Sensory and signal transduction
in the inner ear

Figure 1. Ca²⁺ wave in the organ Corti. One representative experimental record, showing propagating Ca²⁺ transients in fura-2 loaded supporting cells induced by focal ATP application (50 ms, 4µM) on OHCs. (A) Pseudocolor images of Ca²⁺ wave spreading at three different time points. (B) wave front profiles highlighted through frame by frame subtraction of subsequently recorded images. (C) [Ca²⁺]_i profiles from ROIs in A. (D) Distances of wavefront points from the point of stimulus application (white asterisk) plotted versus time yielding wave speed. (E) Amplitude of mean fura-2 ratio signal showing no significant decrease of the wavefront [Ca²⁺]_i with distance from the source of application, thus implicating an active mechanism of propagation.

Figure 1. Ca²⁺ wave in the organ Corti. One representative experimental record, showing propagating Ca²⁺ transients in fura-2 loaded supporting cells induced by focal ATP application (50 ms, 4µM) on OHCs. (A) Pseudocolor images of Ca²⁺ wave spreading at three different time points. (B) wave front profiles highlighted through frame by frame subtraction of subsequently recorded images. (C) [Ca²⁺]_i profiles from ROIs in A. (D) Distances of wavefront points from the point of stimulus application (white asterisk) plotted versus time yielding wave speed. (E) Amplitude of mean fura-2 ratio signal showing no significant decrease of the wavefront [Ca²⁺]_i with distance from the source of application, thus implicating an active mechanism of propagation.
[click image to enlarge]

Stimulation of sensory transduction in the inner ear results in an increased flux of K⁺ from endolymph to perilymph via the hair cells, for deflection of the cells' stereocilia (hair bundle) modulates the open probability of specialized cation-selective mechanosensitive transduction (MET) channels. Though hair cells are the sensors for hearing and balance, several other inner ear cell types play essential roles. For example, cells in the stria vascularis are responsible for maintaining cochlear endolymph at a potential of +80 to +100 mV (the so-called endochlear potential, or EP) relative to perilymph, thus increasing the driving force for K⁺. MET channel activation causes the hair cell to release K⁺ ions that are either dispersed into perilymph or taken up by adjacent supporting cells and are eventually returned to the stria vascularis. Currently we are focussing on selected signal transduction pathways that are deployed in this complex scenario of vertebrate inner ear. We are also working on the diagnosis of molecular defects causing hearing impairment and the development of new preventive and therapeutic tools to ameliorate hearing.

Role of connexin permeability defects in hereditary deafness

In Europe profound deafness affects at least 3 million individuals and at least 6.7 million people are severely deaf. We have recently offered a mechanistic explanation for the pathogenesis of deafness due to connexin mutations. Abnormal or impaired connexin function has been linked to several other diseases, including skin disease, peripheral neuropathies, and cataracts, thus our data may have a more general impact. In collaboration with a team of EU laboratories of the EuroHear consortium we are contributing to create and use mouse models of deafness to advance our understanding of inner ear physiology and pathology and to open the way to genetic and pharmacologic approaches.

Applying a gene therapy approach to the inner ear

At the moment we have good expectations for the efficient expression of a novel bovine virus (BAAV) [Di Pasquale G, Rzadzinska A, Schneider ME, Bossis I, Chiorini JA, Kachar B. Mol Ther. 2005 Jun;11(6):849-55.] A functionality assay based on fluorescence recovery after photobleaching (FRAP) has been utilised to quantify the restoration of gap junctional coupling in a Cx30 KO mouse. The results indicate that this viral carrier affords a potential avenue to gene therapy of the inner ear.

Mechanoelectrical transduction, Ca²⁺ homeostasis and congenital hearing loss (collaboration with E. Carafoli)

Ca²⁺ enters stereocilia of hair cells through mechanoelectrical transduction (MET) channels opened by the deflection of the hair bundle, and is exported back to endolymph by an unusual splicing isoform (w/a) of the PMCA2 pump. Ablation or missense mutations of the pump cause deafness. A novel deafness-inducing missense mutation of PMCA2 (G293S), has been identified in a human family. The aim for the immediate future is to better characterize the activity of normal and mutated PMCA2wa in the Tommy and Oblivion mouse models. In parallel, we will study these mutants of the PMCA2wa in vitro using plasmids for heterologous expression in model cells.

Pursuing technological advances in the field of optical microscopy

In 2005, our group received a 12000 Euro prize in a regional competition for new business ideas. The Foundation for Advanced Biomedical Research subsequently created a spin-off company called Optical and Electronic Solutions (OES) S.r.l. that is now out on the market, trying to attract investors or venture capitalists. We are also considering the development of novel techniques, such as small optical fibers and microlenses (see for example Disseroth et al. J. Neurosci, 2006) that allow to investigate in the live animal, by minimally invasive surgery techniques, cells that are deeply located within the body.

Synoptic CV

Honours

** Selected Publications (VIMM)

** Other Publications (VIMM)

** Additional Publications

Selected Seminars

Contact

Fabio Mammano Venetian Institute of Molecular Medicine Via Orus 2 35129 Padua — Italy

Tel.:  (+39) 049 7923 231 Fax:  (+39) 049 7923 250

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