Elsevier

Hearing Research

Volume 335, May 2016, Pages 94-104
Hearing Research

Research paper
Perinatal thiamine deficiency causes cochlear innervation abnormalities in mice

https://doi.org/10.1016/j.heares.2016.02.006Get rights and content

Highlights

  • Thiamine deprivation was induced during fetal development or early postnatal life.

  • Thiamine-deprivation resulted in significant anomalies in cochlear afferent or efferent innervation.

  • Auditory neuropathy from thiamine deprivation results from loss of inner hair cell synapses in mice.

Abstract

Neonatal thiamine deficiency can cause auditory neuropathy in humans. To probe the underlying cochlear pathology, mice were maintained on a thiamine-free or low-thiamine diet during fetal development or early postnatal life. At postnatal ages from 18 days to 22 wks, cochlear function was tested and cochlear histopathology analyzed by plastic sections and cochlear epithelial whole-mounts immunostained for neuronal and synaptic markers. Although none of the thiamine-deprivation protocols resulted in any loss of hair cells or any obvious abnormalities in the non-sensory structures of the cochlear duct, all the experimental groups showed significant anomalies in the afferent or efferent innervation. Afferent synaptic counts in the inner and outer hair cell areas were reduced, as was the efferent innervation density in both the outer and inner hair cell areas. As expected for primary neural degeneration, the thresholds for distortion product otoacoustic emissions were not affected, and as expected for subtotal hair cell de-afferentation, the suprathreshold amplitudes of auditory brainstem responses were more affected than the response thresholds. We conclude that the auditory neuropathy from thiamine deprivation could be produced by loss of inner hair cell synapses.

Introduction

Auditory neuropathy is a type of sensorineural hearing loss resulting from disorders affecting inner hair cells (IHCs), cochlear sensory neurons or the synapses between them (for review, see Starr and Rance, 2015). It can be congenital (Bahmad et al., 2007, Santarelli et al., 2009, Pangrsic et al., 2010) or acquired (Beutner et al., 2007), and it is diagnosed based on the presence of otoacoustic emissions despite an absent or highly disorganized auditory brainstem response (Hood, 2015). A number of risk factors have been identified. For example a high incidence of auditory neuropathy is found in infants born prematurely (Xoinis et al., 2007). Recently, a study of infant temporal bones revealed that 30% of preterm babies who died in the NICU had selective inner hair cell (IHC) loss, while only 3% of full-term infants presented such a phenotype (Amatuzzi et al., 2011). This high prevalence for such a rare histopathology (Schuknecht, 1993) suggested that selective IHC loss might be a common cause of auditory neuropathy.

Our group recently reported a link between IHC loss and thiamine, a water-soluble vitamin (B1) required for intermediary metabolism. Patients with the syndromic disorder known as Thiamine-Responsive Megaloblastic Anemia (TRMA) have hearing loss in addition to the pathognomonic anemic symptoms (Fleming et al., 1999, Fleming et al., 2001). Genetic analysis revealed a mutation in one of the two transporters involved in trafficking thiamine across cell membranes: SLC19A2. Targeted deletion of SLC19A2 in mice causes auditory neuropathy and selective IHC loss when animals are challenged with a diet low in thiamine (Liberman et al., 2006).

Further evidence for a link between thiamine and auditory neuropathy was provided by a study of children who were bottle-fed from birth with an infant formula completely lacking in thiamine due to a manufacturing error. As infants, these children were hospitalized with a variety of neurological symptoms consistent with thiamine deficiency, collectively known as Wernicke's encephalopathy (Fattal-Valevski et al., 2005). Continued follow-up showed an extraordinarily high percentage of auditory neuropathy (Attias et al., 2012).

Combining all these observations, we wondered if the link between prematurity and auditory neuropathy might reflect an immaturity of thiamine transporters in preterm infants coupled with varying standards for thiamine supplementation during NICU stays (Friel et al., 2001). To gain insight into the effects of thiamine deprivation in the perinatal period, we deprived mice of dietary thiamine during pregnancy and/or nursing. We let the offspring mature to varying ages, measured cochlear function by ABRs and DPOAEs and evaluated the degeneration of hair cells and their afferent and efferent innervation. We found no evidence for selective IHC loss; however, many thiamine-deprived animals showed dramatic abnormalities in the cochlear afferent and efferent innervation, and associated cochlear functional abnormalities consistent with the diagnosis of auditory neuropathy.

Section snippets

Animals, procedure and statistical analysis

CBA/CaJ mice were used in all experiments. To control thiamine intake, a thiamine-free chow was obtained from TestDiet® (TestDiet.com) in powdered form. The formulation was identical, in all other respects, to that established for use in mice by the American Institute of Nutrition. The desired thiamine concentration was adjusted by addition of powdered thiamine diluted in water. The mixture was provided as a moist gruel.

Mice were assigned to one of three groups: 1) Control animals, which were

Results

Normal mouse chow contains 22 mg/kg thiamine. In this study, mice were fed with a chow containing either 1) 22 mg/kg thiamine, 2) 0.2 mg/kg thiamine or 3) 0 mg/kg thiamine. As schematized in Fig. 1, thiamine deprivation was implemented either prenatally or postnatally for durations ranging from 2 to 6 wks. There were no pregnancies when mice were fed 0 mg/kg thiamine for more than 3 wks. Under a low-thiamine diet (0.2 mg/kg), pregnancies went to term only when thiamine restriction lasted less

Cochlear synaptopathy and cochlear development

Here, we investigated the effects of thiamine deficiency on the mouse cochlea during embryonic and postnatal development. We were inspired by a clinical report describing a high rate of auditory neuropathy in newborns fed with a commercial infant formula totally lacking in thiamine (Fattal-Valevski et al., 2005, Attias et al., 2012), also known vitamin B1, a key co-factor in intermediary metabolism. The diagnosis of auditory neuropathy in humans is based on the presence of otoacoustic emissions

Acknowledgments

Research supported by grants from the NIDCD: R21 DC 012599 and P30 DC 05209.

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