Earlier, scientists had hypothesized that the stereocilia
remained rigid because they were supported by a sturdy
backbone made of a crystalline array of cross-linked
parallel filaments composed of the protein actin. Actin is
a robust protein.
NIDCD's investigators followed the preferential
localization of a specific type of actin in the stereocilia
to determine the locus of actin polymerization.
Polymerization is a standard chemical process whereby
individual molecules known as "monomers" self-associate to
form large and often regular aggregates, like the
naturally-occurring silkworm polymers. Using a fluorescent
tag, a method used by molecular biologists to track newly
synthesized proteins in the cell, these NIDCD scientists
demonstrated that the sensory stereocilia of hair cells are
continuously being renewed by a process driven by actin
polymerization and treadmilling at the core of each
Although other scientists have identified these actin
filament bundles as uniformly ordered in the stereocilia,
"Dr. Kachar and his team have seen the remodeling of these
filament bundles by addition of new actin monomers at the
tips of the stereocilium and, for the first time, witnessed
their renewal every 48 hours," notes James F. Battey, Jr.,
M.D., Director of the NIDCD.
The discovery, in an animal model closely related to human
functioning, of this self-renewal has much broader
implications, in that the stereocilium is one of group of
cellular organelles that share a structural backbone of
cross-linked parallel actin filaments formed into a dense
semi crystalline filament bundle. Not only is this an
astounding contribution to auditory and structural
biological research, but it has direct implications for
those who are studying similar cellular organelles
including "Drosophila's" bristles, nurse cell struts,
fertilization cones, and spermatozoa acrosomal structures.
The work described in this week's issue of Nature is being
followed up with additional studies to determine how the
stereocilia can counterbalance the downward movement of the
treadmill. The team proposes that activity from myosins,
mechano-enzymes capable of moving cargo along actin
filaments would allow the membrane associated components of
the stereocilia to remain structurally stable and ready for
mechanical deflection. The limberness of the stereocilia is
key to their ability to respond to deflection and provide
precise information that is interpreted by the brain as the
range of sound.
Dr. Kachar and his team have gained a clearer view of how
hearing is maintained under normal circumstances. They have
also been given new insight into the recovery of hearing
after some instances of noise exposure as well as
information that could help understand the molecular basis
of several genetic, environmental and age-related inner ear
disorders that involve either malformation or the
disruption of stereocilia plasticity.
To schedule an interview with Dr. Kachar or Dr. Battey,
please, call 301-496-7243.###
NIDCD Press Office:
Marin P. Allen, Ph.D.