Ageing causes prominent neurovascular dysfunction associated with loss of astrocytic contacts and gliosis

AIMS

Normal neurovascular coupling, mediated by the fine interplay and communication of cells within the neurovascular unit, is critical for maintaining normal brain activity and cognitive function. This study investigated whether, with advancing age there is disruption of neurovascular coupling and specific cellular components of the neurovascular unit, and whether the effects of increasing amyloid (a key feature of Alzheimer's disease) would exacerbate these changes.

METHODS

Wild-type mice, in which amyloid deposition is absent, were compared to transgenic amyloid precursor protein (APP) littermates (TgSwDI) which develop age-dependent increases in amyloid. Baseline cerebral blood flow and responses to whisker stimulation were measured. Components of the neurovascular unit (astrocytes, end-feet, pericytes, microglia) were measured by immunohistochemistry.

RESULTS

Neurovascular coupling was progressively impaired with increasing age (starting at 12 months) but was not further altered in TgSwDI mice. Aged mice showed reduced vascular pericyte coverage relative to young but this was not related to neurovascular function. Aged mice displayed significant reductions in astrocytic end-feet expression of aquaporin-4 on blood vessels compared to young mice, and a prominent increase in microglial proliferation which correlated with neurovascular function.

CONCLUSIONS

Strategies aimed to restore the loss of astrocytic end feet contact and reduce gliosis may improve neurovascular coupling.

Neuroprotective effect of adenoviral-mediated gene transfer of TIMP-1 and -2 in ischemic brain injury

Gene therapy may be a promising approach for treatment of brain ischemia. We and others previously demonstrated that increased activity of matrix metalloproteinases (MMPs) contributes to the tissue damage that results from ischemic injury. The proteolysis of MMPs is tightly controlled by tissue inhibitors of MMPs (TIMPs). In this study, we examined whether adenoviral-mediated gene transfer of TIMP-1 and TIMP-2 could protect against neuronal damage induced by global cerebral ischemia in mice. An adenovirus expressing TIMP-1 or TIMP-2 (AdTIMP-1 or AdTIMP-2) or a control adenovirus (RAd60) or vehicle was injected into the striatum 3 days before transient global cerebral ischemia. The extent of neuronal damage was quantified 3 days post-ischemia. There was no significant difference in the extent of neuronal damage in vehicle as compared to RAd60-treated mice. In contrast, neuronal damage was reduced, by approximately 50%, after gene transfer of AdTIMP-1 (P<0.001) and AdTIMP-2 (P< 0.01) as compared to controls. This study provides the first in vivo evidence of the protective effects of TIMP-1 and TIMP-2 via gene transfer in global ischemia.

4-Hydroxynonenal immunoreactivity is increased in human hippocampus after global ischemia

Oxidative stress and lipid peroxidation may contribute to the pathology of neurodegenerative disorders such as Alzheimer's disease (AD) and cerebral ischemia. 4-Hydroxynonenal (4-HNE) is a toxic by-product of lipid peroxidation, and immunoreactivity to 4-HNE has been used to examine lipid peroxidation in the pathogenesis of AD and ischemia. This study sought to determine 1) if there are cellular alterations in 4-HNE immunoreactivity in the human hippocampus after global ischemia, and 2) whether possession of an apolipoprotein E (APOE) epsilon4 allele influenced the extent of 4-HNE immunoreactivity. 4-HNE immunoreactivity was assessed semi-quantitatively in the temporal lobe of a group of controls (n = 44) and in a group of patients who had an episode of global ischemia as a result of a cardiorespiratory arrest and subsequently died (n = 56, survival ranged from 1hr to 42 days). There was minimal cellular 4-HNE immunoreactivity in the control group. However, compared to controls, 4-HNE immunoreactivity was significantly increased in neurons (p < 0.0002) and glia (p < 0.0001) in the hippocampal formation after global ischemia. Possession of an APOE epsilon4 allele did not influence the extent of neuronal or glial 4-HNE immunostaining in the control or global ischemia group. There was a significant negative correlation between the extent of neuronal 4-HNE immunoreactivity with survival period after global ischemia (r2 = 0.0801; p < 0.036) and a significant positive correlation between the extent of glial 4-HNE immunoreactivity and survival after global ischemia (r2 = 0.2958; p < 0.0001). The data indicate a marked increase in neuronal and glial 4-HNE. This substantiates a role for lipid peroxidation in the pathogenesis of cerebral ischemia. There was no indication that APOE genotype influenced the extent of 4-HNE immunoreactivity.

Minimal ischaemic neuronal damage and HSP70 expression in MF1 strain mice following bilateral common carotid artery occlusion

Investigation into the influence of specific genes and gene products upon the pathophysiology of cerebral ischaemia has been greatly enhanced by the use of genetically modified mice. A simple model of global cerebral ischaemia in mouse is bilateral common carotid artery occlusion (BCCAo) and the neuropathological impact of BCCAo has been investigated in several mouse strains. Bilateral carotid occlusion produces extensive neuronal damage in C57Bl/6J strain mice and this damage is linked to posterior communicating artery (PcomA) hypoplasticity in the circle of Willis. In the present study, we investigated the effect of BCCAo in MF1 strain mice and compared them with C57Bl/6J mice. The neuropathological consequences of BCCAo were assessed using standard histochemical staining and heat shock protein 70 (HSP70) immunohistochemical staining (to demarcate cells that had been ischaemically stressed). The effect of BCCAo on mean arterial blood pressure (MABP) was also measured. The plasticity of the circle of Willis was recorded using carbon black perfusion. MF1 mice displayed significantly less ischaemic neuronal damage and HSP70 immunoreactivity compared to C57Bl/6J mice following 10-20 min BCCAo. Moreover, ischaemic neuronal damage and HSP70 immunoreactivity in MF1 mice subjected to extended BCCAo (25-45 min) was never as extensive or widespread as that observed in C57Bl/6J mice after 20 min BCCAo. MABP in MF1 mice (102+/-5 mmHg) was significantly higher than in C57Bl/6J mice (87+/-5) during 20 min BCCAo. MABP in MF1 mice during 20 and 40 min (103+/-12 mmHg) BCCAo remained above pre-occlusion values for the entire occlusion period. MF1 mice had significantly greater circle of Willis plasticity (more PcomAs) than C57Bl/6J mice did. These data indicate that MF1 mice are less susceptible to BCCAo than C57Bl/6J mice and that this could be due to maintained increases in MABP during BCCAo and the lower prevalence of abnormalities of the circle of Willis in MF1 mice.

Impaired neuronal plasticity in transgenic mice expressing human apolipoprotein E4 compared to E3 in a model of entorhinal cortex lesion

The apolipoprotein E (APOE) epsilon 4 allele is a major risk factor for late-onset familial and sporadic Alzheimer's disease (AD) and is associated with a poor outcome after brain injury. Each apoE isoform is suggested to have differential effects on neuronal repair mechanisms within the CNS. In the present study, APOE genotype influence on the immediate response to injury and subsequent repair process was examined in a line of transgenic APOE mice possessing human APOE gene insertions (epsilon 3 and epsilon 4). Quantification of synaptophysin and GAP-43 immunoreactivity was used to measure the extent of degeneration and regeneration after entorhinal cortex lesion (ECL). Progressive neurodegenerative decline occurred in the ipsilateral dentate gyrus until day 28 post-ECL which was more severe in APOE epsilon 3 mice compared to APOE epsilon 4 mice. By day 90 post-ECL compensatory sprouting and reactive synaptogenesis had taken place in the dentate gyrus of APOE epsilon 3 mice such that GAP-43 and synaptophysin immunoreactivity had returned to prelesion levels. In contrast, APOE epsilon 4 mice displayed significant deficits in synaptophysin and GAP-43 immunostaining compared to the APOE epsilon 3 mice (P < 0.05). Expansion of the inner molecular layer (IML) was used as a measure of the sprouting index from the commissural-associational pathway and by day 90 post-ECL the IML width in APOE epsilon 3 mice had increased by 45% but only 20% in APOE epsilon 4 mice (P < 0.0001). ApoE immunoreactivity was increased within the neuropil and glia to the same extent in APOE epsilon 3 and APOE epsilon 4 mice post-ECL. There was no significant difference in the deposition and clearance of degeneration products between APOE epsilon 3 and epsilon 4 mice post-ECL. These results indicate that neuronal plasticity is impaired in transgenic mice possessing human APOE epsilon 4 alleles compared to APOE epsilon 3. These isoform-specific differences in plasticity may relate to the severity of AD and poor, long-term recovery after head injury in APOE epsilon 4 individuals.

Targeting expression of hsp70i to discrete neuronal populations using the Lmo-1 promoter: assessment of the neuroprotective effects of hsp70i in vivo and in vitro

Transgenic technology provides a powerful means of studying gene regulation and specific gene function with complex mammalian systems. In this study, the authors exploited the specific and discrete neuronal expression pattern mediated by promoter 1 of the Lmo-1 gene to study the neuroprotective effects of the inducible form of heat shock protein 70kD (hsp70i) in primary hippocampal cultures in a mouse model of global cerebral ischemia. Targeting expression of hsp70i to hippocampal neurons protected these cells significantly from toxic levels of glutamate and oxidative stress (for example, exposure to 10 micromol/L free iron produced a 26% increase in lactate dehydrogenase release from neurons cultured from wild-type mice, but a 7% increase in neurons cultured from hsp70i transgenic mice). Bilateral carotid occlusion (25 minutes) produced significantly less neuronal damage in the caudate nucleus and posterior thalamus in hsp70i transgenic mice than in wild-type littermates (for example, 21% +/- 9.3% and 12.5% +/- 9.0% neuronal damage in lateral caudate nucleus of wild-type and hsp70i transgenic mice, respectively, P < 0.05). The current study highlights the utility of targeted expression of transgenes of interest in cerebral ischemia and demonstrates that expression of hsp70i alone is sufficient to mediate the protection of primary neurons from denaturing stress and that expression of human hsp70i in vivo plays crucial role in determining the fate of neurons after ischemic challenge.

Alterations in ApoE and ApoJ in Relation to Degeneration and Regeneration in a Mouse Model of Entorhinal Cortex Lesion

Apolipoproteins are primarily involved in the transport of lipid and cholesterol within the central nervous system (CNS) and are thought to play a role in synaptic remodeling, repair, and regeneration after brain injury. In the present study, alterations in apolipoproteins E (apoE) and J (apoJ) were examined in the molecular layers of the dentate gyrus after unilateral chemical lesioning of the entorhinal cortex (ECL), at days 0, 1, 3, 7, 28, and 90 days following injury. Alterations in immunostaining for these proteins were assessed in relation to accumulation of silver-labeled degeneration products and alterations in synaptophysin and GAP-43 immunoreactivity. Quantitative analysis of synaptophysin and GAP-43 immunostaining highlighted synaptic loss and fiber degeneration initially (3-7 days post-ECL), with subsequent terminal sprouting and reactive synaptogenesis occurring at longer survival periods (28-90 days post-ECL). Increased apoE and apoJ immunoreactivity was evident first within the neuropil (*P < 0.05 and **P < 0.01) followed by intense glial staining by day 7 post-ECL. By day 28 apoE and apoJ immunostaining had returned almost to baseline levels. However, at day 90 post-ECL, neuropil apoE and apoJ immunoreactivity was dramatically increased compared to contralateral levels (**P < 0.01 and ***P < 0.0001, respectively). Silver-labeled degeneration products were found to be in abundance at day 3 postlesion; however, by day 7 this was reduced leaving only a thin band of material within the MML and at day 90 post-ECL, dentate silver staining was similar to that of controls. The results indicate that apoE and apoJ are upregulated after injury and parallel clearance of cholesterol and lipid debris from the site of injury. This coordinated alteration in apolipoproteins may redistribute lipid material to sprouting fibers to promote neurite extension and may play an important role in long-term plasticity changes following injury.

Increased neuronal damage and apoE immunoreactivity in human apolipoprotein E, E4 isoform-specific, transgenic mice after global cerebral ischaemia

Apolipoprotein E (apoE, protein; APOE, gene) is expressed as three isoforms in humans (E2, E3, E4). The APOE-epsilon4 allele is associated with a poor outcome in patients after head injury of which ischaemic brain damage is a contributor of mortality and morbidity. The aim of the study was to determine whether mice expressing human APOE-epsilon4 displayed more extensive ischaemic neuronal damage 72 h after transient global ischaemia compared with mice which express human APOE-epsilon3. APOE-epsilon3 and -epsilon4 transgenic mice, under the control of a human promoter, were used which express human APOE in neurons and glia. Ischaemic neuronal damage in the CA1 pyramidal cell layer in the APOE-epsilon4 transgenic mice was significantly greater than in the APOE-epsilon3 mice after global ischaemia (36.4+/-8.9%, 18.2+/-7.3%; P<0.05). This was associated with more extensive neuronal apoE immunoreactivity in the CA1 pyramidal cell layer in the APOE-epsilon4 transgenic mice compared with APOE-epsilon3 transgenic mice. In contrast, in the caudate nucleus, there were similar levels of ischaemic neuronal damage in the APOE-epsilon3 and -epsilon4 transgenic mice (39.2 +/-10.1%; 44.6+/-8.4%, P = 0.32). In the caudate, similar numbers of neurons were immunostained for apoE in the APOE-epsilon3 and -epsilon4 transgenic mice. The present study demonstrated that the APOE-epsilon4 allele is associated with an increased vulnerability of a specific brain region to the effects of global ischaemia, which is closely associated with an increase in neuronal apoE. The data extend previous work and are consistent with an association of the APOE-epsilon4 allele with a poor outcome after acute brain injury in humans.

beta-amyloid (Abeta)42(43), abeta42, abeta40 and apoE immunostaining of plaques in fatal head injury

beta-Amyloid (Abeta) deposits are found in the brains of approximately one-third of patients who die within days after a severe head injury; their presence correlating strongly with possession of an apolipoprotein E (apoE)-epsilon4 allele. The aim of the study was to investigate the relationship between Abeta42, Abeta40 and apoE immunostaining of Abeta plaques in the cerebral cortex and the relevance of apoE genotype in 23 fatally head-injured patients. These cases were known to have Abeta deposits from a previous study in which they were examined and semiquantified and related to apoE genotype. In the present study, the temporal cortex was probed using four different antibodies that recognize Abeta42(43), Abeta40 and an antibody to apoE. Abeta42(43)-positive plaques were observed in all of the 23 cases and Abeta40 immunoreactivity in only 11 of the 23 cases. In addition, semiquantitative analysis showed that relatively fewer plaques were detected with anti-Abeta40 than anti-Abeta42(43). ApoE-immunoreactive plaques were identified in 18 of the 23 cases. The number of plaques stained for apoE was relatively less than for Abeta42(43) but greater than for Abeta40. Furthermore, the density of Abeta plaques detected using either Abeta42(43), Abeta40 or apoE antibodies was associated with possession of apoE-epsilon4 in an allele dose-dependent manner. The results are consistent with Abeta42(43) as the initially deposited species in brain parenchyma and provide evidence that apoE is involved in the early stages of amyloid deposition. Further, the findings may be of relevance to the role of apoE genotype in influencing outcome after acute brain injury.

The role of apolipoprotein E in Alzheimer's disease, acute brain injury and cerebrovascular disease: evidence of common mechanisms and utility of animal models

The epsilon 4 allele of apolipoprotein E (APOE denotes gene; apoE denotes protein) is a major risk factor for Alzheimer's disease (AD). More recent evidence indicates an association with a poor outcome after acute brain injury including that due to head trauma and intracerebral hemorrhage. APOE gene polymorphism also influences the risk of hemorrhage in cerebral amyloid angiopathy. These diverse brain disorders seem to have some mechanisms in common. The multiplicity of the roles of apoE within the central nervous system is currently being unraveled. For example, apoE can interact with amyloid beta-protein and tau, proteins central to the pathogenesis of AD. In addition to these effects, it is proposed that one of the major functions of apoE is to mediate neuronal protection, repair and remodeling. In all of the different roles proposed, there are marked apoE-isoform specific differences. Although it remains to be clarified which is the most important mechanism(s) in each disorder in which apoE is involved, these isoform specific differences seem to underly a genetically determined susceptibility to outcome from acute brain injury and to AD with APOE epsilon 4 conferring relative vulnerability. This review focuses on apoE research, from clinical studies to animal models, in AD, acute brain injury and cerebrovascular disease and explores the common mechanisms that may explain some of the complex underlying neurobiology.

Intraventricular infusion of apolipoprotein E ameliorates acute neuronal damage after global cerebral ischemia in mice

The ability of intraventricular infusion of apolipoprotein E (apoE) to reduce neuronal damage after global cerebral ischemia was investigated in apoE-deficient and wild-type mice. ApoE (5 microg/mL lipid-conjugated derived from human plasma; 1 microL/h, continuous infusion) significantly reduced neuronal damage in the caudate nucleus and CA2 pyramidal cell layer by approximately 50% in apoE-deficient mice after global ischemia compared to vehicle infusion. In wild-type mice infused with apoE, there was a trend for ischemic neuronal damage to be reduced. ApoE-infused mice had a marked reduction in 4-hydroxynonenal immunoreactivity, as a marker of lipid peroxidation. The results show that the presence of apoE at or after the time of injury can be neuroprotective, possibly via an anti-oxidant mechanism.

Apolipoprotein E and the response of the brain to injury

Apolipoprotein E (apoE) is an important part of the means by which lipids are transported in the nervous system. This transport system provides injured nerve cells, the cholesterol and phospholipids for the maintenance and repair of membranes, the growth of neurites, dendritic remodelling and synaptogenesis, and the effect of injury to the nervous system is now known in part to be modulated by the various isoforms of apoE. After the demonstration of an association between the apoE epsilon 4 and increased risk of subsequent development of both sporadic and late-onset form Alzheimer's disease, recent studies have provided additional evidence for the possibility that apoE may play an isoform-specific role in determining both the initial response and the subsequent consequences to acute brain injury. Further studies are required to better understand not only the response(s) of the nervous system to injury, but also the relationship between acute injury to the brain and the subsequent development of neurodegenerative disorders.

Increased neuronal damage in apolipoprotein E-deficient mice following global ischaemia

There is accumulating evidence that apolipoprotein E (apoE) plays a role in regulating the response to and outcome following brain injury. The present study compared the histological outcome and recovery following an episode of global ischaemia in apoE-deficient mice and wild-type littermates (12-week-old males, n = 8 per group). Transient global ischaemia was induced for a period of 17 min and the animals were allowed to recover for 72 h. Transient global ischaemia induced selective neuronal degeneration in several brain regions in wild-type mice. There was statistically significant increased ischaemic neuronal damage in apoE-deficient mice compared with wild-type mice in six of the seven regions examined (hippocampal regions CA1, CA3/CA4 and dentate gyrus; thalamus; cortex and caudate nucleus; P < 0.05). The data substantiate a role for apoE in modifying the response of the CNS to acute injury.

Influence of apolipoprotein E genotype on neuronal damage and apoE immunoreactivity in human hippocampus following global ischemia

Apolipoprotein E (apoE) influences the response to and outcome from brain injury possibly through alterations in neuronal repair mechanisms. This study aimed to determine alterations in neuronal and glial apoE after brain injury in patients and sought to determine whether possession of an apoE-epsilon4 allele influences the degree of apoE immunoreactivity or the degree of neuronal damage following brain injury. ApoE immunoreactivity and neuronal damage were semiquantitatively assessed in the temporal lobe of a group of controls (n = 44) and in a group of patients who had an episode of global ischemia and subsequently died (n = 58, survival ranged from 1 hour to 3 months). There was a significant degree of neuronal damage in all hippocampal sectors and in the neocortex of the global ischemia group compared with controls (p < 0.0001). Glial apoE immunoreactivity was significantly increased in hippocampal sectors (CA1, CA2, CA3/CA4, dentate fascia) in the global ischemia group compared with controls (p < 0.01). Neuronal apoE immunoreactivity was significantly increased in all hippocampal sectors (CA1, CA2, CA3/CA4, dentate fascia) and in the neocortex of the global ischemia group compared with controls (p < 0.0001). There was a significant and positive association between the degree of neuronal apoE immunoreactivity and the degree of neuronal damage in the global ischemia cases (r2 = 0.691, p < 0.001) and there was not an association in the control group. Possession of an apoE-epsilon4 allele did not influence the degree of neuronal or glial apoE immunoreactivity or the degree of neuronal damage in the global ischemia cases or the controls. The data indicate apoE is markedly increased in neurons and glia following brain injury. In this study, apoE genotype did not appear to influence neuronal damage, glial apoE or intraneuronal apoE following injury

Apolipoprotein E and the CNS response to injury

Apolipoprotein E (apoE) is a multifunctional protein with an expanding role in the neurobiology of disease. Although originally described in the context of cholesterol metabolism, interest in the neurobiology of apoE has intensified following the association between apoE genotype and risk of developing Alzheimer's disease. Recent clinical observations also suggest that apoE genotype may influence recovery after a variety of neurological insults. Thus, in addition to the study of disease-specific mechanisms by which apoE may modulate susceptibility of developing Alzheimer's disease, there has been an increasing focus on its role in modulating the CNS response to acute injury. Although the neurobiology of apoE in the injured brain remains incompletely defined, there is evidence to suggest neurotrophic, immunomodulatory, and antioxidant effects.

Marked alterations in the cellular localisation and levels of apolipoprotein E following acute subdural haematoma in rat

Apolipoprotein E (apoE) plays a role in the response to acute brain injury, the mechanisms as yet remain unknown. In the present study, alterations in the immunohistochemical localisation of apoE in rat cortex were examined at 30 min, 2 h or 4 h following production of an acute subdural haematoma. Levels of apoE were determined in cortex by immunoblotting at 30 min and 4 h post-haematoma. Extensive areas of ischaemic cell damage were observed in the cortex underlying the haematoma with minimal damage observed in shams. In sham animals, apoE immunoreactivity was confined to astrocytes and their processes. Following the haematoma induction, apoE immunoreactivity was dramatically altered. At 30 min post-haematoma, intense apoE staining was observed in clusters of neuronal perikarya and the neuropil throughout the cortical layers underlying the haematoma and this persisted at 2 h and 4 h post-haematoma. Additionally, at 4 h post-haematoma marked apoE staining of discrete foci within the neuropil closely associated with capillaries was consistently observed in the ipsilateral cortex. Immunoblotting indicated there were no significant alterations in the cortical levels of apoE at 30 min post-haematoma but, at 4 h post-haematoma, there was a significant elevation (27%, P < 0.001) in the levels of apoE in cortex underlying the haematoma compared to control levels. The results indicate that following acute subdural haematoma, a rapid cellular redistribution of apoE occurs and precedes a significant elevation in the levels of apoE. These alterations in apoE may occur, at least initially, as part of the brain's protective response to injury.

Intracortical glutamate perfusion in vivo induces cellular alterations in specific protein kinase C isoforms and amyloid precursor protein

This study investigated the immunostaining of protein kinase C (PKC) isoforms and amyloid precursor protein (APP) in rat brain cortex and determined alterations following an excitotoxic challenge in vivo. Cellular alterations in APP and PKC isoforms (alpha, beta, gamma, delta, epsilon, and zeta) following glutamate perfusion in the rat parietal cortex were compared with NaCl perfusion. In all animals, two histological zones could be defined consistently, a necrotic core and a boundary zone immediately adjacent to the core. Following glutamate and NaCl perfusion, cellular immunoreactivity to PKC isoforms and amino-terminal APP was significantly reduced within the necrotic core. Striking carboxy-terminal APP immunoreactivity was observed in some neurons remaining within the necrotic core. In the boundary of the glutamate lesion, the perikarya of most neurons were intensely immunoreactive to PKC alpha and beta. Furthermore, within the boundary zone, enhanced immunoreactivity within neuronal perikarya was observed to amino-terminal APP and, to a lesser extent, carboxy-terminal APP. Increased immunostaining of PKC alpha and beta and APP at the boundary zone was a consistent feature of intracortical glutamate perfusion and was not observed following NaCl perfusion. There were minimal alterations in PKC isoforms gamma, delta, epsilon, and zeta, in the boundary region following intracortical glutamate or NaCl perfusion. There was no astrocytic response, as detected by GFAP immunoreactivity, at the boundary zone. These findings indicate that there is a topographical relationship between cellular alterations of specific PKC isoforms and APP following an excitotoxic challenge in vivo.

Selective alterations in the cellular distribution of apolipoprotein E immunoreactivity following transient cerebral ischaemia in the rat

The aim of this study was to examine the cellular localization and alterations of apolipoprotein E (apoE) following a transient ischaemic insult using immunohistochemistry. Transient cerebral ischaemia was induced in Wistar rats by occlusion of both carotid arteries with hypotension followed by reperfusion for 4 h (n = 5), 24 h (n = 5) or 72 h (n = 6). In sham-operated animals (n = 9), the carotids were not occluded. In this model, ischaemia for 15 min results in selective neuronal damage in the caudate nucleus and neocortex (24 h after reperfusion) and the hippocampal CA1 pyramidal cells (72 h after reperfusion) while there is minimal damage in other areas such as the CA3 hippocampal region. In sham animals apoE immunoreactivity was confined to astrocytes and their processes. ApoE immunoreactivity was not altered at 4 h post-ischemic reperfusion. At 24 h reperfusion, intense apoE staining of the cytoplasm of astrocytes and neuropil within the caudate and neocortex was observed and at 72 h reperfusion apoE stained neuronal cell bodies within these regions. Within the CA1 region at 24 h reperfusion, there was increased immunoreactivity of the cytoplasm of astrocytes and the neuropil was more intensely stained compared with sham animals. At 72 h reperfusion, intense apoE staining of pyramidal cell bodies and dendrites was consistently observed in the CA1 region of the hippocampus. In contrast, at 72 h reperfusion, apoE staining of astrocytic processes was dramatically reduced in the CA1 region although GFAP staining indicated their preservation. The results demonstrate that following an ischaemic insult apoE is localized to degenerating neurons and their processes. This may indicate an inherent protective response of cells to injury. Alternatively, the results are consistent with the hypothesis that apoE is synthesized and released by astrocytes and taken up by neurons following injury.

Hyperactivation of signal transduction systems in Alzheimer's disease

A compromise or deregulation in signal transduction cascades could adversely affect cellular functions and possibly contribute to cell death. In recent years, it has become increasingly apparent that pronounced activation of neuronal signal transduction systems is a characteristic of AD brain. There is evidence that signal transduction systems play a role in the formation or development of these pathological features of AD. Aberrant activity and localization of components of signaling mechanisms (growth factors, their receptors, protein kinases, phosphoprotein phosphatases, and phosphoproteins) are closely associated with the intracellular accumulation of PHF, the extracellular deposition of amyloid, and the formation of neuritic plaques in AD brain. In particular, immunohistochemical studies reveal increased levels of neuronal staining for APP, possibly an important growth factor in AD, both in frontal cortex and hippocampus. Anti-APP immunostaining is also associated with the neuritic component of plaques. Additionally, PKC(beta II) immunostaining is increased in the neuronal cell body and neuropil of AD samples, particularly in association with plaques, suggesting a postsynaptic involvement of this enzyme. On the other hand, PKC(beta I) immunostaining is associated with axonal staining particularly in the sprouting neurites of plaques. Sprouting neuritic components of plaques are immunopositive with other growth-associated proteins, such as GAP43, MARCKS, and spectrin. Immunoreactivity of other members of signal transduction systems such as Fos and stathmin are all increased in AD hippocampal neurons. On the other hand, several protein kinases and phosphoproteins were immunolocalized to tangles. Thus, the hyperactivation and dysfunction of signal transduction systems could be involved in the pathogenesis of AD.

Focal increases in [3H]forskolin and [3H]phorbol 12,13-dibutyrate binding in the rat brain following lesions of the medial septum

Quantitative autoradiography of [3H]forskolin binding to GS-adenylate cyclase and [3H]phorbol 12,13-dibutyrate (PDBu) binding to protein kinase C (PKC) was examined 21 days following ibotenate lesion of the rat medial septum. A significant reduction (-19%) in [3H]forskolin binding was observed at the lesion site in the medial septum compared to the sham-treated group. A significant increase in [3H]forskolin binding was demonstrated in the polymorph layer of the detante gyrus (19%) in animals with medial septal lesions whilst in all other brain regions, [3H]forskolin binding remained unaltered post-lesion. [3H]PDBu binding was significantly increased in the superficial layers (I-III) of entorhinal cortex (27%) following lesion of the medial septum, and remained unaltered in all other brain regions post-lesion. The nature and location of the alterations (namely elevations) in ligand binding sites remote from the lesioned area are supportive of plastic modifications of second messenger systems following denervation.

Differential alterations of second messenger systems and cerebral glucose use following excitotoxic lesion of rat cerebral cortex

Quantitative autoradiography of [3H]forskolin and [3H]phorbol 12,13 dibutyrate (PDBu) binding was examined 21 days after unilateral lesioning of the rat visual cortex using ibotenic acid. In the same animals, the functional deficit was assessed using quantitative [14C]-2-deoxyglucose autoradiography. [3H]Forskolin binding was significantly reduced in each layer of the lesioned visual cortex by at least 40% compared to the contralateral hemisphere. Significant reductions in [3H]forskolin binding were observed in the superior colliculus (-15%) and dorsal lateral geniculate body (-12%) ipsilateral to the lesioned cortex. [3H]PDBu binding was significantly reduced in the lesioned visual cortex (layers V-VI) by 34%, compared with the control hemisphere. There were no significant alterations in [3H]DPBu binding in any other brain regions. Following ibotenate-induced lesioning of the visual cortex, glucose use was significantly reduced throughout the lesioned cortex by at least 25% with minor alterations in glucose use in the ipsilateral dorsal lateral geniculate body and superior colliculus. The present study highlights the relative robustness of [3H]PDBu binding compared to [3H]forskolin binding after excitotoxic damage to the cerebral cortex and suggests that [3H]forskolin binding sites are present on cortical efferent fibres.

Alterations of functional glucose use and ligand binding to second messenger systems following unilateral orbital enucleation

Quantitative autoradiography was used to examine the effect of lesioning a well-defined glutamatergic system (retinofugal fibres) on [3H]forskolin binding to Gs-adenylate cyclase and [3H]PDBu (phorbol-12,13-dibutyrate) binding to protein kinase C (PKC) in the rat visual system at 1, 5, 10 and 20 days after unilateral orbital enucleation. Local cerebral glucose utilisation was determined in the same animals using quantitative [14C]2-deoxyglucose autoradiography. At 5 days post-lesion, [3H]forskolin binding sites were significantly reduced in the visually-deprived superior colliculus (-14 +/- 1%) and dorsal lateral geniculate body (-8 +/- 2%), and these reductions persisted until 20 days post-lesion. There were no significant alterations in the amount of [3H]PDBu binding in any region in the visually-deprived hemisphere following enucleation. Function-related glucose use was significantly reduced throughout the visual pathway after enucleation. In this study, there was no conclusive evidence of plastic modifications of second messenger systems in the rat visual system despite a general depression of visual function following lesion of retinofugal fibres.

Autoradiographic imaging of [3H]phorbol 12,13-dibutyrate binding to protein kinase C in Alzheimer's disease

Quantitative autoradiography was used to examine the distribution of [3H]phorbol 12,13-dibutyrate ([3H]PDBu) binding to protein kinase C in the middle frontal and temporal cortices and the hippocampal region of nine control and nine elderly subjects with Alzheimer's disease (AD). AD patients had a clinical diagnosis of the disease that was confirmed neuropathologically by the presence of numerous plaques in the hippocampus and cerebral cortex. Choline acetyltransferase (ChAT) activity was significantly reduced in the middle frontal and temporal cortex and in the hippocampus of AD subjects, with the deficit being greater than 60% of control values. Quantitative autoradiographic analysis of [3H]PDBu binding to protein kinase C revealed a heterogeneous pattern in control brain, being particularly high in superficial layers of the cortex and CA1 of the hippocampus. There were no significant differences between control and AD sections in all areas examined within the middle frontal cortex; e.g., layers I-II control, 491 +/- 46 versus AD, 537 +/- 39 pmol/g of tissue; middle temporal cortex, e.g., layers I-II control, 565 +/- 68 versus AD, 465 +/- 72 pmol/g of tissue; and hippocampal formation, e.g., CA1 control, 511 +/- 28 versus AD, 498 +/- 25 pmol/g of tissue. In a parallel study, [3H]PDBu binding to homogenate preparations of control and AD brain confirmed that there was no significant difference in [3H]PDBu binding in either the particulate or the cytosolic fraction. We have demonstrated in a well-defined population of AD patients that [3H]PDBu binding to protein kinase C remains preserved in brain regions that are severely affected by the neuropathological and neurochemical correlates of AD.

Second messenger systems: functional role in cerebrovascular smooth muscle regulation

The role of two second messenger systems in alterations of cerebrovascular smooth muscle tone was examined in feline cerebral arteries using an in vitro preparation of vessel segments and cortical pial vessels in situ. Forskolin, which is known to activate adenylate cyclase, elicited a concentration-dependent relaxation of arteries preconstricted with prostaglandin F2 alpha (PGF2 alpha) (EC50 was approximately 300 nM). Microapplication of forskolin around individual cortical arteries and arterioles in situ elicited a dose-dependent dilatation. The maximum increase in arteriolar calibre was 54 +/- 4% from pre-injection calibre and EC50 was approximately 100 nM. Phorbol 12,13 dibutyrate (PDBu), which activates protein kinase C, elicited strong contractions of cerebral vessels. In vitro, PDBu contracted vessel segments in a concentration-dependent manner (EC50 was approximately 100 nM). Similarly, PDBu elicited potent dose-dependent constriction of pial arterioles in situ. The maximum response to PDBu was a 37 +/- 5% reduction in arteriolar calibre and the concentration eliciting EC50 was approximately 100 nM. These data provide an assessment to capacity of feline cerebral arteries to dilate and contract in response to adenylate cyclase and protein kinase C activation respectively.

Selective alterations of high affinity [3H]forskolin binding sites in Alzheimer's disease: a quantitative autoradiographic study

Quantitative autoradiographic analysis of high affinity [3H]forskolin binding sites was carried out in postmortem brains from normal controls and patients dying with Alzheimer's disease. Choline acetyltransferase (ChAT) activity and senile plaque formation were also quantified. [3H]Forskolin binding was markedly reduced in all layers of middle frontal gyrus in the Alzheimer brains and the deficit correlated with the deficit in ChAT activity in this area. In the hippocampal region [3H]forskolin binding was no different in the Alzheimer brains compared to controls, although ChAT activity was significantly reduced. There was an inconsistent reduction in [3H]forskolin binding in all layers of middle temporal gyrus which did not correlate with the cholinergic deficit. Significant senile plaque formation was observed in all 3 brain regions examined and [3H]forskolin binding did not correlate with plaque formation in any brain region. Thus, while all 3 brain regions were affected by the pathological correlates of Alzheimer's disease, [3H]forskolin binding was consistently reduced only in frontal cortex.