Relationships of Cognitive Measures with Cerebrospinal Fluid but Not Imaging Biomarkers of Alzheimer Disease Vary between Black and White Individuals

OBJECTIVE

Biomarkers of Alzheimer disease vary between groups of self-identified Black and White individuals in some studies. This study examined whether the relationships between biomarkers or between biomarkers and cognitive measures varied by racialized groups.

METHODS

Cerebrospinal fluid (CSF), amyloid positron emission tomography (PET), and magnetic resonance imaging measures were harmonized across four studies of memory and aging. Spearman correlations between biomarkers and between biomarkers and cognitive measures were calculated within each racialized group, then compared between groups by standard normal tests after Fisher's Z-transformations.

RESULTS

The harmonized dataset included at least one biomarker measurement from 495 Black and 2,600 White participants. The mean age was similar between racialized groups. However, Black participants were less likely to have cognitive impairment (28% vs 36%) and had less abnormality of some CSF biomarkers including CSF Aβ42/40, total tau, p-tau181, and neurofilament light. CSF Aβ42/40 was negatively correlated with total tau and p-tau181 in both groups, but at a smaller magnitude in Black individuals. CSF Aβ42/40, total tau, and p-tau181 had weaker correlations with cognitive measures, especially episodic memory, in Black than White participants. Correlations of amyloid measures between CSF (Aβ42/40, Aβ42) and PET imaging were also weaker in Black than White participants. Importantly, no differences based on race were found in correlations between different imaging biomarkers, or in correlations between imaging biomarkers and cognitive measures.

INTERPRETATION

Relationships between CSF biomarkers but not imaging biomarkers varied by racialized groups. Imaging biomarkers performed more consistently across racialized groups in associations with cognitive measures. ANN NEUROL 2024;95:495-506.

A flexible modeling approach for biomarker-based computation of absolute risk of Alzheimer's disease dementia

INTRODUCTION

As Alzheimer's disease (AD) biomarkers rapidly develop, tools are needed that accurately and effectively communicate risk of AD dementia.

METHODS

We analyzed longitudinal data from >10,000 cognitively unimpaired older adults. Five-year risk of AD dementia was modeled using survival analysis.

RESULTS

A demographic model was developed and validated on independent data with area under the receiver operating characteristic curve (AUC) for 5-year prediction of AD dementia of 0.79. Clinical and cognitive variables (AUC = 0.79), and apolipoprotein E genotype (AUC = 0.76) were added to the demographic model. We then incorporated the risk computed from the demographic model with hazard ratios computed from independent data for amyloid positron emission tomography status and magnetic resonance imaging hippocampal volume (AUC = 0.84), and for plasma amyloid beta (Aβ)42/Aβ40 (AUC = 0.82).

DISCUSSION

An adaptive tool was developed and validated to compute absolute risks of AD dementia. This approach allows for improved accuracy and communication of AD risk among cognitively unimpaired older adults.

Racial differences in longitudinal Alzheimer's disease biomarkers among cognitively normal adults

INTRODUCTION

Longitudinal changes in Alzheimer's disease (AD) biomarkers, including cerebrospinal fluid (CSF) analytes, amyloid uptakes from positron emission tomography (PET), structural outcomes from magnetic resonance imaging (MRI), and cognition, have not been compared between Blacks and Whites.

METHODS

A total of 179 Blacks and 1180 Whites who were cognitively normal at baseline and had longitudinal data from at least one biomarker modality were analyzed for the annual rates of change.

RESULTS

CSF amyloid beta (Aβ)42/Aβ40 declined more slowly (P = .0390), and amyloid (PET) accumulated more slowly (P = .0157), in Blacks than Whites. CSF Aβ42 changed in opposite directions over time between Blacks and Whites (P = .0039). The annual increase in CSF total tau and phosphorylated tau181 for Blacks was about half of that for Whites.

DISCUSSION

Longitudinal racial differences in amyloid biomarkers are observed. It will be important to comprehensively and prospectively examine the effects of apolipoprotein E genotype and sociocultural factors on these differences.

Different rates of cognitive decline in autosomal dominant and late-onset Alzheimer disease

As prevention trials advance with autosomal dominant Alzheimer disease (ADAD) participants, understanding the similarities and differences between ADAD and "sporadic" late-onset AD (LOAD) is critical to determine generalizability of findings between these cohorts. Cognitive trajectories of ADAD mutation carriers (MCs) and autopsy-confirmed LOAD individuals were compared to address this question. Longitudinal rates of change on cognitive measures were compared in ADAD MCs (n = 310) and autopsy-confirmed LOAD participants (n = 163) before and after symptom onset (estimated/observed). LOAD participants declined more rapidly in the presymptomatic (preclinical) period and performed more poorly at symptom onset than ADAD participants on a cognitive composite. After symptom onset, however, the younger ADAD MCs declined more rapidly. The similar but not identical cognitive trajectories (declining but at different rates) for ADAD and LOAD suggest common AD pathologies but with some differences.

Development and implementation of an electronic Clinical Dementia Rating and Financial Capacity Instrument-Short Form

INTRODUCTION

To address the need for remote assessments of cognitive decline and dementia, we developed and administered electronic versions of the Clinical Dementia Rating (CDR®) and the Financial Capacity Instrument-Short Form (FCI-SF) (F-CAP^®), called the eCDR and eFCI, respectively.

METHODS

The CDR and FCI-SF were adapted for remote, unsupervised, online use based on item response analysis of the standard instruments. Participants completed the eCDR and eFCI first in clinic, and then at home within 2 weeks.

RESULTS

Of the 243 enrolled participants, 179 (73%) cognitively unimpaired (CU), 50 (21%) with mild cognitive impairment (MCI) or dementia, and 14 (6%) with an unknown diagnosis, 84% and 85% of them successfully completed the eCDR and eFCI, respectively, at home.

DISCUSSION

These results show initial feasibility in developing and administering online instruments to remotely assess and monitor cognitive decline along the CU to MCI/very mild dementia continuum. Validation is an important next step.

Effect of Race on Prediction of Brain Amyloidosis by Plasma Aβ42/Aβ40, Phosphorylated Tau, and Neurofilament Light

BACKGROUND AND OBJECTIVES

To evaluate whether plasma biomarkers of amyloid (Aβ42/Aβ40), tau (p-tau181 and p-tau231), and neuroaxonal injury (neurofilament light chain [NfL]) detect brain amyloidosis consistently across racial groups.

METHODS

Individuals enrolled in studies of memory and aging who self-identified as African American (AA) were matched 1:1 to self-identified non-Hispanic White (NHW) individuals by age, APOE ε4 carrier status, and cognitive status. Each participant underwent blood and CSF collection, and amyloid PET was performed in 103 participants (68%). Plasma Aβ42/Aβ40 was measured by a high-performance immunoprecipitation-mass spectrometry assay. Plasma p-tau181, p-tau231, and NfL were measured by Simoa immunoassays. CSF Aβ42/Aβ40 and amyloid PET status were used as primary and secondary reference standards of brain amyloidosis, respectively.

RESULTS

There were 76 matched pairs of AA and NHW participants (n = 152 total). For both AA and NHW groups, the median age was 68.4 years, 42% were APOE ε4 carriers, and 91% were cognitively normal. AA were less likely than NHW participants to have brain amyloidosis by CSF Aβ42/Aβ40 (22% vs 43% positive; p = 0.003). The receiver operating characteristic area under the curve of CSF Aβ42/Aβ40 status with the plasma biomarkers was as follows: Aβ42/Aβ40, 0.86 (95% CI 0.79-0.92); p-tau181, 0.76 (0.68-0.84); p-tau231, 0.69 (0.60-0.78); and NfL, 0.64 (0.55-0.73). In models predicting CSF Aβ42/Aβ40 status with plasma Aβ42/Aβ40 that included covariates (age, sex, APOE ε4 carrier status, race, and cognitive status), race did not affect the probability of CSF Aβ42/Aβ40 positivity. In similar models based on plasma p-tau181, p-tau231, or NfL, AA participants had a lower probability of CSF Aβ42/Aβ40 positivity (odds ratio 0.31 [95% CI 0.13-0.73], 0.30 [0.13-0.71], and 0.27 [0.12-0.64], respectively). Models of amyloid PET status yielded similar findings.

DISCUSSION

Models predicting brain amyloidosis using a high-performance plasma Aβ42/Aβ40 assay may provide an accurate and consistent measure of brain amyloidosis across AA and NHW groups, but models based on plasma p-tau181, p-tau231, and NfL may perform inconsistently and could result in disproportionate misdiagnosis of AA individuals.

Development of a Comprehensive Battery to Collect Social and Structural Determinants of Health (SSDOH) in Cognitively Normal or Very Mildly Impaired Persons

INTRODUCTION

Research addressing Alzheimer disease and related dementias must examine nonbiological factors influencing the risk for and expression of Alzheimer disease and related dementias. These factors address the interplay of cognition with lived experiences and social and structural determinants of health (SSDOH). However, coordinated measures of SSDOH are limited.

METHODS

The Knight Alzheimer Disease Research Center (ADRC) at Washington University in St. Louis developed and piloted a comprehensive battery to measure SSDOH. One hundred and twelve participants, very mildly cognitively impaired or unimpaired, enrolled in memory studies completed the electronic SSDOH battery. The Clinical Dementia Rating (CDR) determined the presence or absence of cognitive impairment.

RESULTS

Four domains demonstrated above acceptable intraclass correlation scores for test-retest reliability (≥0.70), including adverse childhood events, discrimination, social status, and early education. Twenty very mildly impaired participants completed the electronic pilot study.

CONCLUSION

Our findings indicate that participants with early-stage symptomatic Alzheimer disease are able to participate in electronic SSDOH data collection. In collaboration with the University of Pennsylvania ADRC, we replaced/modified certain assessments to increase intraclass correlation. The resulting battery, Social and Structural Life-courses Influencing Aging and Dementia (SS-DIAD), can serve as a SSDOH collection tool and is currently utilized in cognitively impaired and unimpaired research participants at both ADRCs.

African Americans Have Differences in CSF Soluble TREM2 and Associated Genetic Variants

Objective

To evaluate for racial differences in triggering receptor expressed on myeloid cells 2 (TREM2), a key immune mediator in Alzheimer disease, the levels of CSF soluble TREM2 (sTREM2), and the frequency of associated genetic variants were compared in groups of individuals who self-reported their race as African American (AA) or non-Hispanic White (NHW).

Methods

Community-dwelling older research participants underwent measurement of CSF sTREM2 concentrations and genetic analyses.

Results

The primary cohort included 91 AAs and 868 NHWs. CSF sTREM2 levels were lower in the AA compared with the NHW group (1,336 ± 470 vs 1,856 ± 624 pg/mL, p < 0.0001). AAs were more likely to carry TREM2 coding variants (15% vs 3%, p < 0.0001), which were associated with lower CSF sTREM2. AAs were less likely to carry the rs1582763 minor allele (8% vs 37%, p < 0.0001), located near MS4A4A, which was associated with higher CSF sTREM2. These findings were replicated in an independent cohort of 23 AAs and 917 NHWs: CSF sTREM2 levels were lower in the AA group (p = 0.03), AAs were more likely to carry coding TREM2 variants (22% vs 4%, p = 0.002), and AAs were less likely to carry the rs1582763 minor allele (16% vs 37%, p = 0.003).

Conclusions

On average, AAs had lower CSF sTREM2 levels compared with NHWs, potentially because AAs are more likely to carry genetic variants associated with lower CSF sTREM2 levels. Importantly, CSF sTREM2 reflects TREM2-mediated microglial activity, a critical step in the immune response to amyloid plaques. These findings suggest that race may be associated with risk for genetic variants that influence Alzheimer disease–related inflammation.

Lack of association between acute stroke, post-stroke dementia, race, and β-amyloid status

INTRODUCTION

Stroke and Alzheimer disease share risk factors and often co-occur, and both have been reported to have a higher prevalence in African Americans as compared to non-Hispanic whites. However, their interaction has not been established. The objective of this study was to determine if preclinical Alzheimer disease is a risk factor for stroke and post-stroke dementia and whether racial differences moderate this relationship.

METHODS

This case-control study was analyzed in 2019 using retrospective data from 2007 to 2013. Participants were adults age 65 and older with and without acute ischemic stroke. Recruitment included word of mouth and referrals in Saint Louis, MO, with stroke participants recruited from acutely hospitalized patients and non-stroke participants from community living older adults who were research volunteers. Our assessment included radiologic reads of infarcts, microbleeds, and white matter hyperintensitites (WMH); a Pittsburgh Compound B PET measure of cortical β-amyloid binding; quantitative measures of hippocampal and WMH volume; longitudinal Mini Mental State Examination (MMSE) scores; and Clinical Dementia Rating (CDR) 1 year post-stroke.

RESULTS

A total of 243 participants were enrolled, 81 of which had a recent ischemic stroke. Participants had a mean age of 75, 57% were women, and 52% were African American. Cortical amyloid did not differ significantly by race, stroke status, or CDR post-stroke. There were racial differences in MMSE scores at baseline (mean 26.8 for African Americans, 27.9 for non-Hispanic whites, p = 0.03), but not longitudinally. African Americans were more likely to have microbleeds (32.8% vs 22.6%, p = 0.04), and within the acute stroke group, African Americans were more likely to have small infarcts (75.6% vs 56.8%, p = 0.049).

CONCLUSION

Preclinical Alzheimer disease did not show evidence of being a risk factor for stroke nor predictive of post-stroke dementia. We did not observe racial differences in β-amyloid levels. However, even after controlling for several vascular risk factors, African Americans with clinical stroke presentations had greater levels of vascular pathology on MRI.

Perspective on the "African American participation in Alzheimer disease research: Effective strategies" workshop, 2018

The Washington University School of Medicine Knight Alzheimer Disease Research Center's "African American Participation in Alzheimer Disease Research: Effective Strategies" Workshop convened to address a major limitation of the ongoing scientific progress regarding Alzheimer's disease and related dementias (ADRD): participants in most ADRD research programs overwhelmingly have been limited to non-Hispanic white persons, thus precluding knowledge as to how ADRD may be represented in non-white individuals. Factors that may contribute to successful recruitment and retention of African Americans into ADRD research were discussed and organized into actionable next steps as described within this report.

Item response theory analysis of the Clinical Dementia Rating

INTRODUCTION

The Clinical Dementia Rating (CDR) is widely used in Alzheimer's disease research studies and has well established reliability and validity. To facilitate the development of an online, electronic CDR (eCDR) for more efficient clinical applications, this study aims to produce a shortened version of the CDR, and to develop the statistical model for automatic scoring.

METHODS

Item response theory (IRT) was used for item evaluation and model development. An automatic scoring algorithm was validated using existing CDR global and domain box scores as the reference standard.

RESULTS

Most CDR items discriminate well at mild and very mild levels of cognitive impairment. The bi-factor IRT model fits best and the shortened CDR still demonstrates very high classification accuracy (81%∼92%).

DISCUSSION

The shortened version of the CDR and the automatic scoring algorithm has established a good foundation for developing an eCDR and will ultimately improve the efficiency of cognitive assessment.

Assessment of Racial Disparities in Biomarkers for Alzheimer Disease

Importance

Racial differences in molecular biomarkers for Alzheimer disease may suggest race-dependent biological mechanisms.

Objective

To ascertain whether there are racial disparities in molecular biomarkers for Alzheimer disease.

Design, Setting, and Participants

A total of 1255 participants (173 African Americans) were enrolled from January 1, 2004, through December 31, 2015, in longitudinal studies at the Knight Alzheimer Disease Research Center at Washington University and completed a magnetic resonance imaging study of the brain and/or positron emission tomography of the brain with Pittsburgh compound B (radioligand for aggregated amyloid-β) and/or cerebrospinal fluid (CSF) assays for the concentrations of amyloid-β42, total tau, and phosphorylated tau181. Independent cross-sectional analyses were conducted from April 22, 2016, to August 27, 2018, for each biomarker modality with an analysis of variance or analysis of covariance including age, sex, educational level, race, apolipoprotein E (APOE) ε4 allele status, and clinical status (normal cognition or dementia). All biomarker assessments were conducted without knowledge of the clinical status of the participants.

Main Outcomes and Measures

The primary outcomes were hippocampal volumes adjusted for differences in intracranial volumes, global cerebral amyloid burden as transformed into standardized uptake value ratios (partial volume corrected), and CSF concentrations of amyloid-β42, total tau, and phosphorylated tau181.

Results

Of the 1255 participants (707 women and 548 men; mean [SD] age, 70.8 [9.9] years), 116 of 173 African American participants (67.1%) and 724 of 1082 non-Hispanic white participants (66.9%) had normal cognition. There were no racial differences in the frequency of cerebral ischemic lesions noted on results of brain magnetic resonance imaging, mean cortical standardized uptake value ratios for Pittsburgh compound B, or for amyloid-β42 concentrations in CSF. However, in individuals with a reported family history of dementia, mean (SE) total hippocampal volumes were lower for African American participants than for white participants (6418.26 [138.97] vs 6990.50 [44.10] mm3). Mean (SE) CSF concentrations of total tau were lower in African American participants than in white participants (293.65 [34.61] vs 443.28 [18.20] pg/mL; P < .001), as were mean (SE) concentrations of phosphorylated tau181 (53.18 [4.91] vs 70.73 [2.46] pg/mL; P < .001). There was a significant race by APOE ε4 interaction for both CSF total tau and phosphorylated tau181 such that only APOE ε4-positive participants showed the racial differences.

Conclusions and Relevance

The results of this study suggest that analyses of molecular biomarkers of Alzheimer disease should adjust for race. The lower CSF concentrations of total tau and phosphorylated tau181 in African American individuals appear to reflect a significant race by APOE ε4 interaction, suggesting a differential effect of this Alzheimer risk variant in African American individuals compared with white individuals.

Distinct cytokine profiles in human brains resilient to Alzheimer's pathology

Our group has previously studied the brains of some unique individuals who are able to tolerate robust amounts of Alzheimer's pathological lesions (amyloid plaques and neurofibrillary tangles) without experiencing dementia while alive. These rare resilient cases do not demonstrate the patterns of neuronal/synaptic loss that are normally found in the brains of typical demented Alzheimer's patients. Moreover, they exhibit decreased astrocyte and microglial activation markers GFAP and CD68, suggesting that a suppressed neuroinflammatory response may be implicated in human brain resilience to Alzheimer's pathology. In the present work, we used a multiplexed immunoassay to profile a panel of 27 cytokines in the brains of controls, typical demented Alzheimer's cases, and two groups of resilient cases, which possessed pathology consistent with either high probability (HP, Braak stage V-VI and CERAD 2-3) or intermediate probability (IP, Braak state III-IV and CERAD 1-3) of Alzheimer's disease in the absence of dementia. We used a multivariate partial least squares regression approach to study differences in cytokine expression between resilient cases and both Alzheimer's and control cases. Our analysis identified distinct profiles of cytokines in the entorhinal cortex (one of the earliest and most severely affected brain regions in Alzheimer's disease) that are up-regulated in both HP and IP resilient cases relative to Alzheimer's and control cases. These cytokines, including IL-1β, IL-6, IL-13, and IL-4 in HP resilient cases and IL-6, IL-10, and IP-10 in IP resilient cases, delineate differential inflammatory activity in brains resilient to Alzheimer's pathology compared to Alzheimer's cases. Of note, these cytokines all have been associated with pathogen clearance and/or the resolution of inflammation. Moreover, our analysis in the superior temporal sulcus (a multimodal association cortex that consistently accumulates Alzheimer's pathology at later stages of the disease along with overt symptoms of dementia) revealed increased expression of neurotrophic factors, such as PDGF-bb and basic FGF in resilient compared to AD cases. The same region also had reduced expression of chemokines associated with microglial recruitment, including MCP-1 in HP resilient cases and MIP-1α in IP resilient cases compared to AD. Altogether, our data suggest that different patterns of cytokine expression exist in the brains of resilient and Alzheimer's cases, link these differences to reduced glial activation, increased neuronal survival and preserved cognition in resilient cases, and reveal specific cytokine targets that may prove relevant to the identification of novel mechanisms of brain resiliency to Alzheimer's pathology.

Synaptic Vesicles: Turning Reluctance Into Action

Vesicle availability partly determines the efficacy of synaptic communication in the CNS. The authors recently found that some hippocampal glutamate vesicles exhibit reluctance to exocytose during short, high-frequency action potential trains. These same vesicles can be “coaxed” into exocytosis by increased Ca2+entry, by direct depolarization of synaptic terminals, or by challenge with hypertonic sucrose, a tool used to cause fusion of the population of release-ready synaptic vesicles. Interestingly, the authors did not find evidence of reluctance at hippocampal GABA synapses, suggesting that vesicle reluctance might be a negative feedback mechanism to prevent runaway excitation. It is also possible that synapses exhibit reluctance to retain a dormant population of readily accessible vesicles, ready to respond to triggers such as enhanced Ca2+ influx or neuromodulators. Recent work from the calyx of Held synapse suggests that reluctance might arise from inactivation of Ca2+ channels. The authors review this work, along with several other potential mechanisms of reluctance.

Longitudinal relationships among biomarkers for Alzheimer disease in the Adult Children Study

OBJECTIVE

To determine whether and how longitudinal rates of change in MRI volumetrics, CSF concentrations of Alzheimer-related proteins, molecular imaging of cerebral fibrillar amyloid with PET using the [(11)C] benzothiazole tracer, Pittsburgh compound B (PiB), and cognition were associated among asymptomatic middle-aged to older individuals.

METHODS

Multivariate mixed models for repeated measures were used to assess the correlations on the rates of changes across markers.

RESULTS

Among 209 asymptomatic middle-aged to older individuals longitudinally followed for up to 11 years (mean 6.7 years), a faster intraindividual decrease in CSF Aβ42 was associated with a faster increase in PiB mean cortical standardized uptake value ratio (MCSUVR, p = 0.04), but not others. The rate of change in CSF tau (and Ptau181) was correlated with the rate of change in PiB MCSUVR (p = 0.002), hippocampal volume (p = 0.04), and global cognition (p = 0.008). The rate of change in hippocampal volume was correlated with the rate of change in global cognition (p = 0.04). Only 3 significant correlations were observed at baseline: CSF Aβ42 and PiB MCSUVR (p < 0.001), CSF tau and PiB MCSUVR (p < 0.001), and CSF Aβ42 and global cognition (p = 0.01).

CONCLUSIONS

CSF tau (Ptau181), PiB MCSUVR, and hippocampal volume were all longitudinally correlated with each other, whereas CSF Aβ42 was correlated only with PiB binding. Unlike the baseline values, the longitudinal change in CSF tau (Ptau181) and hippocampal volume were correlated with the longitudinal change in global cognition, validating the role of these biomarkers in Alzheimer disease prevention trials.

Dominantly Inherited Alzheimer Network: facilitating research and clinical trials

The Dominantly Inherited Alzheimer Network (DIAN) is an international registry of individuals at risk for developing autosomal dominant Alzheimer's disease (AD). Its primary aims are to investigate the temporal ordering of AD pathophysiological changes that occur in asymptomatic mutation carriers and to identify those markers that herald the transition from cognitive normality to symptomatic AD. DIAN participants undergo longitudinal evaluations, including clinical and cognitive assessments and measurements of molecular and imaging AD biomarkers. This review details the unique attributes of DIAN as a model AD biomarker study and how it provides the infrastructure for innovative research projects, including clinical trials. The recent design and launch of the first anti-amyloid-beta secondary prevention trial in AD, led by the related DIAN Trials Unit, also are discussed.

Amyloid imaging and CSF biomarkers in predicting cognitive impairment up to 7.5 years later

OBJECTIVES

We compared the ability of molecular biomarkers for Alzheimer disease (AD), including amyloid imaging and CSF biomarkers (Aβ42, tau, ptau181, tau/Aβ42, ptau181/Aβ42), to predict time to incident cognitive impairment among cognitively normal adults aged 45 to 88 years and followed for up to 7.5 years.

METHODS

Longitudinal data from Knight Alzheimer's Disease Research Center participants (N = 201) followed for a mean of 3.70 years (SD = 1.46 years) were used. Participants with amyloid imaging and CSF collection within 1 year of a clinical assessment indicating normal cognition were eligible. Cox proportional hazards models tested whether the individual biomarkers were related to time to incident cognitive impairment. "Expanded" models were developed using the biomarkers and participant demographic variables. The predictive values of the models were compared.

RESULTS

Abnormal levels of all biomarkers were associated with faster time to cognitive impairment, and some participants with abnormal biomarker levels remained cognitively normal for up to 6.6 years. No differences in predictive value were found between the individual biomarkers (p > 0.074), nor did we find differences between the expanded biomarker models (p > 0.312). Each expanded model better predicted incident cognitive impairment than the model containing the biomarker alone (p < 0.005).

CONCLUSIONS

Our results indicate that all AD biomarkers studied here predicted incident cognitive impairment, and support the hypothesis that biomarkers signal underlying AD pathology at least several years before the appearance of dementia symptoms.

Differential requirement for protein synthesis in presynaptic unmuting and muting in hippocampal glutamate terminals

Synaptic function and plasticity are crucial for information processing within the nervous system. In glutamatergic hippocampal neurons, presynaptic function is silenced, or muted, after strong or prolonged depolarization. This muting is neuroprotective, but the underlying mechanisms responsible for muting and its reversal, unmuting, remain to be clarified. Using cultured rat hippocampal neurons, we found that muting induction did not require protein synthesis; however, slow forms of unmuting that depend on protein kinase A (PKA), including reversal of depolarization-induced muting and forskolin-induced unmuting of basally mute synapses, required protein synthesis. In contrast, fast unmuting of basally mute synapses by phorbol esters was protein synthesis-independent. Further studies of recovery from depolarization-induced muting revealed that protein levels of Rim1 and Munc13-1, which mediate vesicle priming, correlated with the functional status of presynaptic terminals. Additionally, this form of unmuting was prevented by both transcription and translation inhibitors, so proteins are likely synthesized de novo after removal of depolarization. Phosphorylated cyclic adenosine monophosphate response element-binding protein (pCREB), a nuclear transcription factor, was elevated after recovery from depolarization-induced muting, consistent with a model in which PKA-dependent mechanisms, possibly including pCREB-activated transcription, mediate slow unmuting. In summary, we found that protein synthesis was required for slower, PKA-dependent unmuting of presynaptic terminals, but it was not required for muting or a fast form of unmuting. These results clarify some of the molecular mechanisms responsible for synaptic plasticity in hippocampal neurons and emphasize the multiple mechanisms by which presynaptic function is modulated.

Presynaptically silent synapses studied with light microscopy

Synaptic plasticity likely underlies the nervous system's ability to learn and remember and may also represent an adaptability that prevents otherwise damaging insults from becoming neurotoxic. We have been studying a form of presynaptic plasticity that is interesting in part because it is expressed as a digital switching on and off of a presynaptic terminal s ability to release vesicles containing the neurotransmitter glutamate. Here we demonstrate a protocol for visualizing the activity status of presynaptic terminals in dissociated cell cultures prepared from the rodent hippocampus. The method relies on detecting active synapses using staining with a fixable form of the styryl dye FM1-43, commonly used to label synaptic vesicles. This staining profile is compared with immunostaining of the same terminals with an antibody directed against the vesicular glutamate transporter 1 (vGluT-1), a stain designed to label all glutamate synapses regardless of activation status. We find that depolarizing stimuli induce presynaptic silencing. The population of synapses that is silent under baseline conditions can be activated by prolonged electrical silencing or by activation of cAMP signaling pathways.

Comparative effects of heterologous TRPV1 and TRPM8 expression in rat hippocampal neurons

Heterologous channel expression can be used to control activity in select neuronal populations, thus expanding the tools available to modern neuroscience. However, the secondary effects of exogenous channel expression are often left unexplored. We expressed two transient receptor potential (TRP) channel family members, TRPV1 and TRPM8, in cultured hippocampal neurons. We compared functional expression levels and secondary effects of channel expression and activation on neuronal survival and signaling. We found that activation of both channels with appropriate agonist caused large depolarizing currents in voltage-clamped hippocampal neurons, exceeding the amplitude responses to a calibrating 30 mM KCl stimulation. Both TRPV1 and TRPM8 currents were reduced but not eliminated by 4 hr incubation in saturating agonist concentration. In the case of TRPV1, but not TRPM8, prolonged agonist exposure caused strong calcium-dependent toxicity. In addition, TRPV1 expression depressed synaptic transmission dramatically without overt signs of toxicity, possibly due to low-level TRPV1 activation in the absence of exogenous agonist application. Despite evidence of expression at presynaptic sites, in addition to somatodendritic sites, TRPM8 expression alone exhibited no effects on synaptic transmission. Therefore, by a number of criteria, TRPM8 proved the superior choice for control over neuronal membrane potential. This study also highlights the need to explore potential secondary effects of long-term expression and activation of heterologously introduced channels.

Adenylyl cyclases 1 and 8 initiate a presynaptic homeostatic response to ethanol treatment

BACKGROUND

Although ethanol exerts widespread action in the brain, only recently has progress been made in understanding the specific events occurring at the synapse during ethanol exposure. Mice deficient in the calcium-stimulated adenylyl cyclases, AC1 and AC8 (DKO), demonstrate increased sedation duration and impaired phosphorylation by protein kinase A (PKA) following acute ethanol treatment. While not direct targets for ethanol, we hypothesize that these cyclases initiate a homeostatic presynaptic response by PKA to reactivate neurons from ethanol-mediated inhibition.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we have used phosphoproteomic techniques and identified several presynaptic proteins that are phosphorylated in the brains of wild type mice (WT) after ethanol exposure, including synapsin, a known PKA target. Phosphorylation of synapsins I and II, as well as phosphorylation of non-PKA targets, such as, eukaryotic elongation factor-2 (eEF-2) and dynamin is significantly impaired in the brains of DKO mice. This deficit is primarily driven by AC1, as AC1-deficient, but not AC8-deficient mice also demonstrate significant reductions in phosphorylation of synapsin and eEF-2 in cortical and hippocampal tissues. DKO mice have a reduced pool of functional recycling vesicles and fewer active terminals as measured by FM1-43 uptake compared to WT controls, which may be a contributing factor to the impaired presynaptic response to ethanol treatment.

CONCLUSIONS/SIGNIFICANCE

These data demonstrate that calcium-stimulated AC-dependent PKA activation in the presynaptic terminal, primarily driven by AC1, is a critical event in the reactivation of neurons following ethanol-induced activity blockade.

Cellular calcium deficiency plays a role in neuronal death caused by proteasome inhibitors

Cytosolic Ca(2+) concentration ([Ca(2+)](i)) is reduced in cultured neurons undergoing neuronal death caused by inhibitors of the ubiquitin proteasome system. Activation of calcium entry via voltage-gated Ca(2+) channels restores cytosolic Ca(2+) levels and reduces this neuronal death (Snider et al. 2002). We now show that this reduction in [Ca(2+)](i) is transient and occurs early in the cell death process, before activation of caspase 3. Agents that increase Ca(2+) influx such as activation of voltage-gated Ca(2+) channels or stimulation of Ca(2+) entry via the plasma membrane Na-Ca exchanger attenuate neuronal death only if applied early in the cell death process. Cultures treated with proteasome inhibitors had reduced current density for voltage-gated Ca(2+) channels and a less robust increase in [Ca(2+)](i) after depolarization. Levels of endoplasmic reticulum Ca(2+) were reduced and capacitative Ca(2+) entry was impaired early in the cell death process. Mitochondrial Ca(2+) was slightly increased. Preventing the transfer of Ca(2+) from mitochondria to cytosol increased neuronal vulnerability to this death while blockade of mitochondrial Ca(2+) uptake via the uniporter had no effect. Programmed cell death induced by proteasome inhibition may be caused in part by an early reduction in cytosolic and endoplasmic reticulum Ca(2+,) possibly mediated by dysfunction of voltage-gated Ca(2+) channels. These findings may have implications for the treatment of disorders associated with protein misfolding in which proteasome impairment and programmed cell death may occur.

Vesicle pool heterogeneity at hippocampal glutamate and GABA synapses

Glutamate and GABA are the major fast excitatory and inhibitory neurotransmitters, respectively, in the CNS. Although glutamate and GABA have clearly distinct postsynaptic actions, we are just beginning to appreciate that presynaptic differences between glutamatergic and GABAergic neurons may contribute to distinct functions of these transmitter systems. We therefore probed possible differences between the functional synaptic vesicle populations of glutamatergic and GABAergic neurons. We examined superecliptic synaptopHluorin (SpH) fluorescence during 20 Hz electrical stimulation in transfected hippocampal neurons and identified the phenotype of SpH-fluorescent synapses with post hoc immunostaining. With 200 stimuli (10 s), individual glutamate synapses displayed considerably more variability in peak SpH fluorescence than GABA synapses, without a strong difference in the mean SpH fluorescence increase. This spatial heterogeneity could not be accounted for by differences in endocytosis, which was nearly constant over these short time periods across glutamate and GABA synapses. Instead, variability in vesicle exocytosis correlated with variability in total vesicle staining and in measures of the total recycling pool size. Differences were also evident using FM1-43 [N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide] uptake. These data support the idea that the population of glutamate synapses exhibits more heterogeneity in release properties than the population of GABA synapses, possibly correlated with glutamatergic synaptic malleability.

Linkage between cellular communications, energy utilization, and proliferation in metastatic neuroendocrine cancers

To identify metabolic features that support the aggressive behavior of human neuroendocrine (NE) cancers, we examined metastatic prostate NE tumors and derived prostate NE cancer (PNEC) cell lines from a transgenic mouse model using a combination of magic angle spinning NMR spectroscopy, in silico predictions of biotransformations that observed metabolites may undergo, biochemical tests of these predictions, and electrophysiological/calcium imaging studies. Malignant NE cells undergo excitation and increased proliferation when their GABA(A), glutamate, and/or glycine receptors are stimulated, use glutamate and GABA as substrates for NADH biosynthesis, and produce propylene glycol, a precursor of pyruvate derived from glycine that increases levels of circulating free fatty acids through extra-NE cell effects. Treatment of nude mice containing PNEC tumor xenografts with (i) amiloride, a diuretic that inhibits Abp1, an enzyme involved in NE cell GABA metabolism, (ii) carbidopa, an inhibitor of dopa decarboxylase which functions upstream of Abp1, plus (iii) flumazenil, a benzodiazepine antagonist that binds to GABA(A) receptors, leads to significant reductions in tumor growth. These findings may be generally applicable: GeneChip data sets from 471 human neoplasms revealed that components of GABA metabolic pathways, including ABP1, exhibit statistically significant increases in their expression in NE and non-NE cancers.

Physiological activity depresses synaptic function through an effect on vesicle priming

Neurons engage compensatory, homeostatic synaptic changes to maintain their overall firing rate. We examined the induction and expression of a persistent presynaptic adaptation. We explored the effect of mild extracellular potassium elevation to increase hippocampal pyramidal neuron spiking over a physiological range. With several days of mild depolarization, glutamate release adapted, as revealed by an increased mismatch between the number of active, FM1-43-positive, glutamatergic synapses and the total number of synapses defined by vesicular glutamate transporter-1 antibody staining. Surprisingly, the adaptation of glutamate terminals was all-or-none; recycling vesicle pool size at remaining active synapses was not significantly altered by the adaptation. Tetrodotoxin (TTX), but not postsynaptic receptor blockade, reversed depolarization-induced adaptation, and TTX added to normal incubation medium increased the number of active synapses, suggesting that normal spiking activity sustains a steady-state percentage of inactive terminals. Chronic mild depolarization depressed EPSCs and decreased the size of the readily releasable pool of vesicles (RRP). Several hours of 10 Hz electrical stimulation also depressed the RRP size, confirming that spiking alone induces adaptation and that strong stimulation induces more rapid presynaptic adaptation. Despite the importance of RRP alteration to the adaptation, ultrastructural experiments revealed no changes in docked or total synaptic vesicle numbers. Furthermore, alpha-latrotoxin induced vesicle release at adapted synapses, consistent with the idea that adaptation resulted from changes in vesicle priming. These results show that glutamatergic neurotransmission persistently adapts to changes in electrical activity over a wide physiological range.

Homeostatic regulation of glutamate release in response to depolarization

Proper nervous system function requires a balance between excitation and inhibition. Systems of homeostasis may have evolved in neurons to help maintain or restore balance between excitation and inhibition, presumably because excessive excitation can cause dysfunction and cell death. This article reviews evidence for homeostatic mechanisms within the hippocampus that lead to differential regulation of glutamate and gamma-aminobutyric acid release in response to conditions of excess depolarization. We recently found differential effects on glutamate release at the level of action potential coupling to transmitter release, vesicular release probability, and vesicle availability. Such differential regulation may help to prevent excitotoxicity and runaway excitation.

Reluctant vesicles contribute to the total readily releasable pool in glutamatergic hippocampal neurons

The size of the readily releasable pool (RRP) of vesicles is critically important for determining the size of postsynaptic currents generated in response to action potentials. However, discrepancies in RRP estimates exist among methods designed to measure RRP size. In glutamatergic hippocampal neurons, we found that hypertonic sucrose application yielded RRP size estimates approximately fivefold larger than values obtained with high-frequency action potential trains commonly assumed to deplete the RRP. This discrepancy was specific for glutamatergic neurons, because no difference was found between sucrose and train estimates of RRP size in GABAergic neurons. A small component of the difference in excitatory neurons was accounted for by postsynaptic receptor saturation. Train estimates of vesicle pool size obtained using more stimuli revealed that action potential-elicited EPSCs did not truly reach a steady state during shorter trains, and RRP estimates were closer to sucrose estimates made in the same neurons. This suggested that reluctant vesicles may contribute to the total available pool. Two additional lines of evidence supported this hypothesis. First, RRP estimates from strongly depolarizing hyperkalemic solutions closely matched those obtained with sucrose. Second, when Ca2+ influx was enhanced during trains, train estimates of pool size matched those obtained with sucrose. These data suggest that glutamatergic hippocampal neurons maintain a heterogeneous population of vesicles that can be differentially released with varying Ca2+ influx, thereby increasing the range of potential synaptic responses.

Reluctant vesicles coaxed into the limelight

Synapses respond to brief, repetitive stimulation with synaptic depression when initial transmitter release probability is high. Vesicle depletion has been a long-standing hypothesis for depression, but results unexplained by the depletion hypothesis have been nagging. In this issue of Neuron, Xu and Wu show that, under some conditions, calcium current inactivation explains stimulus-dependent depression at the calyx of Held.

Astrocytes exert a pro-apoptotic effect on neurons in postnatal hippocampal cultures

We investigated conditions that promote basal and activity-dependent neuronal apoptosis in postnatal rat hippocampal cultures. Low-density mixed cultures of astrocytes and neurons exhibited lower sensitivity than high-density cultures to basal neuronal death and activity-sensitive neuronal death, induced with glutamate receptor blockers, sodium channel blockers, or calcium channel blockers. Although elevations of [Ca(2+)](i) protect neurons from apoptosis, low-density microcultures and mass cultures exhibited only minor differences in resting [Ca(2+)](i) and Ca(2+) current density, suggesting that these variables are unlikely to explain differences in susceptibility. Astrocytes, rather than neurons, were implicated in the neuronal loss. Several candidate molecules implicated in other astrocyte-dependent neurotoxicity models were excluded, but heat inactivation experiments suggested that a heat-labile factor is critically involved. In sum, our results suggest the surprising result that astrocytes can be negative modulators of neuronal survival during development and when the immature nervous system is challenged with drugs that dampen electrical excitability.

Plastic elimination of functional glutamate release sites by depolarization

To examine persisting effects of depolarizing rises in extracellular potassium concentration ([K+](o)) on synapses, we depolarized cells to simulate ischemia-like rises in [K+](o). Elevated [K+](o) for 1-16 hr severely depressed glutamate signaling, while mildly depressing GABA transmission. The glutamate-specific changes were plastic over several hours and involved a decrease in the size of the pool of releasable vesicles. Rather than a reduction of the number of vesicles per release site, the change involved functional elimination of release sites. This change was clearly dissociable from a second effect, depressed probability of transmitter release, which was common to both glutamate and GABA transmission. Thus, while other recent evidence links alteration of the releasable pool size with changes in p(r), our results suggest the two can be independently manipulated. Selective depression of glutamate release may provide an adaptive mechanism by which neurons limit excitotoxicity.

Homeostatic effects of depolarization on Ca2+ influx, synaptic signaling, and survival

Depolarization promotes neuronal survival through moderate increases in Ca(2+) influx, but the effects of survival-promoting depolarization (vs conventional trophic support) on neuronal signaling are poorly characterized. We found that chronic, survival-promoting depolarization, but not conventional trophic support, selectively decreased the somatic Ca(2+) current density in hippocampal and cerebellar granule neurons. Depolarization rearing depressed multiple classes of high-voltage activated Ca(2+) current. Consistent with the idea that these changes also affected synaptic Ca(2+) channels, chronic depolarization presynaptically depressed hippocampal neurotransmission. Six days of depolarization rearing completely abolished glutamate transmission but altered GABA transmission in a manner consistent with the alterations of Ca(2+) current. The continued survival of depolarization-reared neurons was extremely sensitive to the re-establishment of basal culture conditions and was correlated with the effects on intracellular Ca(2+) concentration. Thus, compared with cells reared on conventional trophic factors, depolarization evokes homeostatic changes in Ca(2+) influx and signaling that render neurons vulnerable to cell death on activity reduction.

Ethanol-induced death of postnatal hippocampal neurons

Fetal alcohol exposure causes severe neuropsychiatric problems, but mechanisms of the ethanol-associated changes in central nervous system development are unclear. In vivo, ethanol's interaction with N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid type A (GABA(A)) receptors may cause increased apoptosis in the immature forebrain. We examined whether ethanol affects survival of neonatal hippocampal neurons in primary cultures. A 6-day ethanol exposure killed hippocampal neurons with an LD50 of approximately 25 mM. Elevated extracellular potassium or insulin-related growth factor 1 inhibited cell loss. Although potentiation of GABA(A) receptors or complete block of NMDA receptors also kills hippocampal neurons, pharmacological studies suggest that ethanol's interaction with GABA(A) and NMDA receptors is not sufficient to explain ethanol's effects on neuronal survival. Ca(2+) influx in response to depolarization was depressed >50% by chronic ethanol treatment. We suggest that chronic ethanol may promote neuronal loss through a mechanism affecting Ca(2+) influx in addition to effects on postsynaptic GABA and glutamate receptors.

Intrinsic and extrinsic pathway signaling during neuronal apoptosis: lessons from the analysis of mutant mice

Trophic factor deprivation (TFD)-induced apoptosis in sympathetic neurons requires macromolecular synthesis-dependent BAX translocation, cytochrome c (cyt c) release, and caspase activation. Here, we report the contributions of other intrinsic and extrinsic pathway signals to these processes. Sympathetic neurons expressed all antiapoptotic BCL-2 proteins examined, yet expressed only certain BH3-only and multidomain proapoptotic BCL-2 family members. All coexpressed proapoptotic proteins did not, however, exhibit functional redundancy or compensatory expression, at least in the Bax-/-, Bak-/-, Bim-/-, Bid-/-, and Bad-/- neurons examined. Although the subcellular distribution or posttranslational modification of certain BCL-2 proteins changed with TFD, neither transcriptional nor posttranslational mechanisms regulated the expression or subcellular localization of BID, BAD, or BAK in this paradigm. Despite modest induction of Fas and FasL expression, Fas-mediated signaling did not contribute to TFD-induced apoptosis in sympathetic neurons. Similar findings were obtained with K+ withdrawal-induced apoptosis in cerebellar granule neurons, a model for activity-dependent neuronal survival in the CNS. Thus, expression alone does not guarantee functional redundancy (or compensation) among BCL-2 family members, and, at least in some cells, extrinsic pathway signaling and certain BH3-only proteins (i.e., BID and BAD) do not contribute to BAX-dependent cyt c release or apoptosis caused by TFD.