Publication Date

2010-06-25

Availability

Open access

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Biochemistry and Molecular Biology (Medicine)

Date of Defense

June 2010

First Committee Member

Zafar Nawaz - Committee Chair

Second Committee Member

Jacqueline Sagen - Committee Member

Third Committee Member

Karoline Briegel - Committee Member

Fourth Committee Member

Paul Schiller - Mentor

Fifth Committee Member

Miguel Perez-Pinzon - Mentor

Abstract

Emerging trends in cell-therapy based tissue repair have focused on the renewable source of adult stem cells including human bone marrow-derived mesenchymal stromal cells (hMSCs). Due to immunomodulatory properties as well as a potential to differentiate into cells characteristic of all three germ layers, hMSCs provide a source of immature cells for utilization in cell-therapy based treatments. Marrow isolated adult multilineage inducible (MIAMI) cells are a homogeneous sub-population of hMSCs which maintain self-renewal potential during ex vivo expansion, in addition to efficiently undergoing trans-differentiation into neuron-like cells in vitro. Even though hMSCs have the potential to be used for neural tissue repair, the molecular mechanisms by which they are stimulated to become neuron-like cells have not been fully characterized. Therefore the work described herein focuses on the molecular mechanisms by which MIAMI cells undergo NT-3 dependent neuronal commitment. MIAMI cells express both the full length (FL-) and tyrosine kinase deficient (TKd-) isoforms of the NTRK3 receptor, the primary NT-3 receptor, at the protein level. NT-3 stimulation of MIAMI cells during neuronal commitment induced the phosphorylation of FL-NTRK3, degradation of TKd-NTRK3, downstream activation of the Mek1/2-Erk1/2 signaling cascade, and subsequent up-regulation of a limited number of pro-neuronal genes. These findings were verified using chemical inhibitors to block NTRK autophosphorylation (K252a) and Erk1/2 activation (U0126). TKd-NTRK3 is hypothesized to activate Rac1 upon NT-3 stimulation. Rac1 was found to suppress NT-3 stimulated Erk1/2 phosphorylation, as well as downstream gene expression, as determined using a Rac1 chemical inhibitor. Further characterization confirmed that Rac1b is the predominant Rac1 isoform in MIAMI cells. Rac1b siRNA mediated knock-down resulted in increased expression of the pro-neuronal genes NGN2, MAP2, NFH and NFL during NT-3 stimulation via regulation of Mek1/2-Erk1/2. Rac1b is also involved in NT-3 stimulated cell proliferation, as well as repression of CCND1 and CCNB1 mRNA expression. In an attempt to enhance neuronal differentiation of MIAMI cells, EGF and bFGF were used to pretreat MIAMI cells prior to NT-3 stimulated neuronal commitment. EGF/bFGF pretreatment increased NTRK3 and NTRK1 protein levels along with NT-3 stimulated Erk1/2 phosphorylation. In addition, bFGF versus EGF/bFGF pretreatment restricted the expression of the pro-neuronal transcription factors Ngn2 and Prox1 versus the neural stem cells self-renewal transcription factor Musashi-1, respectively. The culmination of this work provides a model for the NT-3 induced neuronal commitment of MIAMI cells in vitro, as well as insight into the neurogenic potential of MSCs for future applications in cell-therapy based tissue repair.

Keywords

Rac1b Regulation; Neurotrophin Tyrosine Receptor Kinase 3; MIAMI Cells; Neuronal Differentiation; Adult Stem Cells; Mesenchymal Stem Cells; Neurotrophin-3; Neurotrophin Signaling

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