Andres Collazo

Voluntary Faculty at the USC School of Medicine,
Otolaryngology

Section Chief for Inner Ear Development,
Division of Cell Biology and Genetics,
House Ear Institute

Visiting Faculty in Biology,
California Institute of Technology

Research Topics

1. Determine which molecules and regions of the otic placode are required for the normal patterning of the developing inner ear.
2. Understand at the molecular level how placodal cells choose between neuronal, nonsensory or sensory cell fates.
3. The role of the transcription factor Six1 in the formation of mechano-sensory hair cells and neurons.

Research Overview

Sensorineural hearing loss (SNHL) is one of the more common birth defects and approximately 20% of these patients have inner ear malformations that are readily visible using radiological examination. What causes these inner ear malformations and why they should lead to SNHL, as well as balance disorders, is often unknown. The laboratory’s research is divided between two model systems for developmental studies, the frog Xenopus laevis and the zebrafish Danio rerio. The main goal of the frog research is to determine which molecules and regions of the otic placode are required for the normal patterning of the developing inner ear in order to better understand the causes of human inner ear malformations. The goal of our zebrafish studies is to understand at the molecular level how placodal cells choose between neuronal, nonsensory or sensory cell fates. Our research combines embryological manipulations with molecular perturbations of specific cell signaling pathways and single cell lineage analyses, to provide unique insights into how the inner ear is patterned during embryonic development.

We discovered that ablating half of the placode or otocyst along the anterior-posterior (A-P) axis, in the frog Xenopus, results in a high percentage of mirror image duplicated inner ears. The regenerated mirror half is derived from the remaining placode and not surrounding tissues. In zebrafish, the only gene mutations that result in mirror duplicated inner ears are those belonging to the Hedgehog (Hh) signaling pathway. In contrast, mice use Hh and another secreted signaling molecule, Wnt (an amalgam of wingless and int-1, the first mammalian Wnt identified), for dorsal-ventral (D-V) patterning. Blocking Hh signaling in Xenopus, as seen in zebrafish, results in 2 mirror image anterior halves and suggests that Hh signaling is necessary for posterior patterning. The ability to generate mirror duplicated inner ears provides an assay for studying the molecules and regions of the developing inner ear that are required for normal patterning.

Recently, genes belonging to the Pax-Six-Eya-Dach gene regulatory network, well characterized in fly eye development, have been implicated in the development of the inner ear and its ganglion. We have found that the zebrafish homeodomain containing transcription factor six1 (homologous to fly sine oculis) differentially affects hair cell versus neuronal fate in developing ears and is the first gene identified to do so. Six1 gene function is knocked down using antisense morpholino oligonucleotides or increased by injecting mRNA at the single cell stage. Six1 loss of function results in fewer hair cells and more SAG neurons while six1 gain of function results in more hair cells and fewer neurons. The effects of six1 on hair cells and neurons is already apparent when these cells first differentiate suggesting an early role in the development of these two inner ear cell lineages. We are also assaying how mutations in the human SIX1 gene may be functioning at the transcriptional level by introducing these mutations in the highly conserved zebrafish six1 gene and assaying their effects on hair cells and neurons in vivo. At least three mutations of human SIX1 have been identified that lead to branchio-oto-renal (BOR) and branchio-otic (BO) syndromes.