Biomedical subjects

Kristjan R Jessen

Publications and source records attributed to Kristjan R Jessen.

16 recordsLinked to original sources

PubMed2006

A double point mutation in the DNA-binding region of Egr2 switches its function from inhibition to induction of proliferation: A potential contribution to the development of congenital hypomyelinating neuropathy.

Mutations in the DNA-binding domain of EGR2 are associated with severe autosomal dominant forms of peripheral neuropathy. In this study, we show that one such Egr2 mutant (S382R, D383Y), when expressed in Schwann cells in vitro, is not transcriptionally inactive but retains residual wild-type Egr2 functions, including inhibition of transforming growth factor-beta-induced Schwann cell death and an ability to induce the cytoskeletal protein periaxin. More importantly, this mutant Egr2 has aberrant effects in Schwann cells, enhancing DNA synthesis both in the presence and absence of the putative axonal mitogen, beta-neuregulin 1. This is in stark contrast to wild-type Egr2, which causes withdrawal from the cell cycle. Furthermore, mutant Egr2 upregulates cyclin D1 and reduces levels of the cell cycle inhibitor, p27. These observations add significant new evidence to explain how this mutation leads to congenital hypomyelinating neuropathy in humans.

Alleles↗

PubMed2006

A screen for mutations in zebrafish that affect myelin gene expression in Schwann cells and oligodendrocytes.

Myelin is the multi-layered glial sheath around axons in the vertebrate nervous system. Myelinating glia develop and function in intimate association with neurons and neuron-glial interactions control much of the life history of these cells. However, many of the factors that regulate key aspects of myelin development and maintenance remain unknown. To discover new molecules that are important for glial development and myelination, we undertook a screen of zebrafish mutants with previously characterized neural defects. We screened for myelin basic protein (mbp) mRNA by in situ hybridization and identified four mutants (neckless, motionless, iguana and doc) that lacked mbp expression in parts of the peripheral and central nervous systems (PNS or CNS), despite the presence of axons. In all four mutants electron microscopy revealed that myelin-forming glia were present and had formed loose wraps around axons but did not form compact myelin. We found that addition of exogenous retinoic acid (RA) rescued mbp expression in neckless mutant embryos, which lack endogenous RA synthesis. Timed application of the RA synthesis inhibitor DEAB to wild type embryos showed that RA signalling is required at least 48 h before the onset of myelin protein synthesis in both CNS and PNS.

Animals↗

PubMed2006

The structural and functional integrity of peripheral nerves depends on the glial-derived signal desert hedgehog.

We show that desert hedgehog (dhh), a signaling molecule expressed by Schwann cells, is essential for the structural and functional integrity of the peripheral nerve. Dhh-null nerves display multiple abnormalities that affect myelinating and nonmyelinating Schwann cells, axons, and vasculature and immune cells. Myelinated fibers of these mice have a significantly increased (more than two times) number of Schmidt-Lanterman incisures (SLIs), and connexin 29, a molecular component of SLIs, is strongly upregulated. Crossing Dhh-null mice with myelin basic protein (MBP)-deficient shiverer mice, which also have increased SLI numbers, results in further increased SLIs, suggesting that Dhh and MBP control SLIs by different mechanisms. Unmyelinated fibers are also affected, containing many fewer axons per Schwann cell in transverse profiles, whereas the total number of unmyelinated axons is reduced by approximately one-third. In Dhh-null mice, the blood-nerve barrier is permeable and neutrophils and macrophage numbers are elevated, even in uninjured nerves. Dhh-null nerves also lack the largest-diameter myelinated fibers, have elevated numbers of degenerating myelinated axons, and contain regenerating fibers. Transected dhh nerves degenerate faster than wild-type controls. This demonstrates that a single identified glial signal, Dhh, plays a critical role in controlling the integrity of peripheral nervous tissue, in line with its critical role in nerve sheath development (Parmantier et al., 1999). The complexity of the defects raises a number of important questions about the Dhh-dependent cell-cell signaling network in peripheral nerves.

Aging↗

PubMed2006

Gene profiling and bioinformatic analysis of Schwann cell embryonic development and myelination.

To elucidate the molecular mechanisms involved in Schwann cell development, we profiled gene expression in the developing and injured rat sciatic nerve. The genes that showed significant changes in expression in developing and dedifferentiated nerve were validated with RT-PCR, in situ hybridisation, Western blot and immunofluorescence. A comprehensive approach to annotating micro-array probes and their associated transcripts was performed using Biopendium, a database of sequence and structural annotation. This approach significantly increased the number of genes for which a functional insight could be found. The analysis implicates agrin and two members of the collapsin response-mediated protein (CRMP) family in the switch from precursors to Schwann cells, and synuclein-1 and alphaB-crystallin in peripheral nerve myelination. We also identified a group of genes typically related to chondrogenesis and cartilage/bone development, including type II collagen, that were expressed in a manner similar to that of myelin-associated genes. The comprehensive function annotation also identified, among the genes regulated during nerve development or after nerve injury, proteins belonging to high-interest families, such as cytokines and kinases, and should therefore provide a uniquely valuable resource for future research.

Agrin↗

PubMed2006

Desert hedgehog-patched 2 expression in peripheral nerves during Wallerian degeneration and regeneration.

Hedgehog proteins are important in the development of the nervous system. As Desert hedgehog (Dhh) is involved in the development of peripheral nerves and is expressed in adult nerves, it may play a role in the maintenance of adult nerves and degeneration and regeneration after injury. We firstly investigated the Dhh-receptors, which are expressed in mouse adult nerves. The Dhh receptor patched(ptc)2 was detected in adult sciatic nerves using RT-PCR, however, ptc1 was undetectable under the same experimental condition. Using RT-PCR in purified cultures of mouse Schwann cells and fibroblasts, we found ptc2 mRNA in Schwann cells, and at much lower levels, in fibroblasts. By immunohistochemistry, Ptc2 protein was seen on unmyelinated nerve fibers. Then we induced crush injury to the sciatic nerves of wild-type (WT) and dhh-null mice and the distal stumps of injured nerves were analyzed morphologically at different time points and expression of dhh and related receptors was also measured by RT-PCR in WT mice. In dhh-null mice, degeneration of myelinated fibers was more severe than in WT mice. Furthermore, in regenerated nerves of dhh-null mice, minifascicular formation was even more extensive than in dhh-null intact nerves. Both dhh and ptc2 mRNA levels were down-regulated during the degenerative phase postinjury in WT mice, while levels rose again during the phase of nerve regeneration. These results suggest that the Dhh-Ptc2 signaling pathway may be involved in the maintenance of adult nerves and may be one of the factors that directly or indirectly determines the response of peripheral nerves to injury.

Animals↗

PubMed2006

A brief look at glial cells.

Glial cells are numerically the dominant cell type in the central and peripheral nervous system. They are intermixed with the nerve cells and are found in intimate contacts with neuronal cell bodies, dendrites, axons and synaptic contacts. Like neurons, glial cells are a heterogeneous population of cells that differ in developmental origin, molecular composition, structure and activity. When these cells were first discovered some 150 years ago they were viewed as a type of connective tissue support for nerve cells. They are now known to be essential for the development and function of the brain and other parts of the nervous system. They are also central players in a large number of pathological processes. We have therefore moved away from a view of the nervous system as a system of neurons, to the appreciation that it is a neural system where the contributions of both nerve and glial cells are intimately integrated, interdependent and obligatory.

Animals↗

PubMed2005

The origin and development of glial cells in peripheral nerves.

During the development of peripheral nerves, neural crest cells generate myelinating and non-myelinating glial cells in a process that parallels gliogenesis from the germinal layers of the CNS. Unlike central gliogenesis, neural crest development involves a protracted embryonic phase devoted to the generation of, first, the Schwann cell precursor and then the immature Schwann cell, a cell whose fate as a myelinating or non-myelinating cell has yet to be determined. Embryonic nerves therefore offer a particular opportunity to analyse the early steps of gliogenesis from transient multipotent stem cells, and to understand how this process is integrated with organogenesis of peripheral nerves.

Animals↗

PubMed2004

Novel method for studying myelination in vivo reveals that EDTA is a potent inhibitor of myelin protein and mRNA expression during development of the rat sciatic nerve.

To probe the effects of possible inhibitors or enhancers of in vivo myelination, we have modified a technique widely used in studies of the developing neuromuscular system that involves incorporation of test compounds into a silicon rubber solution, which solidifies on contact with air. U-shaped rubber implants are inserted around the sciatic nerve of 1-day-old rats and left in place for 24-48 h. Sections from the region of the nerve lying within the implant, with or without the test compound, are then immunolabeled, examined with in situ hybridization or electron microscopy. Application of EDTA (440 microg/implant) in this way strongly suppressed the levels of the myelin-associated molecules protein P0, myelin basic protein (MBP), and galactocerebroside (Galc). mRNA levels for P0 and the myelin-related transcription factor Krox-20 were also reduced, further supporting association of the EDTA-induced effect with the myelinating Schwann cells. In contrast, no obvious differences were observed in either neurofilament (NF) protein or glial fibrillary acidic protein (GFAP) expression, suggesting absence of influence on axons or nonmyelinating Schwann cells. Despite the severely altered molecular composition of myelin in the presence of EDTA, examination in the electron microscope did not reveal any apparent ultrastructural changes in the myelin sheaths or nerve development. This work introduces a novel method for studying nerve development and shows that EDTA, which chelates divalent cations such as Ca(2+) and Mg(2+), strongly and selectively reduces levels of molecules, which, on postnatal days 1-4, are expressed in myelinating cells at much higher levels than in cells not engaged in myelination.

Animals↗

PubMed2004

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.

Amino Acid Sequence↗

PubMed2004

Krox-20 inhibits Jun-NH2-terminal kinase/c-Jun to control Schwann cell proliferation and death.

The transcription factor Krox-20 controls Schwann cell myelination. Schwann cells in Krox-20 null mice fail to myelinate, and unlike myelinating Schwann cells, continue to proliferate and are susceptible to death. We find that enforced Krox-20 expression in Schwann cells cell-autonomously inactivates the proliferative response of Schwann cells to the major axonal mitogen beta-neuregulin-1 and the death response to TGFbeta or serum deprivation. Even in 3T3 fibroblasts, Krox-20 not only blocks proliferation and death but also activates the myelin genes periaxin and protein zero, showing properties in common with master regulatory genes in other cell types. Significantly, a major function of Krox-20 is to suppress the c-Jun NH2-terminal protein kinase (JNK)-c-Jun pathway, activation of which is required for both proliferation and death. Thus, Krox-20 can coordinately control suppression of mitogenic and death responses. Krox-20 also up-regulates the scaffold protein JNK-interacting protein 1 (JIP-1). We propose this as a possible component of the mechanism by which Krox-20 regulates JNK activity during Schwann cell development.

3T3 Cells↗

PubMed2004

Glial cells.

The nervous system is built from two broad categories of cells, neurones and glial cells. The glial cells outnumber the neurones and the two cell types occupy a comparable amount of space in nervous tissue. The main glial cell types are, in the central nervous system, astrocytes and oligodendrocytes and, in the peripheral nervous system, Schwann cells, enteric glial cells and satellite cells. In the embryo, glial cells form a cellular framework that permits the development of the rest of the nervous system, and regulate neuronal survival and differentiation. The best known function of glia in the adult is the formation of myelin sheaths around axons thus allowing the fast conduction of signalling essential for nervous system function. Glia also maintain appropriate concentrations of ions and neurotransmitters in the neuronal environment. Increasing body of evidence indicates that glial cells are essential regulators of the formation, maintenance and function of synapses, the key functional unit of the nervous system.

Animals↗

PubMed2004

The trunk neural crest and its early glial derivatives: a study of survival responses, developmental schedules and autocrine mechanisms.

Regulation of survival during gliogenesis from the trunk neural crest is poorly understood. Using adapted survival assays, we directly compared crest cells and the crest-derived precursor populations that generate satellite cells and Schwann cells. A range of factors that supports Schwann cells and glial precursors does not rescue crest, with the major exception of neuregulin-1 that rescues crest cells provided they contact the extracellular matrix. Autocrine survival appears earlier in developing satellite cells than Schwann cells. Satellite cells also show early expression of S100beta, BFABP and fibronectin and early survival responses to IGF-1, NT-3 and PDGF-BB that in developing Schwann cells are not seen until the precursor/Schwann cell transition. These experiments define novel differences between crest cells and early glia and show that entry to the glial lineage is an important point for regulation of survival responses. They show that survival mechanisms among PNS glia differ early in development and that satellite cell development runs ahead of schedule compared to Schwann cells in several significant features.

Animals↗

PubMed2003

Regulation of the myelin gene periaxin provides evidence for Krox-20-independent myelin-related signalling in Schwann cells.

We investigated the role of Krox-20 (Egr2), a transcription factor that regulates myelination, in controlling the myelin-associated protein periaxin. In developing Schwann cells, periaxin immunoreactivity appeared at least 2 days before Krox-20-immunopositive nuclei. Consistent with this, in Krox-20 null mice periaxin was upregulated on schedule, albeit to a lower level. In culture Krox-20 and periaxin were upregulated by cAMP as expected for myelin genes. Only those cells with the highest periaxin levels also expressed Krox-20, while other periaxin-positive cells remained Krox-20-negative. Furthermore, cAMP elevated periaxin even in Krox-20 null cells. We also found that in culture enforced Krox-20 expression induced expression of periaxin mRNA and protein in the absence of cAMP elevating agents, and that this induction was inhibited by the co-repressor NAB2. These findings reveal a dual mechanism for periaxin regulation and suggest that the role of Krox-20 is to amplify an earlier Krox-20-independent activation of the periaxin gene. Thus the axonal signals responsible for myelination are only partially transduced in Schwann cells by mechanisms that depend on Krox-20.

Animals↗

PubMed2002

Denervated Schwann cells attract macrophages by secretion of leukemia inhibitory factor (LIF) and monocyte chemoattractant protein-1 in a process regulated by interleukin-6 and LIF.

Injury to peripheral nerves results in the infiltration of immune cells, which remove axonal- and myelin-derived material. Schwann cells could play a key role in this process by regulating macrophage infiltration. We show here that medium conditioned by primary denervated Schwann cells or the Schwannoma cell line RN22 produces chemotactic activity for macrophages. The presence of blocking antibodies to macrophage chemoattractant protein-1 (MCP-1) or leukemia inhibitory factor (LIF) reduced this activity to approximately 35 and 65% of control levels, respectively, and only 15% remained in the presence of both antibodies. The presence of chemotactic LIF in Schwann cell-conditioned medium was confirmed by using cells from lif-/- mice. Although interleukin-6 (IL-6) is not itself a chemotactic factor, we found that medium from il-6-/- nerves showed only 40% of the activity secreted by wild-type nerves. Furthermore, IL-6 rapidly induced LIF mRNA in primary Schwann cells, and LIF rapidly induced MCP-1 mRNA expression. Treatment of RN22 Schwannoma cells with IL-6 or LIF enhanced the secretion of the chemotactic activity of these cells. These observations show that Schwann cells attract macrophages by secreting MCP-1 and LIF. They also provide evidence for an autocrine-signaling cascade involving IL-6, LIF, and MCP-1, which amplifies the Schwann cell-derived chemotactic signals gradually, in agreement with the delayed entry of macrophages to injured nerves.

Animals↗

PubMed2002

beta-Neuregulin and autocrine mediated survival of Schwann cells requires activity of Ets family transcription factors.

Members of the Ets transcription factor family function in many biological processes. We show the presence of Ets transcription factors, most prominently Net, in neonatal rat Schwann cells, and demonstrate Ets-dependent transcription under conditions where the cells are exposed to autocrine signals or autocrine signals plus beta-neuregulin. Using the potent MAPK kinase inhibitor U0126 we also confirm that the MAP kinase pathway, an activator of Ets transcription, is involved in beta-neuregulin mediated Schwann cell survival. Furthermore, we find that expression of dominant negative Ets1 (N70-Ets1) inhibits both the beta-neuregulin and autocrine survival of Schwann cells. In contrast, the survival of Schwann cells mediated by lysophosphatidic acid (LPA) is unaffected by expression of a dominant negative Ets molecule. These data demonstrate that distinct autocrine and beta-neuregulin survival signals converge in their requirement for Ets dependent transcription in Schwann cell survival.

Animals↗

PubMedDate not supplied

Schwann cells as regulators of nerve development.

Myelinating and non-myelinating Schwann cells of peripheral nerves are derived from the neural crest via an intermediate cell type, the Schwann cell precursor [K.R. Jessen, A. Brennan, L. Morgan, R. Mirsky, A. Kent, Y. Hashimoto, J. Gavrilovic. The Schwann cell precursor and its fate: a study of cell death and differentiation during gliogenesis in rat embryonic nerves, Neuron 12 (1994) 509-527]. The survival and maturation of Schwann cell precursors is controlled by a neuronally derived signal, beta neuregulin. Other factors, in particular endothelins, regulate the timing of precursor maturation and Schwann cell generation. In turn, signals derived from Schwann cell precursors or Schwann cells regulate neuronal numbers during development, and axonal calibre, distribution of ion channels and neurofilament phosphorylation in myelinated axons. Unlike Schwann cell precursors, Schwann cells in older nerves survive in the absence of axons, indicating that a significant change in survival regulation occurs. This is due primarily to the presence of autocrine growth factor loops in Schwann cells, present from embryo day 18 onwards, that are not functional in Schwann cell precursors. The most important components of the autocrine loop are insulin-like growth factors, platelet derived growth factor-BB and neurotrophin 3, which together with laminin support long-term Schwann cell survival. The paracrine dependence of precursors on axons for survival provides a mechanism for matching precursor cell number to axons in embryonic nerves, while the ability of Schwann cells to survive in the absence of axons is an absolute prerequisite for nerve repair following injury. In addition to providing survival factors to neurones and themselves, and signals that determine axonal architecture, Schwann cells also control the formation of peripheral nerve sheaths. This involves Schwann cell-derived Desert Hedgehog, which directs the transition of mesenchymal cells to form the epithelium-like structure of the perineurium. Schwann cells thus signal not only to themselves but also to the other cellular components within the nerve to act as major regulators of nerve development.

Animals↗