Heterogeneous occurrence of aquaporin-4 in the ependyma and in the circumventricular organs in rat and chicken

Abstract

Aquaporins are selective water channel proteins critical in volume homeostasis. In the CNS AQP4 predominates, localized mainly in the glia limitans, the perivascular endfeet and ependyma. The present immunofluorescent study reveals the distribution of aquaporin-4 in the circumventricular organs in rat and chicken brains. The ventricular ependyma (especially in the third one), the subfornical organ, the area postrema, the rat pineal body (in part), and the vascular organ of lamina terminalis were marked by intense immunopositivity. Several areas, however, proved to be immunonegative: the central canal, the subcommissural organ, the ependymal zone of the median eminence in rat but its whole thickness in chicken, the subtrochlear organ, and the paraventricular organ. The immunostaining of the lateral septal and subseptal organs were similar to their environment. Results on developing rats suggested that the aquaporin-4 immunonegativity is a secondary phenomenon. Surveying other structural and functional features, no clear explanation of the heterogeneous occurrence of aquaporin-4 was found. The absence of aquaporin-4 seems to correlate with some features of the "ependymal organs" (thickened, pseudostratified ependyma, presence of blood-brain barrier) and with the avoidance of GFAP. On the other hand, the organs rich in aquaporin-4 have features of the "hypendymal organs" (glial and vascular plexus but no blood-brain barrier). There are organs, however, which do not fit into either group completely, i.e. the lateral septal and subseptal organs. Presence of tight junctions coincides with the absence of aquaporin-4 in the ependyma of spinal cord, the subcommissural organ and the ependyma of median eminence.

Immunolocalization of aquaporin-4 in the brain, kidney, skeletal muscle, and gastro-intestinal tract of chicken

“…The colocalization of the AQP4-ir structures with GFAP suggest that this water channel is present in the astrocytic end-feet surrounding the blood vessels and in the astrocytes of the cerebellum. These observations are consistent with previous studies in rat (Nielsen et al 1997;Goren et al 2006) and chicken (Nico et al 2001) in which rat AQP4 antibodies were used. The expression pattern of AQP4 shown in this study is in agreement with the role of this channel in water movement between the brain and cerebrospinal fluid (CSF) compartments, and between the brain and blood (Amiry-Moghaddam et al 2003a).…”

“…Only a few immunohistochemical studies have reported the distribution of AQP4 in the chicken brain by using antibodies to rat AQP4 (Nico et al 2001;Goren et al 2006;Parvas and Bueno 2010). These studies suggest a role of AQP4 in the development and physiology of the bloodbrain barrier (Nico et al 2001;Parvas and Bueno 2010).…”

Prominent brainstem symptoms/signs in patients with neuromyelitis optica in a Taiwanese population

“…20,31 This specific lesion, occurring in the specialized region of the area postrema, shows intense AQP4 expression and has characteristic ''hypendymal'' features; namely, a neurovascular plexus with a loose glial bed, a thin ependymal cover, and an absent blood-brain barrier, which might make the patient vulnerable to attacks. 32 Our observations likely reflect the pathological substrate underlying these clinical and imaging correlates of NMO.…”

Urine concentration and avian aquaporin water channels

“…It is interesting to note that chicken AQP4 mRNA levels increased in the hypothalamus, whereas they decreased in the kidney, after 2 d water deprivation [67]. Immunoreactive AQP4 has been detected in circumventricular organs of chicken brain [24]. Recent evidence suggests that AQP4 may have a role in cell migration [63].…”