Stephen M Strittmatter MD, PhD
Vincent Coates Professor of Neurology and Professor of Neurobiology; Director, Cellular Neuroscience, Neurodegeneration and Repair; Director, Memory Disorders Clinic
Axon Guidance; Axon Regeneration; Spinal Cord Injury; Molecular Biology; Stroke; Neurodegenerative Disease; Alzheimer's Disease; Fronto-Temporal Dementia
Neural Repair and Neuro-Degeneration
Neurological injury frequently interrupts connections while sparing nerve cells. Spinal Cord Injury (SCI) is the epitome of a disconnection syndrome, in which surviving neural tissue fails to function due to lost communication at the level of injury. For the organism to regain function, new pathways must form by growth of cut or surviving nerve fibers. Unfortunately, the growth of axons and the rearrangement of brain circuitry are extremely limited in the adult brain and spinal cord.
We focus on understanding the molecular pathways that limit fiber growth and functional rewiring of neuronal circuits during health and disease. Axonal growth encompasses both neural plasticity and repair. Technically, we utilize chronic in vivo imaging of neuronal connections, genetic alteration of mice and induction of surgical lesions resembling clinical SCI and Stroke. In particular, we have found that the NogoReceptor (NgR1) pathway mediating myelin inhibition of axonal growth plays a role in titrating anatomical plasticity in the adult CNS.
In Alzheimer's Disease and several other neurodegenerative conditions, nerve cells are lost over time. Molecular contributors to this pathology have been discovered by genetic methods, but their mechanism of action has remained poorly understood. We have focused on defining the pathophysiological action of Amyloid-beta (Aß) peptide oligomers in Alzheimer's Disease, and on the role of secreted Progranulin in Fronto-Temporal Dementia. For both of these molecules, interaction with the specific receptors on the neuronal surface is crucial. We utilize receptor ligand binding assays, expression cloning, electrophysiology, genetics and mouse behavior to study these pathways.
Extensive Research Description
NEURAL REAPIR AND PLASTICITYT
Plasticity of Neural Connections
Dynamic alterations of adult brain function, or Neural Plasticity, can occur at the level of metabolites, protein chemistry, synaptic morphology, systems-level activation or anatomical rearrangement. Frank Anatomical Plasticity in the adult mammalian CNS is minimal in comparison to the developing and adolescent brain. Classically, one of the last stages in neural development is the "critical period" during which activity has a pronounced effect on connectivity, prior to entering the anatomically static adult stage. We are exploring the molecular basis for restriction of Anatomical Plasticity in the adult brain and spinal cord.
Nogo Receptor in Plasticity
Monocular deprivation normally alters ocular dominance in the visual cortex only during a postnatal critical period (20 to 32 days postnatal in mice).This is a period when intracortical myelination is reaching adult levels. Therefore, we focused on the role of the myelin inhibitor pathway in plasticity. Mutations in the Nogo-66 receptor (NgR1) affect cessation of ocular dominance plasticity. In NgR1-/- mice, plasticity during the critical period is normal, but it continues abnormally such that ocular dominance at 45 or 120days postnatal is subject to the same plasticity as at juvenile ages. Thus, physiological NgR signaling from myelin-derived Nogo, MAG, and OMgp consolidates the neural circuitry established during experience-dependent plasticity. Our current work explores the anatomical basis of NgR1-regulatedplasticity. We hypothesize that alterations in Anatomical Plasticity underlie the electrophysiological evidence of increased cortical plasticity. We are using in vivo imaging and conditional NgR1 alleles to test the ability of this pathway to titrate adult Anatomical Plasticity in the mouse cerebral cortex. Such regulation may underlie the genetic linkage of NgR1 mutations we have described in human Schizophrenia.
Chronic In Vivo Imaging
Progress in defining the extent and control of Anatomical Plasticity in the adult brain requires high resolution imaging of axons and dendrites over weeks. Fortunately, recent advances in two-photon microscopy and transgenic expression of fluorescent proteins allow repeated imaging of anatomy over weeks in the adult cerebral cortex. These methods have revealed that turnover of axonal varicosities and dendritic spines is most pronounced in the developing and adolescent brain, and decreases in the adult. Specific changes in activity and experience are reported to enhance adult Anatomical Plasticity. We are now testing the role of NgR1 in cortical Anatomical Plasticity using constitutive and conditional gene targeting.
Anatomical Plasticity after Injury
From our studies of stroke and of surgical cuts of the corticospinal tract in the medulla, it is clear that uninjured tracts can be induced to rearrange anatomically after injury to adult CNS. Furthermore,NgR1 limits the extent of this plasticity. Thus studies of Anatomical Plasticity have relevance for Translational Neurology.
Neurological disability has a diverse set of causes. Neuronal tissue can be injured by the loss of blood flow in atherosclerosis (cerebral stroke), by mechanical forces in trauma (spinal cord injury), by neurochemical perturbation in degenerative disease (Alzheimer’s), by microorganisms in infectious diseases or by the immune system in multiple sclerosis. Disease-specific prophylactic approaches seek to prevent tissue damage from occurring in the first place. Unfortunately, few such approaches are successful and many individuals suffer with chronic neurological disabilities. After an injury has occurred, current rehabilitation medicine relies primarily on physical therapy and employs pharmacological agents only to combat secondary complications. Stem cells may someday be adapted to replace lost tissue, but there are many hurdles to the rapid development of such therapy.
We seek to harness the potential of surviving tissue to restore function. To support neurological function, nerve cells must be connected electrically by long cellular fibers, termed axons. In nearly all neurological conditions, a substantial portion of brain and spinal cord is preserved. If remaining healthy tissue can be “rewired” with appropriate axonal connections, improved neurological function can result. The formation of new connections and the recovery of function after injury depend upon new axonal extension from remaining cells. Without treatment, axonal growth is extremely limited in the adult brain and spinal cord, and recovery is typically restricted. Today, no FDA-approved therapeutic promotes new connections between surviving nerve cells.
Nogo Receptor and Myelin inhibition
It has been known for a number of years that the white matter of the CNS inhibits axon growth in the adult brain and spinal cord. A molecular characterization of inhibitors in CNS myelin led to our identification of the protein, Nogo. We also identified a receptor protein for Nogo termed Nogo-66 Receptor, or NgR1. In vitro assays of neurite outgrowth and ligand receptor binding were developed to identify and characterize this pathway. With this ligand receptor pair in hand we have developed peptide and protein inhibitors of Nogo Receptor signaling. Such molecules selectively block the inhibitory effect of myelin on axon growth invitro.
Nogo and Nogo Receptor antagonists have been tested extensively by us and by others in animal SCI models. Molecules with Nogo/NgR1blocking activity promote the growth of axons in the adult spinal cord and promote recovery of walking performance in injured animals without side-effects. We have demonstrated benefit in both acute and chronic spinal contusion models with NgR(310) ecto-Fc protein therapy. Functional recovery of rats after stroke is also enhanced by this treatment. We are working to optimize blockers of this system for clinical use with structural biology, mutagenesis and high-throughput screening methods. We are also working to expand knowledge of NgR1-initiated signal transduction.
Other Limits on Neural Repair
The Nogo/ NgR1 pathway plays a prominent role in vivo, but it is not the only molecular brake on neurological recovery. We are actively studying ephrins, CSPGs, SPRRs, LRRTMs, ROCK, RGMs and other pathways for synergy with the NgR pathway.
In neurodegenerative conditions, neuronal components and brain function are progressively lost. A spectrum of genes has been implicated in these diseases but mechanistic understanding remains sparse. We have focused on the pathophysiology of Aß in Alzheimer's Disease, and Progranulin in Fronto-Temporal Dementia. In both cases, interaction of extracellular disease-associated ligands with the specific receptors on the neuronal surface is crucial, but had not been defined.
Alzheimer's disease (AD) is the most common cause of age-related dementia, affecting more than 25 million people worldwide. The accumulation of insoluble ß-amyloid (Aß) plaques in the brain has long been considered central to the pathogenesis of AD (green in panel to right).However, recent evidence suggests that soluble oligomeric assemblies of Aß may be of greater importance. APP processing yields Aß monomers, which undergo oligomerization, eventually forming amyloid fibrils and plaques. Aß oligomers have been found to be potent synaptotoxins, but the mechanism by which they exert their action had remained elusive. We recently found that cellular prionprotein (PrP-C) is a high-affinity receptor for Aß oligomers, mediating their toxic effects on synaptic plasticity. We hypothesize that the Aß/PrP-C interaction leads to dendritic spine retraction via synaptotoxic action, with subsequent neurotic dystrophy and neurodegenerative pathology. These later steps are then coupled to tauopathy and memory impairment in AD. We are employing biochemical analysis, in vivo imaging of dendrites, genetic investigation and behavioral studies to test this hypothesis. With PrP-C as a molecular target, we have launched a drug discovery program for novel AD therapeutics.
Of inherited Fronto-Temporal Dementia (FTD) cases, haploinsuffiency of Progranulin is the most common etiology. TDP-43 deposition occurs in Progranulin-deficient FTD, as well as in sporadic FTD cases and in ALS. Because Progranulin (PGRN) is a secreted glycoprotein, we searched for high affinity receptors by expression cloning, and idenitifed Sortilin (Sort1). We are now characterizing the role of PGRN/Sort1 interactions in FTD. These studies may provide an accessible cell surface approach to modify FTD and ALS progression.
The PGRN/Sort1 pathway is modified genetically by a lysosomal/endosomal protein TMEM106B in humans. We are studying the moelcualr mechanism of this regulation in cell lines and in mice.