Biomedical subjects

Robert Fern

Publications and source records attributed to Robert Fern.

6 recordsLinked to original sources

PubMed2006

Hypoxia-ischemia preferentially triggers glutamate depletion from oligodendroglia and axons in perinatal cerebral white matter.

Ischemia is implicated in periventricular white matter injury (PWMI), a lesion associated with cerebral palsy. PWMI features selective damage to early cells of the oligodendrocyte lineage, a phenomenon associated with glutamate receptor activation. We have investigated the distribution of glutamate in rat periventricular white matter at post-natal day 7. Immuno-electron microcopy was used to identify O4(+) oligodendroglia in control rats, and a similar approach was employed to stain glutamate in these cells before and after 90 mins of hypoxia-ischemia. This relatively brief period of hypoxia-ischemia produced mild cell injury, corresponding to the early stages of PWMI. Glutamate-like reactivity was higher in oligodendrocytes than in other cell types (2.13+/-0.25 counts/microm(2)), and declined significantly during hypoxia-ischemia (0.93+/-0.15 counts/microm(2): P<0.001). Astrocytes had lower glutamate levels (0.7+/-0.07 counts/microm(2)), and showed a relatively small decline during hypoxia-ischemia. Axonal regions contained high levels of glutamate (1.84+/-0.20 counts/microm(2)), much of which was lost during hypoxia-ischemia (0.72+/-0.20 counts/microm(2): P>0.001). These findings suggest that oligodendroglia and axons are the major source of extracellular glutamate in developing white matter during hypoxia-ischemia, and that astrocytes fail to accumulate the glutamate lost from these sources. We also examined glutamate levels in the choroid plexus. Control glutamate levels were high in both choroid epithelial (1.90+/-0.20 counts/microm(2)), and ependymal cells (2.20+/-0.28 counts/microm(2)), and hypoxia-ischemia produced a large fall in ependymal glutamate (0.97+/-0.08 counts/microm(2): P>0.001). The ependymal cells were damaged by the insult and represent a further potential source of glutamate during ischemia.

Animals↗

PubMed2005

NMDA receptors are expressed in developing oligodendrocyte processes and mediate injury.

Injury to oligodendrocyte processes, the structures responsible for myelination, is implicated in many forms of brain disorder. Here we show NMDA (N-methyl-D-aspartate) receptor subunit expression on oligodendrocyte processes, and the presence of NMDA receptor subunit messenger RNA in isolated white matter. NR1, NR2A, NR2B, NR2C, NR2D and NR3A subunits showed clustered expression in cell processes, but NR3B was absent. During modelled ischaemia, NMDA receptor activation resulted in rapid Ca2+-dependent detachment and disintegration of oligodendroglial processes in the white matter of mice expressing green fluorescent protein (GFP) specifically in oligodendrocytes (CNP-GFP mice). This effect occurred at mouse ages corresponding to both the initiation and the conclusion of myelination. NR1 subunits were found mainly in oligodendrocyte processes, whereas AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)/kainate receptor subunits were mainly found in the somata. Consistent with this observation, injury to the somata was prevented by blocking AMPA/kainate receptors, and preventing injury to oligodendroglial processes required the blocking of NMDA receptors. The presence of NMDA receptors in oligodendrocyte processes explains why previous studies that have focused on the somata have not detected a role for NMDA receptors in oligodendrocyte injury. These NMDA receptors bestow a high sensitivity to acute injury and represent an important new target for drug development in a variety of brain disorders.

Animals↗

PubMed2005

Acute lipopolysaccharide-mediated injury in neonatal white matter glia: role of TNF-alpha, IL-1beta, and calcium.

Bacterial infection is implicated in the selective CNS white matter injury associated with cerebral palsy, a common birth disorder. Exposure to the bacterial endotoxin LPS produced death of white matter glial cells in isolated neonatal rat optic nerve (RON) (a model white matter tract), over a 180-min time course. A delayed intracellular Ca(2+) concentration ([Ca(2+)](i)) rise preceded cell death and both events were prevented by removing extracellular Ca(2+). The cytokines TNF-alpha or IL-1beta, but not IL-6, mimicked the cytotoxic effect of LPS, whereas blocking either TNF-alpha with a neutralizing Ab or IL-1 with recombinant antagonist prevented LPS cytotoxicity. Ultrastructural examination showed wide-scale oligodendroglial cell death in LPS-treated rat optic nerves, with preservation of astrocytes and axons. Fluorescently conjugated LPS revealed LPS binding on microglia and astrocytes in neonatal white and gray matter. Astrocyte binding predominated, and was particularly intense around blood vessels. LPS can therefore bind directly to developing white matter astrocytes and microglia to evoke rapid cell death in neighboring oligodendroglia via a calcium- and cytokine-mediated pathway. In addition to direct toxicity, LPS increased the degree of acute cell death evoked by ischemia in a calcium-dependent manner.

Animals↗

PubMed2004

Acute ischemic injury of astrocytes is mediated by Na-K-Cl cotransport and not Ca2+ influx at a key point in white matter development.

Cerebral palsy is a common birth disorder that frequently involves ischemic-type injury to developing white matter (WM). Dead glial cells are a common feature of this injury and here we describe a novel form of acute ischemic cell death in developing WM astrocytes. Ischemia, modeled by the withdrawal of oxygen and glucose, evoked [Ca2+]i increases and cell death in astrocytes in post-natal day 10 (P10) rat optic nerve (RON). Removing extracellular Ca2+ prevented increases in [Ca2+]i but increased the amount of cell death. Astrocytes showed rapid [Na+]i increases during ischemia and cell death was reduced to control levels by substitution of extracellular Na+ or Cl- or by perfusion with bumetanide, a selective Na-K-Cl cotransport (NKCC) blocker. Astrocytes showed marked swelling during ischemia in the absence of extracellular Ca2+, which was blocked by bumetanide. Raising the extracellular osmolarity to limit water uptake reduced ischemic astrocyte death to control levels. Ultrastructural examination showed that post-ischemic astrocytes had lost their processes and frequently were necrotic, effects partially prevented by bumetanide. At this point in development, therefore, NKCC activation in astrocytes during ischemia produces an osmo-regulatory challenge. Astrocytes can subsequently regulate their cell volume in a Ca2+-dependent fashion but this will require ATP hydrolysis and does not protect the cells against acute cell death.

Acute Disease↗

PubMed2004

Mechanism of acute ischemic injury of oligodendroglia in early myelinating white matter: the importance of astrocyte injury and glutamate release.

In developing CNS white matter (WM), the period of early myelination is characterized by a heightened sensitivity to ischemic injury. Using an in situ (isolated) preparation, we show that the mechanism of acute ischemic injury of immature WM oligodendroglial involves Ca2+ influx though non-NMDA type glutamate receptors (GluRs). The Ca2+-influx and acute cell death that was evoked by ischemic conditions (oxygen and glucose withdrawal) in identified P10 rat optic nerve oligodendroglia were blocked by removing extracellular Ca2+ or by CNQX, a selective non-NMDA GluR antagonist. The selective Na-K-Cl cotransport (NKCC) inhibitor bumetanide was also highly protective, even though NKCC expression is restricted to astrocytes in this tissue. Bumetanide largely prevented the non-NMDA GluR-mediated [Ca2+]i rise evoked by ischemia in oligodendroglia, suggesting that it interfered with ischemic glutamate release. In control WM, glutamate-like reactivity was located mainly in astrocytes and oligodendroglia identified using ultrastructural criteria. In ischemic WM, astrocyte glutamate-like reactivity was reduced, an effect countered by bumetanide. We suggest a model in which NKCC-dependent injury and release of glutamate from astrocytes activates glutamate receptors on oligodendroglia, resulting in Ca2+-influx and acute cell death.

Acute Disease↗

PubMed2003

Variations in spare electron transport chain capacity: The answer to an old riddle?

Several neurological diseases involve focal injury of specific brain structures. Poisons of the electron transport chain complexes (ETCC) can also produce selective injury of brain structures when given systemically and have been implicated in the development of neurological disease. Why ETCC poisons damage particular brain regions is unclear. Calculations of the relative ETCC expression level to glucose utilization rate (GUR) ratio from published observations here reveal that a low ETCC/GUR ratio predisposes a brain structure to injury by a poison of that complex. While GUR can rise with increased neuronal activity, ETCC expression is fixed in the short term. A high ETCC/GUR therefore represents surplus ETCC capacity, allowing for increased ATP generation with short-term increases in demand. A low ETCC/GUR indicates the opposite and will lead to energy failure when the specific ETCC is poisoned. These observations may explain why cyanide, a specific ETCC (IV) inhibitor, can produce selective injury of white matter, which has the lowest ETCC (IV)/GUR found in the brain. They are also consistent with the selective damage of the striatum produced by poisons such as rotenone, a form of injury implicated in Parkinson's disease. The striatum has a low ETCC (I)/GUR ratio, whereas rotenone is a selective ETCC (I) inhibitor.

Animals↗