Loss of cGMP-dependent protein kinase II alters ultrasonic vocalizations in mice, a model for speech impairment in human microdeletion 4q21 syndrome.

Chromosome 4q21 microdeletion leads to a human syndrome that exhibits restricted growth, facial dysmorphisms, mental retardation, and absent or delayed speech. One of the key genes in the affected region of the chromosome is PRKG2, which encodes cGMP-dependent protein kinase II (cGKII). Mice lacking cGKII exhibit restricted growth and deficits in learning and memory, as seen in the human syndrome. However, vocalization impairments in these mice have not been determined. The molecular pathway underlying vocalization impairment in humans is not fully understood. Here, we employed cGKII knockout (KO) mice as a model for the human microdeletion syndrome to test whether vocalizations are affected by loss of the PRKG2 gene. Mice emit ultrasonic vocalizations (USVs) to communicate in social situations, stress, and isolation. We thus recorded ultrasonic vocalizations as a model for human speech. We isolated postnatal day 5-7 pups from the nest to record and analyze USVs and found significant differences in vocalizations of KO mice relative to wild-type and heterozygous mutant mice. KO mice produced fewer calls that were shorter duration and higher frequency. Because neuronal activation in the arcuate nucleus in the hypothalamus is important for the production of animal USVs following isolation from the nest, we assessed neuronal activity in the arcuate nucleus of KO pups following isolation. We found significant reduction of neuronal activation in cGKII KO pups after isolation. Taken together, our studies indicate that cGKII is important for neuronal activation in the arcuate nucleus, which significantly contributes to the production of USVs in neonatal mice. We further suggest cGKII KO mice can be a valuable animal model to investigate pathophysiology of human microdeletion 4q21 syndrome.

Cardiac expression and location of hexokinase changes in a mouse model of pure creatine deficiency.

Creatine kinase (CK) is considered the main phosphotransfer system in the heart, important for overcoming diffusion restrictions and regulating mitochondrial respiration. It is substrate limited in creatine-deficient mice lacking l-arginine:glycine amidinotransferase (AGAT) or guanidinoacetate N-methyltranferase (GAMT). Our aim was to determine the expression, activity, and mitochondrial coupling of hexokinase (HK) and adenylate kinase (AK), as these represent alternative energy transfer systems. In permeabilized cardiomyocytes, we assessed how much endogenous ADP generated by HK, AK, or CK stimulated mitochondrial respiration and how much was channeled to mitochondria. In whole heart homogenates, and cytosolic and mitochondrial fractions, we measured the activities of AK, CK, and HK. Lastly, we assessed the expression of the major HK, AK, and CK isoforms. Overall, respiration stimulated by HK, AK, and CK was ∼25, 90, and 80%, respectively, of the maximal respiration rate, and ∼20, 0, and 25%, respectively, was channeled to the mitochondria. The activity, distribution, and expression of HK, AK, and CK did not change in GAMT knockout (KO) mice. In AGAT KO mice, we found no changes in AK, but we found a higher HK activity in the mitochondrial fraction, greater expression of HK I, but a lower stimulation of respiration by HK. Our findings suggest that mouse hearts depend less on phosphotransfer systems to facilitate ADP flux across the mitochondrial membrane. In AGAT KO mice, which are a model of pure creatine deficiency, the changes in HK may reflect changes in metabolism as well as influence mitochondrial regulation and reactive oxygen species production.NEW & NOTEWORTHY In creatine-deficient AGAT-/- and GAMT-/- mice, the myocardial creatine kinase system is substrate limited. It is unknown whether subcellular localization and mitochondrial ADP channeling by hexokinase and adenylate kinase may compensate as alternative phosphotransfer systems. Our results show no changes in adenylate kinase, which is the main alternative to creatine kinase in heart. However, we found increased expression and activity of hexokinase I in AGAT-/- cardiomyocytes. This could affect mitochondrial regulation and reactive oxygen species production.

Inborn errors of creatine metabolism and epilepsy.

Creatine metabolism disorders include guanidinoacetate methyltransferase (GAMT) deficiency, arginine:glycine amidinotransferase (AGAT) deficiency, and the creatine transporter (CT1-encoded by SLC6A8 gene) deficiency. Epilepsy is one of the main symptoms in GAMT and CT1 deficiency, whereas the occurrence of febrile convulsions in infancy is a relatively common presenting symptom in all the three above-mentioned diseases. GAMT deficiency results in a severe early onset epileptic encephalopathy with development arrest, neurologic deterioration, drug-resistant seizures, movement disorders, mental disability, and autistic-like behavior. In this disorder, epilepsy and associated abnormalities on electroencephalography (EEG) are more responsive to substitutive treatment with creatine monohydrate than to conventional antiepileptic drugs. AGAT deficiency is mainly characterized by mental retardation and severe language disorder without epilepsy. In CT1 deficiency epilepsy is generally less severe than in GAMT deficiency. All creatine disorders can be investigated through measurement of creatine metabolites in body fluids, brain proton magnetic resonance spectroscopy ((1) H-MRS), and molecular genetic techniques. Blood guanidinoacetic acid (GAA) assessment and brain H-MRS examination should be part of diagnostic workup for all patients presenting with epileptic encephalopathy of unknown origin. In girls with learning and/or intellectual disabilities with or without epilepsy, SLC6A8 gene assessment should be part of the diagnostic procedures. The aims of this review are the following: (1) to describe the electroclinical features of epilepsy occurring in inborn errors of creatine metabolism; and (2) to delineate the metabolic alterations associated with GAMT, AGAT, and CT1 deficiency and the role of a substitutive therapeutic approach on their clinical and electroencephalographic epileptic patterns.

AGAT, GAMT and SLC6A8 distribution in the central nervous system, in relation to creatine deficiency syndromes: a review.

Creatine deficiency syndromes, either due to AGAT, GAMT or SLC6A8 deficiencies, lead to a complete absence, or a very strong decrease, of creatine within the brain, as measured by magnetic resonance spectroscopy. While the mammalian central nervous system (CNS) expresses AGAT, GAMT and SLC6A8, the lack of SLC6A8 in astrocytes around the blood-brain barrier limits the brain capacity to import creatine from the periphery, and suggests that the CNS has to rely mainly on endogenous creatine synthesis through AGAT and GAMT expression. This seems contradictory with SLC6A8 deficiency, which, despite AGAT and GAMT expression, also leads to creatine deficiency in the CNS. We present novel data showing that in cortical grey matter, AGAT and GAMT are expressed in a dissociated way: e.g. only a few cells co-express both genes. This suggests that to allow synthesis of creatine within the CNS, at least for a significant part of it, guanidinoacetate must be transported from AGAT- to GAMT-expressing cells, possibly through SLC6A8. This would explain the creatine deficiency observed in SLC6A8-deficient patients. By bringing together creatine deficiency syndromes, AGAT, GAMT and SLC6A8 distribution in CNS, as well as a synthetic view on creatine and guanidinoacetate levels in the brain, this review presents a comprehensive framework, including new hypotheses, on brain creatine metabolism and transport, both in normal conditions and in case of creatine deficiency.

Proton MR spectroscopy in succinic semialdehyde dehydrogenase deficiency.

Succinic semialdehyde dehydrogenase (SSADH) deficiency is a rare hereditary disorder of the CNS catabolism of gamma-aminobutyric acid (GABA), leading to accumulation of the metabolite 4-hydroxybutyrate (GHB). Here the authors report on 1.5 and 3.0 T proton MR spectroscopy in a patient with SSADH deficiency. A characteristic pattern with clearly elevated GABA levels and traces of GHB was found in both the white and the gray matter of the brain. In vivo spectroscopy may be useful for diagnosis and monitoring SSADH deficiency.

Disturbances in histidine metabolism in children with speech abnormalities.

Catabolism of histidine was investigated in 24 patients with different speech and language disorders and with significantly low histidase activity in stratum corneum. No classical histidinemia was found. Biochemical investigation of these patients after loading with L-histidine led to the conclusions that low histidase activity in stratum corneum was connected with: disturbances in folic acid metabolism (2 cases); "atypical histidinemia" (1 case); heterozygotes of histidinemia (2 cases); normal liver histidine metabolism but abnormal in other tissues (18 cases); previously unknown error of histidine metabolism (1 case).