Glutamate receptor, ionotropic, delta 2, also known as GluD2, GluRδ2, or δ2, is a protein that in humans is encoded by the GRID2gene. This protein together with GluD1 belongs to the delta receptor subtype of ionotropic glutamate receptors. They possess 14–24% sequencehomology with AMPA, kainate, and NMDA subunits, but, despite their name, have been found not to directly promote neuronal activation in response to glutamate or various other glutamate agonists.
delta iGluRs have long been considered orphan receptors as their endogenousligand was unknown. They are now believed to bind glycine and D-serine but these do not result in channel opening.
GluD2 induces synaptogenesis through interaction of its N-terminal domain with Cbln1, which in turn interacts with presynaptic neurexins, forming a bridge across cerebellar synapses.
The main functions of GluD2 in synaptic plasticity are carried out by its intracellular C-terminus. This is regulated by D-serine, which binds to the ligand-binding domain and results in changes in the structure of GluD2 without opening the channel in the absence of pre-synaptic connections. Glycine and D-serine can open the channel in GluD2 when bound to cerebellin-1 and neurexin-1β. These changes may signal up to the N-terminal domain or down to the C-terminal domain to alter protein-protein interactions.
Pathology
A heterozygous deletion in GRID2 in humans causes a complicated spastic paraplegia with ataxia, frontotemporal dementia, and lower motor neuron involvement whereas a homozygous biallelic deletion leads to a syndrome of cerebellar ataxia with marked developmental delay, pyramidal tract involvement and tonic upgaze, that can be classified as an ataxia with oculomotor apraxia (AOA) and has been named spinocerebellar ataxia, autosomal recessive type 18 (SCAR18).
A gain of channel function, resulting from a point mutation in mouse GRID2, is associated with the phenotype named 'lurcher', which in the heterozygous state leads to ataxia and motor coordination deficits resulting from selective, cell-autonomous apoptosis of cerebellar Purkinje cells during postnatal development. Mice homozygous for this mutation die shortly after birth from massive loss of mid- and hindbrain neurons during late embryogenesis.
GRID2 has been shown to interact with GOPC, GRIK2, PTPN4 and GRIA1. A possible correlation between GRID2 and the pre-B lymphocyte protein 3 (VPREB3) has been suggested, due to the apparent importance of B-lymphocytes in the origins of cerebellarPurkinje neurons in humans. Morphological studies conducted in GRID2-knockout mice suggest that GRID2 may be present in lymphocytes as well as in the adrenal cortex, however further studies must be conducted to confirm these claims.
↑Araki K, Meguro H, Kushiya E, Takayama C, Inoue Y, Mishina M (December 1993). "Selective expression of the glutamate receptor channel delta 2 subunit in cerebellar Purkinje cells". Biochemical and Biophysical Research Communications. 197 (3): 1267–1276. doi:10.1006/bbrc.1993.2614. PMID7506541.
↑Matsuda K, Yuzaki M (March 2012). "Cbln1 and the δ2 glutamate receptor--an orphan ligand and an orphan receptor find their partners". Cerebellum. 11 (1): 78–84. doi:10.1007/s12311-010-0186-5. PMID20535596. S2CID16612844.
↑Maier A, Klopocki E, Horn D, Tzschach A, Holm T, Meyer R, etal. (February 2014). "De novo partial deletion in GRID2 presenting with complicated spastic paraplegia". Muscle & Nerve. 49 (2): 289–292. doi:10.1002/mus.24096. PMID24122788. S2CID26359325.
↑Utine GE, Haliloğlu G, Salanci B, Çetinkaya A, Kiper PÖ, Alanay Y, etal. (July 2013). "A homozygous deletion in GRID2 causes a human phenotype with cerebellar ataxia and atrophy". Journal of Child Neurology. 28 (7): 926–932. doi:10.1177/0883073813484967. PMID23611888. S2CID206550612.
↑Williams K, Dattilo M, Sabado TN, Kashiwagi K, Igarashi K (May 2003). "Pharmacology of delta2 glutamate receptors: effects of pentamidine and protons". The Journal of Pharmacology and Experimental Therapeutics. 305 (2): 740–748. doi:10.1124/jpet.102.045799. PMID12606689. S2CID83540259.
12Felizola SJ, Katsu K, Ise K, Nakamura Y, Arai Y, Satoh F, etal. (May 2015). "Pre-B Lymphocyte Protein 3 (VPREB3) Expression in the Adrenal Cortex: Precedent for non-Immunological Roles in Normal and Neoplastic Human Tissues". Endocrine Pathology. 26 (2): 119–128. doi:10.1007/s12022-015-9366-7. PMID25861052. S2CID27271366.
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