Carine Bonnard

We identified a new progeroid syndrome with severe neuropathy and intellectual deficits but its underlying cellular and molecular mechanism is unknown. Exome sequencing revealed a homozygous mutation in the IVNS1ABP gene, which encodes IVNS1ABP, an influenza virus non-structural protein-1 binding protein. To investigate disease mechanisms, we generated isogenic induced pluripotent stem cells (iPSCs) from patient fibroblasts and differentiated them into neural progenitor cells (NPCs). Mutant IVNS1ABP fibroblasts, iPSCs, and NPCs exhibited defective cytokinesis, increased DNA damage, and premature cellular senescence. Consistent with these findings, cerebral organoids showed early differentiation of NPCs into neurons. Molecular profiling as well as biochemical and cellular analysis revealed altered binding of mutant IVNS1ABP to actin / actin-associated proteins and dysregulated actin dynamics during cytokinesis. Taken together, we propose that mutant IVNS1ABP dysregulates actin polymerization and organization which is at least partly responsible for the cellular senescence phenotypes in this progeroid neuropathy.

Adipose tissue dysfunction in obesity is a major global public health risk, contributing to insulin resistance and chronic diseases such as diabetes and cardiovascular disorders. Here, we identify a dominant c.37A>G p.(Arg13Gly) variant in the long isoform of CIDEC (CIDEC-L), a key regulator of lipid droplet (LD) size, as the underlying cause of familial obesity. Affected individuals display marked subcutaneous fat accumulation in white adipose tissue (WAT), elevated fat content in brown adipose tissue (BAT) and insulin resistance. Accordingly, patient-derived iPSCs differentiated into white adipocytes exhibit accelerated LD growth, a phenotype mirrored by CIDEC-L ^R13G overexpression. Mechanistically, we find that the p.Arg13Gly variant disrupts the N-terminal structural order of CIDEC-L, shifting its phase separation properties to enable, rather than restrict, lipid exchange through condensation plates between LDs. Notably, knock-in mice with the analogous Cidec-L p.(Arg10Gly) mutation recapitulate the human BAT hypertrophy and exhibit impaired thermogenesis. These findings establish the CIDEC-L ^R13G variant as the first example of a dominantly inherited monogenic obesity driven by a dysfunctional adipocyte LD protein, revealing a critical role for CIDEC-L in restraining fat accumulation and maintaining metabolic health.

Background: Arginase 1 (ARG1) is an enzyme expressed by keratinocytes that drives several functions linked to skin barrier function. However, the mechanisms underpinning keratinocyte ARG1 function in barrier homeostasis have not been fully elucidated. Atopic dermatitis (AD) is linked to impaired skin barrier via altered keratinocyte differentiation and susceptibility to infection. Objectives: To investigate the role of ARG1 in keratinocyte differentiation and antimicrobial responses. Methods: In vitro two-dimensional differentiation assays using ARG knockdown or ARG inhibited keratinocytes were used to explore the function of ARG1 in keratinocyte differentiation and barrier formation. ARG1 was also assessed in an ex vivo model of AD. Results: ARG1 was strongly expressed in the apical layers of human skin, corresponding to high ARG1 expression in late differentiated ­keratinocytes. ARG was downregulated in an ex vivo AD model relative to control, suggesting that altered ARG1 is clinically relevant. ARG1 ­inhibition in keratinocytes led to a significant decrease in the late differentiation markers filaggrin, involucrin and loricrin, and significant downregulation of antimicrobial peptides (AMPs), lipocalin 2, kallikreins and small proline-rich proteins. ARG forms part of the urea cycle and the action of ARG on L-arginine causes the production of L-ornithine and urea. In turn, L-ornithine is catabolized for putrescine production. Supplementation with ARG products, putrescine and urea could rescue late keratinocyte differentiation and AMP expression in ARG-deficient cells. Conclusions: ARG1 activity plays a major role in keratinocyte differentiation and AMP production. ARG1 is downregulated in an AD model, and in cell systems its function can be rescued by the ARG1 downstream products putrescine and urea. Manipulation of the ARG1 pathway may have the potential to be used for the management of skin conditions such as AD.

Summary: Impaired proteostasis can induce protein aggregation which is toxic to neuronal cells, contributing to neurodegeneration and other signs of aging. In this study, we delineate an early-onset progressive neuropathy evoking Giant Axonal Neuropathy 1. The causative gene IVNS1ABP encodes a E3-ubiquitin ligase adaptor which is a close gigaxonin paralogue. Patient-derived fibroblasts, iPSCs, and neural progenitors exhibited hallmarks of protein accumulation and lysosomal dysfunction. Ubiquitome analysis revealed overlapping targets with gigaxonin, including Vimentin and MAP1B. The biallelic correction to the isogenic wildtype state in disease- relevant motor neurons partly rescued cellular vulnerabilities. A newly generated ivns1abpa/b knockout zebrafish model partially recapitulated the human peripheral neuropathy, exhibiting aberrant primary motor neuron axon pathfinding, leading to impaired locomotion. Our findings indicate that IVNS1ABP functions in the same pathway as gigaxonin, ensuring appropriate cellular turnover of critical protein substrates, whose accumulation leads to accelerated aging in discrete cellular lineages.

Objective: Electrical Stimulation Therapy (EST) shows promise for the purpose of accelerating wound healing, but the right electrical stimulation parameters and its mode of action remain unclear. We aim to evaluate the effect of a new EST clinical device on epidermal repair using an in vitro human skin wound model. Approach: We scaled-up a well-established 3D De-Epidermized Dermis-Human Skin Equivalent (DED-HSE) wound model to fit a clinically used device that delivers pre-programmed microcurrent EST. The impact of EST on re-epithelialization of 4-mm circular epidermal wounds was assessed after 4 and 7 days of treatment, using metabolic activity assay, immunohistochemistry (IHC) staining, and RNA in situ hybridization. Results: EST was successfully applied to the wounded in vitro skin model. Large DED-HSEs retained good cell viability for up to 7 days of EST treatment. Excisional wounds subjected to EST for 4 days consistently exhibited faster closure (mean 65.8%, n=9) compared to untreated wounds (mean 49.7%, n=9) (p<0.05). Wounds exposed to EST exhibited significantly longer epithelial tongues (re-epithelialization mean 50.3%, n=9) than untreated wounds (mean 26.2%, n=9) (p<0.001), suggesting faster keratinocyte migration and proliferation. Increased MMP1 transcription (p<0.05) in ES-treated periwound suggests a mechanism for enhanced keratinocyte migration. IHC staining showed advanced epidermal proliferation (p63) and differentiation (K10) in EST-exposed wounds (n=15), as well as stronger attachment of the newly formed epidermis into the dermis compared to untreated controls (n=15) (p<0.001). Innovation: We present a novel approach to assess an EST clinical device designed to stimulate wound healing. Using a scaled-up 3D human skin wound model, we could demonstrate the positive effect of EST on epithelial cell responses and shed light on possible mechanism. Conclusion: Our study provides experimental evidence that microcurrent therapy accelerates wound closure and improves the quantity and quality of re-epithelialization.

The ZAKα-driven ribotoxic stress response (RSR) is activated by ribosome stalling and/or collisions. Recent work demonstrates that RSR also plays a role in innate immunity by activating the human NLRP1 inflammasome. Here, we report that ZAKα and NLRP1 sense bacterial exotoxins that target ribosome elongation factors. One such toxin, diphtheria toxin (DT), the causative agent for human diphtheria, triggers RSR-dependent inflammasome activation in primary human keratinocytes. This process requires iron-mediated DT production in the bacteria, as well as diphthamide synthesis and ZAKα/p38-driven NLRP1 phosphorylation in host cells. NLRP1 deletion abrogates IL-1β and IL-18 secretion by DT-intoxicated keratinocytes, while ZAKα deletion or inhibition additionally limits both pyroptotic and inflammasome-independent non-pyroptotic cell death. Consequently, pharmacologic inhibition of ZAKα is more effective than caspase-1 inhibition at protecting the epidermal barrier in a 3D skin model of cutaneous diphtheria. In summary, these findings implicate ZAKα-driven RSR and the NLRP1 inflammasome in antibacterial immunity and might explain certain aspects of diphtheria pathogenesis.

DRG1 is a highly conserved member of a class of GTPases implicated in ribosome biogenesis and translation. The expression of mammalian DRG1 is elevated in the central nervous system during development, and its function has been implicated in fundamental cellular processes including protein synthesis and cellular proliferation. Using exome sequencing, we identified rare and likely pathogenic germline DRG1 variants including three stop-gained p.Gly54*, p.Arg140*, p.Lys263* and a p.Asn248Phe missense variant. These alleles segregate recessively in four affected individuals from three unrelated families and cause a neurodevelopmental disorder with global developmental delay, microcephaly, short stature and craniofacial anomalies. Using functional assays, we show that these loss-of-function variants: 1) severely disrupt DRG1 mRNA/protein stability in patient-derived fibroblasts, 2) impair it’s GTPase activity in vitro and 3) compromise it’s binding to partner protein ZC3H15. Consistent with the importance of DRG1 in humans, targeted inactivation of Drg1 in mice resulted in pre-weaning lethality. Our work highlights the importance of DRG1 GTPase activity for normal development and underscores the significance of translation factor GTPases in human physiology and homeostasis.

Human NLRP1 (NACHT, LRR, and PYD domain-containing protein 1) is an innate immune sensor predominantly expressed in the skin and airway epithelium. Here, we report that human NLRP1 senses the ultraviolet B (UVB)- and toxin-induced ribotoxic stress response (RSR). Biochemically, RSR leads to the direct hyperphosphorylation of a human-specific disordered linker region of NLRP1 (NLRP1 ^DR ) by MAP3K20/ZAKα kinase and its downstream effector, p38. Mutating a single ZAKα phosphorylation site in NLRP1 ^DR abrogates UVB- and ribotoxin-driven pyroptosis in human keratinocytes. Moreover, fusing NLRP1 ^DR to CARD8, which is insensitive to RSR by itself, creates a minimal inflammasome sensor for UVB and ribotoxins. These results provide insight into UVB sensing by human skin keratinocytes, identify several ribotoxins as NLRP1 agonists, and establish inflammasome-driven pyroptosis as an integral component of the RSR.

Rabenosyn (RBSN) is a conserved endosomal protein necessary for regulating internalized cargo. Here, we present clinical, genetic, cellular and biochemical evidence that two distinct RBSN missense variants are responsible for a novel Mendelian disorder consisting of progressive muscle weakness, facial dysmorphisms, ophthalmoplegia and intellectual disability. Using exome sequencing, we identified recessively acting germline alleles p.Arg180Gly and p.Gly183Arg, which are both situated in the FYVE domain of RBSN. We find that these variants abrogate binding to its cognate substrate phosphatidylinositol 3-phosphate (PI3P) and thus prevent its translocation to early endosomes. Although the endosomal recycling pathway was unaltered, mutant p.Gly183Arg patient fibroblasts show accumulation of cargo tagged for lysosomal degradation. Our results suggest that these variants are separation-of-function alleles, which cause a delay in endosomal maturation without affecting cargo recycling. We conclude that distinct germline mutations in RBSN cause non-overlapping phenotypes with specific and discrete endolysosomal cellular defects.