Alzheimer Disease: Alkon DL

Nelson TJ, Sun MK, Hongpaisan J, Alkon DL. · Blanchette Rockefeller Neurosciences Institute, 9601 Medical Center Drive, Rockville, Maryland 20850 USA. · Eur J Pharmacol. · Pubmed #18402935 No free full text.

Abstract: Protein kinase C (PKC) is involved in synaptic remodeling, induction of protein synthesis, and many other processes important in learning and memory. Activation of neuronal protein kinase C correlates with, and may be essential for, all phases of learning, including acquisition, consolidation, and reconsolidation. Protein kinase C activation is closely tied to hydrolysis of membrane lipids. Phospholipases C and A2 produce 1,2-diacylglycerol and arachidonic acid, which are direct activators of protein kinase C. Phospholipase C also produces inositol triphosphate, which releases calcium from internal stores. Protein kinase C interacts with many of the same pathways as insulin; therefore, it should not be surprising that insulin signaling and protein kinase C activation can both have powerful effects on memory storage and synaptic remodeling. However, investigating the possible roles of insulin in memory storage can be challenging, due to the powerful peripheral effects of insulin on glucose and the low concentration of insulin in the brain. Although peripheral for insulin, synthesized in the beta-cells of the pancreas, is primarily involved in regulating glucose, small amounts of insulin are also present in the brain. The functions of this brain insulin are inadequately understood. Protein kinase C may also contribute to insulin resistance by phosphorylating the insulin receptor substrates required for insulin signaling. Insulin is also responsible insulin-long term depression, a type of synaptic plasticity that is also dependent on protein kinase C. However, insulin can also activate PKC signaling pathways via PLC gamma, Erk 1/2 MAP kinase, and src stimulation. Taken together, the available evidence suggests that the major impact of protein kinase C and its interaction with insulin in the mature, fully differentiated nervous system appears to be to induce synaptogenesis, enhance memory, reduce Alzheimer's pathophysiology, and stimulate neurorepair.

Sun MK, Alkon DL. · Blanchette Rockefeller Neurosciences Institute, 9601 Medical Center Drive, Academic & Research Building, The 3rd floor, Rockville, MD 20850, USA. · Recent Pat CNS Drug Discov. · Pubmed #18221200 No free full text.

Abstract: Activity of protein kinase C (PKC) isozymes plays a critical role in various types of learning and memory. In addition, abnormal functions of PKC signal cascades in neurons represent one of the earliest changes in the brains of patients with Alzheimer's disease (AD) and dementia related to ischemic/stroke events. In preclinical studies, inhibition or impairment of PKC activity leads to compromised learning and memory, whereas an appropriate activation of PKC isozymes has been found to enhance learning and memory and/or to produce antidementic effects. The PKC activators not only increase activity of PKC isozymes and thereby restore PKC signaling activity but also reduce the accumulation of neurotoxic amyloid and tau protein hyperphosphorylation in the brain. These observations strongly suggest that PKC pharmacology may represent an attractive area for the development of cognitive therapeutics and agents against dementia in the future.

Alkon DL, Sun MK, Nelson TJ. · Blanchette Rockefeller Neurosciences Institute, 9601 Medical Center Drive, Rockville, MD 20850, USA. · Trends Pharmacol Sci. · Pubmed #17218018 No free full text.

Abstract: There is strong evidence that protein kinase C (PKC) isozyme signaling pathways are causally involved in associative memory storage. Other observations have indicated that PKC signaling pathways regulate important molecular events in the neurodegenerative pathophysiology of Alzheimer's disease (AD), which is a progressive dementia that is characterized by loss of recent memory. This parallel involvement of PKC signaling in both memory and neurodegeneration indicates a common basis for the origins of both the symptoms and the pathology of AD. Here, we discuss this conceptual framework as a basis for an autopsy-validated peripheral biomarker--and for AD drug design targeting drugs (bryostatin and bryologs) that activate PKC isozymes--that has already demonstrated significant promise for treating both AD neurodegeneration and its symptomatic memory loss.

Sun MK, Alkon DL. · Blanchette Rockefeller Neurosciences Institute, Academic & Research Building, Rockville, Maryland 20850, USA. · Drugs Today (Barc). · Pubmed #16894402 No free full text.

Abstract: The existence of links between Alzheimer's disease and diabetes is an important topic currently under active debate. Establishing such links if they exist and defining their common pathogenesis and pathophysiological mechanisms may lead to new concepts and research directions for the pharmacological treatment of Alzheimer's disease and diabetes. Alzheimer's disease is associated with peripheral and central insulin abnormalities. Cognitive capacities are often impaired in patients with diabetes. There are many mechanisms by which insulin-signaling abnormalities may affect clinical and pathological outcome of Alzheimer's disease. Insulin resistance and dysregulation of the degradation of neurotoxic amyloid and insulin appear at the core of the links between Alzheimer's disease and diabetes. Functions and expression of insulysin, an enzyme involved in the degradation of neurotoxic amyloid peptides and insulin, are usually impaired or reduced in Alzheimer's disease and diabetes. The increased occurrence of insulin resistance in Alzheimer's disease suggests that improving insulin effectiveness and insulysin activity may have therapeutic value in Alzheimer's disease patients and therefore is worth intensive investigation.

Sun MK, Alkon DL. · Blanchette Rockefeller Neurosciences Institute, Rockville, MD 20850, USA. · CNS Drug Rev. · Pubmed #16834754 No free full text.

Abstract: Bryostatin-1 is a powerful protein kinase C (PKC) agonist, activating PKC isozymes at nanomolar concentrations. Pharmacological studies of bryostatin-1 have mainly been focused on its action in preventing tumor growth. Emerging evidence suggests, however, that bryostatin-1 exhibits additional important pharmacological activities. In preclinical studies bryostatin-1 has been shown at appropriate doses to have cognitive restorative and antidepressant effects. The underlying pharmacological mechanisms may involve an activation of PKC isozymes, induction of synthesis of proteins required for long-term memory, restoration of stress-evoked inhibition of PKC activity, and reduction of neurotoxic amyloid accumulation and tau protein hyperphosphorylation. The therapeutic potential of bryostatin-1 as a CNS drug should be further explored.

Nelson TJ, Alkon DL. · Blanchette Rockefeller Neurosciences Institute, Rockville, MD 20850, USA. · Biochem Soc Trans. · Pubmed #16246039 No free full text.

Abstract: Insulin and cholesterol play important roles in basic metabolic processes in peripheral tissues. Both insulin and cholesterol can also act as signalling molecules in the central nervous system that participate in neuronal function, memory and neurodegenerative diseases. A high-cholesterol diet improves spatial memory in experimental animals. beta-Amyloid, the toxic peptide in neurons of AD (Alzheimer's disease) patients, binds cholesterol and catalyses its oxidation to 7beta-hydroxycholesterol, a highly toxic oxysterol that is a potent inhibitor of alpha-PKC (alpha-protein kinase C), an enzyme critical in memory consolidation and synaptic plasticity and implicated in AD. Oxidized cholesterol also can act as a second messenger for insulin. Oxidized low-density lipoprotein inhibits insulin-dependent phosphorylation of the signalling kinases ERK (extracellular-signal-regulated kinase) and PKB/Akt. In sporadic AD patients, insulin levels are decreased, suggesting links between AD and diabetes. Insulin signalling is also important in synaptic plasticity. Insulin receptors are up-regulated and undergo translocation after spatial learning. Insulin modulates the activity of excitatory and inhibitory receptors including the glutamate and gamma-aminobutyric acid receptors and activates two biochemical pathways: the shc-ras-mitogen-activated protein kinase pathway and the PI3K (phosphoinositide 3-kinase)/PKC pathway, both of which are involved in memory processing. These findings point to a convergence at the biochemical level between pathways involved in AD and those important for normal memory.

Sun MK, Alkon DL. · Blânchette Rockefeller Neurosciences Institute, West Virginia University, Johns Hopkins Academic and Research Building, Room 319, 9601 Medical Center Drive, Rockville, MD 20850, USA. · Trends Pharmacol Sci. · Pubmed #11830265 No free full text.

Abstract: Enhancement of memory acquisition and recall represents an important pharmacological goal in the treatment of cognitive disorders. In addition to its involvement in pH regulation, HCO3- reabsorption and CO2 expiration, carbonic anhydrase plays a crucial role in signal processing, long-term synaptic transformation and attentional gating of memory storage. Carbonic anhydrase dysfunction impairs cognition and is associated with mental retardation, Alzheimer's disease and aging. The pharmacological profile of carbonic anhydrase has been refined and specific activators have been developed. In this article, an integrated view of the involvement of carbonic anhydrase activity in synaptic plasticity and cognition will be presented, with particular focus on attentional gating of spatial learning and memory.

Alkon DL. · Laboratory of Adaptive Systems, Basic Neuroscience Program, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA. · J Neurosci Res. · Pubmed #10491569 No free full text.

Abstract: Electrical and chemical signals representing macroscopic "perturbations" in brain networks engage large numbers of transient "microscopic" ionic channel fluctuations in producing long-lasting changes of conductance (and thus potential). Repeated electrical and chemical signals that occur during associative training of living organisms (from mollusc to mammal) can cause ionic conductance changes lasting from days to many weeks. If a stimulus pattern reoccurs with sufficient frequency, voltage-dependent K(+) conductances-responsible for both synaptic and intrinsic membrane currents-become progressively less probabilistic and more deterministic. In effect, more deterministic ion channel functions record in associative memory more deterministic (i.e., higher probability) events in the environment. This memory has been found to be stored within brain networks as ensembles of local dendritic ionic conductance changes distributed throughout brain regions such as the hippocampus and cerebellar cortex. Numerous other studies taken together support the hypothesis that distributed dendritic loci store associative memory, do not involve long-term potentiation, are also loci for Alzheimer's disease (AD) pathophysiology, and can contribute to, if not be responsible for, early memory loss in clinically manifest AD. J. Neurosci. Res. 58:24-32, 1999. Published 1999 Wiley-Liss, Inc.

Khan TK, Alkon DL. · Blanchette Rockefeller Neurosciences Institute, Rockville, MD 20850, USA. · Proc Natl Acad Sci U S A. · Pubmed #16920798 links to  free full text

Abstract: Cognitive impairment has recently been found to correlate with changes in peripheral inflammatory signals such as TNF-alpha and IL-1beta. PKC isozymes regulate levels of TNF-alpha and IL-6 and the release of other cytokines and also show deficits in Alzheimer's disease (AD) brains and skin fibroblasts. Here, we investigate MAPK Erk1 and Erk2 phosphorylation in response to the inflammatory agonist bradykinin, which activates PKC pathways. An internally controlled comparison of Erk1 and Erk2 produced an AD index that accurately distinguished fibroblasts of AD from those of normal controls and of non-AD dementias. This accuracy was demonstrated for Coriell Cell Repository (Coriell Institute of Medical Research, Camden, NJ) samples, as well as for samples analyzed on gels with autopsy diagnostic confirmation. AD Erk1 and Erk2 index values were inversely correlated with disease duration, suggesting maximal efficacy for early diagnosis. Finally, the results also demonstrate that, when the AD index agreed with the clinical diagnosis on the presence of AD, there was a high probability of accuracy based on autopsy validation. Thus, this peripheral molecular biomarker, based on differential Erk1 and Erk2 phosphorylation, could have important clinical utility for providing increased certainty in the positive diagnosis of AD, particularly in the early phase of disease progression.

Zohar O, Pick CG, Cavallaro S, Chapman J, Katzav A, Milman A, Alkon DL. · Blanchette Rockefeller Neurosciences Institute, Johns Hopkins University Academic and Research Building, MD 20850, USA. · Neurobiol Aging. · Pubmed #15917100 No free full text.

Abstract: Plaques found in the brains of patients suffering from Alzheimer's disease (AD) mainly consist of beta-amyloid (Abeta), which is produced by sequential cleaving of amyloid precursor protein (APP) by two proteolytic enzymes, beta- and gamma-secretases. Any change in the fine balance between these enzymes and their substrate may contribute to the etio-pathogenesis of AD. Indeed, the protein level and enzymatic activity of beta-secretase (BACE), but not its mRNA level, were found elevated in brain areas of AD patients who suffer a high load of Abeta plaque formation. Similarly, increased BACE activity but no mRNA change was observed in a transgenic mouse model of AD, tg2576, in which over expression of the Swedish mutated human APP leads to Abeta plaque formation and learning deficits. Based on the recent demonstration of four BACE splice variants with different enzymatic activity, the discrepancy between BACE activity and mRNA expression may be explained by the altered BACE alternative splicing. To test this hypothesis, we studied the expression of all BACE splice variants in different brain areas of tg2576 mice at age of 4 months and 1 year old. We found developmental and regional differences between wild-type and tg2576 mice. Our results indicate that over expression of APP in tg2576 mice leads to the altered alternative splicing of BACE and the increase of its enzymatically more active splice variant (I-501).

Nelson TJ, Alkon DL. · Blanchette Rockefeller Neurosciences Institute, Rockville, Maryland 20850, USA. · J Biol Chem. · Pubmed #15591071 links to  free full text

Abstract: Alzheimer's disease (AD) is characterized by accumulation of the neurotoxic peptide beta-amyloid, which is produced by proteolysis of amyloid precursor protein (APP). APP is a large membrane-bound copper-binding protein that is essential in maintaining synaptic function and may play a role in synaptogenesis. beta-Amyloid has been shown to contribute to the oxidative stress that accompanies AD. Later stages of AD are characterized by neuronal apoptosis. However, the biochemical function of APP and the mechanism of the toxicity of beta-amyloid are still unclear. In this study, we show that both beta-amyloid and APP can oxidize cholesterol to form 7beta-hydroxycholesterol, a proapoptotic oxysterol that was neurotoxic at nanomolar concentrations. 7beta-Hydroxycholesterol inhibited secretion of soluble APP from cultured rat hippocampal H19-7/IGF-IR neuronal cells and inhibited tumor necrosis factor-alpha-converting enzyme alpha-secretase activity but had no effect on beta-site APP-cleaving enzyme 1 activity. 7beta-Hydroxycholesterol was also a potent inhibitor of alpha-protein kinase C, with a K(i) of approximately 0.2 nm. The rate of reaction between cholesterol and beta-amyloid was comparable to the rates of cholesterol-metabolizing enzymes (k(cat) = 0.211 min(-)1). The rate of production of 7beta-hydroxycholesterol by APP was approximately 200 times lower than by beta-amyloid. Oxidation of cholesterol was accompanied by stoichiometric production of hydrogen peroxide and required divalent copper. The results suggest that a function of APP may be to produce low levels of 7-hydroxycholesterol. Higher levels produced by beta-amyloid could contribute to the oxidative stress and cell loss observed in Alzheimer's disease.

Sun MK, Alkon DL. · Blanchette Rockefeller Neurosciences Institute, Rockville, MD 20850, USA. · J Alzheimers Dis. · Pubmed #15345805 No free full text.

Abstract: Alzheimer's disease (AD) is characterized by amyloid plaques, neurofibrillary tangles in the brain, cerebral hypoperfusion/hypometabolism, and amyloid angiopathy. The former two and cell loss occur late in the disease and are probably not the leading causes for the initial memory decline. Cerebral hypoperfusion is a pre-clinical event in AD and represents the most accurate indicator predicting the probable AD patients to develop AD in a future time. However, in young animals, cerebral hypoperfusion as those matching the reduction in AD has no significant effects on learning and memory. Here, we report that association of cerebral hypoperfusion (2-vessel occlusion) with cerebrovascular amyloid (internal-carotid 0.5 mg beta(25-35), an active fragment of Abeta) significantly impaired spatial learning and memory of young adult rats, while neither the same insult alone had significant impact. At the time when the spatial memory was impaired, in vitro recording revealed that the associated cerebral hypoperfusion and internal-carotid amyloid reduced the ability of the hippocampal CA1 network to generate cholinergic theta and the synaptic modification evoked by associative activation of cholinergic and GABAergic inputs. The results suggest that cerebral hypoperfusion and amyloid angiopathy may play an important role as associated events in initiating the early memory decline in AD.

Etcheberrigaray R, Tan M, Dewachter I, Kuipéri C, Van der Auwera I, Wera S, Qiao L, Bank B, Nelson TJ, Kozikowski AP, Van Leuven F, Alkon DL. · NeuroLogic, Inc., Rockville, MD 20850, USA. · Proc Natl Acad Sci U S A. · Pubmed #15263077 links to  free full text

Abstract: Alzheimer's disease (AD) characteristically presents with early memory loss. Regulation of K(+) channels, calcium homeostasis, and protein kinase C (PKC) activation are molecular events that have been implicated during associative memory which are also altered or defective in AD. PKC is also involved in the processing of the amyloid precursor protein (APP), a central element in AD pathophysiology. In previous studies, we demonstrated that benzolactam (BL), a novel PKC activator, reversed K(+) channels defects and enhanced secretion of APP alpha in AD cells. In this study we present data showing that another PKC activator, bryostatin 1, at subnanomolar concentrations dramatically enhances the secretion of the alpha-secretase product sAPP alpha in fibroblasts from AD patients. We also show that BL significantly increased the amount of sAPP alpha and reduced A beta 40 in the brains of APP[V717I] transgenic mice. In a more recently developed AD double-transgenic mouse, bryostatin was effective in reducing both brain A beta 40 and A beta 42. In addition, bryostatin ameliorated the rate of premature death and improved behavioral outcomes. Collectively, these data corroborate PKC and its activation as a potentially important means of ameliorating AD pathophysiology and perhaps cognitive impairment, thus offering a promising target for drug development. Because bryostatin 1 is devoid of tumor-promoting activity and is undergoing numerous clinical studies for cancer treatment in humans, it might be readily tested in patients as a potential therapeutic agent for Alzheimer's disease.

Zhao WQ, Feng C, Alkon DL. · Blanchette Rockefeller Neurosciences Institutes, Rockville, MD 20850, USA. · Neurobiol Dis. · Pubmed #14678762 No free full text.

Abstract: The serine/threonine phosphatase 2A (PP2A) has been implicated in the pathogenesis of Alzheimer's disease (AD) due to its important role in regulating dephosphorylation of the microtubule-associated protein tau and mitogen-activated protein (MAP) kinase. In the present study, we show that PP2A was responsible for dephosphorylation of the extracellular signal-regulated kinase 1/2 (Erk1/2) following its activation by BK stimulation. Abnormal gene and protein expressions of PP2A, as well as its activity, were found to contribute to the abnormally prolonged Erk1/2 phosphorylation in the AD fibroblasts. Inhibition of PP2A with okadiac acid produced enhanced and more lasting Erk1/2 phosphorylation after BK stimulation, whereas FK506, an inhibitor of PP2B and FK-binding protein, inhibited the BK-stimulated Erk1/2 phosphorylation. Furthermore, while the phosphorylated Erk1/2 was concentrated in the nucleus of AC cells, it was mainly distributed in the extranuclear compartments of AD cells. These results suggest that the delayed dephosphorylation of Erk1/2 in AD cells following its BK-stimulated activation may be due to deficits of PP2A activity and impaired nuclear translocation of phosphorylated Erk1/2.

Zohar O, Cavallaro S, D'Agata V, Alkon DL. · Blanchette Rockefeller Neurosciences Institute, West Virginia University, Rockville, MD 20850, USA. · Brain Res Mol Brain Res. · Pubmed #12824056 No free full text.

Abstract: Beta-amyloid (Abeta) is formed by sequential cleaving of the amyloid precursor protein by two proteolytic enzymes, beta- and gamma-secretases. Beta-secretase (BACE) is a type I transmembrane aspartic proteinase that is highly expressed in the mammalian brain. Four alternative splice variants of BACE are currently known and each encodes for a protein isoform with a different enzymatic activity. In Alzheimer's disease (AD) patients, the enzymatic activity and protein levels of BACE are increased in the neocortex, suggesting their differential expression may have a role in Abeta plaque formation. We have determined the differential expression of BACE mRNA and its splice variants in eight regions of the rat and two of the human brain. In humans, the frontal cortex which shows Abeta deposition in AD, expressed three-fold more BACE than the cerebellum and four fold more than the rats' frontal cortex both of which do not form Abeta plaques. The highest BACE levels of rats were found in the frontal cortex and less in other areas. Although most human and rat brain regions expressed all four BACE variants, the human cerebellum did not express the I-457 BACE variant. Human and rat frontal cortex expressed high levels of the I-501 and I-457 variants, but I-432 was highly expressed only in the rat. Species-specific differences were evident between human and rat brain areas, suggesting that BACE transcript variants may have different evolutionary conservation. Differential expression of BACE variants may explain the broad spectrum of phenotypic abnormalities and possible pathogenetic mechanisms underlying Alzheimer's disease.

Zhao WQ, Ravindranath L, Mohamed AS, Zohar O, Chen GH, Lyketsos CG, Etcheberrigaray R, Alkon DL. · Laboratory of Adaptive Systems, National Institute of Neurological Disorder and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA. · Neurobiol Dis. · Pubmed #12460556 No free full text.

Abstract: Mitogen-activated protein kinases (such as Erk1/2) regulate phosphorylation of the microtubule-associated protein tau and processing of the amyloid protein beta, both events critical to the pathophysiology of Alzheimer's disease (AD). Here we report that enhanced and prolonged Erk1/2 phosphorylation in response to bradykinin (BK) was detected in fibroblasts of both familial and sporadic AD, but not age-matched controls (AC). The AD-associated abnormality in Erk1/2 phosphorylation was not seen in fibroblasts from Huntington's disease patients with dementia. The elevation of Erk1/2 phosphorylation occurred immediately after BK stimulation and required an IP3-sensitive Ca(2+) release as well as activation of PKC and c-src as upstream events. Treatment of cells with the PI-3 kinase blocker LY924002 partially inhibited the BK-stimulated Erk1/2 phosphorylation in AC, but had no effect in AD cells, suggesting that the BK-induced Erk1/2 phosphorylation in AD cells is independent of PI-3 kinase. Activation of the cAMP-responsive element binding protein (CREB) monitored as an increase in phosphorylation at Ser-133 was also observed after BK stimulation. Unlike the AD-specific differences for Erk1/2, however, the BK-stimulated CREB phosphorylation was not different between AC and AD cells. Abnormal Erk1/2 activities may alter downstream cellular processes such as gene transcription, amyloid precursor protein processing, and tau protein phosphorylation, which contribute to the pathogenesis of AD. Moreover, detection of AD-specific differences in MAP kinase in peripheral tissues may provide an efficient means for early diagnosis of AD as well as help us to identify therapeutic targets for drug discovery.

Elbaum D, Brzyska M, Bacia A, Alkon DL. · Laboratory of Biophysical Methods, Nencki Institute of Experimental Biology, Warsaw, Poland. · Biochem Biophys Res Commun. · Pubmed #10673360 No free full text.

Abstract: Alzheimer disease (AD) is a heterogeneous disorder with a variety of molecular pathologies converging predominantly on abnormal amyloid deposition particularly in the brain. beta-Amyloid aggregation into senile plaques is one of the pathological hallmarks of AD. beta-Amyloid is generated by a proteolytic cleavage of a large membrane protein, amyloid precursor protein (APP). We have observed a new property of beta-amyloid. The amyloid 1-42 beta fragment, when aggregated, possesses proteolytic and esterase-like activity, in vitro. Three independent methods were used to test the new property of beta-amyloid. While esterase activity involves imidazole catalysis, proteolytic activity is consistent with participation of a serine peptidase triad: catalytic Ser, His and Glu (or Asp). Although the amino acid triad is a necessary requirement for the protease reactivity, it is not sufficient since the secondary structure of the protein significantly contributes to the proteolytic activity. The ability of beta-amyloid to cleave peptide or ester bonds could be thus responsible for either inactivation of other proteins and/or APP proteolysis itself. This property may be responsible for early pathogenesis of AD since there is emerging evidence that non-plaque amyloid is elevated in Alzheimer patients.