Resistance and Resilience to Alzheimer's Disease.

Dementia is a significant public health crisis; the most common underlying cause of age-related cognitive decline and dementia is Alzheimer's disease neuropathologic change (ADNC). As such, there is an urgent need to identify novel therapeutic targets for the treatment and prevention of the underlying pathologic processes that contribute to the development of AD dementia. Although age is the top risk factor for dementia in general and AD specifically, these are not inevitable consequences of advanced age. Some individuals are able to live to advanced age without accumulating significant pathology (resistance to ADNC), whereas others are able to maintain cognitive function despite the presence of significant pathology (resilience to ADNC). Understanding mechanisms of resistance and resilience will inform therapeutic strategies to promote these processes to prevent or delay AD dementia. This article will highlight what is currently known about resistance and resilience to AD, including our current understanding of possible underlying mechanisms that may lead to candidate preventive and treatment interventions for this devastating neurodegenerative disease.

Human microglia show unique transcriptional changes in Alzheimer's disease.

Microglia, the innate immune cells of the brain, influence Alzheimer's disease (AD) progression and are potential therapeutic targets. However, microglia exhibit diverse functions, the regulation of which is not fully understood, complicating therapeutics development. To better define the transcriptomic phenotypes and gene regulatory networks associated with AD, we enriched for microglia nuclei from 12 AD and 10 control human dorsolateral prefrontal cortices (7 males and 15 females, all aged >60 years) before single-nucleus RNA sequencing. Here we describe both established and previously unrecognized microglial molecular phenotypes, the inferred gene networks driving observed transcriptomic change, and apply trajectory analysis to reveal the putative relationships between microglial phenotypes. We identify microglial phenotypes more prevalent in AD cases compared with controls. Further, we describe the heterogeneity in microglia subclusters expressing homeostatic markers. Our study demonstrates that deep profiling of microglia in human AD brain can provide insight into microglial transcriptional changes associated with AD.

Microglia specific deletion of miR-155 in Alzheimer's disease mouse models reduces amyloid-ß pathology but causes hyperexcitability and seizures.

Alzheimer's Disease (AD) is characterized by the accumulation of extracellular amyloid-ß (Aß) as well as CNS and systemic inflammation. Microglia, the myeloid cells resident in the CNS, use microRNAs to rapidly respond to inflammatory signals. MicroRNAs (miRNAs) modulate inflammatory responses in microglia, and miRNA profiles are altered in Alzheimer's disease (AD) patients. Expression of the pro-inflammatory miRNA, miR-155, is increased in the AD brain. However, the role of miR-155 in AD pathogenesis is not well-understood. We hypothesized that miR-155 participates in AD pathophysiology by regulating microglia internalization and degradation of Aß. We used CX3CR1CreER/+ to drive-inducible, microglia-specific deletion of floxed miR-155 alleles in two AD mouse models. Microglia-specific inducible deletion of miR-155 in microglia increased anti-inflammatory gene expression while reducing insoluble Aß1-42 and plaque area. Yet, microglia-specific miR-155 deletion led to early-onset hyperexcitability, recurring spontaneous seizures, and seizure-related mortality. The mechanism behind hyperexcitability involved microglia-mediated synaptic pruning as miR-155 deletion altered microglia internalization of synaptic material. These data identify miR-155 as a novel modulator of microglia Aß internalization and synaptic pruning, influencing synaptic homeostasis in the setting of AD pathology.

A ferret brain slice model of oxygen-glucose deprivation captures regional responses to perinatal injury and treatment associated with specific microglial phenotypes.

Organotypic brain slice models are an ideal technological platform to investigate therapeutic options for hypoxic-ischemic (HI) brain injury, a leading cause of morbidity and mortality in neonates. The brain exhibits regional differences in the response to HI injury in vivo. This can be modeled using organotypic brain slices, which maintain three-dimensional regional structures and reflect the regional differences in injury response. Here, we developed an organotypic whole hemisphere (OWH) slice culture model of HI injury using the gyrencephalic ferret brain at a developmental stage equivalent to a full-term human infant in order to better probe region-specific cellular responses to injury. Each slice encompassed the cortex, corpus callosum, subcortical white matter, hippocampus, basal ganglia, and thalamus. Regional responses to treatment with either erythropoietin (Epo) or the ketone body acetoacetate (AcAc) were highly heterogenous. While both treatments suppressed global injury responses and oxidative stress, significant neuroprotection was only seen in a subset of regions, with others displaying no response or potential exacerbation of injury. Similar regional heterogeneity was seen in the morphology and response of microglia to injury and treatment, which mirrored those seen after injury in vivo. Within each region, machine-learning-based classification of microglia morphological shifts in response to injury predicted the neuroprotective response to each therapy, with different morphologies associated with different treatment responses. This suggests that the ferret OWH slice culture model provides a platform for examining regional responses to injury in the gyrencephalic brain, as well as for screening combinations of therapeutics to provide global neuroprotection after injury.

Glial TDP-43 and TDP-43 induced glial pathology, focus on neurodegenerative proteinopathy syndromes.

Since its discovery in 2006, TAR DNA binding protein 43 (TDP-43) has driven rapidly evolving research in neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD), and limbic predominant age-related TDP-43 encephalopathy (LATE). TDP-43 mislocalization or aggregation is the hallmark of TDP-43 proteinopathy and is associated with cognitive impairment that can be mapped to its regional deposition. Studies in human tissue and model systems demonstrate that TDP-43 may potentiate other proteinopathies such as the amyloid or tau pathology seen in Alzheimer's Disease (AD) in the combination of AD+LATE. Despite this growing body of literature, there remain gaps in our understanding of whether there is heterogeneity in TDP-43 driven mechanisms across cell types. The growing observations of correlation between TDP-43 proteinopathy and glial pathology suggest a relationship between the two, including pathogenic glial cell-autonomous dysfunction and dysregulated glial immune responses to neuronal TDP-43. In this review, we discuss the available data on TDP-43 in glia within the context of the neurodegenerative diseases ALS and FTLD and highlight the current lack of information about glial TDP-43 interaction in AD+LATE. TDP-43 has proven to be a significant modulator of cognitive and neuropathological outcomes. A deeper understanding of its role in diverse cell types may provide relevant insights into neurodegenerative syndromes.

A Subpopulation of Microglia Generated in the Adult Mouse Brain Originates from Prominin-1-Expressing Progenitors.

Microglia maintain brain health and play important roles in disease and injury. Despite the known ability of microglia to proliferate, the precise nature of the population or populations capable of generating new microglia in the adult brain remains controversial. We identified Prominin-1 (Prom1; also known as CD133) as a putative cell surface marker of committed brain myeloid progenitor cells. We demonstrate that Prom1-expressing cells isolated from mixed cortical cultures will generate new microglia in vitro To determine whether Prom1-expressing cells generate new microglia in vivo, we used tamoxifen inducible fate mapping in male and female mice. Induction of Cre recombinase activity at 10 weeks in Prom1-expressing cells leads to the expression of TdTomato in all Prom1-expressing progenitors and newly generated daughter cells. We observed a population of new TdTomato-expressing microglia at 6 months of age that increased in size at 9 months. When microglia proliferation was induced using a transient ischemia/reperfusion paradigm, little proliferation from the Prom1-expressing progenitors was observed with the majority of new microglia derived from Prom1-negative cells. Together, these findings reveal that Prom1-expressing myeloid progenitor cells contribute to the generation of new microglia both in vitro and in vivo Furthermore, these findings demonstrate the existence of an undifferentiated myeloid progenitor population in the adult mouse brain that expresses Prom1. We conclude that Prom1-expressing myeloid progenitors contribute to new microglia genesis in the uninjured brain but not in response to ischemia/reperfusion.SIGNIFICANCE STATEMENT Microglia, the innate immune cells of the CNS, can divide to slowly generate new microglia throughout life. Newly generated microglia may influence inflammatory responses to injury or neurodegeneration. However, the origins of the new microglia in the brain have been controversial. Our research demonstrates that some newly born microglia in a healthy brain are derived from cells that express the stem cell marker Prominin-1. This is the first time Prominin-1 cells are shown to generate microglia.

The pro-inflammatory microRNA miR-155 influences fibrillar ß-Amyloid1-42 catabolism by microglia.

Microglia are the innate immune cells of the central nervous system that adopt rapid functional changes in response to Damage Associated Molecular Patterns, including aggregated ß-Amyloid (Aß) found in Alzheimer's disease (AD). microRNAs (miRNAs) are post-transcriptional modulators that influence the timing and magnitude of microglia inflammatory responses by downregulating the expression of inflammatory effectors. Recent studies implicate miR-155, a miRNA known to regulate inflammatory responses, in the pathogenesis of neurodegenerative disorders including multiple sclerosis, ALS, familial Parkinson's disease, and AD. In this work, we asked if miR-155 expression in microglia modifies cellular behaviors in response to fibrillar Aß1-42 (fAß1-42 ), in vitro. We hypothesized that in microglia, miR-155 expression would impact the internalization and catabolism of extracellular fAß1-42 . Primary microglia stimulated with lipopolysaccharide demonstrate fast upregulation of miR-155 followed by delayed upregulation of miR-146a, an anti-inflammatory miRNA. Conditional overexpression of miR-155 in microglia resulted in significant upregulation of miR-146a. Conditional deletion of miR-155 promoted transit of fAß1-42 to low-pH compartments where catabolism occurs, while miR-155 overexpression decreases fAß1-42 catabolism. Uptake of fAß1-42 across the plasma membrane increased with both up and downregulation of miR-155 expression. Taken together, our results support the hypothesis that inflammatory signaling influences the ability of microglia to catabolize fAß1-42 through interconnected mechanisms modulated by miR-155. Understanding how miRNAs modulate the ability of microglia to catabolize fAß1-42 will further elucidate the role of cellular players and molecular crosstalk in AD pathophysiology.

Early-Onset Familial Alzheimer Disease Variant PSEN2 N141I Heterozygosity is Associated with Altered Microglia Phenotype.

BACKGROUND: Early-onset familial Alzheimer disease (EOFAD) is caused by heterozygous variants in the presenilin 1 (PSEN1), presenilin 2 (PSEN2), and APP genes. Decades after their discovery, the mechanisms by which these genes cause Alzheimer's disease (AD) or promote AD progression are not fully understood. While it is established that presenilin (PS) enzymatic activity produces amyloid-ß (Aß), PSs also regulate numerous other cellular functions, some of which intersect with known pathogenic drivers of neurodegeneration. Accumulating evidence suggests that microglia, resident innate immune cells in the central nervous system, play a key role in AD neurodegeneration. OBJECTIVE: Previous work has identified a regulatory role for PS2 in microglia. We hypothesized that PSEN2 variants lead to dysregulated microglia, which could further contribute to disease acceleration. To mimic the genotype of EOFAD patients, we created a transgenic mouse expressing PSEN2 N141I on a mouse background expressing one wildtype PS2 and two PS1 alleles. RESULTS: Microglial expression of PSEN2 N141I resulted in impaired γ-secretase activity as well as exaggerated inflammatory cytokine release, NFκB activity, and Aß internalization. In vivo, PS2 N141I mice showed enhanced IL-6 and TREM2 expression in brain as well as reduced branch number and length, an indication of "activated" morphology, in the absence of inflammatory stimuli. LPS intraperitoneal injection resulted in higher inflammatory gene expression in PS2 N141I mouse brain relative to controls. CONCLUSION: Our findings demonstrate that PSEN2 N141I heterozygosity is associated with disrupted innate immune homeostasis, suggesting EOFAD variants may promote disease progression through non-neuronal cells beyond canonical dysregulated Aß production.

Selectively Bred Rats Provide a Unique Model of Vulnerability to PTSD-Like Behavior and Respond Differentially to FGF2 Augmentation Early in Life.

Individuals respond differently to traumatic experiences, including their propensity to develop posttraumatic stress disorder (PTSD). Understanding individual differences in PTSD vulnerability will allow the development of improved prevention and treatment options. Here we characterized fear conditioning and extinction in rats selectively bred for differences in their locomotor response to a novel environment. Selectively bred high-responder (bHR) and low-responder (bLR) male rats are known to differ in their emotional reactivity on a range of measures of spontaneous anxiety- and depressive-like behaviors. We demonstrate that bHRs have facilitated extinction learning and retention compared with outbred Sprague Dawley rats, whereas bLRs show reduced extinction learning and retention. This indicates that bLRs are more vulnerable to PTSD-like behavior. Fibroblast growth factor 2 (FGF2) has previously been implicated in the development of these behavioral phenotypes and facilitates extinction learning in outbred animals, therefore we examined the effects of early-life FGF2 on bHR and bLR behavior. FGF2 administered on the day after birth facilitated extinction learning and retention in bHRs, but not in bLRs or control rats, during adulthood. This indicates that, under the current fear conditioning paradigm, early-life FGF2 has protective effects only in resilient animals. This stands in contrast to FGF2's ability to protect vulnerable animals in milder tests of anxiety. These results provide a unique animal model of individual differences in PTSD-like behavior, allowing the study of genetic, developmental, and environmental factors in its expression.

Fibroblast growth factor 9 is a novel modulator of negative affect.

Both gene expression profiling in postmortem human brain and studies using animal models have implicated the fibroblast growth factor (FGF) family in affect regulation and suggest a potential role in the pathophysiology of major depressive disorder (MDD). FGF2, the most widely characterized family member, is down-regulated in the depressed brain and plays a protective role in rodent models of affective disorders. By contrast, using three microarray analyses followed by quantitative RT-PCR confirmation, we show that FGF9 expression is up-regulated in the hippocampus of individuals with MDD, and that FGF9 expression is inversely related to the expression of FGF2. Because little is known about FGF9's function in emotion regulation, we used animal models to shed light on its potential role in affective function. We found that chronic social defeat stress, an animal model recapitulating some aspects of MDD, leads to a significant increase in hippocampal FGF9 expression, paralleling the elevations seen in postmortem human brain tissue. Chronic intracerebroventricular administration of FGF9 increased both anxiety- and depression-like behaviors. In contrast, knocking down FGF9 expression in the dentate gyrus of the hippocampus using a lentiviral vector produced a decrease in FGF9 expression and ameliorated anxiety-like behavior. Collectively, these results suggest that high levels of hippocampal FGF9 play an important role in the development or expression of mood and anxiety disorders. We propose that the relative levels of FGF9 in relation to other members of the FGF family may prove key to understanding vulnerability or resilience in affective disorders.

Aberrant amygdala-frontal cortex connectivity during perception of fearful faces and at rest in generalized social anxiety disorder.

BACKGROUND: Generalized social anxiety disorder (gSAD) is characterized by exaggerated amygdala reactivity to social signals of threat, but if and how the amygdala interacts with functionally and anatomically connected prefrontal cortex (PFC) remains largely unknown. Recent evidence points to aberrant amygdala connectivity to medial PFC in gSAD at rest, but it is difficult to attribute functional relevance without the context of threat processing. Here, we address this by studying amygdala-frontal cortex connectivity during viewing of fearful faces and at rest in gSAD patients. METHODS: Twenty patients with gSAD and 17 matched healthy controls (HCs) participated in functional magnetic resonance imaging of an emotional face matching task and a resting state task. Functional connectivity and psychophysiological interaction analysis were used to assess amygdala connectivity. RESULTS: Compared to HCs, gSAD patients exhibited less connectivity between amygdala and the rostral anterior cingulate cortex (ACC) and dorsolateral prefrontal cortex (DLPFC) while viewing fearful faces. gSAD patients also showed less connectivity between amygdala and rostral ACC at rest in the absence of fearful faces. DLPFC connectivity was negatively correlated with LSASFear (where LSAS is Liebowitz Social Anxiety Scale). CONCLUSIONS: Task and rest paradigms provide unique and important information about discrete and overlapping functional networks. In particular, amygdala coupling to DLPFC may be a phasic abnormality, emerging only in the presence of a social predictor of threat, whereas amygdala coupling to the rostral ACC may reflect both phasic and tonic abnormalities. These findings prompt further studies to better delineate intrinsic and externally evoked brain connectivity in anxiety and depression in relation to amygdala dysfunction.

Functional connectivity tracks clinical deterioration in Alzheimer's disease.

While resting state functional connectivity has been shown to decrease in patients with mild and/or moderate Alzheimer's disease, it is not yet known how functional connectivity changes in patients as the disease progresses. Furthermore, it has been noted that the default mode network is not as homogenous as previously assumed and several fractionations of the network have been proposed. Here, we separately investigated the modulation of 3 default mode subnetworks, as identified with group independent component analysis, by comparing Alzheimer's disease patients to healthy controls and by assessing connectivity changes over time. Our results showed decreased connectivity at baseline in patients versus controls in the posterior default mode network, and increased connectivity in the anterior and ventral default mode networks. At follow-up, functional connectivity decreased across all default mode systems in patients. Our results suggest that earlier in the disease, regions of the posterior default mode network start to disengage whereas regions within the anterior and ventral networks enhance their connectivity. However, as the disease progresses, connectivity within all systems eventually deteriorates.

Failure of anterior cingulate activation and connectivity with the amygdala during implicit regulation of emotional processing in generalized anxiety disorder.

OBJECTIVE: Clinical data suggest that abnormalities in the regulation of emotional processing contribute to the pathophysiology of generalized anxiety disorder, yet these abnormalities remain poorly understood at the neurobiological level. The authors recently reported that in healthy volunteers the pregenual anterior cingulate regulates emotional conflict on a trial-by-trial basis by dampening activity in the amygdala. The authors also showed that this process is specific to the regulation of emotional, compared to nonemotional, conflict. Here the authors examined whether this form of noninstructed emotion regulation is perturbed in generalized anxiety disorder. METHOD: Seventeen patients with generalized anxiety disorder and 24 healthy comparison subjects underwent functional MRI while performing an emotional conflict task that involved categorizing facial affect while ignoring overlaid affect label words. Behavioral and neural measures were used to compare trial-by-trial changes in conflict regulation. RESULTS: Comparison subjects effectively regulated emotional conflict from trial to trial, even though they were unaware of having done so. By contrast, patients with generalized anxiety disorder were completely unable to regulate emotional conflict and failed to engage the pregenual anterior cingulate in ways that would dampen amygdalar activity. Moreover, performance and brain activation were correlated with symptoms and could be used to accurately classify the two groups. CONCLUSIONS: These data demonstrate that patients with generalized anxiety disorder show significant deficits in the noninstructed and spontaneous regulation of emotional processing. Conceptualization of anxiety as importantly involving abnormalities in emotion regulation, particularly a type occurring outside of awareness, may open up avenues for novel treatments, such as by targeting the medial prefrontal cortex.

Disrupted amygdalar subregion functional connectivity and evidence of a compensatory network in generalized anxiety disorder.

CONTEXT: Little is known about the neural abnormalities underlying generalized anxiety disorder (GAD). Studies in other anxiety disorders have implicated the amygdala, but work in GAD has yielded conflicting results. The amygdala is composed of distinct subregions that interact with dissociable brain networks, which have been studied only in experimental animals. A functional connectivity approach at the subregional level may therefore yield novel insights into GAD. OBJECTIVES: To determine whether distinct connectivity patterns can be reliably identified for the basolateral (BLA) and centromedial (CMA) subregions of the human amygdala, and to examine subregional connectivity patterns and potential compensatory amygdalar connectivity in GAD. DESIGN: Cross-sectional study. SETTING: Academic medical center. PARTICIPANTS: Two cohorts of healthy control subjects (consisting of 17 and 31 subjects) and 16 patients with GAD. MAIN OUTCOME MEASURES: Functional connectivity with cytoarchitectonically determined BLA and CMA regions of interest, measured during functional magnetic resonance imaging performed while subjects were resting quietly in the scanner. Amygdalar gray matter volume was also investigated with voxel-based morphometry. RESULTS: Reproducible subregional differences in large-scale connectivity were identified in both cohorts of healthy controls. The BLA was differentially connected with primary and higher-order sensory and medial prefrontal cortices. The CMA was connected with the midbrain, thalamus, and cerebellum. In GAD patients, BLA and CMA connectivity patterns were significantly less distinct, and increased gray matter volume was noted primarily in the CMA. Across the subregions, GAD patients had increased connectivity with a previously characterized frontoparietal executive control network and decreased connectivity with an insula- and cingulate-based salience network. CONCLUSIONS: Our findings provide new insights into the functional neuroanatomy of the human amygdala and converge with connectivity studies in experimental animals. In GAD, we find evidence of an intra-amygdalar abnormality and engagement of a compensatory frontoparietal executive control network, consistent with cognitive theories of GAD.