Neurotrophin receptors in Alzheimer's disease

Abnormal APP, cholinergic and cognitive function in Ts65Dn Down's model mice

We evaluated Ts65Dn Down's syndrome mice and their littermates (LM) at 1-2, 4, and 12 months of age to determine amyloid precursor protein (APP)-related cellular and biochemical changes associated with cognitive deficits. Ts65Dn mice showed cognitive deficits in the Morris water maze compared to LM mice at 4 and 12 months of age. Ts65Dn, but not LM mice, developed a septohippocampal cholinergic neuronal degeneration of choline acetyltransferase (ChAT)-positive neurons at 12 months of age. These cellular changes were compensated by increases in ChAT enzyme activity of remaining cholinergic terminals in the hippocampus. By 12 months of age, Ts65Dn mice had elevations of APP protein levels in the hippocampus compared to their LM. At this age, both Ts65Dn mice and their LM abnormally responded to cholinergic muscarinic M1 agonist treatment in terms of hippocampal APP, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) levels compared to young adult C57BL/6 mice. In summary, the Ts65Dn mice show developmental and progressive age-related behavioral deficits, hippocampal APP, and cholinergic pathology. The relatively better cognitive spatial performance in LM compared to Ts65Dn mice suggests that high APP levels combined with progressive degeneration of the cholinergic system are critical to the pathology and cognitive deficits seen in Ts65Dn mice.

Non-Pharmacological Interventions in Cognitively Impaired and Demented Patients - A Comparison with Cholinesterase Inhibitors

The present paper reviews studies examining the effects of non-pharmacological stimulation, i.e. bright light, physical activity and tactile stimulation (touch), on cognition, affective behaviour, and the sleep-wake rhythm of impaired and demented elderly, both in a qualitative (narrative) and quantitative (meta-analytic) manner. An extensive search through eight bibliographic data bases (PubMed, Web of Science, ERIC, PsychINFO, Psyndex, Cinahl, Biological Abstracts and Rehabdata) was performed up to August 2002. The primary criterion for inclusion in this review was that studies provided sufficient data to calculate effect-sizes. In the qualitative analysis, all three types of stimulation appeared to improve cognitive functioning. Disturbances in behaviour react positively to bright light and tactile stimulation. Bright light was also beneficial to sleep. Tactile stimulation had, moreover, a beneficial influence on the patient-caretaker relationship. A comparison was made with several representative papers published since 1991 on the effects of acetylcholinesterase inhibitors on cognition and behaviour with representative papers on non-pharmacological stimulation interventions. Data indicated that improvements in cognition and affective behaviour by non-pharmacological interventions (d' = 0.32) and by cholinesterase inhibitors (d' = 0.31) were of similar effect-size. Possible mechanisms underlying the non-pharmacological stimulation effects are discussed and suggestions offered for future research.

Failure of brain-derived neurotrophic factor-dependent neuron survival in mouse trisomy 16

The neurotrophin, brain derived neurotrophic factor (BDNF), exerts multiple effects on the development and maintenance of the nervous system, including regulating synaptic plasticity and promoting neuron survival. Here we report the selective failure of BDNF-dependent survival in cultured hippocampal neurons from the trisomy 16 (Ts16) mouse, an animal model of Down syndrome. This failure is accompanied by overexpression of a truncated, kinase-deficient isoform (T1) of the BDNF receptor tyrosine receptor kinase B (trkB). Adenovirus-mediated introduction of exogenous full-length trkB into Ts16 neurons fully restored BDNF-dependent survival, whereas exogenous truncated trkB expression in normal, euploid neurons reproduced the Ts16 BDNF signaling failure. Thus, the failure of Ts16 neurons to respond to BDNF is caused by dysregulation of trkB isoform expression. Such a neurotrophin signaling defect could contribute to developmental and degenerative disorders of the nervous system.

Cortico-hippocampal APP and NGF levels are dynamically altered by cholinergic muscarinic antagonist or M1 agonist treatment in normal mice

To determine whether altered cholinergic neurotransmission can modify the long-term secretion of amyloid precursor protein (APP), endogenous levels of APP and nerve growth factor (NGF), we administered a selective M1 muscarinic receptor agonist (RS86) or the muscarinic antagonist, atropine, for 7 days in vivo into young adult mice (C57BL/6j). The levels of NGF and total APP in the hippocampus, frontal cortex, striatum, parietal cortex and cerebrospinal fluid (CSF) were examined by ELISA and Western blot. We found that this repeated i.m. administration of M1 receptor agonist resulted in decreased total APP levels in the hippocampus, frontal cortex and parietal cortex, and increased secreted alpha-APPs levels in the CSF. M1 agonist treatment also resulted in decreased NGF levels in the hippocampus and CSF. These effects of the M1 muscarinic agonist could be blocked by atropine, which by itself elevated tissue levels of total APP. Interestingly, we found that the decrease of total APP in the hippocampus and striatum after M1 agonist treatment inversely correlated with the change in NGF levels. These data suggest that a sustained increased cholinergic, M1-mediated neurotransmission will enhance secretion of alpha-APPs in CSF and adaptively reduce the levels of total APP and NGF in the corticohippocampal regions of normal mice. The dynamic and adaptive regulation linking total APP and NGF levels in normal adult mice is relevant for understanding the pathophysiology of conditions with cholinergic and APP related pathologies, like Alzheimer's disease and Down's syndrome.

Neurotrophic factor strategies for the treatment of Alzheimer disease

Cholinergic neurons in the nucleus basalis of Meynert are reduced early in the course of Alzheimer disease, and the dysfunction of cholinergic neurons is believed to be primarily responsible for cognitive deficits in the disease. Nerve growth factor has a trophic effect on cholinergic neurons and therefore may have some beneficial effects on the cognitive impairment observed in patients with Alzheimer disease. Experimental studies demonstrated that a continuous infusion of nerve growth factor into the cerebroventricle prevents cholinergic neuron atrophy after axotomy or associated with normal aging and ameliorates cognition impairment in these animals. A clinical study in three patients with Alzheimer disease revealed, however, that a long-term intracerebroventricular infusion of nerve growth factor may have certain potentially beneficial effects, but the continuous intracerebroventricular route of administration is also associated with negative side effects that appear to outweigh the positive effects. Several other strategies have been suggested to provide neurotrophic support to cholinergic neurons. In this article, we review the neurotrophic factor strategies for the treatment of Alzheimer disease.

Neurotrophic factors in Alzheimer's and Parkinson's disease brain

The biomedical literature on the subject of neurotrophic growth factors has expanded prodigiously. This essay reviews neurotrophic factors (NTF) and their receptors in Alzheimer's disease (AD) and Parkinson's disease (PD) brain and recent updates on receptor signaling. The hypotheses for specific NTF involvement in neurodegenerative diseases in human and as potential therapy are based mainly on experimental animal and in vitro models. There are wide gaps in information on regional synthesis and cell contents of NTFs and their receptors in human brain. Observations on AD brain indicate increases in NGF and decreases in BDNF in surviving neurons of hippocampus and certain neocortical regions and decreases in TrkA in cortex and nucleus basalis. In PD brain, the few data available indicate decreases in neuronal content of GDNF and bFGF in surviving substantia nigra dopaminergic neurons. There are very few data regarding age-dependent effects on NTFs and on their receptors in human brain. Since NTFs in neurons are subject to retrograde and, in at least some cases, to anterograde transport from and to target neurons, their effects may be related to synthesis in local or remote sites or to changes in axoplasmic transport. Also, certain NTFs and their receptors are found to be expressed in activated glia. Thus, comparative in situ data for transcription levels and protein contents for NTFs and their receptors in both sites of neuronal origin and termination in human brain are needed to understand their potential roles in treating human diseases.

Overexpression of neurotrophin receptor p75 contributes to the excitotoxin-induced cholinergic neuronal death in rat basal forebrain

Both excitotoxicity and altered trophic factor support have been implicated in the pathogenesis of Alzheimer's disease. To determine whether stimulation of p75, the low-affinity receptor for nerve growth factor, contributes to the excitotoxin-induced apoptotic death of cholinergic neurons, we examined the effect of unilateral kainic acid (KA; PBS vehicle, 1.25, 2.5 and 5.0 nmol) administration into rat basal forebrain on neuronal loss and p75 expression. KA (2. 5 nmol) destroyed 43% of Nissl-stained neurons and 70% of choline acetyltransferase (ChAT)-positive neurons 5 days after injection. Agarose gel electrophoresis revealed that KA (2.5 nmol) induced local internucleosomal DNA fragmentation after 6-48 h. Immunohistochemical analysis further showed that KA (2.5 nmol) augmented p75 immunoreactivity at a time when terminal transferase-mediated deoxyuridine trophosphate (d-UTP)-digoxigenin nick end labeling (TUNEL)-positive nuclei were increased. Many fragmented nuclei were co-labeled with ChAT antibody. The chronic administration of anti-rat p75 or the protein synthesis inhibitor, cycloheximide, but not anti-human p75, substantially reduced the KA-induced destruction of cholinergic neurons and the induction of internucleosomal DNA fragmentation. Anti-rat p75, but not cycloheximide, also reversed the spatial memory impairment produced by KA. These findings suggest that overexpression of p75 contributes to the excitotoxin-induced death of rat basal forebrain cholinergic neurons by an apoptotic-like mechanism.