Research

Caspase-2-mediated cleavage of tau proteins has recently been shown to cause synaptic transmission dysfunction and cognitive impairment in transgenic mice modeling frontotemporal dementia.  Levels of Δtau314, the cleavage product of caspase-2, were also shown to be higher in the brain of individuals with cognitive impairment than cognitively normal individuals.  Because genetic ablation of tau and caspase-2 ameliorates neurological dysfunction in mouse models of Huntington’s disease (HD), I examined the relevance of Δtau314 to human HD and its relation to caspase-2.  In collaboration with Dr. Rocio Gomez-Pastor at the University of Minnesota, Twin Cities, MN, I identified elevated levels of Δtau314 in HD patients and a correlation between caspase-2 and Δtau314.  These findings, for the first time, established a link between Δtau314, a tau species related to cognitive dysfunction, and HD, thus advancing our understanding of the contribution of caspase-2-mediated Δtau314 production to HD pathogenesis.  In another collaborative study with Drs. Ronald Petersen and David Knopman at Mayo Clinic, Rochester, MN, I showed a connection between Δtau314, caspase-2, and AD dementia.  These findings advance our understanding of the contribution of caspase-2-mediated Δtau314 production to cognitive deterioration.

Publications

1. Liu P, Smith BR, Huang ES, Mahesh A, Vonsattel JPG, Petersen AJ, Gomez-Pastor R, Ashe KH. A soluble truncated tau species related to cognitive dysfunction and caspase-2 is elevated in the brain of Huntington’s disease patients. Acta Neuropathologica Communications. 2019;7:111. PubMed PMID: 31358058; PMCID: PMC6664763.

2. Liu P, Smith BR, Montonye ML, Kemper LJ, Leinonen-Wright K, Nelson KM, Higgins L, Guerrero CR, Markowski TW, Zhao X, Petersen AJ, Knopman DS, Petersen RC, Ashe KH. A soluble truncated tau species related to cognitive dysfunction is elevated in the brain of cognitively impaired human individuals. Scientific Reports. 2020;10:3869. PubMed PMID: 32123248; PMCID: PMC7052165.

3. Steuer EL, Kemper LJ, Hlynialuk CJW, Leinonen-Wright K, Montonye ML, Lapcinski IP, Forster CL, Ashe KH, Liu P. Blocking site-specific cleavage of human tau delays progression of disease-related phenotypes in genetically matched tau-transgenic mice modeling frontotemporal dementia. Journal of Neuroscience. 2022;42(23):4737-54. PubMed PMID: 35508385.

Although there is now a general consensus that Aβ oligomers but not amyloid fibrils are detrimental to cognitive function and memory in Alzheimer’s disease (AD), the exact mechanism by which Aβ oligomers exert toxic effects is still unknown.  Using conformation-sensitive antibodies that recognize protein aggregates with unique quaternary structures and a laser microdissection technique that I optimized for protein assays, I led a study addressing the influence of structure and spatiotemporal patterns on the neurological effects of Aβ oligomers.  Our findings indicate the presence of at least two distinct types of Aβ oligomers in the brain of transgenic AD mouse models that influence cognitive function differently.  Importantly, in situ spatial distribution pattern of Aβ oligomers, along with structure, plays a key role in determining their effects on cognition.  These results refine our understanding of the characteristics of Aβ oligomers in vivo, and highlight the importance of attempting to neutralize the highly dispersed Type 1 oligomers to ameliorate cognitive dysfunction.  Our findings have made a positive impact on Aβ therapeutic research performed by other groups (e.g., Knight et al. Neurology Neuroimmunology & Neuroinflammation 2016).

Publications

1. Liu P, Reed MN, Kotilinek LA, Grant MK, Forster CL, Qiang W, Shapiro SL, Reichl JH, Chiang AC, Jankowsky JL, Wilmot CM, Cleary JP, Zahs KR, Ashe KH. Quaternary Structure Defines a Large Class of Amyloid-beta Oligomers Neutralized by Sequestration. Cell Reports. 2015;11(11):1760-71. PubMed PMID: 26051935; PMCID: PMC4494129.

Currently, there are approximately 50 transgenic amyloid precursor protein (APP) mouse models of AD.  These mice exhibit amyloid pathology that mimics that of AD brains, and yet a comparison of the pathological features between widely used mouse lines and between mouse models and human subjects was lacking.  I led a project in which we quantitatively compared neuritic plaque load and density of multiple mouse models and individuals with AD.  Results of this study further our understanding of the diversity of pathological features among model organisms and provide knowledge for developing models to better resemble pathological conditions of human disease.  In addition, I led a study in which we characterized the pathological and behavioral features of a novel transgenic APP mouse model of AD, namely the rTg9191 model, which is a valuable model for investigating the neurological effects of Type 2 Aβ oligomers.

Publications

1. Liu P, Paulson JB, Forster CL, Shapiro SL, Ashe KH, Zahs KR. Characterization of a Novel Mouse Model of Alzheimer's Disease--Amyloid Pathology and Unique β-Amyloid Oligomer Profile. PloS One. 2015;10(5):e0126317. PubMed PMID: 25946042; PMCID: PMC4422728.

2. Liu P, Reichl JH, Rao ER, McNellis BM, Huang ES, Hemmy LS, Forster CL, Kuskowski MA, Borchelt DR, Vassar R, Ashe KH, Zahs KR. Quantitative Comparison of Dense-Core Amyloid Plaque Accumulation in Amyloid-β Protein Precursor Transgenic Mice. Journal of Alzheimer's Disease: JAD. 2017;56(2):743-61. PubMed PMID: 28059792; PMCID: PMC5272806.

Administration of certain nutraceutical supplements to AD mouse models leads to prevention and/or amelioration of cognitive deficits of mice; the underlying molecular mechanism, however, is often unclear.  In collaboration with Dr. Giulio Pasinetti and colleagues at the Mount Sinai School of Medicine, New York, NY, I investigated the effects of grape seed polyphenol extracts (GSPE) on an array of Aβ oligomers.  I found that GSPE treatment significantly decreased brain levels of Aβ*56, a 56-kDa Type 1 Aβ oligomer known to induce memory dysfunction in rodents, without changing the levels of transgenic APP, monomeric Aβ, or other Aβ oligomers.  These results provide the first demonstration that a safe and affordable intervention can lower the levels of a memory-impairing Aβ oligomer in vivo and strongly suggest that GSPE be further tested as a potential prevention and/or therapy for AD.

Publications

1. Liu P, Kemper LJ, Wang J, Zahs KR, Ashe KH, Pasinetti GM. Grape seed polyphenolic extract specifically decreases abeta*56 in the brains of Tg2576 mice. Journal of Alzheimer's Disease: JAD. 2011;26(4):657-66. PubMed PMID: 21743132.

Plasmepsins are a class of aspartic proteases that play an essential role in malaria virulence and pathogenesis, and are considered an important molecular target for novel anti-malarial drug development.  During my PhD, I contributed to understanding the structures of several plasmepsins produced from malarial parasites infecting humans and rodents.  In particular, I designed combinatorial peptide libraries that enabled me to determine subsite preferences of plasmepsin homologs.  Using knowledge thus gained, I designed peptidomimetic inihibitors against different plasmepsins, and the synthesized compounds exhibit high affinity and selectivity to plasmepsins versus homologous human enzymes.  In addition, I determined the atomic structure of a plasmepsin in complex with, for the first time, a peptidomimetic inhibitor using X-ray crystallography.  Findings from these studies further our understanding of the interactions between plasmepsins and inhibitors, and provide leads for novel anti-malarial drug design.

Publications

1. Liu P, Marzahn MR, Robbins AH, Gutierrez-de-Teran H, Rodriguez D, McClung SH, Stevens SM, Jr., Yowell CA, Dame JB, McKenna R, Dunn BM. Recombinant plasmepsin 1 from the human malaria parasite plasmodium falciparum: enzymatic characterization, active site inhibitor design, and structural analysis. Biochemistry. 2009;48(19):4086-99. doi: 10.1021/bi802059r. PubMed PMID: 19271776; PMCID: 2730762.

2. Liu P, Robbins AH, Marzahn MR, McClung SH, Yowell CA, Stevens SM, Jr., Dame JB, Dunn BM. Enzymatic Characterization of Recombinant Food Vacuole Plasmepsin 4 from the Rodent Malaria Parasite Plasmodium berghei. PloS One. 2015;10(10): e0141758. PubMed PMID: 25946042; PMCID: 4422728.

3. Gutierrez-de-Teran H, Nervall M, Ersmark K, Liu P, Janka LK, Dunn B, Hallberg A, Aqvist J. Inhibitor binding to the plasmepsin IV aspartic protease from Plasmodium falciparum. Biochemistry. 2006;45(35):10529-41. doi: 10.1021/bi0609669. PubMed PMID: 16939205.

4. Dell'Agli M, Parapini S, Galli G, Vaiana N, Taramelli D, Sparatore A, Liu P, Dunn BM, Bosisio E, Romeo S. High antiplasmodial activity of novel plasmepsins I and II inhibitors. Journal of Medicinal Chemistry. 2006;49(25):7440-9. doi: 10.1021/jm061033d. PubMed PMID: 17149873.

MyrielCure Award (Ashe-Principal Investigator, Liu-Co-Investigator)
Repairing synapses in neurodegenerative disorders by inhibiting caspase-2
Award Period: 10/01/2022-12/31/2024
Total Direct Costs: $371,334

Funding Source: Myriel, Inc and The Lucas Brothers Foundation

Background: Mounting evidence supports that small aggregated forms of tau, a protein that helps stabilize nerve cells called neurons in the brain, damage brain functions. Understanding the structures of small tau aggregates could provide knowledge to the therapy of brain diseases. However, deciphering structural details of such aggregates is challenging because they are often present in low stability, varying sizes and low abundance in the brain. Dr. Liu and colleagues have recently discovered the presence of higher amounts of a specific small aggregated form of tau in elderly individuals with cognitive impairment than cognitively normal individuals of similar ages. Importantly, they have developed a method to isolate this entity from human brains, taking advantage of its relatively strong stability and uniformity in size.

Research Plan: Dr. Liu and team will study the structure and toxicity of the small tau aggregates isolated from brains of individuals with Alzheimer’s dementia. First, they will use a sensitive device called “mass spectrometer” to determine the composition of the tau aggregates, use microscopes to reveal size and shape, and use other biophysical approaches to unveil additional structural features. 

Second, Dr. Liu and team will assess the impact of the tau aggregates on the process by which neurons communicate with each other. They will introduce the tau aggregates to cultured neurons, and record and analyze signals released and received by neurons. 

Third, the investigators will inject the tau aggregates into the brains of mice and evaluate their effects on cognitive function using an object recognition test.

Impact: Findings of the proposed project could advance our knowledge of how structurally-defined small tau aggregates damage neuronal and cognitive function. This may lead to treatments that benefit various dementias.

Technical Abstract: The microtubule-associated protein tau forms intracellular aggregates of varied morphologies in Alzheimer’s disease (AD) and related dementias (ADRD), which afflict more than 55 million individuals worldwide. Accumulating evidence points that soluble tau oligomers (tauO) play a more significant role in the pathogenesis of ADRD than insoluble tau aggregates. However, high-resolution structures of brain-derived tau related to dementia remain unavailable. We have recently identified and isolated a specific type of tau in human brains, referred to here as deltatau314O. A constituent of deltatau314O is deltatau314, the N-terminal product of caspase-2-mediated tau cleavage at aspartate-314. This tau cleavage triggers synaptic and cognitive dysfunction in models of frontotemporal dementia, and is pathologically relevant to multiple types of dementias. A notable feature of deltatau314O is its high stability when subject to sodium dodecyl sulfate (SDS) and its presence as a single entity when fractionated by SDS-polyacrylamide gel electrophoresis. Importantly, our preliminary data support the relevance of deltatau314O to early AD progression. Our long-term goal is to understand the structure and toxicity of tau originating from the brains of individuals with ADRD, and to use this knowledge to develop therapies that can slow or reverse disease progression. 

The primary goal of the research is to determine the structure and toxicity of deltatau314O. We will pursue three specific aims. 

In aim 1, we investigate the structural characteristics of deltatau314O obtained from individuals with AD dementia and mild cognitive impairment. We hypothesize that deltatau314O exhibits morphologies, sizes, and thioflavin T-binding affinities distinct from tau fibrils. We will use microscopic and spectroscopic methods to reveal its structural characteristics.

In aim 2, we explore the impact of deltatau314O on synaptic transmission in cultured neurons. We hypothesize that deltatau314O weakens excitatory synaptic transmission by reducing the amplitude of miniature excitatory postsynaptic currents(mEPSC). We will apply deltatau314O to cultured neurons, perform patch-clamping, and measure the amplitude and frequency of mEPSC. 

In aim 3, we evaluate the effects of deltatau314O on the cognitive function of healthy mice. We hypothesize that injecting deltatau314O into the hippocampus will result in cognitive impairment. We will inject deltatau314O into mice and evaluate its effects on recognition memory using an object recognition test.

Upon the completion of the research, we expect to have unveiled the structural features of deltatau314O and established its toxicity in synaptic and cognitive function. This will advance our knowledge of the contribution of a structurally-defined, brain-derived tau oligomeric species to cognitive dysfunction and the underlying pathophysiology, pathophysiology, and potentially lead to the development of therapies that can benefit various tau-related disorders.

The Medical School/University of Minnesota Foundation (MS/UMF) (Liu-PI) 

Microtubule-associated protein tau and brain senescence in tauopathies 

Award Period: 12/01/2021-11/30/2022 

Annual Direct Costs: $15,000

NIH 1R56AG073734-01 (Sachs-PI, Liu-Co-I)
Elucidating the biophysics of pre-fibrillar, toxic tau oligomers from amino acid motifs to neuronal dysfunction
Award Period: 09/30/2021-05/31/2023
Annual Direct Costs: $350,000