SLC6A8 (CRT/CT1) — gene review notes

UniProt P48029 (SC6A8_HUMAN). HGNC:11055. Gene at Xq28. 635 aa, 12-TM, SLC6 (SNF/NSS;
TC 2.A.22) family, SLC6A8 subfamily. Disease: X-linked cerebral creatine deficiency
syndrome 1 (CCDS1 / creatine transporter deficiency, CTD).

Core molecular function: creatine:Na+:Cl- symport

• UniProt CATALYTIC ACTIVITY (Rhea RHEA:71831):
  creatine(out) + chloride(out) + 2 Na(+)(out) = creatine(in) + chloride(in) + 2 Na(+)(in)
  i.e. 2 Na+ : 1 Cl- : 1 creatine stoichiometry, electrogenic symport.
• Intestinal study confirms stoichiometry directly: PMID:12433955.
• Heart cDNA in Xenopus oocytes: Na+- and Cl--dependent creatine uptake, Km ~20 uM
  PMID:9882430. Km(Na+) 59 mM, Km(Cl-) 5 mM (UniProt
  BIOPHYSICOCHEMICAL).
• Kidney cDNA: high-affinity creatine uptake Km = 77 uM, NO choline transport
  PMID:7953292.
• Brainstem/spinal cord cDNA: sodium-dependent creatine uptake Km 14.9 uM
  PMID:7945388.
• Electrogenic activity demonstrated electrophysiologically; all CTD mutants lose both
  transport and electrogenic activity PMID:22644605. guanidinopropionate is a substrate analog; phosphocreatine derivative
  gives partial activity.

Substrate specificity

• High specificity for creatine. Distant analogues (GABA, choline, glycine, beta-alanine,
  taurine, betaine) do NOT inhibit creatine uptake PMID:12433955. Close guanidino analogues (guanidinopropionic acid,
  cyclocreatine) compete. So despite being in the GABA/monoamine transporter (SLC6) family,
  it does NOT efficiently transport GABA — it is a creatine transporter. Guanidinoacetate
  (the GAMT substrate precursor) is a poor substrate relative to creatine.

Localization

• Plasma membrane / cell membrane; multi-pass. UniProt SUBCELLULAR LOCATION: Cell membrane
  (PMID:22644605); Apical cell membrane (PMID:12433955).
• All CTD mutants tested were correctly trafficked to plasma membrane PMID:22644605 — so for these
  the defect is in transport function, not localization.
• In polarized epithelia (intestine, kidney) it is apical: PMID:12433955.

Tissue expression / physiology

• Highest in skeletal muscle, kidney, heart; lower in brain and other tissues
  PMID:7953292;
  PMID:7945388.
• UniProt TISSUE SPECIFICITY: "Predominantly expressed in skeletal muscle and kidney. Also
  found in brain, heart, colon, testis and prostate."
• Supplies creatine to the brain via the blood-brain barrier (UniProt FUNCTION, By
  similarity to mouse Q8VBW1). Creatine/phosphocreatine is the cellular ATP buffer in
  tissues with fluctuating energy demand PMID:8661037.

SLC6 family relationship

• Member of Na+/Cl--dependent neurotransmitter symporter (SNF/NSS) family; closely related
  to taurine, GABA and betaine transporters PMID:7953292. This relationship is the basis of the IBA GABA-symporter
  annotation — but the experimentally established substrate is creatine, not GABA.

Disease — CCDS1 / creatine transporter deficiency (CTD), X-linked

• X-linked intellectual disability with severe speech delay, behavioral problems and
  seizures; carrier females may have milder impairment. ~50% of carrier females affected
  PMID:25861866. Many pathogenic missense/in-frame deletion variants reduce or
  abolish creatine transport in functional assays PMID:17465020.

Regulation (non-core)

• SPAK/OSR1 (WNK-controlled) kinases negatively regulate SLC6A8 activity in Xenopus oocytes
  PMID:25531585. Phosphorylated at Thr-42, Thr-617, Thr-620 (large-scale phosphoproteomics).

Structure

• Cryo-EM structures of human SLC6A8 (PDB 9KR7, 9KRH, 9KRI), ~3.3-3.4 A.

Annotation review summary

CORE: GO:0005309 creatine:sodium symporter activity (MF; strong IDA/IMP),
GO:0015881 creatine transmembrane transport (BP), GO:0005886 plasma membrane (CC).
GO:0005308 creatine transmembrane transporter activity (parent MF, NAS) is correct but
less precise than 0005309 → KEEP_AS_NON_CORE/ACCEPT.

OVER-ANNOTATIONS / family-electronic leakage:
- GO:0005332 gamma-aminobutyric acid:sodium:chloride symporter activity (IBA): wrong
substrate — SLC6A8 does not transport GABA; MARK_AS_OVER_ANNOTATED.
- GO:0006836 neurotransmitter transport (IEA InterPro): creatine is not a neurotransmitter;
over-annotation from family signature → MARK_AS_OVER_ANNOTATED.
- GO:0015812 gamma-aminobutyric acid transport (IEA, inferred from GO:0005332):
inherits the wrong-substrate error → MARK_AS_OVER_ANNOTATED.
- GO:0006865 amino acid transport (IBA): creatine is not an amino acid (it's a guanidino
compound); too generic/imprecise → MARK_AS_OVER_ANNOTATED.
- GO:0006936 muscle contraction (TAS PMID:8661155, a genomic-organization paper): indirect
physiological role, not a molecular function of the transporter → KEEP_AS_NON_CORE.
- GO:0035725 sodium ion transmembrane transport (IBA): true (it cotransports Na+) but
generic; KEEP_AS_NON_CORE.
- GO:0016020 membrane (TAS), GO:0006600 creatine metabolic process (TAS Reactome): generic
but acceptable; ACCEPT/KEEP_AS_NON_CORE.
- GO:0016324 apical plasma membrane (IDA PMID:12433955; IEA): correct in polarized epithelia
(apical), ACCEPT but context-specific → KEEP_AS_NON_CORE for the gene as a whole.