Alzheimer's disease (AD), the most common form of dementia, affects 10% of the population aged 65 and older, and up to 50% of people over 85 years old1,2. Patients with AD suffer from a decline in episodic memory with inability to recall the recent past and eventual loss of long-term memories, loss of cognitive function and changes in personality. The major goal of this proposal is to define pathways for the clearance of amyloid-? (A?), whose aggregation into amyloid plaques is a hallmark of AD. Increasing evidence indicates an important role of astrocytes, the most abundant cell type among glial cells in the brain, in A? clearance. This proposal will focus on a potentially novel regulatory mechanism for astrocytic A? uptake by LRP4 (LDLR-related protein 4), an LDLR-like protein that is abundantly and specifically expressed in astrocytes. We and others have demonstrated that LRP4 is critical to the formation of the neuromuscular junction (NMJ) where it serves as a receptor for agrin, a factor from motoneurons. The agrin- LRP4 interaction activates the transmembrane tyrosine kinase MuSK to induce AChR clustering and NMJ formation. Results of our preliminary studies, some of which were published, suggest that LRP4 may play a role in regulating AD pathogenesis. First, LRP4 was selectively expressed in astrocytes in the hippocampus and prefrontal cortex (PFC), both AD-vulnerable regions. Its level in astrocytes was higher than those of LDLR and LRP1, both of which have been implicated in A? uptake. Second, LRP4 was reduced in postmortem brain tissues of AD patients. In accord, deleting LRP4 augmented A? plaques and AD-associated deficits in 5xFAD mice, an AD mouse model. Third, A? uptake was impaired in LRP4 mutant (mt) astrocytes. These results suggest a critical role of LRP4 to astrocytic A? uptake. What is the underlying mechanism(s) by which LRP4 regulates A? uptake? LRP4 is a single transmembrane protein with a large extracellular domain (ECD). Interestingly, we found, in addition to agrin, LRP4 can interact with ApoE, whose ?4 allele (ApoE4) impairs A? clearance. Further pilot studies indicate, first that agrin stimulation could increase A? uptake by astrocytes, in a manner dependent on LRP4. LRP4 could interact with agrin, which has been shown to associate with A?. Therefore, we posit that LRP4 serves as a receptor for ApoE- and agrin-mediated A? uptake. Second, LRP4 could interact with the prorenin receptor (PRR), an auxiliary protein of v-ATPase that is critical to maintaining endolysosomal pH, and is required for its down-regulation. LRP4 deficiency increased PRR levels and caused hyper-acidification of endolysosomes. Therefore, we postulate that PRR upregulation may be a mechanism of LRP4 deficiency. Finally, the agrin-LRP4 interaction could stimulate MuSK in astrocytes. We wondered whether MuSK activation is necessary for agrin-promotion of A? uptake. Based on these results and considerations, we hypothesized that LRP4 is critical to A? uptake by astrocytes via multiple mechanisms. In this proposal, we will determine whether LRP4 deficiency correlates with AD development in AD patients and expedites AD-like pathology in AD mice; determine whether LRP4 regulates astrocytic A? uptake by interacting with ApoE and PRR; and investigate the roles of the agrin- LRP4 pathway in A? uptake by astrocytes. We hope that the proposed studies will not only establish novel cellular functions of astrocytic LRP4 in regulating A? uptake, but also shed new light on AD pathogenesis. Such knowledge is prerequisite to the development of new therapeutic strategies for AD.
This proposal will focus on a potentially novel regulatory mechanism for astrocytic A? uptake by LRP4 (LDLR-related protein 4). The proposed studies will shed new light on AD pathogenesis. Such knowledge is prerequisite to the development of new therapeutic strategies for AD.