Biard, Marie et al. (2021) International Worm Meeting "The role of the extracellular matrix in maintaining neuronal architecture against increased mechanical stress"

Nadour, Malika, Rivollet, Lise, Benard, Claire, Biard, Marie

[International Worm Meeting, 2021]

Little is known about the mechanisms that protect the long-term architectural integrity of the nervous system: formed during embryogenesis, it will later face the stresses of maturation, growth, body movements and accidents. The cell adhesion molecule SAX-7/L1CAM is one of the neuronal maintenance factors identified in C. elegans. Our group has demonstrated that SAX-7S acts post-developmentally to preserve the organization of specific neuronal ganglia and fascicles (Desse et al., bioRxiv). Notably, maintenance defects resulting from the loss of sax-7 can be suppressed by paralysis, highlighting that the mechanical stress associated with body movements may lead to progressive neuronal disorganization. Neuronal disorganization in sax-7 mutants is also supressed by the loss of function of the gene mig-6/papilin, indicating that these two genes affect the maintenance of neuronal organization in opposite ways. MIG-6 is a conserved extracellular matrix glycoprotein containing a "papilin cassette", an assembly of domains also present in extracellular matrix remodeling enzymes. We found that the state of the extracellular matrix is altered in mig-6 mutants, suggesting that a balance between adhesion and flexibility of the peri-neuronal environment is important to maintain the integrity of the nervous system. We therefore hypothesize that mig-6 mutants, which under normal conditions have unaltered neuronal organization, may become defective following increased body movements. To test this, we subjected worms to high mechanical stress and observed that neuronal organization is better preserved in mig-6 mutants compared to wild-type animals under these conditions. This effect is concomitant with a change in the distribution of EMB-9/collagen type IV, one of the main basement membrane proteins. Understanding the fundamental maintenance mechanisms of neuronal architecture could help identify molecular pathways involved in the development of neuropathologies.

Cerdeira, Victoria et al. (2021) International Worm Meeting "Novel neurodevelopmental genes in C. elegans"

Cerdeira, Victoria, Rivollet, Lise, Lanteigne, Cynthia, Benard, Claire, Ares, Myriam

[International Worm Meeting, 2021]

Ongoing research with C. elegans continues to uncover fundamental principles of nervous system development and function. Neuron migration, axon guidance and dendritic spine development are essential to establish functional neural circuits. Our goal here is to study cellular and molecular mechanisms that mediate neuronal development, including that of dendritic spines. For this, we are characterizing several genes named mau (maternal-effect uncoordinated), which are defined by mutations isolated in a screen for maternal-effect viable mutations (Hekimi et al., 1995; our unpublished results). mau mutant animals display morphological and behavioural defects that vary in penetrance and expressivity. Our phenotypic analyses of mau mutants reveal that their locomotion is abnormal and variable over time, with episodes of paralysis, spasms and altered body postures. mau mutants have defective axon guidance, including left/right guidance choice at the ventral midline. Moreover, mau mutants display defects in dendritic spines number and distribution, as well as in synaptic transmission. We are progressing towards molecularly identifying and characterizing the mau mutants, which will provide insights on how these novel genes contribute to neuronal development, including of dendritic spines in vivo.

Nadour, Malika et al. (2019) International Worm Meeting "The extracellular matrix protein MIG-6/papilin mediates the maintenance of neuronal architecture."

St-Louis, Philippe, Rivollet, Lise, Benard, Claire, Nadour, Malika, Thackeray, Andrea

[International Worm Meeting, 2019]

After the initial assembly of the nervous system during embryogenesis, neuronal structures need to persist lifelong for neural circuits to remain functional, in the face of maturation, growth, body movements, and aging. How the nervous system is protected throughout life is not understood. Our research using C. elegans has demonstrated that there are molecular mechanisms actively maintaining the architecture of the nervous system, which act with great cellular specificity (Benard and Hobert, 2009). In neuronal maintenance mutants that we have identified, including in the mutants sax-7/L1CAM, neuronal structures initially develop normally, but subsequently become disorganized. Through our forward genetic screen for modifiers of sax-7 defects, whole genome sequencing, and rescue assays, we have identified the gene mig-6/papilin to mediate the maintenance of neuronal architecture. Indeed, loss of function of mig-6 suppresses the progressive disorganization of sax-7 mutants, suggesting that they play antagonistic roles. Disruption of mig-6 function after embryogenesis alone is sufficient to suppress the defects of sax-7 mutants, highlighting its post-developmental role in this context. Also, we find that the short, but not the long, isoform of the gene mig-6 functions non-autonomously from body wall muscles, and that the thrombospondin type 1 domains appear important in neuronal maintenance. Furthermore, our analysis of extracellular matrix components, by confocal microscopy and FRAP assays, reveal that mig-6 is required for the normal dynamics of collagen type IV/EMB-9 in the extracellular matrix. Thus, the impact of MIG-6 on the state of the extracellular matrix that ensheathes ganglia and fascicles may ensure a balance between the adhesion of neurons to their surrounding environment and the flexibility between them, enabling neurons to endure lifelong stress. Understanding general principles of the maintenance of neuronal architecture may help identify key factors influencing the onset and progression of neurodegenerative conditions.

Nadour, Malika et al. (2021) International Worm Meeting "The extracellular matrix protein MIG-6/papilin mediates the maintenance of neuronal architecture"

Rivollet, Lise, Thackeray, Andrea, Nadour, Malika, St-Louis, Philippe, Benard, Claire, Doitsidou, Maria, Biard, Marie

[International Worm Meeting, 2021]

After the initial assembly of the nervous system during embryogenesis, neuronal circuits need to persist lifelong in the face of maturation, growth, body movements, and aging. How neuronal organization is protected throughout life is not well understood. Our research has demonstrated that molecular mechanisms actively maintain the architecture of the nervous system, acting with great cellular specificity (Benard and Hobert, 2009). sax-7 mutants lack the cell-adhesion molecule SAX-7/L1CAM, and specific neuronal structures that initially develop normally subsequently become disorganized. Through a genetic screen, we uncovered that loss of mig-6/papilin suppresses neuronal disorganization in sax-7 mutants, suggesting antagonistic roles for these genes: whereas SAX-7 mediates adhesion among neurons, MIG-6 may confer increased flexibility between neurons and their surrounding environment. MIG-6/papilin harbors a papilin cassette, composed of thrombospondin type I and lagrin domains, which is shared with ADAMTS metalloproteinases that remodel the extracellular matrix. In neuronal maintenance, mig-6 functions post-developmentally, and the short isoform of mig-6 is secreted from muscles into the extracellular matrix to non-autonomously impact neuronal maintenance in a mig-17/ADAMTS-dependent manner. Loss of mig-6 leads to the accumulation of extracellular collagen type IV/EMB-9 fibrotic-like structures, which do not occur in the wild type, nor in sax-7 mutants. Post-developmental depletion of collagen IV reduces these fibrotic-like structures and reinstates neuronal maintenance defects in sax-7; mig-6 mutants. Moreover, loss of the collagen-IV-crosslinking-extracellular enzyme peroxidasin/PXN-2 also re-establishes sax-7 neuronal maintenance defects in the double mutants sax-7; mig-6, and interestingly, PXN-2 is upregulated in mig-6 mutants. Thus, MIG-6 may ensure a state of flexibility of the extracellular matrix ensheathing neuronal structures that balances neuron-to-neuron adhesion, enabling neuronal architecture to endure lifelong stress. Consistent with this notion, loss of mig-6 bestows enhanced protection of neuronal organization in conditions of increased body movements compared to wild type. Understanding general principles of the maintenance of neuronal architecture and connectivity may help identify key factors influencing the onset and progression of neurodegenerative conditions.

Dima, Raphael et al. (2021) International Worm Meeting "Heparan sulfate proteoglycans, guidance molecules and Rho-family GTPases regulate the number of cellular extensions in developing polarized cells"

Shaye, Daniel, Rivollet, Lise, M. Socovich, Alexandra, Benard, Claire, Bah Tahe, Marianne, A. Chabi, Yann, F. Arena, Anthony, Dima, Raphael

[International Worm Meeting, 2021]

During animal development, precise cellular morphologies are established that enable polarized cells to perform specialized functions within the organism. For instance, the C. elegans excretory-canal cell has four hollow processes (or canals) that extend from the cell soma along the left and right lateral body regions to collect fluid for osmoregulation. Similarly, neurons project processes to connect with appropriate targets. Heparan sulfate proteoglycans (HSPGs) are conserved glycoproteins that play diverse roles, including to regulate interactions between morphogens and guidance cues and their corresponding receptors to elicit cellular responses and orchestrate morphogenetic events. The heparan sulfate (HS) chains attached to HSPG core proteins are polymerized by glycosyltransferases of the exostosin (EXT) family, whose loss causes morphogenetic defects and embryonic lethality across species. Viable hypomorphic mutants of rib-1 and rib-2, which encode the C. elegans HS copolymerase, display the striking phenotype of having supernumerary cellular extensions in a number of polarized cells: several monopolar neurons can develop two full neurites, and the excretory-canal cell can form up to eight projections in these mutants. We define one key HSPG that cell-autonomously controls the number of canals formed by the excretory-canal cell, and find that chemical modifications of its HS chains are important for this function. The supernumerary canals observed in these mutants display the hallmarks of normal canals: they develop at the same time as normal ones, arise from the cell body, and display normal lumen and cytoskeletal (F-actin and microtubule) organization. Through genetic analysis we demonstrate that HSPGs and specific guidance cues and receptors (of the unc-6 and slt-1 pathways) cooperate to regulate the number of formed processes; we also define genetic interactions between HSPGs and downstream Rho-family GTPases known to regulate the cytoskeleton. Our results provide insight into a cellular system operating to guarantee that the proper number of cellular projections is established during polarized cell development, and contribute to understanding the roles of conserved HSPGs in cellular morphogenetic events.