Mechanism of CCR2-Mediated Regulation of Microglial Accumulation in Alzheimer’s Disease

Joseph El Khoury, M.D.

Massachusetts General Hospital

Funded in September, 2006: $200000 for 3 years
LAY SUMMARY . ABSTRACT . BIOGRAPHY . FINDINGS . SELECTED PUBLICATIONS .

LAY SUMMARY

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Do Immune Microglial Cells’ Actions Go from Good to Bad in Alzheimer’s Disease?

This study in “transgenic” mice (having some human cells) will explore how immune microglial cells are recruited to the brain in Alzheimer’s disease.  It also will explore whether initially protective microglial cells then become destructive in the disease’s later stages. 

Microglial cells are the only immune cells that naturally reside in the brain.  In Alzheimer’s disease, through processes not yet understood, additional microglial cells are recruited to the brain.  The researchers suspect that this additional recruitment of microglial cells occurs through an accelerated version of the same process that initially attracts microglia to the brain. Specifically, microglial cells originate in bone marrow, migrate to the bloodstream, and are directed to the brain by substances called “chemokines.” In Alzheimer’s, the researchers hypothesize, a receptor (called CCR2) on the surface of brain cells, binds to chemokines that direct microglia to sites of inflammation in the brain.  Conversely, they found, CCR2 deficiency in animals leads to impaired microglial accumulation in the brain. 

While microglial cells are thought to be protective in the early stages of Alzheimer’s by clearing away deposits of the protein amyloid, scientists do not know whether they are destructive in later disease stages.  Do the toxic substances released by microglia in actually contribute to brain inflammation, cell damage, and neuronal degeneration in late Alzheimer’s? The researchers will study the recruitment of microglia into the brain and their actions at various stages of the disease, in a transgenic mouse model. They will determine whether the CCR2 receptor regulates movement of microglia from bone marrow to blood, from the blood to the brain, or both.  They also will determine whether the CCR2 receptor regulates the movement of microglia within the brain, from healthy areas to sites of amyloid build-up.  The findings should help clarify the role of microglia throughout the disease, and see whether it evolves from protective to destructive. 

Significance:  If the research demonstrates that immune microglial cells are protective early in Alzheimer’s disease, but destructive in late stages, further research could aim at methods for enhancing their recruitment and actions early in the disease, and inhibiting these in late stages.

ABSTRACT

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Mechanism of CCR2-Mediated Regulation of Microglial Accumulation in Alzheimer’s Disease

Alzheimer’s disease (AD) is the most common neurodegenerative disease of the elderly and its prevalence is increasing. The senile plaque is a pathological hallmark of AD and is composed of beta amyloid (Aβ), activated microglia, astyrocytes and degenerating neurons. Microglia are the principal innate immune cells of the brain. Microglia are recruited into the brain and accumulate in senile plaques. However, the exact role of microglia in the pathogenesis of AD has not been resolved. Microglia may play a neuroprotective role because of their ability to clear Aβ, but their subsequent activation and release of cytokines, chemokines, and neurotoxins may contribute to neurodegeneration.

The mechanism of microglial recruitment into normal or AD brains are not established. Published data suggest that in normal brains, microglia originate in the bone marrow, migrate into the blood and then populate the brain. A similar, but accelerated process may be occurring in AD. Chemokines are chemotactic cytokines that bind specific receptors on the surface of the leukocytes and mediate leukocytes accumulation at sites of inflammation. The exact role of chemokines and their receptors in the accumulation of microglia in normal or AD brains is not known. Published and preliminary data from the PI and co-investigator’s laboratories suggest that chemokine receptor CCR2, expressed on monocytes, microglia, astrocytes, and neurons, plays a critical role in microglial accumulation in the APP and PS1-APP mouse models of AD. In preliminary data, we demonstrate that CCR2 deficiency leads to impaired microglial accumulation in the brain, accelerated deposition of Aβ, and increased early mortality in the AD mice.

In this application, the PI, who has a long standing interest in the role of microglia in the innate immune response to Aβ and the identified scavenger receptors as key microglial receptors for Aβ, is collaborating with the co-investigator, an expert in the biology of chemokines, to investigate the role of CCR2 in the pathogenesis of AD and in the accumulation of microglia in AD. Using PS1-APP mice and CCR2-deficient mice and the technique of bone marrow transplantation, we will first confirm that the dramatic early effect we have observed in the CCR2-deficient APP and PS1-APP mice is the result of CCR2 deficiency only in hematopoietically derived cells (i.e. microglia and monocytes).  By performing the bone marrow transplantation at later times, we will also be able to determine if CCR2 controls microglial accumulation throughout the course of the disease. If CCR2 controls microglial recruitment at later time points as well, we will then be able to use these techniques to dissect the exact role of microglia at various stages of AD.

In the second aim, we will determine at what point CCR2 controls the process of microglial recruitment from the bone marrow => blood => brain => senile plaque.  We will determine whether CCR2 regulates the movement of microglial precursors from the bone marrow into the blood, or from the blood into the brain, or both. Finally, we will determine if CCR2 regulates the movement of microglia from the areas of the brain that do not have amyloid towards areas of amyloid accumulation. Specifically, we propose:

1. To determine whether CCR2 deficiency in bone marrow derived brain cells is sufficient to cause increased mortality and Aβ deposition in the PS1-APP mouse model of AD and if the effect of CCR2 deficiency persists throughout the course of the disease.

2. To determine whether CCR2 controls microglial recruitment from the bone marrow into the blood and/or from the blood into the brain of PS1-APP mice, and/or if CCR2 plays a role in the intracerebral migration of microglia into the senile plaque.

The proposed experiments will allow us to understand the role of CCR2 in accumulation of microglia in AD. Since our preliminary data suggests that CCR2 regulates microglial accumulation in AD, these experiments will also clarify the role of microglia in the pathogenesis and progression of AD.

INVESTIGATOR BIOGRAPHIES

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Joseph El Khoury, M.D.

Dr. El Khoury is an Assistant Professor of Medicine at Harvard Medical School and a physician in the Division of Infectious Diseases at the Massachusetts General Hospital. After graduating from medical school, he trained in the laboratory of Dr. Samuel C. Silverstein at Columbia University. While working with Dr. Silverstein, Dr. El Khoury identified microglial scavenger receptors as key microglial receptors that mediate interactions of these brain cells with β-amyloid. Following his initial training in research, Dr. El Khoury returned to clinical medicine and completed a residency in Internal Medicine at the Beth Israel Hospital, and a fellowship in Infectious Diseases at the Massachusetts General Hospital and Brigham and Women’s Hospital. As part of his fellowship training, he worked in the laboratory of Dr. Andrew Luster studying the role of chemokines in the accumulation of microglia in the Alzheimer’s brain.

After joining the faculty at Harvard and MGH, Dr. El Khoury’s laboratory continues to work on this topic and is currently trying to dissect the inflammatory response in Alzheimer’s disease using a variety of cellular and molecular techniques and mouse models of Alzheimer’s disease.

FINDINGS

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Lay Results:
The brain’s immune system has been implicated in the pathogenesis of Alzheimer’s disease for more than 20 years. Microglia, the main immune cells of the brain, are produced in the bone marrow and are recruited into the brain from the peripheral circulation. In Alzheimer’s disease, these immune cells are recruited into the brain at an accelerated rate and accumulate around senile plaques, which contain the pathogenic protein β-amyloid, and are characteristic of this neurodegenerative disease.  However, the role of these microglia in Alzheimer’s disease pathogenesis is not known.  Some studies have suggested that microglia may be neuroprotective by phagocytosing and clearing β-amyloid deposited in the brain of Alzheimer’s patients.  Other studies have suggested that the immune system promotes disease because β-amyloid can activate microglia to secrete neurotoxins that cause neuronal degeneration.

To resolve this question, we used a mouse model of Alzheimer’s disease and mice deficient in the chemokine receptor CCR2.  We found that CCR2 deficiency markedly impaired the recruitment and accumulation of microglia in the brain of Alzheimer’s disease mice. This defect in microglial trafficking resulted in the premature and lethal accumulation of β-amyloid around blood vessels in the brain.  Thus the absence of early microglial accumulation leads to decreased β-amyloid clearance and increased mortality. Therefore, our study provides in vivo evidence that the brain’s immune system plays a protective role in early Alzheimer’s disease by mediating the clearance of β-amyloid.

Our results offer new insight into the understanding of Alzheimer’s disease and suggest new therapeutic strategies.  Our study also demonstrates a new connection between immune cell function and this chronic debilitating disease of great importance to our society. 

Scientific Results:
Microglia are the principal immune cells of the brain. In Alzheimer's disease, these brain mononuclear phagocytes are recruited from the blood and accumulate in senile plaques. However, the role of microglia in Alzheimer's disease has not been resolved. Microglia may be neuroprotective by phagocytosing amyloid-beta (Aβ), but their activation and the secretion of neurotoxins may also cause neurodegeneration. CCR2 is a chemokine receptor expressed on microglia, which mediates the accumulation of mononuclear phagocytes at sites of inflammation. Here we show that CCR2 deficiency accelerates early disease progression and markedly impairs microglial accumulation in a transgenic mouse model of Alzheimer disease (Tg2576). Alzheimer's disease mice deficient in CCR2 accumulated Aβ earlier and died prematurely, in a manner that correlated with CCR2 gene dosage, indicating that absence of early microglial accumulation leads to decreased Aβ clearance and increased mortality. Thus, CCR2-dependent microglial accumulation plays a protective role in the early stages of Alzheimer's disease by promoting Aβ clearance.

SELECTED PUBLICATIONS

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El Khoury J., Toft M., Hickman S.E., Means T.K., Terada K., Geula C., and Luster A.D.  Ccr2 deficiency impairs microglial accumulation and accelerates progression of Alzheimer-like disease.  Nat Med. 2007 Apr;13(4):432-8.