Assessing tissue metabolism by phosphorous-31 magnetic resonance spectroscopy and imaging: a methodology review

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Neuroimaging in mitochondrial disorders

MRI and 1H magnetic resonance spectroscopy (1HMRS) are the main neuroimaging methods to study mitochondrial diseases. MRI can demonstrate seven ‘elementary’ central nervous system (CNS) abnormalities in these disorders, including diffuse cerebellar atrophy, cerebral atrophy, symmetric signal changes in subcortical structures (basal ganglia, brainstem, cerebellum), asymmetric signal changes in the cerebral cortex and subcortical white matter, leukoencephalopathy, and symmetric signal changes in the optic nerve and the spinal cord. These elementary MRI abnormalities can be variably combined in the single patient, often beyond what can be expected based on the classically known clinical-pathological patterns. However, a normal brain MRI is also possible. 1HMRS has a diagnostic role in patients with suspected mitochondrial encephalopathy, especially in the acute phase, as it can detect within the lesions, but also in normal appearing nervous tissue or in the ventricular cerebrospinal fluid (CSF), an abnormally prominent lactate peak, reflecting failure of the respiratory chain with a shift from the Krebs cycle to anaerobic glycolysis. So far, studies correlating MRI findings with genotype in mitochondrial disease have been possible only in small samples and would greatly benefit from data pooling. MRI and 1HMRS have provided important information on the pathophysiology of CNS damage in mitochondrial diseases by enabling in vivo non-invasive assessment of tissue abnormalities, the associated changes of blood perfusion and cellular metabolic derangement. MRI and 1HMRS are expected to serve as surrogate biomarkers in trials investigating therapeutic options in mitochondrial disease.

MRI for Dental Applications

Imaging of hard and soft tissue of the oral cavity is important for dentistry. However, medical computed tomography, cone beam computed tomography (CBCT), nor MRI enables soft and hard tissue imaging simultaneously. Some MRI sequences were shown to provide fast soft and hard tissue imaging of hydrogen, which increased the interest in dental MRI. Recently, MRI allowed direct visualization of cancellous bone, intraoral mucosa, and dental pulp despite that cortical bone and dental roots are indirectly visualized. MRI seems to be adequate for many indications that CBCT is currently used for: implant treatment and inflammatory diseases of the tooth.