CCDS Research Paper Repository & Summaries

ACD Summary: Sun et al. screened for the presence of a CCDS in 3,586 patients who exhibited a developmental delay. Screening involved measuring creatine and guanidinoacetate (GAA) levels in blood, creatine:creatinine levels in urine, and creatine signals in the brain. In total, 14 patients had a confirmed CCDS (6 with GAMT deficiency, 8 with CTD). There were also brain structural differences in some of the patients. In patients with GAMT deficiency, treatment involved a low-protein diet and supplementation with creatine and ornithine. After 2 to 3 weeks of treatment, creatine and GAA levels in blood returned to a normal range; after 6 to 9 months, their brain’s creatine signals increased dramatically. In 3 patients with GAMT deficiency, epilepsy was resolved after 3 to 6 months of treatment. Overall, treatment was associated with a general improvement in the children with GAMT deficiency. For patients with CTD, there was minimal (if any) improvement given supplementation treatment with creatine, glycine, and/or arginine. In conclusion, the authors recommended that patients with a developmental delay, feeding or growth difficulties, and epilepsy should be screened for CCDS.

Link to free article on PubMed: Fourteen cases of cerebral creatine deficiency syndrome in children: a cohort study in China

ACD Summary: Tise et al. described a 20-month-old boy with CTD, diagnosed via urine analyses, genetic testing (which revealed a novel SLC6A8 variant), and magnetic resonance spectroscopy (a noninvasive technique that allows researchers to study the levels of chemical components of the brain’s tissues). The patient began a program of creatine, arginine, and glycine supplementation. At 40 months old (~1 year after starting the supplementation program), the patient showed improved development and weight gain; the authors described the patient as “active and playful… currently no longer requiring or receiving physical or occupational therapy”. Despite limited verbal communication, the patient is able to sign and express his wants and needs, also showing evidence of language comprehension. The authors suggested that this novel SLC6A8 variant may be more receptive to therapy. The authors further emphasized the need for enhanced newborn screening programs and suggest that CTD should be considered in patients showing developmental delays.

Link to free article: Creatine Transporter Deficiency Presenting as Failure to Thrive: A Case Report of a Novel SLC6A8 Variant Causing a Treatable but Likely Underdiagnosed Genetic Disorder

Link to PubMed: PubMed

ACD Summary: Rosko et al. used mouse models of GAMT deficiency to better understand creatine production in the central nervous system (which refers to the brain and spinal cord). Animal models are ultimately helpful for humans because researchers can perform experiments with animals that they would not be ethically allowed to do with people. These authors showed that the main source of the brain’s ability to make creatine is within types of brain cells called oligodendrocytes (pronounced “uh-leh-go-DEN-dro-sites”). These types of cells are responsible for building structures that wrap around nerve cells so that different parts of the brain can work together more efficiently; these structures, called myelin (pronounced “MY-eh-lin”), prevent brain activity from getting lost, like how coats wrapped around our bodies prevent our body heat from escaping. When GAMT was not expressed in these mouse brains, there was a delay in the brain’s ability to create myelin. However, dietary supplementation of creatine did help restart this process of creating myelin. In conclusion, these authors suggest that oligodendrocyte function may be an important mechanism in CCDS.

Link to free article: Cerebral Creatine Deficiency Affects the Timing of Oligodendrocyte Myelination

Link to PubMed: PubMed

ACD Summary: Mabondzo et al. used a mouse model of CTD to better understand (1) which proteins are related to CTD and (2) whether treatment with dodecyl creatine ester (DCE changes the amount of those proteins in the brain. (In March 2021, DCE was suggested to the FDA as a possible treatment for CTD, but it has not yet been approved by the FDA for this purpose.) DCE was given through the nose to a group of mice that did not express the SLC6A8 gene (which needs to be expressed for creatine transport to happen); mice that do not express (or have mutations in) the SLC6A8 gene are called “SLC6A8 knockout” mice. SLC6A8 knockout mice given DCE showed significant improvements in cognition, similar to the mice that did express SLC6A8. In the brains of these same mice, the authors identified 14 different proteins that were altered by SLC6A8 expression and by being given DCE. (Interestingly, 13 of these 14 proteins have been shown to be related to other disorders characterized by intellectual disability, such as autism). For example, in the SLC6A8 knockout mice, there was a large amount of the KIF1A protein, and the mice that showed a greater amount of this protein showed lower cognition. These results suggest that K1F1A (as well as another protein, PLCB1) may be an important mechanism related to CTD and that treatment with DCE may help restore cognitive deficits of CTD.

Link to free article: Dodecyl creatine ester improves cognitive function and identifies key protein drivers including KIF1A and PLCB1 in a mouse model of creatine transporter deficiency

Link to PubMed: PubMed

ACD Summary: Mustafa et al. analyzed levels of guanidinoacetic acid (GAA), creatine, and creatinine in urine in neurotypical children ages 0- to 15-years-old in Egypt. Specifically, the authors looked at the ratio of GAA to creatinine (GAA:creatinine) and the ratio of creatine to creatinine (creatine:creatinine). Typically, GAA:creatinine levels in urine are low in patients with AGAT deficiency, elevated in patients with GAMT deficiency, and normal in patients with CTD. In contrast, creatine:creatinine levels in urine are normal in patients with AGAT or GAMT deficiency and may be elevated in patients with CTD. These ratios are used by physicians to help with diagnosing CCDS. Accordingly, Mustafa et al. wanted to determine how these ratio levels change with age, so they could begin to establish an age-specific reference level for these diagnostic criteria. The authors collected data on 160 children but removed 10 children from analysis because of positive screening for a CCDS. For the remaining 150 neurotypical children, there was a steady decline in GAA:creatinine and creatine:creatinine levels with age, with there being a large difference in these average levels when comparing children less than 4 years old to children between 4 and 15 years old. Accordingly, the reference criteria for diagnosing CCDS may change with age at time of diagnosis, which may be contributing to the underdiagnosis of CCDS.

Link to free article: Age Adjusted Reference Interval for Creatine Deficiency Syndrome Biomarkers by Gas chromatography-Mass Spectrometry

Scientific Abstract: 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.

Link to free article: Current and potential new treatment strategies for creatine deficiency syndromes

Link to PubMed: PubMed

ACD Summary: Shen et al. reported on the genetic analyses of 7 patients with CCDS. The children were between the ages of 2 and 8 years old and all had some form of intellectual disability or motor developmental delay. All patients had low creatine levels and genetic variations on either the SLC6A8 gene or GAMT gene. The authors noted that the definite type of CCDS for these 7 patients remains to be determined. With respect to the genetic testing, the authors identified 12 genetic variations in the 7 patients, with 6 of the variations being novel. The authors concluded that their data expands the spectrum of possible genetic variations underlying CCDS, which may help with future genetic counseling for families.

Link to free article: Identification of novel variations in SLC6A8 and GAMT genes causing cerebral creatine deficiency syndrome

Link to PubMed: PubMed

Scientific Abstract: Introduction: Cerebral creatine deficiency syndromes (CCDS) are a group of potentially treatable neurometabolic disorders. The clinical, genetic profile and follow up outcome of Indian CCDS patients is presented. Materials and methods: This was a retrospective cohort of CCDS patients seen over six-years. Diagnosis was based either on low creatine peak on proton magnetic resonance spectroscopy (MRS) and/or genetic evaluation. Results: Thirteen patients were eligible [8 creatine transporter deficiency (CTD), 4 guanidinoacetate methyltransferase (GAMT) deficiency and 1 could not be classified]. The mean (±SD) age at diagnosis was 7.2(±5.0) years. Clinical manifestations included intellectual disability (ID) with significant expressive speech delay in all. Most had significant behavior issues (8/13) and/or autism (8/13). All had history of convulsive seizures (11/13 had epilepsy; 2 patients only had febrile seizures) and 2/13 had movement disorder. Constipation was the commonest non-neurological manifestation (5/13 patients). Cranial MRI was normal in all CTD patients but showed globus pallidus hyperintensity in all four with GAMT deficiency. MRS performed in 11/13 patients, revealed abnormally low creatine peak. A causative genetic variant (novel mutation in nine) was identified in 12 patients. Three GAMT deficiency and one CTD patient reported neurodevelopmental improvement and good seizure control after creatine supplementation. Conclusion: Intellectual disability, disproportionate speech delay, autism, and epilepsy, were common in our CCDS patients. A normal structural neuroimaging with easily controlled febrile and/or afebrile seizures differentiated CTD from GAMT deficiency patients who had abnormal neuroimaging and often difficult to control epilepsy and movement disorder.

Link to abstract on PubMed: Cerebral creatine deficiency disorders – A clinical, genetic and follow up study from India

ACD Summary: Magnetic resonance spectroscopy (MRS) of the brain is a noninvasive technique that allows researchers to study the levels of chemical components of the brain’s tissues. It was actually the technique used to discover creatine deficiency in the brain. Here, for the first time with respect to CTD), Li et al. used a type of MRS called phosphorus MRS, which provides a better glimpse into energy metabolism in the brain. Even though levels of creatine and phosphocreatine (which are reduced in patients with CTD) are critical for energy metabolism in the brain, these authors did not find evidence to suggest energy metabolism differed in patients with CTD. Because the reduced phosphocreatine may reflect changes in how mitochondria are functioning (i.e., mitochondria are responsible for energy production), these authors suggest that development and application of techniques to enhance the brain’s mitochondrial function may benefit patients with CTD.

Link to free article on PubMed: Oxidative Phosphorylation in Creatine Transporter Deficiency

ACD Summary: This report describes treatment of a 5-year-old boy (diagnosed with CTD at 3.25 years old) with high-dose creatine supplementation, beginning at 4.5 years of age. The dose of creatine supplementation increased over the course of 6 months, up to 1200 mg per kg per day. While there was no significant improvement in cognition function or brain creatine levels, there was improvement in muscle mass, strength, and coordination.

Link to free article: Treatment efficacy of high-dose creatine supplementation in a child with creatine transporter (SLC6A8) deficiency

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ACD Summary: Levin et al. studied the cardiac profile of 18 boys with CTD. To do so, they analyzed the QTc interval, which reflects heart rhythm. Long QTc intervals (and long QT syndrome) may lead to heart conditions, like arrhythmia. One common designation for what counts as a long QTc interval is 450 milliseconds. Here, of the 18 patients with CTD, 7 of them had QTc intervals greater than 470 milliseconds (i.e., prolonged QTc intervals). Similar results were observed in an animal model of CTD. These authors recommend that newly diagnosed CTD patients have an electrocardiogram and an echocardiogram to measure heart rhythm (and to avoid medications that may lead to prolonged QTc intervals).

Link to free article: X-linked creatine transporter deficiency results in prolonged QTc and increased sudden death risk in humans and disease model

Link to PubMed: PubMed

Scientific Abstract: Creatine is pivotal in energy metabolism of the brain. In primary creatine deficiency syndromes, creatine is missing from the brain. Two of them (AGAT and GAMT deficiency) are due to impaired creatine synthesis, and can be treated by creatine supplementation. By contrast, creatine transporter deficiency cannot be treated by such supplementation, since creatine crossing of biological membranes (plasma membrane and blood-brain barrier) is dependent on its transporter. This problem might be overcome by modifying the creatine molecule to allow it to cross biological membranes independently of its transporter. Thus, we designed and synthesized di-acetyl creatine ethyl ester (DAC), a compound that should cross biological membranes independently of the transporter due to its very high lipophilicity. We investigated its ability to increase intracellular creatine levels even after block of creatine transporter, and to counter cell damage induced by transporter block. In our experiments after block of the creatine transporter, DAC was able both to prevent electrophysiological failure and to increase intracellular creatine. Interestingly, it did so in micromolar concentrations, at variance with all the other creatine derivatives that we know of.

Link to abstract on PubMed: Di-acetyl creatine ethyl ester, a new creatine derivative for the possible treatment of creatine transporter deficiency

ACD Summary: Bruun et al. surveyed the physicians of 17 patients with CTD (from 16 unrelated families) on the severity of each patient’s (1) global developmental delay/intellectual disability, (2) seizures, and (3) behavioral disorders. All patients showed global developmental delays/intellectual disabilities; 8 had seizures; 9 had behavioral disorders (with autism spectrum disorder and ADHD being the most common). The authors derived an overall severity score phenotype (which refers to a set of observable traits) by collapsing the 3 individual severity scores: 7 patients had a “mild” phenotype; 6 patients had a “moderate” phenotype; 4 patients had a “severe” phenotype. There was no correlation between overall severity score and age of diagnosis or between overall severity score and urine creatine:creatinine levels. The authors suggested that while combined creatine, arginine, and glycine therapy might have stopped the progression of the disease in males and might have improved the phenotype in females, the overall severity score did not considerably change during treatment.

Link to abstract on PubMed: Treatment outcome of creatine transporter deficiency: international retrospective cohort study

Scientific Abstract: Creatine transporter is currently the focus of renewed interest with emerging roles in brain neurotransmission and physiology, and the bioenergetics of cancer metastases. We here report on amendments of a standard creatine uptake assay which might help clinical chemistry laboratories to extend their current range of measurements of creatine and metabolites in body fluids to functional enzyme explorations. In this respect, short incubation times and the use of a stable-isotope-labeled substrate (D3-creatine) preceded by a creatine wash-out step from cultured fibroblast cells by removal of fetal bovine serum (rich in creatine) from the incubation medium are recommended. Together, these measures decreased, by a first order of magnitude, creatine concentrations in the incubation medium at the start of creatine-uptake studies and allowed to functionally discriminate between 4 hemizygous male and 4 heterozygous female patients with X-linked SLC6A8 deficiency, and between this cohort of eight patients and controls. The functional assay corroborated genetic diagnosis of SLC6A8 deficiency. Gene anomalies in our small cohort included splicing site (c.912G > A [p.Ile260_Gln304del], c.778-2A > G and c.1495 + 2 T > G), substitution (c.407C > T) [p.Ala136Val] and deletion (c.635_636delAG [p.Glu212Valfs*84] and c.1324delC [p.Gln442Lysfs*21]) variants with reduced creatine transporter function validating their pathogenicity, including that of a previously unreported c.1324delC variant. The present assay adaptations provide an easy, reliable and discriminative manner for exploring creatine transporter activity and disease variations. It might apply to drug testing or other evaluations in the genetic and metabolic horizons covered by the emerging functions of creatine and its transporter, in a way, however, requiring and completed by additional studies on female patients and blood-brain barrier permeability properties of selected compounds. As a whole, the proposed assay of creatine transporter positively adds to currently existing measurements of this transporter activity, and determining on a large scale the extent of its exact suitability to detect female patients should condition in the future its transfer in clinical practice.

Link to abstract on PubMed: Functional assessment of creatine transporter in control and X-linked SLC6A8-deficient fibroblasts

ACD Summary: Sharer et al. reported on recommended laboratory methods to diagnose CCDSs. While the authors noted that magnetic resonance spectroscopy (MRS; a noninvasive technique that allows researchers to study the levels of chemical components of the brain’s tissues) can be used, that approach is expensive, requires children to be sedated, and does not specify which CCDS the patient may have. Thus, the authors recommended measuring creatine, guanidinoacetate, and creatinine levels in urine, plasma, or dried blood spots for initial diagnosis. For AGAT or GAMT deficiency, plasma and/or urine can be used; measurements of creatine and guanidinoacetate levels in plasma are specifically recommended. For CTD, urine is the best option. For some female patients with CTD, genetic analyses are also recommended. In general, the authors recommend that the samples be collected when the patients have fasted. The authors also provided a general reference table for relative levels of guanidinoacetate, creatine, and creatine:creatinine ratios in plasma, urine, and cerebrospinal fluid in patients with AGAT deficiency, GAMT deficiency, and CTD. (Cerebrospinal fluid is a fluid that flows through the brain and spinal cord.) The authors did note that creatinine (by itself) has limited value as a diagnostic marker for CCDSs. Additionally, the authors  provided background information on (1) the sources and function of creatine, (2) biochemical/molecular characteristics and clinical descriptions of AGAT, GAMT, and CTD, (3) prevalence and inheritance of CCDSs, and (4) interpretation of test results.

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

Link to PubMed: PubMed

Scientific Abstract: Background: Creatine transporter deficiency (CTD) is an X-linked inborn error of creatine metabolism characterized by reduced intra-cerebral creatine, developmental delay/intellectual disability, (ID), behavioral disturbance, seizures, and hypotonia in individuals harboring mutations in the SLC6A8 gene. Treatment for CTD includes supplementation with creatine, either alone or in combination with creatine precursors (arginine or glycine). Unlike other disorders of creatine metabolism, the efficacy of its treatment remains controversial. Methods: We present our systematic literature review (2001-2013) comprising 7 publications (case series/reports), collectively describing 25 patients who met the inclusion criteria, and 3 additional cases treated at our institution. Definitions were established and extracted data analyzed for cognitive ability, psychiatric and behavioral disturbances, epilepsy, and cerebral proton magnetic resonance spectroscopy measurements at pre- and post-treatment. Results: Treatment regimens varied among the 28 cases: 2 patients received creatine-monohydrate supplementation; 7 patients received L-arginine; 2 patients received creatine-monohydrate and L-arginine; and 17 patients received a combination of creatine-monohydrate, L-arginine and glycine. Median treatment duration was 34.6 months (range 3 months-5 years). Level of evidence was IV. A total of 10 patients (36%) demonstrated response to treatment, manifested by either an increase in cerebral creatine, or improved clinical parameters. Seven of the 28 patients had quantified pre- and post-treatment creatine, and it was significantly increased post-treatment. All of the patients with increased cerebral creatine also experienced clinical improvement. In addition, the majority of patients with clinical improvement had detectable cerebral creatine prior to treatment. 90% of the patients who improved were initiated on treatment before nine years of age. Conclusions: Acknowledging the limitations of this systematic review, we conclude that a proportion of CTD patients show amenability to treatment-particularly milder cases with residual brain creatine, and therefore probable residual protein function. We propose systematic screening for CTD in patients with ID, to allow early initiation of treatment, which currently comprises oral creatine, arginine and/or glycine supplementation. Standardized monitoring for safety and evaluation of treatment effects are required in all patients. This study provides effectiveness on currently available treatment, which can be used to discern effectiveness of future interventions (e.g. cyclocreatine).

Link to abstract on PubMed: Treatment of X-linked creatine transporter (SLC6A8) deficiency: systematic review of the literature and three new cases

Scientific Abstract: Background: Creatine transporter deficiency is a monogenic cause of X-linked intellectual disability. Since its first description in 2001 several case reports have been published but an overview of phenotype, genotype and phenotype–genotype correlation has been lacking. Methods: We performed a retrospective study of clinical, biochemical and molecular genetic data of 101 males with X-linked creatine transporter deficiency from 85 families with a pathogenic mutation in the creatine transporter gene (SLC6A8). Results and conclusions: Most patients developed moderate to severe intellectual disability; mild intellectual disability was rare in adult patients. Speech language development was especially delayed but almost a third of the patients were able to speak in sentences. Besides behavioural problems and seizures, mild to moderate motor dysfunction, including extrapyramidal movement abnormalities, and gastrointestinal problems were frequent clinical features. Urinary creatine to creatinine ratio proved to be a reliable screening method besides MR spectroscopy, molecular genetic testing and creatine uptake studies, allowing definition of diagnostic guidelines. A third of patients had a de novo mutation in the SLC6A8 gene. Mothers with an affected son with a de novo mutation should be counselled about a recurrence risk in further pregnancies due to the possibility of low level somatic or germline mosaicism. Missense mutations with residual activity might be associated with a milder phenotype and large deletions extending beyond the 3′ end of the SLC6A8 gene with a more severe phenotype. Evaluation of the biochemical phenotype revealed unexpected high creatine levels in cerebrospinal fluid suggesting that the brain is able to synthesise creatine and that the cerebral creatine deficiency is caused by a defect in the reuptake of creatine within the neurones.

Link to abstract on PubMed: Phenotype and genotype in 101 males with X-linked creatine transporter deficiency

ACD Summary: Fons et al. studied whether dietary supplementation with creatine monohydrate (CM) or creatine ethyl ester (CEE) (both of which can be purchased commerically in powder form) had clinical benefits. First, the researchers showed that CEE crossed the plasma membrane of fibroblasts (which are types of cells that help connect the body’s tissue together). Thus, they explored whether CEE supplementation would have benefits in patients with CTD. However, after 1 year of treatment with CEE supplementation, while there were no adverse effects, there were also no improvements in communication, daily living skills, socialization, or motor skills in these patients; also, there was not an increase in creatine in the brain. Because there were positive results with the fibroblast cells, there is reason for future research exploring mechanisms of cellular transport of CEE, but these authors do not recommend CEE treatment for patients with CTD.

Link to abstract on PubMed: Response to creatine analogs in fibroblasts and patients with creatine transporter deficiency

Scientific Abstract: Mutations in the creatine transporter gene, SLC6A8 (MIM 30036), located in Xq28, have been found in families with X-linked mental retardation (XLMR) as well as in males with idiopathic mental retardation (MR). In order to estimate the frequency of such mutations in the MR population, a screening of 478 males with MR of unknown cause was undertaken. All 13 exons of SLC6A8 were sequenced using genomic DNA. Six novel potentially pathogenic mutations were identified that were not encountered in at least 588 male control chromosomes: two deletions (p.Asn336del, p.Ile347del) and a splice site alteration (c.1016+2C>T) are considered pathogenic based on the nature of the variant. A mutation (p.Arg391Trp) should be considered pathogenic owing to its localization in a highly conserved region. Two other missense variants (p.Lys4Arg, p.Gly26Arg) are not conserved but were not observed in over 300 male control chromosomes. Their pathogenicity is uncertain. A missense variant (p.Val182Met), was classified as a polymorphism based on a normal creatine/creatinine (Cr:Crn) ratio and cerebral creatine signal in proton magnetic resonance spectroscopy (H-MRS) in the patient. Furthermore, we found 14 novel intronic and neutral variants that were not encountered in at least 280 male control chromosomes and should be considered as unclassified variants. Our findings of a minimum of four pathogenic mutations and two potentially pathogenic mutations indicate that about 1% of males with MR of unknown etiology might have a SLC6A8 mutation. Thus, DNA sequence analysis and/or a Cr:Crn urine screen is warranted in any male with MR of unknown cause.

Link to abstract on PubMed: X-linked creatine transporter (SLC6A8) mutations in about 1% of males with mental retardation of unknown etiology

ACD Summary: In this brief yet seminal study, Salomons et al. reported the first case of CTD. The patient had a mild intellectual disability and a severe speech delay. Magnetic resonance spectroscopy (MRS) revealed that there was no creatine in the brain. (MRS is a noninvasive technique that allows researchers to study the levels of chemical components of the brain’s tissues.) While the absence of a creatine signal is also seen in patients with a GAMT deficiency, this patient’s urine and plasma guanidinoacetate levels were normal, and treatment with creatine monohydrate did not increase creatine in the brain. So, GAMT deficiency was ruled out. The authors found that there was a history of a learning disability in the mother and maternal grandmother and a severe intellectual disability in the mother’s brother. The mother’s sister and mother also had a reduced creatine MRS signal. These data led the authors to speculate that the patient had an X-linked chromosome disorder. (All individuals have at least 1 X chromosome, but those with only 1 X chromosome are more likely to be affected by genetic mutations on the X chromosome.) The authors hypothesized that the patient had a mutation on the creatine-transporter gene SLC6A8, which was confirmed through laboratory analyses of cells from the patient.

Link to free article on PubMed: X-Linked Creatine-Transporter Gene (SLC6A8) Defect: A New Creatine-Deficiency Syndrome

Scientific Abstract: We have generated a stable HEK293 cell line expressing high levels of a creatine transporter (CREAT). This cell line (HEK293-CREAT) was used to study the properties of CREAT in terms of the accumulation and release of creatine. HEK293-CREAT cells accumulated high steady state levels of creatine under saturating creatine levels (approx. 25-fold higher intracellular creatine levels than seen in control cells). The accumulation of high levels of creatine affected [3H]creatine uptake by decreasing the Vmax for transport. High intracellular creatine levels were maintained in the absence of extracellular creatine. External creatine stimulated the release of stored creatine by an exchange mechanism dependent on extracellular Na+. These studies have shown that cellular creatine levels can be affected by the amount of creatine transporter in the membrane and exchange through the creatine transporter. These findings highlight the importance of the creatine transporter in the maintenance of intracellular creatine levels.

Link to abstract on PubMed: Creatine accumulation and exchange by HEK293 cells stably expressing high levels of a creatine transporter