SLC6A9 Gene

SLC6A9 Gene

Overview

<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">SLC6A9 Gene</th>
</tr>
<tr>
<td class="label">Variant Type</td>
<td>Phenotype</td>
</tr>
<tr>
<td class="label">Missense (trafficking)</td>
<td>Moderate</td>
</tr>
<tr>
<td class="label">Missense (transport)</td>
<td>Moderate</td>
</tr>
<tr>
<td class="label">Nonsense/frameshift</td>
<td>Severe</td>
</tr>
<tr>
<td class="label">Splicing</td>
<td>Variable</td>
</tr>
<tr>
<td class="label">Feature</td>
<td>GlyT2 (SLC6A9)</td>
</tr>
<tr>
<td class="label">Cellular location</td>
<td>Presynaptic neuron</td>
</tr>
<tr>
<td class="label">Affinity (Km)</td>
<td>10-20 μM</td>
</tr>
<tr>
<td class="label">Na+ coupling</td>
<td>3 Na+</td>
</tr>
<tr>
<td class="label">Function</td>
<td>Synaptic reuptake</td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>

SLC6A9 encodes Glycine Transporter 2 (GlyT2), a sodium/chloride-dependent glycine transporter primarily expressed in presynaptic terminals of glycinergic neurons in the brainstem and spinal cord. GlyT2 is the major glycine reuptake transporter at inhibitory glycinergic synapses, essential for terminating glycinergic neurotransmission and maintaining glycine homeostasis in the central nervous system[@biche2023][@gomeza2019].

SLC6A9 Gene

Overview

<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">SLC6A9 Gene</th>
</tr>
<tr>
<td class="label">Variant Type</td>
<td>Phenotype</td>
</tr>
<tr>
<td class="label">Missense (trafficking)</td>
<td>Moderate</td>
</tr>
<tr>
<td class="label">Missense (transport)</td>
<td>Moderate</td>
</tr>
<tr>
<td class="label">Nonsense/frameshift</td>
<td>Severe</td>
</tr>
<tr>
<td class="label">Splicing</td>
<td>Variable</td>
</tr>
<tr>
<td class="label">Feature</td>
<td>GlyT2 (SLC6A9)</td>
</tr>
<tr>
<td class="label">Cellular location</td>
<td>Presynaptic neuron</td>
</tr>
<tr>
<td class="label">Affinity (Km)</td>
<td>10-20 μM</td>
</tr>
<tr>
<td class="label">Na+ coupling</td>
<td>3 Na+</td>
</tr>
<tr>
<td class="label">Function</td>
<td>Synaptic reuptake</td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>

SLC6A9 encodes Glycine Transporter 2 (GlyT2), a sodium/chloride-dependent glycine transporter primarily expressed in presynaptic terminals of glycinergic neurons in the brainstem and spinal cord. GlyT2 is the major glycine reuptake transporter at inhibitory glycinergic synapses, essential for terminating glycinergic neurotransmission and maintaining glycine homeostasis in the central nervous system[@biche2023][@gomeza2019].

The SLC6A9 locus is on chromosome 1p34.1 and encodes a 12-transmembrane-domain protein of approximately 638 amino acids. GlyT2 is functionally and anatomically distinct from GlyT1 (SLC6A5): while GlyT1 is primarily astrocytic and regulates ambient extracellular glycine, GlyT2 is presynaptic and mediates high-affinity glycine reuptake at glycinergic nerve terminals. This distinction is critical for understanding the pathophysiology of SLC6A9-related disorders.

Gene And Protein Architecture

SLC6A9 encodes a prototypical SLC6 family member with the canonical 12-transmembrane helix topology:

• N-terminal intracellular domain: Contains regulatory serine residues and motifs for interaction with scaffolding proteins
• Transmembrane core: Forms the substrate-binding pocket (S1 site) with high affinity for glycine
• Extracellular loops: Heavily glycosylated at conserved NXT motifs
• C-terminal intracellular tail: Contains a PDZ-binding motif (Ser-Ser-Leu) for interaction with gephyrin and other synaptic scaffold proteins

Transport Mechanism

GlyT2 operates as a sodium/chloride-coupled high-affinity glycine transporter:

The higher sodium coupling stoichiometry (3 Na+ vs. 2 for most SLC6 members) makes GlyT2 particularly vulnerable to conditions that disrupt sodium gradients.

Physiologic Role In Glycinergic Neurotransmission

Presynaptic Glycine Reuptake

GlyT2 is the primary mechanism for terminating glycinergic neurotransmission:

• Located in presynaptic glycinergic neurons (not astrocytes like GlyT1)
• Mediates high-affinity reuptake of synaptically released glycine
• Maintains the glycine pool for subsequent vesicle filling
• Prevents spillover of glycine to surrounding receptors

Synaptic Vesicle Refilling

The glycine transporter directly supplies glycine to synaptic vesicles:

• Vesicular glycine transporters (VIAAT/VGAT) package glycine into vesicles
• GlyT2-mediated reuptake provides the substrate for this process
• Disruption reduces synaptic glycine stores

Inhibitory Circuit Function

In brainstem and spinal cord, glycinergic inhibition is critical for:

• Motor control and coordination
• Reflex regulation
• Respiratory rhythm generation
• Pain modulation (dorsal horn)

SLC6A9 In Neurodegeneration

Hyperekplexia (Startle Disease)

SLC6A9 is one of the genes causing autosomal recessive hyperekplexia, along with GLRA1, GLRB, and SLC6A5. Pathogenic variants in SLC6A9 account for approximately 5-10% of genetically confirmed hyperekplexia cases[@apparicio2020][@rees2021].

Clinical phenotype:

• Neonatal hypertonia and startle reflex
• Exaggerated startle response to unexpected stimuli
• Apneic episodes in infancy
• Falling episodes triggered by surprise
• Variable developmental delay
• Occasional epilepsy

• Loss-of-function variants reduce presynaptic glycine reuptake
• Impaired glycine clearance from synaptic cleft
• Prolonged glycinergic inhibition affecting brainstem circuits
• Disrupted startle reflex circuit homeostasis

Genotype-Phenotype Correlation

Neuropathic Pain

While not directly a neurodegenerative disease, GlyT2 is a therapeutic target for neuropathic pain[@cull2022]:

• GlyT2 inhibitors enhance glycinergic inhibition in dorsal horn
• Increased glycine tone reduces pain signaling
• Clinical trials of GlyT2 inhibitors for chronic pain have been conducted

Alzheimer's and Parkinson's Disease

Emerging evidence suggests glycine signaling may be relevant to broader neurodegeneration:

Alzheimer's Disease:

• Glycine receptors are expressed in brain regions affected by AD
• Glycinergic inhibition may modulate excitatory toxicity
• GlyT2 function could influence network oscillations important for memory

• Glycinergic signaling in basal ganglia influences motor control
• GlyT2 modulation may affect levodopa-induced dyskinesias
• Spinal GlyT2 influences movement circuits

Clinical Genetics

Variant Spectrum

Over 50 pathogenic SLC6A9 variants have been described:

• Missense variants: Predominantly in transmembrane domains
• Truncating variants: Distributed throughout the gene
• Splice site variants: Cause exon skipping
• Large deletions: Rare but reported

Inheritance Pattern

Autosomal recessive. Both alleles must be mutated for disease expression. Heterozygous carriers are typically asymptomatic but may show subtle electrodiagnostic abnormalities.

Population Genetics

SLC6A9 shows strong evolutionary constraint against loss-of-function variation, consistent with the essential role of GlyT2 in motor circuit function.

Structural Biology

Cryo-EM Structures

Recent cryo-EM studies have revealed:

• Overall architecture similar to other SLC6 family members
• Distinct substrate-binding site geometry
• Sodium binding sites (3 distinct Na+ sites)
• Conformational changes during transport cycle

Comparison with GlyT1

Therapeutic Implications

Hyperekplexia Treatment

• Clonazepam: First-line for suppressing startle and apnea
• Sodium valproate: Alternative for seizures
• Phenytoin: May reduce hypertonia
• Gene therapy: Experimental, AAV-mediated delivery

GlyT2 as Analgesic Target

• GlyT2 inhibitors increase glycinergic inhibition in pain pathways
• Development of selective GlyT2 inhibitors for neuropathic pain
• Avoids the sedation and dependence issues of opioid analgesics

Research Models

Animal Models

• GlyT2 knockout mice: Neonatal lethality due to respiratory failure
• Conditional knockouts: Reveal circuit-specific functions
• Humanized mice: Express patient variants

In Vitro Systems

• HEK293 expression: Functional variant characterization
• Xenopus oocytes: Electrophysiological transport studies
• Primary neuronal cultures: Synaptic function studies

Summary

SLC6A9 encodes GlyT2, the presynaptic high-affinity glycine transporter essential for glycinergic neurotransmission. The gene is associated with autosomal recessive hyperekplexia, where loss-of-function variants cause excessive glycinergic inhibition of brainstem circuits controlling startle reflexes and motor function.

Key aspects for neurodegeneration research include:

See Also

• [SLC6A5 Gene](/genes/slc6a5) - GlyT1, the astrocytic glycine transporter
• [Hyperekplexia](/diseases/hyperekplexia)
• [Glycinergic Neurotransmission](/mechanisms/glycinergic-neurotransmission)
• [Brainstem](/anatomy/brainstem)
• [Motor Control](/mechanisms/motor-control)

External Links

• [NCBI Gene](https://www.ncbi.nlm.nih.gov/gene/6403)
• [OMIM](https://www.omim.org/entry/601020)
• [UniProt](https://www.uniprot.org/uniprot/P48031)
• [Ensembl](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000137727)

Allen Brain Atlas

• [Human Brain Map - SLC6A9 Expression](https://human.brain-map.org/microarray/search/show?search_term=SLC6A9)
• [BrainSpan Transcriptome Atlas](https://brainspan.org/)

References