Determination of Cardiac Histologic Injury in a Transgenic Mouse Model of Heart Failure Expressing a Novel Mitochondrial Progesterone Receptor (PR-M)

1. To develop an automated histological fibrosis quantification protocol. 2. To determine cardiac histologic injury in a transgenic mouse model of heart failure expressing a human mitochondrial progesterone receptor (PR-M). Transgenic mouse study. PR-M is mitochondrial progesterone receptor isoform that increases cellular respiration. An inducible (TET-On) transgenic mouse model expressing human PR-M under the control of the cardiac myosin heavy chain 6 promoter was developed. PR-M expression was induced with oral doxycycline followed in 2 wks by surgical constant thoracic aortic constriction (cTAC) to increase after-load. Intact male and ovariectomized female mice were treated with progesterone in oil (2.5 mg/day SQ) starting 1 wk prior to cTAC. Mice were sacrificed 8 wks after cTAC and cardiac PR-M expression determined with realtime RT-PCR. Cardiac fibrosis and capillaries were identified with Masson's trichrome and immunostaining with lectin respectively, by an investigator blinded to gene expression and treatment. Histologic images were acquired via digital slide scanning. Morphometrics were computed with ImageJ64 (NIH) and Photoshop CS5 (Adobe Systems). Data from the histologic quantification were analyzed with unpaired Student's t-test. Progesterone treated PR-M positive mice (N = 9) showed greater capillary density (P=0.04), greater capillary density to cardiomyocyte width ratio (P=0.02) and decreased cardiac fibrosis (P=0.03) after cTAC compared to progesterone treated PR-M negative mice (N = 17). The mean myocyte width was not different between the 2 groups (P=0.33). A novel computerized fibrosis quantification method for cardiac tissue was developed. Compared to controls, progesterone treated transgenic mice expressing a human mitochondrial progesterone receptor (PR-M) had fewer histologic signs of cardiac injury. These observations support a role for progesterone via a mitochondrial progesterone receptor in cardiac function.