I'd like to put forward the hypothesis that Aβ may compete with pleiotrophin (heparin-binding growth associated molecule) for binding to VEGF165. Yang and colleagues (1) report that binding of Aβ to VEGF165 inhibits both Aβ-induced formation of reactive oxygen species and Aβ aggregation. Pleiotrophin is found to be upregulated on PTEN depletion and also enhances GABAA signaling (2,3). In view of the fact that presenilin-deficient cells and AD brain have reduced PTEN levels and pleiotrophin is found in amyloid plaques in AD and DS (4,5), it would seem we may expect amyloid deposition and enhanced GABAA signaling in both disease states. The study by Craig Garner and colleagues reporting normal cognition in their mouse model following the use of GABAA antagonists is most interesting and would seem to indicate benefit for those with AD as well. It will be interesting to see the results of the clinical trials. Perhaps the study by Nabekura et al. finding that DHA inhibits the GABA response may help to explain the cognitive benefit reported by Cole and Frautschy (6,7).

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I'd like to put forward the hypothesis that Aβ may compete with pleiotrophin (heparin-binding growth associated molecule) for binding to VEGF165. Yang and colleagues (1) report that binding of Aβ to VEGF165 inhibits both Aβ-induced formation of reactive oxygen species and Aβ aggregation. Pleiotrophin is found to be upregulated on PTEN depletion and also enhances GABAA signaling (2,3). In view of the fact that presenilin-deficient cells and AD brain have reduced PTEN levels and pleiotrophin is found in amyloid plaques in AD and DS (4,5), it would seem we may expect amyloid deposition and enhanced GABAA signaling in both disease states. The study by Craig Garner and colleagues reporting normal cognition in their mouse model following the use of GABAA antagonists is most interesting and would seem to indicate benefit for those with AD as well. It will be interesting to see the results of the clinical trials. Perhaps the study by Nabekura et al. finding that DHA inhibits the GABA response may help to explain the cognitive benefit reported by Cole and Frautschy (6,7).

Increased pleiotrophin is also reported in multiple myeloma (8). Should my hypothesis be correct, then you'd expect an increased incidence of dementia and AD associated with that disease. Maybe GABAA inhibition is also indicated with possible silencing of pleiotrophin as a first step.

It's interesting that the Forkhead transcription factors FKHRL1 and FKHR are aberrantly localized to the cytoplasm and cannot activate transcription in PTEN-deficient cells and that inactivation of Rho/ROCK signaling is a prerequisite for FKHR nuclear translocation and myoblast fusion (9,10). This may explain part of the benefit of ROCK inhibition. Is incomplete myoblast fusion a feature of AD, and if so, may it explain the gait abnormalities? It's of interest that DYRK1A, which is upregulated in AD brain, phosphorylates tau and also FKHR and reduces FKHR accumulation in the nucleus (10,11).

References:
1. Yang SP, Kwon BO, Gho YS, Chae CB. Specific interaction of VEGF165 with beta-amyloid, and its protective effect on beta-amyloid-induced neurotoxicity. J Neurochem. 2005 Apr;93(1):118-27. Abstract

2. Li G, Hu Y, Huo Y, Liu M, Freeman D, Gao J, Liu X, Wu DC, Wu H. PTEN deletion leads to up-regulation of a secreted growth factor pleiotrophin. J Biol Chem. 2006 Apr 21;281(16):10663-8. Epub 2006 Feb 28. Abstract

3. Pavlov I, Rauvala H, Taira T. Enhanced hippocampal GABAergic inhibition in mice overexpressing heparin-binding growth-associated molecule. Neuroscience. 2006 May 12;139(2):505-11. Epub 2006 Feb 13. Abstract

4. Zhang H, Liu R, Wang R, Hong S, Xu H, Zhang YW. Presenilins regulate the cellular level of the tumor suppressor PTEN. Neurobiol Aging. 2007 Jan 11; [Epub ahead of print] Abstract

5. Wisniewski T, Lalowski M, Baumann M, Rauvala H, Raulo E, Nolo R, Frangione B. HB-GAM is a cytokine present in Alzheimer's and Down's syndrome lesions. Neuroreport. 1996 Jan 31;7(2):667-71. Abstract

6. Nabekura J, Noguchi K, Witt MR, Nielsen M, Akaike N. Functional modulation of human recombinant gamma-aminobutyric acid type A receptor by docosahexaenoic acid. J Biol Chem. 1998 May 1;273(18):11056-61. Abstract

7. Cole GM, Frautschy SA. Docosahexaenoic acid protects from amyloid and dendritic pathology in an Alzheimer's disease mouse model. Nutr Health. 2006;18(3):249-59. Abstract

8. Yeh HS, Chen H, Manyak SJ, Swift RA, Campbell RA, Wang C, Li M, Lee HJ, Waterman G, Gordon MS, Ma J, Bonavida B, Berenson JR. Serum pleiotrophin levels are elevated in multiple myeloma patients and correlate with disease status. Br J Haematol. 2006 Jun;133(5):526-9. Abstract

9. Nakamura N, Ramaswamy S, Vazquez F, Signoretti S, Loda M, Sellers WR. Forkhead transcription factors are critical effectors of cell death and cell cycle arrest downstream of PTEN. Mol Cell Biol. 2000 Dec;20(23):8969-82. Abstract

10. Woods YL, Rena G, Morrice N, Barthel A, Becker W, Guo S, Unterman TG, Cohen P. The kinase DYRK1A phosphorylates the transcription factor FKHR at Ser329 in vitro, a novel in vivo phosphorylation site. Biochem J. 2001 May 1;355(Pt 3):597-607. Abstract

11. Kimura R, Kamino K, Yamamoto M, Nuripa A, Kida T, Kazui H, Hashimoto R, Tanaka T, Kudo T, Yamagata H, Tabara Y, Miki T, Akatsu H, Kosaka K, Funakoshi E, Nishitomi K, Sakaguchi G, Kato A, Hattori H, Uema T, Takeda M. The DYRK1A gene, encoded in chromosome 21 Down syndrome critical region, bridges between beta-amyloid production and tau phosphorylation in Alzheimer disease. AnticaHum Mol Genet. 2007 Jan 1;16(1):15-23. Epub 2006 Nov 29. Abstract

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It's of interest that Zhao and colleagues (1) found increased efflux of thiamine pyrophosphate in leukemia cells overexpressing the reduced folate carrier. The possibility of reduced cellular thiamine pyrophosphate in DS due to the overexpression of RFC1 is very intriguing. Dodd et al. (2) find increased GABAA and reduced NMDA binding sites in some brain areas in a goat model with thiamine deficiency. It would be interesting to see whether administration of thiamine pyrophosphate in the mouse model of DS would restore cognition as do the GABAA antagonists in the Garner study. Thiamine deficiency has been described as a rare cause of reversible pulmonary hypertension and it makes me wonder whether that might explain the increased risk of this disease as well as congenital ASD and VSD in the DS population (3).

References:
1. Zhao R, Gao F, Wang Y, Diaz GA, Gelb BD, Goldman ID. Impact of the reduced folate carrier on the accumulation of active thiamin metabolites in murine leukemia cells. J Biol Chem. 2001 Jan 12;276(2):1114-8. Abstract

2. Dodd PR, Thomas GJ, McCloskey A, Crane DI, Smith ID. The neurochemical pathology of thiamine deficiency: GABAA and glutamate NMDA receptor binding sites in a goat model. Metab Brain Dis. 1996 Mar;11(1):39-54. Abstract

3. Park JH, Lee JH, Jeong JO, Seong IW, Choi SW. Thiamine deficiency as a rare cause of reversible severe pulmonary hypertension. Int J Cardiol. 2007 Mar 6; [Epub ahead of print] Abstract

View all comments by Mary Reid