EYA1-SIX1 complex in neurosensory cell fate induction in the mammalian inner ear

Abstract

The phosphatase-transactivator EYA1 interacts with the homeodomain protein SIX1 to form transcriptional activation complexes, which play essential roles in regulating cell proliferation, survival and induction of sensory and neuronal differentiation programs during inner ear development. Mutations of the Eya1 and Six1 genes cause profound developmental auditory defects in mice and humans. The molecular mechanisms and developmental processes controlled by the EYA1 and SIX1 complex in inner ear development and neurosensory fate induction are the focus of this review.

Fig. 1.

The EYA-SO/SIX regulatory network. (A) The transcriptional hierarchy among the key regulators in the fly eyes. Feedback loops and protein–protein interactions (double-headed arrows) are indicated. (B) EYA and SIX form transcriptional activation complexes to control cell proliferation and survival as well as to trigger particular differentiation programs. EYA–SIX also interact with other proteins (×), which may vary depending on distinct developmental and cellular contexts to modify the specificity of EYA–SIX function.

Fig. 2.

Schematic drawing of Eya1 and Six1 expression domains in relation to Sox2 expression in the otocyst and developing cochlea, cristae and maculae and their possible roles in sensory cell development. In the otocyst, Eya1 (red), Six1 (red) and Sox2 (green) are co-expressed in the ventral otocyst where the prosensory epithelia are formed. These genes may act together to specify a common prosensory area in the otocyst at this early stage by turning on other sensory genes including Jag1, Lfng, Bmp4 and Fgfs, which are not expressed in Eya1- or Six1-null embryos. In the developing cochlea, at E12.5, the floor of cochlear duct will give rise to the organ of Corti (oc), the greater epithelial ridge (GER) and lesser epithelial ridge (LER). Eya1 and Sox2 are coexpressed in the prosenory progenitors of the OC, and this colocalization suggests that both genes may act together to regulate the cell cycle of the prosensory progenitors. Six1 expression disappears in the postmitotic progenitors but reappears in differentiating hair cells in a pattern similar to that of Atoh1. Once the sensory progenitor cells become postmitotic, they express p27Kip1, and at around E13.5–14.5, Atoh1 is activated in the postmitotic progenitors that become committed to hair cell fate and is coexpressed with Eya1 and Six1 in the differentiating hair cells. However, Sox2 expression becomes restricted to supporting cells and border cells in the GER. Based on published data, Eya1, Six1 and Sox2 act together to regulate hair cell commitment by activating Atoh1; however, Eya1 and Six1 but not Sox2 act to maintain or upregulate Atoh1 expression during hair cell differentiation. Similar expression pattern of these genes is found in the sensory cell development of the cristae and maculae.

Fig. 3.

EYA1–SIX1 complex in inducing hair cell fate. (A) Molecular relationships among the key transcription factors for hair cell differentiation. A direct interaction between EYA1/SIX1/SOX2 proteins coordinately regulates Atoh1 expression, and that POU4F3 is a common downstream factor of the Atoh1-dependent and -independent pathways. Dashed lines indicate that SOX2 may repress Pou4f3 or downstream factors of Pou4f3 to inhibit hair cell differentiation. (B) Possible mechanisms for Atoh1 activation by Sox2/Eya1/Six1. EYA1/SIX1 in collaboration with SOX2 activity in prosensory progenitors can induce Atoh1 activation via direct binding to the Sox and Six binding sites within enhancer A and B, respectively. These three factors may directly interact (model a), or EYA1 may bridge SIX1 and SOX2 (model b). These three factors may also form an active complex to regulate Atoh1 activation via enhancer A (model c), whereas EYA1/SIX1 efficiently upregulate Atoh1 via enhancer B. Question mark indicates that the involvement of the factor is unclear.

Fig. 4.

Possible mechanism for Neurog1 and Neurod1 activation by Eya1/Six1. EYA1/SIX1 in collaboration with the SWI/SNF complex in progenitor cells induce Neurog1—Neurod1 activation. Eya1/Six1 also interact with Neurog1 to regulate Neurod1 activation. In addition, Eya1/Six1 interact with Neurod1 to regulate neuronal differentiation. Sox2 works cooperatively with Eya1/Six1 or the SWI/SNF complex to promote neuronal differentiation. These factors are physically associated.