Knock-out of mitochondrial nucleoside diphosphate kinase NME4 reveals surprising tissue-distribution and remodeled cell signaling, suggestive of multiple functions.

Abstract

The NME protein family (also called nucleoside diphosphate kinases, NDPK or NM23) comprises ten isoforms that are mostly multi-functional and localize to different cellular compartments. Three of them are found in mitochondria: NME3, NME4 and NME6. NME4 binds to the inner membrane, largely facing the intermembrane space, where it locally regenerates GTP and is involved in cardiolipin intermembrane transfer. Loss-of-function mutations in different human cell lines identified NME4 as a metastasis suppressor. However, few is known on NME4 physiology in animals in vivo. Here we characterize a germ line KO mouse model of NME4. While NME4 immunodetection is hampered by the lack of isoform-specific antibodies, comparison of NME levels between wild-type and KO animals allowed a first profiling of NME4 expression in various organs and tissues. Surprisingly, high levels of NME4 were found in uterus, testis, spleen, bladder, and kidney, while NME4 levels in mitochondria-rich organs like liver and heart were low. No compensatory upregulation of mitochondrial NME3 and NME6 was observed. NME4 KO animals occurred to have less mitochondrial mass (as expressed by protein levels, e.g. ATP synthase) in different organs, and a decreased ATP/ADP ratio in liver. Thus, although this organ has few NME4, depletion of these low quantities is sufficient to impact mitochondria. Finally, an unbiased large-scale kinome analysis in liver revealed that lack of NME4 alters cellular signaling cascades. Modified cell signaling was already implicated in the loss of the metastasis suppressor function of NME4. In summary, our data suggest different functions of NME4, with low levels critical for mitochondria and cell signaling in liver and heart, and high levels as in reproductive organs necessary for putative additional functions.