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Is RNA-laden p24 capsid protein a vector for HIV?
Medical Hypotheses (2004) 62, 919–921
http://intl.elsevierhealth.com/journals/mehy
Is RNA-laden p24 capsid protein a vector for HIV? Andrzej W. Lipkowskia,*, Daniel B. Carrb, Victor J. Hrubyc, Masaaki Yoshikawad a
Medical Research Centre, Polish Academy of Sciences, 02106 Warsaw, Poland Departments of Anesthesia and Medicine, Tufts-New England Medical Center, Boston, MA 02111, USA c Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA d Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Uji Kyoto 611-0011, Japan b
Received 24 November 2003; accepted 5 December 2003
Summary The consensus HIV infection pathway, although well supported by abundant experimental evidence, fails to account for infection of cells (e.g., neurons) that lack CD4 and/or coreceptors. We present the hypothesis that p24 capsid protein laden with RNA may be an alternative virulent agent. A novel mechanism of HIV infection may involve binding of the capsid with membrane receptors, followed by internalization of the capsid–receptor complex. This new pathway suggests novel strategies for antiviral pharmacotherapy. c 2004 Elsevier Ltd. All rights reserved.
Introduction Current anti-HIV therapies are based on a consensus mechanism of infection and replication in which the HIV virus enters cells via an interaction with CD4 and coreceptors, then replicates intracellularly to produce new viruses that are subsequently released into the extracellular space [1]. Although this pathway is well supported by abundant experimental evidence, it fails to account for infection of cells (e.g., neurons) that lack CD4 and/ or coreceptors. A number of laboratories are working to fill in observed gaps, according to principles based upon the accepted model of HIV infection. However, further progress is needed. We now propose the hypothesis, that synthesizes *
Corresponding author. E-mail address: [email protected] (A.W. Lipkowski).
existing data, that p24 capsid loaded with RNA may be a virulent agent. P24 protein is found in the serum of HIV-positive individuals. Serum concentrations of this protein frequently correlate better with clinical status such as disease progression than do levels of other HIV-related proteins [2]. Others have reported that p24 alone may form a stable capsid containing RNA [3]. Recently, we observed peptide motifs on the p24 protein that interact with opioid and likely other, unidentified, neuromembrane receptors [4].
Hypothesis Credible published data, supplemented by our recent observations, allows us to propose a novel mechanism of HIV infection in which capsids of p24
0306-9877/$ - see front matter c 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2003.12.035
920
Lipkowski et al.
Testing the hypothesis
Figure 1 Life cycle of HIV-1 virus: (a) consensus mechanism for infection; (b) hypothetical alternative in which HIV capsid infects cells through interaction with membrane G protein-coupled receptors.
proteins laden with RNA, released extracellularly, may act as an infectious pathway in parallel with, and supplementing, that of fully developed HIV viruses. A comparison of (a) the consensus mechanism of HIV infection and (b) a hypothetical infectious pathway involving RNA-laden p24 capsid is depicted in Fig. 1. Because of their lability under typical environmental conditions, p24-derived capsids are unlikely to function as vectors that transmit disease between individuals. Within a host organism, however, they are stable enough to enter (possibly more effectively than the full HIV virus) various tissues. Because the recognition elements on the p24 capsid protein differ from those of the full HIV virus, the former may interact with distinct membrane receptors that are inaccessible to the latter. Because the p24 capsid is smaller than whole virus, the simple mechanism of endocytosis of capsid–receptor complex may suffice for its internalization within host cells. Because RNA-laden p24 capsid carries complete genetic information, fully developed HIV viruses could be formed as a consequence of infection by this mechanism.
The above hypothesis follows from analyses of published data and our modeling of fragments of HIV p24 capsid protein. Before this hypothesis can be viewed as proven, several related questions will require addressing through diverse microbiological and biochemical approaches. The majority of virions circulating in plasma are defective and could liberate the RNA laden p24 capsid. Therefore, it is very likely that there are capsids circulating in the body fluids that lack envelope. However, the confirmation that the RNA laden p24 capsid circulates in the blood of HIV-infected patients is required, perhaps by application of sedimentation techniques. Second, the entry of RNA-laden p24 capsid into cells via membrane receptors should be established. To do so requires preparation of RNA laden p24 capsid, e.g., by expressing it in Escherichia coli [5]. Interaction between RNA laden p24 capsids and membrane receptors may be accomplished in cultured neurocells or other cell lines expressing membrane receptors, including various G-protein coupled receptors, such as neuroblastoma cells [6–8].
Conclusions Twenty years ago the isolation and structure of HIV virus were accomplished, spurring tremendous progress in HIV/AIDS biology and clinical therapies. However, such studies have left questions which cannot be answered on the basis of current understanding of the mechanisms of viral replication. The alternative mechanism now proposed, if confirmed, may shed light on unique virulence of all lentiviruses. Importantly, this hypothesis provides a rationale for the search for new recognition sites of other than viral shell proteins. This search may identify new directions in drug development for HIV disease and other diseases caused by lentiviruses.
References [1] Kilby JM, Eron JJ. Novel therapies based on mechanisms of HIV-1 cell entry. N Engl J Med 2003;348:2228–38. [2] Addo MM, Yu XG, Rathod A, Cohen D, Eldridge RL, Strick D, et al. Comprehensive epitope analysis of human immunodeficiency virus type 1 (HIV-1)-specific T-cell responses directed against the entire expressed HIV-1 genome demonstrate broadly directed responses, but no correlation to viral load. J Virol 2003;77:2081–92.
Is RNA-laden p24 capsid protein a vector for HIV? [3] Gross I, Hohenberger H, Wilk T, Wiegers K, Grattinger M, Muller B, et al. A conformational switch controlling HIV-1 morphogenesis. EMBO J 2000;19:103–13. [4] Lipkowski AW, Kosson P, Misicka A, Carr DB, Hruby VJ, Yoshikawa M, et al. Fragments of HIV GAG polyproteins expressing affinity to opioid receptors. Biopolymers 2003;71:294. [5] Campbell S, Vogt VM. Self-assembly in vitro of purified CANC proteins from Rous sarcoma virus and human immunodeficiency virus type 1. J Virol 1995;69:6487–97.
921 [6] Losada SA, Canto-Nogues C, Munoz-Fernandez A. A new possible mechanism of human immunodeficiency virus type 1 infection of neural cells. Neurobiol Dis 2002;11:469–78. [7] Nagele RG, Andrea MR, Anderson WJ, Wang HY. Intracellular accumulation of eta-amyloid(1-42) in neuron facilitated by the alpha 7 nicotinic acetylcholine receptor in Alzheimer’s disease. Neuroscience 2002;110:199–211. [8] Marie N, Lecoq I, Jauzac P, Allouche S. Differential sorting of human delta-opioid receptors after internalization by peptide and alkaloid agonists. J Biol Chem 2003;278:22795–804.
http://intl.elsevierhealth.com/journals/mehy
Is RNA-laden p24 capsid protein a vector for HIV? Andrzej W. Lipkowskia,*, Daniel B. Carrb, Victor J. Hrubyc, Masaaki Yoshikawad a
Medical Research Centre, Polish Academy of Sciences, 02106 Warsaw, Poland Departments of Anesthesia and Medicine, Tufts-New England Medical Center, Boston, MA 02111, USA c Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA d Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Uji Kyoto 611-0011, Japan b
Received 24 November 2003; accepted 5 December 2003
Summary The consensus HIV infection pathway, although well supported by abundant experimental evidence, fails to account for infection of cells (e.g., neurons) that lack CD4 and/or coreceptors. We present the hypothesis that p24 capsid protein laden with RNA may be an alternative virulent agent. A novel mechanism of HIV infection may involve binding of the capsid with membrane receptors, followed by internalization of the capsid–receptor complex. This new pathway suggests novel strategies for antiviral pharmacotherapy. c 2004 Elsevier Ltd. All rights reserved.
Introduction Current anti-HIV therapies are based on a consensus mechanism of infection and replication in which the HIV virus enters cells via an interaction with CD4 and coreceptors, then replicates intracellularly to produce new viruses that are subsequently released into the extracellular space [1]. Although this pathway is well supported by abundant experimental evidence, it fails to account for infection of cells (e.g., neurons) that lack CD4 and/ or coreceptors. A number of laboratories are working to fill in observed gaps, according to principles based upon the accepted model of HIV infection. However, further progress is needed. We now propose the hypothesis, that synthesizes *
Corresponding author. E-mail address: [email protected] (A.W. Lipkowski).
existing data, that p24 capsid loaded with RNA may be a virulent agent. P24 protein is found in the serum of HIV-positive individuals. Serum concentrations of this protein frequently correlate better with clinical status such as disease progression than do levels of other HIV-related proteins [2]. Others have reported that p24 alone may form a stable capsid containing RNA [3]. Recently, we observed peptide motifs on the p24 protein that interact with opioid and likely other, unidentified, neuromembrane receptors [4].
Hypothesis Credible published data, supplemented by our recent observations, allows us to propose a novel mechanism of HIV infection in which capsids of p24
0306-9877/$ - see front matter c 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2003.12.035
920
Lipkowski et al.
Testing the hypothesis
Figure 1 Life cycle of HIV-1 virus: (a) consensus mechanism for infection; (b) hypothetical alternative in which HIV capsid infects cells through interaction with membrane G protein-coupled receptors.
proteins laden with RNA, released extracellularly, may act as an infectious pathway in parallel with, and supplementing, that of fully developed HIV viruses. A comparison of (a) the consensus mechanism of HIV infection and (b) a hypothetical infectious pathway involving RNA-laden p24 capsid is depicted in Fig. 1. Because of their lability under typical environmental conditions, p24-derived capsids are unlikely to function as vectors that transmit disease between individuals. Within a host organism, however, they are stable enough to enter (possibly more effectively than the full HIV virus) various tissues. Because the recognition elements on the p24 capsid protein differ from those of the full HIV virus, the former may interact with distinct membrane receptors that are inaccessible to the latter. Because the p24 capsid is smaller than whole virus, the simple mechanism of endocytosis of capsid–receptor complex may suffice for its internalization within host cells. Because RNA-laden p24 capsid carries complete genetic information, fully developed HIV viruses could be formed as a consequence of infection by this mechanism.
The above hypothesis follows from analyses of published data and our modeling of fragments of HIV p24 capsid protein. Before this hypothesis can be viewed as proven, several related questions will require addressing through diverse microbiological and biochemical approaches. The majority of virions circulating in plasma are defective and could liberate the RNA laden p24 capsid. Therefore, it is very likely that there are capsids circulating in the body fluids that lack envelope. However, the confirmation that the RNA laden p24 capsid circulates in the blood of HIV-infected patients is required, perhaps by application of sedimentation techniques. Second, the entry of RNA-laden p24 capsid into cells via membrane receptors should be established. To do so requires preparation of RNA laden p24 capsid, e.g., by expressing it in Escherichia coli [5]. Interaction between RNA laden p24 capsids and membrane receptors may be accomplished in cultured neurocells or other cell lines expressing membrane receptors, including various G-protein coupled receptors, such as neuroblastoma cells [6–8].
Conclusions Twenty years ago the isolation and structure of HIV virus were accomplished, spurring tremendous progress in HIV/AIDS biology and clinical therapies. However, such studies have left questions which cannot be answered on the basis of current understanding of the mechanisms of viral replication. The alternative mechanism now proposed, if confirmed, may shed light on unique virulence of all lentiviruses. Importantly, this hypothesis provides a rationale for the search for new recognition sites of other than viral shell proteins. This search may identify new directions in drug development for HIV disease and other diseases caused by lentiviruses.
References [1] Kilby JM, Eron JJ. Novel therapies based on mechanisms of HIV-1 cell entry. N Engl J Med 2003;348:2228–38. [2] Addo MM, Yu XG, Rathod A, Cohen D, Eldridge RL, Strick D, et al. Comprehensive epitope analysis of human immunodeficiency virus type 1 (HIV-1)-specific T-cell responses directed against the entire expressed HIV-1 genome demonstrate broadly directed responses, but no correlation to viral load. J Virol 2003;77:2081–92.
Is RNA-laden p24 capsid protein a vector for HIV? [3] Gross I, Hohenberger H, Wilk T, Wiegers K, Grattinger M, Muller B, et al. A conformational switch controlling HIV-1 morphogenesis. EMBO J 2000;19:103–13. [4] Lipkowski AW, Kosson P, Misicka A, Carr DB, Hruby VJ, Yoshikawa M, et al. Fragments of HIV GAG polyproteins expressing affinity to opioid receptors. Biopolymers 2003;71:294. [5] Campbell S, Vogt VM. Self-assembly in vitro of purified CANC proteins from Rous sarcoma virus and human immunodeficiency virus type 1. J Virol 1995;69:6487–97.
921 [6] Losada SA, Canto-Nogues C, Munoz-Fernandez A. A new possible mechanism of human immunodeficiency virus type 1 infection of neural cells. Neurobiol Dis 2002;11:469–78. [7] Nagele RG, Andrea MR, Anderson WJ, Wang HY. Intracellular accumulation of eta-amyloid(1-42) in neuron facilitated by the alpha 7 nicotinic acetylcholine receptor in Alzheimer’s disease. Neuroscience 2002;110:199–211. [8] Marie N, Lecoq I, Jauzac P, Allouche S. Differential sorting of human delta-opioid receptors after internalization by peptide and alkaloid agonists. J Biol Chem 2003;278:22795–804.