Amyloid-β Shows Another Facet
In this article, Brody et al. showed that concentrations of amyloid-β (Aβ) in brain interstitial fluid (ISF), in vivo, increased following head injury and subarachnoid hemorrhage as neurological status improved. Conversely, concentrations of Aβ fell when neurological status declined. The authors conclude that neuronal activity regulates the concentration of extracellular Aβ, and that declining levels of Aβ reflect depressed neuronal function.

To some extent, Brody et al. underestimate the potential significance of their findings. As emphasized by the authors, observations derived from in vivo studies in human patients are extremely valuable as they relate directly to the human condition and allow the generation of hypotheses that can be tested experimentally. In addition, their studies have produced data regarding the physiological functions of soluble Aβ that are relevant to the role of Aβ in Alzheimer disease (AD).

A number of recent studies suggest that soluble Aβ in the brain may have a more significant role in the pathogenesis...  Read more

Amyloid-β Shows Another Facet
In this article, Brody et al. showed that concentrations of amyloid-β (Aβ) in brain interstitial fluid (ISF), in vivo, increased following head injury and subarachnoid hemorrhage as neurological status improved. Conversely, concentrations of Aβ fell when neurological status declined. The authors conclude that neuronal activity regulates the concentration of extracellular Aβ, and that declining levels of Aβ reflect depressed neuronal function.

To some extent, Brody et al. underestimate the potential significance of their findings. As emphasized by the authors, observations derived from in vivo studies in human patients are extremely valuable as they relate directly to the human condition and allow the generation of hypotheses that can be tested experimentally. In addition, their studies have produced data regarding the physiological functions of soluble Aβ that are relevant to the role of Aβ in Alzheimer disease (AD).

A number of recent studies suggest that soluble Aβ in the brain may have a more significant role in the pathogenesis of AD than the plaques of insoluble Aβ. PET imaging has shown that, as a group, patients with mild cognitive impairment have a similar amount of insoluble Aβ in their brains as patients with AD [3]. Furthermore, removal of Aβ plaques from the brain does not prevent progressive neurodegeneration in AD [2]. Other studies indicate that high levels of soluble Aβ in the brain correlate more closely with cognitive decline in AD than does insoluble Aβ plaque load [5,6] and that removal of Aβ plaques from the brain by Aβ immunotherapy increases the level of soluble Aβ in the brain [8].

The perceived role of soluble Aβ in the pathogenesis of cognitive impairment in AD makes it all the more urgent to take a fresh look at the physiological and pathological roles of Aβ. Little is known about the physiological role of soluble Aβ in the brain in vivo. The result’s of Brody et al. suggest that neurons produce Aβ in response to acute brain injury and that the higher levels of Aβ in the subcortical white matter correlate with improvement of neurological status. But, why should the levels of soluble Aβ rise in those patients showing improvement in neurological status?

Levels of soluble Aβ in the brain depend not only on its level of production but also on the efficiency of elimination of Aβ from the interstitial fluid [11]. Several pathways for elimination of Aβ are recognized; they include degradation by neprilysin and absorption into the blood by various mechanisms [11]. Aβ also drains with brain ISF along basement membranes in the walls of cerebral capillaries and arteries where it may deposit as cerebral amyloid angiopathy (CAA) [11]. The rise in concentration of Aβ in the ISF during the recovery period following acute brain injury in the study of Brody et al. could reflect a reduction of elimination of Aβ as well as its increased production.

Failure of elimination of Aβ along perivascular drainage routes and the development of CAA is observed with increasing age when arteries stiffen [11] and following denervation [1] when vessel tone may be affected. Diffuse vascular injury is well recognized in patients with head injuries [9], and vascular spasm is seen in both head injury and following subarachnoid hemorrhage [7]. Vascular factors may play a role in impeding the elimination of Aβ along artery walls and in the development of CAA following head injury [4].

Brody et al. emphasize that their results correlate with in vitro studies showing that neuronal and synaptic activity dynamically regulates the concentration of extracellular soluble Aβ. However, it is not known whether the effects of the increased production of Aβ are purely local or whether Aβ has an effect on artery walls as it drains out of the brain along perivascular pathways.

The role of Aβ in the pathogenesis of AD requires some re-evaluation. Aβ in its soluble form in the brain seems to be as important as insoluble plaques of Aβ, if not more important, in the pathogenesis of AD. Toxic forms of soluble Aβ appear to have a role in the pathogenesis of AD [10], and the techniques used by Brody et al. may provide the opportunity to study this further in vivo. However, we may ask whether toxicity is the sole role for Aβ in AD. The deposition of fibrillar Aβ in the perivascular drainage pathways in CAA may block the elimination not only of Aβ but also of other brain metabolites. This may lead to loss of homeostasis and neuronal malfunction as a factor in cognitive impairment in AD. Evaluation of the composition of brain ISF may help to answer questions about the quality of the neuronal environment in patients with AD.

References:
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2. Holmes C, Boche D, Wilkinson D, Yadegarfar G, Hopkins V, Bayer A, Jones RW, Bullock R, Love S, Neal JW, Zotova E, Nicoll JA (2008) Long-term effects of Abeta42 immunisation in Alzheimer's disease: follow-up of a randomised, placebo-controlled phase I trial. Lancet 372:216-223. Abstract

3. Kemppainen NM, Aalto S, Wilson IA, Någren K, Helin S, Brück A, Oikonen V, Kailajärvi M, Scheinin M, Viitanen M, Parkkola R, Rinne JO (2007) PET amyloid ligand [11C]PIB uptake is increased in mild cognitive impairment. Neurology 68:1603-1606. Abstract

4. Leclercq PD, Murray LS, Smith C, Graham DI, Nicoll JA, Gentleman SM (2005) Cerebral amyloid angiopathy in traumatic brain injury: association with apolipoprotein E genotype. J Neurol Neurosurg Psychiatry 76:229-233. Abstract

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6. McLean CA, Cherny RA, Fraser FW, Fuller SJ, Smith MJ, Beyreuther K, Bush AI, Masters CL (1999) Soluble pool of Abeta amyloid as a determinant of severity of neurodegeneration in Alzheimer's disease. Ann Neurol 46:860-866. Abstract

7. Oertel M, Boscardin WJ, Obrist WD, Glenn TC, McArthur DL, Gravori T, Lee JH, Martin NA (2005) Posttraumatic vasospasm: the epidemiology, severity, and time course of an underestimated phenomenon: a prospective study performed in 299 patients. J Neurosurg 103:812-824. Abstract

8. Patton RL, Kalback WM, Esh CL, Kokjohn TA, Van Vickle GD, Luehrs DC, Kuo YM, Lopez J, Brune D, Ferrer I, Masliah E, Newel AJ, Beach TG, Castano EM, Roher AE (2006) Amyloid-{beta} Peptide Remnants in AN-1792-Immunized Alzheimer's Disease Patients: A Biochemical Analysis. . Am J Pathol 169:1048-1063. Abstract

9. Pittella JE, Gusmão SN (2003) Diffuse vascular injury in fatal road traffic accident victims: its relationship to diffuse axonal injury. J Forensic Sci 48:626-630. Abstract

10. Walsh DM, Selkoe DJ (2007) Abeta Oligomers - a decade of discovery. J Neurochem 101:1172-1184. Abstract

11. Weller RO, Subash M, Preston SD, Mazanti I, Carare RO (2008) Perivascular Drainage of Amyloid-beta Peptides from the Brain and Its Failure in Cerebral Amyloid Angiopathy and Alzheimer's Disease. Brain Pathol 18:253-266. Abstract

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