sphingosine-1-phosphate and Hearing-Loss

The sphingosine-1-phosphate receptors (S1PRs) are a well-studied class of transmembrane G protein-coupled sphingolipid receptors that mediate multiple cellular processes. However, S1PRs have not been previously reported to be involved in the genetic etiology of human traits. S1PR2 lies within the autosomal-recessive nonsyndromic hearing impairment (ARNSHI) locus DFNB68 on 19p13.2. From exome sequence data we identified two pathogenic S1PR2 variants, c.323G>C (p.Arg108Pro) and c.419A>G (p.Tyr140Cys). Each of these variants co-segregates with congenital profound hearing impairment in consanguineous Pakistani families with maximum LOD scores of 6.4 for family DEM4154 and 3.3 for family PKDF1400. Neither S1PR2 missense variant was reported among ∼120,000 chromosomes in the Exome Aggregation Consortium database, in 76 unrelated Pakistani exomes, or in 720 Pakistani control chromosomes. Both DNA variants affect highly conserved residues of S1PR2 and are predicted to be damaging by multiple bioinformatics tools. Molecular modeling predicts that these variants affect binding of sphingosine-1-phosphate (p.Arg108Pro) and G protein docking (p.Tyr140Cys). In the previously reported S1pr2(-/-) mice, stria vascularis abnormalities, organ of Corti degeneration, and profound hearing loss were observed. Additionally, hair cell defects were seen in both knockout mice and morphant zebrafish. Family PKDF1400 presents with ARNSHI, which is consistent with the lack of gross malformations in S1pr2(-/-) mice, whereas family DEM4154 has lower limb malformations in addition to hearing loss. Our findings suggest the possibility of developing therapies against hair cell damage (e.g., from ototoxic drugs) through targeted stimulation of S1PR2.

Topics: Amino Acid Sequence; Asian People; Chromosomes, Human, Pair 19; Exome; Genes, Recessive; Hearing Loss; Humans; Lod Score; Logistic Models; Lysophospholipids; Models, Molecular; Molecular Sequence Data; Mutation, Missense; Pedigree; Phenotype; Receptors, Lysosphingolipid; Sphingosine; Sphingosine-1-Phosphate Receptors

Spinster homolog 2 (Spns2) acts as a Sphingosine-1-phosphate (S1P) transporter in zebrafish and mice, regulating heart development and lymphocyte trafficking respectively. S1P is a biologically active lysophospholipid with multiple roles in signalling. The mechanism of action of Spns2 is still elusive in mammals. Here, we report that Spns2-deficient mice rapidly lost auditory sensitivity and endocochlear potential (EP) from 2 to 3 weeks old. We found progressive degeneration of sensory hair cells in the organ of Corti, but the earliest defect was a decline in the EP, suggesting that dysfunction of the lateral wall was the primary lesion. In the lateral wall of adult mutants, we observed structural changes of marginal cell boundaries and of strial capillaries, and reduced expression of several key proteins involved in the generation of the EP (Kcnj10, Kcnq1, Gjb2 and Gjb6), but these changes were likely to be secondary. Permeability of the boundaries of the stria vascularis and of the strial capillaries appeared normal. We also found focal retinal degeneration and anomalies of retinal capillaries together with anterior eye defects in Spns2 mutant mice. Targeted inactivation of Spns2 in red blood cells, platelets, or lymphatic or vascular endothelial cells did not affect hearing, but targeted ablation of Spns2 in the cochlea using a Sox10-Cre allele produced a similar auditory phenotype to the original mutation, suggesting that local Spns2 expression is critical for hearing in mammals. These findings indicate that Spns2 is required for normal maintenance of the EP and hence for normal auditory function, and support a role for S1P signalling in hearing.

Topics: Age of Onset; Animals; Anion Transport Proteins; Anterior Eye Segment; Cochlea; Connexin 26; Connexins; Ear, Inner; Hair Cells, Auditory; Hearing Loss; Lysophospholipids; Mice; Organogenesis; Sphingosine; Stria Vascularis; Zebrafish

We sought to demonstrate that tumor necrosis factor (TNF)-α, via sphingosine-1-phosphate signaling, has the potential to alter cochlear blood flow and thus, cause ischemic hearing loss.. We performed intravital fluorescence microscopy to measure blood flow and capillary diameter in anesthetized guinea pigs. To measure capillary diameter ex vivo, capillary beds from the gerbil spiral ligament were isolated from the cochlear lateral wall and maintained in an organ bath. Isolated gerbil spiral modiolar arteries, maintained and transfected in organ culture, were used to measure calcium sensitivity (calcium-tone relationship). In a clinical study, a total of 12 adult patients presenting with typical symptoms of sudden hearing loss who were not responsive or only partially responsive to prednisolone treatment were identified and selected for etanercept treatment. Etanercept (25 mg s.c.) was self-administered twice a week for 12 weeks.. TNF-α induced a proconstrictive state throughout the cochlear microvasculature, which reduced capillary diameter and cochlear blood flow in vivo. In vitro isolated preparations of the spiral modiolar artery and spiral ligament capillaries confirmed these observations. Antagonizing sphingosine-1-phosphate receptor 2 subtype signaling (by 1 μmol/L JTE013) attenuated the effects of TNF-α in all models. TNF-α activated sphingosine kinase 1 (Sk1) and induced its translocation to the smooth muscle cell membrane. Expression of a dominant-negative Sk1 mutant (Sk1(G82D)) eliminated both baseline spiral modiolar artery calcium sensitivity and TNF-α effects, whereas a nonphosphorylatable Sk1 mutant (Sk1(S225A)) blocked the effects of TNF-α only. A small group of etanercept-treated, hearing loss patients recovered according to a 1-phase exponential decay (half-life=1.56 ± 0.20 weeks), which matched the kinetics predicted for a vascular origin.. TNF-α indeed reduces cochlear blood flow via activation of vascular sphingosine-1-phosphate signaling. This integrates hearing loss into the family of ischemic microvascular pathologies, with implications for risk stratification, diagnosis, and treatment.

Topics: Adult; Animals; Calcium; Cochlea; Etanercept; Gerbillinae; Guinea Pigs; Hearing Loss; Humans; Immunoglobulin G; Lysophospholipids; Microvessels; Models, Animal; Phosphotransferases (Alcohol Group Acceptor); Receptors, Tumor Necrosis Factor; Regional Blood Flow; Signal Transduction; Sphingosine; Spiral Ligament of Cochlea; Treatment Outcome; Tumor Necrosis Factor-alpha; Vasoconstriction