Aging Kit Mutant Mice Develop Cardiomyopathy

Both bone marrow (BM) and myocardium contain progenitor cells expressing the c-Kit tyrosine kinase. The aims of this study were to determine the effects of c-Kit mutations on: i. myocardial c-Kit+ cells counts and ii. the stability of left ventricular (LV) contractile function and structure during aging. LV structure and contractile function were evaluated (echocardiography) in two groups of Kit mutant (W/Wv and W41/W42) and in wild type (WT) mice at 4 and 12 months of age and the effects of the mutations on LV mass, vascular density and the numbers of proliferating cells were also determined. In 4 month old Kit mutant and WT mice, LV ejection fractions (EF) and LV fractional shortening rates (FS) were comparable. At 12 months of age EF and FS were significantly decreased and LV mass was significantly increased only in W41/W42 mice. Myocardial vascular densities and c-Kit+ cell numbers were significantly reduced in both mutant groups when compared to WT hearts. Replacement of mutant BM with WT BM at 4 months of age did not prevent these abnormalities in either mutant group although they were somewhat attenuated in the W/Wv group. Notably BM transplantation did not prevent the development of cardiomyopathy in 12 month W41/W42 mice. The data suggest that decreased numbers and functional capacities of c-Kit+ cardiac resident progenitor cells may be the basis of the cardiomyopathy in W41/W42 mice and although defects in mutant BM progenitor cells may prove to be contributory, they are not causal

Mouse LV functional studies.

<p>(A) ejection fraction, and (B) fractional shortening of mice at both young (4 months) and old (12 months) age. Only W41/W42 mice showed impaired LV function at 12 months of age. *: p<0.05 vs age-matched WT mice. #: p<0.05 vs same group at 4 months of age.</p

Impaired c-Kit⁺ cardiac progenitor cell pool in Kit mutant mice.

<p>Shown in panels (A) to (C) are representative immuno-stained images from hearts of WT, W/Wv, and W41/W42 mice, respectively. Quantification of c-kit⁺ cardiac progenitor cells based on immuno-staining images is shown in panel (D). *: p<0.01 vs WT mice.</p

Dual fluorescent immunostaining for vWF-8 and SMA expression.

<p>Representative vascular staining pictures of WT (A&B), W/Wv (C&D), and W41/W42 (E&F) myocardium. (G) Quantification of blood vessel number based on vWF-8 and SMA staining indicated that mutant hearts had significantly fewer capillaries and arteioles than did WT mice, BM transplantation did not increase capillary or arteriolar numbers in any group. *: p<0.05 vs vWF-8 of WT; #: p<0.0 vs vWF-8 of WT+BM; ‡: p<0.05 vs SMA of WT.</p

Effects of BM transplantation on c-kit⁺ cardiac progenitor cell pool of Kit mutant mice.

<p>Shown in panels (A) and (B) are representative c-kit staining of BM transplanted hearts from W/Wv and W41/W42 mice, respectively. Quantification of c-kit⁺ cell pool based on immuno-staining is shown in panel (C). Bone marrow transplantation did not fully restore the impaired c-Kit⁺ cell pool (*: p<0.05 vs WT). Nevertheless, W/Wv mice (but not W41/W42 mice) modestly increased of c-Kit+ cell numbers in response to BM transplantation (#: p<0.05 vs W/Wv).</p

W41/W42 mutant mice had severe cardiac hypertrophy at 12 months of age.

<p>(A) Typical H & E staining pictures of mouse heart tissue sections. (B) The mean cross sectional area of cardiomyocytes in W41/W42 mice was significantly increased. (C) Heart weight/body weight (HW/BW) measurements of W41/W42 hearts were also significantly increased. * p<0.05 vs WT mice.</p