Meta-Biol

• Pathways

Four members of the SLC16A gene family encode classical monocarboxylate transporters MCT1-4. Widely expressed, they all function as proton-dependent transporters of monocarboxylic acids such as lactate and pyruvate and ketone bodies such as acetacetate and beta-hydroxybutyrate. These processes are crucial in the regulation of energy metabolism and acid-base homeostasis.

SLC16A1 encodes MCT1, a ubiquitiously expressed protein. Heterozygous defects in SLC16A1 were found in patients with symptomatic deficiency in lactate transport (SDLT aka erythrocyte lactate transporter defect; MIM:245340), resulting in an acidic intracellular environment and muscle degeneration with the release of myoglobin and creatine kinase (Merezhinskaya et al. 2000). This defect could compromise extreme performance in otherwise healthy individuals.

SLC16A1 is essential for lactate transport in muscle cells. It is also highly enriched in astrocytes and oligodendroglia, neuroglia that support, insulate and provide energy metabolites to axons. Oligodendroglia dysfunction can lead to axon degeneration in several diseases. The cause is unknown but disruption of SLC16A1 transporter produces axon damage and neuron loss in animal and cell culture models. In humans, this transporter is reduced in patients with amyotrophic lateral sclerosis (Lee et al. 2012).

In cancer cells, a common change is the upregulation of glycolysis. The anti-cancer drug candidate 3-bromopyruvate (3-BrPA) can inhibit glycolysis through its uptake into cancer cells via SLC16A1 so it is the main determinant of 3-BrPA sensitivity in these cells (Birsoy et al. 2013).

Proteins with transporting functions can be roughly classified into 3 categories: ATP hydrolysis-coupled pumps, ion channels, and transporters. Pumps utilize the energy released by ATP hydrolysis to power the movement of substrates across the membrane against their electrochemical gradient. Channels in their open state can transfer substrates (ions or water) down their electrochemical gradient at an extremely high efficiency (up to 108 s-1). Transporters facilitate the movement of a specific substrate either against or with their concentration gradient at a lower speed (about 102 -104 s-1); as generally believed, conformational change of the transporter protein is involved in the transfer process. Diseases caused by defects in these transporter proteins are detailed in this section. Disorders associated with ABC transporters and SLC transporters are annotated here (Dean 2005).

Biological processes are captured in Reactome by identifying the molecules (DNA, RNA, protein, small molecules) involved in them and describing the details of their interactions. From this molecular viewpoint, human disease pathways have three mechanistic causes: the inclusion of microbially-expressed proteins, altered functions of human proteins, or changed expression levels of otherwise functionally normal human proteins.

The first group encompasses the infectious diseases such as influenza, tuberculosis and HIV infection. The second group involves human proteins modified either by a mutation or by an abnormal post-translational event that produces an aberrant protein with a novel function. Examples include somatic mutations of EGFR and FGFR (epidermal and fibroblast growth factor receptor) genes, which encode constitutively active receptors that signal even in the absence of their ligands, or the somatic mutation of IDH1 (isocitrate dehydrogenase 1) that leads to an enzyme active on 2-oxoglutarate rather than isocitrate, or the abnormal protein aggregations of amyloidosis which lead to diseases such as Alzheimer's.

Infectious diseases are represented in Reactome as microbial-human protein interactions and the consequent events. The existence of variant proteins and their association with disease-specific biological processes is represented by inclusion of the modified protein in a new or variant reaction, an extension to the 'normal' pathway. Diseases which result from proteins performing their normal functions but at abnormal rates can also be captured, though less directly. Many mutant alleles encode proteins that retain their normal functions but have abnormal stabilities or catalytic efficiencies, leading to normal reactions that proceed to abnormal extents. The phenotypes of such diseases can be revealed when pathway annotations are combined with expression or rate data from other sources.

Depending on the biological pathway/process immediately affected by disease-causing gene variants, non-infectious diseases in Reactome are organized into diseases of signal transduction by growth factore receptors and second messengers, diseases of mitotic cell cycle, diseases of cellular response to stress, diseases of programmed cell death, diseases of DNA repair, disorders of transmembrane transporters, diseases of metabolism, diseases of immune system, diseases of neuronal system, disorders of developmental biology, disorders of extracellular matrix organization, and diseases of hemostatis.