Adipocytokines and CD34+ Progenitor Cells in Alzheimer's Disease

BACKGROUND: Alzheimer's disease (AD) and atherosclerosis share common vascular risk factors such as arterial hypertension and hypercholesterolemia. Adipocytokines and CD34(+) progenitor cells are associated with the progression and prognosis of atherosclerotic diseases. Their role in AD is not adequately elucidated. METHODS AND FINDINGS: In the present study, we measured in 41 patients with early AD and 37 age- and weight-matched healthy controls blood concentrations of adiponectin and leptin by enzyme linked immunoabsorbent assay and of CD34(+) progenitor cells using flow cytometry. We found significantly lower plasma levels of leptin in AD patients compared with the controls, whereas plasma levels of adiponectin did not show any significant differences (AD vs. control (mean ± SD): leptin:8.9 ± 5.6 ng/mL vs.16.3 ± 15.5 ng/mL;P = 0.038; adiponectin:18.5 ± 18.1 µg/mL vs.16.7 ± 8.9 µg/mL;P = 0.641). In contrast, circulating CD34(+) cells were significantly upregulated in AD patients (mean absolute cell count ± SD:253 ± 51 vs. 203 ± 37; P = 0.02) and showed an inverse correlation with plasma levels of leptin (r =  -0.248; P = 0.037). In logistic regression analysis, decreased leptin concentration (P = 0.021) and increased number of CD34(+) cells (P = 0.036) were both significantly associated with the presence of AD. According to multifactorial analysis of covariance, leptin serum levels were a significant independent predictor for the number of CD34(+) cells (P = 0.002). CONCLUSIONS: Our findings suggest that low plasma levels of leptin and increased numbers of CD34(+) progenitor cells are both associated with AD. In addition, the results of our study provide first evidence that increased leptin plasma levels are associated with a reduced number of CD34(+) progenitor cells in AD patients. These findings point towards a combined involvement of leptin and CD34(+) progenitor cells in the pathogenesis of AD. Thus, plasma levels of leptin and circulating CD34(+) progenitor cells could represent an important molecular link between atherosclerotic diseases and AD. Further studies should clarify the pathophysiological role of both adipocytokines and progenitor cells in AD and possible diagnostic and therapeutic applications

Cholinergic agonists stimulate secretion of soluble full-length amyloid precursor protein in neuroendocrine cells

The AP peptide of Alzheimer disease is derived from the proteolytic processing of the amyloid precursor proteins (APP), which are considered type I transmembrane glycoproteins. Recently, however, soluble forms of full-length APP were also detected in several systems including chromaffin granules. In this report we used antisera specific for the cytoplasmic sequence of APP to show that primary bovine chromaffin cells secrete a soluble APP, termed solAPPcyt, of an apparent molecular mass of 130 kDa. This APP was oversecreted from Chinese hamster ovary cells transfected with a full-length APP cDNA indicating that solAPPcyt contained both the transmembrane and A beta sequence. Deglycosylation of solAPPcyt showed that it contained both N- and O-linked sugars, suggesting that this APP was transported through the endoplasmic reticulum-Golgi pathway. Secretion of solAPPcyt from primary chromaffin cells was temperature-, time-, and energy-dependent and was stimulated by cell depolarization in a Ca2+-dependent manner. Cholinergic receptor agonists, including acetylcholine, nicotine, or carbachol, stimulated the rapid secretion of solAPPcyt, a process that was inhibited by cholinergic antagonists. Stimulation of solAPPcyt secretion was paralleled by a stimulation of secretion in catecholamines and chromogranin A, indicating that secretion of solAPPcyt was mediated by chromaffin granule vesicles, Taken together, our results show that release of the potentially amyloidogenic solAPPcyt is an active cellular process mediated by both the constitutive and regulated pathways. solAPPcyt was also detected in human cerebrospinal fluid. Combined with the neuronal physiology of chromaffin cells, our data suggest that cholinergic agonists may stimulate the release of this APP in neuronal synapses where it may exert its biological function(s). Moreover, vesicular or secreted solAPPcyt may serve as a soluble precursor of A beta

Release of nontransmembrane full-length Alzheimer's amyloid precursor protein from the lumenar surface of chromaffin granule membranes

We previously demonstrated the presence of a soluble form of full- length Alzheimer's amyloid precursor protein (APP) in the lumen of adrenal medullary chromaffin granules (CG). Furthermore, full-length APP is released from CG membranes in vitro at pH 9.0 by an enzymatic mechanism, sensitive to protease inhibitors [Vassilacopoulou et al. (1995) J. Neurochem. 64, 2140- 2146]. In this study, we found that when intact CG were subjected to exogenous trypsin, a fraction of APP was not digested, consistent with an intragranular population of APP. To examine the substrate-product relationship between membrane and soluble full-length APP, we labeled CG transmembrane APP with 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID), a lipophilic probe, specific for membrane-spanning domains of proteins. APP released from the membranes at pH 9.0 was not labeled with [125I]TID. In addition, this APP was not biotinylated in intact CG. Combined, the results indicate that APP released from CG membranes derives from a unique nontransmembrane population of membrane-associated APP, located in the lumenal side of CG membranes. Dithiobis(succinimidylpropionate) (DSP) cross-linking indicated that APP in CG is situated in close proximity with other proteins, possibly with APP itself. APP complexes were also detected under nonreducing conditions, without DSP cross-linking. These results, combined with our previous studies, indicate that full-length APP within CG exists as three different populations: (I) transmembrane, (II) membrane- associated/nontransmembrane, and (III) soluble. The existence of nontransmembrane populations suggests that putative γ-secretase cleavage sites of APP, assumed to be buried within the lipid bilayer, could be accessible to proteolysis in a soluble intravesicular environment