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.

无法提供满足身体需求的能量会限制功能储备能力，并且在某些应激时期（例如缺血）下可能会导致不可逆的细胞和组织损伤。这种匹配对于代谢率高且快速波动的组织（例如心脏）至关重要。线粒体是满足细胞需求的主要 ATP 供应商。线粒体使用的燃料穿过线粒体内膜输送到基质，并产生电子源，其氧化还原势能反过来被电子传输链利用。电子通量反映在耗氧量上。电子流释放的能量用于将质子从基质中穿过线粒体内膜，形成梯度，其质子动力驱动 ATP 合酶产生 ATP。这种“上游”调节被称为“推动”机制。仍然缺乏 ATP 合酶控制机制的完整描述。已提出两种通用机制作为关键调节剂：1）ADP 和 Pi 浓度；细胞质中 ATP 的利用/水解增加了 ADP 和 Pi 流向线粒体的量，因此用于 ATP 生产的可用底物量增加； 2）Ca2+浓度； ATP 利用/水解与游离胞质钙和线粒体 Ca2+ 的变化相关，后者控制参与 ATP 产生的某些线粒体反应速率决定酶的 Ca2+ 依赖性激活。在能量需求较高的情况下，出现的问题是，与作用于 ATP 合酶的“推”机制信号并行，是否也可以促进电子传递链氧化还原通量，从而提高 ATP 生产的效率。这种效应模拟了对上游通量的明显额外“拉力”，这导致呼吸作用按特定比例增加。迄今为止，尚未证明这种由 Ca2+ 及其靶标调节的“拉动”机制。 心肌细胞响应于电刺激而导致细胞质 Ca2+ 的周期性“增加”而收缩。由于收缩功的水平，ATP 被收缩元件按比例利用。因此，从需求角度来看，Ca2+是一种直接效应器，可以很好地在能量匹配调节机制中发挥作用。细胞质和线粒体 Ca2+ 之间也显示出相关性。 Ca2+ 通过线粒体单向转运蛋白进入线粒体，并被线粒体 Na+/Ca2+ 交换器挤出。这导致线粒体 Ca2+ 积累，以响应刺激频率或 Ca2+ 瞬态幅度的增加。因此，线粒体中的 Ca2+ 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 Ca2+ 可以激活参与 ATP 生成的线粒体酶。 因此，电刺激过程中线粒体 Ca2+ 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，进而调节能量产生。还已知 Ca2+ 环境可以调节分离的线粒体悬浮液中的线粒体体积，这引发了一个问题：通过增加电刺激 Ca2+ 循环而产生的 Ca2+ 生理变化是否会以这种方式起作用。我们发现，虽然增加电刺激，生理性 Ca2+ 循环在从休息过渡到低或高工作负荷后不久（2.5 分钟）并没有明显改变“舒张期”线粒体长轴和短轴尺寸（即体积），但它仍然在这两种情况下都会导致细胞呼吸的增加（进而促进能量的产生）。这些结果与其他人在分离的线粒体模型中观察到的结果形成对比。此外，我们发现控制心脏线粒体 ATP 供应的机制包括“推”和“拉”机制，并且“拉”机制直接针对 ATP 合酶。我们发现“拉”机制是由线粒体 Ca2+ 控制的，并且可以通过药理学调节线粒体体积来进一步促进。在低心脏工作负荷下，“推”机制足以匹配 ATP 供应和需求，并且线粒体跨膜 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，发现调节性线粒体体积增加的药理诱导可促进线粒体 Ca2+ 进入，从而进一步推动呼吸，而在较高的工作负荷下，线粒体 Ca2+ 进入不需要这种促进，反过来又是充分和必要的驱动呼吸的“推”和“拉”效应。此外，还发现药物增强线粒体 Ca2+ 积累（不改变胞质 Ca2+）可以促进呼吸。 人们正在研究促进这些“推”和“拉”机制，作为逆转 ATP 供需匹配信号缺陷的潜在治疗方法，例如困扰数百万人，尤其是老年人的心力衰竭。