This Week in The Journal

Angiotensin Receptors in the Paraventricular Nucleus

Annette D. de Kloet, Lei Wang, Soledad Pitra, Helmut Hiller, Justin A. Smith, et al.

Angiotensin II is a key regulator of blood pressure and water balance, and it also contributes to physiological and hormonal responses to stressors. These effects are mediated primarily by angiotensin 1a receptors (AT1aRs), which are expressed throughout the body and in several brain regions. AT1aR expression is particularly pronounced in components of the hypothalamic–pituitary–adrenal axis, which promotes physiological responses to environmental stress. In the paraventricular nucleus (PVN) of the hypothalamus, AT1aRs are expressed in parvocellular neurons that produce corticotrophin-releasing hormone (CRH), which stimulates adrenocorticotropic hormone (ACTH) release from the pituitary and downstream release of corticosterone from the adrenal glands.

Angiotensin II levels and AT1aR expression increase in both the brain and the periphery under stressful conditions, and AT1aR antagonists reduce many of the physiological effects of restraint and isolation stress in rodents (Saavedra 2005 Cell Mol Neurobiol 25:485). How different types of AT1aR-expressing neurons contribute to these responses is poorly understood, however. To address this question, de Kloet, Wang, et al. created transgenic mice that express fluorescent or light-activated proteins under the control of the AT1aR promoter and characterized AT1aR-expressing neurons in the PVN.

Consistent with previous work, these neurons were concentrated in the parvocellular neurosecretory subdivision of the PVN, and they projected densely to the median eminence. Almost all AT1aR-expressing neurons were glutamatergic, and ∼55% expressed CRH, while ∼24% expressed thyrotropin-releasing hormone (TRH).

Although few AT1aR-expressing neurons were active (as indicated by fos expression) under baseline conditions, acute restraint stress activated approximately one third of the neurons. Optical activation of AT1aR-expressing neurons in the PVN increased circulating levels of ACTH, corticosterone, thyroid-stimulating hormone (TSH), and thyroxine, as well as increasing systolic blood pressure. In contrast, inhibition of AT1aR-expressing neurons decreased levels of corticosterone and TSH after an acute stressor, and it increased the amount of time mice spent in the open arm of an elevated plus maze, suggesting inhibition of these neurons had anxiolytic-like effects.

These results confirm that AT1aR-expressing neurons of the PVN are involved in stress responses, and they provide additional evidence that targeting brain AT1aRs might be an effective treatment for anxiety disorders. Future work should investigate whether both CRH- and TRH-expressing neurons are among the population activated by restraint stress, what other stressors activate these and other AT1aR-expressing neurons, and whether these subpopulations make different contributions to anxiety-like behaviors.

Pejvakin Function in Hair Cells

Marcin Kazmierczak, Piotr Kazmierczak, Anthony W. Peng, Suzan L. Harris, Prahar Shah, et al.

(see pages 3447–3464)

Mutations in DFNB59, which encodes pejvakin, cause profound deafness. Although point mutations in pejvakin have been proposed to impair function primarily in spiral ganglion neurons (Delmaghani et al. 2006 Nat Genet 38:770), a truncation mutation was shown to affect only hair cells (Ebermann et al. 2007 Hum Mutat 28:571). Nonetheless, disruption of peroxisome function and oxidative damage has been hypothesized to underlie dysfunction in both cell types (Delmaghani et al. 2015 Cell 163:894). Work by Kazmierczak, Kazmierczak et al. raises doubts about previous hypotheses, however.

A fluorescent protein (magenta) labels spiral ganglion neurons. Knocking out pejvakin in these neurons had no effect on auditory function. Green is anti-tubulin antibody. See Kazmierczak, Kazmierczak et al. for details.

Whereas knocking out DFNB59 selectively in mouse hair cells caused severe hearing loss, neuron-specific knockout had no obvious effect on auditory brainstem responses. Inner hair cells were present in pejvakin-null mice, but they were often missing fourth-row stereocilia, and some second-row stereocilia had abnormally elongated tips. Outer hair cells also lost stereocilia, and the innermost row of outer hair cells began to degenerate by postnatal day 30. Consistent with a loss of stereocilia, the current induced by deflection of the stereocilia bundle was reduced in pejvakin-deficient hair cells.

To further elucidate the function of pejvakin, GFP-tagged protein was expressed in hair cells. GFP-pejvakin was localized predominantly in the rootlets at the base of hair cell stereocilia, where it colocalized with the rootlet protein TRIOBP. Notably, deafness-linked point mutations disrupted GFP-pejvakin localization, such that it was present uniformly in the cytoplasm. Contrary to previous work, however, GFP-tagged pejvakin did not colocalize with peroxisome proteins.

These results suggest that pejvakin is a component of stereocilia rootlets in hair cells. Rootlets anchor stereocilia to the actin cortex in the hair cell body, and they are thought to contribute to stereocilia elasticity. Although rootlets formed in the absence of pejvakin, the protein appears to be necessary for maintaining stereocilia health. Future studies should elucidate the function of pejvakin in rootlets and seek explanations for the discrepancies between this and previous studies.