New mechanism for HIV infection of neurons discovered

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THE LANCETNeurology Vol 2 February 2003 http://neurology.thelancet.com 71

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In a technological tour de force, twogroups of neuroscientists haveindependently developed a method forvisualising the turnover of spines—thetiny protrusions from dendrites—inlive mice over a period of months.Although previous studies done in vitrohad shown that spines are dynamicstructures that can grow in response tointense synaptic stimulation, until nowthe use of light microscopy to studyspines in vivo seemed impossible.

The two groups of researchers, oneled by Karel Svoboda (Cold SpringHarbor Laboratory, New York, USA)and the other by Wen-Biao Gan (NewYork University School of Medicine,USA), both used mice that express afluorescent protein in a few neurons inthe cerebral cortex. This allowed themorphology of individual neurons tobe tracked over time. They both used amultiphoton based imaging approachto excite the fluorescent protein byshining an infrared laser through awindow in the mice’s skulls. Infrared

light, in contrast to visible light, canpenetrate several hundreds ofmicrometres into the brain, far enoughto reach the upper layers of the cortex.

Interestingly, however, the twogroups came to different conclusionsabout the stability of spines in adultanimals. Svoboda’s team identifiedthree populations of spines withdifferent turnover rates; the most stablegroup of spines had a lifetime of justover 3 months (Nature 2002; 420:788–94). By contrast, Gan’s groupfound that 96% of spines remained

stable over a month-long periodimplying a half-life of more than 13months (Nature 2002 420: 812–16).These differences may be because thetwo groups examined different corticalareas—the barrel cortex (which receivessensory inputs from the whiskers) andthe primary visual cortex—or becausethe mice used were of different ages.

“Since ours are the first glimpses ofsynapses in vivo over long times we arein a priviledged position, sort of likeearly astronomers or astronauts, andanything we find in this strange world isnew and informative”, says Svoboda.Gan also thinks that the techniqueholds great promise for the future: “Webelieve that the long-term imagingapproach that we developed in ourstudy should provide a very powerfulapproach for studying not only basicbrain functions, but also disease-relatedlong-term structural changes in thebrain, such as Alzheimer’s disease,Huntington’s disease, or stroke”. James Butcher

Long-term mapping of dendritic spines now possible

Spanish scientists have shown thatHIV infects human neuroblastomacells, apparently requiring only CCR5or CXCR4 chemokine receptors to doso (Neurobiol Dis 2002, doi:10.1006/nbdi.2002.0566). This finding suggeststhat the virus might be capable ofinfecting neurons.

40 to 50% of patients with AIDShave neurological problems, includingencephalopathy in children and AIDSdementia complex in adults, butwhether their neurons are actuallyinfected is unclear. To date,neurological damage has largely beenattributed to HIV-infected microgliareleasing tumour necrosis factor-�,nitric oxide, or some toxin. Unlikeneurons, microglia express CD4, themajor receptor implicated in HIVaccess to T cells—the virus gains entryby linking to CD4 and one of a seriesof co-receptors. The new work, how-ever, suggests that the chemokine co-receptors CCR5 or CXCR4 are all thatis needed for HIV to infect neuroblast-oma cells. “Cells we thought were not

infected appear to be infected, and co-receptors might be the mainreceptors”, explains team leader Maria Angeles Muñoz-Fernández(Laboratory of ImmunomolecularBiology, Gregorio Marañón Hospital,Madrid, Spain).

The researchers took CD4-negativeneuroblastoma cells and challengedthem with HIV-1. They subsequentlybecame infected, as shown by theappearance of large quantities of theviral protein Ag-p24. They then testedwhether blocking the co-receptorscould prevent this infection.Antibodies to galactosylceramide andnucleolin—known co-receptors inother cell types but also expressed byneuroblastoma cells—failed to inhibitHIV infection, clearing them ofinvolvement in viral entry. Neither wasHIV infection inhibited whenchemokine receptors were blockedwith their normal ligands. However,few chemokine receptors are held atthe cell surface at any one time, mostare found in reservoirs inside the cell.

To increase their presence in themembrane, heparan sulphate, aproteoglycan found in all cell types,was added. Although viral entry wasinhibited by heparan sulphate alone,infection was further inhibited whencombined with the CCR5 and CXCR4ligands. Moreover, incubation of thecells with either anti-CCR5 or anti-CXCR4 antibodies strongly inhibitedinfection. “CCR5 or CXCR4 mighttherefore be used by different HIVstrains to gain entry to neural cells—including neurons. Antiretroviraldrugs that could cross the blood–brainbarrier and block them might helpprevent neurological problems”, saysMuñoz-Fernández.

“These are intriguing results”,comments Santos Mañes (NationalCenter for Biotechnology, Madrid,Spain). “However, heparan sulphate onits own partially inhibited viral entry inthese neuroblastoma cells, suggestingthat it too may play an important rolein HIV-1 entry in some cells.”Adrian Burton

New mechanism for HIV infection of neurons discovered

Windows into the brain

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