Irx3 and Irx5 - Novel Regulatory Factors of Postnatal Hypothalamic Neurogenesis

Abstract

The hypothalamus is a brain region that exhibits highly conserved anatomy across vertebrate species and functions as a central regulatory hub for many physiological processes such as energy homeostasis and circadian rhythm. Neurons in the arcuate nucleus of the hypothalamus are largely responsible for sensing of peripheral signals such as leptin and insulin, and are critical for the regulation of food intake and energy expenditure. While these neurons are mainly born during embryogenesis, accumulating evidence have demonstrated that neurogenesis also occurs in postnatal-adult mouse hypothalamus, particularly in the first two postnatal weeks. This second wave of active neurogenesis contributes to the remodeling of hypothalamic neuronal populations and regulation of energy homeostasis including hypothalamic leptin sensing. Radial glia cell types, such as tanycytes, are known to act as neuronal progenitors in the postnatal mouse hypothalamus. Our recent study unveiled a previously unreported radial glia-like neural stem cell (RGL-NSC) population that actively contributes to neurogenesis in the postnatal mouse hypothalamus. We also identified Irx3 and Irx5, which encode Iroquois homeodomain-containing transcription factors, as genetic determinants regulating the neurogenic property of these RGL-NSCs. These findings are significant as IRX3 and IRX5 have been implicated in FTO-associated obesity in humans, illustrating the importance of postnatal hypothalamic neurogenesis in energy homeostasis and obesity. In this review, we summarize current knowledge regarding postnatal-adult hypothalamic neurogenesis and highlight recent findings on the radial glia-like cells that contribute to the remodeling of postnatal mouse hypothalamus. We will discuss characteristics of the RGL-NSCs and potential actions of Irx3 and Irx5 in the regulation of neural stem cells in the postnatal-adult mouse brain. Understanding the behavior and regulation of neural stem cells in the postnatal-adult hypothalamus will provide novel mechanistic insights in the control of hypothalamic remodeling and energy homeostasis.

Epigenetic modifier Kdm6a/Utx controls the specification of hypothalamic neuronal subtypes in a sex-dependent manner

“…Our data show a requirement of Kdm6a for Ascl1 mRNA expression equally important in male and female hypothalamic neurons, suggesting a key regulatory role for Kdm6a in neurogenesis and neuronal diversification in the hypothalamus through the transcriptional control of a proneural master gene such as Ascl1. These results are not only relevant considering the proneural functions of Ascl1 during hypothalamic development in utero but could also be of critical interest in the exciting, more recent, and rapidly developing field of postnatal-adult hypothalamic neurogenesis (Kokoeva et al, 2005(Kokoeva et al, , 2007Hourai and Miyata, 2013;Yoo and Blackshaw, 2018), where Ascl1 expression has been proposed to regulate the switch from the quiescent to the active state of neural stem cells (Sueda et al, 2019;Dou et al, 2021;Son et al, 2021). However, ChIP-qPCR assays found neither H3K27me3 enrichment nor any change in H3K27me3 levels after Kdm6a knockdown at the Ascl1 promoter, indicating that Kdm6amediated mechanisms other than histone demethylation are acting to promote Ascl1 transcription in both male and female hypothalamic neurons.…”

Evaluating the mouse neural precursor line, SN4741, as a suitable proxy for midbrain dopaminergic neurons

“…When shifted to the non-permissive temperature, SN4741 cells appear to robustly differentiate, exemplified by a decrease in the expression of Nestin ( Nes ), a neural stem cell marker ( Figure 1F ). Additional transcriptional changes at this non-permissive temperature include an increase in the expression of a neural marker CUGBP Elav-Like Family Member 5 ( Celf5 ) 43 , as well as transcripts that have been found to regulate neural stem cell self-renewal (Inhibitor of DNA Binding 2, Id2; High Mobility Group AT-Hook 2, Hmga2 ) 44,45 , neurogenesis (Iroquois Homeobox 3, Irx3 ) 46 , and arborization of neurons (Sodium Voltage-Gated Channel Beta Subunit 1, Scn1b ) 47 , indicating that these cells may be differentiating into neural precursor cells ( Figure 1G ). Furthermore, Cadherin 13 ( Cdh13) , a modulator of GABAergic neurons, is significantly upregulated at this non-permissive temperature ( Figure 1G ), while the expression of a variety of DA neuron markers fail to be detected in either the permissive or non-permissive temperatures.…”

“…When shifted to the non-permissive temperature, SN4741 cells appear to robustly differentiate, exemplified by a decrease in the expression of Nestin (Nes), a neural stem cell marker (Figure 1F). Additional transcriptional changes at this non-permissive temperature include an increase in the expression of a neural marker CUGBP Elav-Like Family Member 5 (Celf5) [43], as well as genes that have been found to regulate neural stem cell self-renewal (Inhibitor of DNA Binding 2, Id2; High Mobility Group AT-Hook 2, Hmga2) [44,45], neurogenesis (Iroquois Homeobox 3, Irx3) [46], and arborization of neurons (Sodium Voltage-Gated Channel Beta Subunit 1, Scn1b) [47],…”

Epigenetic Modifier Kdm6a/Utx Controls the Specification of Hypothalamic Neuronal Subtypes in a Sex-Dependent Manner

“…Our data show a requirement of Kdm6a for Ascl1 mRNA expression equally important in male and female hypothalamic neurons, suggesting a key regulatory role for Kdm6a in neurogenesis and neuronal diversification in hypothalamus through the transcriptional control of a proneural master gene such as Ascl1. These results are not only relevant considering the proneural functions of Ascl1 during hypothalamic development in utero, but could also be of critical interest in the exciting, more recent and rapidly developing field of postnatal-adult hypothalamic neurogenesis (Kokoeva et al, 2005;Hourai and Miyata, 2013;Yoo and Blackshaw, 2018), where Ascl1 expression has been proposed to regulate the switch from the quiescent to the active state of neural stem cells (Sueda et al, 2019;Dou et al, 2021;Son et al, 2021). Additionally, we found higher mRNA levels of the dopaminergic molecular marker Th in female than in male-derived cultures and a female-specific requirement of Kdm6a for the expression of this sexually dimorphic pattern.…”