Huntington's disease (HD) is a neurodegenerative disease caused by the expansion of a poly-glutamine (poly-QTo determine whether the disease-causing poly-Q mutation in Htt affects synapse development, we next investigated the synaptic connectivity in a full-length knock-in mouse model of HD, the zQ175 mouse. Similar to the cortical conditional knock-outs, we found excessive excitatory synapse formation and maturation in the cortices of P21 zQ175, which was lost by 5 weeks. Together, our findings reveal that cortical Htt is required for the correct establishment of cortical and striatal excitatory circuits, and this function of Htt is lost when the mutant Htt is present.

The discovery of a fetal origin for tissue-resident macrophages (trMacs) has inspired an intense search for the mechanisms underlying their development. Here, we performed in vivo lineage tracing of cells with an expression history of IL7Rα, a marker exclusively associated with the lymphoid lineage in adult hematopoiesis. Surprisingly, we found that Il7r-Cre labeled fetalderived, adult trMacs. Labeling was almost complete in some tissues and partial in others. The putative progenitors of trMacs, yolk sac (YS) erythromyeloid progenitors, did not express IL7R, and YS hematopoiesis was unperturbed in IL7R-deficient mice. In contrast, tracking of IL7Rα message levels, surface expression, and Il7r-Cremediated labeling across fetal development revealed dynamic regulation of Il7r mRNA expression and rapid upregulation of IL7Rα surface protein upon transition from monocyte to macrophage within fetal tissues. Fetal monocyte differentiation in vitro produced IL7R + macrophages, supporting a direct progenitor-progeny relationship. Additionally, blockade of IL7R function during late gestation specifically impaired the establishment of fetal-derived trMacs in vivo. These data provide evidence for a distinct function of IL7Rα in fetal myelopoiesis and identify IL7R as a novel regulator of trMac development.

Highlights d Loss of Htt in striatal neurons recapitulates key features of Huntington's disease d Striatal neurons require Htt for survival during aging d Deletion of Htt from striatal neurons disrupts synaptic connectivity