Anne B. Rocher

Stress-Induced Alterations in Prefrontal Cortical Dendritic Morphology Predict Selective Impairments in Perceptual Attentional Set-Shifting

Stressful life events have been implicated clinically in the pathogenesis of mental illness, but the neural substrates that may account for this observation remain poorly understood. Attentional impairments symptomatic of these psychiatric conditions are associated with structural and functional abnormalities in a network of prefrontal cortical structures. Here, we examine whether chronic stress-induced dendritic alterations in the medial prefrontal cortex (mPFC) and orbital frontal cortex (OFC) underlie impairments in the behaviors that they subserve. After 21 d of repeated restraint stress, rats were tested on a perceptual attentional set-shifting task, which yields dissociable measures of reversal learning and attentional set-shifting, functions that are mediated by the OFC and mPFC, respectively. Intracellular iontophoretic injections of Lucifer yellow were performed in a subset of these rats to examine dendritic morphology in layer II/III pyramidal cells of the mPFC and lateral OFC. Chronic stress induced a selective impairment in attentional set-shifting and a corresponding retraction (20%) of apical dendritic arbors in the mPFC. In stressed rats, but not in controls, decreased dendritic arborization in the mPFC predicted impaired attentional set-shifting performance. In contrast, stress was not found to adversely affect reversal learning or dendritic morphology in the lateral OFC. Instead, apical dendritic arborization in the OFC was increased by 43%. This study provides the first direct evidence that dendritic remodeling in the prefrontal cortex may underlie the functional deficits in attentional control that are symptomatic of stress-related mental illnesses.

Chronic behavioral stress induces apical dendritic reorganization in pyramidal neurons of the medial prefrontal cortex

Both the hippocampus and the medial prefrontal cortex (mPFC) play an important role in the negative feedback regulation of hypothalamic-pituitary-adrenal (HPA) activity during physiologic and behavioral stress. Moreover, chronic behavioral stress is known to affect the morphology of CA3c pyramidal neurons in the rat, by reducing total branch number and length of apical dendrites. In the present study, we investigated the effects of behavioral stress on the mPFC, using the repeated restraint stress paradigm. Animals were perfused after 21 days of daily restraint, and intracellular iontophoretic injections of Lucifer Yellow were carried out in pyramidal neurons of layer II/III of the anterior cingulate cortex and prelimbic area. Cellular reconstructions were performed on apical and basal dendrites of pyramidal neurons in layer II/III of the anterior cingulate and prelimbic cortices. We observed a significant reduction on the total length (20%) and branch numbers (17%) of apical dendrites, and no significant reduction in basal dendrites. These cellular changes may impair the capacity of the mPFC to suppress the response of the HPA axis to stress, and offer an experimental model of stress-induced neocortical reorganization that may provide a structural basis for the cognitive impairments observed in post-traumatic stress disorder.

Diet-induced insulin resistance promotes amyloidosis in a transgenic mouse model of Alzheimer's disease

Recent epidemiological evidence indicates that insulin resistance, a proximal cause of Type II diabetes [a non‐insulin dependent form of diabetes mellitus (NIDDM)], is associated with an increased relative risk for Alzheimers disease (AD). In this study we examined the role of dietary conditions leading to NIDDM‐like insulin resistance on amyloidosis in Tg2576 mice, which model AD‐like neuropathology. We found that diet‐induced insulin resistance promoted amyloidogenic β‐amyloid (Aβ) Aβ1–40 and Aβ1–42 peptide generation in the brain that corresponded with increased γ‐secretase activities and decreased insulin degrading enzyme (IDE) activities. Moreover, increased Aβ production also coincided with increased AD‐type amyloid plaque burden in the brain and impaired performance in a spatial water maze task. Further exploration of the apparent interrelationship of insulin resistance to brain amyloidosis revealed a functional decrease in insulin receptor (IR)‐mediated signal transduction in the brain, as suggested by decreased IR β‐subunit (IRβ) Y1162/1163 autophosphorylation and reduced phosphatidylinositol 3 (PI3)‐kinase/pS473‐AKT/Protein kinase (PK)‐B in these same brain regions. This latter finding is of particular interest given the known inhibitory role of AKT/PKB on glycogen synthase kinase (GSK)‐3α activity, which has previously been shown to promote Aβ peptide generation. Most interestingly, we found that decreased pS21‐GSK‐3α and pS9‐GSK‐ 3β phosphorylation, which is an index of GSK activation, positively correlated with the generation of brain C‐terminal fragment (CTF)‐γ cleavage product of amyloid precursor protein, an index of γ‐secretase activity, in the brain of insulin‐resistant relative to normoglycemic Tg2576 mice. Our study is consistent with the hypothesis that insulin resistance may be an underlying mechanism responsible for the observed increased relative risk for AD neuropathology, and presents the first evidence to suggest that IR signaling can influence Aβ production in the brain.

Changes in the structural complexity of the aged brain

Structural changes of neurons in the brain during aging are complex and not well understood. Neurons have significant homeostatic control of essential brain functions, including synaptic excitability, gene expression, and metabolic regulation. Any deviations from the norm can have severe consequences as seen in aging and injury. In this review, we present some of the structural adaptations that neurons undergo throughout normal and pathological aging and discuss their effects on electrophysiological properties and cognition. During aging, it is evident that neurons undergo morphological changes such as a reduction in the complexity of dendrite arborization and dendritic length. Spine numbers are also decreased, and because spines are the major sites for excitatory synapses, changes in their numbers could reflect a change in synaptic densities. This idea has been supported by studies that demonstrate a decrease in the overall frequency of spontaneous glutamate receptor‐mediated excitatory responses, as well as a decrease in the levels of α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid and N‐methyl‐d‐aspartate receptor expression. Other properties such as γ‐aminobutyric acid A receptor‐mediated inhibitory responses and action potential firing rates are both significantly increased with age. These findings suggest that age‐related neuronal dysfunction, which must underlie observed decline in cognitive function, probably involves a host of other subtle changes within the cortex that could include alterations in receptors, loss of dendrites, and spines and myelin dystrophy, as well as the alterations in synaptic transmission. Together these multiple alterations in the brain may constitute the substrate for age‐related loss of cognitive function.

REPEATED STRESS ALTERS DENDRITIC SPINE MORPHOLOGY IN THE RAT MEDIAL PREFRONTAL CORTEX

Anatomical alterations in the medial prefrontal cortex (mPFC) are associated with hypothalamopituitary adrenal (HPA) axis dysregulation, altered stress hormone levels, and psychiatric symptoms of stress‐related mental illnesses. Functional imaging studies reveal impairment and shrinkage of the mPFC in such conditions, and these findings are paralleled by experimental studies showing dendritic retraction and spine loss following repeated stress in rodents. Here we extend this characterization to how repeated stress affects dendritic spine morphology in mPFC through the utilization of an automated approach that rapidly digitizes, reconstructs three dimensionally, and calculates geometric features of neurons. Rats were perfused after being subjected to 3 weeks of daily restraint stress (6 hours/day), and intracellular injections of Lucifer Yellow were made in layer II/III pyramidal neurons in the dorsal mPFC. To reveal spines in all angles of orientation, deconvolved high‐resolution confocal laser scanning microscopy image stacks of dendritic segments were reconstructed and analyzed for spine volume, surface area, and length using a Rayburst‐based automated approach (8,091 and 8,987 spines for control and stress, respectively). We found that repeated stress results in an overall decrease in mean dendritic spine volume and surface area, which was most pronounced in the distal portion of apical dendritic fields. Moreover, we observed an overall shift in the population of spines, manifested by a reduction in large spines and an increase in small spines. These results suggest a failure of spines to mature and stabilize following repeated stress and are likely to have major repercussions on function, receptor expression, and synaptic efficacy. J. Comp. Neurol. 507:1141–1150, 2008.

Reversibility of apical dendritic retraction in the rat medial prefrontal cortex following repeated stress.

Apical dendritic retraction and axospinous synapse loss in the medial prefrontal cortex (PFC) are structural alterations that result from repeated restraint stress. Such changes in this brain region may be associated with impaired working memory, altered emotionality, and inability to regulate hypothalamic-pituitary adrenal activity, which in turn may underlie stress-related mental illnesses. In the present study, we examined the persistence of these stress-induced dendritic alterations in the medial PFC following the cessation of stress. Animals received either daily restraint stress for a 3-week period and were then allowed to recover for another 3 weeks, restraint stress for 3 or 6 weeks, or no restraint. Following perfusion and fixation, intracellular iontophoretic injections of Lucifer Yellow were performed in layer II/III pyramidal neurons in slices from the medial PFC, and apical and basal dendritic arbors were reconstructed in three dimensions. We observed a significant reduction in apical dendritic length and branch number following 3 or 6 weeks of repeated stress compared to 3-week stress/3-week recovery. These results suggest that stress-induced dendritic plasticity in the medial PFC is reversible and may have implications for the functional recovery of medial PFC function following prolonged psychological stress.

Caloric restriction attenuates β-amyloid neuropathology in a mouse model of Alzheimer's disease

This study was designed to explore the possibility that caloric restriction (CR) may benefit Alzheimers disease (AD) by preventing β‐amyloid (Aβ) neuropathology pivotal to the initiation and progression of the disease. We report that a CR dietary regimen prevents Aβ peptides generation and neuritic plaque deposition in the brain of a mouse model of AD neuropathology through mechanisms associated with promotion of anti‐amyloidogenic α‐secretase activity. Study findings support existing epidemiological evidence indicating that caloric intake may influence risk for AD and raises the possibility that CR may be used in preventative measures aimed at delaying the onset of AD amyloid neuropathology.

The nature and effects of cortical microvascular pathology in aging and Alzheimer's disease

Abstract Age-related and amyloid-induced pathology of the cerebral microvasculature have been implicated as potential contributing factors to the pathogenesis of Alzheimers disease (AD). The microvasculature plays a crucial role in maintaining brain homeostasis and deterioration of its integrity may have deleterious effects on brain function in AD, possibly leading to neurofibrillary degeneration, plaque formation, and cell loss. Brain vessels possess peculiar anatomical and physiological properties owing to their role in the exchange processes of various substances between blood and brain, which are highly regulated for the maintenance of ionic homeostasis of the neuronal environment. Here we review neuropathological aspects of cortical microvessels in aging and AD in relationship to known cardiovascular risk factors and their possible impact on the cognitive decline seen in late-onset dementia.

Dendritic vulnerability in neurodegenerative disease: insights from analyses of cortical pyramidal neurons in transgenic mouse models

In neurodegenerative disorders, such as Alzheimer’s disease, neuronal dendrites and dendritic spines undergo significant pathological changes. Because of the determinant role of these highly dynamic structures in signaling by individual neurons and ultimately in the functionality of neuronal networks that mediate cognitive functions, a detailed understanding of these changes is of paramount importance. Mutant murine models, such as the Tg2576 APP mutant mouse and the rTg4510 tau mutant mouse have been developed to provide insight into pathogenesis involving the abnormal production and aggregation of amyloid and tau proteins, because of the key role that these proteins play in neurodegenerative disease. This review showcases the multidimensional approach taken by our collaborative group to increase understanding of pathological mechanisms in neurodegenerative disease using these mouse models. This approach includes analyses of empirical 3D morphological and electrophysiological data acquired from frontal cortical pyramidal neurons using confocal laser scanning microscopy and whole-cell patch-clamp recording techniques, combined with computational modeling methodologies. These collaborative studies are designed to shed insight on the repercussions of dystrophic changes in neocortical neurons, define the cellular phenotype of differential neuronal vulnerability in relevant models of neurodegenerative disease, and provide a basis upon which to develop meaningful therapeutic strategies aimed at preventing, reversing, or compensating for neurodegenerative changes in dementia.

Age-related vascular pathology in transgenic mice expressing presenilin 1-associated familial Alzheimer's disease mutations.

Mutations in the presenilin 1 (PS1) gene are the most commonly recognized cause of familial Alzheimers disease (FAD). Besides senile plaques, neurofibrillary tangles, and neuronal loss, Alzheimers disease (AD) is also accompanied by vascular pathology. Here we describe an age-related vascular pathology in two lines of PS1 FAD-mutant transgenic mice that mimics many features of the vascular pathology seen in AD. The pathology was especially prominent in the microvasculature whose vessels became thinned and irregular with the appearance of many abnormally looped vessels as well as string vessels. Stereologic assessments revealed a reduction of the microvasculature in the hippocampus that was accompanied by hippocampal atrophy. The vascular changes were not congophilic. Yet, despite the lack of congophilia, penetrating vessels at the cortical surface were often abnormal morphologically and microhemorrhages sometimes occurred. Altered immunostaining of blood vessels with basement membrane-associated antigens was an early feature of the microangiopathy and was associated with thickening of the vascular basal laminae and endothelial cell alterations that were visible ultrastructurally. Interestingly, although the FAD-mutant transgene was expressed in neurons in both lines of mice, there was no detectable expression in vascular endothelial cells or glial cells. These studies thus have implications for the role of neuronal to vascular signaling in the pathogenesis of the vascular pathology associated with AD.