Browsing by Subject "Autoregulation"

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    Autoregulatory and structural control of CaMKII substrate specificity
    (2016-09) Johnson, Derrick Ethan; Hudmon, Andy; Hurley, Thomas D.; Hoang, Quyen Q.; Gallagher, Patricia
    Calcium/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a multimeric holoenzyme composed of 8–14 subunits from four closely related isoforms (α, β, γ, δ). CaMKII plays a strategic, multifunctional role in coupling the universal second messenger calcium with diverse cellular processes including metabolism, cell cycle control, and synaptic plasticity. CaMKII exhibits broad substrate specificity, targeting numerous substrates with diverse phosphorylation motifs. Binding of the calcium sensor CaM to the autoregulatory domain (ARD) of CaMKII functions to couple kinase activation with calcium signaling. Important sites of autophosphorylation, namely T287 and T306/7 (δ isoform numbering), reside within the ARD and control either CaM dependence or ability to bind to CaMKII respectively, thus determining various activation states of the kinase. Because autophosphorylation is critical to the function of CaMKII in vivo, we sought to determine the relationship between the activation state of the kinase and substrate selectivity. We show that the ARD of activated CaMKII tunes substrate selectivity by competing for substrate binding to the catalytic domain, thus functioning as a selectivity filter. Specifically, in the absence of T287 autophosphorylation, substrate phosphorylation is limited to high-affinity, consensus substrates. T287 autophosphorylation restores maximal kinase activation and broad substrate selectivity by disengaging ARD filtering. The unique multimeric architecture of CaMKII is an ideal sensor which encodes calcium-spike frequency into graded levels of subunit activation/autophosphorylation within the holoenzyme. We find that differential activation states of the holoenzyme produce distinct substrate phosphorylation profiles. Maximal holoenzyme activation/autophosphorylation leads to further broadening of substrate specificity beyond the effect of autophosphorylation alone, which is consistent with multivalent avidity. Thus, the ability of calcium-spike frequency to regulate T287 autophosphorylation and holoenzyme activation permits cellular activity to dictate switch-like behavior in substrate selectivity that is required for diverse cellular responses by CaMKII.
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    Local metabolic hypothesis is not sufficient to explain coronary autoregulatory behavior
    (Springer Nature, 2018-08-02) Kiel, Alexander M.; Goodwill, Adam G.; Baker, Hana E.; Dick, Gregory M.; Tune, Johnathan D.; Cellular and Integrative Physiology, School of Medicine
    The local metabolic hypothesis proposes that myocardial oxygen tension determines the degree of autoregulation by increasing the production of vasodilator metabolites as perfusion pressure is reduced. Thus, normal physiologic levels of coronary venous PO2, an index of myocardial oxygenation, are proposed to be required for effective autoregulation. The present study challenged this hypothesis through determination of coronary responses to changes in coronary perfusion pressure (CPP 140-40 mmHg) in open-chest swine in the absence (n = 7) and presence of euvolemic hemodilution (~ 50% reduction in hematocrit), with (n = 5) and without (n = 6) infusion of dobutamine to augment MVO2. Coronary venous PO2 decreased over similar ranges (~ 28-15 mmHg) as CPP was lowered from 140 to 40 mmHg in each of the groups. However, coronary venous PO2 was not associated with changes in coronary blood flow (r = - 0.11; P = 0.29) or autoregulatory gain (r = - 0.29; P = 0.12). Coronary zero-flow pressure (Pzf) was measured in 20 mmHg increments and determined to be directly related to vascular resistance (r = 0.71; P < 0.001). Further analysis demonstrated that changes in coronary blood flow remained minimal at Pzf > 20 mmHg, but progressively increased as Pzf decreased below this threshold value (r = 0.68; P < 0.001). Coronary Pzf was also positively correlated with autoregulatory gain (r = 0.43; P = 0.001). These findings support that coronary autoregulatory behavior is predominantly dependent on an adequate degree of underlying vasomotor tone, independent of normal myocardial oxygen tension.