Effects of salt loading on the organisation of microtubules in rat magnocellular vasopressin neurones

Abstract

Magnocellular vasopressin (VP) neurones are activated by increases in blood osmolality, leading to the secretion of VP into the circulation to promote water retention in the kidney, thus constituting a key mechanism for the regulation of body fluid homeostasis. However, chronic high salt intake can lead to excessive activation of VP neurones and increased circulating levels of VP, contributing to an elevation in blood pressure. Multiple extrinsic factors, such as synaptic inputs and glial cells, modulate the activity of VP neurones. Moreover, magnocellular neurones are intrinsically osmosensitive, and are activated by hypertonicity in the absence of neighbouring cells or synaptic contacts. Hypertonicity triggers cell shrinking, leading to the activation of VP neurones. This cell‐autonomous activation is mediated by a scaffold of dense somatic microtubules, uniquely present in VP magnocellular neurones. Treating isolated magnocellular neurones with drugs modulating microtubule stability modifies the sensitivity of neuronal activation in response to acute hypertonic stimuli. However, whether the microtubule network is altered in conditions associated with enhanced neuronal activation and increased VP release, such as chronic high salt intake, remains unknown. We examined the organisation of microtubules in VP neurones of the supraoptic and paraventricular hypothalamic nuclei (SON and PVN, respectively) of rats subjected to salt‐loading (drinking 2% NaCl for 7 days). Using super‐resolution imaging, we found that the density of microtubules in magnocellular VP neurones from the SON and PVN was significantly increased, whereas the density and organisation of microtubules remain unchanged in other hypothalamic neurones, as well as in neurones from other brain areas (e.g., hippocampus, cortex). We propose that the increase in microtubule density in magnocellular VP neurones in salt‐loading promotes their enhanced activation, possibly contributing to elevated blood pressure in this condition.

Osmoregulation and the Hypothalamic Supraoptic Nucleus: From Genes to Functions

“…Super-resolution images obtained by structured illumination microscopy have revealed that MNCs contain many actin filaments and microtubules in their somata, with exclusive organization compared with cells in other brain regions, such as the cortex, as well as with hippocampal and other hypothalamic neurons ( Prager-Khoutorsky et al, 2014 ). Additionally, the density/array of actin filaments and microtubules are substantially changed in response to salt-loading ( Barad et al, 2020 ; Hicks et al, 2020 ), reinforcing the hypothesis that these elements participate in the mechanotransduction process. Based on the assumption that cytoskeleton components can redistribute forces across the lipid bilayer ( Ingber, 1997 ), Zhang et al (2007) proposed that osmosensory transduction depends on the amount and structure of F-actin filaments present in MNCs.…”

“…As we discuss in the coming sections, under chronic dehydration, the MNCs experience dramatic transcriptomic remodeling, which includes changes in the expression of several genes encoding plasma membrane channels and transporters ( Hindmarch et al, 2006 ; Greenwood M. P. et al, 2015 ; Johnson et al, 2015 ; Pauža et al, 2021 ). Another important adaptation is the increased complexity of the cytoskeleton array observed during sustained hyperosmolality ( Barad et al, 2020 ; Hicks et al, 2020 ), whose crucial role for osmosensitivity was previously demonstrated by Prager-Khoutorsky et al (2014) , also discussed later in this review. Thus, the new molecular phenotype of the MNCs under chronic osmotic challenge might also help understanding how they are able to ensure the continuous AVP and OXT secretion, essential for survival during dehydration, despite the incessant plasma membrane insertion due to the vesicle fusion.…”

Advances in the neurophysiology of magnocellular neuroendocrine cells

“…Interestingly, Hicks et al demonstrated recently that the density of microtubules in magnocellular VP neurones increases in response to salt loading. The organisation of microtubules in other brain areas remains unchanged following salt loading, and only microtubule cytoskeleton in magnocellular SON and PVN neurones is modulated by this condition . Because an enhancement of microtubule density is sufficient to elevate the autonomous osmosensitivity of VP magnocellular neurones, the increase in microtubule density following salt loading can facilitate the activation of magnocellular VP neurones, contributing to excessive VP release and elevated blood pressure in this condition.…”

“…By contrast to actin filaments, which form a thin subcortical layer in these neurones, microtubules create a highly complex three‐dimensional network of filaments that occupy the entire cytoplasm of neuronal somata (Figure B). Super‐resolution imaging of microtubules in situ in different areas of the rodent brain revealed that magnocellular VP neurones from the SON and paraventricular nucleus (PVN) feature a unique microtubule structure that is strikingly different from the rectilinear microtubules commonly observed in other types of neurones, and from the typical pattern of centrosome‐divergent microtubules found in somatic cells . Thus, this interwoven microtubule network appears to be a unique feature of the magnocellular neurosecretory cells.…”

“…Moreover, stabilising each of these cytoskeletal networks amplifies the activation of neurones by changes in osmolality. Interestingly, Hicks et al demonstrated recently that the density of microtubules in magnocellular VP neurones increases in response to salt loading. The organisation of microtubules in other brain areas remains unchanged following salt loading, and only microtubule cytoskeleton in magnocellular SON and PVN neurones is modulated by this condition .…”