May 2010 Fact Sheet
Leading neuroscientists, clinicians, neurobiologists, and pediatric neurologists came together for a symposium entitled “Injury to the Preterm Brain and Cerebral Palsy” in conjunction with the 37th Annual Meeting of the Child Neurology Society on November 5th, 2008. This symposium was supported by the National Institutes of Health, the Child Neurology Society, the Kennedy Krieger Institute, and the Cerebral Palsy International Research Foundation. Recently a summary of the symposium was published in the Journal of Child Neurology by Michael Babcock, Felina Kostova and Drs. Donna Ferriero, Michael Johnson, Jan Brunstrom, Henrik Hagberg and Bernard Maria. The second session was on molecular mechanisms and animal models of injury to the preterm brain.
Apoptotic Mechanisms of Cell Death in the Immature Brain
Dr. Hagberg of Goteborg University discussed how there is tremendous complexity in the pathophysiology of hypoxic-ischemic (HI) brain injury, one of the most common causes of CP. After the initial injury, there is a regenerative and compensatory response in which mitochondrial respiration and glucose metabolism fully recover in brain tissue. This recovery is often followed by a secondary deterioration that signifies a secondary injury. Dr. Hagberg’s research focuses on apoptotic mechanisms occurring between the first and second injury because it may be the most clinically effective time to initiate therapy.
The intrinsic pathway is a mechanism of apoptosis and is likely responsible for secondary injuries due to a HI event in the premature brain. An internal injury to the cell causes the Bax protein to migrate to the outside of the cell, activating the intrinsic pathway. Once this pathway is activated, the mitochondrial membrane becomes permeable releasing cytochrome 3. The apoptosome is then assembled which activates Caspases 3 and 9, which leads to an expanding cascade of proteolytic activity resulting in digestion of structural proteins in the cytoplasm, degradation of chromosomal DNA, and then phagocytosis of the cell.
Dr. Hagberg is focusing on preventing permeability of the mitochondrial membrane as his therapeutic target. He has found that to block mitochondrial outer membrane permeability and provide neural protection to the immature brain the Bax-dependent channel must be blocked. Caspase -2 is an upstream regulator and has shown in-vitro, that inhibition of caspase -2 can block Bax-dependent mitochondrial outer membrane permeability. Further a selective caspase-2 inhibitor has been developed and has shown neuroprotective effects in animal models of excitotoxicity, hypoxia-ischemia and stroke.
Experimental Models of White Matter Injury
Dr. Steven Back discussed animal models of white matter injury. Much progress has been made in determining triggers of brain injury, the affected cell types and the molecular pathways leading to damage. Current animal models reflect this progress and demonstrate that most common pathology seen in preterm infants is noncystic focal or diffuse white matter lesions. One of the most important mechanisms of injury to white matter in the infant brain is ischemia reperfusion and the preoligodendrocyte, precursor to the myelin producing oligodendrocyte, is the most affected cell type.
Dr. Back has developed a global ischemia model in preterm fetal sheep. At 70% gestation, fetal sheep brain development looks very similar to that of a human fetus at 24 to 28 weeks. Using this model they have demonstrated the diffuse white matter lesions, with little gray matter involvement. With increased ischemia there is damage found in the cortex and basal ganglia, perhaps reflecting the wide spectrum of pathology seen in patients with developmental disorders.
In addition, Dr. Back and colleagues have demonstrated that ischemic injury in fetal sheep white matter injury is not uniform, but varying in severity corresponding to where preoligodendrocytes are more likely to be found. In other words, the topography of white matter injury is related both to the maturational stage and distribution of susceptible cells, and oligodendrocyte maturation confers resistance to injury.
There is a surviving population of preoligodendrocytes after acute-hypoxic injury, however, they do not differentiate to regenerate oligodendrocytes in chronic lesions. This may be due to glial scaring in white matter lesions. Glial scars contain among many things, hyaluronic acid, a molecule known to block maturation of preoligodendrocytes. Further, there is an increase in the number of preoligodendrocytes in chronic lesions, but they appear to have lost functionality and do not mature into myelinating cells even with axons present.
Excitotoxic Mechanisms of White Matter Injury
Dr. Francis Jenkins of Children’s Hospital in Boston discussed the role of excitotoxicity and developmental patterns of neurotransmitter expression as a potential mechanism of pre-term brain injury. Glutamate is the major excitatory neurotransmitter in the brain and glutamate receptors are present in neuronal synapses and on glial cells. The presence of glutamate receptors is developmentally regulated; they first appear on radial glia and subplate neurons, then on preoligodendrocytes , microglia and cortex; and then finally on neuronal synapses at term. Thus this unique pattern of appearance of receptors in the preterm brain potentially provides a window of therapeutic opportunity.
Glutamate receptors are involved in many signaling cascades, particularly for excitatory neurotransmitters involving calcium and downstream signaling cascades. If there is too much excitation, however, these cascades can lead to upregulation of calcium-dependent pathways involved in free radical formation and apoptosis. Many studies have shown a pooling of extracellular glutamate in response to hypoxia/ischemia and infection. In addition, the developing brain has glutamate receptors that have upregulated activity due to importance of excitability in the developing brain. However, this increases the risk of excitotoxicity given the glutamate receptors increased activity and the high levels of extracellular glutamate in response to a hypoxic/ischemic or sepsis event.
Given this, a potential therapeutic target may be glutamate receptor antagonism. Studies have demonstrated that NBQX, an AMPA receptor antagonist, preserved white matter in an animal treated after a hypoxic/ischemic event. NBQX is not currently available for human use, however, another AMPA receptor antagonist, topiramate, has FDA approval and has shown preservation of white matter in treated animals after hypoxia/ischemia. Finally, a NMDA receptor antagonist, being studied in dementia trials, has demonstrated cell survival improvement in animals treated after hypoxia/ischemia and long-term improvement evidenced by a decreased cortical thinning.