Environmental Factor, March 2009, National Institute of Environmental Health Sciences
Study Explores the Mechanisms of Age-Related Neurodegeneration
By Shweta Trivedi
A new study by researchers at NIEHS sheds light on the mechanisms involved in the accumulation of a neurotoxic protein, amyloid-beta42 (AΒ42), which is implicated in Alzheimer's disease (AD) and other age-related neurodegeration. According to the investigators, their report is the first to demonstrate the role of raft aggregation-induced phagocytosis specifically in the brain macrophage or microglia and to elucidate the brain-specific mechanisms influencing its activation.
The investigators, working in the NIEHS Laboratory of Neurobiology (LN), explored a process known as raft aggregation that is essential for efficient phagocytosis, or the removal of cellular debris, in brain cells. The researchers also identified specific components of raft aggregation that they conclude could be "manipulated or used as targets for therapeutic intervention in the treatment of neurodegeneration."
The study, by first author and Postdoctoral Fellow Dixie-Ann Persaud-Sawin, Ph.D., Lynna Banach and Neurotoxicology Group head and Principal Investigator Jean Harry, Ph.D., was published in the February issue of the journal Glia.
Phagocytosis is critical for the clearance of potentially harmful proteins and the maintenance of brain homeostasis. A defect in this scavenging of debris in the brain may lead to excessive deposits of AΒ42 - a protein associated with the development of proteinopathies and neurodegeneration that are the hallmark characteristics of AD. This deposition triggers microglial activation and increased production of proinflamatory cytokines, higher levels of amyloid precursor protein (APP) - triggering even more deposition of AΒ42 - and transcription of apolipoprotein E (APOE).
The process of phagocytosis involves the identification of debris material and the initiation of molecular and morphological changes that result in receptor recruitment and formation of a phagocytic cup for clearing the material from the microglia. Harry's group is interested in exploring the mechanism by which rafts may regulate microglial phagocytosis for insight into the development and progression of neurodegenerative disease.
Rafts are liquid-ordered microdomains in the membrane of a cell made up of specific proteins - the most important being the raft structural protein Flotillin1 (Flot1) and the cell-surface glycoprotein CD36 - cholesterol and sphingolipids, such as GM1 ganglioside. Rafts are involved in the orchestration of important cell signaling cascades and cell processes, among them apoptosis, cytokine signaling and phagocytosis. Although previous studies have demonstrated that rafts are critical in phagocytosis by peripheral macrophages, the unconventional phagocytosis that functions in the specialized macrophages found in the brain had not previously been well characterized.
To test their hypothesis that phagocytic activity in microglia employs raft activation similar to that seen in peripheral macrophages, the team treated cells directly with AΒ42 and induced AΒ42 production in rodent microglia cultures with celecoxib, a COX-2-selective non-steroidal anti-inflammatory drug (NSAID). Prolonged treatment allowed the researchers to evaluate changes in the effects on raft integrity and microglial phagocytosis with AΒ42 accumulation over time and to determine an AΒ42 threshold that is comparable to levels found in human patients with early to mid-stage AD.
Utilizing filipin, a cholesterol-sequestering agent, to induce loss of Flot1 and disrupt rafts, the authors demonstrated that shifts in that protein to non-raft fractions led to inhibition of raft aggregation and phagocytosis. They also determined that high AΒ42 disrupted raft localization of CD36, which is critical for normal phagocytic function.
"Along with identifying these candidate proteins for follow-up investigation, this study has important implications for the clinic," observed Persaud-Sawin, "and may provide clues as to why NSAID treatment in AD and other inflammatory neurodegenerative diseases has not been more successful."
In their conclusion, the authors speculated about how their novel approach to studying raft aggregation could possibly help investigators gain insight into other neurodegenerative conditions. "Prion diseases such as Creutzfeld-Jacob disease (CJD) and bovine spongiform encephalopathy require rafts for protein processing," they said, "and the over-expression of α-synuclein in Parkinson's disease can upregulate the raft-related caveolar protein, caveolin 1 and activate microglia."
Citation: Persaud-Sawin DA, Banach L, Harry GJ (http://www.ncbi.nlm.nih.gov/pubmed/18756527?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum) . 2009. Raft aggregation with specific receptor recruitment is required for microglial phagocytosis of AΒ42. Glia 57(3):320-335.
(Shweta Trivedi, Ph.D., is a postdoctoral fellow in the Laboratory of Respiratory Biology Environmental Genetics Group)