Fig. 1

a Image of adult (P35) PV-IRES-Cre; Ai14-TdTomato mouse brain. The cerebellum is visibly pink due to high TdTomato expression. Black arrowhead denotes lobule X, only lobule without visible RFP expression. b Cartoon outlining the architecture of cerebellar cortex (adapted with modifications from Cerminara et al., Nat. Rev. Neurosci. 2015). Inhibitory neurons (Purkinje projection neurons and molecular layer interneurons, MLIs) are localized to the molecular layer, while excitatory granule cells (GCs) are located in the granule cell layer. c Immunostaining of P12 PV-IRES-Cre; Ai14-TdTomato cerebellum. Entire cerebellar cortex is globally TdTomato + with the exception of scattered dark, TdTomato-soma within the molecular layer (ML; containing inhibitory neurons (−); teal bar) and granule cell layer (GCL; containing excitatory neurons ( +); magenta bar). d Immunostaining for RFP in PV-IRES-Cre; TdTomato at P9 (top) and P38 (bottom) show TdTomato + granule cells within the GCL (containing mature granule cells; magenta bar) and internal granule cell layer (IGL; containing granule cell progenitors and precursors; yellow bar). e TdTomato ISH staining for PV-T2A-Cre; Ai14-TdTomato mouse shows similar positive Cre expression within granule cells (from the Allen Brain Institute). f PV-IRES-Cre was crossed to the mTmG mouse line, express membrane-targeted GFP upon Cre recombination (Muzumdar et al., Genesis, 2007). Cerebellar cortex of PV-IRES-Cre; mTmG mouse shows Cre reporter expression in granule cells in the GCL and EGL (insets). High GFP expression in the ML arises from granule cell axons. g Lobule specific activation of PV-IRES-Cre in granule cells. Immunostaining for TdTomato, PV, and VGLUT1 are shown for subset of cerebellar folia. VGLUT1 labels GC axon terminals within the molecular layer, and is present across lobules. Zoomed in view of IGL (bottom) show lobule-specific Cre recombination patterns. The majority of granule cells within folia I, III and V are TdTomato + . Few granule cells in folia X are TdTomato + . h Immunostaining for PV and TdTomato demonstrate delayed onset of Cre expression in inhibitory interneurons of interest. Migratory MLIs in the upper ML (yellow arrowheads and inset) are TdTomato (magenta) negative despite being positive for PV (green). Maturing MLIs in the lower ML (blue arrowheads and inset) are Cre and PV positive. i Quantifications demonstrating lobule-specific nature of PV-Cre recombination. TdTomato + (top) or GFP + (bottom) cells were quantified as a proportion of DAPI + nuclei from the external granule cell layer (EGL) at P9. EGL was chosen as opposed to the internal granule cell layer as it contains almost exclusively granule cell progenitors and precursors, except small numbers of migrating molecular layer interneurons. Error bars represent the standard deviation. j UMAP for scRNA-Seq showing Pvalb (PV) expression within granule cell progenitors between E13–E15. Top panel shows granule cells highlighted, with color codes representing animal age. Bottom panel shows PV expression, with high expression colocalizing with the E13–E15 timepoints. k Single molecule fluorescence in situ hybridization images for Zic1 (magenta) and PV (yellow) in the E13.5 rhombic lip where granule cell progenitors (GCPs) reside. Zic1 labels proliferating GCPs. PV mRNA is observed in a subset of Zic1 + GCPs