The primary focus of our research group is on the molecular mechanisms of neurodegeneration with an emphasis on the role of mitochondrial dysfunction. We have a longstanding interest in the pathogenic processes in Huntington’s disease and Alzheimer’s disease, and more recently in stroke.  For our studies we use a wide variety of different approaches from in vitro enzyme assays with purified proteins, to studies in whole animals.  This broad-based approach allows us to translate what we learn about a process or signaling pathway at the molecular level to the in vivo situation.  Each of the 3 areas of research that are ongoing in our lab is discussed briefly below.

Two hallmarks of the Alzheimer’s disease brain are the intracellular neurofibrillary tangles composed primarily of the protein tau in a pathologically modified state and the extracellular senile plaques composed primarily of the Aβ peptide.  There is compelling evidence that aberrant posttranslational processing of tau is central to the disease process. Nonetheless, the upstream events that result in pathological changes in tau and the downstream mediators of the cell death processes in Alzheimer’s disease have not been fully elucidated. Therefore a major focus in our lab is first to understand the signaling cascades that are dysregulated in Alzheimer’s disease brain and result in tau pathology, and second to identify the downstream targets of tau.  Currently we are investigating how abnormal phosphorylation and proteolytic processing of tau in Alzheimer’s disease may make tau “toxic”. In addition we have a strong interest in determining how tau affects mitochondrial dynamics and function as there is exciting new data suggesting that the mitochondria may be a crucial downstream target of tau and contribute to the neurodegenerative processes.

Our lab has a well-established interest in understanding the regulation and function of transglutaminase 2 (TG2) in neuronal cell death and survival. Recently we found that TG2 in its capacity as a scaffold protein binds HIF1β, attenuates HIF signaling and protects neurons from cell death induced by oxygen and glucose deprivation (OGD).  In addition, there is data to suggest that the mitochondria play an important role in the HIF response. Further, TG2 has been shown to modulate mitochondrial function.  Therefore we are investigating how TG2 attenuates HIF signaling and protects against OGD-induced cell death and the role of mitochondria in this process.

Huntington’s disease is an autosomal dominant neurodegenerative disease caused by a pathological expansion of the polyglutamine domain in the huntingtin protein. There is convincing evidence that mitochondrial dysfunction plays a pivotal role in the pathogenesis of Huntington’s disease.  In recent studies, we have found that mitochondria from mutant huntingtin-expressing striatal cells take up significantly less calcium than mitochondria from wild type cells and are significantly more sensitive to calcium-induced decreases in respiration.  Further, studies from other labs have shown that expression of the co-activator PGC-1α, which plays a key role in the expression of numerous mitochondrial genes, is significantly decreased in Huntington’s disease. Therefore we are now investigating: (1) whether mutant huntingtin impairs the ability of mitochondria to appropriately maintain pH and regulate redox status and  if this contributes to the calcium handling defects that result in respiratory deficits and increased sensitivity to calcium-induced permeability transition pore opening, (2) whether transcriptional repression of PGC-1α by mutant huntingtin compromises mitochondrial metabolism and function and (3) whether activation of PGC-1α ameliorates mitochondrial dysfunction in mouse Huntington’s disease models.

Stoothoff, W.H. and Johnson, G.V.W. 2005. Tau phosphorylation: physiological and  pathological consequences, Biochimica Biophysica Acta,1739:280-297.

Bailey, C.D.C., Tucholski, J. and Johnson, G.V.W. 2005. Transglutaminases in neurodegenerative disorders in, "Transglutaminases: the family of enzymes with diverse functions", editors, K. Mehta and R. Eckert, Karger Publishers, Progress in Experimental Tumor Research., 38:139-157.

Stoothoff, W.H., Cho, J.-H., McDonald, R.P. and Johnson, G.V.W. 2005. FRAT-2 preferentially increases GSK3β-mediated phosphorylation of primed sites which results in enhanced tau phosphorylation, Journal of Biological Chemistry, 280:270-276.

Bailey, C.D.C and Johnson, G.V.W. 2006.The protective effects of cystamine in the R6/2 Huntington’s disease mouse involve mechanisms other than the inhibition of tissue transglutaminase, Neurobiology of Aging, 27:871-879.

Johnson, G.V.W. 2006. Tau phosphorylation and proteolysis: insights and perspectives.  Journal of  Alzheimer's Disease, 9:243-250.

Milakovic, T. and Johnson, G.V.W. 2005. Mitochondrial respiration and ATP production are significantly impaired in striatal cells expressing mutant huntingtin, Journal of Biological Chemistry, 280:30773-30782.

Choo, Y.S., Mao, Z., and Johnson, G.V.W. 2005 Lesort, M. Increased glutathione levels in cortical and striatal mitochondria of the R6/2 Huntington's disease mouse model, Neuroscience Letters, 386:63-68.

Tucholski, J., Roth, K.A. and Johnson, G.V.W. 2006. Tissue transglutaminase overexpression in the brain potentiates excitotoxicity-induced hippocampal damage, Journal of Neurochemistry, 97:582-594. 

Mi, K., Dolan, P.J. and Johnson, G.V.W. 2006. The low density lipoprotein receptor-related protein 6 interacts with glycogen synthase kinase 3 and attenuates activity, Journal of Biological Chemistry, 284-1:4787-4794.   

Van Raansdonk J.M., Pearson, J. Bailey, C.D.C., Rogers, D.A., Johnson, G.V.W., Hayden, M.R. and Leavitt, B.R. 2005. Cystamine treatment is neuroprotective in the YAC128 mouse model of Huntington's disease, Journal of Neurochemistry, 95:210-220. 

Ding, H., Matthews, T.A. and Johnson, G.V.W. 2006. Site-specific phosphorylation and caspase cleavage differentially impacts tau-microtubule interactions and tau aggregation, Journal of Biological Chemistry, 281:19107-19114. 

Milakovic, T., Quintanilla, R. A. and Johnson, G.V.W. 2006. Mutant huntingtin expression induces mitochondrial calcium handling defects in clonal striatal cells: functional consequences, Journal of Biological Chemistry, 281:34785-34795. 

Mi, K. and Johnson, G.V.W. 2006. The role of tau phosphorylation in the pathogenesis of Alzheimer’s disease, Current Alzheimer Research, 3:449-463. 

Chun, W. and Johnson, G.V.W. 2007. The role of tau phosphorylation and cleavage in neuronal cell death, Frontiers in Bioscience, 12:733-756. 

Ruan, Q. and Johnson, G.V.W. 2007. Transglutaminase 2 in neurodegenerative disorders, Frontiers in Bioscience, 12:891-904.

Clodfelder-Miller, B.J., Zmijewska, A.A., Johnson, G.V.W. and Jope, R.S. 2006. Tau is hyperphosphorylated at multiple sites in mouse brain in vivo after streptozotocin-induced insulin deficiency. Diabetes, 55:3320-3325. 

Hunter, J.M., Lesort, M. and Johnson, G.V.W. 2007.  Mutant huntingtin alterations of the proteasome system, Journal of Neuroscience Research, in press.  

Mi, K. and Johnson, G.V.W. 2007. Regulated proteolytic processing of LRP6 results in release of its intracellular domain, Journal of Neurochemistry, in press. 

Mookherjee, P., Quintanilla, R.,   Roh, M.-S.,  Zmijewska, A.A., Jope, R.S., and Johnson, G.V.W. 2007. Mitochondrial-targeted active Akt protects SH-SY5Y neuroblastoma cells from staurosporine-induced apoptotic cell death, Journal of Cellular Biochemistry, in press.  

Ruan, Q., Quintanilla, R. A. and Johnson, G.V.W. 2007. Type 2 transglutaminase differentially modulates striatal cell death in the presence of wild-type or mutant huntingtin, Journal of Neurochemistry, in press.