Usually, HIV attacks the lymphocytes of the immune system, binding specifically
to receptors know as CD4 molecules. Neurons and other brain cells are not
know to possess these receptors and so would seem to be safe.

But Yaffa Mizrachi and her colleagues at St Luke’s-Roosevelt Hospital
believe they have located a receptor which allows HIV to enter cells in
the brain. The receptor is on the membrane of neurons and glial cells, the
support cells of the brain. The researchers reported their results at the
20th annual meeting of the Society for Neuroscience in St Louis last week.

Until now, researchers have debated whether dementia is caused by HIV
itself or by one of the opportunistic infections that strike people with
AIDS. No one has known how HIV enters the brain. Mizrachi believes that
HIV uses a protein on its outer coat, known as gp120.

In the test tube, Mizrachi was able to successfully infect brain cells
with gp120 via a receptor other than CD4. Once it has attached itself to
the cells, the viral protein entered by fusing with the membrane.

Mizrachi is now trying to characterise the receptor that allows entry,
and locate the gene that codes for it. She believes it might then be possible
to devise a treatment for AIDS patients with dementia that would block these
receptors, leaving the virus with no place to link up.

But researchers are still puzzled about how the virus harms brain cells
once it enters Luke’s, once HIV is integrated into brain cells, it does
not replicate well enough to do the sort of damage it does in the cells
of the immune system. ‘It is probably not killing cells,’ says Volsky. Instead,
it must be causing some other action, he says.

Stuart Lipton at Harvard University has suggested how this might work.
He has discovered that gp120 appears to make nerve cells more sensitive
to glutamate, which they use for communication. Too much glutamate, which
occurs in cases of stroke or trauma, is believed to allow calcium in cells
to reach harmful levels. Oversensitivity to glutamate could cause the motor
abnormalities and reasoning disabilities associated with AIDS dementia,
says Lipton.

Over the next two months, all the telescope’s other instruments will
be tested as well. Then NASA will advise astronomers how to adapt their
experiments to get the best data possible. ‘We expect now that about 50
to 60 per cent of the original science can be done,’ says Ed Weiler, NASA’s
programme scientist for Hubble. Most of the loss involves a wide field camera.
A faint object camera, a photometer and spectrometers will also be affected,
but not so seriously.

‘It’s the deep sky astronomy, looking back through time, that is most
affected,’ says Weiler. Objects of most interest are typically some 10 billion
light years away. The blurring of the images caused by the spherical aberration
of the primary mirror does not destroy resolution of objects; astronomers
can still distinguish objects within 0.10 arc seconds of each other. But
the ‘encircled energy’ captured by Hubble’s instruments, a measure of an
object’s brightness, is expected to remain only 15 to 20 per cent, down
from a projected 70 per cent. ‘The energy is being blurred out,’ Weiler
explains.

NASA will use a computer to ‘deemphasise the fuzz’ and enhance the bright
core of images, says Weiler. This will erase any ‘haloes’ created by the
mirror’s aberration. The tests will also help NASA to design a replacement
for the faulty mirror, to be launched by the space shuttle in 1993.

NASA concluded last month that a faulty optical reference device used
to measure the surface of the mirror during its manufacture caused the aberration
(This Week, 18 August).

The error in the device measured about one millimetre, which is consistent
with the amount of blurring in the images the telescope is sending back
to Earth. The board of inquiry investigating the ‘single but costly error’
by the mirror’s manufacturer has not determined how the slip-up occurred.
They are investigating the possibility that washers on the baseplate of
the lens assembly, which are 1.3 millimetres across, had been improperly
installed.