We propose to investigate why central visual pathways fail to regenerate after injury and how their repair can be enhanced. We will focus on why retinal ganglion cell axons fail to regenerate after optic nerve injury. Many studies point to multifactorial cause of retinal ganglion cell axon regenerative failure. On the one hand adult retinal ganglion cells have an intrinsically limited axon growth potential. On the other, the optic nerve environment after nerve injury is strongly inhibitory to regenerating axons as a result of inhibitory cues deriving from both degenerating myelin and reactive astrocytes. We do not yet have an in depth understanding of the molecular basis of these phenomena. The proposed experiments address a longstanding central question in the field, which is why degenerating myelin, which is strongly inhibitory to axon regeneration, is cleared robustly by phagocytosis after PNS injury but not after CNS injury. Schwann cells play a critical role in clearning degenerating PNS myelin but the relevant phagocytic pathways are not yet known. We have recently discovered that Schwann cells in the PNS and astrocytes in the CNS express the same repertoire of phagocytic pathways including the Mertk/Axl and the Megf10/LRP1 pathways. This is surprising as only Schwann cells perform robust myelin clearance after nerve injury. Furthermore, we have established that at least one of these pathways, the Mertk pathway, is required for clearance of PNS myelin debris. The proposed experiments are designed to elucidate the molecular mechanisms of Schwann cell-mediated myelin debris clearance and to investigate why these same pathways do not mediate myelin clearance after CNS injury. Using conditional knockout mice, in which each pathway is deleted specifically in Schwann cells, we will test the hypothesis that the Mertk/Axl and Megf10/LRP1 phagocytic pathways are required for glial-mediated clearance of degenerating (inhibitory) PNS myelin. We will then test several hypotheses for why astrocyte phagocytosis of myelin is disabled in the injured optic nerve, either because the required phagocytic pathways are downregulated in astrocytes after optic nerve injury or because degenerating CNS myelin is resistant to glial-mediated phagocytosis. Our ultimate goal is to determine how phagocytic clearance of degenerating myelin can be activated in optic nerve astrocytes in order to develop new treatments to promote retinal ganglion cell axon regeneration after injury in ocular diseases including glaucoma, retinal ischemia, optic neuritis, and optic neuropathy.

Public Health Relevance

Our goal is to understand why retinal ganglion cells and other CNS axons do not regenerate after injury and how their regeneration can be enhanced. The proposed experiments are designed to test the hypothesis that specific glial phagocytic pathways robustly clear degenerating PNS but not CNS myelin. We will elucidate the relevant phagocytic pathways in Schwann cells, investigate why these same pathways which are present in astrocytes do not phagocytose degenerating CNS myelin, and then test whether more robust glial phagocytosis of degenerating CNS myelin will promote retinal ganglion cell regeneration and optic nerve repair.

Agency
National Institute of Health (NIH)
Institute
National Eye Institute (NEI)
Type
Research Project (R01)
Project #
2R01EY011310-17A1
Application #
8695187
Study Section
Neurodifferentiation, Plasticity, and Regeneration Study Section (NDPR)
Program Officer
Steinmetz, Michael A
Project Start
1996-01-22
Project End
2015-03-31
Budget Start
2014-04-01
Budget End
2015-03-31
Support Year
17
Fiscal Year
2014
Total Cost
$396,169
Indirect Cost
$146,169
Name
Stanford University
Department
Biology
Type
Schools of Medicine
DUNS #
009214214
City
Stanford
State
CA
Country
United States
Zip Code
94305
Brosius Lutz, Amanda; Chung, Won-Suk; Sloan, Steven A et al. (2017) Schwann cells use TAM receptor-mediated phagocytosis in addition to autophagy to clear myelin in a mouse model of nerve injury. Proc Natl Acad Sci U S A 114:E8072-E8080
Mandemakers, Wim (2014) Immunopanning of retrograde-labeled corticospinal motor neurons from early postnatal rodents. Cold Spring Harb Protoc 2014:375-88
Lutz, Amanda Brosius (2014) Purification of Schwann cells. Cold Spring Harb Protoc 2014:1234-6
Lutz, Amanda Brosius (2014) Purification of schwann cells from the neonatal and injured adult mouse peripheral nerve. Cold Spring Harb Protoc 2014:1312-9
Steketee, Michael B; Oboudiyat, Carly; Daneman, Richard et al. (2014) Regulation of intrinsic axon growth ability at retinal ganglion cell growth cones. Invest Ophthalmol Vis Sci 55:4369-77
Wang, Jack T; Barres, Ben A (2012) Axon degeneration: where the Wlds things are. Curr Biol 22:R221-3
Wang, Jack T; Medress, Zachary A; Barres, Ben A (2012) Axon degeneration: molecular mechanisms of a self-destruction pathway. J Cell Biol 196:7-18
Rivlin-Etzion, Michal; Zhou, Kaili; Wei, Wei et al. (2011) Transgenic mice reveal unexpected diversity of on-off direction-selective retinal ganglion cell subtypes and brain structures involved in motion processing. J Neurosci 31:8760-9
Winzeler, Alissa M; Mandemakers, Wim J; Sun, Matthew Z et al. (2011) The lipid sulfatide is a novel myelin-associated inhibitor of CNS axon outgrowth. J Neurosci 31:6481-92
Osterhout, Jessica A; Josten, Nicko; Yamada, Jena et al. (2011) Cadherin-6 mediates axon-target matching in a non-image-forming visual circuit. Neuron 71:632-9

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