Succinic semialdehyde dehydrogenase: biochemical-molecular-clinical disease mechanisms, redox regulation, and functional significance

Abstract

Succinic semialdehyde dehydrogenase (SSADH; aldehyde dehydrogenase 5a1, ALDH5A1; E.C. 1.2.1.24; OMIM 610045, 271980) deficiency is a rare heritable disorder that disrupts the metabolism of the inhibitory neurotransmitter 4-aminobutyric acid (GABA). Identified in conjunction with increased urinary excretion of the GABA analog gamma-hydroxybutyric acid (GHB), numerous patients have been identified worldwide and the autosomal-recessive disorder has been modeled in mice. The phenotype is one of nonprogressive neurological dysfunction in which seizures may be prominently displayed. The murine model is a reasonable phenocopy of the human disorder, yet the severity of the seizure disorder in the mouse exceeds that observed in SSADH-deficient patients. Abnormalities in GABAergic and GHBergic neurotransmission, documented in patients and mice, form a component of disease pathophysiology, although numerous other disturbances (metabolite accumulations, myelin abnormalities, oxidant stress, neurosteroid depletion, altered bioenergetics, etc.) are also likely to be involved in developing the disease phenotype. Most recently, the demonstration of a redox control system in the SSADH protein active site has provided new insights into the regulation of SSADH by the cellular oxidation/reduction potential. The current review summarizes some 30 years of research on this protein and disease, addressing pathological mechanisms in human and mouse at the protein, metabolic, molecular, and whole-animal level.

FIG. 1.

Schematic diagram of GABA formation and catabolism in mammals. The block in heritable SSADH (ALDH5A1) deficiency is indicated by the cross-hatched box. Decarboxylation of glutamate (catalyzed by glutamic acid decarboxylase; GAD) produces GABA (arrow depicts its elevation in patients and null mice). Succinate semialdehyde is normally oxidized to succinate, thus moving the carbon skeleton of GABA into the TCA cycle for further metabolism. When blocked, accumulated SSA can be converted to gamma-hydroxybutyrate (double upward arrows indicating the significant increase in both mice and humans with ALDH5A1 deficiency). Gamma-hydroxybutyrate, at increased levels, inhibits presynaptic DA release and enhances DA turnover. AKR7A2, aldo-keto reductase 7A2; Aldh5a1, aldehyde dehydrogenase 5a1; DA, dopamine; GABA, gamma-aminobutyrate; GABA-T, GABA-transaminase; GHB, gamma-hydroxybutyric acid; GLase, glutaminase; HOT, hydroxyacid-oxoacid transhydrogenase (catalyzing the cofactor-independent conversion of GHB to succinic semialdehyde with coupled conversion of 2-oxoglutarate to D-2-hydroxyglutaric acid); SSA, succinic semialdehyde; SSADH, succinic semialdehyde dehydrogenase; TCA, tricarboxylic acid.

FIG. 2.

Schematic diagram of the GABAergic synapse. The contribution of the astrocyte (AC) to GABA uptake and metabolism is depicted in the upper right quadrant of the figure. The pertinent processes in synaptic transmission are diagrammed as synthesis, vesicular formation, release into the synaptic cleft, receptor interaction, reuptake, and metabolism. Not all steps are shown. GABA(A), GABA(A) receptor; GABA(B), GABA(B) receptor; GABA-T, GABA-transaminase; GAD, glutamate decarboxylase (pyridoxine dependent); GAT, GABA transporter (many are known, see text). Sources of glutamate primarily include 2-oxoglutarate from the TCA cycle, as well as glutamine (producing glutamate by the action of L-glutaminase in the neuronal presynaptic terminal). As well, the nitrogen acceptor for the GABA-T reaction is 2-oxoglutarate, which yields one molecule of glutamate for each molecule of GABA catabolized through SSA. Note that glutamine represents the key shuttle form for glutamate and GABA between neuronal terminals and astrocytes.

FIG. 3.

Schematic diagram of receptor interactions for GABA. GABA(C) receptors are predominantly located in ocular tissue. Ca, calcium; cAMP, cyclic adenosine monophosphate; Cl, chloride; K, potassium.

FIG. 4.

T-2 weighted MRI demonstrates characteristic symmetrical dentatopallidoluysian pattern of SSADH deficiency. (A) Arrowheads show globus pallidus interna (black) and externa (white). (B) Long arrows show subthalamic nucleus. (C) Long white arrows show dentate nucleus. MRI, magnetic resonance imaging.

FIG. 5.

Mid-sagittal MRI in SSADH deficiency showing cerebellar vermian atrophy (highlighted by the black arrow).

FIG. 6.

MRI reveals the variant pattern of speckled, asymmetric pallidal signal hyperintensity (highlighted by dark arrow) in a 7-year-old boy with SSADH deficiency.

FIG. 7.

Compounds employed therapeutically in Aldh5a1-deficient mice (and occasionally in human patients) with demonstrated efficacy. Full IUPAC names include (a) NCS-382 = (2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene) ethanoic acid; (b) CGP-35348 = (3-Aminopropyl)(diethoxymethyl)phosphinic acid; (c) Vigabatrin = (RS)-4-aminohex-5-enoic acid; (d) Taurine = 2-aminoethanesulfonic acid; and (e) SGS-742 (also CGP-36742, see Ciba-Geigy Pharmaceuticals) = 3-aminopropyl-n-butyl phosphinic acid. For comparison, GABA is shown in the bottom right corner (f, 4-aminobutyric acid). Note that b–e have structural correlations with the GABA backbone, although both SGS-742 and CGP-35348 are phosphinic acid derivatives. Similarly, NCS-382 has structural characteristics corresponding to GHB on the top portion of the heptane ring (more closely related to 4-hydroxycrotonic acid). Figure adapted from structures available on Wikipedia (www.wikipedia.org).

FIG. 8.

Biochemical perturbations identified in patients and knockout mice with inherited Aldh5a1 deficiency. The site of the block in human Aldh5a1-deficient patients and in knockout mice is indicated by the cross-hatched box. Arrows depict the qualitative level of abnormality (downward, decreased; upward, increased). α-KG, α-ketoglutarate; D-2-HG, D-2-hydroxyglutarate; DHHA, 4,5-dihydroxyhexanoic acid; GABA, 4-aminobutyrate. Numbered enzymes include 1, glutamine synthase; 2, glutaminase; 3, glutamic acid decarboxylase; 4, carnosinase (not completely characterized); 5, L-arginine:glycine amidinotransferase (likely, but not proven); 6, GABA-transaminase; 7, HOT, hydroxyacid-oxoacid transhydrogenase (catalyzing the cofactor-independent conversion of GHB to SSA with coupled conversion of 2-oxoglutarate to D-2-hydroxyglutaric acid); 8, AKR7A2, aldo-keto reductase 7A2; 9, pyruvate dehydrogenase reaction (most likely, not examined in any detail); 10, SSA dehydrogenase.

FIG. 9.

Hypothetical formation of 4,5-dihydroxyhexanoic acid from SSA and a putative 2-carbon activated species, such as acetyl-CoA (shown). Lactonization of 4,5-dihydroxyhexanoic acid can readily occur in aqueous solution. GABA, 4-aminobutyrate; SCoA, coenzyme A.

FIG. 10.

Schematic of potential interactions resulting in myelin abnormalities in SSADH-deficient mice. Circled dashes represent regions of downregulation, whereas upward or downward pointing arrows indicate metabolite concentrations in mutant mice. Increased GABA/GHB result in use-dependent effects on GABA receptors and altered signaling via the mitogen-activated protein kinase pathway. These changes, coupled with decreases in brain neuroactive steroids in SSADH-deficient mice (78), may provide an explanation for low plasmalogen and myelin levels in mutant mice. 3βHSD, 3β-hydroxysteroid dehydrogenase; GABA, 4-aminobutyrate; GABA(A)R and GABA(B)R, GABA receptors; MAPK, mitogen-activated protein kinase. Figure adapted from Donarum et al. (45).

FIG. 11.

Representative murine electrocorticograms obtained from implated electrodes. (A) Baseline ECoG recorded in a 16-day-old wild-type control mouse indicating an uneventful baseline with 35–50 μV and 5–7 Hz oscillations. (B) ECoG of 16-day-old SSADH-deficient mouse revealing 250–300 μV, 5–7 Hz spike-and-wave discharge lasting 3–6 s on average (arrows). (C) ECoG recording in 20-day-old mutant mouse revealed a transition from absence to a sustained rhythmic onset of generalized 600 μV at 5 Hz followed by 1.5–2 Hz spike-and-wave discharge associated with tonic-clonic seizures (arrow). ECoG, electrocorticogram; LF, left frontal; LP, left parietal; RF, right frontal; RP, right parietal leads, identical for A and C also. Figure adapted from Cortez et al. (37).

FIG. 12.

Documented and hypothesized pathophysiological alterations in heritable SSADH deficiency. Arrows indicate the direction and magnitude of metabolic disturbances. Dashed arrows indicate proposed and/or hypothesized disturbances. D-2-HG, D-2-hydroxyglutarate; 5-HT, 5-hydroxytryptamine (serotonin); GABA, gamma-aminobutyric acid; GABA_R, GABA receptors; Gln, glutamine; GSH, glutathione; HC, homocarnosine.

FIG. 13.

Crystal structure of human SSADH. Shown is the tetrameric structure of human SSADH. SSADH functions as a tetramer as for other aldehyde dehydrogenases. The tetrameric structure is shown as a ribbon diagram showing one dimer in light blue and orange and the other dimer in magenta and cyan. (To see this illustration in color the reader is referred to the web version of this article at www.liebertonline.com/ars).

FIG. 14.

Monomeric structure of human SSADH. A monomeric protein is presented in the asymmetric unit, and depicted as ribbon representation in which the substrate and NAD bound in the protein are represented with sphere model in cyan and orange colors, respectively. (To see this illustration in color the reader is referred to the web version of this article at www.liebertonline.com/ars).

FIG. 15.

Structural changes of the dynamic catalytic loop upon redox changes. Structural changes of the dynamic catalytic loop are shown in the figure. The catalytic loops of the oxidized and the reduced forms of the wild-type SSADH (colored magenta and cyan, respectively) are superimposed. The disulfide bond in the oxidized form, and the two cysteine residues in the reduced form of SSADH, are shown in a stick model and labeled accordingly. (To see this illustration in color the reader is referred to the web version of this article at www.liebertonline.com/ars).

FIG. 16.

Blockade of the SSA binding site by the dynamic catalytic loop in the oxidized form. The catalytic loop of the oxidized form of SSADH is presented with magenta color. SSA substrate bound to the reduced form of the protein is shown with stick model in cyan color. The disulfide bond formation in the oxidized form of SSADH blocks the binding of SSA. (To see this illustration in color the reader is referred to the web version of this article at www.liebertonline.com/ars).

FIG. 17.

Substrate binding properties of human SSADH. SSA and NAD are shown as a stick model with yellow and orange colors, respectively. Residues involved in the binding of SSA and catalytic residues are presented as stick model with green color, and labeled accordingly. The hydrogen bonds involved in SSA binding are presented with red dotted lines. (To see this illustration in color the reader is referred to the web version of this article at www.liebertonline.com/ars).

FIG. 18.

Comparison of the catalytic loops between human and EcSSADH. Stabilization of the catalytic loops between human and EcSSADH is compared. Unlike human SSADH, the catalytic loop of E. coli SSADH is well stabilized by the strong hydrogen bonds with the neighboring loop, resulting in no disulfide bond formation. The catalytic loop and the connecting loop (β15–β16) of EcSSADH are shown in magenta, and those of human SSADH are in cyan. Three water molecules involved in a hydrogen bond network for the stabilization of the EcSSADH catalytic loop are shown in red spheres; hydrogen bonds that form between the catalytic loop and the connecting loop (β15-β16) are depicted with black dotted lines. EcSSADH = Escherichia coli SSADH. (To see this illustration in color the reader is referred to the web version of this article at www.liebertonline.com/ars).