LABORATORY OF BIOLOGICAL DIVERSITYSAKUTA Group
Assistant Professor:
Technical Assistant :SAKUTA, Hiraki
KODAMA, Akiko

The homeostatic osmoregulation of body fluids (such asplasma and cerebrospinal fluid (CSF)) is vital to life. This isbecause substantial changes in cell volumes due to hypertonicity or hypotonicity cause irreversible damage to organs andlead to lethal neurological trauma. Water deprivation (loss ofwater from the body) elevates the concentration of Na+ ([Na+])and osmolality in body fluids. Animals exhibit prominent andeffective responses to water deprivation, including behavioral
responses, such as inducing water intake and avoiding sodium(Na), along with vasopressin-induced reductions in urine
volumes. The aim of our research group is to reveal the brain
systems for body-fluid homeostasis.

body fluids are presented in Figure 1. When [Na+] in plasma
and CSF increases, Nax channels in glial cells in the OVLT
are activated, leading to the synthesis of epoxyeicosatrienoic
acids (EETs) in Nax-positive glial cells. EETs released fromNax-positive glial cells function as gliotransmitters to activate
neurons bearing TRPV4 channels in the OVLT, which areinvolved in the stimulation of water-intake behavior.

Figure 1. Schematic drawings of sensing mechanisms of body-fluid conditions in the OVLT responsible for the induction of water intake. AA,arachidonic acid.

II. Identification of novel sensors involved inwater intake control

Visual overview: Two postulated systems for sensing body-fluid conditions

I. Thirst control by Nax and TRPV4[Na+] is the main factor influencing osmolality in vivo,
and is continuously monitored in the brain to be maintained
within a physiological range. We have shown that Nax,
which structurally resembles voltage-gated sodium channels(Nav1.1–1.9), is the brain [Na+] sensor to detect increases in [Na+] in body fluids. Nax is preferentially expressedin specific glial cells of sensory circumventricular organs(sCVOs) including the subfornical organ (SFO) and organum
vasculosum lamina terminalis (OVLT). We have found thatNax signals in these brain regions deficient in a blood-brain
barrier are involved in the control of salt intake.
We demonstrated that Nax signals are also involved in thecontrol of water intake behavior. The signaling mechanisms in
the OVLT for water-intake induction by increases in [Na+] inWater intake by Nax-KO mice after an ICV injection ofhypertonic NaCl solution was small, but still approximately
half that of WT mice. It was noteworthy that this was significantly higher than that of Nax-KO and WT mice after an ICV
injection of an equimolar hypertonic sorbitol solution. Thesefindings suggest the existence of another unknown [Na +]sensor and osmosensor. In order to identify the novel sensors
involved in water intake control, we performed RNA-seq
analysis of OVLT and identified many candidates for saidsensors. As a result, we revealed that SLC9A4 in the OVLT
functions as a [Na+] sensor for the control of water intake
behavior among these candidates, and that the signaling
pathway originating from this sensor is independent of theNax/TRPV4 pathway, another [Na+]-sensing pathway for thecontrol of water intake (Figure 1). Our experimental results
suggested that SLC9A4-positive neurons are activated viaASIC1a in a H+-dependent manner. Our experimental results
also revealed that water intake induced by the increase in[Na+] in CSF was completely lost in slc9a4-knockdown
Nax-KO mice. Thus, water intake induced by [Na+] increases
in body fluids may be explained by the Nax/TRPV4 and
SLC9A4/ASIC1a pathways. In addition to these two [Na+]dependent pathways that induce water intake, another signaling pathway originating from the osmosensor may be independently involved in the induction of water intake. We arenow examining the functional roles of remaining candidates
in water intake to clarify the osmosensing system.

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