Cyclocreatine treatment improves cognition in mice with creatine transporter deficiency

Rescue of myocytes and locomotion through AAV2/9-2YF intracisternal gene therapy in a rat model of creatine transporter deficiency

Creatine deficiency syndromes (CDS), caused by mutations in GATM (AGAT), GAMT, and SLC6A8, mainly affect the central nervous system (CNS). CDS show brain creatine (Cr) deficiency, intellectual disability with severe speech delay, behavioral troubles, epilepsy, and motor dysfunction. AGAT/GAMT-deficient patients lack brain Cr synthesis but express the Cr transporter SLC6A8 at the blood-brain barrier and are thus treatable by oral supplementation of Cr. In contrast, no satisfactory treatment has been identified for Cr transporter deficiency (CTD), the most frequent of CDS. We used our Slc6a8Y389C CTD rat model to develop a new AAV2/9-2YF-driven gene therapy re-establishing the functional Slc6a8 transporter in rat CNS. We show, after intra-cisterna magna AAV2/9-2YF-Slc6a8-FLAG vector injection of postnatal day 11 pups, the transduction of Slc6a8-FLAG in cerebellum, medulla oblongata, and spinal cord as well as a partial recovery of Cr in these brain regions, together with full prevention of locomotion defaults and impairment of myocyte development observed in Slc6a8Y389 C/y male rats. While more work is needed to correct those CTD phenotypes more associated with forebrain structures, this study is the first demonstrating positive effects of an AAV-driven gene therapy on CTD and thus represents a very encouraging approach to treat the so-far untreatable CTD.

TREM2 deficiency aggravates renal injury by promoting macrophage apoptosis and polarization via the JAK-STAT pathway in mice

The triggering receptor expressed on myeloid cells 2 (TREM2) is an immune receptor that affects cellular phenotypes by modulating phagocytosis and metabolism, promoting cell survival, and counteracting inflammation. Its role in renal injury, in particular, unilateral ureteral obstruction (UUO) or ischemia-reperfusion injury (IRI)-induced renal injury remains unclear. In our study, WT and Trem2-/- mice were employed to evaluate the role of TREM2 in renal macrophage infiltration and tissue injury after UUO. Bone marrow-derived macrophages (BMDM) from both mouse genotypes were cultured and polarized for in vitro experiments. Next, the effects of TREM2 on renal injury and macrophage polarization in IRI mice were also explored. We found that TREM2 expression was upregulated in the obstructed kidneys. TREM2 deficiency exacerbated renal inflammation and fibrosis 3 and 7 days after UUO, in association with reduced macrophage infiltration. Trem2-/- BMDM exhibited increased apoptosis and poorer survival compared with WT BMDM. Meanwhile, TREM2 deficiency augmented M1 and M2 polarization after UUO. Consistent with the in vivo observations, TREM2 deficiency led to increased polarization of BMDM towards the M1 proinflammatory phenotype. Mechanistically, TREM2 deficiency promoted M1 and M2 polarization via the JAK-STAT pathway in the presence of TGF-β1, thereby affecting cell survival by regulating mTOR signaling. Furthermore, cyclocreatine supplementation alleviated cell death caused by TREM2 deficiency. Additionally, we found that TREM2 deficiency promoted renal injury, fibrosis, and macrophage polarization in IRI mice. The current data suggest that TREM2 deficiency aggravates renal injury by promoting macrophage apoptosis and polarization via the JAK-STAT pathway. These findings have implications for the role of TREM2 in the regulation of renal injury that justify further evaluation.

TREM2-IGF1 Mediated Glucometabolic Enhancement Underlies Microglial Neuroprotective Properties During Ischemic Stroke

Microglia, the major resident immune cells in the central nervous system, serve as the frontline soldiers against cerebral ischemic injuries, possibly along with metabolic alterations. However, signaling pathways involved in the regulation of microglial immunometabolism in ischemic stroke remain to be further elucidated. In this study, using single-nuclei RNA sequencing, a microglial subcluster up-regulated in ischemic brain tissues is identified, with high expression of Igf1 and Trem2, neuroprotective transcriptional signature and enhanced oxidative phosphorylation. Microglial depletion by PLX3397 exacerbates ischemic brain damage, which is reversed by repopulating the microglia with high Igf1 and Trem2 phenotype. Mechanistically, Igf1 serves as one of the major down-stream molecules of Trem2, and Trem2-Igf1 signaling axis regulates microglial functional and metabolic profiles, exerting neuroprotective effects on ischemic stroke. Overexpression of Igf1 and supplementation of cyclocreatine restore microglial glucometabolic levels and cellular functions even in the absence of Trem2. These findings suggest that Trem2-Igf1 signaling axis reprograms microglial immunometabolic profiles and shifts microglia toward a neuroprotective phenotype, which has promising therapeutic potential in treating ischemic stroke.

Suggestion of creatine as a new neurotransmitter by approaches ranging from chemical analysis and biochemistry to electrophysiology

The discovery of a new neurotransmitter, especially one in the central nervous system, is both important and difficult. We have been searching for new neurotransmitters for 12 y. We detected creatine (Cr) in synaptic vesicles (SVs) at a level lower than glutamate and gamma-aminobutyric acid but higher than acetylcholine and 5-hydroxytryptamine. SV Cr was reduced in mice lacking either arginine:glycine amidinotransferase (a Cr synthetase) or SLC6A8, a Cr transporter with mutations among the most common causes of intellectual disability in men. Calcium-dependent release of Cr was detected after stimulation in brain slices. Cr release was reduced in Slc6a8 and Agat mutants. Cr inhibited neocortical pyramidal neurons. SLC6A8 was necessary for Cr uptake into synaptosomes. Cr was found by us to be taken up into SVs in an ATP-dependent manner. Our biochemical, chemical, genetic, and electrophysiological results are consistent with the possibility of Cr as a neurotransmitter, though not yet reaching the level of proof for the now classic transmitters. Our novel approach to discover neurotransmitters is to begin with analysis of contents in SVs before defining their function and physiology.

"Heads Up" for Creatine Supplementation and its Potential Applications for Brain Health and Function

There is emerging interest regarding the potential beneficial effects of creatine supplementation on indices of brain health and function. Creatine supplementation can increase brain creatine stores, which may help explain some of the positive effects on measures of cognition and memory, especially in aging adults or during times of metabolic stress (i.e., sleep deprivation). Furthermore, creatine has shown promise for improving health outcome measures associated with muscular dystrophy, traumatic brain injury (including concussions in children), depression, and anxiety. However, whether any sex- or age-related differences exist in regard to creatine and indices of brain health and function is relatively unknown. The purpose of this narrative review is to: (1) provide an up-to-date summary and discussion of the current body of research focusing on creatine and indices of brain health and function and (2) discuss possible sex- and age-related differences in response to creatine supplementation on brain bioenergetics, measures of brain health and function, and neurological diseases.

Thermogenic Modulation of Adipose Depots: A Perspective on Possible Therapeutic Intervention with Early Cardiorenal Complications of Metabolic Impairment

Cardiovascular complications of diabetes and obesity remain a major cause for morbidity and mortality worldwide. Despite significant advances in the pharmacotherapy of metabolic disease, the available approaches do not prevent or slow the progression of complications. Moreover, a majority of patients present with significant vascular involvement at early stages of dysfunction prior to overt metabolic changes. The lack of disease-modifying therapies affects millions of patients globally, causing a massive economic burden due to these complications. Significantly, adipose tissue inflammation was implicated in the pathogenesis of metabolic syndrome, diabetes, and obesity. Specifically, perivascular adipose tissue (PVAT) and perirenal adipose tissue (PRAT) depots influence cardiovascular and renal structure and function. Accumulating evidence implicates localized PVAT/PRAT inflammation as the earliest response to metabolic impairment leading to cardiorenal dysfunction. Increased mitochondrial uncoupling protein 1 (UCP1) expression and function lead to PVAT/PRAT hypoxia and inflammation as well as vascular, cardiac, and renal dysfunction. As UCP1 function remains an undruggable target so far, modulation of the augmented UCP1-mediated PVAT/PRAT thermogenesis constitutes a lucrative target for drug development to mitigate early cardiorenal involvement. This can be achieved either by subtle targeted reduction in UCP-1 expression using innovative proteolysis activating chimeric molecules (PROTACs) or by supplementation with cyclocreatine phosphate, which augments the mitochondrial futile creatine cycling and thus decreases UCP1 activity, enhances the efficiency of oxygen use, and reduces hypoxia. Once developed, these molecules will be first-in-class therapeutic tools to directly interfere with and reverse the earliest pathology underlying cardiac, vascular, and renal dysfunction accompanying the early metabolic deterioration. SIGNIFICANCE STATEMENT: Adipose tissue dysfunction plays a major role in the pathogenesis of metabolic diseases and their complications. Although mitochondrial alterations are common in metabolic impairment, it was only recently shown that the early stages of metabolic challenge involve inflammatory changes in select adipose depots associated with increased uncoupling protein 1 thermogenesis and hypoxia. Manipulating this mode of thermogenesis can help mitigate the early inflammation and the consequent cardiorenal complications.

Diagnosis and Treatment of X-Linked Creatine Transporter Deficiency: Case Report and Literature Review

(1) Background: X-linked creatine transporter deficiency (CTD) (OMIM 300036) is a rare group of inherited metabolic disorders characterized by global developmental delay/intellectual disability (GDD/ID), seizures, autistic behavior, and movement disorders. Pathogenic variants in the SLC6A8 gene, located at Xq28, are causative of the disease, leading to impaired creatine transport into the brain. Supplementation with creatine and its precursors, glycine and arginine, has been attempted, yet the treatment efficacy remains controversial. (2) Methods: Here we report a de novo SLC6A8 variant in a boy aged 3 years 9 months presenting with GDD, autistic behavior, and epilepsy. Elevated urinary creatine/creatinine ratio and diminished creatine peak on brain MR spectroscopy suggested the diagnosis of CTD. Genetic sequencing revealed a de novo hemizygous frameshift variant (NM_005629: c.1136_1137del, p. Glu379ValfsTer85). Creatine supplementation therapy was initiated after definitive diagnosis. Electroencephalography and MR spectroscopy were monitored during follow-up in concurrence with neuropsychological evaluations. The clinical phenotype and treatment response of CTD were summarized by systematic view of the literature. (3) Results: In silico analysis showed this variant to be deleterious, probably interfering with substrate binding and conformational changes during creatine transport. Creatine supplementation therapy led to seizure cessation and modest cognitive improvement after half-year's treatment. (4) Conclusions: This case highlights the importance of MR spectroscopy and metabolic screening in males with GDD/ID, allowing for early diagnosis and therapeutic intervention. Mechanistic understanding and case-per-se analysis are required to enable precision treatment for the patients.

Creatine transporter-deficient rat model shows motor dysfunction, cerebellar alterations, and muscle creatine deficiency without muscle atrophy

Creatine (Cr) is a nitrogenous organic acid and plays roles such as fast phosphate energy buffer to replenish ATP, osmolyte, antioxidant, neuromodulator, and as a compound with anabolic and ergogenic properties in muscle. Cr is taken from the diet or endogenously synthetized by the enzymes arginine:glycine amidinotransferase and guanidinoacetate methyltransferase, and specifically taken up by the transporter SLC6A8. Loss-of-function mutations in the genes encoding for the enzymes or the transporter cause creatine deficiency syndromes (CDS). CDS are characterized by brain Cr deficiency, intellectual disability with severe speech delay, behavioral troubles, epilepsy, and motor dysfunction. Among CDS, the X-linked Cr transporter deficiency (CTD) is the most prevalent with no efficient treatment so far. Different animal models of CTD show reduced brain Cr levels, cognitive deficiencies, and together they cover other traits similar to those of patients. However, motor function was poorly explored in CTD models, and some controversies in the phenotype exist in comparison with CTD patients. Our recently described Slc6a8^Y389C knock-in rat model of CTD showed mild impaired motor function, morphological alterations in cerebellum, reduced muscular mass, Cr deficiency, and increased guanidinoacetate content in muscle, although no consistent signs of muscle atrophy. Our results indicate that such motor dysfunction co-occurred with both nervous and muscle dysfunctions, suggesting that muscle strength and performance as well as neuronal connectivity might be affected by this Cr deficiency in muscle and brain.

Effects of Delivering Guanidinoacetic Acid or Its Prodrug to the Neural Tissue: Possible Relevance for Creatine Transporter Deficiency

The creatine precursor guanidinoacetate (GAA) was used as a dietary supplement in humans with no adverse events. Nevertheless, it has been suggested that GAA is epileptogenic or toxic to the nervous system. However, increased GAA content in rodents affected by guanidinoacetate methyltransferase (GAMT) deficiency might be responsible for their spared muscle function. Given these conflicting data, and lacking experimental evidence, we investigated whether GAA affected synaptic transmission in brain hippocampal slices. Incubation with 11.5 μM GAA (the highest concentration in the cerebrospinal fluid of GAMT-deficient patients) did not change the postsynaptic compound action potential. Even 1 or 2 mM had no effect, while 4 mM caused a reversible decrease in the potential. Guanidinoacetate increased creatine and phosphocreatine, but not after blocking the creatine transporter (also used by GAA). In an attempt to allow the brain delivery of GAA when there was a creatine transporter deficiency, we synthesized diacetyl guanidinoacetic acid ethyl ester (diacetyl-GAAE), a lipophilic derivative. In brain slices, 0.1 mM did not cause electrophysiological changes and improved tissue viability after blockage of the creatine transporter. However, diacetyl-GAAE did not increase creatine nor phosphocreatine in brain slices after blockage of the creatine transporter. We conclude that: (1) upon acute administration, GAA is neither epileptogenic nor neurotoxic; (2) Diacetyl-GAAE improves tissue viability after blockage of the creatine transporter but not through an increase in creatine or phosphocreatine. Diacetyl-GAAE might give rise to a GAA–phosphoGAA system that vicariates the missing creatine–phosphocreatine system. Our in vitro data show that GAA supplementation may be safe in the short term, and that a lipophilic GAA prodrug may be useful in creatine transporter deficiency.

Current and potential new treatment strategies for creatine deficiency syndromes

Creatine deficiency syndromes (CDS) are inherited metabolic disorders caused by mutations in GATM, GAMT and SLC6A8 and mainly affect central nervous system (CNS). AGAT- and GAMT-deficient patients lack the functional brain endogenous creatine (Cr) synthesis pathway but express the Cr transporter SLC6A8 at blood-brain barrier (BBB), and can thus be treated by oral supplementation of high doses of Cr. For Cr transporter deficiency (SLC6A8 deficiency or CTD), current treatment strategies benefit one-third of patients. However, as their phenotype is not completely reversed, and for the other two-thirds of CTD patients, the development of novel more effective therapies is needed. This article aims to review the current knowledge on Cr metabolism and CDS clinical aspects, highlighting their current treatment possibilities and the most recent research perspectives on CDS potential therapeutics designed, in particular, to bring new options for the treatment of CTD.

Formaldehyde toxicity in age-related neurological dementia

The primordial small gaseous molecules, such as: NO, CO, H₂S and formaldehyde (FA) are present in the brains. Whether FA as well as the other molecules participates in brain functions is unclear. Recently, its pathophysiological functions have been investigated. Notably, under physiological conditions, learning activity induces a transient generation of hippocampal FA, which promotes memory formation by enhancing N-methyl-D-aspartate (NMDA)-currents. However, ageing leads to FA accumulation in brain for the dysregulation of FA metabolism; and excessive FA directly impairs memory by inhibiting NMDA-receptor. Especially, in Alzheimer's disease (AD), amyloid-beta (Aβ) accelerates FA accumulation by inactivating alcohol dehydrogenase-5; in turn, FA promotes Aβ oligomerization, fibrillation and tau hyperphosphorylation. Hence, there is a vicious circle encompassing Aβ assembly and FA generation. Even worse, FA induces Aβ deposition in the extracellular space (ECS), which blocks the medicines (dissolved in the interstitial fluid) flowing into the damaged neurons in the deep cortex. However, phototherapy destroys Aβ deposits in the ECS and restores ISF flow. Coenzyme Q10, which scavenges FA, was shown to ameliorate Aβ-induced AD pathological phenotypes, thus suggesting a causative relation between FA toxicity and AD. These findings suggest that the combination of these two methods is a promising strategy for treating AD.

HIF-Dependent CKB Expression Promotes Breast Cancer Metastasis, Whereas Cyclocreatine Therapy Impairs Cellular Invasion and Improves Chemotherapy Efficacy

Simple Summary

Targeting dysregulated cellular metabolism is a promising avenue to treat metastatic disease. The aim of our study was to identify genes downstream of the hypoxia-inducible factor (HIF)-1 transcription factor that are amenable to therapeutic intervention to treat metastatic breast cancer (MBC). We identified creatine kinase, brain isoform (CKB) as an HIF-dependent gene that strongly promotes invasion and metastasis in estrogen-receptor (ER)-negative breast cancer models. Deletion of Ckb also repressed glycolysis and mitochondrial respiration, leading to a reduction in intracellular ATP. Either the deletion of Ckb or inhibition of creatine kinase (CK) activity using the creatine analog cyclocreatine (cCr) repressed cell invasion, the formation of invadopodia and lung metastasis. In addition, when paired with paclitaxel or doxorubicin, cCr enhanced growth inhibition in an additive or synergistic manner. cCr may be an effective anti-metastatic agent in ER-negative, HIF-1α-positive breast cancers, targeting both cellular metabolism and motility, particularly when paired with conventional cytotoxic agents.

Abstract

The oxygen-responsive hypoxia inducible factor (HIF)-1 promotes several steps of the metastatic cascade. A hypoxic gene signature is enriched in triple-negative breast cancers (TNBCs) and is correlated with poor patient survival. Inhibiting the HIF transcription factors with small molecules is challenging; therefore, we sought to identify genes downstream of HIF-1 that could be targeted to block invasion and metastasis. Creatine kinase brain isoform (CKB) was identified as a highly differentially expressed gene in a screen of HIF-1 wild type and knockout mammary tumor cells derived from a transgenic model of metastatic breast cancer. CKB is a cytosolic enzyme that reversibly catalyzes the phosphorylation of creatine, generating phosphocreatine (PCr) in the forward reaction, and regenerating ATP in the reverse reaction. Creatine kinase activity is inhibited by the creatine analog cyclocreatine (cCr). Loss- and gain-of-function genetic approaches were used in combination with cCr therapy to define the contribution of CKB expression or creatine kinase activity to cell proliferation, migration, invasion, and metastasis in ER-negative breast cancers. CKB was necessary for cell invasion in vitro and strongly promoted tumor growth and lung metastasis in vivo. Similarly, cyclocreatine therapy repressed cell migration, cell invasion, the formation of invadopodia and lung metastasis. Moreover, in common TNBC cell line models, the addition of cCr to conventional cytotoxic chemotherapy agents was either additive or synergistic to repress tumor cell growth.

Evaluation of chronic toxicity of cyclocreatine in beagle dogs after oral gavage administration for up to 23 weeks

Cyclocreatine (LUM-001) was evaluated for chronic toxicity (23 weeks) in beagle dogs to support clinical development in patients with creatine transporter deficiency (CTD) disorder. Deionized water (vehicle control) or cyclocreatine was administered by oral gavage twice daily (12 ± 1 h apart) at 20, 40 and 75 mg/kg/dose followed by a recovery period. Due to severe toxicity, the study was terminated earlier than the planned 39 weeks of dosing. Animals in the 20, 40 and 75 mg/kg/dose groups completed 160, 106, and 55 days of dosing, respectively, followed by 30, 55 and 106 days of a recovery period, respectively. Three (25%), 7 (58%), and 7 (58%) animals were euthanized and/or found dead in the 40, 80, and 150 mg/kg/day dose groups, respectively. Clinical signs observed were inappetence, frequent emesis, stool abnormalities, weight loss, lethargy and respiratory distress. Histopathological evaluation revealed congestion, edema, cellular infiltration, fibrin, and/or hemorrhage in the lungs of all dose groups. Additionally, animals in all cyclocreatine treatment groups had perinuclear cytoplasmic vacuoles in the heart, kidneys, skeletal and smooth muscles. After the recovery period, the vacuoles were still observed in the cardiac and renal tissues. Cyclocreatine was absorbed rapidly with mean Tmax within 1 to 2 h and half-life ranged between 2.17 and 2.79 h on Day 1, however, on the final day of dosing, it ranged between 5.80 and 8.77 h (males) and 10.3 to 13.1 h (females). To conclude, in this study the lungs, kidneys, heart, skeletal and smooth muscles were identified as the target organs of cyclocreatine toxicity in beagle dogs.

Creatine transporter deficiency impairs stress adaptation and brain energetics homeostasis

The creatine transporter (CrT) maintains brain creatine (Cr) levels, but the effects of its deficiency on energetics adaptation under stress remain unclear. There are also no effective treatments for CrT deficiency, the second most common cause of X-linked intellectual disabilities. Herein, we examined the consequences of CrT deficiency in brain energetics and stress-adaptation responses plus the effects of intranasal Cr supplementation. We found that CrT-deficient (CrT^–/y) mice harbored dendritic spine and synaptic dysgenesis. Nurtured newborn CrT^–/y mice maintained baseline brain ATP levels, with a trend toward signaling imbalance between the p-AMPK/autophagy and mTOR pathways. Starvation elevated the signaling imbalance and reduced brain ATP levels in P3 CrT^–/y mice. Similarly, CrT^–/y neurons and P10 CrT^–/y mice showed an imbalance between autophagy and mTOR signaling pathways and greater susceptibility to cerebral hypoxia-ischemia and ischemic insults. Notably, intranasal administration of Cr after cerebral ischemia increased the brain Cr/N-acetylaspartate ratio, partially averted the signaling imbalance, and reduced infarct size more potently than intraperitoneal Cr injection. These findings suggest important functions for CrT and Cr in preserving the homeostasis of brain energetics in stress conditions. Moreover, intranasal Cr supplementation may be an effective treatment for congenital CrT deficiency and acute brain injury.

1H-MR Spectroscopy of the Early Developmental Brain, Neonatal Encephalopathies, and Neurometabolic Disorders

MRI interpretations of the pediatric brain are often challenging for general radiologists and clinicians because MR  signals and morphology are continuously changing in the developing brain. Furthermore, the developing brain reacts differently to injuries, resulting in imaging characteristics that differ from those of the mature brain. Proton magnetic resonance spectroscopy (¹H-MRS) is a non-invasive method for assessing neurological abnormalities at the microscopic level and measures in vivo brain metabolites using a clinical MR machine. In MR examinations of the pediatric brain, ¹H-MRS demonstrates its powerful diagnostic capability when MRI is insufficient for diagnostic features. MRI and ¹H-MRS may be complementary tools for diagnosing and monitoring diseases. However, there is currently no consensus on how to include ¹H-MRS in clinical MR examinations. An overview of the clinical implementation of ¹H-MRS for the assessment of early pediatric developmental brains as well as the diagnosis, prognostification, and disease monitoring of various non-neoplastic brain disorders, including neonatal encephalopathies and neurometabolic/neurodegenerative diseases, was provided herein. Qualitative and quantitative ¹H-MRS is a powerful non-invasive tool for accessing various brain metabolites to confirm age appropriate peaks and detect abnormal peaks or deficient or reduced peaks, which may facilitate the identification of metabolic and neurodegenerative disorders as well as damage associated with hypoxic-ischemic encephalopathy (HIE). Moreover, ¹H-MRS has potential as a biomarker for monitoring therapeutic efficacy in metabolic diseases and neonatal HIE. It also provides insights into the pathophysiologies of various disorders, which may facilitate the use of novel therapeutic approaches. Therefore, ¹H-MRS needs to be included more frequently in routine clinical MR examinations of pediatric patients with neurological disorders.

The Role of Preclinical Models in Creatine Transporter Deficiency: Neurobiological Mechanisms, Biomarkers and Therapeutic Development

Creatine (Cr) Transporter Deficiency (CTD) is an X-linked metabolic disorder, mostly caused by missense mutations in the SLC6A8 gene and presenting with intellectual disability, autistic behavior, and epilepsy. There is no effective treatment for CTD and patients need lifelong assistance. Thus, the research of novel intervention strategies is a major scientific challenge. Animal models are an excellent tool to dissect the disease pathogenetic mechanisms and drive the preclinical development of therapeutics. This review illustrates the current knowledge about Cr metabolism and CTD clinical aspects, with a focus on mainstay diagnostic and therapeutic options. Then, we discuss the rodent models of CTD characterized in the last decade, comparing the phenotypes expressed within clinically relevant domains and the timeline of symptom development. This analysis highlights that animals with the ubiquitous deletion/mutation of SLC6A8 genes well recapitulate the early onset and the complex pathological phenotype of the human condition. Thus, they should represent the preferred model for preclinical efficacy studies. On the other hand, brain- and cell-specific conditional mutants are ideal for understanding the basis of CTD at a cellular and molecular level. Finally, we explain how CTD models might provide novel insight about the pathogenesis of other disorders, including cancer.

Creatine transport and pathological changes in creatine transporter deficient mice

The severe impact on brain function and lack of effective therapy for patients with creatine (Cr) transporter deficiency motivated the generation of three ubiquitous Slc6a8 deficient mice (-/y). While each mouse knock-out line has similar behavioral effects at 2 to 3 months of age, other features critical to the efficient use of these mice in drug discovery are unclear or lacking: the concentration of Cr in brain and heart differ widely between mouse lines, there are limited data on histopathologic changes, and no data on Cr uptake. Here, we determined survival, measured endogenous Cr and uptake of its deuterium-labeled analogue Cr-d3 using a liquid chromatography coupled with tandem mass spectrometry assay, and performed comprehensive histopathologic examination on the Slc6a8^-/y mouse developed by Skelton et al. Our results show that Slc6a8^-/y mice have widely varying organ-specific uptake of Cr-d3, significantly diminished growth with the exception of brain, progressive vacuolar myopathy, and markedly shortened lifespan.

Use of an animal model of disease for toxicology enables identification of a juvenile no observed adverse effect level for cyclocreatine in creatine transporter deficiency

In standard general toxicology studies in two species to support clinical development, cyclocreatine, a creatine analog for the treatment of creatine transporter deficiency, caused deaths, convulsions, and/or multi-organ pathology. The potential translatability of these findings to patients was evaluated by comparing toxicity of cyclocreatine in wild-type mice to creatine transporter-deficient mice, a model of the human disease. A biodistribution study indicated greater accumulation of cyclocreatine in the brains of wild-type mice, consistent with its ability to be transported by the creatine transporter. Subsequent toxicology studies confirmed greater sensitivity of wild-type mice to cyclocreatine-induced toxicity. Exposure at the no observed adverse effect level in creatine transporter-deficient (554 μg*hr/ml) mice exceeded exposure at the maximum tolerated dose in wild-type (248 μg*hr/ml) mice. When dosed at 300 mg/kg/day for 3 months, cyclocreatine-related mortality, convulsions, and multi-organ pathology were observed in wild-type mice whereas there were no adverse findings in creatine transporter-deficient mice. Brain vacuolation was common to both strains. Although transporter-deficient mice appeared to be more sensitive, the finding had no functional correlates in this strain. The results highlight the importance of considering models of disease for toxicology in cases where they may be relevant to assessing safety in the intended patient population.

The influence of the R47H triggering receptor expressed on myeloid cells 2 variant on microglial exosome profiles

Variants in the triggering receptor expressed on myeloid cells 2 gene are linked with an increased risk of dementia, in particular the R47Hhet triggering receptor expressed on myeloid cells 2 variant is linked to late-onset Alzheimer's disease. Using human induced pluripotent stem cells-derived microglia, we assessed whether variations in the dynamics of exosome secretion, including their components, from these cells might underlie some of this risk. We found exosome size was not altered between common variant controls and R47Hhet variants, but the amount and constitution of exosomes secreted were different. Exosome quantities were rescued by incubation with an ATP donor or with lipids via a phosphatidylserine triggering receptor expressed on myeloid cells 2 ligand. Following a lipopolysaccharide or phagocytic cell stimulus, exosomes from common variant and R47Hhet microglia were found to contain cytokines, chemokines, APOE and triggering receptor expressed on myeloid cells 2. Differences were observed in the expression of CCL22, IL-1β and triggering receptor expressed on myeloid cells 2 between common variant and R47Hhet derived exosomes. Furthermore unlike common variant-derived exosomes, R47Hhet exosomes contained additional proteins linked to negative regulation of transcription and metabolic processes. Subsequent addition of exosomes to stressed neurones showed R47Hhet-derived exosomes to be less protective. These data have ramifications for the responses of microglia in Alzheimer's disease and may point to further targets for therapeutic intervention.

A new rat model of creatine transporter deficiency reveals behavioral disorder and altered brain metabolism

Creatine is an organic compound used as fast phosphate energy buffer to recycle ATP, important in tissues with high energy demand such as muscle or brain. Creatine is taken from the diet or endogenously synthetized by the enzymes AGAT and GAMT, and specifically taken up by the transporter SLC6A8. Deficit in the endogenous synthesis or in the transport leads to Cerebral Creatine Deficiency Syndromes (CCDS). CCDS are characterized by brain creatine deficiency, intellectual disability with severe speech delay, behavioral troubles such as attention deficits and/or autistic features, and epilepsy. Among CCDS, the X-linked creatine transporter deficiency (CTD) is the most prevalent with no efficient treatment so far. Different mouse models of CTD were generated by doing long deletions in the Slc6a8 gene showing reduced brain creatine and cognitive deficiencies or impaired motor function. We present a new knock-in (KI) rat model of CTD holding an identical point mutation found in patients with reported lack of transporter activity. KI males showed brain creatine deficiency, increased urinary creatine/creatinine ratio, cognitive deficits and autistic-like traits. The Slc6a8^Y389C KI rat fairly enriches the spectrum of CTD models and provides new data about the pathology, being the first animal model of CTD carrying a point mutation.

Cyclocreatine treatment ameliorates the cognitive, autistic and epileptic phenotype in a mouse model of Creatine Transporter Deficiency

Creatine Transporter Deficiency (CTD) is an inborn error of metabolism presenting with intellectual disability, behavioral disturbances and epilepsy. There is currently no cure for this disorder. Here, we employed novel biomarkers for monitoring brain function, together with well-established behavioral readouts for CTD mice, to longitudinally study the therapeutic efficacy of cyclocreatine (cCr) at the preclinical level. Our results show that cCr treatment is able to partially correct hemodynamic responses and EEG abnormalities, improve cognitive deficits, revert autistic-like behaviors and protect against seizures. This study provides encouraging data to support the potential therapeutic benefit of cyclocreatine or other chemically modified lipophilic analogs of Cr.

The Creatine Transporter Unfolded: A Knotty Premise in the Cerebral Creatine Deficiency Syndrome

Creatine provides cells with high-energy phosphates for the rapid reconstitution of hydrolyzed adenosine triphosphate. The eponymous creatine transporter (CRT1/SLC6A8) belongs to a family of solute carrier 6 (SLC6) proteins. The key role of CRT1 is to translocate creatine across tissue barriers and into target cells, such as neurons and myocytes. Individuals harboring mutations in the coding sequence of the human CRT1 gene develop creatine transporter deficiency (CTD), one of the pivotal underlying causes of cerebral creatine deficiency syndrome. CTD encompasses an array of clinical manifestations, including severe intellectual disability, epilepsy, autism, development delay, and motor dysfunction. CTD is characterized by the absence of cerebral creatine, which implies an indispensable role for CRT1 in supplying the brain cells with creatine. CTD-associated variants dramatically reduce or abolish creatine transport activity by CRT1. Many of these are point mutations that are known to trigger folding defects, leading to the retention of encoded CRT1 proteins in the endoplasmic reticulum and precluding their delivery to the cell surface. Misfolding of several related SLC6 transporters also gives rise to detrimental pathologic conditions in people; e.g., mutations in the dopamine transporter induce infantile parkinsonism/dystonia, while mutations in the GABA transporter 1 cause treatment-resistant epilepsy. In some cases, folding defects are amenable to rescue by small molecules, known as pharmacological and chemical chaperones, which restore the cell surface expression and transport activity of the previously non-functional proteins. Insights from the recent molecular, animal and human case studies of CTD add toward our understanding of this complex disorder and reveal the wide-ranging effects elicited upon CRT1 dysfunction. This grants novel therapeutic prospects for the treatment of patients afflicted with CTD, e.g., modifying the creatine molecule to facilitate CRT1-independent entry into brain cells, or correcting folding-deficient and loss-of-function CTD variants using pharmacochaperones and/or allosteric modulators. The latter justifies a search for additional compounds with a capacity to correct mutation-specific defects.

Evaluation of chronic toxicity of cyclocreatine, a creatine analog, in Sprague Dawley rat after oral gavage administration for up to 26 weeks

Cyclocreatine (LUM-001), a creatine analog, was evaluated for its nonclinical toxicity in Sprague Dawley (SD) rats. Deionized water as a vehicle control article or cyclocreatine was administered by oral gavage twice daily (approximately 12 ± 1 h apart) at 30, 100 and 300 mg/kg/dose levels in rats up to 26 weeks followed by a 28-day recovery period. Due to an increased incidence of seizures, the 600 mg/kg/day dose group males were dosed only for 16-weeks followed by a 14-week recovery period. Thirteen males and four females from 600 mg/kg/day dose group were sacrificed at interim on Day 113 to study plausible brain lesions and not due to moribundity. There was a dose dependent increase in the number of seizure incidences in ≥60 mg/kg/day males and 600 mg/kg/day females. Microscopically, higher incidences of vacuoles in the brain at 600 mg/kg/day in both sexes, thyroid follicular atrophy and follicular cell hypertrophy at ≥200 mg/kg/day in males and 600 mg/kg/day in females, and seminiferous tubular degeneration and/or interstitial edema in testes at ≥200 mg/kg/day were observed. Mean plasma half-life of cyclocreatine was between 3.5 and 6.5 h. In conclusion, chronic administration of cyclocreatine by oral gavage in Sprague Dawley rats induced the seizures and microscopic lesions in the brain, testes and thyroid. Based on the results of this study the highest tested dose of 600 mg/kg/day (mean C_max of 151.5 μg/mL; AUC_0-24 of 1970 h*μg/mL) was considered the maximum tolerated dose (MTD) in SD rats.

Novel translational phenotypes and biomarkers for creatine transporter deficiency

Creatine transporter deficiency is a metabolic disorder characterized by intellectual disability, autistic-like behaviour and epilepsy. There is currently no cure for creatine transporter deficiency, and reliable biomarkers of translational value for monitoring disease progression and response to therapeutics are sorely lacking. Here, we found that mice lacking functional creatine transporter display a significant alteration of neural oscillations in the EEG and a severe epileptic phenotype that are recapitulated in patients with creatine transporter deficiency. In-depth examination of knockout mice for creatine transporter also revealed that a decrease in EEG theta power is predictive of the manifestation of spontaneous seizures, a frequency that is similarly affected in patients compared to healthy controls. In addition, knockout mice have a highly specific increase in haemodynamic responses in the cerebral cortex following sensory stimuli. Principal component and Random Forest analyses highlighted that these functional variables exhibit a high performance in discriminating between pathological and healthy phenotype. Overall, our findings identify novel, translational and non-invasive biomarkers for the analysis of brain function in creatine transporter deficiency, providing a very reliable protocol to longitudinally monitor the efficacy of potential therapeutic strategies in preclinical, and possibly clinical, studies.

Cyclocreatine Transport by SLC6A8, the Creatine Transporter, in HEK293 Cells, a Human Blood-Brain Barrier Model Cell, and CCDSs Patient-Derived Fibroblasts

PURPOSE

Cyclocreatine, a creatine analog, is a candidate drug for treating patients with cerebral creatine deficiency syndromes (CCDSs) caused by creatine transporter (CRT, SLC6A8) deficiency, which reduces brain creatine level. The purpose of this study was to clarify the characteristics of cyclocreatine transport in HEK293 cells, which highly express endogenous CRT, in hCMEC/D3 cells, a human blood-brain barrier (BBB) model, and in CCDSs patient-derived fibroblasts with CRT mutations.

METHODS

Cells were incubated at 37°C with [¹⁴C]cyclocreatine (9 μM) and [¹⁴C]creatine (9 μM) for specified periods of times in the presence or absence of inhibitors, while the siRNAs were transfected by lipofection. Protein expression and mRNA expression were quantified using targeted proteomics and quantitative PCR, respectively.

RESULTS

[¹⁴C]Cyclocreatine was taken up by HEK293 cells in a time-dependent manner, while exhibiting saturable kinetics. The inhibition and siRNA knockdown studies demonstrated that the uptake of [¹⁴C]cyclocreatine by both HEK293 and hCMEC/D3 cells was mediated predominantly by CRT as well as [¹⁴C]creatine. In addition, uptake of [¹⁴C]cyclocreatine and [¹⁴C]creatine by the CCDSs patient-derived fibroblasts was found to be largely reduced.

CONCLUSION

The present study suggests that cyclocreatine is a CRT substrate, where CRT is the predominant contributor to influx of cyclocreatine into the brain at the BBB. Our findings provide vital insights for the purposes of treating CCDSs patients using cyclocreatine.

Phosphocyclocreatine is the dominant form of cyclocreatine in control and creatine transporter deficiency patient fibroblasts

Creatine transporter deficiency (CTD) is a metabolic disorder resulting in cognitive, motor, and behavioral deficits. Cyclocreatine (cCr), a creatine analog, has been explored as a therapeutic strategy for the treatment of CTD. We developed a rapid, selective, and accurate HILIC ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method to simultaneously quantify the intracellular concentrations of cCr, creatine (Cr), creatine-d3 (Cr-d3), phosphocyclocreatine (pcCr), and phosphocreatine (pCr). Using HILIC-UPLC-MS/MS, we measured cCr and Cr-d3 uptake and their conversion to the phosphorylated forms in primary human control and CTD fibroblasts. Altogether, the data demonstrate that cCr enters cells and its dominant intracellular form is pcCr in both control and CTD patient cells. Therefore, cCr may replace creatine as a therapeutic strategy for the treatment of CTD.

Deletion of the creatine transporter gene in neonatal, but not adult, mice leads to cognitive deficits

Creatine (Cr) is a guanidino compound that provides readily available phosphate pools for the regeneration of spent adenosine triphosphate (ATP). The lack of brain Cr causes moderate to severe intellectual disability, language impairment, and epilepsy. The most prevalent cause of Cr deficiency are mutations in the X-linked SLC6A8 (Creatine transporter; CrT) gene, known as CrT deficiency (CTD). One of the most critical areas that need to be addressed is whether Cr is necessary for brain development. To address this concern, the Slc6a8 gene was knocked out in either neonatal (postnatal day (P)5) or adult (P60) mice using a tamoxifen-inducible Cre recombinase driven by the human ubiquitin C (UBC) promoter. Mice were tested in the Morris water maze, novel, object recognition, and conditioned fear 60 days after Slc6a8 deletion. In addition, overnight locomotor activity was analyzed. Mice that had the gene deleted on P5 showed deficits in the Morris water maze and novel object recognition, while there were no deficits in P60 knockout mice. Interestingly, the P5 knockout mice showed hyperactivity during the dark phase; however, when examining control mice, the effect was due to the administration of tamoxifen from P5 to 10. Taken together, the results of this study show that Cr is necessary during periods of brain development involved in spatial and object learning. This study also highlights the continued importance of using proper control groups for behavioral testing.

Slc6a8-Mediated Creatine Uptake and Accumulation Reprogram Macrophage Polarization via Regulating Cytokine Responses

Macrophage polarization is accompanied by drastic changes in L-arginine metabolism. Two L-arginine catalytic enzymes, iNOS and arginase 1, are well-characterized hallmark molecules of classically and alternatively activated macrophages, respectively. The third metabolic fate of L-arginine is the generation of creatine that acts as a key source of cellular energy reserve, yet little is known about the role of creatine in the immune system. Here, genetic, genomic, metabolic, and immunological analyses revealed that creatine reprogrammed macrophage polarization by suppressing M(interferon-γ [IFN-γ]) yet promoting M(interleukin-4 [IL-4]) effector functions. Mechanistically, creatine inhibited the induction of immune effector molecules, including iNOS, by suppressing IFN-γ-JAK-STAT1 transcription-factor signaling while supporting IL-4-STAT6-activated arginase 1 expression by promoting chromatin remodeling. Depletion of intracellular creatine by ablation of the creatine transporter Slc6a8 altered macrophage-mediated immune responses in vivo. These results uncover a previously uncharacterized role for creatine in macrophage polarization by modulating cellular responses to cytokines such as IFN-γ and IL-4.

Facile High-Performance Liquid Chromatography Mass Spectrometry Method for Analysis of Cyclocreatine and Phosphocyclocreatine in Complex Mixtures of Amino Acids

Creatine transporter deficiency (CTD) is caused by a defect in the X-linked creatine transporter SLC6A8 gene leading to severe neurologic and physiologic conditions. Cyclocreatine and phosphocyclocreatine supplementation is seen as a potential treatment, but the presence of these compounds within commercially available dietary supplements presents the risk of self-medication. High-performance liquid chromatography-mass spectrometry (HPLC-MS) is an excellent technique to assess composition of complex amino acid mixtures. Herein, we have developed a facile HPLC-MS method using a cyano column in hydrophilic interaction liquid chromatography (HILIC) mode with isocratic elution over 4 min to identify the main components of two commercially available dietary supplements. The relative standard deviation (RSD) for retention time and extracted ion integrated area are <0.3% and 4%, respectively, showing excellent reproducibility. Cyclocreatine and phosphocyclocreatine were not detectable within the dietary supplements, even at ppm levels, demonstrating the power and importance of the developed HPLC-MS method in analyzing complex mixtures.

Dodecyl creatine ester-loaded nanoemulsion as a promising therapy for creatine transporter deficiency

Creatine transporter (CrT) deficiency is an X-linked intellectual disability caused by mutations of CrT. Aim: This work focus on the preclinical development of a new therapeutic approach based on a microemulsion (ME) as drug delivery system for dodecyl creatine ester (DCE). Materials & methods: DCE-ME was prepared by titration method. Novel object recognition (NOR) tests were performed before and after DCE-ME treatment on Slc6a8^-/y mice. Results: Intranasal administration with DCE-ME improved NOR performance in Slc6a8^-/y mice. Slc6a8^-/y mice treated with DCE-ME had increased striatal ATP levels mainly in the striatum compared with vehicle-treated Slc6a8^-/y mice which was associated with increased expression of synaptic markers. Conclusion: These results highlight the potential value of DCE-ME as promising therapy for creatine transporter deficiency.

Rescue by 4-phenylbutyrate of several misfolded creatine transporter-1 variants linked to the creatine transporter deficiency syndrome

Diseases arising from misfolding of SLC6 transporters have been reported over recent years, e.g. folding-deficient mutants of the dopamine transporter and of the glycine transporter-2 cause infantile/juvenile Parkinsonism dystonia and hyperekplexia, respectively. Mutations in the coding sequence of the human creatine transporter-1 (hCRT-1/SLC6A8) gene result in a creatine transporter deficiency syndrome, which varies in its clinical manifestation from epilepsy, mental retardation, autism, development delay and motor dysfunction to gastrointestinal symptoms. Some of the mutations in hCRT-1 occur at residues, which are highly conserved across the SLC6 family. Here, we examined 16 clinically relevant hCRT-1 variants to verify the conjecture that they were misfolded and that this folding defect was amenable to correction. Confocal microscopy imaging revealed that the heterologously expressed YFP-tagged mutant CRTs were trapped in the endoplasmic reticulum (ER), co-localised with the ER-resident chaperone calnexin. In contrast, the wild type hCRT-1 reached the plasma membrane. Preincubation of transiently transfected HEK293 cells with the chemical chaperone 4-phenylbutyrate (4-PBA) restored ER export and surface expression of as well as substrate uptake by several folding-deficient CRT-1 mutants. A representative mutant (hCRT-1-P544L) was expressed in rat primary hippocampal neurons to verify pharmacochaperoning in a target cell: 4-PBA promoted the delivery of hCRT-1-P544L to the neurite extensions. These observations show that several folding-deficient hCRT-1 mutants can be rescued. This proof-of-principle justifies the search for additional pharmacochaperones to restore folding of 4PBA-unresponsive hCRT-1 mutants. Finally, 4-PBA is an approved drug in paediatric use: this provides a rationale for translating the current insights into clinical trials. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Mitochondrial and calcium perturbations in rat CNS neurons induce calpain-cleavage of Parkin: Phosphatase inhibition stabilizes pSer65Parkin reducing its calpain-cleavage

Mitochondrial impairment and calcium (Ca⁺⁺) dyshomeostasis are associated with Parkinson's disease (PD). When intracellular ATP levels are lowered, Ca⁺⁺-ATPase pumps are impaired causing cytoplasmic Ca⁺⁺ to be elevated and calpain activation. Little is known about the effect of calpain activation on Parkin integrity. To address this gap, we examined the effects of mitochondrial inhibitors [oligomycin (Oligo), antimycin and rotenone] on endogenous Parkin integrity in rat midbrain and cerebral cortical cultures. All drugs induced calpain-cleavage of Parkin to ~36.9/43.6 kDa fragments. In contrast, treatment with the proinflammatory prostaglandin J2 (PGJ2) and the proteasome inhibitor epoxomicin induced caspase-cleavage of Parkin to fragments of a different size, previously shown by others to be triggered by apoptosis. Calpain-cleaved Parkin was enriched in neuronal mitochondrial fractions. Pre-treatment with the phosphatase inhibitor okadaic acid prior to Oligo-treatment, stabilized full-length Parkin phosphorylated at Ser⁶⁵, and reduced calpain-cleavage of Parkin. Treatment with the Ca⁺⁺ ionophore A23187, which facilitates Ca⁺⁺ transport across the plasma membrane, mimicked the effect of Oligo by inducing calpain-cleavage of Parkin. Removing extracellular Ca⁺⁺ from the media prevented oligomycin- and ionophore-induced calpain-cleavage of Parkin. Computational analysis predicted that calpain-cleavage of Parkin liberates its UbL domain. The phosphagen cyclocreatine moderately mitigated Parkin cleavage by calpain. Moreover, the pituitary adenylate cyclase activating peptide (PACAP27), which stimulates cAMP production, prevented caspase but not calpain-cleavage of Parkin. Overall, our data support a link between Parkin phosphorylation and its cleavage by calpain. This mechanism reflects the impact of mitochondrial impairment and Ca⁺⁺-dyshomeostasis on Parkin integrity and could influence PD pathogenesis.

A Nervous System-Specific Model of Creatine Transporter Deficiency Recapitulates the Cognitive Endophenotype of the Disease: a Longitudinal Study

Mutations in creatine (Cr) transporter (CrT) gene lead to cerebral creatine deficiency syndrome-1 (CTD), an orphan neurodevelopmental disorder presenting with brain Cr deficiency, intellectual disability, seizures, movement and autistic-like behavioral disturbances, language and speech impairment. We have recently generated a murine model of CTD obtained by ubiquitous deletion of 5-7 exons in the CrT gene. These mice showed a marked Cr depletion, associated to early and progressive cognitive impairment, and autistic-like defects, thus resembling the key features of human CTD. Given the importance of extraneural dysfunctions in neurodevelopmental disorders, here we analyzed the specific role of neural Cr in the CTD phenotype. We induced the conditional deletion of Slc6a8 gene in neuronal and glial cells by crossing CrT floxed mice with the Nestin::Cre recombinase Tg (Nes-cre) 1Kln mouse. We report that nervous system-specific Cr depletion leads to a progressive cognitive regression starting in the adult age. No autistic-like features, including repetitive and stereotyped movements, routines and rituals, are present in this model. These results indicate that Cr depletion in the nervous system is a pivotal cause of the CTD pathological phenotype, in particular with regard to the cognitive domain, but extraneural actors also play a role.

Oligodendrocyte Bioenergetics in Health and Disease

The human brain weighs approximately 2% of the body; however, it consumes about 20% of a person’s total energy intake. Cellular bioenergetics in the central nervous system involves a delicate balance between biochemical processes engaged in energy conversion and those responsible for respiration. Neurons have high energy demands, which rely on metabolic coupling with glia, such as with oligodendrocytes and astrocytes. It has been well established that astrocytes recycle and transport glutamine to neurons to make the essential neurotransmitters, glutamate and GABA, as well as shuttle lactate to support energy synthesis in neurons. However, the metabolic role of oligodendrocytes in the central nervous system is less clear. In this review, we discuss the energetic demands of oligodendrocytes in their survival and maturation, the impact of altered oligodendrocyte energetics on disease pathology, and the role of energetic metabolites, taurine, creatine, N-acetylaspartate, and biotin, in regulating oligodendrocyte function.

A Mouse Model of Creatine Transporter Deficiency Reveals Impaired Motor Function and Muscle Energy Metabolism

Creatine serves as fast energy buffer in organs of high-energy demand such as brain and skeletal muscle. L-Arginine:glycine amidinotransferase (AGAT) and guanidinoacetate N-methyltransferase are responsible for endogenous creatine synthesis. Subsequent uptake into target organs like skeletal muscle, heart and brain is mediated by the creatine transporter (CT1, SLC6A8). Creatine deficiency syndromes are caused by defects of endogenous creatine synthesis or transport and are mainly characterized by intellectual disability, behavioral abnormalities, poorly developed muscle mass, and in some cases also muscle weakness. CT1-deficiency is estimated to be among the most common causes of X-linked intellectual disability and therefore the brain phenotype was the main focus of recent research. Unfortunately, very limited data concerning muscle creatine levels and functions are available from patients with CT1 deficiency. Furthermore, different CT1-deficient mouse models yielded conflicting results and detailed analyses of their muscular phenotype are lacking. Here, we report the generation of a novel CT1-deficient mouse model and characterized the effects of creatine depletion in skeletal muscle. HPLC-analysis showed strongly reduced total creatine levels in skeletal muscle and heart. MR-spectroscopy revealed an almost complete absence of phosphocreatine in skeletal muscle. Increased AGAT expression in skeletal muscle was not sufficient to compensate for insufficient creatine transport. CT1-deficient mice displayed profound impairment of skeletal muscle function and morphology (i.e., reduced strength, reduced endurance, and muscle atrophy). Furthermore, severely altered energy homeostasis was evident on magnetic resonance spectroscopy. Strongly reduced phosphocreatine resulted in decreased ATP/Pi levels despite an increased inorganic phosphate to ATP flux. Concerning glucose metabolism, we show increased glucose transporter type 4 expression in muscle and improved glucose clearance in CT1-deficient mice. These metabolic changes were associated with activation of AMP-activated protein kinase – a central regulator of energy homeostasis. In summary, creatine transporter deficiency resulted in a severe muscle weakness and atrophy despite different compensatory mechanisms.

Cognitive deficits and increases in creatine precursors in a brain-specific knockout of the creatine transporter gene Slc6a8

Creatine transporter (CrT; SLC6A8) deficiency (CTD) is an X-linked disorder characterized by severe cognitive deficits, impairments in language and an absence of brain creatine (Cr). In a previous study, we generated floxed Slc6a8 (Slc6a8 ^flox ) mice to create ubiquitous Slc6a8 knockout (Slc6a8^-/y ) mice. Slc6a8^-/y mice lacked whole body Cr and exhibited cognitive deficits. While Slc6a8^-/y mice have a similar biochemical phenotype to CTD patients, they also showed a reduction in size and reductions in swim speed that may have contributed to the observed deficits. To address this, we created brain-specific Slc6a8 knockout (bKO) mice by crossing Slc6a8^flox mice with Nestin-cre mice. bKO mice had reduced cerebral Cr levels while maintaining normal Cr levels in peripheral tissue. Interestingly, brain concentrations of the Cr synthesis precursor guanidinoacetic acid were increased in bKO mice. bKO mice had longer latencies and path lengths in the Morris water maze, without reductions in swim speed. In accordance with data from Slc6a8 ^-/y mice, bKO mice showed deficits in novel object recognition as well as contextual and cued fear conditioning. bKO mice were also hyperactive, in contrast with data from the Slc6a8 ^-/y mice. The results show that the loss of cerebral Cr is responsible for the learning and memory deficits seen in ubiquitous Slc6a8^-/y mice.

TREM2 Maintains Microglial Metabolic Fitness in Alzheimer's Disease

Elevated risk of developing Alzheimer's disease (AD) is associated with hypomorphic variants of TREM2, a surface receptor required for microglial responses to neurodegeneration, including proliferation, survival, clustering, and phagocytosis. How TREM2 promotes such diverse responses is unknown. Here, we find that microglia in AD patients carrying TREM2 risk variants and TREM2-deficient mice with AD-like pathology have abundant autophagic vesicles, as do TREM2-deficient macrophages under growth-factor limitation or endoplasmic reticulum (ER) stress. Combined metabolomics and RNA sequencing (RNA-seq) linked this anomalous autophagy to defective mammalian target of rapamycin (mTOR) signaling, which affects ATP levels and biosynthetic pathways. Metabolic derailment and autophagy were offset in vitro through Dectin-1, a receptor that elicits TREM2-like intracellular signals, and cyclocreatine, a creatine analog that can supply ATP. Dietary cyclocreatine tempered autophagy, restored microglial clustering around plaques, and decreased plaque-adjacent neuronal dystrophy in TREM2-deficient mice with amyloid-β pathology. Thus, TREM2 enables microglial responses during AD by sustaining cellular energetic and biosynthetic metabolism.

Quantification of cyclocreatine in mouse and rat plasma using hydrophilic-interaction ultra-performance liquid chromatography-tandem mass spectrometry

An accurate, rapid and selective method was developed to quantify cyclocreatine in mouse and rat plasma using hydrophilic interaction (HILIC) ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The plasma samples were prepared by protein precipitation with acetonitrile:methanol (70:30). Chromatographic separation was performed on a HILIC BEH amide column (2.1mm×50mm, 1.7μm) with a 3min gradient elution at a flow rate of 0.5mL/min. For mass spectrometric detection, selected reaction monitoring (SRM) was used; the SRM transitions were m/z 144→98 and m/z 144→56 for cyclocreatine and m/z 148→102 for the internal standard (D4-cyclocreatine) in the positive ionization mode. No endogenous components interfered with the analysis of cyclocreatine and the internal standard in mouse and rat plasma. Plasma calibration curves were constructed in the range of 0.01-25μM. The correlation coefficient of the calibration curves was greater than 0.99. The mean intraday assay accuracy for all quality control (QC) replicates was between 93 and 105%. The mean intraday assay precision (CV%) was 1.9-11% for all QC levels. The HILIC-UPLC-MS/MS method was successfully applied in pharmacokinetic (PK) studies of cyclocreatine in mice and rats for the first time. After a single 30mg/kg oral administration in mice and rats, the AUC_0-∞ (area under the curve) was 84.1μgh/mL and 91.7±18.0μgh/mL, respectively.

Abnormal N-Glycosylation of a Novel Missense Creatine Transporter Mutant, G561R, Associated with Cerebral Creatine Deficiency Syndromes Alters Transporter Activity and Localization

Cerebral creatine deficiency syndromes (CCDSs) are caused by loss-of-function mutations in creatine transporter (CRT, SLC6A8), which transports creatine at the blood-brain barrier and into neurons of the central nervous system (CNS). This results in low cerebral creatine levels, and patients exhibit mental retardation, poor language skills and epilepsy. We identified a novel human CRT gene missense mutation (c.1681 G>C, G561R) in Japanese CCDSs patients. The purpose of the present study was to evaluate the reduction of creatine transport in G561R-mutant CRT-expressing 293 cells, and to clarify the mechanism of its functional attenuation. G561R-mutant CRT exhibited greatly reduced creatine transport activity compared to wild-type CRT (WT-CRT) when expressed in 293 cells. Also, the mutant protein is localized mainly in intracellular membrane fraction, while WT-CRT is localized in plasma membrane. Western blot analysis revealed a 68 kDa band of WT-CRT protein in plasma membrane fraction, while G561R-mutant CRT protein predominantly showed bands at 55, 110 and 165 kDa in crude membrane fraction. The bands of both WT-CRT and G561R-mutant CRT were shifted to 50 kDa by N-glycosidase treatment. Our results suggest that the functional impairment of G561R-mutant CRT was probably caused by incomplete N-linked glycosylation due to misfolding during protein maturation, leading to oligomer formation and changes of cellular localization.

Fifteen-year follow-up of Italian families affected by arginine glycine amidinotransferase deficiency

BACKGROUND

Arginine:glycine amidinotransferase deficiency (AGAT-d) is a very rare inborn error of creatine synthesis mainly characterized by absence of brain Creatine (Cr) peak, intellectual disability, severe language impairment and behavioural disorder and susceptible to supplementary Cr treatment per os. Serial examinations by magnetic resonance spectroscopy are required to evaluate Cr recovery in brain during treatment of high doses of Cr per os, which have been proved beneficial and effective in treating main clinical symptoms. A long term study with detailed reports on clinical, neurochemical and neuropsychological outcomes of the first Italian patients affected by AGAT-d here reported can represent a landmark in management of this disorder thus enhancing medical knowledge and clinical practice.

RESULTS

We have evaluated the long term effects of Cr supplementation management in four Italian patients affected by AGAT-d, correlating specific treatments with serial clinical, biochemical and magnetic resonance spectroscopy examinations as well as the neuropsychological outcome by standardized developmental scales. Consecutive MRS examinations have confirmed that Cr depletion in AGAT-d patients is reversible under Cr supplementation. Cr treatment is considered safe and well tolerated but side effects, including weight gain and kidney stones, have been reported.

CONCLUSIONS

Early treatment prevents adverse developmental outcome, while patients diagnosed and treated at an older age showed partial but significant cognitive recovery with clear improvements in adaptive functioning.

Laboratory diagnosis of creatine deficiency syndromes: a technical standard and guideline of the American College of Medical Genetics and Genomics

Disclaimer: These ACMG Standards and Guidelines are intended as an educational resource for clinical laboratory geneticists to help them provide quality clinical laboratory genetic services. Adherence to these standards and guidelines is voluntary and does not necessarily assure a successful medical outcome. These Standards and Guidelines should not be considered inclusive of all proper procedures and tests or exclusive of others that are reasonably directed to obtaining the same results. In determining the propriety of any specific procedure or test, clinical laboratory geneticists should apply their professional judgment to the specific circumstances presented by the patient or specimen. Clinical laboratory geneticists are encouraged to document in the patient's record the rationale for the use of a particular procedure or test, whether or not it is in conformance with these Standards and Guidelines. They also are advised to take notice of the date any particular guideline was adopted, and to consider other relevant medical and scientific information that becomes available after that date. It also would be prudent to consider whether intellectual property interests may restrict the performance of certain tests and other procedures.Cerebral creatine deficiency syndromes are neurometabolic conditions characterized by intellectual disability, seizures, speech delay, and behavioral abnormalities. Several laboratory methods are available for preliminary and confirmatory diagnosis of these conditions, including measurement of creatine and related metabolites in biofluids using liquid chromatography-tandem mass spectrometry or gas chromatography-mass spectrometry, enzyme activity assays in cultured cells, and DNA sequence analysis. These guidelines are intended to standardize these procedures to help optimize the diagnosis of creatine deficiency syndromes. While biochemical methods are emphasized, considerations for confirmatory molecular testing are also discussed, along with variables that influence test results and interpretation.Genet Med 19 2, 256-263.

Creatine in the central nervous system: From magnetic resonance spectroscopy to creatine deficiencies

Creatine (Cr) is an important organic compound acting as intracellular high-energy phosphate shuttle and in energy storage. While located in most cells where it plays its main roles in energy metabolism and cytoprotection, Cr is highly concentrated in muscle and brain tissues, in which Cr also appears to act in osmoregulation and neurotransmission. This review discusses the basis of Cr metabolism, synthesis and transport within brain cells. The importance of Cr in brain function and the consequences of its impaired metabolism in primary and secondary Cr deficiencies are also discussed. Cr and phosphocreatine (PCr) in living systems can be well characterized using in vivo magnetic resonance spectroscopy (MRS). This review describes how ¹H MRS allows the measurement of Cr and PCr, and how ³¹P MRS makes it possible to estimate the creatine kinase (CK) rate constant and so detect dynamic changes in the Cr/PCr/CK system. Absolute quantification by MRS using creatine as internal reference is also debated. The use of in vivo MRS to study brain Cr in a non-invasive way is presented, as well as its use in clinical and preclinical studies, including diagnosis and treatment follow-up in patients.

Creatine Defects and Central Nervous System

Creatine deficiency syndromes are a group of disorders of creatine (Cr) synthesis and transport characterized by intellectual disability, language delay, epilepsy, autism spectrum disorder, and movement disorders secondary to decrease of Cr concentration in the brain. Synthesis defects are treatable, therefore an early diagnosis and treatment is essential. The aim of this article is to review the Cr metabolism and function in the central nervous system. We describe the optimal diagnostic protocol in Cr deficiency syndromes based on biochemical methods, neuroradiological (1H-MRS), and molecular analysis. Finally, a treatment approach of the different Cr deficiency syndromes is described.

Creatine Transporter Deficiency: Screening of Males with Neurodevelopmental Disorders and Neurocognitive Characterization of a Case

OBJECTIVE

Creatine transporter deficiency (CTD) is an X-linked, neurometabolic disorder associated with intellectual disability that is characterized by brain creatine (Cr) deficiency and caused by mutations in SLC6A8, the Cr transporter 1 protein gene. CTD is identified by elevated urine creatine/creatinine (Cr/Crn) ratio or reduced Cr peak on brain magnetic resonance spectroscopy; the diagnosis is confirmed by decreased Cr uptake in cultured fibroblasts, and/or identification of a mutation in the SLC6A8 gene. Prevalence studies suggest this disorder may be underdiagnosed. We sought to identify cases from a well-characterized cohort of children diagnosed with neurodevelopmental disorders.

METHOD

Urine screening for CTD was performed on a cohort of 46 males with autism spectrum disorder (ASD) and 9 males with a history of non-ASD developmental delay (DD) classified with intellectual disability.

RESULTS

We identified 1 patient with CTD in the cohort based on abnormal urine Cr/Crn, and confirmed the diagnosis by the identification of a novel frameshift mutation in the SLC6A8 gene. This patient presented without ASD but with intellectual disability, and was characterized by a nonspecific phenotype of early language delay and DD that persisted into moderate-to-severe intellectual disability, consistent with previous descriptions of CTD.

CONCLUSION

Identification of patients with CTD is possible by measuring urine Cr and Crn levels and the current case adds to the growing literature of neurocognitive deficits associated with the disorder that affect cognition, language and behavior in childhood.