Efficient in vivo ³¹P magnetization transfer approach for noninvasively determining multiple kinetic parameters and metabolic fluxes of ATP metabolism in the human brain

Abstract

ATP metabolism is controlled mainly by ATP synthase (ATP ase ) and creatine kinase (CK) reactions that regulate cerebral ATP production, transportation, and utilization. These coupled reactions constitute a chemical exchange metabolic network of PCr7ATP7Pi characterized by two forward and two reverse reaction fluxes, which can be studied noninvasively by in vivo 31 P MRS combined with magnetization transfer (MT). However, it is still debated whether current MT approaches can precisely determine all of these fluxes. We developed and tested a modified in vivo 31 P MT approach based on a multiple single-site saturation (MSS) technique to study the entire PCr7ATP7Pi network in human occipital lobe at 7T. Our results reveal that 1) the MSS MT approach can explicitly determine all four reaction fluxes with a minimal number of The primary functions of brain cells are excitation and conduction, which are reflected by constant electrophysiological activity in the brain. The cerebral bioenergetics that support sustained electrophysiological activity are ultimately driven by a variety of biochemical processes that maintain the normal function and structural integrity of the brain (1). Of these processes, the most fundamental for supporting various cellular activities is adenosine triphosphate (ATP) metabolism in living cells (2). The majority of ATP is formed from adenosine diphosphate (ADP) and inorganic phosphate (Pi) in the mitochondria through oxidative phosphorylation catalyzed by the ATP synthase (ATP ase ) enzyme, as illustrated by Fig. 1a (3,4). The highly demanding biochemical processes involving ATP production and utilization in the brain cause rapid chemical cycling among ATP, ADP, and Pi (see Fig. 1a). These processes are also accompanied by another important chemical reaction involving phosphocreatine (PCr) and creatine kinase (CK). PCr acts as an ATP reservoir and carrier, and transfers energy from the mitochondria to sites of ATP utilization in the cytosol through reversible CK reactions, ultimately maintaining a stable cellular ATP level (4,5). These two chemical exchange reactions (i.e., PCr7ATP and Pi7ATP) play central roles in regulating ATP metabolism and maintaining normal ATP functionality, both of which are crucial for cerebral bioenergetics and brain function in the healthy brain as well as in neurodegenerative diseases. Moreover, the ATP ase and CK reactions are tightly coupled together, leading to a complex three-31 P-spin chemical exchange kinetic network (i.e., PCr7ATP7Pi) as depicted in Fig. 1b. Thus, it is essential to develop a noninvasive, reliable technique that is capable of assessing the entire kinetic network of PCr7ATP7Pi and associated ATP metabolic fluxes in situ, particularly in the human brain.Measuring all kinetic parameters involved in the PCr7ATP7Pi network requires extensive information, including three steady-state phosphate metabolite concentrations (i.e., [ATP], [PCr], and [Pi]) and four pseudo-firstorder chemical reaction rate constants (forward and reverse rate constants fo...

Tightly coupled brain activity and cerebral ATP metabolic rate

“…Multiplication of this CMRO 2 value with 2ϫ P:O ratio with a P:O ratio of 2.4 (20, 33) yields a rate of 10.6 mol⅐g Ϫ1 ⅐min Ϫ1 for net ATP synthesis by oxidative phosphorylation, in excellent agreement with F f,ATPase values of 9.7 Ϯ 1.2 mol⅐g Ϫ1 ⅐min Ϫ1 measured by 31 P MT under the same animal condition in this study. This result further validates the conclusion reached in the awake human brain (9,10) and in the rat brain with sodium pentobarbital anesthesia (13). The significance of the current data are particularly important, because the CMRO 2 and F f,ATPase were examined under identical experimental conditions, albeit in different animals, thus removing potential sources of error in the comparison.…”

“…2. However, the intrinsic spin lattice relaxation times of Pi (T 1,Pi ) and PCr (T 1,PCr ) in the absence of exchange with ATP have to be determined at a given magnetic field strength and are, in general, independent of the physiologic condition changes (9,10). In this study, we have conducted a progressive saturation transfer experiments (see Materials and Methods and Eq.…”

“…The brain ATP metabolism is mainly controlled by ATPase and creatine kinase (CK) reactions that are coupled together and constitute a complex chemical exchange system involving ATP, phosphocreatine (PCr), and intracellular inorganic phosphate (Pi) (i.e., a PCr^ATP^Pi chemical exchange system) (7)(8)(9)(10). One vital function of this ATP metabolic network is to maintain a stable cellular ATP concentration by adjusting the reaction rates to ensure a continuous energy supply for sustaining electrophysiological activity and maintaining normal function in the brain.…”

“…Logically, both the kinetics of ATP metabolism and the associated chemical exchange rates should be more sensitive to the brain activity and energy states than the steady-state ATP concentration; they should provide an essential measure of cerebral bioenergetics under different brain activity states. The sole approach able to noninvasively and directly assess the cerebral ATP metabolic rates is in vivo 31 P magnetic resonance spectroscopy (MRS) combined with the magnetization transfer (MT) method (9)(10)(11)(12)(13)(14)(15)(16).…”

1H NMR spectroscopy of rat brain in vivo at 14.1Tesla: Improvements in quantification of the neurochemical profile

“…The obtained concentrations of PCho and GPC can in principle be compared with 31 P NMR data, where PCho and GPC peaks have been resolved. We are aware only of studies on human brain [20] and on brain of newborn piglets [21] showing resolved PCho and GPC peaks. In these 31 P NMR spectra, peak intensities of GPC and PCho agree well with the GPC/PCho ratio found at 14.1 T. The importance of the upfield spectral region for the quantification of PCho and GPC is demonstrated by the LCModel fit at 14.1 T. When only the spectral range from 0 to 4.1 ppm was used (Table 1), standard deviations dramatically increased and the mean PCho and GPC concentrations changed.…”