In this report, Cheng and coworkers describe an interaction between herpes simplex virus type 1 (HSV1) and amyloid precursor protein (APP) that alters the cellular distribution and trafficking of APP. Impressively, they have used a variety of methods, including fixed and live-cell fluorescence microscopy and immunoelectron microscopy, to demonstrate colocalization between HSV1 and APP. These results are interesting, given resurgence in interest surrounding HSV1 as a putative environmental risk factor for AD.

The authors have nicely demonstrated that HSV1 and APP colocalize, and that this interaction increases APP expression and alters APP distribution within cells. Yet, beyond this, it is unclear whether the reported interaction with APP is 1) specific to HSV1 versus other viruses, or 2) has functional implications for APP or HSV1 biology. Regarding the former, perhaps high-level infection of cells (at multiplicity of infection of 10, which was used in this report) with other viruses (or even herpesviruses other than HSV1) may also produce interactions with APP and/or other...  Read more

In this report, Cheng and coworkers describe an interaction between herpes simplex virus type 1 (HSV1) and amyloid precursor protein (APP) that alters the cellular distribution and trafficking of APP. Impressively, they have used a variety of methods, including fixed and live-cell fluorescence microscopy and immunoelectron microscopy, to demonstrate colocalization between HSV1 and APP. These results are interesting, given resurgence in interest surrounding HSV1 as a putative environmental risk factor for AD.

The authors have nicely demonstrated that HSV1 and APP colocalize, and that this interaction increases APP expression and alters APP distribution within cells. Yet, beyond this, it is unclear whether the reported interaction with APP is 1) specific to HSV1 versus other viruses, or 2) has functional implications for APP or HSV1 biology. Regarding the former, perhaps high-level infection of cells (at multiplicity of infection of 10, which was used in this report) with other viruses (or even herpesviruses other than HSV1) may also produce interactions with APP and/or other APP-like glycoproteins. For me, though, the key question is whether these interactions alter APP or HSV1 biology. For example, does knocking down APP as the authors have done in this paper alter HSV1 infectivity (by plaque assay)? Does HSV1 versus mock infection impact amyloidogenic APP metabolism? Finally, that APP is exclusively present within the trans-Golgi network in the authors’ wild-type, mock-infected cells is curious, given decades of work in neuronal cells showing membranous/endosomal/lysosomal localization of APP. Does APP trafficking behave differently in the authors’ epithelial cell culture system compared with neuronal models? In my view, these are the sorts of penetrating biological questions that should be addressed in future studies.

View all comments by Terrence Town

Whether abnormal axonal transport is a cause or a consequence of the neuronal pathology in Alzheimer’s disease (AD) is largely debated (1). The recent paper from the Bearer lab suggests that perturbed axonal transport of amyloid-β precursor protein (APP) could be at the core of the pathogenic process in AD. As proposed by the authors, the disruption of the normal transport and—consequently—localization of APP could result from infection with herpes simplex virus (HSV). More specifically, by using APP—a potential anchor for the microtubule motor, kinesin-1—to achieve its own intracellular transport needs required for infectivity, the virus also modifies the normal transport route of APP, and thus its processing and function. How these events lead to the neuronal pathology and the lesions that characterize AD is not addressed, but the findings open the door for speculative thoughts. There are, however, some defined questions that this study raises. For example, why are APP levels upregulated to considerable high extent upon HSV infection? Is APP kept away from the compartments...  Read more

Whether abnormal axonal transport is a cause or a consequence of the neuronal pathology in Alzheimer’s disease (AD) is largely debated (1). The recent paper from the Bearer lab suggests that perturbed axonal transport of amyloid-β precursor protein (APP) could be at the core of the pathogenic process in AD. As proposed by the authors, the disruption of the normal transport and—consequently—localization of APP could result from infection with herpes simplex virus (HSV). More specifically, by using APP—a potential anchor for the microtubule motor, kinesin-1—to achieve its own intracellular transport needs required for infectivity, the virus also modifies the normal transport route of APP, and thus its processing and function. How these events lead to the neuronal pathology and the lesions that characterize AD is not addressed, but the findings open the door for speculative thoughts. There are, however, some defined questions that this study raises. For example, why are APP levels upregulated to considerable high extent upon HSV infection? Is APP kept away from the compartments where its processing and degradation normally occur? Is all intracellular trafficking perturbed, not only that of APP? It should be, since the reorganization—or disorganization—of the microtubule cytoskeleton, as revealed by the provided images, appears to be global. What is the consequence of altered trafficking of APP? Does it have an effect on processing, on the intracellular accumulation and secretion of APP-derived fragments, including the amyloid-β peptide? Or does it simply alter APP’s yet to be identified function, by mislocalization?

The interpretation of the data from this study relies on the assumption that APP is involved in the recruitment of the kinesin-1 motor to the cargo vesicle that is hijacked by the virus particles; this highly debated aspect (2) is not addressed in this paper. The notion that at least part of APP could serve—directly or indirectly—as an anchor for kinesin-1 (3-5) provides support for such an interpretation. Moreover, even if only a small fraction of APP normally serves such a role, viral infection could lead to post-translational modifications (e.g., phosphorylation, prolyl isomerisation, and O-glycosylation) in the cytoplasmic domain of APP that could either impair or enhance its interaction with motor containing protein complexes. Phosphorylation of APP at Thr668 is known to facilitate recruitment of kinesin-1 via the JNK-interacting protein-1 (JIP-1) (5,6). In addition, phosphorylation of threonine and tyrosine residues in APP appears to be a general mechanism for regulating its interaction with known binding partners (7). In this context, of note is that the cytoplasmic O-linked N-acetylglucosamine O-GlcNAc transferase capable of adding N-acetylglucosamine to APP (8) also targets the transcriptional co-regulator, HCF-1 (9), a host-cell factor implicated in HSV infection. Could the two events be related? It is too early to speculate. In any case, the fact that the outgoing capsids travel with APP is certainly relevant for both HSV infection and AD, even if it turns out that—in this case—APP is not the anchor that recruits the motor to the vesicle. The HSV-APP connection is still there, although more mysterious.

In his comment to this paper, Terrence Town points out that the authors find APP almost exclusively present within the trans-Golgi network (TGN), and not in endosomes. However, in our studies, we found it difficult to determine the site of residence of APP at light microscopy level based exclusively on colocalization with organelle markers. This is because the TGN, the endosomal recycling compartment, the pericentrosomal periciliary compartment—to give just a few examples—are all localized to overlapping regions, and often share “marker” proteins. Thus, part of the detected APP in this study could actually be localized to some other exocytic-endocytic compartments, in addition to TGN.

Certainly, this interesting and informative study adds to the fascinating, but far from being solved, mystery of what causes, or predisposes to, AD.

References:
1. Muresan, V. and Z. Muresan, Is abnormal axonal transport a cause, a contributing factor or a consequence of the neuronal pathology in Alzheimer's disease? Future Neurology, 2009. 4(6): p. 761-773. Abstract

2. Lazarov, O., et al., Axonal transport, amyloid precursor protein, kinesin-1, and the processing apparatus: revisited. J Neurosci, 2005 25(9): p. 2386-95. Abstract

3. Inomata, H., et al., A scaffold protein JIP-1b enhances amyloid precursor protein phosphorylation by JNK and its association with kinesin light chain 1. J Biol Chem, 2003 278(25): p. 22946-55. Abstract

4. Matsuda, S., Y. Matsuda, and L. D'Adamio, Amyloid beta protein precursor (AbetaPP), but not AbetaPP-like protein 2, is bridged to the kinesin light chain by the scaffold protein JNK-interacting protein 1. J Biol Chem, 2003 278(40): p. 38601-6. Abstract

5. Muresan, Z. and V. Muresan, Coordinated transport of phosphorylated amyloid-beta precursor protein and c-Jun NH2-terminal kinase-interacting protein-1. J Cell Biol, 2005. 171(4): p. 615-25. Abstract

6. Muresan, Z. and V. Muresan, c-Jun NH2-terminal kinase-interacting protein-3 facilitates phosphorylation and controls localization of amyloid-beta precursor protein. J Neurosci, 2005. 25(15): p. 3741-51. Abstract

7. Tamayev, R., D. Zhou, and L. D'Adamio, The interactome of the amyloid beta precursor protein family members is shaped by phosphorylation of their intracellular domains. Mol Neurodegener, 2009 4: p. 28. Abstract

8. Griffith, L.S., M. Mathes, and B. Schmitz, Beta-amyloid precursor protein is modified with O-linked N-acetylglucosamine. Journal of neuroscience research, 1995. 41(2): p. 270-8. Abstract

9. Capotosti, F., et al., O-GlcNAc transferase catalyzes site-specific proteolysis of HCF-1. Cell, 2011. 144(3): p. 376-88. Abstract

View all comments by Zoia Muresan
View all comments by Virgil Muresan

Three more papers supporting the concept of a viral involvement in AD have recently been published. Santana et al. (1) found, as we did, that Aβ accumulation occurred in HSV1-infected cultures. They used APP-transfected human neuroblastoma cells (SK-N-MC) and detected a sevenfold increase (over mock-infected) in Aβ two hours post-infection (p.i.), well before the start of viral DNA replication (at about four hours p.i.), whereas we detected an increase after about six hours (2). This apparent difference is probably due to our using a 10-fold lower HSV1 dose and/or to a cell-type difference (we used non-APP-transfected SH-SY5Y neuroblastoma cells). In fact, as Santana et al. point out, a much lower HSV1 dose is probably more relevant to “physiological” HSV1 infection.

Information about the stage of infection is important in relation to antivirals: Agents such as acyclovir (ACV) and foscarnet (FOS) act by preventing viral DNA replication, so if viral damage relevant to AD occurs before replication—say, during virus binding to or entry into the cell—these agents would reduce...  Read more

Three more papers supporting the concept of a viral involvement in AD have recently been published. Santana et al. (1) found, as we did, that Aβ accumulation occurred in HSV1-infected cultures. They used APP-transfected human neuroblastoma cells (SK-N-MC) and detected a sevenfold increase (over mock-infected) in Aβ two hours post-infection (p.i.), well before the start of viral DNA replication (at about four hours p.i.), whereas we detected an increase after about six hours (2). This apparent difference is probably due to our using a 10-fold lower HSV1 dose and/or to a cell-type difference (we used non-APP-transfected SH-SY5Y neuroblastoma cells). In fact, as Santana et al. point out, a much lower HSV1 dose is probably more relevant to “physiological” HSV1 infection.

Information about the stage of infection is important in relation to antivirals: Agents such as acyclovir (ACV) and foscarnet (FOS) act by preventing viral DNA replication, so if viral damage relevant to AD occurs before replication—say, during virus binding to or entry into the cell—these agents would reduce the damage only indirectly, by reducing the number of viral progeny and, hence, the extent of viral spread.

Santana et al. showed that FOS treatment did not visibly affect Aβ staining, even at the high concentration of 1.3 mM (1), suggesting that Aβ production induced by HSV1 is independent of viral DNA replication. Our recent experiments show similarly that HSV1-induced Aβ accumulation is viral DNA replication independent (Wozniak et al., submitted). We found that FOS and ACV decreased Aβ levels significantly (depending on the concentration used), but that levels were not reduced to those in mock-infected cells. The fact that Aβ level was reduced by FOS does contrast with the findings of Santana et al., but the result likely reflects our infection conditions (1 pfu/cell for 16 hours), which allow the virus to replicate and spread to additional cells; thus, the decrease we observe probably occurs through diminished viral spread.

The second paper, by Lerchundi et al. (3), examined the cleavage of tau in HSV1-infected cultures of neurons and astrocytes from embryonic or neonatal mice. Previously, they found that in mouse primary neuronal cultures, the virus reduced neuronal viability, caused neurite alterations and changes in cytoskeletal dynamics, and that these effects were prevented by 50 μM ACV treatment before or during infection (4,5). Also, HSV1 triggered hyperphosphorylation of tau at S202, T205, S396, and S404 (we subsequently confirmed phosphorylation at S202, S396, and S404, and detected it also at T212 and S214 [6]). Their recent study showed that HSV1 caused caspase-3-induced cleavage of tau at D421 (3), an occurrence associated with neurodegeneration. This cleavage, as well as the tau hyperphosphorylation, occurred during the first four hours p.i., and it was not significantly reduced by 50 μM ACV treatment, suggesting that these effects are independent of viral DNA replication. We, too, have investigated the effect of antiviral agents on tau phosphorylation, specifically phosphorylation at S214 and T212 (Wozniak et al., submitted). However, at these sites, unlike those examined by Lerchundi et al. (3), we found that 50 μM ACV reduces the HSV1-induced phosphorylation to the levels in mock-infected cells, suggesting that the phosphorylation at these sites is dependent on viral DNA replication. Thus, it appears that phosphorylation of tau by HSV1 is a complex process with both viral DNA replication-dependent and independent sites.

In the third paper, Cheng et al. (7) built on earlier findings (8) showing that HSV1 and amyloid precursor protein (APP) interact during transport of the virus. They show, using a comprehensive series of experiments, that the interaction between HSV1 and APP is specific (other similar proteins are less frequently associated with the virus) and meaningful (APP transport is slowed and APP location is altered in infected cells, thereby possibly affecting APP function).

All three papers stress that their data support a role of HSV1 in the development of AD, thus adding to the weight of evidence provided by earlier research (see review [9]) and three other recent studies on HSV1 and AD (10-12).

References:
1. Santana S, Recuero M, Bullido MJ, Valdivieso F, Aldudo J. Herpes simplex virus type I induces the accumulation of intracellular ß-amyloid in autophagic compartments and the inhibition of the non-amyloidogenic pathway in human neuroblastoma cells. Neurobiol Aging. 2011 Jan 25. 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 Dec 18;429(2-3):95-100. Abstract

3. Lerchundi R, Neira R, Valdivia S, Vio K, Concha MI, Zambrano A, Otth C. Tau cleavage at D421 by caspase-3 is induced in neurons and astrocytes infected with herpes simplex virus type 1. J Alzheimers Dis. 2011 Jan 1;23(3):513-20. Abstract

4. Zambrano A, Solis L, Salvadores N, Cortés M, Lerchundi R, Otth C. Neuronal cytoskeletal dynamic modification and neurodegeneration induced by infection with herpes simplex virus type 1. J Alzheimers Dis. 2008 Jul;14(3):259-69. Abstract

5. Otth C et al. Journal of Neurochemistry. 2009;110:45.

6. Wozniak MA, Frost AL, Itzhaki RF. Alzheimer's disease-specific tau phosphorylation is induced by herpes simplex virus type 1. J Alzheimers Dis. 2009;16(2):341-50. Abstract

7. Cheng SB, Ferland P, Webster P, Bearer EL. Herpes simplex virus dances with amyloid precursor protein while exiting the cell. PLoS One. 2011;6(3):e17966. 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 Dec;2(6):305-18. Abstract

9. Wozniak MA, Itzhaki RF. Antiviral agents in Alzheimer's disease: hope for the future? Ther Adv Neurol Disord. 2010 May;3(3):141-52. Abstract

10. Piacentini R, Civitelli L, Ripoli C, Marcocci ME, De Chiara G, Garaci E, Azzena GB, Palamara AT, Grassi C. HSV-1 promotes Ca(2+)-mediated APP phosphorylation and Aß accumulation in rat cortical neurons. Neurobiol Aging. 2010 Jul 31. Abstract

11. Lukiw WJ, Cui JG, Yuan LY, Bhattacharjee PS, Corkern M, Clement C, Kammerman EM, Ball MJ, Zhao Y, Sullivan PM, Hill JM. Acyclovir or Aß42 peptides attenuate HSV-1-induced miRNA-146a levels in human primary brain cells. Neuroreport. 2010 Oct 6;21(14):922-7. Abstract

12. Porcellini E, Carbone I, Ianni M, Licastro F. Alzheimer's disease gene signature says: beware of brain viral infections. Immun Ageing. 2010;7:16. Abstract

View all comments by Ruth Itzhaki
View all comments by Matthew Wozniak