Amyloid precursor protein in the cerebral cortex is rapidly and persistently induced by loss of subcortical innervation.

Abstract

Lesons of the cholinergic nucleus basalls of Meynert elevate the ex Wivo synthesis of ( amyloid precursor protein (f-APP) In the cerebral cortex, a major projection region. We have found that this elevation is reflc by Icased levels of f-APP mRNA. The induction is rapid (occurring 60 min after placement of the lsion) and perstent (remaning for at least 45 days after esloning (-APP in that numerous other proteins, including glial fibrillary acidic protein, were not affected.We have now characterized this lesion-induced (-APP expression more completely and report that (i) the elevated ex vivo synthesis is due to increased levels of(3-APP mRNA; (ii) the (-APP response to the lesion is rapid (exhibited within 60 min of lesioning) and persistent (remaining as long as 45 days post-lesion); (iii) the induction is reversible, requiring the attenuation of neurotransmitter release but not the loss of the subcortical neurons; (iv) subcortical lesions of the cortically projecting noradrenergic and serotonergic systems similarly induce cortical (-APP expression; and, finally, (v) other general perturbations of central nervous system function fail to induce the elevated (-APP synthesis response. These results suggest a cause and effect relationship between lesioninduced neurotransmitter deficits and (-APP induction, providing a possible linkage between subcortical neurotransmitter system deficits and amyloid deposition in the AD brain. METHODSPlacement of Lesions. Adult (-8 weeks old) male SpragueDawley rats (t225-250 g) purchased from Charles River Breeding Laboratories were subcortically lesioned at the following sites: (i) unilateral lesions of the nbM with N-methyl-D-aspartate (NMDA, 50 nM) as the excitotoxin as described (3); (ii) unilateral lesions of the ascending noradrenergic bundle (ANB) with 6-hydroxydopamine (2 p1 of a 4 iLg/ml solution) as described (4); (iii) dorsal raphe nucleus (DRN) lesions with 5,7-dihydroxytryptamine (50 mM) as described (5). Controls for the nbM and ANB lesions were the contralateral cortices, whereas controls for the DRN lesions were sham-operated animals. This latter shamoperated group also served as controls for any nonspecific contralateral effects produced by the unilateral nbM and ANB lesions.TransIent Inhibition of Corticl Acetylchoine (ACh) Release. A total of 12 male Long Evans hooded rats (300-325 g) was used in this experiment. Each rat was anesthetized with chloral hydrate (400 mg/kg, i.p.) and was positioned in a Kopfstereotaxic apparatus. A 28-gauge cannula was lowered into the region of the nbM (Bregma -0.5, ±3.0, -7.7). One-half of the rats were randomly assigned to group I and also received a 2-mm microdialysis probe (CMA/12; Bioanalytical Systems, West Lafayette, IN) that was stereotaxically directed at the frontal cortex at coordinates Bregma

A critical analysis of the ‘amyloid cascade hypothesis’

“…Hence, experimental lesions that damage the nucleus basalis in rat brain elevate APP synthesis in the cerebral cortex suggesting APP production is a specific response to loss of functional innervation of the cortex [138]. Chemically induced lesions of the brain produce similar results.…”

“…Chemically induced lesions of the brain produce similar results. For example, lesions of the nucleus basalis using N-methyl D-aspartate (NMDA) elevate APP synthesis in cortical polysomes [138] and, in areas of brain damaged by kainite [70], APP was recorded in dystrophic neurites adjacent to the lesion. In addition, intrathecal or intraparenchymal injections of a toxin induced the formation of APP in hippocampal neurons subsequent to neuronal damage [68].…”

Amyloid β‐Peptide Inhibits High‐Affinity Choline Uptake and Acetylcholine Release in Rat Hippocampal Slices

“…Earlier studies have reported that lesioning of the basal forebrain cholinergic neurons or a transient inhibition of cortical ACh release elevates the synthesis of APP in the cerebral cortex (Iverfeldt et al, 1993;Wallace et al, 1993). Furthermore, the agonist-induced activation of muscarinic m 1 and m5 receptor subtypes, which are coupled to protein kinase C transduction pathways, has also been shown to increase the secretion of soluble APP derivatives along with a reduction in the amyloidogenic A/I peptides (Buxbaum et al, 1992;Nitsch et al, 1992).…”

Secreted beta-amyloid precursor protein stimulates mitogen-activated protein kinase and enhances tau phosphorylation.

“…Cleavage at another site results in secretion of the extracellular domain (secreted (-APP), which appears in the medium of cultured cells and in cerebrospinal fluid (2)(3)(4). Amounts of (-APP increase during neuronal differentiation (5)(6)(7) and in response to injury (8)(9)(10), suggesting possible physiologic roles for this molecule. Biological activities have been attributed to A(3 (11)(12)(13) and to secreted (-APP (14)(15)(16)(17)(18).…”

“…Following treatment, cells were washed twice with ice-cold phosphatebuffered saline with 1 mM sodium orthovanadate and then scraped into 100 A4 of immunoprecipitation buffer (50 mM NaCl/1 mM EDTA/5 mM EGTA/10 mM Tris HCl, pH 7.6) with 0.2% Nonidet P-40, 0.1% deoxycholate, and protease and phosphatase inhibitors (1 mM phenylmethylsulfonyl fluoride, 1 mM benzamidine, 10 pg of aprotinin per ml, 10 pg of pepstatin A per ml, 10 ug of leupeptin per ml, 1 mM sodium orthovanadate, 10 mM sodium pyrophosphate, 20 mM sodium fluoride). Lysates were centrifuged to remove cellular debris, normalized to 50-100 pug of protein (42), and precipitated for 1 hr at 0°C with 2.5 j4 of anti-MAPK/ERK-1 antiserum kindly provided by J. Blenis (27).…”