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R.
Roy Baker
Professor
B.Sc., University of Toronto, 1969
Ph.D., University of Toronto, 1973
Max Planck Institute for Biophysical Chemistry
(Gottingen), 1973-75
Montreal Neurological Institute (McGill University),
1976 |
Medical
Sciences Building, Room 5219A
416-978-6921
roy.baker@utoronto.ca |
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Phospholipid and Glyceride
Metabolism in Neuronal Nuclei and Ischemic Brain
Research Synopsis
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Of the tissues in the body,
central nervous tissue is the most highly specialized
and complex. It is highlighted by the neuronal cell- the
transmitter of nervous impulses- with its central perikaryon,
the focus of the cell's synthetic machinery, the forest
of receptive dendrites, the elongated axon and numerous
synaptic terminals.
In our laboratory we are studying the metabolism of phospholipids
and neutral glycerides (diacylglycerols and triacylglycerols)
in various subcellular membrane fractions of brain. The
techniques of lipid analysis, enzyme assays and tissue
fractionation are employed. We have several areas of research
interest.
We have found, of several subcellular fractions prepared
from cerebral cortex, that nuclear membranes are remarkably
active in lipid metabolism. The nuclear membranes are
found as a nuclear envelope, with outer and inner membranes
joined at many points to form nuclear pores. By density
gradient centrifugation neuronal nuclei can be purified
from homogenates of cerebral cortex. The nuclear membranes
have quite high lipid acylation rates, particularly in
the acylation of lysophospholipids and diacylglycerols.
The neuronal nuclear membranes have a remarkably sizeable
pool of diacylglycerols which may participate in nuclear
signalling events.
We have also demonstrated that the biologically active
phospholipid, platelet-activating factor (PAF), a central
player in inflammatory responses such as those encountered
in stroke, is made at the neuronal nuclear membrane. PAF
can regulate transcriptional events leading to the synthesis
of cyclooxygenase 2 (COX 2) and its synthesis at the nuclear
membranes is of considerable interest with respect to
the production of inflammatory eicosanoids observed in
brain ischemia. It is of interest that one pathway for
PAF formation, the acetylation of lyso PAF, is found at
very high specific activity in the nuclear membrane, as
this pathway is the one associated with biological response
and cellular stimulation. We have studied a number of
enzymes involved in PAF metabolism and ultimately hope
to control or block this event by specific inhibitors,
lessening the damage that PAF can bring during brain ischemia.
Of particular interest to us are the regulatory mechanisms
that control PAF synthesis. For example we have found
that ATP, the energy carrier in cells, will inhibit the
formation of PAF, indicating that periods of ischemia
that see precipitous declines in ATP may well allow a
window for PAF generation.
As we have interests in stroke, we are also studying animal
models of brain ischemia in which the bloodflow to the
brain is cut off for certain periods of time and reperfusion
is then initiated. This insult to the brain results in
selective neuronal loss some days after the ischemic period,
particularly in a brain region known as the hippocampus,
a centre of memory and learning. As PAF likely plays a
role in the pathology associated with these events we
are currently studying the responses of brain slices prepared
from ischemic tissue in efforts to follow PAF metabolism
during neurodegeneration. We plan to quantitate levels
of PAF and its metabolites by mass spectrometric analyses
and to follow the metabolism of lyso PAF and other PAF
precursors in these slices. The metabolism of free fatty
acids in ischemic brain is also of interest as these compounds
accumulate rapidly in brain tissue deprived of glucose
and oxygen. Fatty acids are used in acylation mechanisms
that utilize lyso PAF to form 1-alkyl-2-acyl species of
choline phospholipids. The control of acylation is of
critical importance in regulating lyso PAF acetylation,
as the acetylation and acylation paths are competing for
the same substrate.
Nuclei are also known for their metabolism of polyphophoinositides
such as PIP2 and PIP. We have found that the most prominent
phosphorylation product of neuronal nuclei is phosphatidic
acid, a lipid which also has biological activity in cells.
As neuronal nuclei have a significant pool of diacylglycerols,
formed primarily by the degradation of PIP2, we are interested
in the regulation of diacylglycerol turnover and the diacylglycerol
kinase that generates phosphatidic acid. Diacylglycerols
may also be formed from other phospholipids to supply
the activity of diacylglycerol lipase in nuclei, itself
a significant source of nuclear free fatty acids. The
effect of brain ischemia on these paths that surround
diacylglycerols in nuclei is of considerable interest,
and one that we will be following in animal models of
stroke.
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Selected Publications
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MgATP may depress de novo
neuronal nuclear PAF generation by promoting the formation
of alkylacylglycerophosphate, an inhibitor of alkylglycerophosphate
acetyltransferase.
Baker, R.R. and Chang, H.-Y. (2002) Biochim. Biophys.
Acta (in press)
Phosphatidic acid is the prominent product of endogenous
neuronal nuclear lipid phosphorylation, an activity enhanced
by sphingosine, linked to phospholipase C and associated
with the nuclear envelope.
Baker, R.R. and Chang, H.-Y. (2001) Biochim. Biophys.
Acta 1534, 110-120
The regulation of CoA-independent transacylation reactions
in neuronal nuclei by lysophospholipid, free fatty acid
and lysophospholipase. The control of nuclear lyso platelet-activating
factor metabolism.
Baker, R.R. and Chang, H.-Y. (2000) Mol. Cell Biochem.
215, 135-144.
A metabolic path for the degradation of lysophosphatidic
acid, an inhibitor of lysophosphatidylcholine lysophospholipase
in neuronal nuclei of cerebral cortex.
Baker, R.R. and Chang, H.-Y. (2000) Biochim. Biophys.
Acta 1483, 58-68.
Lipid acetylation reactions.
Baker, R.R..(2000) Neurochem. Res. 25, 677-683. |
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