Lucidity in dementia: A perspective from the NIA.

In this issue of Alzheimer's & Dementia, Mashour et al. propose the intriguing hypothesis that some manifestations of late-stage dementia are reversible, albeit transiently. Calling this phenomenon paradoxical lucidity, their paper follows a 2018 workshop sponsored by the National Institute on Aging that assessed the state of knowledge on lucidity in dementia and identified areas ripe for further study. The National Institute on Aging has since released two funding opportunity announcements (RFA-AG-20-016 and RFA-AG-20-017) to establish the building blocks of such a research program. The potential challenges of conducting such studies are matched by the potential opportunities to open a novel window onto our understanding of dementia. Initial findings from this research may eventually lead to studies that uncover novel mechanisms underlying cognitive decline, identify potential preventive or therapeutic approaches for individuals with dementia, offer more effective strategies for caregivers, and perhaps even expand our understanding of the nature of personhood and consciousness.

Altered expression of the Cdk5 activator-like protein, Cdk5α, causes neurodegeneration, in part by accelerating the rate of aging.

Aging is the greatest risk factor for neurodegeneration, but the connection between the two processes remains opaque. This is in part for want of a rigorous way to define physiological age, as opposed to chronological age. Here, we develop a comprehensive metric for physiological age in Drosophila, based on genome-wide expression profiling. We applied this metric to a model of adult-onset neurodegeneration, increased or decreased expression of the activating subunit of the Cdk5 protein kinase, encoded by the gene Cdk5α, the ortholog of mammalian p35. Cdk5α-mediated degeneration was associated with a 27-150% acceleration of the intrinsic rate of aging, depending on the tissue and genetic manipulation. Gene ontology analysis and direct experimental tests revealed that affected age-associated processes included numerous core phenotypes of neurodegeneration, including enhanced oxidative stress and impaired proteostasis. Taken together, our results suggest that Cdk5α-mediated neurodegeneration results from accelerated aging, in combination with cell-autonomous neuronal insults. These data fundamentally recast our picture of the relationship between neurodegeneration and its most prominent risk factor, natural aging.

Of proneurotrophins and their antineurotrophic effects.

Neurotrophins perform essential processes throughout neural development. They signal through Trk receptor proteins, typically in association with a "low affinity" p75(NTR) pan-neurotrophin co-receptor. Neurotrophins are synthesized as proproteins; the pro domains are removed proteolytically to yield the mature, presumably functional forms of the neurotrophins. Recent findings, however, have revealed a positive role for the proneurotrophins themselves. The proproteins bind with high affinity to the p75(NTR) pan-neurotrophin receptor in the absence of Trks to initiate a separate set of signaling cascades that actively oppose the effects of the mature growth factors. These experiments suggest that the balance between pro- and mature neurotrophin plays a critical role in tuning downstream signaling. This view changes the neurotrophin field substantially and also points to the broader idea that the potential activities of precursor proteins deserve a closer look.

Peripheral administration of poly I:C disrupts contextual fear memory consolidation and BDNF expression in mice.

In the current study, administration of poly I:C induced a deficit in contextual, but not auditory-cue, fear memory consolidation. This memory deficit coincided with a decrease in hippocampal and cortical BDNF mRNA expression. These results extend prior work, and suggest that a single peripheral injection of poly I:C disrupts contextual fear memory consolidation processes in adult mice, and that these deficits may potentially be mediated by diminished BDNF expression.

Prolonged elevation in hippocampal Aß and cognitive deficits following repeated endotoxin exposure in the mouse.

Alzheimer's disease (AD) is characterized by neuronal cell death and atrophy in regions of the adult brain, including the hippocampus and cortex, due to formation of amyloid beta (Aß) plaques and neurofibrillary tangles. The presence of these pathologies can limit normal signaling properties and ultimately lead to learning and memory deficits. Chronic inflammation has been implicated in the onset and progression of these AD-related pathologies. Our study was designed to assess the effects of peripheral inflammation on pathologies associated with AD by using the bacterial endotoxin lipopolysaccharide (LPS). C57BL/6J mice were given intraperitoneal injections of LPS or saline for 1, 3, or 7 consecutive days. Hippocampal tissue from animals receiving LPS contained significantly higher levels of Aß1-42, a peptide component of AD plaques, than did those from saline control animals. Central and peripheral pro-inflammatory cytokine levels were increased following a single injection of LPS, but retuned to baseline levels before cognitive testing began. We show that one injection of LPS leads to sickness behavior, but 7 consecutive days does not, indicating tolerance to the endotoxin. Cognitive testing was then conducted to determine if whether deficits from increased Aß1-42 was evident. Results from both Morris water maze and contextual fear conditioning revealed cognitive deficits in LPS-treated mice. In summary, multiple injections of LPS resulted in increased Aß1-42 in the hippocampus and cognitive deficits in mice.

At the Fulcrum in Health and Disease: Cdk5 and the Balancing Acts of Neuronal Structure and Physiology.

Cdk5 has been implicated in a multitude of processes in neuronal development, cell biology and physiology. These influence many neurological disorders, but the very breadth of Cdk5 effects has made it difficult to synthesize a coherent picture of the part played by this protein in health and disease. In this review, we focus on the roles of Cdk5 in neuronal function, particularly synaptic homeostasis, plasticity, neurotransmission, subcellular organization, and trafficking. We then discuss how disruption of these Cdk5 activities may initiate or exacerbate neural disorders. A recurring theme will be the sensitivity of Cdk5 sequelae to the precise biological context under consideration.

Age exacerbates sickness behavior following exposure to a viral mimetic.

Poly I:C, a viral mimetic, is a synthetic double-stranded RNA that is known to cause activation of the innate immune system, resulting in the emergence of sickness behaviors in otherwise healthy adult mice. However, the way in which such effects of poly I:C manifest themselves in aged mice are not currently known. We hypothesized that poly I:C administration would lead to burrowing deficits, but that these deficits would be exaggerated in aged subjects (19-months old) compared to young subjects (4-months old) that received the same dose. In order to associate these behavioral decrements with inflammatory factors, we measured mRNA expression of IL-1ß and IL-6 in the hippocampus and parietal cortex and peripheral protein expression of IL-6, TNF-α, MCP-1, MIP-1α, and IL-1ß in the serum. After exposure to poly I:C, aged subjects demonstrated significant impairments in their burrowing behavior, compared to younger subjects administered the same dose. These behavioral decrements coincided with increased expression of IL-6 among animals exposed to poly I:C and increased expression of IL-1ß among aged animals in the hippocampus and cortex. Furthermore, we observed an increase in peripheral poly I:C-induced IL-6, TNF-α, MCP-1, and MIP-1α, but not IL-1ß. These results indicate that virus-mediated immune activation in the aging body can lead to increased sickness behavior. Furthermore, these data indicated a possible dissociation between the effects of poly I:C on sickness behaviors in aged mice, with central expression of IL-1ß potentially playing a role in age-related impairments.

Peripheral bacterial endotoxin administration triggers both memory consolidation and reconsolidation deficits in mice.

Peripherally administered inflammatory stimuli, such as lipopolysaccharide (LPS), induce the synthesis and release of proinflammatory cytokines and chemokines in the periphery and the central nervous system, and trigger a variety of neurobiological responses. Indeed, prior reports indicate that peripheral LPS administration in rats disrupts contextual fear memory consolidation processes, potentially due to elevated cytokine expression. We used a similar, but partially olfaction-based, contextual fear conditioning paradigm to examine the effects of LPS on memory consolidation and reconsolidation in mice. Additionally, interleukin-1ß (IL-1ß), brain-derived neurotrophic factor (BDNF), and zinc finger (Zif)-268 mRNA expression in the hippocampus and the cortex, along with peripheral cytokines and chemokines, were assessed. As hypothesized, LPS administered immediately or 2 h, but not 12 h, post-training impaired memory consolidation processes that support the storage of the conditioned contextual fear memory. Additionally, as hypothesized, LPS administered immediately following the fear memory trace reactivation session impaired memory reconsolidation processes. Four hours post-injection, both central cytokine and peripheral cytokine and chemokine levels were heightened in LPS-treated animals, with a simultaneous decrease in BDNF, but not Zif-268, mRNA. Collectively, these data reinforce prior work showing LPS- and cytokine-related effects on memory consolidation, and extend this work to memory reconsolidation.

The effects of age on lipopolysaccharide-induced cognitive deficits and interleukin-1ß expression.

An acute LPS challenge immediately following day 1 of shuttlebox training triggered exacerbated central IL-1ß production and disrupted memory consolidation and/or further acquisition of the task in 18-month-old mice, compared to 4-month-old controls. These deficits cannot be attributed to alterations in sickness behavior. The findings suggest that age and immune activation combine to impair learning and memory consolidation processes, and that increased central IL-1ß production may play a role.

Influence of prenatal stress on behavioral, endocrine, and cytokine responses to adulthood bacterial endotoxin exposure.

Prior research suggests that prenatal stress, among other effects, can lead to hyper-reactivity of the offspring's hypothalamic-pituitary-adrenal (HPA) axis and alterations in immune function. These stress-induced changes have been linked to a greater propensity to develop depression or anxiety disorders. Furthermore, prenatally stressed offspring may be more susceptible to certain diseases. The immune alterations induced by prenatal stress exposure may disrupt the normal communication between the immune system, endocrine system, and central nervous system, potentially making prenatally stressed individuals more vulnerable to the negative aspects of immune activation, including cytokine-induced cognitive deficits and anxiety. The present study investigated whether prenatal stress would exaggerate these detrimental effects of peripheral immune activation. We hypothesized that prenatally stressed subjects would be hypersensitive to endotoxin administration and would therefore show exaggerated learning deficits, increased anxiety-like behavior, and increased peripheral and central interleukin-1beta (IL-1beta) levels. The observed results only partially supported our hypotheses, as prenatally stressed subjects showed evidence, albeit modest, of increased anxiety-like behavior following endotoxin administration relative to non-stressed controls. While prenatal stress exposure or lipopolysaccharide (LPS) administration independently impaired learning, the data failed to support the hypothesis that prenatally stressed subjects would show exaggerated cognitive deficits, engendered via enhanced peripheral and central IL-1beta levels, following immune activation. Collectively, the data suggest that although prenatal stress exposure led to increases in anxiety-like behavior following endotoxin exposure, it did not appear to increase susceptibility to LPS-induced cognitive decline or elevations in proinflammatory cytokine production.