Apolipoprotein E in Alzheimer's disease trajectories and the next-generation clinical care pathway.

Alzheimer's disease (AD) is a complex, progressive primary neurodegenerative disease. Since pivotal genetic studies in 1993, the ε4 allele of the apolipoprotein E gene (APOE ε4) has remained the strongest single genome-wide associated risk variant in AD. Scientific advances in APOE biology, AD pathophysiology and ApoE-targeted therapies have brought APOE to the forefront of research, with potential translation into routine AD clinical care. This contemporary Review will merge APOE research with the emerging AD clinical care pathway and discuss APOE genetic risk as a conduit to genomic-based precision medicine in AD, including ApoE's influence in the ATX(N) biomarker framework of AD. We summarize the evidence for APOE as an important modifier of AD clinical-biological trajectories. We then illustrate the utility of APOE testing and the future of ApoE-targeted therapies in the next-generation AD clinical-diagnostic pathway. With the emergence of new AD therapies, understanding how APOE modulates AD pathophysiology will become critical for personalized AD patient care.

Insoluble Tau From Human FTDP-17 Cases Exhibit Unique Transmission Properties In Vivo.

One hallmark of neurodegenerative diseases is the intracellular accumulation of hyperphosphorylated tau protein, a neuronal microtubule-associated protein, into structures known as neurofibrillary tangles. Tauopathies are heterogeneous neurodegenerative diseases caused by the misfolding of the tau protein. It has been previously shown that the tau protein can spread from cell to cell in a prion-like manner. Tauopathies can be sporadic or familial, with the identification of pathogenic mutations in the microtubule-associated protein tau gene on chromosome 17 in the familial cases. Different frontotemporal dementia with parkinsonism-17 (FTDP-17) cases are associated with varying clinical presentations and types of neuropathology. We previously demonstrated that insoluble tau extracted from sporadic tauopathy human brains contain distinct tau strains, which underlie the heterogeneity of these diseases. Furthermore, these tau strains seeded tau aggregates that resemble human tau neuropathology in nontransgenic and 6hTau mice in vivo. Here, we show insoluble tau from human brains of FTDP-17 cases transmit different patterns of neuronal and glial tau pathology in vivo, similar to the sporadic tauopathies. This suggests that each of these tau mutations has unique properties that underlie the heterogeneity of FTDP-17 cases.

Transmission of tauopathy strains is independent of their isoform composition.

The deposition of pathological tau is a common feature in several neurodegenerative tauopathies. Although equal ratios of tau isoforms with 3 (3R) and 4 (4R) microtubule-binding repeats are expressed in the adult human brain, the pathological tau from different tauopathies have distinct isoform compositions and cell type specificities. The underlying mechanisms of tauopathies are unknown, partially due to the lack of proper models. Here, we generate a new transgenic mouse line expressing equal ratios of 3R and 4R human tau isoforms (6hTau mice). Intracerebral injections of distinct human tauopathy brain-derived tau strains into 6hTau mice recapitulate the deposition of pathological tau with distinct tau isoform compositions and cell type specificities as in human tauopathies. Moreover, through in vivo propagation of these tau strains among different mouse lines, we demonstrate that the transmission of distinct tau strains is independent of strain isoform compositions, but instead intrinsic to unique pathological conformations.

Human tau pathology transmits glial tau aggregates in the absence of neuronal tau.

Tauopathies are characterized by abnormal accumulation of tau protein in neurons and glia. In Alzheimer's disease (AD), tau aggregates in neurons, while in corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP), tau also aggregates in astrocytes and oligodendrocytes. We previously demonstrated that human CBD and PSP tauopathy lysates (CBD-tau and PSP-tau) contain distinct tau strains that propagate neuronal and glial tau aggregates in nontransgenic (nonTg) mouse brain. Yet the mechanism of glial tau transmission is unknown. Here, we developed a novel mouse model to knock down tau in neurons to test for glial tau transmission. While oligodendroglial tau pathology propagated across the mouse brain in the absence of neuronal tau pathology, astrocytic tau pathology did not. Oligodendroglial tau aggregates propagated along white matter tracts independently of neuronal axons, and resulted in oligodendrocyte cell loss. Thus, glial tau pathology has significant functional consequences independent of neuronal tau pathology.

Impact of TREM2 risk variants on brain region-specific immune activation and plaque microenvironment in Alzheimer's disease patient brain samples.

Identification of multiple immune-related genetic risk factors for sporadic AD (sAD) have put the immune system center stage in mechanisms underlying this disorder. Comprehensive analysis of microglia in different stages of AD in human brains revealed microglia activation to follow the progression of AD neuropathological changes and requiring the co-occurrence of beta-Amyloid (Aß) and tau pathology. Carriers of AD-associated risk variants in TREM2 (Triggering receptor expressed on myeloid cells 2) showed a reduction of plaque-associated microglia and a substantial increase in dystrophic neurites and overall pathological tau compared with age and disease stage matched AD patients without TREM2 risk variants. These findings were substantiated by digital spatial profiling of the plaque microenvironment and targeted gene expression profiling on the NanoString nCounter system, which revealed striking brain region dependent differences in immune response patterns within individual cases. The demonstration of profound brain region and risk-variant specific differences in immune activation in human AD brains impacts the applicability of immune-therapeutic approaches for sAD and related neurodegenerative diseases.

Humanization of the entire murine Mapt gene provides a murine model of pathological human tau propagation.

In cortical regions of brains from individuals with preclinical or clinical Alzheimer's disease (AD), extracellular ß-amyloid (Aß) deposition precedes the aggregation of pathological intracellular tau (the product of the gene microtubule-associated protein tau (MAPT)). To our knowledge, current mouse models of tauopathy reconstitute tau pathology by overexpressing mutant human tau protein. Here, through a homologous recombination approach that replaced the entire murine Mapt gene with the human ortholog, we developed knock-in mice with humanized Mapt to create an in vivo platform for studying human tauopathy. Of note, the humanized Mapt expressed all six tau isoforms present in humans. We next cross-bred the MAPT knock-in mice with single amyloid precursor protein (App) knock-in mice to investigate the Aß-tau axis in AD etiology. The double-knock-in mice exhibited higher tau phosphorylation than did single MAPT knock-in mice but initially lacked apparent tauopathy and neurodegeneration, as observed in the single App knock-in mice. We further observed that tau humanization significantly accelerates cell-to-cell propagation of AD brain-derived pathological tau both in the absence and presence of Aß-amyloidosis. In the presence of Aß-amyloidosis, tau accumulation was intensified and closely associated with dystrophic neurites, consistently showing that Aß-amyloidosis affects tau pathology. Our results also indicated that the pathological human tau interacts better with human tau than with murine tau, suggesting species-specific differences between these orthologous pathogenic proteins. We propose that the MAPT knock-in mice will make it feasible to investigate the behaviors and characteristics of human tau in an animal model.

TREM2 function impedes tau seeding in neuritic plaques.

Variants in the triggering receptor expressed on myeloid cells 2 (TREM2) have been associated with increased risk for sporadic, late-onset Alzheimer's disease. Here we show that germline knockout of Trem2 or the TREM2R47H variant reduces microgliosis around amyloid-ß plaques and facilitates the seeding and spreading of neuritic plaque tau aggregates. These findings demonstrate a key role for TREM2 and microglia in limiting the development of peri-plaque tau pathologies.

Amyloid-ß plaques enhance Alzheimer's brain tau-seeded pathologies by facilitating neuritic plaque tau aggregation.

Alzheimer's disease (AD) is characterized by extracellular amyloid-ß (Aß) plaques and intracellular tau inclusions. However, the exact mechanistic link between these two AD lesions remains enigmatic. Through injection of human AD-brain-derived pathological tau (AD-tau) into Aß plaque-bearing mouse models that do not overexpress tau, we recapitulated the formation of three major types of AD-relevant tau pathologies: tau aggregates in dystrophic neurites surrounding Aß plaques (NP tau), AD-like neurofibrillary tangles (NFTs) and neuropil threads (NTs). These distinct tau pathologies have different temporal onsets and functional consequences on neural activity and behavior. Notably, we found that Aß plaques created a unique environment that facilitated the rapid amplification of proteopathic AD-tau seeds into large tau aggregates, initially appearing as NP tau, which was followed by the formation and spread of NFTs and NTs, likely through secondary seeding events. Our study provides insights into a new multistep mechanism underlying Aß plaque-associated tau pathogenesis.

Pathological Tau Strains from Human Brains Recapitulate the Diversity of Tauopathies in Nontransgenic Mouse Brain.

Pathological tau aggregates occur in Alzheimer's disease (AD) and other neurodegenerative tauopathies. It is not clearly understood why tauopathies vary greatly in the neuroanatomical and histopathological patterns of tau aggregation, which contribute to clinical heterogeneity in these disorders. Recent studies have shown that tau aggregates may form distinct structural conformations, known as tau strains. Here, we developed a novel model to test the hypothesis that cell-to-cell transmission of different tau strains occurs in nontransgenic (non-Tg) mice, and to investigate whether there are strain-specific differences in the pattern of tau transmission. By injecting pathological tau extracted from postmortem brains of AD (AD-tau), progressive supranuclear palsy (PSP-tau), and corticobasal degeneration (CBD-tau) patients into different brain regions of female non-Tg mice, we demonstrated the induction and propagation of endogenous mouse tau aggregates. Specifically, we identified differences in tau strain potency between AD-tau, CBD-tau, and PSP-tau in non-Tg mice. Moreover, differences in cell-type specificity of tau aggregate transmission were observed between tau strains such that only PSP-tau and CBD-tau strains induce astroglial and oligodendroglial tau inclusions, recapitulating the diversity of neuropathology in human tauopathies. Furthermore, we demonstrated that the neuronal connectome, but not the tau strain, determines which brain regions develop tau pathology. Finally, CBD-tau- and PSP-tau-injected mice showed spatiotemporal transmission of glial tau pathology, suggesting glial tau transmission contributes to the progression of tauopathies. Together, our data suggest that different tau strains determine seeding potency and cell-type specificity of tau aggregation that underlie the diversity of human tauopathies.SIGNIFICANCE STATEMENT Tauopathies show great clinical and neuropathological heterogeneity, despite the fact that tau aggregates in each disease. This heterogeneity could be due to tau aggregates forming distinct structural conformations, or strains. We now report the development of a sporadic tauopathy model to study human tau strains by intracerebrally injecting nontransgenic mice with pathological tau enriched from human tauopathy brains. We show human tau strains seed different types and cellular distributions of tau neuropathology in our model that recapitulate the heterogeneity seen in these human diseases.

The use of mouse models to study cell-to-cell transmission of pathological tau.

Tau protein aggregates are found in a variety of neurodegenerative diseases known as tauopathies. Emerging evidence shows tau can propagate from cell-to-cell by seeding endogenous tau to aggregate. Studies in tau transgenic mice showed intracerebrally injecting misfolded tau seeds initiates and transmits tau pathology across the mouse brain. However, transgenic mice that overexpress human tau with disease-associated mutations do not fully recapitulate sporadic tauopathies. Here, we present our method for developing a sporadic tauopathy model using pathological tau extracted from human Alzheimer's disease (AD) brains. We describe a novel method for sequentially extracting tau pathologies from human AD brain in high yield and purity. We then describe how to intracerebrally inject this extract into a nontransgenic mouse brain and analyze the transmission of tau pathology. This novel sporadic tauopathy model can be used to study the transmission of tau aggregates and test new tau-directed therapies.

Unique pathological tau conformers from Alzheimer's brains transmit tau pathology in nontransgenic mice.

Filamentous tau aggregates are hallmark lesions in numerous neurodegenerative diseases, including Alzheimer's disease (AD). Cell culture and animal studies showed that tau fibrils can undergo cell-to-cell transmission and seed aggregation of soluble tau, but this phenomenon was only robustly demonstrated in models overexpressing tau. In this study, we found that intracerebral inoculation of tau fibrils purified from AD brains (AD-tau), but not synthetic tau fibrils, resulted in the formation of abundant tau inclusions in anatomically connected brain regions in nontransgenic mice. Recombinant human tau seeded by AD-tau revealed unique conformational features that are distinct from synthetic tau fibrils, which could underlie the differential potency in seeding physiological levels of tau to aggregate. Therefore, our study establishes a mouse model of sporadic tauopathies and points to important differences between tau fibrils that are generated artificially and authentic ones that develop in AD brains.

Peripubertal Stress With Social Support Promotes Resilience in the Face of Aging.

The peripubertal period of development is a sensitive window, during which adverse experiences can increase the risk for presentation of cognitive and affective dysfunction throughout the lifespan, especially in women. However, such experiences in the context of a supportive social environment can actually ameliorate this risk, suggesting that resilience can be programmed in early life. Affective disorders and cognitive deficits commonly emerge during aging, with many women reporting increased difficulty with prefrontal cortex (PFC)-dependent executive functions. We have developed a mouse model to examine the interaction between peripubertal experience and age-related changes in cognition and stress regulation. Female mice were exposed to peripubertal chronic stress, during which they were either individually housed or housed with social interaction. One year after this stress experience, mice were examined in tasks to access their cognitive ability and flexibility in stress reactive measures. In a test of spatial memory acquisition and reversal learning where aged females normally display a decreased performance, the females that had experienced stress with social interaction a year earlier showed improved performance in reversal learning, a measure of cognitive flexibility. Because peripuberty is a time of major PFC maturation, we performed transcriptomic and biochemical analysis of the aged PFC, in which long-term changes in microRNA expression and in myelin proteins were found. These data suggest that stress in the context of social support experienced over the pubertal window can promote epigenetic reprogramming in the brain to increase the resilience to age-related cognitive decline in females.

Lack of influence of DAT1 and DRD2 gene variants on antidepressant response in generalized anxiety disorder.

OBJECTIVE: Although antidepressant drugs are used as first-line intervention to treat patients with generalized anxiety disorder (GAD), only one-third of patients respond positively to treatment. In our study, we investigated whether functional genetic polymorphisms in the dopamine active transporter 1 (DAT1) and dopamine receptor D2 (DRD2) may play a role in antidepressant treatment response in GAD. METHODS: We examined 156 patients diagnosed with GAD who received venlafaxine Extended-Release (XR) treatment as part of an 18-month relapse-prevention study to determine whether variation in these genes had an effect on treatment response after 6 months of open-label treatment. Genotypes were obtained for rs1076560 (DRD2), rs1800497 (DRD2), rs2550948 (DAT1), and a variable number tandem repeat in the 3' untranslated region of the DAT1 gene using standard methods. RESULTS: Results show that none of the tested variants were associated with treatment response to venlafaxine XR in GAD. Genotype and allele frequencies did not differ statistically significantly between responders and non-responders using either the Hamilton Anxiety or Clinical Global Impressions of Improvement Scale at 6 months. CONCLUSIONS: Although we detected no association in our sample, future studies using larger samples and more comprehensive gene coverage are needed to evaluate potential effects of dopaminergic variants on antidepressant treatment response in anxiety disorders.

Genetic polymorphisms in the PACAP and PAC1 receptor genes and treatment response to venlafaxine XR in generalized anxiety disorder.

The pituitary adenylate cyclase-activating peptide (PACAP) and its receptor (PAC1) are involved in stress response and anxiety. Genotypes for PACAP/PAC1 were examined for effects on treatment response to venlafaxine XR in generalized anxiety disorder. The Asp54Gly (rs2856966) variant in the PACAP gene was associated with better treatment outcome.

Neuronal calcium sensor-1 and cocaine addiction: a genetic association study in African-Americans and European Americans.

Genes involved in drug reward pathways are plausible candidates for susceptibility to substance use disorders. Given the prominent role of dopamine in drug reward, dopamine receptor-interacting proteins (DRIPs) such as the neuronal calcium sensor-1 (NCS-1) protein have been hypothesized to play a role in the pathophysiology of cocaine addiction (CA). In this study, we investigated whether genetic variants in the NCS-1 gene confer risk to CA. We genotyped 8 SNPs (rs4837479, rs7849345, rs3824544, rs10819611, rs947513, rs2277200, rs7873936 and rs1342043) in our discovery sample (cases n = 796, controls n = 416) of African descent. Confirmation of associated or trending SNPs (rs7849345, rs10819611, rs1342043) was attempted using a replication sample of African American (AA) ethnicity (cases n = 335, controls n = 336) and European-American (EA) ancestry (cases n = 336, controls n = 656). Secondary sex specific analysis was also carried out for each SNP in both AA and EA individuals. Genotyping of the discovery cohort showed significant genotypic (p = 0.0005, corrected q-value) as well as allelic (p = 0.005, corrected q-value) associations of rs1342043 with CA in AAs; however, this marker could not be confirmed in either the AA or EA replication sample. Combined analysis of all AA samples (n = 1883) for rs1342043 showed a significant association with CA (genotypic p = 0.0001, allelic p = 0.002) with a gender specific effect for males (allelic p = 0.005, genotypic p = 0.0003). Our data suggest that genetic variants in the NCS-1 gene contribute to susceptibility of CA in individuals of African descent.

Variation in the catechol-O-methyltransferase (COMT) gene and treatment response to venlafaxine XR in generalized anxiety disorder.

Antidepressant drugs are the preferred choice for the treatment of generalized anxiety disorder (GAD). However, the choice of pharmacotherapy is determined on a trial-and-error basis, as the underlying mechanisms of treatment response are unknown. We examined whether the COMT gene, which has been known to play a role in antidepressant treatment response in major depressive disorder (MDD), has a pharmacogenetic effect in antidepressant treatment response in GAD. In our study, 156 patients diagnosed with GAD received venlafaxine XR treatment as part of an 18-month relapse prevention study. Genotypes were obtained for the COMT functional variant rs4680 (Val158Met) for all patients; however, pharmacogenetic analysis was only conducted for the European American population (n=112). We found no significant association between our primary Hamilton Anxiety Scale outcome measure and rs4680. However, we did find a nominally significant allelic association between this variant and a secondary treatment outcome measure (CGI-I) in our European American population (n=112). Furthermore, we show a slight dominant effect of the A-allele with the CGI-I measure in the European American population indicating a possible pharmacogenetic role of rs4680 in antidepressant treatment outcome in GAD. Further studies in a larger population are needed to confirm this effect.

Pharmacogenetics of antidepressant drugs: current clinical practice and future directions.

While antidepressants are widely used to treat mood and anxiety disorders, only half of the patients will respond to antidepressant treatment and only one-third of patients experience a full remission of symptoms. The identification of genetic biomarkers that predict antidepressant-treatment response can improve current clinical practice. This is an emerging field known as pharmacogenetics, which comprises of genetic studies on both the pharmacokinetics and pharmacodynamics of treatment response. Recent studies on antidepressant-treatment response have focused on both aspects of pharmacogenetics research, identifying new candidate genes that may predict better treatment response for patients. This paper reviews recent findings on the pharmacogenetics of antidepressant drugs and future clinical applications. Ultimately, these studies should lead to the use of genetic testing to guide the use of antidepressants in clinical practice.

Association analysis between the Val66Met polymorphism in the brain-derived neurotrophic factor (BDNF) gene and treatment response to venlafaxine XR in generalized anxiety disorder.

While antidepressant drugs are used to treat generalized anxiety disorder (GAD), patients vary greatly in their treatment response. Evidence shows genetic factors may play a role in treatment response in GAD. We examined whether the BDNF gene, which has been shown to play a role in antidepressant treatment response in major depressive disorder (MDD), also has an effect in GAD. In our study, 155 patients diagnosed with GAD received venlafaxine XR treatment as part of an 18-month relapse prevention study. Genotypes were obtained for the BDNF functional variant rs6265 (Val66Met) for the entire sample (n=155); however, only the European American (EA) population was considered (n=111) for pharmacogenetic analysis. We did not find a significant association between rs6265 and antidepressant treatment response in our GAD population. Future studies in larger populations will need to be conducted to further elucidate the pharmacogenetic role of this variant in anxiety disorders.

TLR3-mediated NF-{kappa}B signaling in human esophageal epithelial cells.

Despite its position at the front line against ingested pathogens, very little is presently known about the role of the esophageal epithelium in host innate immune defense. As a key player in the innate immune response, Toll-like receptor (TLR) signaling has not been well characterized in human esophageal epithelial cells. In the present study, we investigated the inflammatory response and signaling pathways activated by TLR stimulation of human esophageal cells in vitro. Using quantitative RT-PCR, we profiled the expression pattern of human TLRs 1-10 in primary esophageal keratinocytes (EPC2), immortalized nontransformed esophageal keratinocytes (EPC2-hTERT), and normal human esophageal mucosal biopsies and found that TLRs 1, 2, 3, and 5 were expressed both in vivo and in vitro. Using the cytokine IL-8 as a physiological read out of the inflammatory response, we found that TLR3 is the most functional of the expressed TLRs in both primary and immortalized esophageal epithelial cell lines in response to its synthetic ligand polyinosinic polycytidylic acid [poly(I:C)]. Through reporter gene studies, we show that poly(I:C)-induced NF-kappaB activation is critical for the transactivation of the IL-8 promoter in vitro and that nuclear translocation of NF-kappaB occurs at an early time point following poly(I:C) stimulation of esophageal epithelial cells. Importantly, we also show that poly(I:C) stimulation induces the NF-kappaB-dependent esophageal epithelial expression of TLR2, leading to enhanced epithelial responsiveness of EPC2-hTERT cells to TLR2 ligand stimulation, suggesting an important regulatory role for TLR3-mediated NF-kappaB signaling in the innate immune response of esophageal epithelial cells. Our findings demonstrate for the first time that TLR3 is highly functional in the human esophageal epithelium and that TLR3-mediated NF-kappaB signaling may play an important regulatory role in esophageal epithelial homeostasis.