Single photon emission-computed tomography (SPECT) for functional investigation of the proximal tubule in conscious mice

Abstract

Noninvasive analysis of renal function in conscious mice is necessary to optimize the use of mouse models. In this study, we evaluated whether single photon emission-computed tomography (SPECT) using specific radionuclear tracers can be used to analyze changes in renal proximal tubule functions. The tracers included 99m TC-dimercaptosuccinic acid ( 99m Tc-DMSA), which is used for cortex imaging; 99m Tc-mercaptoacetyltriglycine ( 99m Tc-MAG3), used for dynamic renography; and 123 I-␤2-microglobulin, which monitors receptor-mediated endocytosis.99m Tc-DMSA SPECT imaging was shown to delineate the functional renal cortex with a ϳ1-mm spatial resolution and accumulated in the cortex reaching a plateau 5 h after injection. The cortical uptake of 99m Tc-DMSA was abolished in Clcn5 knockout mice, a model of proximal tubule dysfunction. Dynamic renography with 99m Tc-MAG3 in conscious mice demonstrated rapid extraction from blood, renal accumulation, and subsequent tubular secretion. Anesthesia induced a significant delay in the 99m Tc-MAG3 clearance. The tubular reabsorption of 123 I-␤2-microglobulin was strongly impaired in the Clcn5 knockout mice, with defective tubular processing and loss of the native tracer in urine, reflecting proximal tubule dysfunction. Longitudinal studies in a model of cisplatin-induced acute tubular injury revealed a correlation between tubular recovery and 123 I-␤2-microglobulin uptake. These data show that SPECT imaging with well-validated radiotracers allows in vivo investigations of specific proximal tubule functions in conscious mice.receptor-mediated endocytosis; tubular secretion; ClC-5; Dent's disease; radionuclear tracers TRANSGENIC MOUSE STRAINS ARE widely used to investigate the structure and function of developing and mature kidneys and to model human disorders. Thus far, functional studies into disease mechanisms essentially require animal death and ex vivo investigations that are potentially limited by various factors including low fertility rates, gender effects (e.g., X-linked transmission), or limited life span in some models. These restrictions complicate the design of intervention studies and require the development of large colonies, increasing costs and raising ethical questions. To circumvent some of these limitations, efforts are being made to develop in vivo functional imaging techniques in mice. In particular, the possibility to perform quantitative and/or dynamic molecular imaging studies using high-resolution positron emission tomography (PET) or single photon emission-computed tomography (SPECT) has emerged recently (23).Among its numerous applications, SPECT may be useful for noninvasive investigation of renal function in the mouse by means of dynamic imaging, parenchymal scanning, and quantification of filtration, secretion, and reabsorption events (8). In particular, the possibility to use different radiotracers for SPECT imaging provides the opportunity to investigate various transport processes taking place in the proximal tubule (PT), a segment that plays a ...

Dynamic and Static Small-Animal SPECT in Rats for Monitoring Renal Function After 177Lu-Labeled Tyr3-Octreotate Radionuclide Therapy

“…3B). The clearance of 99m Tc-MAG3 is not megalin or cubilin receptor-dependent, as discovered in Clcn5 knockout mice (26) and confirmed by us in megalin-deficient mice (data not shown), but is mediated by organic anion transport. This transport system is expressed in cells at the basolateral side of the convoluted proximal tubules and appeared to be inactive beyond 100 d after PRRT.…”

“…A role of megalin in the tubular reabsorption of 99m Tc-DMSA after glomerular filtration was confirmed by our observation that only 35%-55% of 99m Tc-DMSA renal uptake was found when we imaged kidney-specific megalin-deficient mice (32), compared with wild-type mice (data not shown). Also in Clcn5 knockout mice, a model of Dent disease showing defective megalin-and cubilin-mediated endocytosis, heavily reduced 99m Tc-DMSA uptake was observed, which matched results described in studies in patients with Dent disease or Fanconi syndrome (25,26,33). Therefore, the conclusion drawn by Müller-Suur et al that only peritubular extraction was responsible for renal 99m Tc-DMSA uptake has been contradicted by these observations (22).…”

“…Renographic 99m Tc-MAG3 studies demonstrated more efficient elimination of the radioactivity from the kidneys in conscious mice than in anesthetized ones, indicating that the metabolic state of the animals is a confounding factor during dynamic renal imaging (26). Although the clearance rate of 99m Tc-MAG3 and 111 In-DTPA in our current study in anesthetized rats might not reflect the true physiologic state, the 3 groups of rats to be compared were imaged under equal, controlled conditions.…”

“…Recently, Bioscan developed highsensitivity apertures with enlarged pinhole diameters, offering improved time resolution. In the Linoview study (26), continuous dynamic SPECT was performed using 15-s frames for 30 min after injection of 99m Tc-MAG3, and the newly developed U-SPECT dedicated small-animal SPECT camera (Milabs) also will enable the acquisition of shorter frames than were used in this study (29).…”

“…The 2 postulated mechanisms of 99m Tc-DMSA uptake are peritubular extraction or absorption of plasma protein-bound 99m Tc-DMSA directly from the blood into the proximal tubular cells or glomerular filtration, followed by tubular reabsorption. Discussions on the relative contribution of each of these pathways are ongoing (20)(21)(22)(23)(24)(25)(26). Dynamic imaging of the clearance of radiolabeled diethylenetriaminepentaacetic acid (DTPA) monitors glomerular filtration, because this small molecule is cleared only via this route.…”