Apoptosis, or programmed cell death, is an important process for a myriad of different reasons.
It is vital during embryogenesis and later development, for normal functioning of the immune system,
and in tissue homeostasis. In addition, the abnormal regulation of apoptosis is thought to play an important
role in a number of human diseases, including cancer, neurological degenerative diseases, autoimmunity,
and AIDS. The Clem lab has two main interests: one is understanding how the process of apoptosis is
regulated at the molecular level, and the second is understanding how apoptosis functions in anti-viral
defense. We are especially interested in mechanisms by which viruses are able to manipulate apoptosis
in their host cells for their own benefit. Many different viruses do this, including baculoviruses, poxviruses,
adenoviruses, papovaviruses, and
herpesviruses. The ba
culoviruses contain genes which block apoptosis
in their host cells, leading to increased viral replication and virulence in both cell culture and in insects. The
virus we work with for the most part is called Autographa californica multicapsid nucleopolyhedrovirus,
or AcMNPV. The entire circular DNA genome of AcMNPV has been sequenced, and it is 133,894 bp
in length, with the potential to encode around 150 proteins. Only around half of these proteins have been
studied functionally. AcMNPV carries an anti-apoptotic gene, p35, which is a specific inhibitor of caspases.
Caspases are an evolutionarily conserved family of cysteine proteases that are centrally involved in
carrying out apoptosis. Expression of
p35 during AcMNPV replication prevents an apoptotic response
by the host cells. The P35 protein is also able to block apoptosis in nematodes,
Drosophila, and
vertebrate cells, making it a useful tool in the study of cell death.
Other baculoviruses carry genes called
iap (Inhibitor of APoptosis) genes, which also block apoptosis in a variety of organisms. Cellular iap
homologs have been discovered in Drosophila and mammals, and these cellular homologs also function to
prevent apoptosis.
Another area of interest is the role of apoptosis as an anti-viral defense response in insects. Little is known
about insect immunity against virus infection. Mutant viruses lacking p35 are severely disabled in their ability
to infect insect larvae, and this defect can be corrected by expression of an iap gene, suggesting that apoptosis
plays a role in host defense. We have used wild type and p35 mutant viruses expressing the green fluorescent
protein (GFP) to
track virus infection in infected larvae, and we have shown that infection
with p35 mutant
virus
correlates with apoptosis in the infected insect.