White matter injury: Studies on basic mechanisms and treatment design

In the CNS, axons are critically important for transmitting information between neurons with high fidelity and reliability. A number of prevalent human diseases such as stroke, spinal cord injury, traumatic brain injury and demyelination (e.g. multiple sclerosis) can severely disrupt segments of axonal tracts, resulting in significant morbidity and mortality. The focus of our lab has been the study of the pathophysiology of mammalian CNS myelinated axons. A number of inter-related projects are designed to further expand our knowledge of white matter injury mechanisms.

1. White Matter “Excitotoxicity”: the unexpected role of glutamate Although CNS white matter is devoid of synapses, neurotransmitters such as glutamate and glycine are released during injury (Micu et al., Nat Med, 2007), and activate critically located receptors on axons, myelin and oligodendrocytes (Micu et al., Nature, 2006; Ouardouz et al, Ann Neurol., 2009; Salter & Fern, Nature, 2005; Karadottir et al., Nature, 2005). Using a unique 2-photon laser-scanning microscopy technique developed in our lab (Micu et al., Nat Med, 2007), this project examines in greater detail the mechanisms of release of glutamate and glycine, their target receptors, and the physiological roles and pathological consequences of neurotransmitter-dependent signaling in this unexpected location. We are proposing that there exists another type synapse, the axo-myelinic synapse that allows axons to communicate with their overlying myelin sheaths:

Proposed "synapse" between axon and its myelin sheath
From Stys & Lipton, TiPS, 2007

2. Intracellular Ca Dynamics in Myelinated Axons This project focuses on the storage and release of intra-axonal Ca in CNS
axons in response to physiological and pathological (e.g. anoxia, ischemia)
stimuli. The role of intracellular Ca stores and mitochondria are of particular
interest. This project relies heavily on our ability to image Ca in
myelinated fibers. Results to date show a fascinating and complex arrangement of signaling molecules along the internode under the myelin sheath: these “nancomplexes” undoubtedly play important roles in axonal physiology and injury (Ouardouz et al., Neuron, 2003; Ouardouz et al., Ann Neurol, 2009a & 2009b).

3. Mechanisms of axonal spheroid formation

4. Role of microglia in CNS white matter injury

5. Neuroprotection in acute spinal cord injury

6. Neurotransmitter modulation to improve outcome in EAE, a model of MS

7. Role of glutamate receptors in peripheral nerve physiology and pathophysiology

8. Coherent Anti-Stokes Raman (CARS) Microscopy: advanced structural and chemical imaging of myelin

9. Schmitt-Lanterman incisures in myelinated axons: what is their purpose?

10. Local intra-axonal protein synthesis: control mechanisms and injury responses

11. A fiber-optic based miniature “exoscope” for minimally invasive CARS and multiphoton fluorescence imaging of neural tissue

12. Using fluorescence spectroscopy and laser polarization to study the nanostructural organization of living myelin

Myelin is a unique multilamellar structure that is vital for the normal transmission of signals in the CNS and peripheral nerves. Subtle alterations in the nanostructural organization of the membrane wraps can destabilize the myelin sheath and lead to demyelination. We are using fluorescence spectroscopy to study the nano-architecture of myelin both in living ex vivo tissue as well as in the intact animal in vivo. Here is an example of a node of Ranvier from a sciatic nerve in a mouse that expresses YFP in the axon; the myelin was labeled with a fluorescent dye, and the nerve imaged using 2-photon microscopy. Spectroscopic characteristics of the dye emission, together with polarization-resolved images, provide unique insights into the health of this vital element:

2-photon micrograph of a sciatic nerve node of Ranvier (Image by Dr. D. Stirling)
2-photon image of a node of Ranvier in sciatic nerve of a YFP mouse in vivo. Image by Dr. D. Stirling. 13. Advanced optical engineering and laser design: pushing the limits of microscopy for biomedical imaging