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Alexei Koudinov, Elena Kezlya, Natalia Koudinova, Temirbolat Berezov Amyloid-beta, tau protein, and oxidative changes as a physiological compensatory mechanism to maintain CNS plasticity under Alzheimer's disease and other neurodegenerative conditions. Journal of Alzheimer’s Disease. 2009 18(2): 381-400. Unabridged Notedited Original Author Edition. Available at: http://alzheimercode.blogspot.com
Alzheimer's Amyloid beta and its' precursor molecule APP implicated in the function of peripheral synapses and the neuromuscular junctions (NMJ) pathology
Further investigation is warranted to elucidate the normal role for amyloid beta protein (Abeta) and its’ precursor at peripheral synapses, neuromuscular junctions (NMJ) and NMJ pathologies (ex. inclusion-body myositis, IBM). Two basic observations include the study published more then a decade ago [[48]]. This was the first demonstration of APP and Abeta concentration postsynaptically at human neuro-muscular junctions, that led authors to conclude that “APP may have a role in normal junction biology and possibly in some diseases affecting NMJs”. Another study later showed that APP homolog in Drosophila (APPL) promotes synapse differentiation at the neuromuscular junction [[49]], leading the authors to propose a model “by which APPL, in conjunction with activity-dependent mechanisms, regulates synaptic structure and number”.
Recent comprehensive study performed analyses of neurotransmission in mature neuromuscular synapse of APP deficient mice. This research “found that APP deletion led to reduced paired-pulse facilitation [PPF, a measure of synaptic transmission and synaptic vesicles recycling efficacy] and increased depression of synaptic transmission with repetitive stimulation. Readily releasable pool size and total releasable vesicles were not affected, but probability of release was significantly increased. Strikingly, the amount of asynchronous release, a measure sensitive to presynaptic calcium concentration, was dramatically increased, and pharmacological studies revealed that it was attributed to aberrant activation of N- and L-type Ca(2+) channels.” Authors therefore proposed that APP modulates synaptic transmission at the NMJ by ensuring proper Ca(2+) channel function [[50]].
Another study [[51]] reported on an essential role of APP family of proteins in the development of neuromuscular synapses. “Mice deficient in APP and its homolog APP-like protein 2 (APLP2) exhibited aberrant apposition of presynaptic marker proteins with postsynaptic acetylcholine receptors and excessive nerve terminal sprouting. The number of synaptic vesicles at presynaptic terminals was dramatically reduced. [In agreement with the previous study by Yang L et al.[[Ref. 50]] ], these structural abnormalities were accompanied by defective neurotransmitter release and a high incidence of synaptic failure [[51]]. This is also in accord with the reduction of the synaptic vesicle density, active zone size, and docked vesicle number per active zone in submandibular ganglion synapses of mice lacking APP and APPLP2 demonstrated in lab animals with double gene deletions [[52]].
Another recent report demonstrated that APP is essential in regulating the presynaptic expression and activity of the high-affinity choline transporter (CHT), a molecule that mediates the rate-limiting step of cholinergic synaptic transmission in both the NMJ and central cholinergic neurons. Loss of APP was the cause of the aberrant localization of CHT at the neuromuscular synapses and reduced CHT activity at cholinergic projections. [[53]]
Interestingly, abnormal accumulation of APP and Abeta epitopes (as well as excessively phosphorylated tau in the form of paired helical filaments, PHF, see below) in vacuolated muscle fibers (VMF) is a characteristic feature of not just AD, but also the neuromuscular pathology such as IBM [[54, 55]]. Also important is the observation that in IBM there is an abnormal accumulation of a number of lipoprotein receptors (such as LDLR, VLDLR, LRP) and cholesterol within IBM vacuolated muscle fibers [[54]]. However, LDLR and VLDLR are also expressed at normal NMJ, suggesting “physiologic roles for them in transsynaptic signaling pathways and increased internalization of lipoproteins” at NMJ. Similarly, based on our physiological research with rat hippocampal slices, we earlier proposed that there is an activity-dependent demand for lipoprotein cholesterol and phospholipids at the central synapses [[56]]. Most important, the study by Jaworska-Wilczynska [[54]] confirms that there is a peripheral application of the functional crosstalk between Abeta, lipoproteins and cholesterol, that our own explanation of Abeta involvement in AD is about (see below).
References:
48. V. Askanas, WK. Engel, RB. Alvarez. Strong immunoreactivity of beta-amyloid precursor protein, including the beta-amyloid protein sequence, at human neuromuscular junctions. Neurosci. Lett. (1992) 143: 96-100.
49. L. Torroja, M. Packard, M. Gorczyca , K. White, V. Budnik. The Drosophila beta-amyloid precursor protein homolog promotes synapse differentiation at the neuromuscular junction. J. Neurosci. (1999) 19: 7793-7803.
50. L. Yang, B. Wang, C. Long, G. Wu, H. Zheng. Increased asynchronous release and aberrant calcium channel activation in amyloid precursor protein deficient neuromuscular synapses. Neuroscience (2007) 149(4):768-78.
51. P. Wang, G. Yang, DR. Mosier, P. Chang, T. Zaidi, YD. Gong, NM. Zhao, B. Dominguez, KF. Lee, WB. Gan, H. Zheng. Defective neuromuscular synapses in mice lacking amyloid precursor protein (APP) and APP-Like protein 2. J. Neurosci. (2005) 25(5):1219-25.
52. G. Yang, YD. Gong, K. Gong, WL. Jiang, E. Kwon, P. Wang, H. Zheng, XF. Zhang, WB. Gan, NM. Zhao. Reduced synaptic vesicle density and active zone size in mice lacking amyloid precursor protein (APP) and APP-like protein 2. Neurosci. Lett. (2005) 384(1-2):66-71.
53. B. Wang, L. Yang, Z. Wang, H. Zheng. Amyolid precursor protein mediates presynaptic localization and activity of the high-affinity choline transporter. Proc. Natl. Acad. Sci. USA (2007) 104(35):14140-5.
54. M. Jaworska-Wilczynska, GM. Wilczynski, WK. Engel, DK. Strickland, KH. Weisgraber, V. Askanas. Three lipoprotein receptors and cholesterol in inclusion-body myositis muscle. Neurology (2002) 58: 438-445.
55. AR. Koudinov, NV. Koudinova, U. Beisiegel. Cholesterol homeostasis failure at neuromuscular junctions and CNS synapses: a unifying cause of synaptic degeneration? Neurology online. (2002) Available at neurology.org
56. AR. Koudinov, NV. Koudinova. Essential role for cholesterol in synaptic plasticity and neuronal degeneration. FASEB J. (2001) 15: 1858-1860. Available at fasebj.org , also available as a slide show at koudinov.info (to be reprinted at Alzheimer's Code):
Link to this publication: amyloid-beta-app-implicated-in-synapse
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