the role of islet1 in the development of the auditory and vestibular pathways
TRANSCRIPT
The Role of The Role of Islet1Islet1 in the in the Development of the Auditory Development of the Auditory
and Vestibular Pathwaysand Vestibular Pathways
Nicole DoddNicole Dodd
1515thth May 2014 May 2014Laboratory of Molecular PathogeneticsLaboratory of Molecular Pathogenetics
Institute of Biotechnology AS CRInstitute of Biotechnology AS CR
•Rationale
•Auditory and Vestibular systems
•Transcription factor Islet1
•Overexpression Model
•What we know so far..
•Results
•Molecular mechanisms?
•Conclusions thus far & future work
Overview
Rationale
• Unravel the role of Islet1 in neurosensory development
• Due to early KO lethality, role in neurosensory development is unclear
• Propose Islet1 molecular interactions
Inner Ear
Auditory apparatus - Cochlea
Both the bony labyrinth and the cochlear duct are coiled in a ‘snail-like’ shape
Vestibular system – vestibule & 3 semicircular canals
Structures dedicated to sense of balance
The Inner Ear: Auditory
•Hair cells transduce vibrations into receptor potentials•Inner hair cells communicate via the neurotransmitter glutamate
• A specific wave frequency causes fibres to rapidly vibrate
• Short waves (lower frequencies) at the basal part, long waves (higher frequencies) at the apical segments
The Inner Ear: Vestibular
3 major components:•Peripheral sensory apparatus•Central processor•Mechanism for motor output
Peripheral apparatus sends information to the CNS – specifically the vestibular nucleus and the cerebellum
The Vestibulocochlear Nerve VIII
The vestibulocochlear nerve enters the brainstem at the pontomedullary junction
• Each lobule receives different sensory input
• Lobules III, IV and V receive major afferent input through the ventral spinocerebellar tract
• Lobule X has extensive vestibular input
Cerebellum
Numbered I to X, rostral to caudal
• Represents 10% of brain volume, but contains over half the neurons
• One of the first structures to differentiate
• Coordination centre, fine tunes movement/balance
Transcription Factor Islet1
• Insulin Gene Enhancer Protein
• LIM domain for protein/protein interactions, homeodomain for binding to DNA control elements
• Expressed in multiple organs
• Expression studies suggest Islet1 plays a role in cell fate decision of neurosensory cells in the inner ear
• Islet1 KO is lethal at E10.5
• Exact role in neurosensory development
unknown
Pronuclear injection of Pax2-isl1 construct
Pax2 promoter
LIM
Homeodomain
DNA binding
Protein binding
Islet1
FVB
T2
T1 -/+
-/+
LIM
T1/52
T2/300
Pax2-Isl1 transgenic mouse: What we know so far…
• Reduced survival rate
• Circling phenotype consistent with vestibular dysfunction
• Reduced hearing, especially in mid-range
• Deaf by 6 monthsN
um
ber
of
pu
ps/
litte
r
Transgenic Islet1 expression at P3
Lobule X Islet1 Pax2
WT
Tg+/-
• Islet1 not endogenously expressed in the cerebellum
• White cells indicate transgenic Islet1 positive cells
• Pax2 endogenously expressed by a subset of cerebellar GABAergic interneurons and their precursors
• Pax2 expressed from E12 until the end of cerebellar development (postnatal day 15)
ABR data shows auditory defects
• Increase rate of hearing loss
• Hearing loss more profound in midrange frequencies
• More profound in the 6-9 month group
WT
Tg+/-
Nissl stain
A
B
Decrease in number of SG neurons
Calretinin staining of the Vestibular ganglion
WT Tg+/-
B
• No calretinin positive cells in Tg
• Apoptosis?
• Differentiation?
• Calcium regulation?
Decrease number of Unipolar Brush Cells
Lobule IX & X calretinin
WT
Tg+/-
• UBCs express the calcium binding protein calretinin
• Excitory glutamatergic interneuron found in the granule layer of cerebellar cortex
• Abundant in regions liked to vestibular input especially lobules X & a portion of IX
• Amplify inputs form vestibular ganglia and nuclei
Pax2 Calbindin
Decrease size of cerebellum
WT
Tg+/-
• Overall size reduced
• Disruption in Purkinje cell layer
• Purkinje cells control the output of the cerebellum
Lipophilic dye-tracing of neuronal fibres
• Each neuron is surrounded by a lipid bilayer that allows filling those profiles with lipophilic dyes
• Allows selective labelling of distinct fibre types• Multicolour dye approach to verify projections
otherwise indistinguishable• Antibody ‘stains’ can lead to false negatives in
mutant mice owing to limited presence of antigen
Professor Bernd Fritzsch, leading expert in developmental neuroscience & specialist in carbocyanine dyes
Lipophilic dye tracing
Ut
Ac Hc
CN
VN
Sa
Sg
FN
Vestibular > Cb & Bs
Ac
FN
Ut
Sa
AE
AE
Ig
Eff
Cerebella injection
WT
Tg+/-
VIIIth
Cb
Bs
FN
PC
Cb
VIIIth
S
S/I VG
PC
Bs
Inner ear injection Cerebellar injection
WT
Tg+/-
Lipophilic dye tracing
Molecular mechanisms?
• Unique combinations of LIM-HD TFs form a transcriptional ‘LIM Code’
• Contributes to cell type specification
• Calcium binding proteins in homeostasis- No calretinin expression in tg vestibular
ganglion- Decrease expression in UBCs- No expression difference in auditory system
Conclusions
•Peripheral - hearing defects
•Central - projection aberration
•Vestibular ganglion - calretinin negative
•Reduction in UBCs in the cerebellum
•Reduction in overall size
•Initial premature, more rapid differentiation in the presence of additional islet1
•Overexpression effects are context dependent
Future work
• Quantify calretinin expression and intensity profile
• Determine stage differences occur
• Neuronal fibre projection atP1 using dye-tracing vestibular to cerebellumE10.5 (tubulin ICH)
• qPCR quantify islet1 OE in inner ear and cerebellum
• RNDr. Gabriela Pavlínková, Ph.D. (Institute of
Biotechnology AS CR)
Ing. Romana Bohuslavová, Michaela Lišáková
• Prof. Bernd Fritzsch (University of Iowa)
• Prof. Josef Syka (Institute of Experimental Medicine AS
CR): Mgr. Tetyana Chumak, Dr. Daniela Buckiova,
Acknowledgements