Control Mechanisms for Matching ATP Supply and Demand in Heart Mitochondria

心脏线粒体中 ATP 供需匹配的控制机制

• 批准号: 8148203
• 负责人: Steven Sollott
• 金额: $ 32.64万
• 依托单位: NATIONAL INSTITUTE ON AGING
• 依托单位国家: 美国
• 资助国家: 美国
• 起止时间: 至
• 项目状态: 未结题
• 来源: https://reporter.nih.gov/project-details/8148203
• 关键词: Heart Mitochondria

Failure to supply energy to match the body's demands limits the functional reserve capacity, and under certain periods of stress, such as ischemia, can lead to irreversible cell and tissue damage. This matching is critical in tissues with high and rapidly fluctuating metabolic rates such as the heart. Mitochondria are the main ATP suppliers to meet cellular demands. The fuel used by mitochondria is transported across the inner mitochondrial membrane to the matrix and produces a source of electrons whose redox-potential energy is, in turn, harnessed by the electron transport chain. The flux of electrons is reflected in oxygen consumption. The energy released from this electron flow is used to transport protons out of the matrix across the inner mitochondrial membrane forming a gradient whose proton-motive force drives ATP synthase to make ATP. This "upstream" regulation is known as the "push" mechanism. A complete description of the ATP synthase control mechanisms is still lacking. Two general mechanisms have been suggested to serve as key regulators: 1) ADP and Pi concentrations; ATP utilization/hydrolysis in the cytosol increases ADP and Pi fluxes to mitochondria and hence the amount of available substrates for ATP production increases; 2) Ca2+ concentration; ATP utilization/hydrolysis is coupled to changes in free cytosolic calcium and mitochondrial Ca2+, the latter controlling Ca2+-dependent activation of certain mitochondrial reaction-rate-determining enzymes taking part in ATP production. At high levels of energy demand the question arises whether parallel to the "push" mechanism signals acting on ATP synthase could also facilitate the electron transport chain redox flux, enhancing the efficiency of ATP production. This effect simulates an apparent additional "pull" on the upstream flux, which causes as a specific proportionate increase in respiration. Proof of such a "pull" mechanism regulated by Ca2+ and its target has not been demonstrated to-date. Cardiomyocytes contract in response to driven cyclic 'increases' in cytosolic Ca2+ in a response to electrically stimulation. As a consequence of the levels of contractile work, ATP is proportionately utilized by the contractile elements. Therefore, from the demand point of view Ca2+ is a direct effector that might be well positioned to play a role in the energy matching regulatory mechanisms. A correlation has also been shown between cytosolic and mitochondrial Ca2+. Ca2+ enters the mitochondria through the mitochondrial uniporter and is extruded by the mitochondrial Na+/Ca2+ exchanger. This results in mitochondrial Ca2+ accumulation in response to an increase in stimulation frequency or Ca2+ transient amplitude. Therefore, Ca2+ levels in the mitochondria reflect changes in both myocardial work and ATP consumption and, hence, the demand for ATP. It was shown that mitochondrial Ca2+ can activate the mitochondrial enzymes taking part in ATP production. Therefore, changes in mitochondrial Ca2+ during electrical stimulation are linked to changes in ATP supply and demand. We and others have shown that small changes in mitochondrial volume can regulate respiration and in turn energy production. It is also known that the Ca2+ environment may regulate mitochondrial volume in isolated mitochondrial suspension raising the question whether physiological changes in Ca2+ via increasing electrically stimulated Ca2+ cycling would act in this way. We found that while increasing electrically stimulated, physiological Ca2+ cycling does not detectibly change the 'diastolic' mitochondrial long- and short-axis dimensions (i.e, volume) shortly (2.5 min) after the transition from rest to low or higher workloads, it nevertheless caused an increase in cell respiration (and in turn facilitated energy production) in both conditions. These results were in contrast to that observed by others in the isolated mitochondria models. Additionally, we found that the mechanisms that control ATP supply from the hearts mitochondria consist of both 'push' and 'pull' mechanisms and that 'pull' mechanism directly targets ATP synthase. We identified that the 'pull' mechanism is controlled by mitochondrial Ca2+ and can be further facilitated by pharmacologically regulating mitochondrial volume. At low cardiac workload, the 'push' mechanism is sufficient to match ATP supply and demand, and the mitochondrial transmembrane ADP/Pi gradient is presumably sufficient to drive the 'push' and 'pull' mechanisms. However, under the same experimental conditions, pharmacological induction of a regulatory mitochondrial volume increase was found to facilitate mitochondrial Ca2+ entry responsible for further pushing respiration, whereas at higher workloads, mitochondrial Ca2+ entry did not require such facilitation, and in turn was sufficient and essential to drive both "push" and 'pull' effects on respiration. Moreover, pharmacologically-enhanced mitochondrial Ca2+ accumulation (without changing cytosolic Ca2+) was also found to push respiration. Facilitation of these 'push' and 'pull' mechanisms is being examined as a potential treatment to reverse signaling defects in matching ATP supply and demand, such as occurs in heart failure which afflicts millions of people, especially the elderly population.

不能提供满足身体需求的能量限制了功能储备能力，在某些应激时期，如缺血，可能会导致不可逆转的细胞和组织损伤。这种匹配在心脏等代谢率高且波动迅速的组织中至关重要。线粒体是满足细胞需求的主要ATP供给者。线粒体使用的燃料通过线粒体内膜运输到基质，并产生电子源，其氧化还原势能反过来被电子传输链利用。电子的流量反映在氧气消耗上。这种电子流释放的能量被用来将质子从基质中输送到线粒体内膜，形成一个梯度，其质子动力驱动ATP合成酶合成ATP。这种“上游”监管被称为“推动”机制。对三磷酸腺苷合成酶的调控机制仍缺乏完整的描述。已经提出了两种主要的调节机制：1)ADP和PI浓度；胞质中的ATP利用/水解增加了ADP和PI流向线粒体的通量，从而增加了ATP产生的可用底物的数量；2)钙浓度；ATP利用/水解与细胞内游离钙和线粒体钙的变化相耦合，后者控制参与ATP产生的某些线粒体反应速率决定酶对钙的依赖激活。在高水平的能量需求下，与作用于ATP合成酶的“推”机制信号平行的问题是否也能促进电子传递链的氧化还原通量，从而提高ATP的生产效率。这一效应模拟了对上游通量的明显的额外“拉力”，这将导致特定比例的呼吸增加。到目前为止，还没有证据证明这种受钙离子及其靶点调控的“拉动”机制。 心肌细胞收缩是对细胞内钙离子周期性增加的反应，这是对电刺激的反应。作为收缩功水平的结果，ATP被收缩元素成比例地利用。因此，从需求的角度来看，钙离子是一个直接的效应器，可能在能量匹配调节机制中发挥很好的作用。细胞质和线粒体的钙离子之间也显示出相关性。Ca~(2+)通过线粒体单转运体进入线粒体，并由线粒体Na~+/Ca~(2+)交换器排出。这会导致线粒体在刺激频率或钙瞬变幅度增加时产生钙积聚。因此，线粒体中的钙离子水平反映了心肌做功和ATP消耗的变化，从而反映了对ATP的需求。结果表明，线粒体Ca~(2+)能激活参与ATP产生的线粒体酶。因此，电刺激时线粒体Ca~(2+)的变化与ATP供需的变化有关。 我们和其他人已经证明，线粒体体积的微小变化可以调节呼吸，进而调节能量生产。在分离的线粒体悬浮液中，钙环境可以调节线粒体的体积，这也提出了一个问题，即通过增加电刺激的钙循环，钙离子的生理变化是否会起到这种作用。我们发现，虽然增加了电刺激，但生理钙循环在从静息状态转变为低或高工作负荷后不久(2.5分钟)并没有明显改变线粒体的舒张期长轴和短轴尺寸(即体积)，但在这两种情况下它仍然导致细胞呼吸的增加(反过来又促进了能量的产生)。这些结果与其他人在分离的线粒体模型中观察到的结果相反。此外，我们还发现，控制心肌线粒体ATP供应的机制包括“推”和“拉”两种机制，而“拉”机制直接针对ATP合成酶。我们发现，“拉”机制是由线粒体的钙离子控制的，并可以通过药物调节线粒体的体积来进一步促进。在较低的心脏负荷下，“推”机制足以匹配ATP供需，线粒体跨膜ADP/PI梯度可能足以驱动“推”和“拉”机制。然而，在相同的实验条件下，发现药物诱导的调节性线粒体体积增加促进了线粒体钙进入，负责进一步推动呼吸，而在更高的工作负荷下，线粒体钙进入不需要这种促进，反过来又充分和必要地驱动对呼吸的“推”和“拉”效应。此外，药物增强的线粒体钙积累(不改变细胞内钙离子)也被发现可以促进呼吸。促进这些“推”和“拉”机制正在被研究为一种潜在的治疗方法，以扭转匹配ATP供需的信号缺陷，例如发生在困扰数百万人，特别是老年人的心力衰竭中。