Mechanism of metabolic stroke and spontaneous cerebral hemorrhage in glutaric aciduria type I

Background

Metabolic stroke is the rapid onset of lasting central neurological deficit associated with decompensation of an underlying metabolic disorder. Glutaric aciduria type I (GA1) is an inherited disorder of lysine and tryptophan metabolism presenting with metabolic stroke in infancy. The clinical presentation includes bilateral striatal necrosis and spontaneous subdural and retinal hemorrhages, which has been frequently misdiagnosed as non-accidental head trauma. The mechanisms underlying metabolic stroke and spontaneous hemorrhage are poorly understood.

Results

Using a mouse model of GA1, we show that metabolic stroke progresses in the opposite sequence of ischemic stroke, with initial neuronal swelling and vacuole formation leading to cerebral capillary occlusion. Focal regions of cortical followed by striatal capillaries are occluded with shunting to larger non-exchange vessels leading to early filling and dilation of deep cerebral veins. Blood–brain barrier breakdown was associated with displacement of tight-junction protein Occludin.

Conclusion

Together the current findings illuminate the pathophysiology of metabolic stroke and vascular compromise in GA1, which may translate to other neurometabolic disorders presenting with stroke.

Interrupting the mechanisms of brain injury in a model of maple syrup urine disease encephalopathy

Maple syrup urine disease (MSUD) was first recognized as an inherited lethal encephalopathy beginning in the first week of life and associated with an unusual odor in the urine of affected children. It was later confirmed as a deficiency of branched-chain keto acid dehydrogenase (BCKDH), which is the second step in branched-chain amino acid (BCAA) breakdown. MSUD is characterized by BCAA and branched-chain keto acid (BCKA) accumulation. BCAAs are essential amino acids and powerful metabolic signals with severe consequences of both deprivation and accumulation. Treatment requires life-long dietary restriction and monitoring of BCAAs. However, despite excellent compliance, children commonly suffer metabolic decompensation during intercurrent illness resulting in life-threatening cerebral edema and dysmyelination. The mechanisms underlying brain injury have been poorly understood. Recent studies using newly developed mouse models of both classic and intermediate MSUD have yielded insight into the consequences of rapid BCAA accumulation. Additionally, these models have been used to test preliminary treatments aimed at competing with blood-brain barrier transport of BCAA using norleucine. Assessment of biochemical changes with and without treatment suggests different roles for BCAA and BCKA in the mechanism of brain injury.

Mouse model of encephalopathy and novel treatment strategies with substrate competition in glutaric aciduria type I

Glutaric aciduria type I (GA-1) results from an inherited defect in a common step of lysine, hydroxylysine and tryptophan metabolism. This defect is associated with an age-dependent susceptibility to encephalopathy commonly preceded by non-specific childhood illnesses or fasting. The brain injury that develops with encephalopathic crisis in GA-1 is anatomically and symptomatically similar to Huntington's disease, affecting the striatum. The mechanism of injury remains poorly understood. Recently, an animal model of GA-1 encephalopathy was developed by providing GA-1 mice with added dietary lysine. This model shows age-dependent susceptibility similar to the human disease. Enhanced lysine accumulation and utilization in the immature brain correlates with increased glutaric acid levels and age-dependent susceptibility. Neurotransmitter and Krebs cycle intermediate depletion in this model represent novel findings toward uncovering the mechanism of neuronal injury. Additionally this mouse model is responsive to glucose analogous to human GA-1 and provides insight toward the mechanism of this effect. Together these findings led to a new treatment strategy of competing with brain lysine uptake that shows promising results. This research serves as a model for understanding blood brain barrier amino acid transport at critical stages of development and may help advance understanding of brain injury and development of treatments in other IEMs including urea cycle disorders.

Hepatocyte transplantation (HTx) corrects selected neurometabolic abnormalities in murine intermediate maple syrup urine disease (iMSUD)

Skvorak et al. [1] demonstrated the therapeutic efficacy of HTx in a murine model of iMSUD, confirming significant metabolic improvement and survival. To determine the effect of HTx on extrahepatic organs, we examined the metabolic effects of HTx in brain from iMSUD animals. Amino acid analysis revealed that HTx corrected increased ornithine, partially corrected depleted glutamine, and revealed a trend toward alloisoleucine correction. For amino acid and monoamine neurotransmitters, decreased GABA was partially corrected with HTx, while the l-histidine dipeptide of GABA, homocarnosine, was decreased in iMSUD mice and hypercorrected following HTx. Elevated branched-chain amino acids (BCAA; leucine, isoleucine, and valine) in MSUD can deplete brain tyrosine and tryptophan (the precursors of monoamine neurotransmitters, dopamine (DA) and serotonin (5-hydroxytryptamine; 5-HT)) through competition via the large neutral amino acid transporter. HTx corrected decreased DA levels and the DA metabolite, 3-methoxytyramine, and partially corrected the DA intermediate 3,4-dihydroxyphenylacetate (DOPAC) and 5-HT levels, despite normal tyrosine and tryptophan levels in iMSUD mouse brain. We further observed enhanced intracellular turnover of both DA and 5-HT in iMSUD mouse brain, both of which partially corrected with HTx. Our results suggest new pathomechanisms of neurotransmitter metabolism in this disorder and support the therapeutic relevance of HTx in iMSUD mice, while providing proof-of-principle that HTx has corrective potential in extrahepatic organs.

Dual mechanism of brain injury and novel treatment strategy in maple syrup urine disease

Maple syrup urine disease (MSUD) is an inherited disorder of branched-chain amino acid metabolism presenting with life-threatening cerebral oedema and dysmyelination in affected individuals. Treatment requires life-long dietary restriction and monitoring of branched-chain amino acids to avoid brain injury. Despite careful management, children commonly suffer metabolic decompensation in the context of catabolic stress associated with non-specific illness. The mechanisms underlying this decompensation and brain injury are poorly understood. Using recently developed mouse models of classic and intermediate maple syrup urine disease, we assessed biochemical, behavioural and neuropathological changes that occurred during encephalopathy in these mice. Here, we show that rapid brain leucine accumulation displaces other essential amino acids resulting in neurotransmitter depletion and disruption of normal brain growth and development. A novel approach of administering norleucine to heterozygous mothers of classic maple syrup urine disease pups reduced branched-chain amino acid accumulation in milk as well as blood and brain of these pups to enhance survival. Similarly, norleucine substantially delayed encephalopathy in intermediate maple syrup urine disease mice placed on a high protein diet that mimics the catabolic stress shown to cause encephalopathy in human maple syrup urine disease. Current findings suggest two converging mechanisms of brain injury in maple syrup urine disease including: (i) neurotransmitter deficiencies and growth restriction associated with branched-chain amino acid accumulation and (ii) energy deprivation through Krebs cycle disruption associated with branched-chain ketoacid accumulation. Both classic and intermediate models appear to be useful to study the mechanism of brain injury and potential treatment strategies for maple syrup urine disease. Norleucine should be further tested as a potential treatment to prevent encephalopathy in children with maple syrup urine disease during catabolic stress.

Mechanism of age-dependent susceptibility and novel treatment strategy in glutaric acidemia type I

Glutaric acidemia type I (GA-I) is an inherited disorder of lysine and tryptophan metabolism presenting with striatal lesions anatomically and symptomatically similar to Huntington disease. Affected children commonly suffer acute brain injury in the context of a catabolic state associated with nonspecific illness. The mechanisms underlying injury and age-dependent susceptibility have been unknown, and lack of a diagnostic marker heralding brain injury has impeded intervention efforts. Using a mouse model of GA-I, we show that pathologic events began in the neuronal compartment while enhanced lysine accumulation in the immature brain allowed increased glutaric acid production resulting in age-dependent injury. Glutamate and GABA depletion correlated with brain glutaric acid accumulation and could be monitored in vivo by proton nuclear magnetic resonance (1H NMR) spectroscopy as a diagnostic marker. Blocking brain lysine uptake reduced glutaric acid levels and brain injury. These findings provide what we believe are new monitoring and treatment strategies that may translate for use in human GA-I.

A diet-induced mouse model for glutaric aciduria type I

In the autosomal recessive human disease, glutaric aciduria type I (GA-1), glutaryl-CoA dehydrogenase (GCDH) deficiency disrupts the mitochondrial catabolism of lysine and tryptophan. Affected individuals accumulate glutaric acid (GA) and 3-hydroxyglutaric acid (3-OHGA) in the serum and often suffer acute striatal injury in childhood. Prior attempts to produce selective striatal vulnerability in an animal model have been unsuccessful. We hypothesized that acute striatal injury may be induced in GCDH-deficient (Gcdh-/-) mice by elevated dietary protein and lysine. Here, we show that high protein diets are lethal to 4-week-old and 8-week-old Gcdh-/- mice within 2-3 days and 7-8 days, respectively. High lysine alone resulted in vasogenic oedema and blood-brain barrier breakdown within the striatum, associated with serum and tissue GA accumulation, neuronal loss, haemorrhage, paralysis, seizures and death in 75% of 4-week-old Gcdh-/- mice after 3-12 days. In contrast, most 8-week-old Gcdh-/- mice survived on high lysine, but developed white matter lesions, reactive astrocytes and neuronal loss after 6 weeks. Thus, the Gcdh-/- mouse exposed to high protein or lysine may be a useful model of human GA-1 including developmentally dependent striatal vulnerability.