Tanycytes of the hypothalamic median eminence form a diet-responsive neurogenic niche

Abstract

Adult hypothalamic neurogenesis has recently been reported, but the cell of origin and the function of these newborn neurons are unknown. Using genetic fate mapping, we found that median eminence tanycytes generate newborn neurons. Blocking this neurogenesis altered the weight and metabolic activity of adult mice. These findings reveal a previously unreported neurogenic niche in the mammalian hypothalamus with important implications for metabolism.

Figure 1. Hypothalamic Proliferative Zone

PD19 mice received BrdU (PD10 18), and coronal ventrobasal hypothalamic sections were examined for BrdU immunostaining. (a) Hypothalamic proliferative zone (HPZ): enriched BrdU⁺ cell population along the MEm ependymal layer of third ventricle (3V) floor. (b) Quantification of BrdU⁺ ependymal layer cells juxtaposing hypothalamic nuclei (n=4; mean±s.e.m.; *=p<0.01): median eminence (MEm: 8488.5±1134.8 cells/mm²), arcuate nucleus (ArcN: 66.5±54.0 cells/mm²), ventromedial nucleus (VMH: 298.1±58.3 cells/mm²), dorsomedial nucleus (DMH: 307.6±90.3 cells/mm²). (c) HPZ BrdU⁺ cells are β2-tanycytes: ependymal layer position, radial-glial like morphology, and co-expression of tanycyte-enriched markers like Nestin. Dashed box: higher magnification z-stack reconstruction. Sections counterstained with DAPI (blue), a nuclear marker. Scale bar = 50μm (a); 10μm (c).

Figure 2. The median eminence is a neurogenic niche

(a) MEm neuronal distribution in PD35 mice. (b) PD35 mice received BrdU (PD10 18); MEm immunostained for BrdU and pan-neuronal marker Hu. Hu⁺BrdU⁺ co-expression adjacent to HPZ. (c) Age and diet-dependent effects of MEm neurogenesis. Newborn Hu⁺BrdU⁺ MEm neurons quantified 1 month following BrdU administration. (n=4; mean±s.e.m.; *=p<0.05) (d) Nestin:CreER^R26stopYFP mice permanently labels tanycytes and their progeny with yellow fluorescent protein (YFP) after 4-hydroxytamoxifen (4-OHT) induction. (e) PD7 Nestin:CreER^R26stopYFP mice treated with 4-OHT 72 hrs prior demonstrate specificity of tanycytic labeling via co-expression of YFP and Nestin⁺ tanycytes. (f) PD35 Nestin:CreER^R26stopYFP mice were 4-OHT induced at PD4. Hu⁺YFP⁺ neurons present following 1 month post-induction, but not following a 72 hr interval (SFig. 5g). Sections counterstained with DAPI (blue), a nuclear marker. Scale bars: 50μm (a); 20μm (b,e); 10μm (f).

Figure 3. Median eminence neurogenesis regulates metabolism

(a) Experimental diagram of CT-guided irradiation of HPZ. (b) Superimposition of dosimetry-film acquired with 1mm radiation beam in phantom with an X-ray of a real mouse subject (yellow line). White circle (arrow) indicates 10Gy radiation dose focally targeted towards HPZ. (c) Radiation targeting accuracy confirmed by γH2AX immunostaining, an indicator of radiation dose. γH2AX immunostaining limited to the HPZ within the ventrobasal hypothalamus. No radiation exposure observed within the SVZ and the SGZ. Scale bar = 200μm (d) Attenuated weight gain in HFD-fed irradiated mice compared with parallel sham controls (n=9). (e) Irradiation inhibits MEm neurogenesis by ~85% (n=4). (f) Higher energy expenditure observed in irradiated mice (n=11). (g) Higher total activity observed in irradiated mice (n=11). mean±s.e.m *=p<0.05; **=p<0.01