In the journal Cell, researchers report the serendipitous discovery that the initial development of fat requires a "remodeling" protein that must first make way for the lipid-laden tissue's growth. An enzyme that "chews" through collagen fibers sculpts the matrix surrounding fat cells, allowing them to expand and mature, the team reports.
In the absence of the collagen-busting "membrane-anchored metalloproteinase" (MT1-MMP), would-be fat cells, or preadipocytes, fail to break through the fibrous extracellular matrix, they found. Moreover, the cells entrapment within a dense collagen meshwork disrupts their structure. That structural change, in turn, stalls the genetic programs critical for full-blown fat development, leading to sweeping changes in gene activity.
In contrast, they report, fat cells deficient for MT1-MMP develop apparently normally when placed on the flat surface of a laboratory dish.
"When small fat cells embedded in what is essentially a 3D molecular cage receive the appropriate external signals during development, it appears that they change their geometry--remodeling the surrounding matrix using an enzyme to chew the fibers," said the study's senior author Stephen Weiss of the University of Michigan, Ann Arbor.
For reasons that are not entirely clear, that shape change leads to a dramatic shift in gene expression and cell behavior, he added. In the absence of the collagen-rich 3-dimensional matrix, the MT1-MMP enzyme apparently becomes irrelevant.
The study identifies MT1-MMP as a "heretofore unsuspected 3-dimensional matrix specific regulator of white adipose tissue development and function," the researchers said. White adipose tissue is a type of fat that serves as a primary depot for energy stores.
Moreover, the findings in developing mice raise the possibility that the enzyme might also play an important role in the remodeling of extracellular barriers in adult animals as they gain or burn fat, he said.
In the beginning, the study's lead author Tae-Hwa Chun, also of the University of Michigan, wasn't particularly interested in fat, Weiss noted. Rather, he had set out to examine the effects of MT1-MMP on blood vessels in mice deficient for the enzyme.
Chun soon noticed, however, that the mice seemed to have an unexpected lack of white adipose tissue. Indeed, they later confirmed that the only white fat the mice had consisted of abnormally small "mini-adipocytes."
Weiss first suspected the mutant mice were simply malnourished as a result of their other deficiencies. However, the seemingly normal quantities of heat-generating brown fat retained by the developing mice challenged that notion.
To get to the root of the problem, the team screened the activity of genes in the white fat cells of normal mice versus those deficient for MT1-MMP.
"That was our first really big surprise," Weiss said. "From the loss of a single enzyme, we expected focused changes in gene expression. Instead, we saw broad changes in many, many genes."
The finding raised a big question, he said: "Why would a little cutting enzyme have such a grand effect on seemingly unrelated genes?"
They next isolated undifferentiated preadipocytes from mice lacking the collagenase in an attempt to zero in on the defects. In the 2-dimensional environment of the laboratory dish, however, the fat precursors turned into mature fat cells "with no problem," leading the team to question whether the defect truly resided within the fat cells or was perhaps a result of deficiencies in neighboring cells, such as blood vessels or nerves, Weiss said.
When the researchers transplanted the mutant preadipocytes into otherwise normal mice, however, their defect again surfaced.
The findings confirmed that the problem was "cell autonomous," Weiss said, meaning that it stemmed from the fat cells themselves. Indeed, microscopic images of white fat tissue taken from the mutant mice revealed a marked increase in collagen, with levels elevated by more than 10-fold.
Further in vitro study of preadipocytes confirmed that normal fat formation within the context of a 3-dimensional extracellular matrix requires a burst in MT1-MMP activity that relaxes surrounding collagen.
"In 3-D culture, we posit that in the absence of establishing an optimal cell shape and level of tension with the surrounding matrix of native and cleaved collagen, signaling cascades critical to the normal fat-generating program fail to engage properly," the researchers wrote. "Interestingly, despite a required role for MT1-MMP in regulating collagen in white adipose tissue, brown fat was spared, presumably as a consequence of its low collagen content," they added.
The findings in young mice might have additional implications for changes in fat composition later in life as animals lose or gain weight, the researchers suggest.
The MT1-MMP collagenase is known to be regulated after birth in response to fat intake, they noted. Furthermore, high-fat diets have been reported to induce local losses in the collagen content of affected fat pads, and mice expressing low levels of collagen can display enlarged fat cells.
As such, they suggest, MT1-MMP may well act as a protein-degrading "rheostat," controlling the activity and function of genes in white fat tissue throughout life by remodeling surrounding extracellular matrix barriers as adipocytes accumulate or break down lipids in response to changing energy demands.
The researchers plan to test the idea in older mice with reduced, rather than absent, MT1-MMP levels. The question would be whether such animals, when fed a high-fat diet, would have trouble gaining weight, Weiss said.
The new findings might also point to a general paradigm, Weiss added. "Many cell types embedded in extracellular networks, including cancer cells, might have to remodel in order to control their gene expression profiles," he said.