Laura Feltri: Of Schwann cells, matrix, and family

Laura Feltri (1963–2023) has been a pioneer in the study of extracellular matrix in peripheral nervous system myelination.

Prostaglandin D2 synthase controls Schwann cells metabolism

We previously reported that in the absence of Prostaglandin D2 synthase (L-PGDS) peripheral nerves are hypomyelinated in development and that with aging they present aberrant myelin sheaths. We now demonstrate that L-PGDS expressed in Schwann cells is part of a coordinated program aiming at preserving myelin integrity. In vivo and in vitro lipidomic, metabolomic and transcriptomic analyses confirmed that myelin lipids composition, Schwann cells energetic metabolism and key enzymes controlling these processes are altered in the absence of L-PGDS. Moreover, Schwann cells undergo a metabolic rewiring and turn to acetate as the main energetic source. Further, they produce ketone bodies to ensure glial cell and neuronal survival. Importantly, we demonstrate that all these changes correlate with morphological myelin alterations and describe the first physiological pathway implicated in preserving PNS myelin. Collectively, we posit that myelin lipids serve as a reservoir to provide ketone bodies, which together with acetate represent the adaptive substrates Schwann cells can rely on to sustain the axo-glial unit and preserve the integrity of the PNS.

Structured wound angiogenesis instructs mesenchymal barrier compartments in the regenerating nerve

Organ injury stimulates the formation of new capillaries to restore blood supply raising questions about the potential contribution of neoangiogenic vessel architecture to the healing process. Using single-cell mapping, we resolved the properties of endothelial cells that organize a polarized scaffold at the repair site of lesioned peripheral nerves. Transient reactivation of an embryonic guidance program is required to orient neovessels across the wound. Manipulation of this structured angiogenic response through genetic and pharmacological targeting of Plexin-D1/VEGF pathways within an early window of repair has long-term impact on configuration of the nerve stroma. Neovessels direct nerve-resident mesenchymal cells to mold a provisionary fibrotic scar by assembling an orderly system of stable barrier compartments that channel regenerating nerve fibers and shield them from the persistently leaky vasculature. Thus, guided and balanced repair angiogenesis enables the construction of a "bridge" microenvironment conducive for axon regrowth and homeostasis of the regenerated tissue.

IL-1β+ macrophages fuel pathogenic inflammation in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is a lethal disease with high resistance to therapies1. Inflammatory and immunomodulatory signals co-exist in the pancreatic tumour microenvironment, leading to dysregulated repair and cytotoxic responses. Tumour-associated macrophages (TAMs) have key roles in PDAC2, but their diversity has prevented therapeutic exploitation. Here we combined single-cell and spatial genomics with functional experiments to unravel macrophage functions in pancreatic cancer. We uncovered an inflammatory loop between tumour cells and interleukin-1β (IL-1β)-expressing TAMs, a subset of macrophages elicited by a local synergy between prostaglandin E2 (PGE2) and tumour necrosis factor (TNF). Physical proximity with IL-1β+ TAMs was associated with inflammatory reprogramming and acquisition of pathogenic properties by a subset of PDAC cells. This occurrence was an early event in pancreatic tumorigenesis and led to persistent transcriptional changes associated with disease progression and poor outcomes for patients. Blocking PGE2 or IL-1β activity elicited TAM reprogramming and antagonized tumour cell-intrinsic and -extrinsic inflammation, leading to PDAC control in vivo. Targeting the PGE2-IL-1β axis may enable preventive or therapeutic strategies for reprogramming of immune dynamics in pancreatic cancer.

Quantification of perineural invasion in pancreatic ductal adenocarcinoma: proposal of a severity score system

Perineural invasion (PNI) is a common feature in pancreatic ductal adenocarcinoma (PDAC) and correlates with an aggressive tumor behavior already at early stages of disease. PNI is currently considered as a "present vs. absent" feature, and a severity score system has not yet been established. The aim of the present study was thus to develop and validate a score system for PNI and to correlate it with other prognostic features. In this monocentric retrospective study, 356 consecutive PDAC patients (61.8% upfront surgery patients, 38.2% received neoadjuvant therapy) were analyzed. PNI was scored as follows: 0: absent; 1: the presence of neoplasia along nerves < 3 mm in caliber; and 2: neoplastic infiltration of nerve fibers ≥ 3 mm and/or massive perineural infiltration and/or the presence of necrosis of the infiltrated nerve bundle. For every PNI grade, the correlation with other pathological features, disease-free survival (DFS), and disease-specific survival (DSS) were analyzed. Uni- and multivariate analysis for DFS and DSS were also performed. PNI was found in 72.5% of the patients. Relevant trends between PNI score and tumor differentiation grade, lymph node metastases, vascular invasion, and surgical margins status were found. The latter was the only parameter statistically correlated with the proposed score. The agreement between pathologists was substantial (Cohen's K 0.61). PNI severity score significantly correlated also with decreased DFS and DSS at univariate analysis (p < 0.001). At multivariate analysis, only the presence of lymph node metastases was an independent predictor of DFS (HR 2.235 p < 0.001). Lymph node metastases (HR 2.902, p < 0.001) and tumor differentiation grade (HR 1.677, p = 0.002) were independent predictors of DSS. Our newly developed PNI score correlates with other features of PDAC aggressiveness and proved to have a prognostic role though less robust than lymph nodes metastases and tumor differentiation grade. A prospective validation is needed.

Oligodendrocyte-lineage cell exocytosis and L-type prostaglandin D synthase promote oligodendrocyte development and myelination

In the developing central nervous system, oligodendrocyte precursor cells (OPCs) differentiate into oligodendrocytes, which form myelin around axons. Oligodendrocytes and myelin are essential for the function of the central nervous system, as evidenced by the severe neurological symptoms that arise in demyelinating diseases such as multiple sclerosis and leukodystrophy. Although many cell-intrinsic mechanisms that regulate oligodendrocyte development and myelination have been reported, it remains unclear whether interactions among oligodendrocyte-lineage cells (OPCs and oligodendrocytes) affect oligodendrocyte development and myelination. Here, we show that blocking vesicle-associated membrane protein (VAMP) 1/2/3-dependent exocytosis from oligodendrocyte-lineage cells impairs oligodendrocyte development, myelination, and motor behavior in mice. Adding oligodendrocyte-lineage cell-secreted molecules to secretion-deficient OPC cultures partially restores the morphological maturation of oligodendrocytes. Moreover, we identified L-type prostaglandin D synthase as an oligodendrocyte-lineage cell-secreted protein that promotes oligodendrocyte development and myelination in vivo. These findings reveal a novel autocrine/paracrine loop model for the regulation of oligodendrocyte and myelin development.

Beyond Wrapping: Canonical and Noncanonical Functions of Schwann Cells

Schwann cells in the peripheral nervous system (PNS) are essential for the support and myelination of axons, ensuring fast and accurate communication between the central nervous system and the periphery. Schwann cells and related glia accompany innervating axons in virtually all tissues in the body, where they exhibit remarkable plasticity and the ability to modulate pathology in extraordinary, and sometimes surprising, ways. Here, we provide a brief overview of the various glial cell types in the PNS and describe the cornerstone cellular and molecular processes that enable Schwann cells to perform their canonical functions. We then dive into discussing exciting noncanonical functions of Schwann cells and related PNS glia, which include their role in organizing the PNS, in regulating synaptic activity and pain, in modulating immunity, in providing a pool of stem cells for different organs, and, finally, in influencing cancer.

Dysregulated copper transport in multiple sclerosis may cause demyelination via astrocytes

Demyelination is a key pathogenic feature of multiple sclerosis (MS). Here, we evaluated the astrocyte contribution to myelin loss and focused on the neurotrophin receptor TrkB, whose up-regulation on the astrocyte finely demarcated chronic demyelinated areas in MS and was paralleled by neurotrophin loss. Mice lacking astrocyte TrkB were resistant to demyelination induced by autoimmune or toxic insults, demonstrating that TrkB signaling in astrocytes fostered oligodendrocyte damage. In vitro and ex vivo approaches highlighted that astrocyte TrkB supported scar formation and glia proliferation even in the absence of neurotrophin binding, indicating TrkB transactivation in response to inflammatory or toxic mediators. Notably, our neuropathological studies demonstrated copper dysregulation in MS and model lesions and TrkB-dependent expression of copper transporter (CTR1) on glia cells during neuroinflammation. In vitro experiments evidenced that TrkB was critical for the generation of glial intracellular calcium flux and CTR1 up-regulation induced by stimuli distinct from neurotrophins. These events led to copper uptake and release by the astrocyte, and in turn resulted in oligodendrocyte loss. Collectively, these data demonstrate a pathogenic demyelination mechanism via the astrocyte release of copper and open up the possibility of restoring copper homeostasis in the white matter as a therapeutic target in MS.

Enhanced axonal neuregulin-1 type-III signaling ameliorates neurophysiology and hypomyelination in a Charcot-Marie-Tooth type 1B mouse model

Charcot-Marie-Tooth (CMT) neuropathies are a group of genetic disorders that affect the peripheral nervous system with heterogeneous pathogenesis and no available treatment. Axonal neuregulin 1 type III (Nrg1TIII) drives peripheral nerve myelination by activating downstream signaling pathways such as PI3K/Akt and MAPK/Erk that converge on master transcriptional regulators of myelin genes, such as Krox20. We reasoned that modulating Nrg1TIII activity may constitute a general therapeutic strategy to treat CMTs that are characterized by reduced levels of myelination. Here we show that genetic overexpression of Nrg1TIII ameliorates neurophysiological and morphological parameters in a mouse model of demyelinating CMT1B, without exacerbating the toxic gain-of-function that underlies the neuropathy. Intriguingly, the mechanism appears not to be related to Krox20 or myelin gene upregulation, but rather to a beneficial rebalancing in the stoichiometry of myelin lipids and proteins. Finally, we provide proof of principle that stimulating Nrg1TIII signaling, by pharmacological suppression of the Nrg1TIII inhibitor tumor necrosis factor-alpha-converting enzyme (TACE/ADAM17), also ameliorates the neuropathy. Thus, modulation of Nrg1TIII by TACE/ADAM17 inhibition may represent a general treatment for hypomyelinating neuropathies.

Ablation of neuronal ADAM17 impairs oligodendrocyte differentiation and myelination

Myelin, one of the most important adaptations of vertebrates, is essential to ensure efficient propagation of the electric impulse in the nervous system and to maintain neuronal integrity. In the central nervous system (CNS), the development of oligodendrocytes and the process of myelination are regulated by the coordinated action of several positive and negative cell-extrinsic factors. We and others previously showed that secretases regulate the activity of proteins essential for myelination. We now report that the neuronal α-secretase ADAM17 controls oligodendrocyte differentiation and myelin formation in the CNS. Ablation of Adam17 in neurons impairs in vivo and in vitro oligodendrocyte differentiation, delays myelin formation throughout development and results in hypomyelination. Furthermore, we show that this developmental defect is, in part, the result of altered Notch/Jagged 1 signaling. Surprisingly, in vivo conditional loss of Adam17 in immature oligodendrocytes has no effect on myelin formation. Collectively, our data indicate that the neuronal α-secretase ADAM17 is required for proper CNS myelination. Further, our studies confirm that secretases are important post-translational regulators of myelination although the mechanisms controlling CNS and peripheral nervous system (PNS) myelination are distinct.

Nerves and Pancreatic Cancer: New Insights into a Dangerous Relationship

Perineural invasion (PNI) is defined as the presence of neoplastic cells along nerves and/or within the different layers of nervous fibers: epineural, perineural and endoneural spaces. In pancreatic cancer-particularly in pancreatic ductal adenocarcinoma (PDAC)-PNI has a prevalence between 70 and 100%, surpassing any other solid tumor. PNI has been detected in the early stages of pancreatic cancer and has been associated with pain, increased tumor recurrence and diminished overall survival. Such an early, invasive and recurrent phenomenon is probably crucial for tumor growth and metastasis. PNI is a still not a uniformly characterized event; usually it is described only dichotomously ("present" or "absent"). Recently, a more detailed scoring system for PNI has been proposed, though not specific for pancreatic cancer. Previous studies have implicated several molecules and pathways in PNI, among which are secreted neurotrophins, chemokines and inflammatory cells. However, the mechanisms underlying PNI are poorly understood and several aspects are actively being investigated. In this review, we will discuss the main molecules and signaling pathways implicated in PNI and their roles in the PDAC.

Neuregulin 1 type III improves peripheral nerve myelination in a mouse model of congenital hypomyelinating neuropathy

Myelin sheath thickness is precisely regulated and essential for rapid propagation of action potentials along myelinated axons. In the peripheral nervous system, extrinsic signals from the axonal protein neuregulin 1 (NRG1) type III regulate Schwann cell fate and myelination. Here we ask if modulating NRG1 type III levels in neurons would restore myelination in a model of congenital hypomyelinating neuropathy (CHN). Using a mouse model of CHN, we improved the myelination defects by early overexpression of NRG1 type III. Surprisingly, the improvement was independent from the upregulation of Egr2 or essential myelin genes. Rather, we observed the activation of MAPK/ERK and other myelin genes such as peripheral myelin protein 2 and oligodendrocyte myelin glycoprotein. We also confirmed that the permanent activation of MAPK/ERK in Schwann cells has detrimental effects on myelination. Our findings demonstrate that the modulation of axon-to-glial NRG1 type III signaling has beneficial effects and improves myelination defects during development in a model of CHN.

The Complex Work of Proteases and Secretases in Wallerian Degeneration: Beyond Neuregulin-1

After damage, axons in the peripheral nervous system (PNS) regenerate and regrow following a process termed Wallerian degeneration, but the regenerative process is often incomplete and usually the system does not reach full recovery. Key steps to the creation of a permissive environment for axonal regrowth are the trans-differentiation of Schwann cells and the remodeling of the extracellular matrix (ECM). In this review article, we will discuss how proteases and secretases promote effective regeneration and remyelination. We will detail how they control neuregulin-1 (NRG-1) activity at the post-translational level, as the concerted action of alpha, beta and gamma secretases cooperates to balance activating and inhibitory signals necessary for physiological myelination and remyelination. In addition, we will discuss the role of other proteases in nerve repair, among which A Disintegrin And Metalloproteinases (ADAMs) and gamma-secretases substrates. Moreover, we will present how matrix metalloproteinases (MMPs) and proteases of the blood coagulation cascade participate in forming newly synthetized myelin and in regulating axonal regeneration. Overall, we will highlight how a deeper comprehension of secretases and proteases mechanism of action in Wallerian degeneration might be useful to develop new therapies with the potential of readily and efficiently improve the regenerative process.

Non-cell-autonomous function of DR6 in Schwann cell proliferation

Death receptor 6 (DR6) is an orphan member of the TNF receptor superfamily and controls cell death and differentiation in a cell-autonomous manner in different cell types. Here, we report an additional non-cell-autonomous function for DR6 in the peripheral nervous system (PNS). DR6-knockout (DR6 KO) mice showed precocious myelination in the PNS Using an in vitro myelination assay, we demonstrate that neuronal DR6 acts in trans on Schwann cells (SCs) and reduces SC proliferation and myelination independently of its cytoplasmic death domain. Mechanistically, DR6 was found to be cleaved in neurons by "a disintegrin and metalloprotease 10" (ADAM10), releasing the soluble DR6 ectodomain (sDR6). Notably, in the in vitro myelination assay, sDR6 was sufficient to rescue the DR6 KO phenotype. Thus, in addition to the cell-autonomous receptor function of full-length DR6, the proteolytically released sDR6 can unexpectedly also act as a paracrine signaling factor in the PNS in a non-cell-autonomous manner during SC proliferation and myelination. This new mode of DR6 signaling will be relevant in future attempts to target DR6 in disease settings.

DRG Neuron/Schwann Cells Myelinating Cocultures

Our understanding of the processes controlling peripheral nervous system myelination have been significantly benefited by the development of an in vitro myelinating culture system in which primary Schwann cells are cocultured together with primary sensory neurons. In this chapter, we describe the protocol currently used in our laboratories to establish Schwann cells neuronal myelinating cocultures. We also include a detailed description of the various substrates that can be used to establish it.

Laminin 211 inhibits protein kinase A in Schwann cells to modulate neuregulin 1 type III-driven myelination

Myelin is required for proper nervous system function. Schwann cells in developing nerves depend on extrinsic signals from the axon and from the extracellular matrix to first sort and ensheathe a single axon and then myelinate it. Neuregulin 1 type III (Nrg1III) and laminin α2β1γ1 (Lm211) are the key axonal and matrix signals, respectively, but how their signaling is integrated and if each molecule controls both axonal sorting and myelination is unclear. Here, we use a series of epistasis experiments to show that Lm211 modulates neuregulin signaling to ensure the correct timing and amount of myelination. Lm211 can inhibit Nrg1III by limiting protein kinase A (PKA) activation, which is required to initiate myelination. We provide evidence that excessive PKA activation amplifies promyelinating signals downstream of neuregulin, including direct activation of the neuregulin receptor ErbB2 and its effector Grb2-Associated Binder-1 (Gab1), thereby elevating the expression of the key transcription factors Oct6 and early growth response protein 2 (Egr2). The inhibitory effect of Lm211 is seen only in fibers of small caliber. These data may explain why hereditary neuropathies associated with decreased laminin function are characterized by focally thick and redundant myelin.

Unraveling gene expression profiles in peripheral motor nerve from amyotrophic lateral sclerosis patients: insights into pathogenesis

The aim of the present study is to investigate the molecular pathways underlying amyotrophic lateral sclerosis (ALS) pathogenesis within the peripheral nervous system. We analyzed gene expression changes in human motor nerve diagnostic biopsies obtained from eight ALS patients and seven patients affected by motor neuropathy as controls. An integrated transcriptomics and system biology approach was employed. We identified alterations in the expression of 815 genes, with 529 up-regulated and 286 down-regulated in ALS patients. Up-regulated genes clustered around biological process involving RNA processing and protein metabolisms. We observed a significant enrichment of up-regulated small nucleolar RNA transcripts (p = 2.68*10-11) and genes related to endoplasmic reticulum unfolded protein response and chaperone activity. We found a significant down-regulation in ALS of genes related to the glutamate metabolism. Interestingly, a network analysis highlighted HDAC2, belonging to the histone deacetylase family, as the most interacting node. While so far gene expression studies in human ALS have been performed in postmortem tissues, here specimens were obtained from biopsy at an early phase of the disease, making these results new in the field of ALS research and therefore appealing for gene discovery studies.

Niacin-mediated Tace activation ameliorates CMT neuropathies with focal hypermyelination

Charcot–Marie–Tooth (CMT) neuropathies are highly heterogeneous disorders caused by mutations in more than 70 genes, with no available treatment. Thus, it is difficult to envisage a single suitable treatment for all pathogenetic mechanisms. Axonal Neuregulin 1 (Nrg1) type III drives Schwann cell myelination and determines myelin thickness by ErbB2/B3‐PI3K–Akt signaling pathway activation. Nrg1 type III is inhibited by the α‐secretase Tace, which negatively regulates PNS myelination. We hypothesized that modulation of Nrg1 levels and/or secretase activity may constitute a unifying treatment strategy for CMT neuropathies with focal hypermyelination as it could restore normal levels of myelination. Here we show that in vivo delivery of Niaspan, a FDA‐approved drug known to enhance TACE activity, efficiently rescues myelination in the Mtmr2^−/− mouse, a model of CMT4B1 with myelin outfoldings, and in the Pmp22^+/− mouse, which reproduces HNPP (hereditary neuropathy with liability to pressure palsies) with tomacula. Importantly, we also found that Niaspan reduces hypermyelination of Vim (vimentin)^−/− mice, characterized by increased Nrg1 type III and Akt activation, thus corroborating the hypothesis that Niaspan treatment downregulates Nrg1 type III signaling.

Spatial mapping of juxtacrine axo-glial interactions identifies novel molecules in peripheral myelination

Cell–cell interactions promote juxtacrine signals in specific subcellular domains, which are difficult to capture in the complexity of the nervous system. For example, contact between axons and Schwann cells triggers signals required for radial sorting and myelination. Failure in this interaction causes dysmyelination and axonal degeneration. Despite its importance, few molecules at the axo-glial surface are known. To identify novel molecules in axo-glial interactions, we modified the ‘pseudopodia' sub-fractionation system and isolated the projections that glia extend when they receive juxtacrine signals from axons. By proteomics we identified the signalling networks present at the glial-leading edge, and novel proteins, including members of the Prohibitin family. Glial-specific deletion of Prohibitin-2 in mice impairs axo-glial interactions and myelination. We thus validate a novel method to model morphogenesis and juxtacrine signalling, provide insights into the molecular organization of the axo-glial contact, and identify a novel class of molecules in myelination.

Neuron–glia interactions are critical in the nervous system, where they result in the extension of glial pseudopodia. Poitelon et al. isolate these protrusions using an in vitro assay, and, by characterising their proteomes, identify Prohibitin-2 as a regulator of myelination.

New insights on Schwann cell development

In the peripheral nervous system, Schwann cells are glial cells that are in intimate contact with axons throughout development. Schwann cells generate the insulating myelin sheath and provide vital trophic support to the neurons that they ensheathe. Schwann cell precursors arise from neural crest progenitor cells, and a highly ordered developmental sequence controls the progression of these cells to become mature myelinating or nonmyelinating Schwann cells. Here, we discuss both seminal discoveries and recent advances in our understanding of the molecular mechanisms that drive Schwann cell development and myelination with a focus on cell-cell and cell-matrix signaling events.

Iron homeostasis in peripheral nervous system, still a black box?

SIGNIFICANCE

Iron is the most abundant transition metal in biology and an essential cofactor for many cellular enzymes. Iron homeostasis impairment is also a component of peripheral neuropathies.

RECENT ADVANCES

During the past years, much effort has been paid to understand the molecular mechanism involved in maintaining systemic iron homeostasis in mammals. This has been stimulated by the evidence that iron dyshomeostasis is an initial cause of several disorders, including genetic and sporadic neurodegenerative disorders.

CRITICAL ISSUES

However, very little has been done to investigate the physiological role of iron in peripheral nervous system (PNS), despite the development of suitable cellular and animal models.

FUTURE DIRECTIONS

To stimulate research on iron metabolism and peripheral neuropathy, we provide a summary of the knowledge on iron homeostasis in the PNS, on its transport across the blood-nerve barrier, its involvement in myelination, and we identify unresolved questions. Furthermore, we comment on the role of iron in iron-related disorder with peripheral component, in demyelinating and metabolic peripheral neuropathies.

Peripheral nerve morphogenesis induced by scaffold micropatterning

Several bioengineering approaches have been proposed for peripheral nervous system repair, with limited results and still open questions about the underlying molecular mechanisms. We assessed the biological processes that occur after the implantation of collagen scaffold with a peculiar porous micro-structure of the wall in a rat sciatic nerve transection model compared to commercial collagen conduits and nerve crush injury using functional, histological and genome wide analyses. We demonstrated that within 60 days, our conduit had been completely substituted by a normal nerve. Gene expression analysis documented a precise sequential regulation of known genes involved in angiogenesis, Schwann cells/axons interactions and myelination, together with a selective modulation of key biological pathways for nerve morphogenesis induced by porous matrices. These data suggest that the scaffold's micro-structure profoundly influences cell behaviors and creates an instructive micro-environment to enhance nerve morphogenesis that can be exploited to improve recovery and understand the molecular differences between repair and regeneration.

Vimentin regulates peripheral nerve myelination

Myelination is a complex process that requires coordinated Schwann cell-axon interactions during development and regeneration. Positive and negative regulators of myelination have been recently described, and can belong either to Schwann cells or neurons. Vimentin is a fibrous component present in both Schwann cell and neuron cytoskeleton, the expression of which is timely and spatially regulated during development and regeneration. We now report that vimentin negatively regulates myelination, as loss of vimentin results in peripheral nerve hypermyelination, owing to increased myelin thickness in vivo, in transgenic mice and in vitro in a myelinating co-culture system. We also show that this is due to a neuron-autonomous increase in the levels of axonal neuregulin 1 (NRG1) type III. Accordingly, genetic reduction of NRG1 type III in vimentin-null mice rescues hypermyelination. Finally, we demonstrate that vimentin acts synergistically with TACE, a negative regulator of NRG1 type III activity, as shown by hypermyelination of double Vim/Tace heterozygous mice. Our results reveal a novel role for the intermediate filament vimentin in myelination, and indicate vimentin as a regulator of NRG1 type III function.

Bace1 processing of NRG1 type III produces a myelin-inducing signal but is not essential for the stimulation of myelination

Myelin sheath thickness is precisely adjusted to axon caliber, and in the peripheral nervous system, neuregulin 1 (NRG1) type III is a key regulator of this process. It has been proposed that the protease BACE1 activates NRG1 dependent myelination. Here, we characterize the predicted product of BACE1-mediated NRG1 type III processing in transgenic mice. Neuronal overexpression of a NRG1 type III-variant, designed to mimic prior cleavage in the juxtamembrane stalk region, induces hypermyelination in vivo and is sufficient to restore myelination of NRG1 type III-deficient neurons. This observation implies that the NRG1 cytoplasmic domain is dispensable and that processed NRG1 type III is sufficient for all steps of myelination. Surprisingly, transgenic neuronal overexpression of full-length NRG1 type III promotes hypermyelination also in BACE1 null mutant mice. Moreover, NRG1 processing is impaired but not abolished in BACE1 null mutants. Thus, BACE1 is not essential for the activation of NRG1 type III to promote myelination. Taken together, these findings suggest that multiple neuronal proteases collectively regulate NRG1 processing. © 2011 Wiley Periodicals, Inc.

Type III neuregulin-1 promotes oligodendrocyte myelination

The axonal signals that regulate oligodendrocyte myelination during development of the central nervous system (CNS) have not been established. In this study, we have examined the regulation of oligodendrocyte myelination by the type III isoform of neuregulin-1 (NRG1), a neuronal signal essential for Schwann cell differentiation and myelination. In contrast to Schwann cells, primary oligodendrocytes differentiate normally when cocultured with dorsal root ganglia (DRG) neurons deficient in type III NRG1. However, they myelinate type III NRG1-deficient neurites poorly in comparison to wild type cultures. Type III NRG1 is not sufficient to drive oligodendrocyte myelination as sympathetic neurons are not myelinated even with lentiviral-mediated expression of NRG1. Mice haploinsufficient for type III NRG1 are hypomyelinated in the brain, as evidenced by reduced amounts of myelin proteins and lipids and thinner myelin sheaths. In contrast, the optic nerve and spinal cord of heterozygotes are myelinated normally. Together, these results implicate type III NRG1 as a significant determinant of the extent of myelination in the brain and demonstrate important regional differences in the control of CNS myelination. They also indicate that oligodendrocyte myelination, but not differentiation, is promoted by axonal NRG1, underscoring important differences in the control of myelination in the CNS and peripheral nervous system (PNS).

Interleukin-11 potentiates oligodendrocyte survival and maturation, and myelin formation

Mechanisms that regulate oligodendrocyte survival and myelin formation are an intense focus of research into myelin repair in the lesions of multiple sclerosis (MS). Although demyelination and oligodendrocyte loss are pathological hallmarks of the disease, increased oligodendrocyte numbers and remyelination are frequently observed in early lesions, but these diminish as the disease course progresses. In the current study, we used a microarray-based approach to investigate genes regulating repair in MS lesions, and identified interleukin-11 (IL-11) as an astrocyte-derived factor that potentiates oligodendrocyte survival and maturation, and myelin formation. IL-11 was induced in human astrocyte cultures by the cytokines IL-1beta and TGFbeta1, which are both prominently expressed in MS plaques. In MS tissue samples, IL-11 was expressed by reactive astrocytes, with expression particularly localized at the myelinated border of both active and silent lesions. Its receptor, IL-11R alpha, was expressed by oligodendrocytes. In experiments in human cultures in vitro, IL-11R alpha localized to immature oligodendrocytes, and its expression decreased during maturation. In cultures treated with IL-11, we observed a significant increase in oligodendrocyte number, and this was associated with enhanced oligodendrocyte survival and maturation. Importantly, we also found that IL-11 treatment was associated with significantly increased myelin formation in rodent CNS cocultures. These data are the first to implicate IL-11 in oligodendrocyte viability, maturation, and myelination. We suggest that this pathway may represent a potential therapeutic target for oligodendrocyte protection and remyelination in MS.

Neuregulin-1 type III determines the ensheathment fate of axons

The signals that determine whether axons are ensheathed or myelinated by Schwann cells have long been elusive. We now report that threshold levels of neuregulin-1 (NRG1) type III on axons determine their ensheathment fate. Ensheathed axons express low levels whereas myelinated fibers express high levels of NRG1 type III. Sensory neurons from NRG1 type III deficient mice are poorly ensheathed and fail to myelinate; lentiviral-mediated expression of NRG1 type III rescues these defects. Expression also converts the normally unmyelinated axons of sympathetic neurons to myelination. Nerve fibers of mice haploinsufficient for NRG1 type III are disproportionately unmyelinated, aberrantly ensheathed, and hypomyelinated, with reduced conduction velocities. Type III is the sole NRG1 isoform retained at the axon surface and activates PI 3-kinase, which is required for Schwann cell myelination. These results indicate that levels of NRG1 type III, independent of axon diameter, provide a key instructive signal that determines the ensheathment fate of axons.

Characterization of a Schwann cell enhancer in the myelin basic protein gene

Oligodendrocytes and Schwann cells use diverse regulatory elements to transcribe myelin basic protein (Mbp). For example, an enhancer 9.0 kb upstream of Mbp (MbpSCE1) activated either the proximal Mbp or hsp68 promoters only in Schwann cells in transgenic mice. Here, we analyze MbpSCE1 in vitro and in vivo and show that MbpSCE1 also activates another myelin gene, Mpz, specifically in Schwann cells in transgenic mice. Surprisingly, although MbpSCE1 behaves as an enhancer in Schwann cells, we show here for the first time that it also activates transcription robustly in cultured oligodendrocytes, but only from its original genomic position. Diverse nuclear proteins binding at distinct combinations of DNA elements in the two cell types may account for cell-specific MbpSCE1 function. A further surprise is that MbpSCE1 activation does not depend on consensus binding sites for the myelin-associated PPARbeta or Krox 20 transcription factors. Finally, chromatin context augments activation of MbpSCE1. Therefore, MbpSCE1 is active in both Schwann cells and oligodendrocytes, but by diverse mechanisms.

Identification and Characterization of ZFP-57, a Novel Zinc Finger Transcription Factor in the Mammalian Peripheral Nervous System

To isolate new zinc finger genes expressed at early stages of peripheral nerve development, we have used PCR to amplify conserved zinc finger sequences. RNA from rat embryonic day 12 and 13 sciatic nerves, a stage when nerves contain Schwann cell precursors, was used to identify several genes not previously described in Schwann cells. One of them, zinc finger protein (ZFP)-57, proved to be the homologue of a mouse gene found in F9 teratocarcinoma cells. Its mRNA expression profile within embryonic and adult normal and transected peripheral nerves, and its distribution in the rest of the nervous system is described. High levels of expression are seen in embryonic nerves and spinal cord. These drop rapidly during the first few weeks after birth, a pattern mirrored in other parts of the nervous system. ZFP-57 localizes to the nucleus of Schwann and other cells. The sequence contains an N-terminal Krüppel-associated box (KRAB) domain and ZFP-57 constructs containing green fluorescent protein reveal that the protein colocalizes with heterochromatin protein 1alpha to centromeric heterochromatin in a characteristic speckled pattern in NIH3T3 cells. The KRAB domain is required for this localization, because constructs lacking it target the protein to the nucleus but not to the centromeric heterochromatin. When fused to a heterologous DNA binding domain, the KRAB domain of ZFP-57 represses transcription, and full-length ZFP-57 represses Schwann cell transcription from myelin basic protein and P(0) promoters in co-transfection assays. Zfp-57 mRNA is up-regulated in Schwann cells in response to leukemia inhibitory factor and fibroblast growth factor 2.

Expression of laminin receptors in schwann cell differentiation: evidence for distinct roles

Schwann cells require laminin-2 throughout nerve development, because mutations in the alpha2 chain in dystrophic mice interfere with sorting of axons before birth and formation of myelin internodes after birth. Mature Schwann cells express several laminin receptors, but their expression and roles in development are poorly understood. Therefore, we correlated the onset of myelination in nerve and synchronized myelinating cultures to the appearance of integrins and dystroglycan in Schwann cells. Only alpha6beta1 integrin is expressed before birth, whereas dystroglycan and alpha6beta4 integrin appear perinatally, just before myelination. Although dystroglycan is immediately polarized to the outer surface of Schwann cells, alpha6beta4 appears polarized only after myelination. We showed previously that Schwann cells lacking beta1 integrin do not relate properly to axons before birth. Here we show that the absence of beta1 before birth is not compensated by other laminin receptors, whereas coexpression of both dystroglycan and beta4 integrin is likely required for beta1-null Schwann cells to myelinate after birth. Finally, both beta1-null and dystrophic nerves contain bundles of unsorted axons, but they are predominant in different regions: in spinal roots in dystrophic mice and in nerves in beta1-null mice. We show that differential compensation by laminin-1, but not laminin receptors may partially explain this. These data suggest that the action of laminin is mediated by beta1 integrins during axonal sorting and by dystroglycan, alpha6beta1, and alpha6beta4 integrins during myelination.

MEBA derepresses the proximal myelin basic protein promoter in oligodendrocytes

The central nervous system expression of myelin basic protein (MBP) is restricted to oligodendrocytes and is developmentally regulated; these regulatory features are transcriptionally mediated. We have previously shown that the proximal 149 nucleotides of the MBP promoter were both necessary and sufficient to activate the transcription of MBP in cultured oligodendrocytes, but not in other cell types. Sequences within the distal portion of this promoter, which contains a nuclear factor 1 (NF1) binding site, repressed activation of the MBP promoter in Cos-7 cells, but not in oligodendrocytes. We now describe a sequence upstream of and partially overlapping the NF1 site that activates the MBP promoter in oligodendrocytes, but not in Cos-7 cells. A protein complex binds to this site, designated MEBA (myelinating glia-enriched DNA binding activity), and is enriched in nuclear extracts prepared from the brain, oligodendrocytes, and Schwann cells. The amount of MEBA parallels MBP expression and myelinogenesis in the developing brain and parallels new MBP expression as purified oligodendrocytes differentiate. Mutational analyses of binding and function distinguish MEBA, an activator, from NF1, a repressor of MBP transcription, and suggest that MEBA consists of at least two proteins. Because the binding sites of MEBA and NF1 overlap, we suggest that MEBA may either compete with or modify NF1 binding, thereby activating the MBP promoter in oligodendrocytes.

Transduced fibroblasts and metachromatic leukodystrophy lymphocytes transfer arylsulfatase A to myelinating glia and deficient cells in vitro

Metachromatic leukodystrophy (MLD) is a lysosomal storage disease, caused by deficiency of arylsulfatase A (ASA), that manifests primarily in the white matter of the nervous system. Currently, no specific treatment exists that will reverse its fatal outcome. Replacement therapy has been hampered by the blood-brain barrier (BBB). To circumvent this problem we designed an ex vivo gene therapy strategy that includes the retrovirus-mediated ASA transduction of cells, such as activated lymphocytes, that are able to traverse the BBB or other membranes of the CNS. For this purpose, two recombinant retroviruses based on the pLXSN vector were produced, containing the wild-type ASA cDNA or a pseudodeficiency ASA cDNA, which encodes a smaller enzyme with normal activity. After transduction, ASA activity increased more than 100-fold in fibroblasts from an MLD patient. Furthermore, ASA-transduced MLD PBLs expressed 30 times higher ASA activity when compared with control PBLs. Moreover, cell culture experiments demonstrated that transduced fibroblasts could efficiently transfer ASA to deficient cells across a transwell barrier, whereas transduced MLD lymphocytes could transfer ASA to deficient fibroblasts only by direct cell-to-cell contact. Finally, ASA was taken up by normal oligodendrocytes and Schwann cells, the target myelinating glial cells for therapy in MLD. These data suggest possible short-term strategies for transfer of ASA into the CNS via transduced autologous cells while long-term strategies, related to autologous transduced bone marrow transplant, take effect in patients.

A minimal human MBP promoter-lacZ transgene is appropriately regulated in developing brain and after optic enucleation, but not in shiverer mutant mice

Previous studies, both in vitro and in vivo, suggest that small portions of the mouse myelin basic protein (MBP) promoter are sufficient to activate regulated expression of MBP. To confirm our previous in vitro studies, we prepared transgenic mice with short regions of the human MBP promoter fused to the lacZ reporter gene. We found that 750 nucleotides of the proximal human MBP promoter is sufficient to activate oligodendrocyte-specific, developmentally regulated expression of lacZ in three independent lines. This promoter, however, does not activate expression of lacZ in Schwann cells in peripheral nerve or in adult mouse brain. The relative levels of beta-galactosidase specific activity, mRNA, and transcription parallel those of MBP mRNA during myelinogenesis. Thus, we exploited this transgene as a quantitative tool to evaluate the response to stimuli known to affect myelination. Transgene expression is reduced 75 % after optic enucleation, as previously reported for levels of MBP mRNA, indicating that axons signal to this portion of the proximal MBP promoter to fully activate MBP expression during myelinogenesis. Instead, in adult shiverer mice, another setting in which MBP transcription is modulated, transgene expression is not increased, in contrast to the increased transcriptional activation of MBP previously reported in these mice. These data suggest that the regulatory region that mediates transcriptional activation of the MBP gene is modular, since discrete subregions are required for activation in Schwann cells, during myelinogenesis in oligodendrocytes, during maintenance myelination in adult brain, and in the dysmyelinating mutant shiverer mouse.