macromolecules in the cytoplasm and nucleoplasm has profound implications for cellular biochemistry. How intracellular density distributions vary and affect cellular physiology remains largely unknown. Here, we show that the nucleus is less dense than the cytoplasm and that living systems establish and maintain a constant density ratio between these compartments. Using label-free biophotonics and theory, we show that nuclear density is set by a pressure balance across the nuclear envelope in vitro, in vivo and during early development. Nuclear transport establishes a specific nuclear proteome that exerts a colloid osmotic pressure, which, assisted by entropic chromatin pressure, draws water into the nucleus. Using C. elegans, we show that while nuclear-to-cytoplasmic (N/C) volume ratios change during early development, the N/C density ratio is robustly maintained. We propose that the maintenance of a constant N/C density ratio is the biophysical driver of one of the oldest tenets of cell biology: the N/C volume ratio. In summary, this study reveals a previously unidentified homeostatic coupling of macromolecular densities that drives cellular organization with implications for pathophysiologies such as senescence and cancer.

biophysics↗