Dynamic CREB family activity drives segmentation and posterior polarity specification in mammalian somitogenesis.

The segmented body plan of vertebrates is prefigured by reiterated embryonic mesodermal structures called somites. In the mouse embryo, timely somite formation from the presomitic mesoderm (PSM) is controlled by the "segmentation clock," a molecular oscillator that triggers progressive waves of Notch activity throughout the PSM. Notch clock activity is suppressed in the posterior PSM by FGF signaling until it crosses a determination front at which its net activity is sufficiently high to effect segmentation. Here, Notch and Wnt signaling directs somite anterior/posterior (A/P) polarity specification and boundary formation via regulation of the segmentation effector gene Mesoderm posterior 2. How Notch and Wnt signaling becomes coordinated at this front is incompletely defined. Here we show that the activity of the cAMP responsive element binding protein (CREB) family of transcription factors exhibits Wnt3a-dependent oscillatory behavior near the determination front and is in unison with Notch activity. Inhibition of CREB family in the mesoderm causes defects in somite segmentation and a loss in somite posterior polarity leading to fusions of vertebrae and ribs. Among the CREB family downstream genes, several are known to be regulated by Wnt3a. Of those, we show that the CREB family occupies a conserved binding site in the promoter region of Delta-like 1, encoding a Notch ligand, in the anterior PSM as a mechanism to specify posterior identity of somites. Together, these data support that the CREB family acts at the determination front to modulate Wnt signaling and strengthen Notch signaling as a means to orchestrate cells for somite segmentation and anterior/posterior patterning.

CSF1R inhibitors induce a sex-specific resilient microglial phenotype and functional rescue in a tauopathy mouse model.

Microglia are central to pathogenesis in many neurological conditions. Drugs targeting colony-stimulating factor-1 receptor (CSF1R) to block microglial proliferation in preclinical disease models have shown mixed outcomes, thus the therapeutic potential of this approach remains unclear. Here, we show that CSF1R inhibitors given by multiple dosing paradigms in the Tg2541 tauopathy mouse model cause a sex-independent reduction in pathogenic tau and reversion of non-microglial gene expression patterns toward a normal wild type signature. Despite greater drug exposure in male mice, only female mice have functional rescue and extended survival. A dose-dependent upregulation of immediate early genes and neurotransmitter dysregulation are observed in the brains of male mice only, indicating that excitotoxicity may preclude functional benefits. Drug-resilient microglia in male mice exhibit morphological and gene expression patterns consistent with increased neuroinflammatory signaling, suggesting a mechanistic basis for sex-specific excitotoxicity. Complete microglial ablation is neither required nor desirable for neuroprotection and therapeutics targeting microglia must consider sex-dependent effects.

A novel vector for transgenesis in the rat CNS.

The larger brain of the rat enables a much greater repertoire of complex behaviors than mice, likely making rats preferential for investigating neurodegeneration. Because molecular tools for specific expression of transgenes in the rat brain are sparse, we chose Prnp encoding the prion protein (PrP) to develop a novel vector to drive transgene expression in the rat brain. We compared the rat Prnp sequence with mouse and Syrian hamster Prnp sequences, identifying conserved genetic elements and hypothesizing that these elements would be able to drive neuronal transgene expression. We investigated this by generating a vector termed RaPrnp that encompasses portions of the rat Prnp gene. Importantly, we replaced the rat Prnp open reading frame (ORF) with a cloning site for rapid and seamless In-Fusion cloning. To validate the in vivo neuronal specificity of the RaPrnp vector in rats, we generated stable RaPrnp-LacZ/enhanced green fluorescent protein (EGFP) transgenic (Tg) rat lines, which led to robust LacZ activity and high EGFP fluorescence in the central nervous system of embryos and adult animals. Next, we restored the rat Prnp ORF and generated multiple Tg(RaPrnp-PrP) lines, demonstrating that overexpression of Prnp accelerates the onset of scrapie. While the incubation time in wild-type (WT) rats was 175 ± 3 days post inoculation (dpi), one line, Tg2919, overexpressed RaPrPC at 4.4-fold and exhibited a reduced incubation time of 149 ± 2 dpi. The second line, Tg2922, overexpressed RaPrPC at 9.7-fold compared with WT animals and had an incubation time of 112 ± 0 dpi. Tg2922 rats inoculated with rat RML showed extensive vacuolation of the brainstem in contrast to WT and Tg2919 animals in which vacuolation was most prominent in the hippocampus and striatum as well as the motor and sensory cortices. It is possible that construction of Tg rats with modified phenotypes will prove more advantageous than mice for neurodegeneration studies.