Myelination, myelinopathies and remyelination
Myelin sheath is a protective membrane that wraps around part of certain nerve cells. It affects how fast signals travel through those nerve cells. It is a plasma membrane extension of specialised glial cells that wraps around axons. These cell are oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Myelin disruption underlies several human diseases, such as Multiple sclerosis and Charcot-Marie-Tooth.
Our teams evaluate mechanisms involved in initial myelination, in myelinopathies and in remyelination that restaure myelin during early phases of myelionopathies.
Axo-glial communication in Schwann cell development and peripheral myelination
Our broad aim is to comprehend how the different cellular players within the nervous system interact to elaborate a functional neuronal entity. We mainly focus on the axo-glial communication, a fundamental process that orchestrates the assembly of the myelin sheath. In order to gain insight into this mechanism, we use the simplicity and high optical quality of the zebrafish embryo to monitor early and most fundamental behaviour of developing neuronal and glial cells. Some of the questions we beg: i) What are the molecular signallings that shape the peripheral myelinating glia? We have initiated a differential screen looking for genes that are dysregulated in the absence of Schwann cells using zebrafish. This screen helped us in identifying several new candidates that regulate different aspects of peripheral nervous system development. ii) How do Schwann cells migrate and divide along growing axons in order to myelinate? We use pharmacological and genetics tools, laser ablation, in vivo time-lapse imaging and transmitted electron microscopy to study the different molecular and cellular aspects that shape Schwann cell migration, division and behaviour.
Deciphering Signaling pathways in Schwann cell Development and peripheral myelination
PI : Marcel Tawk, Cindy Degerny (Team 2)
Our laboratory has initiated a differential screen looking for genes that are dysregulated in the absence of Schwann cells. This led us to identify novel players and signaling pathways required for Schwann cell development and myelination.
Elmo1: a novel signaling pathway required for Schwann cell development and myelination
We describe a novel and previously unknown role for engulfment and cell motility 1 gene (elmo1) in neuronal survival and timely Schwann cell myelination in the peripheral nervous system. Elmo1 is one of the missing links that conveys an extracellular signal to Schwann cells intracellular cytoskeleton to mediate radial sorting via Rac1 and allow Schwann cells to sort out axons, a key step for peripheral myelination.
Mikdache A, Fontenas L, Albadri S, Revenu C, Loisel-Duwattez J, Lesport E, Degerny C, Del Bene F, Tawk M. Elmo1 function, linked to Rac1 activity, regulates neuronal numbers and myelination in zebrafish. Cell Mol Life Sci. 2019, Jun 3. doi: 10.1007/s00018-019-03167-5.
Novel ON/OFF mechanism that links temporal control of Schwann cells division to radial sorting and myelination via Laminin/cAMP pathway
Schwann cell migrate along peripheral axons and divide intensively to generate the right number of cells prior to axonal ensheathment; however, little is known regarding the temporal and molecular control of their division and its impact on myelination. We have identified Sil, a spindle pole protein associated with autosomal recessive primary microcephaly (MCPH), as essential for temporal mitotic exit of Schwann cell. In sil-deficient cassiopeia (csp-/-) mutants, Schwann cell fail to radially sort and myelinate peripheral axons. Elevation of cAMP, but not Rac1 activity in csp-/- restores myelin ensheathment. Most importantly, we show a significant decrease in Laminin expression within csp-/- posterior lateral line nerve and that forcing Laminin2 expression in csp-/- fully restores Schwann cell ability to myelinate.
Time-lapse imaging revealed an important role for sil in Schwann cell metaphase progression that delayed mitotic exit and caused a complete loss of axonal ensheathment as revealed by transmission electron microscopy (TEM). Treating csp-/- with forskolin that binds to adenyl cyclase and elevates the levels of cAMP or forcing Laminin2 expression was sufficient to fully restore peripheral myelination.
We also show that Schwann cell are a major source of Laminin within peripheral nerves in zebrafish. Our clonal analysis shows that forcing Laminin expression autonomously within Schwann cell is enough to secrete Laminin in the extracellular space and restore normal Mbp expression within the vicinity of these cells in csp-/- embryos.
This work provides evidence of a novel ON/OFF mechanism that links temporal control of Schwann cells division to radial sorting and myelination via Laminin/cAMP pathway in vivo.
Mikdache A, Boueid MJ, Lesport E, Delespierre B, Loisel-Duwatetez J, Degerny C, Tawk M. Timely Schwann Cell division drives peripheral myelination in vivo via the Laminin/cAMP pathway. Development. 2022 Sep 1;149(17):dev200640. Doi: 10.1242/dev.200640.
Dissecting molecular and cellular mechanisms of human peripheral neuropathies
The ubiquitous second messenger cAMP controls a variety of cellular responses in different cell types and Schwann cells are no exception since their survival, proliferation, and differentiation are all dependent on cAMP, a phenomenon documented as early as 1970. Several cAMP upstream and downstream key molecular players have been discovered recently, however, what converts ATP in cAMP within Schwann cells and provides them with this essential messenger remains unknown.
We have identified a mutation in adcy6 (adenyl cyclase 6, that catalyzes the conversion of ATP into cAMP) gene as responsible for the lack of myelin that results in arthrogryposis. The latter is a congenital (present at birth) disease characterized by joint contracture in two or more areas of the body. Children born with this disease have abnormal fibrosis of the muscle tissue causing muscle shortening, and therefore are unable to perform active extension and flexion in the affected joints. Our unit established a strong, yet previously unappreciated role, of defective myelin and nodes of Ranvier components in causing arthrogryposis.
It has been shown that a gpr126 (G protein receptor 126) mutation leads to severe arthrogryposis during fetal development due to defective myelination. Indeed, GPR126 is essential for PNS myelination in zebrafish and mice. Moreover, several results point to a role of GPR126 protein in activating G proteins and elevating cAMP within Schwann cells in order to drive SC myelination. The fact that the adcy6 mutation in humans leads to the exact same phenotype observed in gpr126 patients is quite intriguing. Furthermore, given the fact that ADCY6 is an adenyl cyclase required for cAMP synthesis, and that GPR126 function is dependent on cAMP elevation, it is possible that these two proteins are part of the Schwann cell signaling pathway required for peripheral myelination. It is also possible that ADCY6 is required within neurons/axons for cAMP activity that influences axo-glial communication and therefore peripheral myelination. It is important to note that these two hypotheses are not mutually exclusive.
Our current research investigates how Adenyl cyclase, more specifically ADCY6, contributes to Schwann cell myelination. We also evaluate whether i. GPR126 and ADCY6, both essential for PNS myelination, communicate within Schwann cells. ii. Whether ADCY6 is the main (or only) adenyl cyclase protein that is required for PNS myelination and if this pathway is well conserved across different species.
This research will help us unfold the underlying mechanisms for peripheral defects in humans with ADCY6 mutations. The ultimate goal is to unravel novel and fundamental signaling pathways required for Schwann cell function, a key step for any future drug treatment strategy in peripheral neuropathies and arthrogryposis. It relies on studies involving two animal models: zebrafish and mice.
Novel targets for therapeutic approaches of myelin diseases in the central nervous system
In the central nervous system, oligodendrocytes form insulating myelin sheaths around axons, ensuring rapid electrical communication between neurons and providing trophic support. Damaged myelin can be partly replaced by spared oligodendrocytes and the recruitment of oligodendrocyte progenitor cells (OPC), followed by their differentiation into myelinating oligodendrocytes. This process is known as remyelination, myelin regeneration or myelin repair. In demyelinating diseases such as multiple sclerosis, remyelination often remains insufficient or fails, and enhancing the regenerative capacity of myelin is now regarded as a promising therapeutic option.
Signaling pathways that drive the differentiation of OPC into mature oligodendrocytes and the formation of new myelin are potential drug targets for remyelination-enhancing therapies.
Hedgehog signaling is a major pathway in both developmental myelination and myelin repair. Elucidating its molecular mechanisms and identifying its neural cell targets should thus open new perspectives for regenerating myelin sheaths.
When considering remyelination therapies, an important but often neglected factor is the hormonal environment of neural cells. Steroid hormones are indeed important regulators of the proliferation and differentiation of OPC and myelin formation by interacting with pro-myelination signaling pathways such as Hedgehog signaling. Moreover, reduced levels of gonadal steroids in multiple sclerosis patients have been associated with neurological disability and worse clinical outcomes.
Cooperation of the hedgehog and androgen signaling pathways during myelination
PI : Elisabeth Traiffort (Team 1)
Our current projects aim to dissect the molecular and cellular mechanisms involved in the processes of myelin production by oligodendrocytes during development and following demyelination of the central nervous system. Two signaling pathways are currently under investigation. These are the pathways induced respectively by secreted Hedgehog proteins and by the steroid hormone testosterone. The project is based on the genetic and pharmacological manipulation of these pathways during development and in different mouse models of demyelination. The unit also focuses on local inflammatory processes associated with myelin pathologies and other neurological pathologies. The final objective is the identification of new therapeutic targets in the field of myelin diseases and inflammatory pathologies of the central nervous system.
The Hedgehog signaling pathway is the first signaling pathway that was identified in the genetic control of early embryo development. From 1998, E. Traiffort revealed the persistence of Hedgehog pathway in the postnatal and adult central nervous system. Her group recently demonstrated a cooperation between Hedgehog and testosterone signaling in myelination. Both pathways exert complementary activities during postnatal oligodendrogenesis. In adults, their cooperation promotes the differentiation of OPC into mature myelin forming oligodendrocytes, as well as a higher preservation of axonal integrity and lower neuroinflammation. In experimental autoimmune encephalomyelitis (EAE), an animal model most commonly used to investigate multiple sclerosis pathology, both pathways ameliorate disease symptoms in a concerted manner.
Laouarem Y, Kassoussi A, Zahaf A, Hutteau-Hamel T, Mellouk A, Bobé P, Mattern C, Schumacher M, Traiffort E (2021) Functional cooperation of the hedgehog and androgen signaling pathways during developmental and repairing myelination. Glia 69:1369-1392.
Testosterone is well-known as a male hormone produced by the testes. However, it is less appreciated that testosterone, at lower levels, is also en essential hormone in women. So far, the remyelinating, neuroprotective and anti-inflammatory actions of testosterone have been studied in male mice and men suffering from multiple sclerosis. However, androgen effects mediated by the androgen receptor (AR) have been only poorly studied in females. Recently, we have shown significant AR expression in demyelinated lesions from female mice and women with multiple sclerosis. In female mice, androgens and estrogens act in a synergistic way while androgens drive microglia responses towards myelin regeneration. Our work also uncovered major sex-dependent molecular differences regarding androgen effects during remyelination. Most genes down-regulated by androgens in female mice are related to inflammation, whereas those predominantly down-regulated in males are related to lipid metabolic processes. Thus, androgens are essential for proper myelin regeneration in females and therapeutic approaches of demyelinating diseases need to consider male-female differences.
Zahaf A, Kassoussi A, Hutteau-Hamel T, Mellouk A, Marie C, Zoupi L, Tsouki F, Mattern C, Bobé P, Schumacher M, Williams A, Parras C, Traiffort E (2023) Androgens show sex-dependent differences in myelination in immune and non-immune murine models of CNS demyelination. Nature Communications 14:1592.
Elucidating molecular mechanisms implicated in pro-myelinating Hedgehog signaling is a prerequisite for targeting this pathways in remyelination therapies. The transduction of the Hedgehog signal involves a major receptor complex associating the transmembrane protein Patched and its co-receptors. Hedgehog binding to Patched relieves repression of the G protein-coupled receptor Smoothened (Smo), which then triggers either an intracellular signaling cascade involving Gli transcription factors or noncanonical mechanisms. Our work has identified the Patched co-receptor BOC as a crucial partner in developmental and repairing myelination.
Zakaria M, Ferent J, Hristovska I, Laouarem Y, Zahaf A, Kassoussi A, Mayeur ME, Pascual O, Charron F, Traiffort E (2019) The Shh receptor Boc is important for myelin formation and repair. Development 146.
The role of steroid hormones in myelination: New keys to understand the sexual differentiation of myelin
Our laboratory has been pioneering in demonstrating a role of steroid hormones in myelination. Pro-myelinating actions of progesterone have been first demonstrated for Schwann cells in the peripheral nervous system, and then for oligodendrocytes in the central nervous system. These were among the first observations of steroid actions in the nervous system going beyond classical reproductive functions. The impact of the steroid hormonal milieu on the myelination process may have its roots in the concomitant evolutionary appearance of myelin and the diversification of steroid receptors in hinge-jawed vertebrates.
Testosterone can exert persistent differentiating effects on the brain, qualified as organizational, by acting on immature neuronal circuits during sensitive developmental periods.
PI : Abdel Ghoumari (Team 1)
We have shown that neonatal testosterone, acting via brain androgen receptors (AR), exerts persistent organizing effects on the structure of myelin sheaths in the male mouse brain. As a result, adult male mice have thicker myelin sheaths, a greater density of oligodendrocytes and shorter myelinated axonal segments (internodes) when compared with females.
Structural sex differences in the myelin sheaths may impact on their integrity and vulnerability to immune attacks and may contribute sex-related differences in the incidence and progressions of demyelinating diseases such as multiple sclerosis.
Abi Ghanem C, Degerny C, Hussain R, Liere P, Pianos A, Tourpin S, Habert R, Macklin WB, Schumacher M, Ghoumari AM (2017) Long-lasting masculinizing effects of postnatal androgens on myelin governed by the brain androgen receptor. PLoS Genet 13:e1007049.
PI : Elisabeth Traiffort (Team 1)
It is well-known that Testosterone is a male hormone produced by the testes. However, it is less appreciated that testosterone, at lower levels, is also en essential hormone in women. So far, the remyelinating, neuroprotective and anti-inflammatory actions of testosterone have been studied in male mice and men suffering from multiple sclerosis. However, androgen effects mediated by the androgen receptor (AR) have been only poorly studied in females. Recently, we have shown significant AR expression in demyelinated lesions from female mice and women with multiple sclerosis. In female mice, androgens and estrogens act in a synergistic way while androgens drive microglia responses towards myelin regeneration. Our work also uncovered major sex-dependent molecular differences regarding androgen effects during remyelination. Most genes down-regulated by androgens in female mice are related to inflammation, whereas those predominantly down-regulated in males are related to lipid metabolic processes. Thus, androgens are essential for proper myelin regeneration in females and therapeutic approaches of demyelinating diseases need to consider male-female differences.
Zahaf A, Kassoussi A, Hutteau-Hamel T, Mellouk A, Marie C, Zoupi L, Tsouki F, Mattern C, Bobé P, Schumacher M, Williams A, Parras C, Traiffort E (2023) Androgens show sex-dependent differences in myelination in immune and non-immune murine models of CNS demyelination. Nature Communications 14:1592.
Genetics and neurodevelopment
PI : Judith Melki (Team 2)
Our main research is focused on deciphering the genetic basis of several fetal and infantile diseases including arthrogryposis, a heterogeneous group of fetal neuromuscular disorders diseases, peripheral neuropathies and congenital anomaly of the cerebral vasculature. To support gene discovery, our have set up a genomics facility including a server for in-house bioinformatics analysis of SNPs and data from whole or targeted exome sequencing. Functional genomics are performed using various cellular or animal models to help understand how discovered gene mutations affect phenotypes.
This approach should lead to the identification of possibly novel signaling pathways involved in the process of development and/ maintenance of the neuromuscular system, peripheral myelination and brain vasculature in human. Moreover, our approaches should open new avenue in the genetic investigation of these diseases.