The
long-term goal of my research program is to understand how cells
establish and maintain organelle identity and function. Why
am I interested in this?
INTRODUCTION: The hallmark of the eukaryotic cell is the
presence of membrane-bound organelles, which compartmentalize
the cell and segregate reactions that are often incompatible.
To illustrate this point, the endoplasmic reticulum (ER) is a
labyrinth-like membrane network enriched in specific chaperonin
proteins that aid new proteins to fold correctly. In contrast,
lysosomes are small, spherical, highly acidic organelles enriched
in acid-hydrolases that degrade proteins.
The
distinct biophysical, biochemical and functional properties of
each organelle constitute its identity, which is endowed by its
molecular composition. In turn, the molecular composition of an
organelle results from a complex network of processes like membrane
trafficking and molecular targeting. Intriguingly, organelles
incessantly exchange content by membrane trafficking, but despite
this constant exchange, organelles retain their identity. For
example, while the ER and lysosomes are unreservedly distinct,
they are part of a continuous bidirectional membrane trafficking
"freeway" that includes the Golgi and endosomes. In other words,
organelles don't merge into one hybrid structure!
Underpinning
the importance of organelle function are the serious ailments
arising when organelles malfunction, which range from various
hereditary conditions like the lysosome-based Niemann-Pick Disease
to cancer. Therefore, I am interested in unraveling the molecular
mechanisms that establish and maintain organelle identity.
Currently,
we are focused on phosphoinositides (PtdInsPs), which are key
determinants of organelle identity and function. These are signaling
phospholipids that govern myriad functions in the cell including
membrane traffic between organelles, membrane deformation, and
the cytoskeleton. In fact, there are seven PtdInsPs, each with
a stereotypical intracellular distribution. This distribution
is a central element of the molecular code that establishes organelle
identity.
What
do PtdInsPs do? The presence of a specific PtdInsP species within
a membrane helps to recruit and anchor cognate proteins (PtdInsP
effector proteins) to that membrane. By this very method, PtdInsPs
help to control the molecular composition of an organelle!
CURRENT
RESEARCH GOALS: my research is divided into three major branches:
i) I am interested in the molecular mechanisms that enable cells
to regulate, locate and coordinate PtdInsP kinases and phosphatases
that synthesize, degrade and interconvert PtdInsPs. These enzymes
determine where, when and how much PtdInsP is made or eliminated
in cells.
ii)
I am interested in how PtdInsP effector proteins are recruited
and perform their function. There are myriad PtdInsP effectors
ranging from ion channels, to cytoskeleton proteins, protein kinases
and phosphatases, adaptor proteins, GTPase regulating proteins,
membrane fusion and fission proteins, etc.
iii)
I am interested in how PtdInsP signaling interfaces with other
architects of organelle function including Rab and ARF GTPases.
These regulators give specificity to PtdInsP signaling.
CURRENT
RESEARCH MODELS AND APPROACHES: To study these processes,
my research program currently uses two biological model systems:
i) We use yeast genetics and biochemistry to dissect the regulation
of PtdInsPs.
ii)
We use phagosomes as model organelles to study organelle identity
processes. Phagosomes are organelles containing large extracellular
particles (like bacteria) engulfed during phagocytosis. Phagosomes
are interesting because they undergo phagosome maturation, a programmed
process during which phagosomes change identity - from a plasma
membrane-like organelle to a lysosome-like organelle. Therefore,
they are perfect to study mechanisms of organelle identity, and
as a bonus, we improve our understanding of an important immune
process.
Finally,
in order to address questions related to these biological processes,
we employ a multi-disciplinary approach that uses genetics, molecular
biology, biochemistry, cell biology and bioinformatics.
