Prominent microglial activation in cortical white matter is selectively associated with cortical atrophy in primary progressive aphasia.

AIMS: Primary progressive aphasia (PPA) is a clinical syndrome characterized by selective language impairments associated with focal cortical atrophy favouring the language dominant hemisphere. PPA is associated with Alzheimer's disease (AD), frontotemporal lobar degeneration (FTLD) and significant accumulation of activated microglia. Activated microglia can initiate an inflammatory cascade that may contribute to neurodegeneration, but their quantitative distribution in cortical white matter and their relationship with cortical atrophy remain unknown. We investigated white matter activated microglia and their association with grey matter atrophy in 10 PPA cases with either AD or FTLD-TDP pathology. METHODS: Activated microglia were quantified with optical density measures of HLA-DR immunoreactivity in two regions with peak cortical atrophy, and one nonatrophied region within the language dominant hemisphere of each PPA case. Nonatrophied contralateral homologues of the language dominant regions were examined for hemispheric asymmetry. RESULTS: Qualitatively, greater densities of activated microglia were observed in cortical white matter when compared to grey matter. Quantitative analyses revealed significantly greater densities of activated microglia in the white matter of atrophied regions compared to nonatrophied regions in the language dominant hemisphere (P < 0.05). Atrophied regions of the language dominant hemisphere also showed significantly more activated microglia compared to contralateral homologues (P < 0.05). CONCLUSIONS: White matter activated microglia accumulate more in atrophied regions in the language dominant hemisphere of PPA. While microglial activation may constitute a response to neurodegenerative processes in white matter, the resultant inflammatory processes may also exacerbate disease progression and contribute to cortical atrophy.

Differential Neurotoxicity Related to Tetracycline Transactivator and TDP-43 Expression in Conditional TDP-43 Mouse Model of Frontotemporal Lobar Degeneration.

Frontotemporal lobar degeneration (FTLD) is among the most prevalent dementias of early-onset. Pathologically, FTLD presents with tauopathy or TAR DNA-binding protein 43 (TDP-43) proteinopathy. A biallelic mouse model of FTLD was produced on a mix FVB/129SVE background overexpressing wild-type human TDP-43 (hTDP-43) using tetracycline transactivator (tTA), a system widely used in mouse models of neurological disorders. tTA activates hTDP-43, which is placed downstream of the tetracycline response element. The original study on this transgenic mouse found hippocampal degeneration following hTDP-43 expression, but did not account for independent effects of tTA protein. Here, we initially analyzed the neurotoxic effects of tTA in postweaning age mice of either sex using immunostaining and area measurements of select brain regions. We observed tTA-dependent toxicity selectively in the hippocampus affecting the dentate gyrus significantly more than CA fields, whereas hTDP-43-dependent toxicity in bigenic mice occurred in most other cortical regions. Atrophy was associated with inflammation, activation of caspase-3, and loss of neurons. The atrophy associated with tTA expression was rescuable by the tetracycline analog, doxycycline, in the diet. MRI studies corroborated the patterns of atrophy. tTA-induced degeneration was strain-dependent and was rescued by moving the transgene onto a congenic C57BL/6 background. Despite significant hippocampal atrophy, behavioral tests in bigenic mice revealed no hippocampally mediated memory impairment. Significant atrophy in most cortical areas due solely to TDP-43 expression indicates that this mouse model remains useful for providing critical insight into co-occurrence of TDP-43 pathology, neurodegeneration, and behavioral deficits in FTLD.SIGNIFICANCE STATEMENT The tTA expression system has been widely used in mice to model neurological disorders. The technique allows investigators to reversibly turn on or off disease causing genes. Here, we report on a mouse model that overexpresses human TDP-43 using tTA and attempt to recapitulate features of TDP-43 pathology present in human FTLD. The tTA expression system is problematic, resulting in dramatic degeneration of the hippocampus. Thus, our study adds a note of caution for the use of the tTA system. However, because FTLD is primarily characterized by cortical degeneration and our mouse model shows significant atrophy in most cortical areas due to human TDP-43 overexpression, our animal model remains useful for providing critical insight on this human disease.

Accumulation and age-related elevation of amyloid-ß within basal forebrain cholinergic neurons in the rhesus monkey.

Basal forebrain cholinergic neurons (BFCN) are selectively vulnerable to damage and loss in a number of neurodegenerative disorders that afflict the elderly, particularly Alzheimer's disease. The reasons for this selective vulnerability remain poorly understood. Given that intraneuronal accumulation of the amyloid-ß peptide (Aß) has been shown to exert deleterious effects on neurons, we tested potential accumulation of Aß within BFCN in rhesus monkeys, which like the human display age-related accumulation of this peptide in plaques. The non-isoform-specific Aß antibodies 1282 and 6E10 and the specific antibodies to 1-40 amino acid isoform of Aß (Aß1-40) and 1-42 amino acid isoform of Aß (Aß1-42) species were used in immunohistochemical experiments of basal forebrain in young and aged rhesus monkeys. All four antibodies visualized cortical plaques in the same sections in which BFCN were examined, in aged but not in young animals. The basal forebrain region within which the BFCN are localized was virtually free of plaques. Appreciable Aß immunoreactivity was present within the nucleus basalis of Meynert-cholinergic cell group 4 (nbM-Ch4), the major component of BFCN, with all antibodies used. Quantitation of optical density indicated significant age-related increases in immunoreactivity in nbM-Ch4 neurons with the Aß1-40 (p<0.002) and 1282 (p<0.03) antibodies. Immunoreactivity for 6E10 displayed a small, non-significant age-related increase in nbM-Ch4 neurons (p>0.05). No age-related changes were detected in Aß1-42 immunoreactivity in these neurons. Unlike the BFCN, cortical neurons within the same sections were virtually devoid of Aß immunoreactivity, particularly with isoform-specific antibodies. Both smooth and granular intraneuronal Aß immunoreactivity, reminiscent of endosomal/lysosomal packaged peptide, were observed within nbM-Ch4 neurons. In some nbM-Ch4 neurons, 1282 immunoreactivity had the appearance of large peptide aggregates. Significant accumulation and age-related increase of Aß in BFCN is likely to interfere with the normal functioning of these neurons. It remains to be determined if similar accumulation of Aß occurs in human BFCN.

Asymmetric TDP-43 distribution in primary progressive aphasia with progranulin mutation.

OBJECTIVE: Primary progressive aphasia (PPA) results from an asymmetric degeneration of the language dominant (usually left) hemisphere and can be associated with the pathology of Alzheimer disease (AD) or frontotemporal lobar degeneration (FTLD). This study aimed to investigate whether the anatomic distribution of TDP-43 inclusions displayed a corresponding leftward asymmetry in a patient with PPA with a mutation in the progranulin gene and FTLD pathology. METHODS: Brain tissue from a 65-year-old patient with PPA and progranulin mutation was analyzed using immunohistochemical methods for TDP-43. Analysis was performed in the superior temporal gyrus, inferior temporal gyrus, inferior parietal lobule, orbitofrontal cortex, entorhinal cortex, and dentate gyrus. Neuronal intranuclear inclusions, neuronal cytoplasmic inclusions, and dystrophic neurites were quantified using modified stereologic analysis. Analysis of variance was used to determine significant effects. RESULTS: All 3 types of inclusions predominated on the left side of analyzed cortical regions. They were also more frequent in language areas than in memory-related areas. CONCLUSION: These results demonstrate a phenotypically concordant distribution of abnormal TDP-43 inclusions in primary progressive aphasia (PPA). This contrasts with PPA cases with Alzheimer pathology where no consistent leftward asymmetry of neurofibrillary degeneration or amyloid deposition has been demonstrated despite the leftward asymmetry of the atrophy, and where neurofibrillary tangles show a greater density in memory than language areas despite the predominantly aphasic phenotype. This case suggests that the TDP-43 inclusions in PPA-frontotemporal lobar degeneration are more tightly linked to neuronal death and dysfunction than neurofibrillary and amyloid deposits in PPA-Alzheimer disease.

Effects of lipids and aging on the neurotoxicity and neuronal loss caused by intracerebral injections of the amyloid-beta peptide in the rat.

The influence of diet and age on the area of lesion and on the neuronal density in the cerebral cortex was studied in rats following local injections of the amyloid-beta peptide (Abeta1-40) in PBS vehicle into the left frontal and cingulate cortices and compared with effects of injections of PBS alone into the corresponding regions of the right hemisphere The experiments were carried out in two groups of animals: one group of young adult rats and a second group of aged rats. Each group of animals, depending on the diet received, was divided into high-cholesterol, high-fat, and a control group. In order to evaluate the interaction of Abeta/PBS-cholesterol and of Abeta/PBS-fat, animals without dietary manipulation receiving Abeta and PBS injection were used as controls. The results showed that the greatest area of lesion was at Abeta injection sites in the high-cholesterol fed group of aged animals. The results also revealed a significant variance in the neuronal density by group and by injection type. Thus, high-cholesterol fed animals showed a greater reduction in neuronal density at Abeta and PBS-injected sites than that seen in the high-fat or control groups. The results also indicate that the loss of neurons at the Abeta injection site exceeds that seen in the PBS-injected area. The greatest reduction in the neuronal density was found at Abeta-injected site in the high-cholesterol fed group of aged animals. In conclusion, our findings indicate an interaction between lipids, age, and Abeta neurotoxicity, and might provide insights into the basic mechanisms involved in a short-term (acute-to-subchronic) response to Abeta peptide.

Amyloid beta peptide-induced cholinergic fibres loss in the cerebral cortex of the rat is modified by diet high in lipids and by age.

The influence of diet and age on the effects of intracerebral injection of beta-amyloid peptide (Abeta1-40) in vehicle phosphate-buffered saline (PBS) and on the effects of vehicle alone on cholinergic fibres of the cerebral cortex was studied in rats. The experiments were carried in two groups of animals: one group of young adult rats and a second group of aged rats. Each group of animals, depending on the diet received, was divided into high-cholesterol, high-fat, and a control diet group. In order to evaluate the interaction of Abeta/PBS-cholesterol and of Abeta/PBS-fat, animals without dietary manipulation receiving Abeta and PBS injection were used as controls. High-cholesterol fed animals showed a statistically significant reduction of 49.62% in the number of cholinergic fibres at the Abeta injection site as compared with that at PBS injection site, while the high-fat and control animals showed a significant reduction of 28.13 and 26.81%, respectively. In all diet groups, the loss of cholinergic fibres caused by Abeta as compared to that caused by PBS injection was significantly greater in aged rats in comparison with that observed in the young animals. Furthermore, the results of a multivariate linear regression model revealed that the greatest reduction in cholinergic fibres was in the high-cholesterol fed animals (35 fibres/mm) as compared with that seen in the high-fat and control animals. A significantly greater reduction was also observed at Abeta injection site (28 fibres/mm) as compared with that caused by PBS injection, and a reduction of 16 cholinergic fibres per mm was found in aged animals as compared to that seen in young adult rats. These results show that high-cholesterol diet enhances the toxicity of Abeta peptide and that this is also age-dependent. Therefore, this study increases the evidences of the role of cholesterol in the pathology of Alzheimer's disease (AD).

Selective age-related loss of calbindin-D28k from basal forebrain cholinergic neurons in the common marmoset (Callithrix jacchus).

A significant number of the cholinergic neurons in the basal forebrain of the primate, but not the rodent brain contain the calcium binding protein calbindin-D28k (CB). Previous experiments in our laboratory have demonstrated a substantial age-related loss of CB from the human basal forebrain cholinergic neurons (BFCN). The present study investigated the possible age-related loss of CB from the BFCN in a non-human primate species, the common marmoset (Callithrix jacchus). Quantitative analysis of matching sections as well as unbiased stereological determination of neuronal number were used in 16 adult marmosets ranging in age between 2 and 15 years. No significant changes were observed in the number of choline acetyltransferase-positive BFCN when a group of young animals (< or =4 years) was compared with a 6-8-year-old group and a 9-15-year-old group. Similarly, no age-related changes were observed in Nissl-stained magnocellular basal forebrain (putatively cholinergic) neurons. In contrast, the BFCN of the two older groups of animals displayed a significant loss of CB. The age-related loss of CB occurred in all sectors of the BFCN, but was greatest in the anterior sector of this cell group. The CB loss was neurochemically specific since the BFCN in the older groups of animals continued to express other markers such as high and low affinity neurotrophin receptors. The age-related loss of CB from the marmoset BFCN was also regionally selective as CB positive neurons in other structures, such as the cerebral cortex and the striatum displayed no apparent age-related changes. These results indicate that the marmoset BFCN display a significant and selective age-related loss of CB reminiscent of that observed in the human. Therefore, the common marmoset represents an appropriate animal model in which the consequences of BFCN CB loss can be investigated in depth. Loss of CB from the aged BFCN is likely to reduce the capacity of these neurons to buffer intracellular calcium and to leave them vulnerable to insults which can result in increased calcium levels. The vulnerability of the CB-negative BFCN in the aged marmoset to various insults which disturb calcium homeostasis remains to be investigated.

GnRH-immunoreactive fiber changes with unilateral amygdala-kindled seizures.

PURPOSE: To assess whether unilateral amygdala seizures are associated with a change in the number and lateral distribution of gonadotropin releasing hormone (GnRH)-staining fibers in the ventromedial hypothalamus of female rats. METHODS: The study compared three groups of female rats: (1) amygdala seizures induced by focal injection of kainic acid (KA); (2) saline injected controls; and (3) nai;ve controls. The animals were sacrificed at 4 weeks in the diestrus phase. GnRH fibers were counted in the ventromedial hypothalamus and compared among groups. RESULTS: GnRH fiber counts were significantly lower in KA than saline and nai;ve animals ipsilaterally but not contralaterally. CONCLUSIONS: This finding may support a potential mechanism by which (1) temporolimbic epilepsy may promote the development of reproductive endocrine disorders and (2) the laterality of the epilepsy may influence the particular nature of the reproductive endocrine disorder.

Selective cell loss in Edinger-Westphal in asymptomatic elders and Alzheimer's patients.

Exaggerated pupillary response to a low concentration of cholinergic antagonists has been suggested as an early marker for Alzheimer's Disease (AD). To examine the anatomic basis of this phenomenon, we determined possible neuropathological changes in the Edinger-Westphal (EW) nucleus, a midbrain neural center with a significant functional role in the control of pupil size. Stereologically determined neuronal numbers within the EW were counted in individuals with pathologically confirmed AD, control cases with no AD-type pathology, and subjects with AD pathology not meeting diagnostic criteria for AD. The EW of AD patients displayed a marked and striking neuronal loss when compared with controls. In contrast, the number of neurons in the somatic portion of the nucleus of the third cranial nerve (NCNIII) remained intact. The EW in brains from clinically normal individuals with evidence of early AD-type pathology also displayed a significant and selective loss of neurons. The magnitude of EW neuronal loss in the latter group was smaller than that observed in AD. These findings suggest that pupillary hypersensitivity in AD may be caused by abnormalities in the EW. Neuronal loss and pathology within the EW in a subpopulation of clinically silent controls with pathologic findings consistent with early-stage AD constitutes a possible explanation for the reported exaggerated pupil response in some normal elderly subjects.

Cyto- and chemoarchitecture of basal forebrain cholinergic neurons in the common marmoset (Callithrix jacchus).

The cyto- and chemoarchitecture of basal forebrain cholinergic neurons (BFCN) was investigated in the lower primate, the common marmoset (Callithrix jacchus). A large population of magnocellular, hyperchromic, and choline acetyltransferase (ChAT)-positive neurons was detected in the marmoset basal forebrain. The distribution of these neurons was similar to those in higher primates. Thus, ChAT-positive neurons were observed in the medial septum (Ch2), the vertical (Ch2) and horizontal (Ch3) limbs of the diagonal band of Broca, and the nucleus basalis of Meynert (Ch4). The Ch4 complex was relatively well differentiated and displayed distinct sectors. We detected anterior (Ch4a, with a medial and a lateral subdivision), intermediate (Ch4i, with a dorsal and a ventral subdivision), and posterior (Ch4p) sectors in the marmoset Ch4. The Ch4i was relatively small while the Ch4p was large. Similar to the rodent, the marmoset Ch1 extended quite a distance posteriorly, and the Ch4p displayed a major interstitial component distributed within the globus pallidus, its medullary laminae, and the internal capsule. Virtually all of the marmoset BFCN displayed acetylcholinesterase activity, and low affinity (p75(NTR)) and high affinity (Trk) neurotrophin receptor immunoreactivity. A majority contained immunoreactivity for calbindin-D(28K) and calretinin. Many of the Ch4 neurons also displayed tyrosine hydroxylase immunoreactivity. The BFCN lacked galanin immunoreactivity, but were innervated by galanin-positive fibers. None of the marmoset BFCN were NADPH-d-positive. Thus, the BFCN display major anatomical and biochemical differences in the marmoset when compared with higher primates. The marmoset BFCN also display many characteristics common to other primates. This fact, combined with the relatively short life span of the marmoset, indicates that this species may be ideal for studies of age-related changes in the BFCN.

Increased susceptibility to generalized seizures after immunolesions of the basal forebrain cholinergic neurons in rats.

We investigated whether basal forebrain cholinergic neurons influence the expression of generalized seizures. Animals received intracerebroventricular injections of saporin (lesioned) or saline (controls) and were tested for susceptibility to flurothyl- or pentylenetetrazole-induced seizures. Lesioned rats had significantly shorter latencies to onset of generalized tonic-clonic seizures than controls. Our findings suggest that basal forebrain cholinergic neurons may participate in the modulation of generalized seizures.

Selective vulnerability of spinal cord motor neurons to non-NMDA toxicity.

We previously reported that alpha-motor neurons in organotypic cultures of rat spinal cord (OTC-SC) are resistant to excitotoxicity induced through NMDA receptors. Here we describe the effects of non-NMDA glutamate receptor agonists kainic acid (KA) and quisqualic acid (QUIS) on motor neurons in OTC-SC. Large ventral horn acetylcholinesterase-positive neurons (VHANs), most of which are motor neurons, were quite sensitive to QUIS and KA toxicity and displayed losses of 95% and 94%, respectively. Small VHANs were reduced by 41% and 61% only. Identical results were obtained in cultures stained for non-phosphorylated neurofilaments. These observations demonstrate that alpha-motor neurons are considerably more sensitive to KA and QUIS than to NMDA toxicity. The proposed excitotoxic mechanism of ALS, therefore, is most likely mediated through non-NMDA glutamate receptors.

Motor neurons are rich in non-phosphorylated neurofilaments: cross-species comparison and alterations in ALS.

The localization and distribution of non-phosphorylated neurofilaments (NP-NF) in the upper and lower motor neurons was investigated in the rat, the common marmoset, the rhesus monkey and man using the SMI-32 antibody. Within the spinal cord of all species studied, the most intense NP-NF immunoreactivity was observed within the ventral horn alpha-motor neurons. Concurrent staining for the cholinergic marker choline acetyltransferase (ChAT) demonstrated that virtually all of the ChAT-positive alpha-motor neurons contain NP-NF immunoreactivity. Although NP-NF staining was also observed in other neurons within the ventral and intermediate horns, these neurons were loosely scattered and contained a considerably lower staining intensity. The only other prominent NP-NF staining in the spinal cord occurred within the neurons of the dorsal nucleus of Clark and the intermediolateral cell column. Phosphorylated neurofilament (P-NF) immunoreactivity was found primarily in neuronal processes. Occasionally, a solitary motor neuron contained weak P-NF immunoreactivity. Within the brainstem, neurons in all cranial nerve motor nuclei contained intense NP-NF immunoreactivity. The distribution and apparent density of NP-NF immunoreactive neurons in these nuclei was virtually identical to that observed for neurons immunoreactive for ChAT. NP-NF immunoreactive neurons of relatively lower intensity were found in many other regions of the brainstem. All of the giant Betz cells of layer (L) V in the motor cortex contained dark NP-NF immunoreactivity. Within the spinal cord of amyotrophic lateral sclerosis (ALS) patients, both Nissl and NP-NF staining demonstrated the dramatic loss of alpha-motor neurons characteristic of this disorder. Some of the remaining motor neurons contained intense P-NF immunoreactivity. These observations suggest that NP-NF immunoreactivity is a good marker for motor neurons in health and disease and may be a useful tool for studies of motor neuron degeneration (MND).

Expressions of amyloid precursor protein, synaptophysin and presenilin-1 in the different areas of the developing cerebellum of rat.

This study reveals the expressions of Alzheimer's disease-related amyloid precursor protein, presenilin-1, and a presynaptic marker protein, synaptophysin, in the archi-, paleo- and neocerebellum during the postnatal development of the rat. The Western blot results demonstrate a gradual increase in the soluble amyloid precursor protein level in the archicerebellum during the first 3 weeks, while in the neo- and paleocerebellum the levels reach a plateau as early as the 1st week. Immunohistochemically, the protein is present in the deep part of the external granule cell layer and the internal granule cell layer in the newborn animal, while in 3-week-old animals the staining appears mainly in the perikarya and dendrites of the Purkinje cells. The level of synaptophysin increases progressively from postnatal day 7 up to 3 weeks in the archi- and paleocerebellum, and up to 6 weeks in the neocerebellum. Immunohistochemically, the amyloid precursor protein staining appears first in the inner part of the molecular layer and in the internal granule cell layer. In a 3-week-old animal, synaptophysin staining is present in all areas of the cerebellar molecular layer and in the internal granule cell layer. The presenilin-1 immunohistochemical reaction appeared equally in the archi-, paleo- and neocerebellum. Much of the staining is present in the glial cells and Purkinje cells. Less immunoreactivity is observed in the Golgi cells and granule cells. It is concluded that the postnatal expressions of soluble and membrane-bound amyloid precursor protein, synaptophysin and presenilin-1 are regulated differently during the ontogenetical development of the archi-, paleo- and neocerebellum of rat. Further, the amyloid precursor protein and presenilin-1 may be present in cells which do not degenerate in Alzheimer's disease.

Focal pathology in the Edinger-Westphal nucleus explains pupillary hypersensitivity in Alzheimer's disease.

Patients who suffer from Alzheimer's disease (AD) and a sub-population of community-dwelling elders show an exaggerated pupillary reaction to dilute tropicamide, a cholinergic antagonist. This finding may serve as an early diagnostic marker of AD. Here we report a likely pathological basis for this hypersensitive pupillary response. Our observations indicate that the Edinger-Westphal nucleus (EW), a known center for the control of pupillary function, is a selective target of Alzheimer pathology early in the course of the disease. In all AD cases examined, the EW contained plaques and tangles. In contrast, the adjacent somatic portion of the oculomotor complex was virtually spared of pathology. Early pathology in the EW is likely to initiate a cascade of events that may give rise to pupillary hypersensitivity.

Abnormalities of neural circuitry in Alzheimer's disease: hippocampus and cortical cholinergic innervation.

Severe pathology in Alzheimer's disease (AD) results in marked disruption of cortical circuitry. Formation of neurofibrillary tangles, neuronal loss, decrease in dendritic extent, and synaptic depletion combine to halt communication among various cortical areas, resulting in anatomic isolation and fragmentation of many cortical zones. The clinical manifestation of this disruption is severe and debilitating cognitive dysfunction, often accompanied by psychiatric and behavioral disturbances and a diminished ability to perform activities of daily living. However, different cortical circuits are not equally vulnerable to AD pathology. In particular, two cortical systems that appear to be involved in the neural processing of memory are selectively vulnerable to degeneration in AD. One consists of connections between the hippocampus and its neighboring cortical structures within the temporal lobe. The second is the cortical cholinergic system that originates in neurons within the basal forebrain and innervates the entire cortical mantle. The circuitry in these systems shows early and severe degenerative changes in the course of AD. The selective vulnerability of these circuits is the probable reason for the early and marked loss of memory observed in these patients. This review presents current knowledge of the general pattern of cortical circuitry, followed by a summary of abnormalities of this circuitry in AD. The cortical circuits that exhibit selective pathology in AD are described in greater detail. Therapeutic implications of the abnormal circuitry in AD are also discussed. For therapies to be effective, early diagnosis of AD is necessary. Future efforts at AD therapy must be combined with an equally intense effort to develop tools capable of early diagnosis of AD, preferably at a preclinical stage before the onset of cognitive symptoms.

Aging renders the brain vulnerable to amyloid beta-protein neurotoxicity.

The formation of fibrillar deposits of amyloid beta protein (Abeta) in the brain is a pathological hallmark of Alzheimer's disease (AD). A central question is whether Abeta plays a direct role in the neurodegenerative process in AD. The involvement of Abeta in the neurodegenerative process is suggested by the neurotoxicity of the fibrillar form of Abeta in vitro. However, mice transgenic for the Abeta precursor protein that develop amyloid deposits in the brain do not show the degree of neuronal loss or tau phosphorylation found in AD. Here we show that microinjection of plaque-equivalent concentrations of fibrillar, but not soluble, Abeta in the aged rhesus monkey cerebral cortex results in profound neuronal loss, tau phosphorylation and microglial proliferation. Fibrillar Abeta at plaque-equivalent concentrations is not toxic in the young adult rhesus brain. Abeta toxicity in vivo is also highly species-specific; toxicity is greater in aged rhesus monkeys than in aged marmoset monkeys, and is not significant in aged rats. These results suggest that Abeta neurotoxicity in vivo is a pathological response of the aging brain, which is most pronounced in higher order primates. Thus, longevity may contribute to the unique susceptibility of humans to Alzheimer's disease by rendering the brain vulnerable to Abeta neurotoxicity.

Relationship between plaques, tangles, and loss of cortical cholinergic fibers in Alzheimer disease.

Recent observations in our laboratory have indicated substantial and systematic regional variations in the loss of cortical cholinergic fibers in Alzheimer disease (AD). Previous attempts to study the relationship between cortical cholinergic loss and the density of cortical pathological lesions have resulted in conflicting findings. Furthermore, most reports have correlated density of plaques and tangles with the residual level of cholinergic innervation rather than its loss. The purpose of the present study was to determine the relationship between loss of cholinergic axons and density of tangles and beta-amyloid (Abeta) deposits in various cortical areas of AD brains. Abeta deposits and tangles were observed throughout the cerebral cortex. Quantitative analysis revealed almost no correlation between loss of cholinergic fibers and the density of Abeta deposits. Qualitative observations revealed similar results when cored and neuritic plaques were considered separately. By contrast, cholinergic fiber loss displayed a significant correlation with the density of tangles (r = 0.52-0.79). However, in a few areas, such as the cingulate cortex, tangle density appeared to be unrelated to the loss of cholinergic fibers. These results indicate that cortical cholinergic denervation in AD is related to cytoskeletal pathology. However, the lack of a perfect relationship with cytoskeletal pathology implicates additional factors in the cholinergic pathology of AD.

Age-related loss of calbindin from human basal forebrain cholinergic neurons.

Loss of basal forebrain cholinergic neurons (BFCN) occurs in many age-related neurological diseases. Although age is the common risk factor in these disorders, no consistent age-related changes have been reported in the human BFCN. We investigated age-related alterations in choline acetyltransferase (ChAT), low-affinity nerve growth factor receptor (p75LNGFR) and calbindin-D28k (CalBP) immunoreactivity in the human BFCN. No significant age-related changes were observed in ChAT or p75LNGFR immunoreactivity. By contrast, normal aging was accompanied by a selective, substantial and significant loss of CalBP immunoreactivity from the BFCN. Other CalBP-positive neurons were unchanged. Loss of the calcium buffering capacity conferred by CalBP may leave the BFCN vulnerable to damage in neurodegenerative disorders.

Retrograde degeneration and colchicine protection of basal forebrain cholinergic neurons following hippocampal injections of an immunotoxin against the P75 nerve growth factor receptor.

Intracerebroventricular injection of 192 IgG antibody against the p75LNGFR rat low affinity nerve growth factor receptor conjugated with saporin, a ribosome inactivating protein, has been shown to destroy the p75LNGFR-expressing cholinergic neurons of the basal forebrain. We injected this immunotoxin into the hippocampus and studied its retrograde effect upon the cholinergic neurons of the medial septum and the vertical limb of the diagonal band of Broca. Seven days after injection, there was a nearly total depletion of cholinergic axons within the hippocampus. This depletion was associated with a marked and significant decrease in the number of cholinergic neurons of the ipsilateral medial septum and the vertical limb of the diagonal band of Broca. At longer survival times, these changes were more pronounced. Parvalbumin-positive, GABAergic neurons within the same areas of the basal forebrain were not affected by immunotoxin injections. Injections of saporin alone had no effect upon cholinergic neurons. Simultaneous injection of colchicine with the immunotoxin resulted in a significant reduction of retrograde degeneration of cholinergic neurons and relative preservation of hippocampal cholinergic axons. These observations suggest that 192 IgG-saporin is transported retrogradely from the hippocampus to the cholinergic neurons in the medial septum and the vertical limb of the diagonal band of Broca and provide a model for retrograde degeneration of basal forebrain cholinergic neurons following cortically based toxic-pathologic processes.