Creatine metabolism in patients with urea cycle disorders

Arginase deficiency masked by cerebral palsy and coagulopathy-Three varied presentations of Latin American origin

Arginase deficiency (ARG1-D) is an autosomal recessive inborn error of metabolism that is often misdiagnosed. Classic presentation of ARG1-D includes progressive symptoms of spasticity, delayed development, cognitive impairment, protein avoidance, and seizures. Patients who present atypically may evade diagnosis and require a thoughtful diagnostic workup. Here, we discuss three females of Latin American origin with differing clinical presentations, but who all have the same intronic pathogenic variant in ARG1. Importantly, we found that each case included elevated coagulopathy on laboratory testing and discussed one case in particular with manifestation of bleeding. When diagnosed early, treatment is favorable and can prevent progressive decline. While many states have added ARG1-D to their expanded newborn screening panels, still many states and countries do not screen for ARG1-D, and it can be missed in a healthy newborn. We aim to bring awareness to not only the classic presentation as a necessary consideration for otherwise unexplained spastic diplegia but also to the varied presentations of ARG1-D.

Investigation of molecular details of a bacterial cationic amino acid transporter (GkApcT) during arginine transportation using molecular dynamics simulation and umbrella sampling techniques

CONTEXT

Cationic amino acid transporters (CATs) facilitate arginine transport across membranes and maintain its levels in various tissues and organs, but their overexpression has been associated with severe cancers. A recent study identified the alternating access mechanism and critical residues involved in arginine transportation in a cationic amino acid transporter from Geobacillus kaustophilus (GkApcT). Here, we used molecular dynamics (MD) simulation methods to investigate the transportation mechanism of arginine (Arg) through GkApcT. The results revealed that arginine strongly interacts with specific binding site residues (Thr43, Asp111, Glu115, Lys191, Phe231, Ile234, and Asp237). Based on the umbrella sampling, the main driving force for arginine transport is the polar interactions of the arginine with channel-lining residues. An in-depth description of the dissociation mechanism and binding energy analysis brings valuable insight into the interactions between arginine and transporter residues, facilitating the design of effective CAT inhibitors in cancer cells.

METHODS

The membrane-protein system was constructed by uploading the prokaryotic CAT (PDB ID: 6F34) to the CHARMM-GUI web server. Molecular dynamics simulations were done using the GROMACS package, version 5.1.4, with the CHARMM36 force field and TIP3P water model. The MM-PBSA approach was performed for determining the arginine binding free energy. Furthermore, the hotspot residues were identified through per-residue decomposition analysis. The characteristics of the channel such as bottleneck radius and channel length were analyzed using the CaverWeb 1.1 web server. The proton wire inside the transporter was investigated based on the classic Grotthuss mechanism. We also investigated the atomistic details of arginine transportation using the path-based free energy umbrella sampling technique (US).

Guanidinoacetate (GAA) is a potent GABAA receptor GABA mimetic: Implications for neurological disease pathology

Impairment of excretion and enzymatic processing of nitrogen, for example, because of liver or kidney failure, or with urea cycle and creatine synthesis enzyme defects, surprisingly leads to primarily neurologic symptoms, yet the exact mechanisms remain largely mysterious. In guanidinoacetate N-methyltransferase (GAMT) deficiency, the guanidino compound guanidinoacetate (GAA) increases dramatically, including in the cerebrospinal fluid (CSF), and has been implicated in mediating the neurological symptoms in GAMT-deficient patients. GAA is synthesized by arginine-glycine amidinotransferase (AGAT), a promiscuous enzyme that not only transfers the amidino group from arginine to glycine, but also to primary amines in, for example, GABA and taurine to generate γ-guanidinobutyric acid (γ-GBA) and guanidinoethanesulfonic acid (GES), respectively. We show that GAA, γ-GBA, and GES share structural similarities with GABA, evoke GABA_A receptor (GABA_A R) mediated currents (whereas creatine [methylated GAA] and arginine failed to evoke discernible currents) in cerebellar granule cells in mouse brain slices and displace the high-affinity GABA-site radioligand [³ H]muscimol in total brain homogenate GABA_A Rs. While γ-GBA and GES are GABA agonists and displace [³ H]muscimol (EC₅₀ /IC₅₀ between 10 and 40 μM), GAA stands out as particularly potent in both activating GABA_A Rs (EC₅₀ ~6 μM) and also displacing the GABA_A R ligand [³ H]muscimol (IC₅₀ ~3 μM) at pathophysiologically relevant concentrations. These findings stress the role of substantially elevated GAA as a primary neurotoxic agent in GAMT deficiency and we discuss the potential role of GAA in arginase (and creatine transporter) deficiency which show a much more modest increase in GAA concentrations yet share the unique hyperexcitability neuropathology with GAMT deficiency. We conclude that orthosteric activation of GABA_A Rs by GAA, and potentially other GABA_A R mimetic guanidino compounds (GCs) like γ-GBA and GES, interferes with normal inhibitory GABAergic neurotransmission which could mediate, and contribute to, neurotoxicity.