Once thought to be immune-privileged, over the past few decades the central nervous system has been found to be intricately tied to the immune system. In many pathological states of the CNS, the immune system has a major causative role or exacerbates the disease processes. Alzheimer's disease, a major focus of the Landreth Laboratory, involves a robust immune response. Microglia, the brain's macrophage, are the first line of defense for the CNS innate immune system. Over many years the laboratory has defined many of the microglial receptors that are responsible for the immune response to the amyloid plaques of AD, a hallmark of the disease. My interest and focus lie in understanding the role that microglia and their innate immune receptors play in either exacerbating or attenuating the disease process. Specifically, I have focused on a class of kinases, the Interleukin-1 receptor-associated kinases, or IRAKs, that are critical to the innate immune response. I am investigating their role in modulating the activation status and response of microglia as Alzheimer's disease progresses using genetic mouse models and various in vitro assays.
Apolipoprotein E (ApoE) is the major risk factor for late onset Alzheimer's disease. It is the predominant apolipoprotein and a critical component of high density lipoproteins (HDL) in the brain. In humans, there are three common ApoE alleles (E2, E3, and E4), which differ in their ability to form HDL particles. Inheritance of ApoE4 confers increased risk, earlier onset and poorer clinical outcomes for AD. Our lab recently demonstrated that both intracellular and extracullar degradation of Aß by microglia is facilitated in the presence of ApoE. Importantly, the degradation is ApoE isoform- and lipidation status-dependent, in which the ApoE4 isoform and poorly lipidated ApoE are less efficient in promoting proteolytic degradation of Aß. Understanding the molecular mechanism of ApoE-mediated facilitation of Aß degradation is not only essential for resolving the biological function of ApoE in Aß catabolism, but also provides insights for development of new therapeutic drugs for AD.
I am investigating the molecular mechanism that underlies these effects of ApoE by using enzyme kinetics, biochemical interference and genetic manipulation. A growing body of evidence suggests that dysregulated cholesterol metabolism may be involved in the pathogenesis of AD. Many of genetic variants impeding cholesterol homeostasis in human also result in AD-like pathology. Epidemiologic studies suggest a link between elevated plasma cholesterol levels and increased risk of AD and that the use of the cholesterol synthesis lowering drugs is associated with reduced prevalence of AD later in life. Since maintaining brain cholesterol homeostasis is one of the important functions of ApoE, I am investigating the impact of cholesterol homeostasis on the Aß degradation. I hypothesize that ApoE promotes proteolysis of Aß by modulating intracellular membrane cholesterol levels. My data shows that the promotion of Aß degradation is a common feature of HDL apolipoproteins, including ApoE, ApoA-I as well as a small peptide mimicking the lipid binding structure of ApoA-I. Reduction of cellular cholesterol by these apolipoproteins resulted in enhanced Aß degradation. Conversely, elevated cellular cholesterol levels are associated with the impaired Aß clearance by microglia. However, the changes in Aß degrading ability are not dependent on regulating the levels of Aß degrading enzymes. To our surprise, the effects are possibly originated from the changes in the endocytic trafficking efficiency of internalized Aß as a result of modifying cellular cholesterol levels. These findings demonstrate a direct role of cholesterol in AD.
One of the characteristics of the Alzheimer’s disease (AD) brain is the accumulation of amyloid ß (Aß) in the brain. Our lab has recently found that treatment of mouse models of AD with agonists of Liver X Receptors (LXRs), results in the proteolytic degradation and clearance of Aß from the brain. LXRs form obligate heterodimers with other nuclear receptors to form transcriptionally active complexes which drive the expression of genes, such as ApoE and the lipid transporter ABCA1, that function in Aß clearance. The capacity of ApoE to facilitate Aß degradation is dependent not only on ApoE isoform but also on its lipidation status, which is determined by ABCA1 activity. LXR agonists act to increase clearance of Aß by driving the expression of ABCA1 and ApoE. I am interested in the therapeutic potential of LXR agonists in AD mouse models, and how to administer these agonists to achieve the greatest benefit.
Neuro-cardio-facial cutaneous (NCFC) syndromes are genetic disorders arising from mutations in the signaling pathway which activates the ERK MAP kinases. These syndromes are associated with cognitive impairment and psychiatric disease resulting from early perturbations in CNS development. I am interested in potential mechanisms by which aberrant ERK signaling disrupts cortical development. The ERKs are involved in cell proliferation, differentiation, as well as learning and memory. In my research, I use mice models with telencephalic ERK1/2 deletions to investigate why ERK activity is essential during cortical development. In my research I demonstrate that mice lacking ERK1 and ERK2 lack crucial balance between neurogenic and proliferative progenitor divisions, resulting in aberrant layering of the adult cortex. Mechanistically, deletion of ERK1/2 effects apical and basal progenitor proliferation by altering the length and dynamics of the cell cycle. Specifically, elongation of G1 results in a switch where neurogenic divisions are favored at the expense of proliferation. The precocious neurogenesis causes premature depletion of the progenitor pool reducing the number of pyramidal neurons in the mature cortex. These early embryonic deficits affect global cortical circuitry altering normal physiology and behavior in the adult mice.
Experimental mouse models with perturbations in ERK1/2 signaling resulting in altered cortical morphology present a unique opportunity to directly study molecular and cellular mechanisms linked to cortical development, physiology and animal behavior. I hope to establish a mechanistic explanation and further our understanding of the link between loss of ERK1/2 and their consequences on development and the pathophysiology and behavioral abnormalities associated with disorders of the RAS/MAPK pathway including DiGeorge Syndrome and autism spectrum disorders .
Alzheimer’s Disease is a neurodegenerative disease characterized by the slow buildup of insoluble plaques comprised of fibrillar Aß. Microglia, the brain’s resident macrophages, migrate to the plaque and invest processes deep within it. These plaque associated microglia become classically activated into an M1 state. They express proinflammatory cytokines to attract more microglia and reactive oxygen species in an attempt to destroy the plaque. While this M1 response is effective in destroying an invading pathogen, it creates an inflammatory environment. This chronic inflammation renders the M1 microglia incapable of phagocytosing and degrading the amyloid plaques. As a result, the AD brain is riddled with plaques surrounded by phagocytically incompetent M1 microglia, thus worsening AD pathogenesis. Interestingly, other tissue specific macrophages such as those in the lung or liver are capable of resolving inflammation, repairing damaged tissue, and phagocytosing cellular debris by converting to an “alternatively activated” M2 state. M2 microglia could be of benefit in AD due to their anti-inflammatory and phagocytic capabilities. However, M2 activation in microglia has not been well studied.
It has recently been appreciated that activation of the nuclear receptors PPARγ and PPARδ drives M2 macrophage polarization. Treating monocytes with PPARγ agonist pioglitazone causes them to differentiate into M2 macrophages, while PPARδ is vital for clearing cellular debris. The related nuclear receptor LXR enhances phagocytosis of fAß. Macrophages lacking PPARδ or LXR are incompetent phagocytes. Interestingly, treating AD model mice with LXR or PPAR agonists improves cognition and decreases plaque load, but the mechanism of fAß degradation has not been well established. Since these nuclear receptor agonists can drive M2 activation in peripheral tissue macrophages, I am studying the effects of LXR and PPAR agonists on the activation state and phagocytic competency of microglia, which is still unknown. I am investigating whether these agonists will drive M2 activation in microglia, thus reducing inflammation in AD model mice treated with agonists compared to untreated controls. Furthermore, I predict that treated microglia will take up and degrade fAß more effectively than untreated microglia. If they do, it would explain the decrease in plaque load of AD models treated with these agonists.
Alzheimer’s disease is associated with the inability to effectively clear ß-amyloid from the brain, a process that is normally reliant upon Apolipoprotein E. ApoE expression is transcriptionally induced through the coordinate action of the nuclear receptors PPARγ:RXR and LXR:RXR. The nuclear receptors, LXR and PPARγ form obligate heterodimers with retinoid X receptors (RXRs) to form functional transcription factors and are termed “permissive” as their transcriptional activity can be activated by ligation of either member of the receptor pair. PPARγ:RXR and LXR:RXR participate in a linked metabolic pathway, acting in a feed-forward manner to not only induce the expression of apoE, but also induce expression of its lipid transporters and the genes for the nuclear receptors themselves. Previous work in the laboratory has shown that lipidation status of ApoE is directly correlated with the ability to degrade ß-amyloid. In addition, the AD brain is also characterized by a microglial-mediated inflammatory response that impairs Aß clearance and exacerbates the primary disease processes. It is of particular significance that the PPARs and LXRs act to promote phagocytosis, to suppress inflammation and stimulate the conversion of macrophages/microglia into ‘alternative’ activation states that favor Aß clearance. My research project focuses on RXRs and their role in ameliorating AD mouse model pathologies. We hypothesize that administration of the RXR agonists will act to enhance normal Aß clearance mechanisms through the simultaneous activation of the PPARs and LXRs in the brain, leading to reduced brain Aß by enhancing clearance of the peptide via apoE-dependent mechanisms as well as via phagocytic mechanisms.
The peroxisome proliferator-activated receptor-gamma (PPARγ) is a ligand activated nuclear receptor that exhibits potent anti-inflammatory actions and functions to regulate insulin sensitivity . Previous studies carried out in our lab have shown that treatment of AD mouse models with PPARγ agonists can reduce brain levels of soluble Aß42. My research project focuses on the mechanisms through which PPARγ facilitates the proteolytic degradation of these soluble Aß species.
Recently, our lab has shown that activation of another nuclear receptor the Liver X Receptor (LXR) facilitates the proteolytic degradation of Aß in an ApoE dependent mechanism through the induction of target genes essential to reverse cholesterol transport. These genes include the ATP Cassette Binding Transporter A1 (ABCA1) and ApoE. ABCA1 regulates cholesterol efflux via lipidation of apolipoproteins such as ApoE and lipidation of ApoE particles augments the degradation of soluble Aß species. Since, PPARγ has been shown to induce the expression of LXR we hypothesize that PPARγ may be acting though an LXR pathway, by inducing the expression of ABCA1 and ApoE to reduce amyloid pathology in AD.
Neuro-cardio-facio cutaneous (NCFC) syndromes are a collection of congenital conditions that arise from mutations in the signaling elements of the Erk pathway. One of the common features seen in these conditions are olfactory deficits, however, the reason for these deficits is not clear. We have a created an Erk1 knockout and conditional knockout of the Erk2 isoform that results in complete loss of function of one or both isoforms in the brain of the developing mouse. The mutant mice are anosmic, in addition to having learning and behavioral deficits. Previous work on the developing cortex has found that the lack of Erk signaling induces radial glia, the neural stem cells, to generate neurons rather than undergoing self-renewing divisions. The deficits we have identified are during embryonic brain development, but the behavioral tests have been performed on adult mice. This raises the question of the relative contributions of the developmental perturbations of brain structure versus the role of Erks in normal neuronal physiology. In order to answer this question, we are examining how the loss of Erk signaling affects the hippocampus and olfactory bulb during development as well as postnatally. The olfactory bulb and the hippocampus are the ideal areas of the brain to study because they are needed for olfaction and learning, respectively. In addition to their role in olfaction and behavior, the olfactory bulb and hippocampus are the primary sites of adult neurogenesis. We hope to elucidate some of the mechanisms involved during embryonic and adult neurogenesis.