MOLECULAR BIOLOGY OF THE EXTRACELLULAR MATRIX 
Jeffrey F. Bonadio, M.D., Assistant Investigator 
The long-term goal of Dr. Bonadio's research ef- 
fort is to understand how the extracellular matrix 
contributes to the structure and function of tissues. 
The extracellular matrix evolved to protect cells 
and hold them in spatial arrangements required for 
anatomical and physiological functions. The extent 
and organization of the matrix at different anatomi- 
cal sites is a reflection of the specialized functions 
of cells and their interactions with the environment. 
In the adult, the matrix plays a role in wound heal- 
ing and contributes to the ability of organs to bear 
mechanical loads, e.g., as occur in the musculoskel- 
etal system during locomotion, the uterus during 
parturition, the lung during respiration, and the car- 
diovascular system during circulation. 
Dr. Bonadio's laboratory is investigating the mo- 
lecular basis of load bearing in the mammalian skele- 
ton. Type I collagen is particularly abundant in 
skeletal tissues and may represent a primary load- 
bearing element. Dr. Bonadio and his colleagues ini- 
tially sought to define the mechanical role of colla- 
gen by creating genetic deletions in the germline of 
mice. Afoi^ mouse strains were generated in the labo- 
ratory of Dr. Rudolf Jaenisch by exposing mouse em- 
bryos to the Moloney murine leukemia virus. In 
Movl3 mice the retrovirus integrated within the 
first intron of the «1 (I) collagen gene. With the ex- 
ception of embryonic odontoblasts and a small sub- 
population (~5%) of osteoblasts, the proviral insert 
prevents initiation of transcription at the collagen 
locus. Mice homozygous for the null mutation pro- 
duce no type I collagen and die in utero because of 
the decreased structural integrity of cardiovascular 
tissues. In heterozygous Movl3 mice, the block in 
transcription initiation is associated with a 50% re- 
duction in type I collagen production by connective 
tissue cells and a 50% decrease in tissue collagen 
content. Dr. Bonadio and his colleagues found that 
the collagen deficiency is associated with hearing 
loss and with morphological and functional connec- 
tive tissue defects. 
A subsequent analysis of the transgenic mouse 
strain Movl3 included a successful attempt to res- 
cue the collagen-deficient phenotype. These exper- 
iments argue convincingly that type I collagen nor- 
mally contributes to the relative ductility of cortical 
bone, an important property that allows repeated 
loading without macroscopic failure. By contribut- 
ing to ductility, type I collagen allows bone to be 
used repeatedly and yet avoid macroscopic failure: 
following the application of a large mechanical 
load, bone will plastically deform before failure be- 
cause of its collagen content. Type I collagen is 
highly conserved in evolution, and one reason may 
be the fundamental contribution it makes to the ma- 
terial properties of the skeleton. The latter observa- 
tion was dramatically underscored by the fact that 
the M0VI3 phenotype was successfully comple- 
mented by the human «1 (I) collagen gene. 
Dr. Bonadio's laboratory currently is attempting 
to generate a mouse strain deficient in fibrillin, an 
extracellular matrix molecule that participates in 
the formation of ubiquitous 10- to 12-nm microfi- 
brils. Members of the laboratory have cloned and 
characterized the full-length cDNA sequence of a 
mouse fibrillin gene. Preliminary evidence suggests 
that there are at least two other members of the 
mouse fibrillin gene family. With colleagues the lab- 
oratory is working to generate fibrillin-deficient 
mouse strains by targeted disruption of the genetic 
locus, using the technique of homologous recombi- 
nation in embryonic stem cells. If this work is suc- 
cessful, founder mice will then be used to breed 
mutant strains that are either homozygous or hetero- 
zygous for the nonfunctional fibrillin allele. The 
mutant mice will be analyzed in a manner similar to 
M0VI3 mice. This project was supported in part by a 
grant from the National Institutes of Health. 
Dr. Bonadio and his colleagues recently demon- 
strated that small changes in skeletal geometry can 
lead to a significant improvement in mechanical 
function. Over a two-month period, cortical bone 
cells of the mouse femur were stimulated to synthe- 
size, deposit, and mineralize a small amount of new 
bone along periosteal surfaces. The deposition of 
new bone increased cross-sectional geometry, and 
this increase in turn led to a dramatic increase in 
strength. This work suggests that regulation of the 
pattern of gene expression in osteogenic cells may 
represent a rational approach to treating skeletal fra- 
gility, an important problem in industrialized coun- 
tries. The goal of this therapy would be to 
strengthen long bone by altering its geometry ac- 
cording to a rational design. This project was sup- 
poned in part by a grant from the National Institutes 
of Health. 
The laboratory has sought to test the validity of 
this therapeutic strategy in animal model systems. 
Toward this end the laboratory has identified several 
candidate genes whose products will stimulate the 
metabolism of periosteal bone cells, and the appro- 
priate gene transfer constructs have been prepared. 
CELL BIOLOGY AND REGULATION 29 
