Biology subjects

Lortat-Jacob, H.

Publications and source records attributed to Lortat-Jacob, H..

6 recordsLinked to original sources

bioRxiv2026

Glycosaminoglycan presentation-dependent regulation of glial responses by Orion during Drosophila neuronal pruning

Neuronal pruning refines neural circuits by eliminating unnecessary connections to ensure proper nervous system function. In Drosophila mushroom body gamma-neuron remodeling, the chemokine-like protein Orion is required for astrocyte infiltration and phagocytosis. This study investigated whether glial responses depend on how Orion is presented. We found that Orion interacts with glycosaminoglycans (GAG) associated to proteoglycans, which are essential for proper axon pruning. Tethering Orion to the gamma-neuron plasma membrane, mimicking glycosaminoglycan binding, was sufficient to induce astrocyte infiltration into axon bundles. However, membrane-bound Orion alone could not efficiently support phagocytosis, which additionally requires a soluble and mobile Orion pool. We also identified exosomes carrying GAG-bound Orion as potential mediators of neuronal remodeling. These findings provide the first evidence that glial infiltration can be uncoupled from phagocytosis, demonstrating that the mode of Orion presentation regulates distinct glial mechanisms during neuronal remodeling.

neuroscience↗

bioRxiv2026

ST8SIA4-mediated polysialylation is critical for CCR2-driven monocyte egress from the bone marrow

Polysialylation is a rare post-translational protein modification essential for brain development and synaptic plasticity, where it fine-tunes cell-cell and cell-matrix interactions to direct neuronal migration, neurite outgrowth and synaptogenesis. Although polysialic acid is also expressed on circulating leukocytes, its functions in the immune system remain largely unexplored. Guided by analysis of publicly available human genomic data showing that naturally occurring variants of ST8SIA4, which encodes one of the two enzymes that mediate polysialylation, associate with reduced circulating monocyte counts, we investigated the in vivo role of this enzyme in monocyte biology. Using St8sia4-deficient mice, we show that ST8SIA4-dependent polysialylation is essential for CCR2-mediated egress of inflammatory monocytes from the bone marrow at steady state and during Mycobacterium tuberculosis infection. We confirm NCAM1 as the principal polysialylated protein in inflammatory monocytes and demonstrate that Ncam1-deficient mice phenocopy the monocyte defects observed in St8sia4-deficient animals. Mechanistically, loss of ST8SIA4-dependent polysialylation impairs engagement and internalization of the CCR2 ligands CCL2 and CCL7, accompanied by disrupted CCR2 surface organization and extensive cytoskeletal remodeling. Together, these findings identify polysialylation as a previously unrecognized regulator of CCR2 function and monocyte mobilization, with broad implications for immune surveillance and inflammatory responses. Highlights- Human ST8SIA4 variants are associated with reduced blood monocyte and lymphocyte counts and increased risk of SLE. - St8sia4-/- mice exhibit monocytopenia with impaired CCR2-mediated egress of inflammatory monocytes from the bone marrow both at steady state and during Mycobacterium tuberculosis infection. - ST8SIA4 polysialylates NCAM1 (CD56) in inflammatory monocytes, and Ncam1-/- - mice phenocopy the monocytopenia observed in St8sia4-/- mice. - Loss of ST8SIA4 and polysialylation impairs CCR2-mediated binding and endocytosis of CCL2 and CCL7 by monocytes. - Polysialic acid itself does not function as a co-receptor for these chemokines, revealing a new paradigm for its role in immune cell trafficking. - Proper CCR2 surface distribution and function, as well as cytoskeletal organization in inflammatory monocytes, depend on ST8SIA4-mediated polysialylation.

immunology↗

bioRxiv2022

Structure of the human heparan sulfate polymerase complex EXT1-EXT2

Heparan sulfates are complex polysaccharides that mediate the interaction with a broad range of protein ligands at the cell surface. A key step in heparan sulfate biosynthesis is catalyzed by the bi-functional glycosyltransferases EXT1 and EXT2, which generate the glycan backbone consisting of repeating N-acetylglucosamine and glucuronic acid units. The molecular mechanism of heparan sulfate chain polymerization remains, however, unknown. Here, we present the cryo-electron microscopy structure of human EXT1-EXT2, which reveals the formation of a tightly packed hetero-dimeric complex harboring four glycosyltransferase domains with their catalytic sites facing in opposite directions. Along with in vitro activity assays using fluorescently labeled and chemically defined substrates, these findings provide a molecular insight into donor substrate recognition and demonstrate that the glycosyltransferase reactions are highly specific. A combination of in vitro and in cellulo mutational studies was used to dissect the functional role of the four catalytic sites. While EXT1 is able to catalyze both glycosyltransferase reactions, EXT2 harbors only N-acetylglucosamine transferase activity. Our results provide mechanistic insight into heparan sulfate chain elongation as a non processive process and lay the cornerstone for future studies on EXT1-EXT2 function in health and disease.

molecular biology↗

bioRxiv2021

Extracellular endosulfatase Sulf-2 harbours a chondroitin/dermatan sulfate chain that modulates its enzyme activity

Sulfs represent a class of unconventional sulfatases, which differ from all other members of the sulfatase family by their structures, catalytic features and biological functions. Through their specific endosulfatase activity in extracellular milieu, Sulfs provide an original post-synthetic regulatory mechanism for heparan sulfate complex polysaccharides and have been involved in multiple physiopathological processes, including cancer. However, Sulfs remain poorly characterized enzymes, with major discrepancies regarding their in vivo functions. Here we show that human Sulf-2 (HSulf-2) features a unique polysaccharide post-translational modification. We identified a chondroitin/dermatan sulfate glycosaminoglycan (GAG) chain, attached to the enzyme substrate binding domain. We found that this GAG chain affects enzyme/substrate recognition and tunes HSulf-2 activity in vitro and in vivo using a mouse model of tumorigenesis and metastasis. In addition, we showed that mammalian hyaluronidase acted as a promoter of HSulf-2 activity by digesting its GAG chain. In conclusion, our results highlight HSulf-2 as a unique proteoglycan enzyme and its newly-identified GAG chain as a critical non-catalytic modulator of the enzyme activity. These findings contribute in clarifying the conflicting data on the activities of the Sulfs and introduce a new paradigm into the study of these enzymes.

biochemistry↗

bioRxiv2020

Axon-secreted chemokine-like Orion is a signal for astrocyte infiltration during neuronal remodeling

The remodeling of neurons is a conserved fundamental mechanism underlying nervous system maturation and function. Glial cells are known to clear neuronal debris but also to have an active role in the remodeling process. Developmental axon pruning of Drosophila memory center neurons occurs by a degenerative process mediated by infiltrating astrocytes. However, how these glial processes are recruited by the axons is unknown. In an unbiased screen, we identified a new gene (orion) which is necessary for both the pruning of some axons and removal of the resulting debris. Orion is secreted from the neurons and bears some features common to the chemokines, a family of chemoattractant cytokines. Thus, chemokine involvement in neuron/glial cell interaction is an evolutionarily ancient mechanism. We propose that Orion is the neuronal signal that elicits astrocyte infiltration required for developmental neuronal remodeling.

neuroscience↗

bioRxiv2019

Semaphorin 3A binding to chondroitin sulfate E enhances the biological activity of the protein, and cross-links and rigidifies glycosaminoglycan matrices

Semaphorin 3A (Sema3A) is a secreted protein that signals to cells through binding to neuropilin and plexin receptors and provides neurons with guidance cues key for axon pathfinding, and also controls cell migration in several other biological systems. Sema3A interacts with glycosaminoglycans (GAGs), an interaction that could localize the protein within tissues and involves the C-terminal domain of the protein. This domain comprises several furin cleavage sites that are processed during secretion and in previous works have hampered recombinant production of full-length wild type Sema3A, and the biochemical analysis of Sema3A interaction with GAGs. In this work, we have developed a strategy to purify the full-length protein in high yield and identified two sequences in the C-terminal domain, KRDRKQRRQR and KKGRNRR, which confer to the protein sub nM affinity for chondroitin sulfate and heparan sulfate polysaccharides. Using chemically defined oligosaccharides and solid phase binding assays, we report that Sema3A recognizes a (GlcA-GalNAc4S6S)2 motif but not a (GlcA2S-GalNAc6S)2 motif and is thus highly specific for type E chondroitin sulfate. Functionally, we found that Sema3A rigidified CS-E films that mimic the GAG presentation within extracellular matrices (ECMs), suggesting that Sema3A may have a previously unidentified function to cross-link and thus stabilize GAG-rich ECMs. Finally, we demonstrated that the full-length Sema3A is more potent at inhibiting neurite outgrowth than the truncated or mutant forms that were previously purified and that the GAG binding sites are required to achieve full activity. The results suggest that Sema3A can rigidify and cross-link GAG matrices, implicating Sema3A could function as an extracellular matrix organizer in addition to binding to and signaling through its cognate cell surface receptors.

biochemistry↗