Macroautophagy is a major mechanism for intracellular protein degradation. It begins with the engulfment of part of the cytoplasm, including intracellular organelles, by double membrane vacuolar structures, the autophagosomes; these structures then fuse with lysosomes, thus creating the autolysosomes in which the intracellular constituents are degraded. Observations in a number of neurodegenerative diseases, including Alzheimer disease, indicate that there is an extensive accumulation of autophagic vacuoles in affected tissues. Whether this represents an induction of the process of macroautophagy or an inhibition of the conversion of autophagosomes to mature lysosomes has been unclear.

Boland et al. now report, using a number of careful imaging and biochemical tools, that in cultured primary cortical neurons induction of macroautophagy through inhibition of mTOR leads to little accumulation of double-membrane LC3-II-positive autophagosomes, as these are rapidly converted to autolysosomes. In contrast, inhibition of lysosomal proteolysis or disruption of...  Read more

Macroautophagy is a major mechanism for intracellular protein degradation. It begins with the engulfment of part of the cytoplasm, including intracellular organelles, by double membrane vacuolar structures, the autophagosomes; these structures then fuse with lysosomes, thus creating the autolysosomes in which the intracellular constituents are degraded. Observations in a number of neurodegenerative diseases, including Alzheimer disease, indicate that there is an extensive accumulation of autophagic vacuoles in affected tissues. Whether this represents an induction of the process of macroautophagy or an inhibition of the conversion of autophagosomes to mature lysosomes has been unclear.

Boland et al. now report, using a number of careful imaging and biochemical tools, that in cultured primary cortical neurons induction of macroautophagy through inhibition of mTOR leads to little accumulation of double-membrane LC3-II-positive autophagosomes, as these are rapidly converted to autolysosomes. In contrast, inhibition of lysosomal proteolysis or disruption of autophagosome-lysosome fusion led to abundant accumulation of double-membrane structures, most of them positive for LC3-II, indicating inhibition of macroautophagy-dependent degradation. These latter structures resemble those seen in Alzheimer disease brains, as well as in the brains of APP/PS1 Tg mice. The authors conclude that disruption of the process downstream of autophagosome formation, rather than induction of macroautophagy, is likely to be the main determinant leading to the marked vacuolar accumulation seen in AD.

The findings raise a number of questions:

First, what is the nature of the presumed defect in the “maturation” of the autophagic pathway? One possibility is that there is a defect in the fusion of autophagosomes into autolysosomes. This could be due to a general defect in axonal transport or to yet unknown properties specific to the budding vacuoles. Alternatively, the problem could lie within the lysosomes. Lysosomal dysfunction in cultured neurons, as shown convincingly in this study, but also in experimental animals or in humans, leads to accumulation of autophagic vacuoles. Lysosomal dysfunction as a primary event could also, in part, explain the defects in endocytosis observed early in the course of AD.

A second question, perhaps more important from the point of view of potential therapies, is whether this accumulation of autophagic vacuoles is beneficial or detrimental, and whether preventing or inducing it would be a good therapeutic strategy against neurodegeneration. The data from this paper indicate that the presence of autophagosomes in AD is a readout for a dysfunction of the downstream protein degradation pathway within lysosomes. Therefore, it would make little sense to further induce the macroautophagy pathway in this setting as a therapeutic strategy, especially since autophagic vacuoles aid Aβ production (Yu et al., 2005). It would be better, in fact, to find ways to boost either the fusion event or lysosomal function in general.

Of course, the AD disease process could have additional effects on the autophagic/lysosomal pathway, as the authors themselves recognize. In fact, a recent manuscript (Pickford et al., 2008) has suggested that an early event in AD is the reduction of beclin-1, a protein that promotes macroautophagy; modulation of beclin-1 influenced the phenotype of APP transgenic mice, suggesting that the reduction of beclin-1, and the resultant reduction in macroautophagic degradation, observed in AD could have detrimental effects. Although seemingly contradictory, these reports both highlight the fact that there is dysfunction of the autophagic/lysosomal pathway in AD, perhaps at various levels. Lysosomal dysfunction with marked induction of autophagic vacuoles has been reported previously by us in the setting of aberrant α-synuclein expression in a PC12 cell model (Stefanis et al., 2001). Therefore, alterations of this pathway may play a significant role in various neurodegenerative conditions and could represent therapeutic targets.

View all comments by Leonidas Stefanis

The Pickford et al. study adds strong support to an emerging view that autophagic-lysosomal impairment in AD can contribute to Aβ pathology and also to neurodegeneration through additional Aβ-independent mechanisms, which might be shared by other neurological diseases across the lifespan (1). The deficiencies Pickford and colleagues identified in the initial “sequestration” stages of autophagy compound other defects. We previously reported the clearance of Aβ-generating autophagic vacuoles that lead to vacuole accumulation—even in the presence of possibly slowed autophagosome formation as implied by the current findings.

Protein/vesicular trafficking defects in AD tend to be viewed from the focused perspective of how APP metabolism is altered, but, as this and other recent studies imply, the trafficking/handling of many proteins is affected by alterations of endosomes, autophagic compartments, and lysosomes, which are increasingly being linked to AD-related genetic factors (e.g., presenilin, SorLA, APP duplication, etc.). These more global effects on neuronal function are...  Read more

The Pickford et al. study adds strong support to an emerging view that autophagic-lysosomal impairment in AD can contribute to Aβ pathology and also to neurodegeneration through additional Aβ-independent mechanisms, which might be shared by other neurological diseases across the lifespan (1). The deficiencies Pickford and colleagues identified in the initial “sequestration” stages of autophagy compound other defects. We previously reported the clearance of Aβ-generating autophagic vacuoles that lead to vacuole accumulation—even in the presence of possibly slowed autophagosome formation as implied by the current findings.

Protein/vesicular trafficking defects in AD tend to be viewed from the focused perspective of how APP metabolism is altered, but, as this and other recent studies imply, the trafficking/handling of many proteins is affected by alterations of endosomes, autophagic compartments, and lysosomes, which are increasingly being linked to AD-related genetic factors (e.g., presenilin, SorLA, APP duplication, etc.). These more global effects on neuronal function are the "elephant in the room" in most current discussions of altered protein/vesicular trafficking in AD and deserve consideration as factors relevant to AD pathogenesis in their own right. In this regard, the endosomal-autophagic lysosomal system dysfunction being recognized in a growing number of other neurodegenerative diseases may well inform us about the pathogenic significance of such impairments in AD.

References:
1. Nixon RA, Yang DS, Lee JH. Neurodegenerative lysosomal disorders—a continuum from development to late age. Autophagy. 2008 May 12;4(5). Abstract

View all comments by Ralph Nixon

Another Herpes Virus-Alzheimer’s Disease Connection: Beclin Beckons
This study not only strengthens the link between AD and autophagy by relating it to a reduced beclin 1 activity in the diseased brain. It also strengthens, indirectly, another link which we proposed (1), namely, among herpes simplex virus type 1 (HSV1), autophagy, and AD—thus extending the striking HSV1-amyloid connection that we recently discovered (2). HSV1 infects, and then resides lifelong, in the peripheral nervous system (PNS) of most humans in a latent state and is reactivated periodically by events such as stress; it then causes damage—cold sores—in some of those infected.

We detected HSV1 DNA some 18 years ago in the brain of many elderly humans (3), and subsequently showed that in brain, as in the PNS, it reactivates from latency (4), possibly recurrently, triggered presumably by stress, systemic infection, etc. Further, we found that HSV1 in ApoE-ε4 carriers’ brains conferred a strong risk of AD (5), and we suggested that brain damage caused on viral reactivation was greater in ApoE-ε4...  Read more

Another Herpes Virus-Alzheimer’s Disease Connection: Beclin Beckons
This study not only strengthens the link between AD and autophagy by relating it to a reduced beclin 1 activity in the diseased brain. It also strengthens, indirectly, another link which we proposed (1), namely, among herpes simplex virus type 1 (HSV1), autophagy, and AD—thus extending the striking HSV1-amyloid connection that we recently discovered (2). HSV1 infects, and then resides lifelong, in the peripheral nervous system (PNS) of most humans in a latent state and is reactivated periodically by events such as stress; it then causes damage—cold sores—in some of those infected.

We detected HSV1 DNA some 18 years ago in the brain of many elderly humans (3), and subsequently showed that in brain, as in the PNS, it reactivates from latency (4), possibly recurrently, triggered presumably by stress, systemic infection, etc. Further, we found that HSV1 in ApoE-ε4 carriers’ brains conferred a strong risk of AD (5), and we suggested that brain damage caused on viral reactivation was greater in ApoE-ε4 carriers, leading to the development of AD (5,6). Since then, several studies by others have linked HSV1 to AD, detecting a close homology between a sequence in Aβ and one of the viral glycoproteins (7), and showing that APP associates with HSV1 during axonal transport (8). Also, recent studies have indicated that ApoE affects HSV1 (9-12), determining its transport in tissues and its expression. (Indeed, we have found that ApoE influences outcome of infection by several other viruses, including occurrence of cold sores in ApoE-ε4 carriers—paralleling the CNS situation—see, e.g., [6].)

Our recent data reveal that HSV1 infection of cultured cells causes a large accumulation of Aβ (2) and also AD-like tau phosphorylation, and infection of mice causes Aβ accumulation in brain.

Our recent hypothesis proposes that excess Aβ produced by HSV1 action is inadequately removed by autophagy because of viral hindrance, thus allowing plaque formation to occur (1). This was based on the fact that cells infected by HSV1 attempt to demolish the intruder by an autophagic process called xenophagy, but to evade this, the viral-encoded neurovirulence protein, ICP34.5, binds to beclin and inhibits its autophagic function (13). This ability to overcome xenophagy is shared by various other infectious agents, e.g., HIV (14) (and some microbes even use autophagy for their own advantage [15,16]), but HSV1 is the only pathogen (or indeed inflammation-producing agent) detected so far in many normal elderly human brains, and is therefore the only agent in a position to cause AD-like damage—as indeed we have shown it to do in cells and mice. In fact, one of the authors of the present study, too, has discussed the possibility that viral inhibition of autophagy might contribute to “non-infectious” neurodegenerative diseases such as AD (17).

References:
1. Itzhaki RF, Cosby SL, Wozniak MA. Herpes simplex virus type 1 and Alzheimer's disease: the autophagy connection. J Neurovirol. 2008;14:1-4. Abstract

2. Wozniak MA, Itzhaki RF, Shipley SJ, Dobson CB. Herpes simplex virus infection causes cellular beta-amyloid accumulation and secretase upregulation. Neurosci Lett. 2007;429:95-100. Abstract

3. Jamieson GA, Maitland NJ, Wilcock GK, Craske J, Itzhaki RF. Latent herpes simplex virus type 1 in normal and Alzheimer's disease brains. J Med Virol. 1991;33:224-7. Abstract

4. Wozniak MA, Shipley SJ, Combrinck M, Wilcock GK, Itzhaki RF. Productive herpes simplex virus in brain of elderly normal subjects and Alzheimer's disease patients. J Med Virol. 2005;75:300-6. Abstract

5. Itzhaki RF, Lin WR, Shang D, Wilcock GK, Faragher B, Jamieson GA. Herpes simplex virus type 1 in brain and risk of Alzheimer's disease. Lancet. 1997;349:241-4. Abstract

6. Itzhaki RF, Wozniak MA. Herpes Simplex Virus Type 1 in Alzheimer's disease: The Enemy Within. J Alzheimers Dis. 2008;13:393-405. Abstract

7. Cribbs DH, Azizeh BY, Cotman CW, LaFerla FM. Fibril formation and neurotoxicity by a herpes simplex virus glycoprotein B fragment with homology to the Alzheimer's A beta peptide. Biochemistry. 2000;39:5988-94. Abstract

8. Satpute-Krishnan P, DeGiorgis JA, Bearer EL. Fast anterograde transport of herpes simplex virus: role for the amyloid precursor protein of Alzheimer's disease. Aging Cell. 2003;2:305-18. Abstract

9. Burgos JS, Ramirez C, Sastre I, Bullido MJ, Valdivieso F. ApoE4 is more efficient than E3 in brain access by herpes simplex virus type 1. Neuroreport. 2003;14:1825-7. Abstract

10. Burgos JS, Ramirez C, Sastre I, Valdivieso F. Effect of apolipoprotein E on the cerebral load of latent herpes simplex virus type 1 DNA. J Virol. 2006;80:5383-7. Abstract

11. Bhattacharjee PS, Neumann DM, Stark D, Thompson HW, Hill JM. Apolipoprotein E modulates establishment of HSV-1 latency and survival in a mouse ocular model. Curr Eye Res. 2006;31:703-8. Abstract

12. Miller RM, Federoff HJ. Isoform-specific effects of ApoE on HSV immediate early gene expression and establishment of latency. Neurobiol Aging. 2008;29:71-7. Abstract

13. Orvedahl A, Alexander D, Tallóczy Z, Sun Q, Wei Y, Zhang W, Burns D, Leib DA, Levine B. HSV-1 ICP34.5 confers neurovirulence by targeting the Beclin 1 autophagy protein. Cell Host Microbe. 2007;1:23-35. Abstract

14. Zhou D, Spector S. Human immunodeficiency virus type-1 infection inhibits autophagy. AIDS. 2008;22:695-9. Abstract

15. Colombo MI. Pathogens and autophagy: subverting to survive. Cell Death Differ. 2005;12 Suppl 2:1481-3. Abstract

16. Wileman T. Aggresomes and autophagy generate sites for virus replication. Science. 2006;312:875-8. Abstract

17. Orvedahl A, Levine B. Autophagy and viral neurovirulence. Cell Microbiol. 2008 May 22. [Epub ahead of print] Abstract

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