Distribution, levels and phosphorylation of Raf-1 in Alzheimer's disease.
Extracellular signal-regulated kinase (ERK), a member of the mitogen-activated protein kinase pathway, has been increasingly implicated in the pathogenesis of Alzheimer's disease due to its critical role in brain function. While we previously demonstrated that ERK is activated in Alzheimer's disease, the upstream cascade leading to its activation had not been fully examined. In this study, we focused on Raf-1, one of the physiological activators of the ERK pathway. Raf-1 is activated by phosphorylation at Ser338 and Tyr340/341 and inhibited by phosphorylation at Ser259. Interestingly, phosphorylation at all three sites on Raf-1 was increased as evidenced by both immunocytochemistry and immunoblot analysis in Alzheimer's disease brains compared to age-matched controls. Both phospho-Raf-1 (Ser259) and phospho-Raf-1 (Ser338) were localized to intracytoplasmic granular structures, whereas phospho-Raf-1 (Tyr340/341) was localized to neurofibrillary tangles and granules in pyramidal neurons in Alzheimer's disease hippocampus. There is extensive overlap between phospho-Raf-1 (Ser338) and phospho-Mek1/2, the downstream effector of Raf-1, suggestive of a mechanistic link. Additionally, increased levels of Raf-1 are associated with Ras and MEK1 in Alzheimer's disease as evidenced by its coimmunoprecipitation with Ras and Mek1, respectively. Based on these findings, we speculate that Raf-1 is activated to effectively mediate Ras-dependent signals in Alzheimer's disease.
Mei
Matthew
M
Department of Pathology, Case Western Reserve University, and MetroHealth Medical Center, Cleveland, Ohio 44106, USA.
Su
Bo
B
Harrison
Kristen
K
Chao
Mark
M
Siedlak
Sandra L
SL
Previll
Laura A
LA
Jackson
LaMia
L
Cai
Dan X
DX
Zhu
Xiongwei
X
D000368:Aged; D000369:Aged, 80 and over; D000544:Alzheimer Disease / Q000378:metabolism* / Q000473:pathology / Q000503:physiopathology; D000595:Amino Acid Sequence / Q000502:physiology; D015415:Biomarkers; D001921:Brain / Q000378:metabolism* / Q000473:pathology / Q000503:physiopathology; D003594:Cytoplasmic Granules / Q000378:metabolism; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D006624:Hippocampus / Q000378:metabolism / Q000473:pathology / Q000503:physiopathology; D006801:Humans; D007150:Immunohistochemistry; D048369:MAP Kinase Kinase 1 / Q000378:metabolism; D020935:MAP Kinase Signaling System / Q000502:physiology*; D016874:Neurofibrillary Tangles / Q000378:metabolism / Q000473:pathology; D010766:Phosphorylation; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism*; D017966:Pyramidal Cells / Q000378:metabolism / Q000473:pathology; D012694:Serine / Q000378:metabolism; D015398:Signal Transduction; D014443:Tyrosine / Q000378:metabolism; D015854:Up-Regulation; D018631:ras Proteins / Q000378:metabolism*
D015415:Biomarkers; D014443:Tyrosine; D012694:Serine; D019908:Proto-Oncogene Proteins c-raf; D048049:Extracellular Signal-Regulated MAP Kinases; D048369:MAP Kinase Kinase 1; C482125:MAP2K1 protein, human; D018631:ras Proteins