Kristina Becker

A novel recessive hyperekplexia allele GLRA1 (S231R): genotyping by MALDI-TOF mass spectrometry and functional characterisation as a determinant of cellular glycine receptor trafficking

Hyperekplexia or startle disease (stiff baby syndrome, STHE) is a hereditary neurological disorder characterised by an exaggerated startle response and infantile muscle hypertonia. Several autosomal dominant and recessive forms of the disorder have been associated with point mutations in GLRA1, the human gene encoding the α1 subunit of the inhibitory glycine receptor. Here, we describe a recessive point mutation (C1073G) in exon 7 of GLRA1 leading to an amino acid exchange of serine 231 to arginine in transmembrane region TM1. The mutation was detectable by restriction digest analysis of genomic PCR amplimers by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS). Genotyping of family members was performed using an allele specific primer extension assay in combination with MALDI-TOF-MS and confirmed by conventional DNA sequencing. These studies demonstrate the broad applicability of MALDI-TOF-MS as a comparative screening tool applicable to the analysis of allelic gene variants. In comparison to the wild type α1 subunit, biochemical, electrophysiological, and confocal microscopy data indicate a reduced integration of functional α1S231R glycine receptors into the cell surface membrane upon recombinant expression. Apparently, the amino acid exchange S231R influences glycine receptor biogenesis and cellular trafficking by introducing a positive charge into transmembrane region TM1.

The respiratory rhythm in mutant oscillator mice.

Since glycinergic inhibition is important for respiratory rhythm generation in mature mammals, we tested the hypothesis that the loss of glycine receptors during postnatal development (P17-P23) of homozygous mutant oscillator mice (spd(ot)/spd(ot)) may result in serious impairment of respiratory rhythm. We measured breathing in a plethysmographic recording chamber on conscious oscillator mice and used an in situ perfused brainstem preparation to record phrenic nerve activity, as well as membrane properties of respiratory neurones. The deletion of glycinergic inhibition did not result in failure of respiratory rhythm: homozygous mutant oscillator mice continue to generate a disturbed respiratory rhythm until death. Postsynaptic activity and membrane potential trajectories of respiratory neurones revealed a persistence of GABAergic inhibition and changes in respiratory rhythm and pattern generation.

Transient Expression of NMDA Receptor Subunit NR2B in the Developing Rat Heart

Abstract: NMDA receptors represent a subtype of the ionotropicglutamate receptor family, comprising three classes of subunits (NR1, NR2A‐D,NR3), which exhibit distinct patterns of regional and developmental expressionin the CNS. Recently, some NMDA receptor subunits have also been described inadult extraneuronal tissues and keratinocytes. However, their developmentalexpression patterns are currently unknown. With use of RT‐PCR and western blotanalysis, the expression of NMDA receptor subunit NR2B was investigated in thedeveloping rat heart. NR2B mRNA and protein were detected in heart tissue ofrats from embryonic day 14 until postnatal day 21 but disappeared 10 weeksafter birth. In contrast, no NMDA receptor subunit NR1,α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid receptor subunitGluR2, or anchoring postsynaptic density protein‐95 could be detected in ratheart at any developmental stage. Confocal microscopy of cultured cardiacmyocytes (CMs) from neonatal rats revealed distinct NR2B staining mainly ofintracellular structures. However, no functional NMDA receptor could bedetected on CMs by whole‐cell recordings. In conclusion, high concentrationsof NR2B protein can be detected in early rat heart development, but itsfunction still remains elusive.

Multifunctional basic motif in the glycine receptor intracellular domain induces subunit-specific sorting.

The strychnine-sensitive glycine receptor (GlyR) is a ligand-gated ion channel that mediates fast synaptic inhibition in the vertebrate central nervous system. As a member of the family of Cys-loop receptors, it assembles from five homologous subunits (GlyRα1–4 and -β). Each subunit contains an extracellular ligand binding domain, four transmembrane domains (TM), and an intracellular domain, formed by the loop connecting TM3 and TM4 (TM3–4 loop). The TM3–4 loops of the subunits GlyRα1 and -α3 harbor a conserved basic motif, which is part of a potential nuclear localization signal. When tested for functionality by live cell imaging of green fluorescent protein and β-galactosidase-tagged domain constructs, the TM3–4 loops of GlyRα1 and -α3, but not of GlyRα2 and -β, exhibited nuclear sorting activity. Subunit specificity may be attributed to slight amino acid alterations in the basic motif. In yeast two-hybrid screening and GST pulldown assays, karyopherin α3 and α4 were found to interact with the TM3–4 loop, providing a molecular mechanism for the observed intracellular trafficking. These results indicate that the multifunctional basic motif of the TM3–4 loop is capable of mediating a karyopherin-dependent intracellular sorting of full-length GlyRs.

Functional Complementation of Glra1spd-ot, a Glycine Receptor Subunit Mutant, by Independently Expressed C-Terminal Domains

The oscillator mouse (Glra1spd-ot) carries a 9 bp microdeletion plus a 2 bp microinsertion in the glycine receptor α1 subunit gene, resulting in the absence of functional α1 polypeptides from the CNS and lethality 3 weeks after birth. Depending on differential use of two splice acceptor sites in exon 9 of the Glra1 gene, the mutant allele encodes either a truncated α1 subunit (spdot-trc) or a polypeptide with a C-terminal missense sequence (spdot-elg). During recombinant expression, both splice variants fail to form ion channels. In complementation studies, a tail construct, encoding the deleted C-terminal sequence, was coexpressed with both mutants. Coexpression with spdot-trc produced glycine-gated ion channels. Rescue efficiency was increased by inclusion of the wild-type motif RRKRRH. In cultured spinal cord neurons from oscillator homozygotes, viral infection with recombinant C-terminal tail constructs resulted in appearance of endogenous α1 antigen. The functional rescue of α1 mutants by the C-terminal tail polypeptides argues for a modular subunit architecture of members of the Cys-loop receptor family.

Expression of glycine receptors and gephyrin in the rat cochlea

The cochlear efferent feedback system exerts direct impact on cochlear nerve activity and balances interaural sensitivity. So far, acetylcholine, GABA and dopamine are known to be transmitters of the inhibitory efferent system. Despite the wealth of information about glycinergic neurotransmission in the central auditory system, the inhibitory glycine receptor (GlyR) has not yet been regarded as a target molecule of efferent transmission in the cochlea. Using RT-PCR, in situ hybridization and immunohistochemistry, we show that GlyRα3, GlyRβ and gephyrin are expressed in the organ of Corti and spiral ganglion neurons. Furthermore, two alternative splice variants of GlyRα3, corresponding to the long (α3_L) and short (α3_K) human isoforms, could be distinguished. The localization of glycine receptors below inner hair cells and in outer hair cells of the adult cochlea suggests that these inhibitory receptors may serve as target molecules of the efferent olivocochlear bundle.

Effects of functional knock-out of α1 glycine-receptors on breathing movements in oscillator mice

The effects of a deficiency of glycinergic inhibition deriving from mutations of the glycine-receptor gene Glra1 on the breathing pattern of oscillator mice were studied. We compared the development of breathing frequency, tidal volume and minute ventilation from control mice (wild type- and heterozygous oscillator mice) with those of homozygous oscillator mice during early postnatal periods from p9 until p21. The changes of ventilation were correlated with body-weight and changes in blood-pH. During the second to third weeks of postnatal development, breathing frequency increased from 310 to 445.4 mm-1 in control mice. Oscillator mice reached a maximal value of 313.3 min-1 at p18 followed by a fast decrease to 233.0 min-1. This decrease is caused by a prolongation of expiratory duration. Tidal volume showed a steady increase from 6.6 to 15.1 microliters in control animals. In comparison, oscillator mice showed significant lower values after p14. After p15, minute ventilation of oscillator mice declined as compared with control animals leading to respiratory acidosis at p20.

DARPP-32 binds to tra2-beta1 and influences alternative splicing.

The majority of human genes undergo alternative splicing, which is frequently altered in response to physiological stimuli. DARPP-32 (dopamine and cAMP regulated phosphoprotein, 32kDa) is a component of PKA-dependent signaling pathways. Here we show that DARPP-32 binds directly to the splicing factor tra2-beta1 (transformer 2). DARPP-32 changes the usage of tra2-beta1 dependent alternative exons in a concentration-dependent manner, suggesting that the DARPP-32:tra2-beta1 interaction is a molecular link between signaling pathways and pre-mRNA processing.

The novel hyperekplexia allele GLRA1(S267N) affects the ethanol site of the glycine receptor

Mutations in the GLRA1 gene, which encodes the α1-subunit of the inhibitory glycine receptor (GlyR), are the underlying causes in the majority of cases of hereditary startle disease (OMIM no. 149400). GlyRs are modulated by alcohols and volatile anesthetics, where a specific amino acid at position 267 has been implicated in receptor modulation. We describe a hyperekplexia family carrying the novel dominant missense allele GLRA1(S267N), that affects agonist responses and ethanol modulation of the mutant receptor. This study implies that a disease-related receptor allele carries the potential to alter drug responses in affected patients.

Spinal inhibitory synaptic transmission in the glycine receptor mouse mutant spastic

Inhibitory glycine receptor (GlyR) and GABA(A) receptor (GABA(A)R)-mediated synaptic transmission was examined in two strains of the GlyR mutant mouse spastic and the respective wild types. The mutants display a mild and a severe neurological phenotype. Electrically evoked postsynaptic whole-cell currents were recorded from alpha-motoneurons in lumbar spinal cord slices. Amplitudes of GlyR-mediated IPSCs were significantly reduced in the severe phenotype in comparison to the respective wild type and the mild phenotype mutants. Surprisingly, amplitudes of GABA(A)R-mediated IPSCs were also significantly reduced in both mutants. Fast time constants of the decay phase of IPSCs were slightly reduced for the GlyR-mediated IPSCs and significantly larger for the GABA(A)R-mediated IPSCs in both mutant strains.