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The primary research interest of my group is to understand molecular mechanism for the reduced synaptic connections in Alzheimer’s disease (AD) leading to loss of memory, a seminal feature of AD. AD is characterized by the presence of intracellular neurofibrillary tangles and extracellular amyloid plaques believed to be responsible for loss of synapses. The long-term objective of my research is to discover molecular targets which may modulate generation of amyloid beta peptide (Ab), the core constituent of amyloid plaques. Additionally, we are interested to investigate whether critical regulators of spine generation and maintenance can be used as therapeutic targets in AD since loss of synapses is a crucial factor for the cognitive deficits in AD.
In spite of rigorous research efforts worldwide, currently there is not a single drug available that can effectively reverse or even slow down the loss of memory in patients with AD. Therefore our goal is to use proof-of-concept molecules which modulate Ab generation and/or number of spines to discover lead compounds using varieties of small molecule and peptide libraries by high throughput screening (HTS). To accomplish these objectives we will use histological, pharmacological, biochemical and molecular approaches by using varieties of techniques including immunoprecipitation, yeast two-hybrid screening, in vivo lentiviral delivery, transgenic mouse models, knockout mouse models, high throughput robotics screening and TR-FRET imaging.
Our current research focus is to identify every protein that modulates the generation of toxic amyloid beta. We recently identified and demonstrated that RanBP9 robustly increases secretion of amyloid beta in varieties of cultured cells from both wild type APP (APPwt) and APP with Swedish mutation (APPswe). Although the exact mechanism is not yet clear, RanBP9 appears to increase amyloid beta generation through beta secretase processing of APP in the endocytic pathway because RanBP9 reduced cell surface APP, increased sAPP beta levels and accelerated APP internalization and association with lipid rafts. Remarkably, knockdown of endogenous RanBP9 using siRNAs in primary neuronal cultures led to marked reduction in amyloid beta levels suggesting that RanBP9 is an essential regulatory molecule for amyloid beta generation. More importantly, full-length RanBP9 (RanBP9-FL) and its 60 kDa-proteolytic product (RanBP9-N60) are significantly increased in APP J20 mice and Alzheimer’s disease brains respectively. These multiple evidences clearly suggest that RanBP9 plays critical role in generating amyloid beta.
Probing deeper in to the mechanisms, we have also shown that RanBP9 increases amyloid beta by binding to APP, the precursor of amyloid beta, LRP (low-density lipoprotein receptor-related protein), an APP binding protein and the beta secretase, the enzyme responsible for making the first cut in generating the amyloid beta. To gain a better insight on the functional significance of RanBP9 in vivo, we generated RanBP9 transgenic mice and crossed with APdE9 double transgenic mice (expressing Mo/HuAPP695swe and presenilin 1 with exon 9 deletion, PS1-dE9) to obtain RanBP9/APdE9 triple transgenic mice. Results from these triple transgenic mice revealed that RanBP9 in fact increases amyloid beta levels measured by ELISA as well as APP CTF levels detected by immunoblots. We further validated the increased amyloidogenic processing of APP and amyloid beta generation by showing increased number of amyloid plaques in triple transgenic mice compared to double transgenic mice (Fig. 1).
RanBP9 increases amyloid plaques in the transgenic mice
RanBP9 is a multidomain scaffolding protein implicated in the integration of varieties of cell surface receptors with intracellular signaling targets. Previous studies suggest that RanBP9 drastically reduces the growth and branching of neurites from dorsal root ganglion (DRG) neurons and cerebellar primary neurons. Also, RanBP9 can regulate cell morphology and adhesion. The majority of RanBP9 is localized to the nucleus but a significant amount can be found in the interior surface of the cell membrane and cytoplasm. In primary hippocampal neurons, we found RanBP9 not only in the cell body but also throughout the complex network of neurites with a punctate appearance (Fig. 2).
Determining the role of RanBP9 in the loss of synapses
The loss of synapses is an early event in the progression of Alzheimer’s disease which most closely correlates with cognitive decline. Therefore, in addition to studying the effects of RanBP9 on the formation of amyloid plaques, our lab is also determining the role of RanBP9 in the loss of dendritic spines and synapses. Additionally, we want to expand our goal to identify and test whether molecules which are critical for the formation and maintenance of spines are able to reverse the loss of synapses and cognitive impairment, even in the presence of amyloid plaques in the brain.
Our ultimate goal is to identify small molecule inhibitors that disrupt interactions between RanBP9 and APP/LRP/β-secretase to prevent initial production of amyloid beta so that all the subsequent alterations associated with amyloid plaques such as loss of synapses and memory impairment are completely prevented. Our objective is to screen millions of acyclic and heterocyclic small molecules mixed in combinatorial libraries readily available in our institute by high throughput robotics screening.