Knockout mice: Is it just genetics? Effect of enriched housing on fibulin-4+/- mice

Background. Fibulin-4 is an extracellular matrix protein expressed by vascular smooth muscle cells that is essential for maintaining arterial integrity. Fibulin-4-/- mice die just before birth due to arterial hemorrhage, but fibulin-4+/- mice appear to be outwardly normal. Experiments were therefore performed to determine whether fibulin-4+/- mice display arterial pathologies on a microscopic scale. After preliminary experiments were performed, a second purpose developed, which was to test the hypothesis that any observed pathologies would be ameliorated by housing the animals in enriched cages. Methodology. Fibulin-4+/- and wild-type mice were housed either four/cage in standard cages or two per cage in larger cages, each cage containing a tunnel and a wheel. After three weeks the mice were sacrificed, and the aortas perfusion-fixed and excised for light and electron microscopy. Principle Findings. When the mice were in standard cages, localized regions of disorganized extracellular matrix and collagen fibers consistently appeared between some of the medial smooth muscle cells in the fibulin-4+/- mice. In the wild-type mice, the smooth muscle cells were closely connected to each other and the media was more compact. The number of disorganized regions per square mm was significantly greater for fibulin-4+/- mice (172±43 (SEM)) than for wild-type mice (15±8) (p<0.01, n = 8). When the mice were in enriched cages, the fibulin-4+/- mice showed significantly fewer disorganized regions than those in standard cages (35±12) (p<0.05, n = 8). The wild type mice also showed fewer disorganized regions (3±2), but this difference was not significant. Conclusions. These results indicate that arterial pathologies manifested in fibulin-4+/- mice can be reduced by enriching the housing conditions, and imply that appropriate environments may counteract the effects of some genetic deficiencies

Wild Type Mice, Number of Gaps per square mm (Standard versus Enriched Housing)

<p>No significant difference (Statistical significance obtained using Mann-Whitney Rank Sum Test)</p

Enriched Housing (two per cage), Number of Gaps per square mm (Wild Type Mice versus Fibulin-4^+/− Mice)

<p>Statistical significance, p = 0.008 (Statistical significance obtained using Mann-Whitney Rank Sum Test)</p

Figure 2

<p>Electron micrographs of transverse ultra-thin sections of aortas from fibulin-4^+/− mice housed in a standard cage (panel A) and an enriched cage (panel B).</p

Standard Housing (four per cage), Number of Gaps per square mm (Wild Type Mice versus Fibulin-4^+/− Mice)

<p>Statistical significance, p = 0.006 (Statistical significance obtained using Mann-Whitney Rank Sum Test)</p

Fibulin-4^+/− Mice, Number of Gaps per square mm (Standard versus Enriched Housing)

<p>Statistical significance, p = 0.016 (Statistical significance obtained using Mann-Whitney Rank Sum Test)</p

Figure 3

<p>Electron micrographs of transverse ultra-thin sections of aortas from wild-type mice housed in a standard cage (panel A) and an enriched cage (panel B).</p