Human Polyomavirus-Associated Cerebral Disorders in the Post-HAART Era

Strength in Diversity: Nuclear Export of Viral RNAs

The nuclear export of cellular mRNAs is a complex process that requires the orchestrated participation of many proteins that are recruited during the early steps of mRNA synthesis and processing. This strategy allows the cell to guarantee the conformity of the messengers accessing the cytoplasm and the translation machinery. Most transcripts are exported by the exportin dimer Nuclear RNA export factor 1 (NXF1)-NTF2-related export protein 1 (NXT1) and the transcription-export complex 1 (TREX1). Some mRNAs that do not possess all the common messenger characteristics use either variants of the NXF1-NXT1 pathway or CRM1, a different exportin. Viruses whose mRNAs are synthesized in the nucleus (retroviruses, the vast majority of DNA viruses, and influenza viruses) exploit both these cellular export pathways. Viral mRNAs hijack the cellular export machinery via complex secondary structures recognized by cellular export factors and/or viral adapter proteins. This way, the viral transcripts succeed in escaping the host surveillance system and are efficiently exported for translation, allowing the infectious cycle to proceed. This review gives an overview of the cellular mRNA nuclear export mechanisms and presents detailed insights into the most important strategies that viruses use to export the different forms of their RNAs from the nucleus to the cytoplasm.

PPAR agonist-mediated protection against HIV Tat-induced cerebrovascular toxicity is enhanced in MMP-9-deficient mice

The strategies to protect against the disrupted blood-brain barrier (BBB) in HIV-1 infection are not well developed. Therefore, we investigated the potential of peroxisome proliferator-activated receptor (PPAR) agonists to prevent enhanced BBB permeability induced by HIV-1-specific protein Tat. Exposure to Tat via the internal carotid artery (ICA) disrupted permeability across the BBB; however, this effect was attenuated in mice treated with fenofibrate (PPARα agonist) or rosiglitazone (PPARγ agonist). In contrast, exposure to GW9662 (PPARγ antagonist) exacerbated Tat-induced disruption of the BBB integrity. Increased BBB permeability was associated with decreased tight junction (TJ) protein expression and activation of ERK1/2 and Akt in brain microvessels; these effects were attenuated by cotreatment with fenofibrate but not with rosiglitazone. Importantly, both PPAR agonists also protected against Tat-induced astrogliosis and neuronal loss. Because disruption of TJ integrity has been linked to matrix metalloproteinase (MMP) activity, we also evaluated Tat-induced effects in MMP-9-deficient mice. Tat-induced cerebrovascular toxicity, astrogliosis, and neuronal loss were less pronounced in MMP-9-deficient mice as compared with wild-type controls and were further attenuated by PPAR agonists. These results indicate that enhancing PPAR activity combined with targeting MMPs may provide effective therapeutic strategies in brain infection by HIV-1.

CNS-immune reconstitution inflammatory syndrome in the setting of HIV infection, part 1: overview and discussion of progressive multifocal leukoencephalopathy-immune reconstitution inflammatory syndrome and cryptococcal-immune reconstitution inflammatory syndrome

SUMMARY

While uncommon, CNS-IRIS developing after the initiation of HAART in the setting of HIV-related severe immunosuppression is characterized by an intense inflammatory reaction to dead or latent organisms or to self-antigens due to a heightened but dysregulated immune response. While this reaction can range from mild to fulminating, encompassing a very wide clinical spectrum, it is important to recognize because changes in medical management may be necessary to prevent neurologic decline and even death. Once contained, however, this inflammatory response can be associated with improved patient outcome as immune function is restored. Among the infectious organisms that are most commonly associated with CNS-IRIS are the JC virus and Cryptococcus organisms, which will be the subject of this review. CD8 cell infiltration in the leptomeninges, perivascular spaces, blood vessels, and even parenchyma seems to be the pathologic hallmark of CNS-IRIS. While recognition of CNS-IRIS may be difficult, the onset of new or progressive clinical symptoms, despite medical therapy and despite improved laboratory data, and the appearance on neuroimaging studies of contrast enhancement, interstitial edema, mass effect, and restricted diffusion in infections not typically characterized by these findings in the untreated HIV-infected patient should raise the strong suspicion for CNS-IRIS. While CNS-IRIS is a diagnosis of exclusion, the neuroradiologist can play a critical role in alerting the clinician to the possibility of this syndrome.