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Mitochondria are thought to have evolved from ancient prokaryotes living inside other cells as endosymbionts. One vestige of this evolutionary history is that the mitochondrial organelle still maintains and expresses its own DNA (mtDNA). Our laboratory has a long-standing and continuing interest in this extrachromosomal genome, mainly in the areas of mtDNA replication and transcription.
MtDNA encodes essential proteins involved in oxidative phosphorylation, and the production of ATP is a well-known function of the mitochondria. Therefore, mtDNA expression defects are most commonly associated with bioenergetic deficiencies. However, ATP production is not the only function of the mitochondria.
Mitochondria are also involved in a variety of signaling pathways, including apoptosis, survival, growth, development, and immune responses, among others. The details of many of these signaling pathways remain obscure. Nonetheless, it appears that the assimilation of a symbiotic prokaryote through evolutionary time has involved varied and unique mechanisms of communication between mitochondria and other intracellular and extracellular compartments.
Some of our current research interests are at
the interface between the molecular dynamics of mtDNA and
extramitochondrial signaling events. MtDNA is organized within "nucleoids," which are
loosely packaged bundles of mtDNA, RNA, and various proteins. Accumulating
evidence suggests that nucleoids may represent the "activity unit" for mtDNA
location, replication, and expression. Molecular signals to and from the
mitochondrial genome are mediated by the components of the nucleoid. Most of the
questions in this area remain fundamental. Is the nucleoid composition
homogenous, or is it tailored to local needs within a cell? Which signals are
directly mediated by the nucleoid components? What subset of nucleoids is
associated with the inner mitochondrial membrane or with contiguous cytoskeletal
structures, where they might be better positioned for intracellular
communication? We are using various fluorescently tagged proteins and the newly
developed PALM imaging system in this effort (in collaboration with Eric Betzig
and others at JFRC). The high spatial resolution provided by PALM provides a
unique opportunity in this regard, since the small physical dimensions of
mitochondria have previously precluded such an analysis. This and other
high-resolution optical techniques may elaborate the physical parameters of the
nucleoid in different cell types and physiological situations.
An outstanding problem in mitochondrial biology is the lack of a
transformation system for introducing new or altered mtDNA genes to the intracellular
organelle environment. There have been attempts to achieve this through
ballistic approaches or by associating mtDNA elements with nucleus-encoded
mitochondrial proteins that are known to be imported into mitochondria. A recent publication has
suggested that linear forms of mtDNA may be imported, perhaps at natural protein
import sites. It has also been reported that cellular 5S ribosomal RNA is
normally imported in mitochondria. We will try to exploit these initial
findings in attempts to develop a competent mitochondrial transformation system
that would represent a powerful tool to study the fine details of mtDNA
function, as well as provide a mechanism to "rescue" cells that have impaired
respiratory capacity due to inherited mutations in their native mitochondrial
genomes.
We are also exploring model systems in which to study in a more general fashion the roles of the mitochondria in synapse development, maintenance, and degeneration. This is in context with neuronal networking themes at JFRC.