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Molecular Mechanisms Underlying Neuronal Pathology in the Spinal Cord and Cerebellum: The Role of Plasma Membrane Calcium ATPase

by
Michael Kurnellas
Neuroscience Program

B.A. 2000, Drew University
M.S. 2004, Graduate School of Biomedical Sciences-UMDNJ


Thesis Advisor: Stella Elkabes, Ph.D.

Associate Professor
Department of Neurology & Neurosciences

Monday, August 24, 2009
10:00 a.m., Cancer Center-G-Level


Abstract

Plasma membrane calcium ATPase isoform 2 (PMCA2) is a calcium pump found predominantly in the central nervous system (CNS) and the heart. Evidence suggests that the dysfunction of this pump is involved in neural pathology, but its precise contribution to neuronal injury and degeneration is not well understood. The goal of the studies in this thesis is to elucidate the distinct involvement of PMCA2 in neuronal dysfunction in the spinal cord and the cerebellum, two CNS regions where the pump is very highly expressed and is likely to play a critical role as suggested by our earlier investigations and those of others (Nicot et al., 2003; 2005; Kozel et al., 1998).
In the spinal cord, the absence or partial reduction of PMCA2 expression results in loss of spinal cord neurons and especially motor neurons. This is indicated by our studies on knockdown of PMCA2 in spinal cord neuronal cultures and on mice with mutations in the PMCA2 gene affecting the expression or function of the pump (Kurnellas et al., 2005; 2009; Souayah et al., 2008).
These findings could have relevance to neurodegeneration in multiple sclerosis (MS) because earlier studies in this laboratory had shown that the levels of neuronal PMCA2 mRNA and protein are reduced in the inflamed lumbar spinal cord at onset of symptoms in mice with experimental autoimmune encephalomyelitis (EAE), an animal model of MS (Nicot et al., 2003; 2005). In agreement with this concept, we now found that an antagonist of -amino-3-hydroxy-5-methylisoxazole-4-propionate acid (AMPA)/kainate receptors, which has been shown to ameliorate clinical deficits and prevent axonal injury and neuronal loss in EAE (Smith et al., 2000; Pitt et al., 2000), restores PMCA2 levels when the treatment is initiated after the onset of symptoms or even at peak of the disease. Further investigations on the triggers and mechanisms that cause a suppression of PMCA2 expression indicated that kainic acid, an agonist of the AMPA/kainate receptors, decreases PMCA2 protein levels in spinal cord neuronal cultures and this reduction can be prevented by a calpain inhibitor. Finally, studies aiming at the elucidation of molecular events downstream to PMCA2 identified collapsin response mediator protein 1 (CRMP1) as an effector mediating neuronal death. CRMP1 levels decrease after silencing of PMCA2 expression and prior to spinal cord neuronal loss, in vitro. Knockdown of CRMP1 also causes loss of spinal cord neurons, in culture. A correlation exists between PMCA2 and CRMP1 levels in the spinal cord of mice with EAE, with and without treatment with the AMPA/kainate receptor antagonist, a novel finding that could implicate CRMP1 in this disease. Lower CRMP1 levels also corresponds with decreased PMCA2 levels in neurons exposed to kainic acid, in vitro. As CRMPs mediate microtubule assembly, a reduction in CRMP1 levels could perturb assembly of cytoskeletal elements leading to cell pathology.
In contrast to spinal cord neurons, the lack of PMCA2 does not lead to death of Purkinje cells (PC) in the cerebellum of PMCA2-null mice, but results in morphological and potentially, functional abnormalities (Kozel et al., 1998). In our investigations, we found that PMCA2 associates with metabotropic glutamate receptor 1 (mGluR1) and its downstream effectors, inositol-1,4,5-triphosphate receptor 1 (IP3R1) and Homer 3. This indicates that PMCA2 is a component of the mGluR1 signaling complex. Thus, in the cerebellum, PMCA2 could function not only as a calcium pump but also as a modulator of mGluR1 signaling. The lack of PMCA2 results in decreased mGluR1, IP3R1 and Homer 3 protein levels, which could be a cause of PC dysfunction as mGluR1 is an essential modulator of plasticity at the parallel fiber-PC synapse.
Overall, the findings of these investigations highlight the distinct and specific roles of PMCA2 in different neuronal populations and the importance of this calcium pump in the health and pathology of the CNS.


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