Stress induces behavioral abnormalities by increasing expression of phagocytic receptor MERTK in astrocytes to promote synapse phagocytosis.

Childhood neglect and/or abuse can induce mental health conditions with unknown mechanisms. Here, we identified stress hormones as strong inducers of astrocyte-mediated synapse phagocytosis. Using in vitro, in vivo, and human brain organoid experiments, we showed that stress hormones increased the expression of the Mertk phagocytic receptor in astrocytes through glucocorticoid receptor (GR). In post-natal mice, exposure to early social deprivation (ESD) specifically activated the GR-MERTK pathway in astrocytes, but not in microglia. The excitatory post-synaptic density in cortical regions was reduced in ESD mice, and there was an increase in the astrocytic engulfment of these synapses. The loss of excitatory synapses, abnormal neuronal network activities, and behavioral abnormalities in ESD mice were largely prevented by ablating GR or MERTK in astrocytes. Our work reveals the critical roles of astrocytic GR-MERTK activation in evoking stress-induced abnormal behaviors in mice, suggesting GR-MERTK signaling as a therapeutic target for stress-induced mental health conditions.

LGI1 governs neuritin-mediated resilience to chronic stress.

Depression is accompanied by neuronal atrophy and decreased neuroplasticity. Leucine-rich glioma-inactivated protein 1 (LGI1), a metastasis suppressor, plays an important role in the development of CNS synapses. We found that LGI1 expression was reduced in the hippocampi of mice that underwent chronic unpredictable stress (CUS), and could be rescued by the antidepressant, fluoxetine. Recombinant soluble neuritin, an endogenous protein previously implicated in antidepressant-like behaviors, elevated hippocampal LGI1 expression in a manner dependent on histone deacetylase 5 (HDAC5) phosphorylation. Accordingly, Nrn1 flox/flox ;Pomc-cre (Nrn1 cOE) mice, which conditionally overexpress neuritin, displayed increases in hippocampal LGI1 level under CUS and exhibited resilience to CUS that were blocked by hippocampal depletion of LGI1. Interestingly, neuritin-mediated LGI1 expression was inhibited by HNMPA-(AM)3, an insulin receptor inhibitor, as was neuritin-mediated HDAC5 phosphorylation. We thus establish hippocampal LGI1 as an effector of neurite outgrowth and stress resilience, and suggest that HDAC5-LGI1 plays a critical role in ameliorating pathological depression.

Side branching and luminal lineage commitment by ID2 in developing mammary glands.

Mammary glands develop through primary ductal elongation and side branching to maximize the spatial area. Although primary ducts are generated by bifurcation of terminal end buds, the mechanism through which side branching occurs is still largely unclear. Here, we show that inhibitor of DNA-binding 2 (ID2) drives side branch formation through the differentiation of K6+ bipotent progenitor cells (BPs) into CD61+ luminal progenitor cells (LPs). Id2-null mice had side-branching defects, along with developmental blockage of the differentiation of K6+ BPs into CD61+ LPs. Notably, CD61+ LPs were found in budding and side branches, but not in terminal end buds. Hormone reconstitution studies using ovariectomized MMTV-hemagglutinin-nuclear localized sequence-tagged Id2 transgenic mice revealed that ID2 is a key mediator of progesterone, which drives luminal lineage differentiation and side branching. Our results suggest that CD61 is a marker of side branches and that ID2 regulates side branch formation by inducing luminal lineage commitment from K6+ BPs to CD61+ LPs.

Survival and differentiation of mammary epithelial cells in mammary gland development require nuclear retention of Id2 due to RANK signaling.

RANKL plays an essential role in mammary gland development during pregnancy. However, the molecular mechanism by which RANK signaling leads to mammary gland development is largely unknown. We report here that RANKL stimulation induces phosphorylation of Id2 at serine 5, which leads to nuclear retention of Id2. In lactating Id2Tg; RANKL(-/-) mice, Id2 was not phosphorylated and was localized in the cytoplasm. In addition, in lactating Id2(S5A)Tg mice, Id2(S5A) (with serine 5 mutated to alanine) was exclusively localized in the cytoplasm of mammary epithelial cells (MECs), while endogenous Id2 was localized in the nucleus. Intriguingly, nuclear expression of Id2(S5A) rescued increased apoptosis and defective differentiation of MECs in RANKL(-/-) mice. Our results demonstrate that nuclear retention of Id2 due to RANK signaling plays a decisive role in the survival and differentiation of MECs during mammary gland development.

Essential role of CR6-interacting factor 1 (Crif1) in E74-like factor 3 (ELF3)-mediated intestinal development.

Although terminal differentiation of intestinal epithelium is essential for the efficient digestion and absorption of nutrients, little is known about the molecular mechanisms underlying this process. Recent studies have shown that Elf3 (E74-like factor 3), a member of the ETS transcription factor family, has an essential role in the terminal differentiation of absorptive enterocytes and mucus-secreting goblet cells. Here, we demonstrated that Crif1 (CR6-interacting factor 1) functions as transcriptional coactivator of Elf3 in intestinal epithelium differentiation. The intestinal epithelium-specific Crif1-deficient mice died soon after birth and displayed severe alterations of tissue architecture in the small intestine, including poor microvillus formation and abnormal differentiation of absorptive enterocytes. Strikingly, these phenotypes are largely similar to that of Elf3-deficient mice, suggesting that Elf3 signaling in the intestinal epithelium depends on the Crif1 expression. We dissected this relationship further and found that Crif1 indeed interacted with Elf3 through its ETS DNA binding domain and enhanced the transcriptional activity of Elf3 by regulating the DNA binding activity. Knockdown of Crif1 by RNA interference conversely attenuated the transcriptional activity of Elf3. Consistently, the expression level of Tgf-betaRII (transforming growth factor beta type II receptor), a critical target gene of Elf3, was dramatically reduced in the Crif1-deficient mice. Our results reveal that Crif1 is a novel and essential transcriptional coactivator of Elf3 for the terminal differentiation of absorptive enterocytes.

Crif1 is a novel transcriptional coactivator of STAT3.

Signal transducer and activator of transcription 3 (STAT3) is a transcriptional factor that performs a broad spectrum of biological functions in response to various stimuli. However, no specific coactivator that regulates the transcriptional activity of STAT3 has been identified. Here we report that CR6-interacting factor 1 (Crif1) is a specific transcriptional coactivator of STAT3, but not of STAT1 or STAT5a. Crif1 interacts with STAT3 and positively regulates its transcriptional activity. Crif1-/- embryos were lethal around embryonic day 6.5, and manifested developmental arrest accompanied with defective proliferation and massive apoptosis. The expression of STAT3 target genes was markedly reduced in a Crif1-/- blastocyst culture and in Oncostatin M-stimulated Crif1-deficient MEFs. Importantly, the key activities of constitutively active STAT3-C, such as transcription, DNA binding, and cellular transformation, were abolished in the Crif1-null MEFs, suggesting the essential role of Crif1 in the transcriptional activity of STAT3. Our results reveal that Crif1 is a novel and essential transcriptional coactivator of STAT3 that modulates its DNA binding ability, and shed light on the regulation of oncogenic STAT3.

Receptor activator of NF-kappaB ligand regulates the proliferation of mammary epithelial cells via Id2.

Receptor activator of NF-kappaB ligand (RANKL) is a key regulator for mammary gland development during pregnancy. RANKL-deficient mice display impaired development of lobulo-alveolar mammary structures. Similar mammary gland defects have been reported in mice lacking Id2. Here we report that RANKL induces the proliferation of mammary epithelial cells via Id2. RANKL triggers marked nuclear translocation of Id2 in mammary epithelial cells. In vivo studies further demonstrated the defective nuclear translocation of Id2, but the normal expression of cyclin D1, in the mammary epithelial cells of rankl-/- mice. In vitro studies with nuclear localization sequence-tagged Id2 revealed that the nuclear localization of Id2 itself is critical for the downregulation of p21 promoter activity. Moreover, RANKL stimulation failed to induce cell growth and to downregulate p21 expression in Id2-/- mammary epithelial cells. Our results indicate that the inhibitor of helix-loop-helix protein, Id2, is critical to control the proliferation of mammary epithelial cells in response to RANKL stimulation.