TARRYTOWN, NY--An essential step in the life cycle of HIV is entry
into the cell, allowing delivery of the viral genetic information to
the target cell cytoplasm. Attachment and fusion are the critical
events that make this possible.
A new HIV assay, developed by Progenics Pharmaceuticals, Inc. and
known as ProSys, permits real-time determination of
glycoprotein-mediated membrane fusion.
Such research could help lay the groundwork for development of
therapeutic agents to specifically inhibit HIV membrane fusion, says
Graham P. Allaway, PhD, head of Progenics' Therapeutic Development Group.
The assay system works by a technique known as resonance energy
transfer (REF). The assay measures the fusion of two plasma
membranes, one labeled with a fluorescent dye that expresses the HIV
envelope glycoprotein (gp120-gp41) and one with a dye that expresses
the CD4 receptor. Fusion results in the close association of the dyes
in the plasma membranes and is detected by the transfer of energy
between the dyes.
Researchers used ProSys to analyze HIV-1 membrane fusion with CD4+
target cell lines in both a laboratory-adapted HIV strain and a
primary isolate. They found that membrane fusion mediated by the
envelope glycoproteins of these viruses had very similar properties (Journal of Virology
"In particular," Dr. Allaway says, "the degree and
kinetics of membrane fusion were similar. Membrane fusion mediated by
both isolates occurred at neutral pH and was dependent on the
presence of divalent cations."
In the primary HIV isolate, fusion was inhibited at levels of soluble
CD4 and CD4-IgG2 (a CD4-Ig fusion protein) similar to those required
to neutralize the virus. However, in the laboratory strain, higher
levels of CD4 and CD4-IgG2 were required to inhibit than to
neutralize membrane fusion. "This finding suggests that the
mechanisms of membrane fusion inhibition and neutralization of HIV-1
are distinct," he commented.
Dr. Allaway's co-workers in the study were Virginia Litwin, Kirsten
Nagashima, Andrew Ryder, Chun-Huey Chang, Jeffrey Carver, William
Olson, Karl Hasel, and Paul Maddon of Progenics, and Marc Alizon of
the Institut Cochin de Genetique Moleculaire, Paris.