Abstract
Purpose::
Tropomodulin1 (Tmod1) binds tropomyosin (TM) and caps actin filaments in the lens fiber cell plasma membrane skeleton. To investigate the function of Tmod1 in the mouse lens, we analysed the consequences of genetic deletion of Tmod1 for fiber cell morphology, organization and lens function.
Methods::
Tmod1 null mouse lenses were obtained from a Tmod1 knockout mouse in which the embryonic lethality of the homozygous null mice was rescued by expression of a Tmod1 transgene in the heart. Lens development and morphology were analysed by immunofluorescence staining of cryosections, and levels of Tmod1, TMs, actin and membrane skeleton proteins by biochemical fractionation and western blotting. Lens fiber cell morphology and packing organization was analysed by thin section electron microscopy (TEM) and scanning electron microsopy (SEM), and lens optical properties by laser scan analysis.
Results::
Tmod1 is expressed in cortical but not nuclear fiber cells of mouse lenses, where it is predominantly associated with fiber cell lateral membranes along their broad and narrow sides. In the absence of Tmod1, lens embryonic and postnatal development is normal, including timing and location of organelle loss. By 4-7 months of age, lenses lacking Tmod1 demonstrate disordered anterior fiber cell layer packing, with abnormal cell shapes and sizes and gaps between cells, as well as grossly disorganized membrane protrusions along the fiber cell lateral membranes. The optical quality of Tmod1 null lenses from older mice is compromised based on increased back focal length and back focal length variability relative to wild-type lenses. Western blotting shows levels of a short gamma-TM isoform are decreased, while levels of a long TM isoform are unchanged in the absence of Tmod1. Levels of membrane skeleton proteins, beta2-spectrin, ankyrinB, and alpha-adducin, are also reduced in the absence of Tmod1, while levels of N-cadherin, beta-catenin, ezrin, and MIP26 are not.
Conclusions::
The absence of Tmod1 leads to aberrant fiber cell membrane protrusions, abnormal fiber cell shapes and disordered packing in adult mouse lenses, resulting in an age-dependent loss of optical quality. Tmod1 may be required to stabilize gamma-TM-coated actin filaments, promoting assembly and stability of the spectrin-based membrane skeleton, thereby maintaining normal fiber cell morphology and interactions.
Keywords: cytoskeleton • cell adhesions/cell junctions • cataract