Detection of cortical gray matter lesion in the late phase of mild hypoxic–ischemic injury by manganese-enhanced MRI
Introduction
Neonatal hypoxic–ischemic (H–I) brain injury is a major cause of chronic neurological disorders. In premature infants, the periventricular tissue is particularly vulnerable to hypoxia–ischemia (Back, 2006, Vexler and Ferriero, 2001). This injury is commonly known as periventricular leukomalacia (PVL) and is the predominant cause of cerebral palsy associated with the developmental deficits in motor, sensory, visual or cognitive functions in later life (Back, 2006, Inder et al., 1999, Inder et al., 2005, Woodward et al., 2006).
Recent advances in neonatal care have markedly improved the survival rate and reduced the incidence of cystic PVL (accounting only for less than 5%), whereas focal or diffuse noncystic PVL injury is becoming a predominant lesion form (Back, 2006, Inder et al., 2003a). Moreover, the high frequency of this noncystic disorder is found to closely correlate with the high frequency (∼ 50%) of later cognitive and behavioral deficits in premature infants with very low birth weights (Inder et al., 2005). Recent observations in the premature infants suggest that the basis for the cognitive and related deficits may not relate directly to the white matter (WM) injury. Several studies using quantitative volumetric magnetic resonance imaging (MRI) have revealed the cortical gray matter (GM) volume reduction among the adolescent children who had been born preterm, correlating with impaired IQ (Peterson et al., 2000), memory (Isaacs et al., 2000), numeracy (mathematical skills) (Isaacs et al., 2001), and global cognitive functions (Nosarti et al., 2002). These cognitive deficits suggest that the structures affected are principally GM, especially cerebral cortical GM (Inder et al., 2005).
At present, certain pathogenesis of noncystic PVL, such as the GM volume reduction during juvenile phase of this disease, remains unclear (Back, 2006). The primary reason is that the histopathological study in noncystic PVL has been hampered by limitation in specimen collection from premature infants. However, classic studies that established the neurodevelopmental parallels across species have provided the foundation for the use of animal models to study neonatal brain injury (Clancy et al., 2001, Northington, 2006). Several neonatal rodent models have been developed to study the pathophysiology of neonatal H–I brain injury, reproducing many of the anatomical features of PVL (Northington, 2006, Qiao et al., 2004, Rothstein and Levison, 2005, Uehara et al., 1999). It has been shown that a short duration of H–I insult at a slightly lower body temperature can produce a reliable neonatal model with an injury in both WM and GM, without cystic necrotic lesion (Meng et al., 2005, Meng et al., 2006, Qiao et al., 2004).
On the other hand, in vivo diagnosis of GM lesion in noncystic PVL is imperative for the necessary optimal clinical management and treatment strategies. In current clinic, the principal method for imaging the premature brain, cranial ultrasonography, has a low sensitivity for detection of noncystic PVL (Inder et al., 2003a). MRI is a clinically attractive means to assess neonatal encephalopathy, especially noncystic PVL, because of its ability to provide detailed structural as well as metabolic and functional information. Quantitative volumetric MRI technique has been used in cohort studies to assess the reduction of tissue volumes in cerebral cortical GM in premature infants with noncystic PVL (Inder et al., 2003b). However, utility of such MR analysis is limited in revealing subtle alterations in sequence of cerebral structural development, particularly in structures such as the GM (Inder et al., 2005). Moreover, the previous studies demonstrated that the sensitivity of H–I lesion detection depends on both the type of MRI techniques employed and the timing of the study relative to onset of injury. Some lesions in neonatal encephalopathy tend to return to normal appearance in T1-weighted images (T1WI), T2-weighted images (T2WI), diffusion-weighted images (DWI) and MR spectroscopy during the first 2 weeks after injury (Barkovich et al., 2006, Kuker et al., 2004, Mader et al., 2002). In 7-day-old (P7) rat models with mild H–I insult, certain transient changes in GM in T2WI, apparent diffusion coefficient (ADC) map, and cerebral perfusion imaging were observed in acute phase. The changes of MRI appearances normalized more quickly in cortical GM (within 48 h) than in WM despite the histological abnormality (Meng et al., 2006). Thus these conventional T1WI, T2WI, and DWI techniques are of limited clinical value in diagnosis of noncystic PVL because their detection of GM injury is difficult and only feasible during acute phase. Therefore, it is highly desirable to develop a new MRI method that is capable of detecting this transient and subtle change associated with GM injury in noncystic PVL.
Divalent manganese ion (Mn2+) as an analogue for calcium ion (Ca2+) has been used as a cellular contrast agent for tracing neuronal pathways and study of ischemia in adult neural tissues (Aoki et al., 2003, Aoki et al., 2004a, Pautler et al., 2003). Mn is essential to the development and function of the brain in physiological and pathological states (Crossgrove and Zheng, 2004, Takeda, 2003). Mn metal is bound to glutamine synthetase (GS) which is a glia (astrocytes and oligodendrocytes) specific enzyme for regulating the extracellular glutamate and ammonia and reducing glutamate excitotoxicity (Fujioka et al., 2003, Suarez et al., 2002, Wedler and Denman, 1984) and to the mitochondrial Mn-superoxide dismutase (Mn-SOD) enzyme which acts against cellular oxidative stress (Bidmon et al., 1998, Fujioka et al., 2003, Lindenau et al., 2000). Studies have reported that accumulation of endogenous Mn in the form of Mn-SOD and GS in delayed neurodegeneration after transient focal ischemia could be detected by MRI in adult rat and human (Fujioka et al., 2003, Yang et al., 2007). Recently, our preliminary study of neonatal rat with severe H–I injury demonstrated that exogenous Mn could provide enhanced MEMRI detection of the delayed cortical and striatal degeneration that correlated with increased local GS and Mn-SOD production (Yang et al., 2007).
In this study, we hypothesized that local Mn accumulation would occur as the co-factor for increased GS and Mn-SOD induction in neonatal rat brain with mild H–I injury after systemic Mn2+ administration, allowing enhanced detection of GM injury particularly in the later stage. Such MEMRI approach may be valuable in studying the subtle GM abnormality related to the excess oxidative stress and glutamate excitotoxicity that cause local Mn2+ buildup in brain lesions and diagnosis of such GM injury after acute phase in noncystic PVL.
Section snippets
Animal preparation
All animal experiments were approved by the local animal research ethics committee. Pregnant Sprague–Dawley rats were obtained approximately 2 days before parturition, and their litters were culled to 9 to 13 pups. Neonate rats were kept with their mother in regular light/dark cycle for 7 days after birth. A total of 20 P7 rats, ranging from 12 to 16 g in weight, underwent unilateral ligation of right common carotid artery (double ligation followed by severing the artery in between) via a
Results
All 20 neonatal rats induced with mild H–I injury exhibited lesions on the parietal cortex with minimal atrophy in the hemisphere ipsilateral to carotid artery ligation side without a cystic lesion.
Discussion
In this study, we employed in vivo MEMRI to observe the progression and permanence of the GM injuries during mild H–I injury in a neonatal rat model. The MRI findings were compared and correlated to the histological examination for H&E staining and manganese binding GS and Mn-SOD. For the first time, this study demonstrated the potential utility of MEMRI for detection of GM injuries in a rodent model of noncystic PVL.
Conclusion
MEMRI with systemic Mn2+ injection during and after the acute phase of mild H–I injury provides an enhanced and prolonged detection of cortical gray matter lesions in mild H–I neonatal model that parallels the clinical PVL. This in vivo Mn-induced MRI enhancement was found to correlate with increased immunoactivities of Mn-binding GS and Mn-SOD enzymes. These findings suggest the potential utility of MEMRI in detecting the GM lesions that are otherwise undetectable using the conventional MRI
Acknowledgments
The authors would like to thank Dr. Pek-Lan Khong from the Department of Diagnostic Radiology, Drs. Pik-to Cheung and Shu-Leong, and Mr. Andrew Chu from the Department of Medicine, The University of Hong Kong for their technical assistance. This work was supported in part by Hong Kong Research Grant Council and The University of Hong Kong CRCG grant.
References (57)
- et al.
In vivo detection of neuroarchitecture in the rodent brain using manganese-enhanced MRI
NeuroImage
(2004) - et al.
Manganese dipyridoxyl diphosphate: MRI contrast agent with antioxidative and cardioprotective properties?
Biochem. Biophys. Res. Commun.
(1999) - et al.
Translating developmental time across mammalian species
Neuroscience
(2001) - et al.
Interaction among substrates, inhibitors and Mn2+ bound to glutamine synthetase as studied by NMR relaxation rate measurements
Arch. Biochem. Biophys.
(1987) - et al.
In vivo MRI reveals the dynamics of pathological changes in the brains of cathepsin D-deficient mice and correlates changes in manganese-enhanced MRI with microglial activation
Magn. Reson. Imaging.
(2007) - et al.
Transient ADC change precedes persistent neuronal death in hypoxic–ischemic model in immature rats
Brain Res.
(2006) - et al.
Defining the nature of the cerebral abnormalities in the premature infant: a qualitative magnetic resonance imaging study
J. Pediatr.
(2003) - et al.
Glutamine synthetase immunoreactivity in two types of mouse brain glial cells
Brain Res.
(1992) - et al.
Brain glutamine synthetase increases following cerebral ischemia in the rat
Brain Res.
(1992) - et al.
Magnetic resonance imaging of differential gray versus white matter injury following a mild or moderate hypoxic–ischemic insult in neonatal rats
Neurosci. Lett.
(2004)