Sox2 expression in Schwann cells inhibits myelination in vivo and induces influx of macrophages to the nerve.

Correct myelination is crucial for the function of the peripheral nervous system. Both positive and negative regulators within the axon and Schwann cell function to ensure the correct onset and progression of myelination during both development and following peripheral nerve injury and repair. The Sox2 transcription factor is well known for its roles in the development and maintenance of progenitor and stem cell populations, but has also been proposed in vitro as a negative regulator of myelination in Schwann cells. We wished to test fully whether Sox2 regulates myelination in vivo and show here that, in mice, sustained Sox2 expression in vivo blocks myelination in the peripheral nerves and maintains Schwann cells in a proliferative non-differentiated state, which is also associated with increased inflammation within the nerve. The plasticity of Schwann cells allows them to re-myelinate regenerated axons following injury and we show that re-myelination is also blocked by Sox2 expression in Schwann cells. These findings identify Sox2 as a physiological regulator of Schwann cell myelination in vivo and its potential to play a role in disorders of myelination in the peripheral nervous system.

Merlin controls the repair capacity of Schwann cells after injury by regulating Hippo/YAP activity.

Loss of the Merlin tumor suppressor and activation of the Hippo signaling pathway play major roles in the control of cell proliferation and tumorigenesis. We have identified completely novel roles for Merlin and the Hippo pathway effector Yes-associated protein (YAP) in the control of Schwann cell (SC) plasticity and peripheral nerve repair after injury. Injury to the peripheral nervous system (PNS) causes a dramatic shift in SC molecular phenotype and the generation of repair-competent SCs, which direct functional repair. We find that loss of Merlin in these cells causes a catastrophic failure of axonal regeneration and remyelination in the PNS. This effect is mediated by activation of YAP expression in Merlin-null SCs, and loss of YAP restores axonal regrowth and functional repair. This work identifies new mechanisms that control the regenerative potential of SCs and gives new insight into understanding the correct control of functional nerve repair in the PNS.

X-rays only when you want them: optimized pump-probe experiments using pseudo-single-bunch operation.

Laser pump-X-ray probe experiments require control over the X-ray pulse pattern and timing. Here, the first use of pseudo-single-bunch mode at the Advanced Light Source in picosecond time-resolved X-ray absorption experiments on solutions and solids is reported. In this mode the X-ray repetition rate is fully adjustable from single shot to 500 kHz, allowing it to be matched to typical laser excitation pulse rates. Suppressing undesired X-ray pulses considerably reduces detector noise and improves signal to noise in time-resolved experiments. In addition, dose-induced sample damage is considerably reduced, easing experimental setup and allowing the investigation of less robust samples. Single-shot X-ray exposures of a streak camera detector using a conventional non-gated charge-coupled device (CCD) camera are also demonstrated.

Loss of SOX10 function contributes to the phenotype of human Merlin-null schwannoma cells.

Loss of the Merlin tumour suppressor causes abnormal de-differentiation and proliferation of Schwann cells and formation of schwannoma tumours in patients with neurofibromatosis type 2. Within the mature peripheral nerve the normal development, differentiation and maintenance of myelinating and non-myelinating Schwann cells is regulated by a network of transcription factors that include SOX10, OCT6 (now known as POU3F1), NFATC4 and KROX20 (also known as Egr2). We have examined for the first time how their regulation of Schwann cell development is disrupted in primary human schwannoma cells. We find that induction of both KROX20 and OCT6 is impaired, whereas enforced expression of KROX20 drives both myelin gene expression and cell cycle arrest in Merlin-null cells. Importantly, we show that human schwannoma cells have reduced expression of SOX10 protein and messenger RNA. Analysis of mouse SOX10-null Schwann cells shows they display many of the characteristics of human schwannoma cells, including increased expression of platelet derived growth factor receptor beta (PDGFRB) messenger RNA and protein, enhanced proliferation, increased focal adhesions and schwannoma-like morphology. Correspondingly, reintroduction of SOX10 into human Merlin-null cells restores the ability of these cells to induce KROX20 and myelin protein zero (MPZ), localizes NFATC4 to the nucleus, reduces cell proliferation and suppresses PDGFRB expression. Thus, we propose that loss of the SOX10 protein, which is vital for normal Schwann cell development, is also key to the pathology of Merlin-null schwannoma tumours.

Regulation of Schwann cell differentiation and proliferation by the Pax-3 transcription factor.

Pax-3 is a paired domain transcription factor that plays many roles during vertebrate development. In the Schwann cell lineage, Pax-3 is expressed at an early stage in Schwann cells precursors of the embryonic nerve, is maintained in the nonmyelinating cells of the adult nerve, and is upregulated in Schwann cells after peripheral nerve injury. Consistent with this expression pattern, Pax-3 has previously been shown to play a role in repressing the expression of the myelin basic protein gene in Schwann cells. We have studied the role of Pax-3 in Schwann cells and have found that it controls not only the regulation of cell differentiation but also the survival and proliferation of Schwann cells. Pax-3 expression blocks both the induction of Oct-6 and Krox-20 (K20) by cyclic AMP and completely inhibits the ability of K20, the physiological regulator of myelination in the peripheral nervous system, to induce myelin gene expression in Schwann cells. In contrast to other inhibitors of myelination, we find that Pax-3 represses myelin gene expression in a c-Jun-independent manner. In addition to this, we find that Pax-3 expression alone is sufficient to inhibit the induction of apoptosis by TGFß1 in Schwann cells. Expression of Pax-3 is also sufficient to induce the proliferation of Schwann cells in the absence of added growth factors and to reverse K20-induced exit from the cell cycle. These findings indicate new roles for the Pax-3 transcription factor in controlling the differentiation and proliferation of Schwann cells during development and after peripheral nerve injury.

Pseudo-single-bunch with adjustable frequency: a new operation mode for synchrotron light sources.

We present the concept and results of pseudo-single-bunch (PSB) operation--a new operational mode at the advanced light source--that can greatly expand the capabilities of synchrotron light sources to carry out dynamics and time-of-flight experiments. In PSB operation, a single electron bunch is displaced transversely from the other electron bunches using a short-pulse, high-repetition-rate kicker magnet. Experiments that require light emitted only from a single bunch can stop the light emitted from the other bunches using a collimator. Other beam lines will only see a small reduction in flux due to the displaced bunch. As a result, PSB eliminates the need to schedule multibunch and timing experiments during different running periods. Furthermore, the time spacing of PSB pulses can be adjusted from milliseconds to microseconds with a novel "kick-and-cancel" scheme, which can significantly alleviate complications of using high-power choppers and substantially reduce the rate of sample damage.

EphB signaling directs peripheral nerve regeneration through Sox2-dependent Schwann cell sorting.

The peripheral nervous system has astonishing regenerative capabilities in that cut nerves are able to reconnect and re-establish their function. Schwann cells are important players in this process, during which they dedifferentiate to a progenitor/stem cell and promote axonal regrowth. Here, we report that fibroblasts also play a key role. Upon nerve cut, ephrin-B/EphB2 signaling between fibroblasts and Schwann cells results in cell sorting, followed by directional collective cell migration of Schwann cells out of the nerve stumps to guide regrowing axons across the wound. Mechanistically, we find that cell-sorting downstream of EphB2 is mediated by the stemness factor Sox2 through N-cadherin relocalization to Schwann cell-cell contacts. In vivo, loss of EphB2 signaling impaired organized migration of Schwann cells, resulting in misdirected axonal regrowth. Our results identify a link between Ephs and Sox proteins, providing a mechanism by which progenitor cells can translate environmental cues to orchestrate the formation of new tissue.

Laser seeding of the storage-ring microbunching instability for high-power coherent terahertz radiation.

We report the first observation of laser seeding of the storage-ring microbunching instability. Above a threshold bunch current, the interaction of the beam and its radiation results in a coherent instability, observed as a series of stochastic bursts of coherent synchrotron radiation (CSR) at terahertz frequencies initiated by fluctuations in the beam density. We have observed that this effect can be seeded by imprinting an initial density modulation on the beam by means of laser "slicing." In such a situation, most of the bursts of CSR become synchronous with the pulses of the modulating laser and their average intensity scales exponentially with the current per bunch. We present detailed experimental observations of the seeding effect and a model of the phenomenon. This seeding mechanism also creates potential applications as a high-power source of CSR at terahertz frequencies.

Tailored terahertz pulses from a laser-modulated electron beam.

We present a new method to generate steady and tunable, coherent, broadband terahertz radiation from a relativistic electron beam modulated by a femtosecond laser. We have demonstrated this in the electron storage ring at the Advanced Light Source. Interaction of an electron beam with a femtosecond laser pulse copropagating through a wiggler modulates the electron energies within a short slice of the electron bunch with about the same duration of the laser pulse. The bunch develops a longitudinal density perturbation due to the dispersion of electron trajectories, and the resulting hole emits short pulses of temporally and spatially coherent terahertz pulses synchronized to the laser. We present measurements of the intensity and spectra of these pulses. This technique allows tremendous flexibility in shaping the terahertz pulse by appropriate modulation of the laser pulse.

Explicit symplectic integrator for s-dependent static magnetic field.

This paper reports our recent work on explicit symplectic integration techniques for the charged particle motion in an s-dependent static magnetic field. Using the extended phase space, symplectic integrators can be developed for Hamiltonians with or without the paraxial approximation using either the space or time as an independent variable. This work extends the successful element-by-element tracking method for studying single-particle nonlinear dynamics to a set of s-dependent magnetic elements. Important applications of this work include the studies of the charged particle dynamics in a storage ring with various insertion devices, superconducting magnets, large aperture magnets with significant fringe fields, and solenoid magnets in the interaction region. Consequently, this work is expected to make an impact on design and optimal operation of existing and future light source rings and high energy physics accelerators.

Maps for distributions and their time evolution.

Many dynamical stochastic processes occur "on top" of a deterministic process. We present a method which uses the trajectory of the deterministic process as basis functions for quasiarbitrary distributions. A map for the stochastic process can then be computed. This may have applications in electron storage rings or other devices perturbed by a small stochasticity. In this paper we will look only at the most elementary applications of the method.