Graduation Year

2011

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Medical Sciences

Major Professor

Keith R. Pennypacker

Keywords

Hypoxia, Leukemia Inhibitory Factor, Metallothionein 3, Oxidative Stress, Peroxiredoxin 4, Stroke

Abstract

Oligodendrocytes (OL)s are the dominant cell type in the white matter and are integral for synaptic transmission essential for proper neuronal communication between brain areas. Previous studies have shown that intravenous administration of the mononuclear fraction of human umbilical cord blood (HUCB) cells in rat models of stroke reduced white matter injury, gray matter injury and behavioral deficits. Yet the mechanisms used by HUCB cells remain unknown in ischemic injury. These studies will investigate both in vitro and in vivo approaches to elucidate this mechanism in OLs. When mature primary OLs were coincubated with HUCB cells, HUCB cells secreted soluble factors that reduced cell death in OLs exposed to OGD. Microarray analysis revealed that HUCB cell treatment induced OL gene changes. These changes included genes involved in cell proliferation, signaling, anti-oxidant activity, and myelination. To extend these findings, the middle cerebral artery occlusion (MCAO) model was used to assess the expression profile of protein products of gene changes observed in vitro. The in vivo data mirrored in vitro data in that metallothionein 3 (Mt3), peroxiredoxin 4 (Prdx4), myelin oligodendrocyte glycoprotein (Mog), U2AF homology kinase 1(Uhmk1), and insulin induce gene 1(Insig1) were upregulated in OLs of the white matter tract adjacent to the infarct. Furthermore, double immunofluorescence staining determined that OLs expressed these proteins. Other reports have shown that HUCB cells secrete soluble factors related to cellular protection, including interleukin 6 (IL-6), interleukin 8 (IL-8), and interleukin 10 (IL-10). Other factors are known for their proliferative actions, such as vascular endothelial growth factor (VEGF), BDNF, platelet derived growth factor B (PDGF-B), leukemia inhibitory factor (LIF), and granulocyte colony stimulating factor (GCSF) all of which converge on the Akt survival pathway. Given these findings we hypothesize that Akt activation is integral to HUCB cell mediated OL protection. In models of excitotoxicity, the addition of Akt inhibitor IV blocked HUCB cell mediated protection in OL cultures exposed to 24 hrs OGD. In vivo, HUCB cell treatment increased Akt activation, antioxidant protein expression and decreased caspase 3 cleavage in the external capsule in a time dependent manner. The next series of experiments determine whether the soluble factors secreted by HUCB cells could replace HUCB cells as treatment. LIF expression is increased in HUCB cells as compared to peripheral blood and as previously mentioned, LIF is secreted by HUCB cells. Additionally, LIF rescued OLs from spinal cord and experimental autoimmune encephalomyelitis injury. Thus LIF was investigated. LIF protected OL subjected to 24 hr OGD, increased antioxidant Prdx4 gene expression and reduced reactive oxygen species production. Additionally the inclusion of Akt inhibitor IV blocked LIF induced OL protection. Similar results were obtained when GCSF was evaluated. All these findings indicate that HUCB cell mediated OL/white matter protection is due to the soluble factors secreted by the mononuclear population of these cells. These soluble factors including LIF activate cellular machinery leading to enhanced cellular survival. Here we found a specific survival pathway activated by soluble factors released from HUCB cells, leading to Akt activation. Akt activation arrests stroke induced apoptosis and reduced the expansion of the infarct, promoting functional recovery from acute ischemic injury.

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