Targeted disruption of the murineNhe1locus induces ataxia, growth retardation, and seizures

Abstract

In most cells, the ubiquitously expressed Na+/H+exchanger isoform 1 (NHE1) is thought to be a primary regulator of pH homeostasis, cell volume regulation, and the proliferative response to growth factor stimulation. To study the function of NHE1 during embryogenesis when these cellular processes are very active, we targeted the Nhe1 gene by replacing the sequence encoding transmembrane domains 6 and 7 with the neomycin resistance gene. NHE activity assays on isolated acinar cells indicated that the targeted allele is functionally null. Although the absence of NHE1 is compatible with embryogenesis, Nhe1 homozygous mutants (−/−) exhibit a decreased rate of postnatal growth that is first evident at 2 wk of age. At this time, Nhe1 −/− animals also begin to exhibit ataxia and epileptic-like seizures. Approximately 67% of the −/− mutants die before weaning. Postmortem examinations frequently revealed an accumulation of a waxy particulate material inside the ears, around the eyes and chin, and on the ventral surface of the paws. Histological analysis of adult tissues revealed a thickening of the lamina propria and a slightly atrophic glandular mucosa in the stomach.

Traditional and emerging roles for the SLC9 Na+/H+ exchangers

“…Inhibition of NHE1 transport in granulocytes does not affect chemotaxis and chemokinesis [63]. Furthermore, NHE1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 knock-out (KO) mice exhibit normal embryogenesis, suggesting redundant or compensatory mechanisms [34,15].…”

“…Cox et al described a spontaneous mutation ("swe" -slow-wave epilepsy) that arose in the Jackson laboratories which resulted in a truncation of the protein in the transmembrane domain with a subsequent NHE-null phenotype [34]. Bell et al generated a traditional NHE1 KO, achieved by homologous recombination resulting in deletion of NHE1 transmembrane domains 6 and 7 [15]. Loss of NHE1 was compatible with embryogenesis, however, in contrast to the Swe mice the NHE1 KO mice exhibited a decreased rate of postnatal growth and high mortality with only ~10% of mice surviving 5 weeks after birth [15].…”

“…Bell et al generated a traditional NHE1 KO, achieved by homologous recombination resulting in deletion of NHE1 transmembrane domains 6 and 7 [15]. Loss of NHE1 was compatible with embryogenesis, however, in contrast to the Swe mice the NHE1 KO mice exhibited a decreased rate of postnatal growth and high mortality with only ~10% of mice surviving 5 weeks after birth [15].…”

Proton production, regulation and pathophysiological roles in the mammalian brain

“…and osmotic cell shrinkage [31] . NHE1 deficiency leads to growth retardation, ataxia, and seizures [2] as well as abnormal cell morphology and adhesion [32] . It is reported that the asymmetrical expression and activation of NHE1 in cells generates a local intracellular pH gradient and thus induces or at least supports the rearrangement of the cytoskeleton by affecting cofilin, an actin-binding protein that increases the recycling of actin monomers [1] .…”

“…However, only few of these changes have physiological relevance in the brain [1] . Elimination or inhibition of pH-sensitive proteins, such as Na + /H + exchangers (NHEs) [2] , monocarboxylate transporters (MCTs) [3] , and acid-sensing ion channels (ASICs) [4] , results in seizures, ataxia, impaired acid secretion or sour-sensing in mice, suggesting that protons are involved in signal transduction or the maintenance of cellular function. In particular, recent studies in C. elegans demonstrated that protons even directly serve as transmitters during muscle contraction [5,6] .…”