Janet M. Boggs

Pressure Overload–Induced Alterations in Fibrillar Collagen Content and Myocardial Diastolic Function Role of Secreted Protein Acidic and Rich in Cysteine (SPARC) in Post–Synthetic Procollagen Processing

Background— Chronic pressure overload causes myocardial hypertrophy, increased fibrillar collagen content, and abnormal diastolic function. We hypothesized that one determinant of these pressure overload–induced changes is the extracellular processing of newly synthesized procollagen into mature collagen fibrils. We further hypothesized that secreted protein acidic and rich in cysteine (SPARC) plays a key role in post–synthetic procollagen processing in normal and pressure-overloaded myocardium. Methods and Results— To determine whether pressure overload–induced changes in collagen content and diastolic function are affected by the absence of SPARC, age-matched wild-type (WT) and SPARC-null mice underwent either transverse aortic constriction (TAC) for 4 weeks or served as nonoperated controls. Left ventricular (LV) collagen content was measured histologically by collagen volume fraction, collagen composition was measured by hydroxyproline assay as soluble collagen (1 mol/L NaCl extractable) versus insoluble collagen (mature cross-linked collagen), and collagen morphological structure was examined by scanning electron microscopy. SPARC expression was measured by immunoblot. LV, myocardial, and cardiomyocyte structure and function were assessed by echocardiographic, papillary muscle, and isolated cardiomyocyte studies. In WT mice, TAC increased LV mass, SPARC expression, myocardial diastolic stiffness, fibrillar collagen content, and soluble and insoluble collagen. In SPARC-null mice, TAC increased LV mass to an extent similar to WT mice. In addition, in SPARC-null mice, TAC increased fibrillar collagen content, albeit significantly less than that seen in WT TAC mice. Furthermore, the proportion of LV collagen that was insoluble was less in the SPARC-null TAC mice (86±2%) than in WT TAC mice (99±2%, P<0.05), and the proportion of collagen that was soluble was greater in the SPARC-null TAC mice (14±2%) than in WT TAC mice (1±2%, P<0.05) As a result, myocardial diastolic stiffness was lower in SPARC-null TAC mice (0.075±0.005) than in WT TAC mice (0.045±0.005, P<0.05). Conclusions— The absence of SPARC reduced pressure overload–induced alterations in extracellular matrix fibrillar collagen and diastolic function. These data support the hypothesis that SPARC plays a key role in post–synthetic procollagen processing and the development of mature cross-linked collagen fibrils in normal and pressure-overloaded myocardium.

Thromboxane A2 receptor density increases during chronic exposure to thromboxane A2 receptor antagonists after porcine carotid bypass

Domestic swine (n=12 in each group) were randomized to daily treatment with the thromboxane A2 (TXA2) receptor antagonist BMS-180291 (group I), aspirin (group II), or no drug (group III) prior to prosthetic carotid graft implantation. Platelet and arterial wall receptor density were measured by equilibrium binding using 125I-BOP. At 6 weeks, means (s.e.m.) platelet receptor density (pmol/mg) had increased in groups I (3.3(0.6) versus 1.8(0.3); P<0.05) and II (2.6(0.6) versus 1.7(0.2); P<0.05), but not in group III (1.3(0.3) versus 1.2(0.2)). Aortic membrane TXA2 receptor density (fmol/mg) was significantly greater (P<0.05) in groups I (150(50)) and II (68(10)) compared with group III (39(6)). Chronic exposure to a TXA2 receptor antagonist or aspirin is associated with increased platelet and aortic receptor density in pigs.

The role of endoplasmic reticulum stress in hippocampal insulin resistance

Metabolic syndrome, which includes hypertension, hyperglycemia, obesity, insulin resistance, and dyslipidemia, has a negative impact on cognitive health. Endoplasmic reticulum (ER) stress is activated during metabolic syndrome, however it is not known which factor associated with metabolic syndrome contributes to this stress. ER stress has been reported to play a role in the development of insulin resistance in peripheral tissues. The role of ER stress in the development of insulin resistance in hippocampal neurons is not known. In the current study, we investigated ER stress in the hippocampus of 3 different mouse models of metabolic syndrome: the C57BL6 mouse on a high fat (HF) diet; apolipoprotein E, leptin, and apolipoprotein B-48 deficient (ApoE 3KO) mice; and the low density lipoprotein receptor, leptin, and apolipoprotein B-48 deficient (LDLR 3KO) mice. We demonstrate that ER stress is activated in the hippocampus of HF mice, and for the first time, in ApoE 3KO mice, but not LDLR 3KO mice. The HF and ApoE 3KO mice are hyperglycemic; however, the LDLR 3KO mice have normal glycemia. This suggests that hyperglycemia may play a role in the activation of ER stress in the hippocampus. Similarly, we also demonstrate that impaired insulin signaling is only present in the HF and ApoE 3KO mice, which suggests that ER stress may play a role in insulin resistance in the hippocampus. To confirm this we pharmacologically induced ER stress with thapsigargin in human hippocampal neurons. We demonstrate for the first time that thapsigargin leads to ER stress and impaired insulin signaling in human hippocampal neurons. Our results may provide a potential mechanism that links metabolic syndrome and cognitive health.

Preoperative thromboxane A2/prostaglandin H2 receptor activity predicts early graft thrombosis

PURPOSE This study was carried out to determine whether early failure of infrainguinal bypass grafts is associated with increased expression of platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors. A prospective correlation of preoperative platelet TXA2/PGH2 receptor-mediated activity with lower extremity graft patency was sought. METHODS Twenty-five patients who underwent infrainguinal bypass surgery for limb salvage were studied at an inpatient academic tertiary referral center and Department of Veterans Affairs Medical Center. Outcome measures were primary graft patency rate at 3 months, platelet TXA2/PGH2 receptor activity by equilibrium binding with 125I-BOP, and aggregation to the TXA2-mimetic U46619. RESULTS Preoperative platelet TXA2/PGH2 receptor density was higher (Bmax, 3100 +/- 1300 vs 1500 +/- 1100 sites/platelet [mean +/- SD]; p = 0.004) in the five patients who had graft thrombosis within 3 months. The EC50 for U46619 was lower (26 +/- 6 nmol/L vs 57 +/- 30 nmol/L; p < 0.05) in these patients as well, confirming the functional effect of the increased receptor density. Early graft thrombosis was more likely in patients with a platelet TXA2/PGH2 receptor density greater than 3000 sites/platelet (odds ratio, 76; 95% confidence interval, 3.9 to 1500) or an EC50 for U46619 less than 30 nmol/L (odds ratio, 16; 95% confidence interval, 1.4 to 180). CONCLUSIONS Elevated platelet TXA2/PGH2 receptor levels and enhanced sensitivity of platelet aggregation to TXA2 predict early arterial graft thrombosis. Specific TXA2/PGH2 receptor antagonism may prevent one of the mechanisms that contributes to early graft occlusion.

Pressure-Overload Induced Alterations in Fibrillar Collagen Content and Myocardial Diastolic Function: Role of SPARC in Post-Synthetic Procollagen Processing

Background— Chronic pressure overload causes myocardial hypertrophy, increased fibrillar collagen content, and abnormal diastolic function. We hypothesized that one determinant of these pressure overload–induced changes is the extracellular processing of newly synthesized procollagen into mature collagen fibrils. We further hypothesized that secreted protein acidic and rich in cysteine (SPARC) plays a key role in post–synthetic procollagen processing in normal and pressure-overloaded myocardium. Methods and Results— To determine whether pressure overload–induced changes in collagen content and diastolic function are affected by the absence of SPARC, age-matched wild-type (WT) and SPARC-null mice underwent either transverse aortic constriction (TAC) for 4 weeks or served as nonoperated controls. Left ventricular (LV) collagen content was measured histologically by collagen volume fraction, collagen composition was measured by hydroxyproline assay as soluble collagen (1 mol/L NaCl extractable) versus insoluble collagen (mature cross-linked collagen), and collagen morphological structure was examined by scanning electron microscopy. SPARC expression was measured by immunoblot. LV, myocardial, and cardiomyocyte structure and function were assessed by echocardiographic, papillary muscle, and isolated cardiomyocyte studies. In WT mice, TAC increased LV mass, SPARC expression, myocardial diastolic stiffness, fibrillar collagen content, and soluble and insoluble collagen. In SPARC-null mice, TAC increased LV mass to an extent similar to WT mice. In addition, in SPARC-null mice, TAC increased fibrillar collagen content, albeit significantly less than that seen in WT TAC mice. Furthermore, the proportion of LV collagen that was insoluble was less in the SPARC-null TAC mice (86±2%) than in WT TAC mice (99±2%, P<0.05), and the proportion of collagen that was soluble was greater in the SPARC-null TAC mice (14±2%) than in WT TAC mice (1±2%, P<0.05) As a result, myocardial diastolic stiffness was lower in SPARC-null TAC mice (0.075±0.005) than in WT TAC mice (0.045±0.005, P<0.05). Conclusions— The absence of SPARC reduced pressure overload–induced alterations in extracellular matrix fibrillar collagen and diastolic function. These data support the hypothesis that SPARC plays a key role in post–synthetic procollagen processing and the development of mature cross-linked collagen fibrils in normal and pressure-overloaded myocardium.

Pressure Overload–Induced Alterations in Fibrillar Collagen Content and Myocardial Diastolic Function

Background— Chronic pressure overload causes myocardial hypertrophy, increased fibrillar collagen content, and abnormal diastolic function. We hypothesized that one determinant of these pressure overload–induced changes is the extracellular processing of newly synthesized procollagen into mature collagen fibrils. We further hypothesized that secreted protein acidic and rich in cysteine (SPARC) plays a key role in post–synthetic procollagen processing in normal and pressure-overloaded myocardium. Methods and Results— To determine whether pressure overload–induced changes in collagen content and diastolic function are affected by the absence of SPARC, age-matched wild-type (WT) and SPARC-null mice underwent either transverse aortic constriction (TAC) for 4 weeks or served as nonoperated controls. Left ventricular (LV) collagen content was measured histologically by collagen volume fraction, collagen composition was measured by hydroxyproline assay as soluble collagen (1 mol/L NaCl extractable) versus insoluble collagen (mature cross-linked collagen), and collagen morphological structure was examined by scanning electron microscopy. SPARC expression was measured by immunoblot. LV, myocardial, and cardiomyocyte structure and function were assessed by echocardiographic, papillary muscle, and isolated cardiomyocyte studies. In WT mice, TAC increased LV mass, SPARC expression, myocardial diastolic stiffness, fibrillar collagen content, and soluble and insoluble collagen. In SPARC-null mice, TAC increased LV mass to an extent similar to WT mice. In addition, in SPARC-null mice, TAC increased fibrillar collagen content, albeit significantly less than that seen in WT TAC mice. Furthermore, the proportion of LV collagen that was insoluble was less in the SPARC-null TAC mice (86±2%) than in WT TAC mice (99±2%, P<0.05), and the proportion of collagen that was soluble was greater in the SPARC-null TAC mice (14±2%) than in WT TAC mice (1±2%, P<0.05) As a result, myocardial diastolic stiffness was lower in SPARC-null TAC mice (0.075±0.005) than in WT TAC mice (0.045±0.005, P<0.05). Conclusions— The absence of SPARC reduced pressure overload–induced alterations in extracellular matrix fibrillar collagen and diastolic function. These data support the hypothesis that SPARC plays a key role in post–synthetic procollagen processing and the development of mature cross-linked collagen fibrils in normal and pressure-overloaded myocardium.