Brain mitochondria contain aquaporin water channels: evidence for the expression of a short AQP9 isoform in the inner mitochondrial membrane

Abstract

Aquaporins are a family of water channels found in animals, plants, and microorganisms. A subfamily of aquaporins, the aquaglyceroporins, are permeable for water as well as certain solutes such as glycerol, lactate, and urea. Here we show that the brain contains two isoforms of AQP9--an aquaglyceroporin with a particularly broad substrate specificity--and that the more prevalent of these isoforms is expressed in brain mitochondria. The mitochondrial AQP9 isoform is detected as an approximately 25 kDa band in immunoblots. This isoform is likely to correspond to a new AQP9 mRNA that is obtained by alternative splicing and has a shorter ORF than the liver isoform. Subfractionation experiments and high-resolution immunogold analyses revealed that this novel AQP9 isoform is enriched in mitochondrial inner membranes. AQP9 immunopositive mitochondria occurred in astrocytes throughout the brain and in a subpopulation of neurons in the substantia nigra, ventral tegmental area, and arcuate nucleus. In the latter structures, the AQP9 immunopositive mitochondria were located in neurons that were also immunopositive for tyrosine hydroxylase, as demonstrated by double labeling immunogold electron microscopy. Our findings suggest that mitochondrial AQP9 is a hallmark of astrocytes and midbrain dopaminergic neurons. In physiological conditions, the flux of lactate and other metabolites through AQP9 may confer an advantage by allowing the mitochondria to adjust to the metabolic status of the extramitochondrial cytoplasm. We hypothesize that the complement of mitochondrial AQP9 in dopaminergic neurons may relate to the vulnerability of these neurons in Parkinson's disease.

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“…In rats, they further reported a prominent expression of AQP9 in catecholaminergic cell bodies and proximal fibers in the brain stem and hypothalamus but not the habenula or in other monoaminergic cell groups (17). Amiry-Moghaddam et al (16) further demonstrated that AQP9 was present in rat plasma membranes and inner mitochondrial membranes of dopaminergic neurons and astrocytes in the SN, VTA and arcuate nucleus, using immunogold electron microscopy.…”

“…AQP4 is also expressed in ependymal cells of the ventricles but is primarily associated with astrocytic cell membranes (20). Subpopulations of astrocytes also express AQP9 in plasma membranes as well as in inner mitochondrial membranes (16). Moreover, AQP9 is the only aquaporin expressed in neurons, where it is located in the plasma membrane and the inner mitochondrial membrane (16).…”

“…Subpopulations of astrocytes also express AQP9 in plasma membranes as well as in inner mitochondrial membranes (16). Moreover, AQP9 is the only aquaporin expressed in neurons, where it is located in the plasma membrane and the inner mitochondrial membrane (16). AQP9 and AQP4 are discussed in detail below.…”

“…One study performed in rats has shown that AQP9 is expressed as two isoforms, one ~30 kDa isoform equivalent to the liver isoform anchored in the cell membrane of dopaminergic neurons and astrocytes, and a larger pool of a ~25 kDa isoform found in the inner membrane of mitochondria of these cells (16). The shorter isoform was detected by Western blotting of rat brain mitochondrial inner membranes, and a sequence analysis of PCR gels revealed that the short isoform had a shorter open reading frame with a start codon associated with perfect Kozak consensus site, likely obtained by alternative splicing.…”

“…AQP9 is an aquaglyceroporin with particularly broad substance permeability, transporting water, glycerol, monocarboxylates and arsenite, among others (10)(11)(12)(13)(14). This water channel has recently been found to be selectively expressed in dopaminergic plasma membranes as well as mitochondrial inner membranes in the SN (15)(16)(17), indicating that it may serve as a toxin influx route into these cells, thereby accounting for an increased susceptibility to cell death. AQP4, on the other hand, is an orthodox aquaporin exclusively permeable to water.…”