The function of the cerebral cortex depends on various types of interneurons (cortical interneurons [cINs]) and their appropriate allocation to the cortical layers. Caudal ganglionic eminence-derived cINs (cGE-cINs) are enriched in superficial layers. Developmental mechanisms directing cGE-cINs toward superficial layers remain poorly understood. We examine how developmental and final positioning of cGE-cINs are influenced by the Cxcl12, Cxcr4, Ackr3 module, the chief attractant system guiding medial ganglionic eminence-derived cINs (mGE-cINs). We find that Cxcl12 attracts cGE-cINs through Cxcr4 and supports their layer-specific positioning in the developing cortex. This requires the prevention of excessive Cxcr4 stimulation by Ackr3-mediated Cxcl12 sequestration. Postnatally, Ackr3 confines Cxcl12 action to the marginal zone. Unlike mGE-cINs, cGE-cINs continue to express Cxcr4 at early postnatal stages, which permits cGE-cINs to become positioned in the forming layer 1. Thus, chemoattraction by Cxcl12 guides cGE-cINs and holds them in superficial cortical layers.

Phosphorylation of heptahelical receptors is thought to regulate G protein signaling, receptor endocytosis, and non-canonical signaling via recruitment of β-arrestins. We investigated chemokine receptor functionality under phosphorylation-deficient and β-arrestin-deficient conditions by studying interneuron migration in the embryonic cortex. This process depends on CXCL12, CXCR4, G protein signaling and on the atypical CXCL12 receptor ACKR3. We found that phosphorylation was crucial, whereas β-arrestins were dispensable for ACKR3-mediated control of CXCL12 levels in vivo. Cortices of mice expressing phosphorylation-deficient ACKR3 exhibited a major interneuron migration defect, which was accompanied by excessive activation and loss of CXCR4. Cxcl12-overexpressing mice mimicked this phenotype. Excess CXCL12 caused lysosomal CXCR4 degradation, loss of CXCR4 responsiveness, and, ultimately, similar motility defects as Cxcl12 deficiency. By contrast, β-arrestin deficiency caused only a subtle migration defect mimicked by CXCR4 gain of function. These findings demonstrate that phosphorylation regulates atypical chemokine receptor function without β-arrestin involvement in chemokine sequestration and non-canonical signaling.

Biallelic mutations in DONSON, an essential gene encoding for a replication fork protection factor, were linked to skeletal abnormalities and microcephaly. To better understand DONSON function in corticogenesis, we characterized Donson expression and consequences of conditional Donson deletion in the mouse telencephalon. Donson was widely expressed in the proliferation and differentiation zones of the embryonic dorsal and ventral telencephalon, which was followed by a postnatal expression decrease. Emx1-Cre-mediated Donson deletion in progenitors of cortical glutamatergic neurons caused extensive apoptosis in the early dorsomedial neuroepithelium, thus preventing formation of the neocortex and hippocampus. At the place of the missing lateral neocortex, these mutants exhibited a dorsal extension of an early-generated paleocortex. Targeting cortical neurons at the intermediate progenitor stage using Tbr2-Cre evoked no apparent malformations, whereas Nkx2.1-Cre-mediated Donson deletion in subpallial progenitors ablated 75% of Nkx2.1-derived cortical GABAergic neurons. Thus, the early telencephalic neuroepithelium depends critically on Donson function. Our findings help explain why the neocortex is most severely affected in individuals with DONSON mutations and suggest that DONSON-dependent microcephaly might be associated with so far unrecognized defects in cortical GABAergic neurons. Targeting Donson using an appropriate recombinase is proposed as a feasible strategy to ablate proliferating and nascent cells in experimental research.