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Biomedical subjects

Michael V Johnston

Publications and source records attributed to Michael V Johnston.

At least 19 recordsLinked to original sources
PubMed2007

Sex and the pathogenesis of cerebral palsy.

Cerebral palsy (CP) and related developmental disorders are more common in males than in females, but the reasons for this disparity are uncertain. Males born very preterm also appear to be more vulnerable to white matter injury and intraventricular hemorrhage than females. Experimental studies in adult animals and data from adult patients with stroke indicate that sex hormones such as estrogens provide protection against hypoxic-ischemic injury, and the neonatal brain is also influenced by these hormones. However, hormonal influences on the fetus and neonates are substantially different from those on adults. Recent data from neonatal rodents subjected to hypoxia-ischemia also demonstrate differences between males and females. Knockout of the gene for poly (ADP-ribose) polymerase (PARP-1), a major step in the cascade of injury, protected male but not female mouse pups from hypoxic-ischemic injury. Other reports demonstrated major differences between male and female neurons grown separately in cell culture, suggesting that sex differences in the fetal or neonatal period result from intrinsic differences in cell death pathways. This new information indicates that there are important neurobiological differences between males and females with respect to their response to brain injuries. This information is relevant to understanding the pathogenesis of CP as well as to the design of future clinical trials of potential neuroprotective strategies.

Animals↗
PubMed2006

Vascular response to hypoxic preconditioning in the immature brain.

We hypothesized that hypoxic preconditioning (PC) modifies the microvasculature in the immature brain and thereby affects the cerebral blood flow (CBF) during a subsequent hypoxic-ischemic (HI) insult. On postnatal day 6 rats were exposed to hypoxia (36 degrees C, 8.0% O2) or normoxia for 3 h. Unilateral HI (unilateral carotid ligation and 8% hypoxia) was induced 24 h later. Cortical CBF was measured with the 14C-iodoantipyrine technique (at the end of HI) or with laser Doppler flowmetry (Perimed PF5001) before and during HI. At 0, 2, 8, and 24 h cerebral cortex was sampled and analyzed with gene arrays (Affymetrix 230 2.0). L-nitroarginine or vehicle was administrated before hypoxic PC or 30 mins before HI followed by CBF measurement (laser Doppler) during subsequent HI. Twenty-four hours after PC animals were perfusion-fixed and brains immunolabeled for von Willebrand factor and vascular density was determined by stereological quantification. The decrease in CBF during HI was attenuated significantly in PC versus control animals (P<0.01), as detected by both techniques. Several vascular genes (Angpt2, Adm, Apln, Vegf, Flt1, Kdr, Pdgfra, Agtrap, Adora2a, Ednra, serpine1, caveolin, Id1, Prrx1, Ero1l, Acvrl1, Egfl7, Nudt6, Angptl4, Anxa2, and NOS3) were upregulated and a few (Csrp2, Adora2b) were downregulated after PC. A significant increase in vascular density (P<0.05) was seen after PC. Nitric oxide synthase inhibition did not affect CBF during HI after PC. In conclusion, hypoxic PC upregulates vascular genes, increases vascular density and attenuates the decrease of CBF during a subsequent HI, which could contribute to tolerance.

Animals↗
PubMed2006

Dextromethorphan protects male but not female mice with brain ischemia.

The non-competitive N-methyl-D-aspartate receptor antagonist dextromethorphan is protective against some types of brain injury. Unilateral carotid ligation in postnatal day 12 CD1 mice produces ischemic brain injury. To evaluate the neuroprotective potential of dextromethorphan against ischemic injury in the immature brain, seven litters of postnatal day 12 CD1 mice received either dextromethorphan or vehicle after a unilateral carotid ligation. Only the male pups were protected, and brain injury was unchanged in the female pups treated with dextromethorphan. These results suggest that dextromethorphan neuroprotection against ischemic injury in the immature brain is sex-dependent.

Animals↗
PubMed2006

Valproic acid prevents brain injury in a canine model of hypothermic circulatory arrest: a promising new approach to neuroprotection during cardiac surgery.

BACKGROUND: The anticonvulsant valproic acid (sodium valproate, Depacon) acts as a neuroprotectant in rodents, but has never been tested in larger animals. We used valproate in our canine model of hypothermic circulatory arrest to evaluate its neuroprotective benefit in complex cardiac surgical cases. METHODS: Thirteen dogs pretreated with valproate before 2 hours of hypothermic circulatory arrest survived for 24 hours (n = 7) or 72 hours (n = 6). Thirteen control animals (placebo only) also survived for 24 hours (n = 7) or 72 hours (n = 6) after hypothermic circulatory arrest. Blinded clinical neurologic evaluation was performed daily until sacrifice using the Pittsburgh Canine Neurologic Scoring System. Brains were harvested for blinded histopathologic analysis by a neuropathologist to determine the extent of apoptosis and necrosis in 11 brain regions (Total Brain Cell Death Score: 0 = normal, 99 = extensive neuronal death in all regions). Quantification of N-acetyl-aspartate, an established marker for brain injury, was performed with mass spectrometry. RESULTS: Valproate dogs scored significantly better than control animals on clinical neurologic evaluation. Histopathologic examination revealed that valproate animals demonstrated less neuronal damage (by Total Brain Cell Death Score) than control animals at both 24 hours (16.4 versus 11.4; p = 0.03) and 72 hours (21.7 versus 17.7; p = 0.07). At 72 hours, the entorhinal cortex, an area involved with learning and memory, was significantly protected in valproate dogs (p < 0.05). Furthermore, the cortex, hippocampus, and cerebellum demonstrated preservation of near-normal N-acetyl-aspartate levels after valproate pretreatment. CONCLUSIONS: These data demonstrate clinical, histologic, and biochemical improvements in dogs pretreated with valproate before hypothermic circulatory arrest. This commonly used drug may offer a promising new approach to neuroprotection during cardiac surgery.

Animals↗
PubMed2006

Noninvasive assessment of brain injury in a canine model of hypothermic circulatory arrest using magnetic resonance spectroscopy.

BACKGROUND: Studies have confirmed the neuroprotective effect of diazoxide in canines undergoing hypothermic circulatory arrest (HCA). A decreased N-acetyl-asparate:choline (NAA:Cho) ratio is believed to reflect the severity of neurologic injury. We demonstrated that noninvasive measurement of NAA:Cho with magnetic resonance spectroscopy facilitates assessment of neuronal injury after HCA and allows for evaluation of neuroprotective strategies. METHODS: Canines underwent 2 hours of HCA at 18 degrees C and were observed for 24 hours. Animals were divided into three groups (n = 15 in each group): normal (unoperated), HCA (HCA only), and HCA+diazoxide (pharmacologic treatment before HCA). The NAA:Cho ratios were obtained 24 hours after HCA by spectroscopy. Brains were immediately harvested for fresh tissue NAA quantification by mass spectrometry. Separate cohorts of HCA (n = 16) and HCA+diazoxide (n = 23) animals were kept alive for 72 hours for daily neurologic assessment. RESULTS: Cortical NAA:Cho ratios were significantly decreased in HCA versus normal animals (1.01 +/- 0.29 versus 1.31 +/- 0.23; p = 0.004), consistent with severe neurologic injury. Diazoxide pretreatment limited neurologic injury versus HCA alone, reflected in a preserved NAA:Cho ratio (1.21 +/- 0.27 versus 1.01 +/- 0.29; p = 0.05). Data were substantiated with fresh tissue NAA extraction. A significant decrease in cortical NAA was observed in HCA versus normal (7.07 +/- 1.9 versus 8.54 +/- 2.1 micromol/g; p = 0.05), with maintenance of normal NAA levels after diazoxide pretreatment (9.49 +/- 1.1 versus 7.07 +/- 1.9 micromol/g; p = 0.0002). Clinical neurologic scores were significantly improved in the HCA+diazoxide group versus HCA at all time points. CONCLUSIONS: Neurologic injury remains a significant complication of cardiac surgery and is most severe after HCA. Magnetic resonance spectroscopy assessment of NAA:Cho ratios offers an early, noninvasive means of potentially evaluating neurologic injury and the effect of neuroprotective agents.

Animals↗
PubMed2006

Cerebral palsy.

Cerebral palsy (CP) is a group of disorders of movement and posture resulting from nonprogressive disturbances of the fetal or neonatal brain. More than 80% of cases of CP in term infants originate in the prenatal period; in premature infants, both prenatal or postnatal causes contribute. The most prevalent pathological lesion seen in CP is periventricular white matter injury (PWMI) resulting from vulnerability of the immature oligodendrocytes (pre-OLs) before 32 wk of gestation. PWMI is responsible for the spastic diplegia form of CP and a spectrum of cognitive and behavioral disorders. Oxidative stress and excitotoxicity resulting from excessive stimulation of ionotropic glutamate receptors on preOLs are the most prominent molecular mechanisms for PWMI. Asphyxia around the time of birth in term infants accounts for less than 15% of CP in developed countries but the incidence is higher in underdeveloped areas. Asphyxia causes a different pattern of brain injury and CP than is seen after preterm injuries. This type of CP is associated with the clinical syndrome of hypoxic-ischemic encephalopathy shortly after the insult, and the cortex, basal ganglia, and brainstem are selectively vulnerable to injury. Experimental models indicate that neurons in the neonatal brain are more likely to die by delayed apoptosis extending over days to weeks than those in the adult brain. Neurons die by glutamate-mediated excitotoxicity involving downstream caspase-dependent and caspase-independent cell death pathways. Recent reports indicate that males and females preferentially utilize different pathways. Clinical trials indicate that mild hypothermia reduces death or disability in term infants following asphyxia and basic research suggests that this approach might be combined with pharmacological strategies in the future.

Asphyxia Neonatorum↗
PubMed2005

MeCP2 expression and function during brain development: implications for Rett syndrome's pathogenesis and clinical evolution.

Most cases of Rett syndrome (RTT) are associated with mutations of the transcriptional regulator MeCP2. On the basis of molecular structure, ontogeny, and subcellular and regional distribution, MeCP2 appears to be a link between synaptic activity and neuronal transcription. Integrating data on MeCP2 neurobiology, RTT neurobiology, MeCP2 mutational patterns in RTT and other disorders, histone profiles of relevance to RTT, and genotype-phenotype correlations in RTT, we update here our synaptic hypothesis of RTT. We postulate that MeCP2 dysfunction leads to abnormal brain development through maladjustment of neuronal gene expression to synaptic and other extra-cellular signals, mainly during the critical period of synaptic maturation. RTT phenotype will develop, only if severe MeCP2 dysfunction is present during early neuronal differentiation. Two models are proposed for explaining general and regional neuronal abnormalities in RTT and the phenotypical outcome of MeCP2 dysfunction, respectively.

Animals↗
PubMed2005

Excitotoxicity in perinatal brain injury.

Excitotoxicity is an important mechanism involved in perinatal brain injuries. Glutamate is the major excitatory neurotransmitter, and most neurons as well as many oligodendrocytes and astrocytes possess receptors for glutamate. Perinatal insults such as hypoxia-ischemia, stroke, hypoglycemia, kernicterus, and trauma can disrupt synaptic function leading to accumulation of extracellular glutamate and excessive stimulation of these receptors. The activities of certain glutamate receptor/channel complexes are enhanced in the immature brain to promote activity-dependent plasticity. Excessive stimulation of glutamate receptor/ion channel complexes triggers calcium flooding and a cascade of intracellular events that results in apoptosis and/or necrosis. Recent research suggests that some of these intracellular pathways are sexually dimorphic. Age dependent expression of different glutamate receptor subtypes with varying abilities to flux calcium has been associated with special patterns of selective vulnerability at different gestational ages. For example, selective injury to the putamen, thalamus and cerebral cortex from near total asphyxia in term infants may be related to excessive activation of neuronal NMDA and AMPA type glutamate receptors, while brainstem injury may be related primarily to stimulation of neuronal AMPA/kainate receptors. In contrast, periventricular leukomalacia in premature infants has been linked to expression of AMPA/kainate receptors on immature oligodendrocytes. Insight into the molecular pathways that mediate perinatal brain injuries could lead to therapeutic interventions.

Age Factors↗
PubMed2005

Rett syndrome and neuronal development.

The clinical signs of Rett syndrome, as well as neuropathology and brain imaging, suggest that the disorder disrupts neuronal circuits. Studies using receptor autoradiography demonstrate abnormalities in the density of excitatory glutamate and inhibitory gamma-aminobutyric acid (GABA) synaptic receptors in postmortem brain from young female subjects with Rett syndrome. MeCP2, the protein that is abnormal in most female individuals with Rett syndrome, is expressed predominantly in neurons and appears during development at the time of synapse formation. Studies of nasal epithelium from patients with Rett syndrome show that the maturation of olfactory receptor neurons is impeded prior to the time of synapse formation. Recent reports indicate that MeCP2 controls the expression of brain-derived neurotrophic factor and the DNA-binding homeobox protein Dlx5. Brain-derived neurotrophic factor enhances glutamate neurotransmission at excitatory synapses, whereas Dlx5 is expressed in most GABAergic neurons and stimulates the synthesis of GABA. Taken together, this information supports the hypothesis that Rett syndrome is a genetic disorder of synapse development, especially synapses that use glutamate and GABA as neurotransmitters.

Brain↗
PubMed2005

Immature mouse unilateral carotid ligation model of stroke.

Cerebral palsy in humans results from a diverse group of disorders that produce nonprogressive motor impairments in the developing brain. Stroke is an important cause of hemiparetic cerebral palsy in neonates and young children. We recently developed a new immature mouse model of stroke that demonstrates seizures, the severity of which correlates with brain injury. This model has strengths compared with other immature rodent models of ischemic injury, such as relative technical ease and the presence of seizures, as is seen in humans. This model also has relative weaknesses, such as the inability to titrate the severity of the injury with different periods of hypoxia. In addition, more work is needed to delineate the long-term consequences of the insult in this new model.

Animals↗
PubMed2004

Activity-dependent expression of Egr1 mRNA in somatosensory cortex of developing rats.

The rat barrel field in somatosensory cortex is a well-characterized model of neocortical development, with activity-dependent and activity-independent components. Egr1 encodes an inducible transcription factor that is required for certain forms activity-dependent plasticity. This study examines Egr1 mRNA expression in the developing barrel field under basal conditions and after short-term deprivation or stimulation of whiskers. Egr1 mRNA was measured with in situ hybridization at postnatal Day (P) 6, P9, P12, P15, and P21. For short-term deprivation, whiskers were trimmed close to the skin and Egr1 mRNA was examined 3 hr later. For controlled stimulation of a single whisker, surrounding whiskers were trimmed, a wire was glued to the designated whisker, and animals were placed in an AC magnetic field pulsed at 2 Hz, 10 mT rms for 15 min. Egr1 mRNA was examined 30 min later. At P6, basal Egr1 mRNA in the barrel field was very low and was increased only slightly by stimulation (P < 0.05). At each of the later ages, there was a large increase in Egr1 mRNA in stimulated versus deprived barrels (P < 0.001). Egr1 mRNA expression after whisker stimulation increased exponentially with age through P15 (P < 0.001) and then declined between P15 and P21. The onset of Egr1 responses to whisker stimulation at P9 and the striking increase in activity-dependent Egr1 mRNA expression in the second postnatal week suggest that this transcription factor may play a role in activity-dependent processes that occur in this developmental period, such as maturation of barrel cortex circuitry.

Animals↗
PubMed2004

Minocycline worsens hypoxic-ischemic brain injury in a neonatal mouse model.

Hypoxic-ischemic encephalopathy (HIE) is a leading cause of mortality and morbidity during the perinatal period, and currently no therapeutic drug is available. Minocycline, an antibiotic, has recently been shown to have neuroprotective effects distinct from its antimicrobial effect in several neurological disorders including ischemic brain injury. We examined the effect of minocycline on neonatal hypoxic-ischemic brain injury by using histologic scoring in both mouse and rat models. Mouse (C57Bl/6) and rat (SD) pups were exposed to a unilateral hypoxic-ischemic insult at 8 and 7 days of age, respectively. Minocycline hydrochloride was administered according to protocols that were reported to provide neuroprotection in adult or neonatal rats. Seven days after the insult, we examined brain injury in Nissl stained sections. Although minocycline ameliorated brain injury in the developing rat, it increased injury in the developing mouse. This detrimental effect in the mouse was consistent across different regions (cortex, striatum, and thalamus), with both single and multiple injection protocols and with both moderate and high-dose treatment (P < 0.05). The mechanism of the contrasting effects in mouse and rat is not clear and remains to be elucidated. Minocycline has been used as an antibiotic in the clinical setting for decades; therefore, it may be considered for use in infants with hypoxic-ischemic brain damage, based on prior reports of neuroprotection in the rat. However, it is important to examine this drug carefully before clinical use in human infants, taking our data in the mouse model into consideration.

Analysis of Variance↗
PubMed2004

A new model of stroke and ischemic seizures in the immature mouse.

Ischemic brain injury from stroke is an important cause of disability in infants and children, but current experimental models for the disorder are complex. These preparations require occlusion of small intracerebral vessels or common carotid artery ligation combined with exposure to reduced levels of oxygen. Unilateral carotid artery ligation alone was sufficient to cause brain injury in more than 70% of 12-day-old CD1 mice. Using a blinded behavioral rating scale of seizure activity in mice, a direct, highly significant correlation between the severity of seizures over the 4-hour period after ligation and the severity of histologic brain injury 7 days later (Spearman's rho = 0.835, P < 0.001) was documented. This study presents the first model of stroke in immature mice produced by unilateral carotid artery ligation alone, and the first to demonstrate a clear correlation between acute ischemia-induced seizures and brain injury. This new model should be useful for examining the pathogenesis of stroke in the immature brain and the potential contribution of seizures to final outcome.

Age Factors↗
PubMed2004

Clinical disorders of brain plasticity.

Clinical disorders of brain plasticity are common in the practice of child neurology. Children have an enhanced capacity for brain plasticity compared to adults as demonstrated by their superior ability to learn a second language or their capacity to recover from brain injuries or radical surgery such as hemispherectomy for epilepsy. Basic mechanisms that support plasticity during development include persistence of neurogenesis in some parts of the brain, elimination of neurons through apoptosis or programmed cell death, postnatal proliferation and pruning of synapses, and activity-dependent refinement of neuronal connections. Brain plasticity in children can be divided into four types: adaptive plasticity that enhances skill development or recovery from brain injury; impaired plasticity associated with cognitive impairment; excessive plasticity leading to maladaptive brain circuits; and plasticity that becomes the brain's 'Achilles' Heel' because makes it vulnerable to injury. A broad group of pediatric neurologic disorders can be understood in terms of their impact on fundamental mechanisms for brain plasticity. These include neurofibromatosis, tuberous sclerosis, Fragile X syndrome, other inherited forms of mental retardation, cretinism, Coffin-Lowry syndrome, lead poisoning, Rett syndrome, epilepsy, hypoxic-ischemic encephalopathy and cerebral palsy.

Animals↗
PubMed2004

PARP-1 gene disruption in mice preferentially protects males from perinatal brain injury.

Poly(ADP-ribose) polymerase-1 is over-activated in the adult brain in response to ischemia and contributes to neuronal death, but its role in perinatal brain injury remains uncertain. To address this issue, 7-day-old wild-type (wt) and PARP-1 gene deficient (parp+/- and parp-/-) Sv129/CD-1 hybrid mice were subjected to unilateral hypoxia-ischemia and histologic damage was assessed 10 days later by two evaluators. Poly(ADP-ribose) polymerase-1 knockout produced moderate but significant (p < 0.05) protection in the total group of animals, but analysis by sex revealed that males were strongly protected (p < 0.05) in contrast to females in which there was no significant effect. Separate experiments demonstrated that PARP-1 was activated over 1-24 h in both females and males after the insult in neonatal wt mice and rats using immnocytochemistry and western blotting for poly(ADP-ribose). Brain levels of NAD+ were also significantly reduced, but the decrease of NAD+ during the early post-hypoxia-ischemia (HI) phase was only seen in males. The results indicate that hypoxia-ischemia activates Poly(ADP-ribose) polymerase-1 in the neonatal brain and that the sex of the animal strongly influences its role in the pathogenesis of brain injury.

Animals↗