Genetic deletion of genes in the cerebellar rhombic lip lineage can stimulate compensation through adaptive reprogramming of ventricular zone-derived progenitors

Abstract

BackgroundThe cerebellum is a foliated posterior brain structure involved in coordination of motor movements and cognition. The cerebellum undergoes rapid growth postnataly due to Sonic Hedgehog (SHH) signaling-dependent proliferation of ATOH1+ granule cell precursors (GCPs) in the external granule cell layer (EGL), a key step for generating cerebellar foliation and the correct number of granule cells. Due to its late development, the cerebellum is particularly vulnerable to injury from preterm birth and stress around birth. We recently uncovered an intrinsic capacity of the developing cerebellum to replenish ablated GCPs via adaptive reprogramming of Nestin-expressing progenitors (NEPs). However, whether this compensation mechanism occurs in mouse mutants affecting the developing cerebellum and could lead to mis-interpretation of phenotypes was not known.MethodsWe used two different approaches to remove the main SHH signaling activator GLI2 in GCPs: 1) our mosaic mutant analysis with spatial and temporal control of recombination (MASTR) technique to delete Gli2 in a small subset of GCPs; 2) An Atohl-Cre transgene to delete Gli2 in most of the EGL. Genetic Inducible Fate Mapping (GIFM) and live imaging were used to analyze the behavior of NEPs after Gli2 deletion.ResultsMosaic analysis demonstrated that SHH-GLI2 signaling is critical for generating the correct pool of granule cells by maintaining GCPs in an undifferentiated proliferative state and promoting their survival. Despite this, inactivation of GLI2 in a large proportion of GCPs in the embryo did not lead to the expected dramatic reduction in the size of the adult cerebellum. GIFM uncovered that NEPs do indeed replenish GCPs in Gli2 conditional mutants, and then expand and partially restore the production of granule cells. Furthermore, the SHH signaling-dependent NEP compensation requires Gli2, demonstrating that the activator side of the pathway is involved.ConclusionWe demonstrate that a mouse conditional mutation that results in loss of SHH signaling in GCPs is not sufficient to induce long term severe cerebellum hypoplasia. The ability of the neonatal cerebellum to regenerate after loss of cells via a response by NEPs must therefore be considered when interpreting the phenotypes of conditional mutants affecting GCPs.

Impaired SNF2L Chromatin Remodeling Prolongs Accessibility at Promoters Enriched for Fos/Jun Binding Sites and Delays Granule Neuron Differentiation

“…In this regard, a rare population of Nestin-expressing progenitor cells (NEPs) that migrate from the second germinal zone (VZ lining the fourth ventricle) and reside in the deep part of the EGL could be affected by loss of Snf2l function. These NEPs are typically quiescent but can be activated to replenish granule neurons following injury in the postnatal period (Li et al, 2013;Wojcinski et al, 2017Wojcinski et al, , 2019. In such a scenario, the lack of Snf2l in this cell population would alleviate their quiescence and activate their proliferation.…”

Injury-induced ASCL1 expression orchestrates a transitory cell state required for repair of the neonatal cerebellum

“…When the proliferating GCPs are depleted in the newborn cerebellum either via genetic approaches or by irradiation, the external granule layer (EGL) recovers within a week and the cerebellum grows to almost a normal size ( 12 , 13 , 15 , 16 ). A key aspect of the regenerative process involves BgL-NEPs undergoing adaptive reprograming to replenish the lost GCPs.…”

ASCL1 drives induction of a transitory cell state required for repair of the injured neonatal brain

“…The late development of the CB leads to increased susceptibility to injury around birth and cerebellar hypoplasia is the second leading risk factor of autism spectrum disorders (Wang et al, 2014). Interestingly, the newborn rodent CB can efficiently replenish at least two of its main cell types when they are ablated (Altman and Anderson, 1971;Altman et al, 1969;Bayin et al, 2018;Wojcinski et al, 2017;Wojcinski et al, 2018), and one repair process involves unexpected progenitor plasticity and a glial to neural fate switch after injury. The molecular mechanisms that drive progenitor plasticity in vivo however are unknown in the neonatal CB.…”

“…When the proliferating GCPs are depleted in the newborn CB either via genetic approaches or by irradiation, the EGL recovers within a week and the CB grows to almost a normal size (Altman and Anderson, 1971;Altman et al, 1969;Wojcinski et al, 2017;Wojcinski et al, 2018). A key aspect of the regenerative process involves BgL-NEPs, thought to be gliogenic, undergoing adaptive reprograming to replenish the lost GCPs.…”

“…A key aspect of the regenerative process involves BgL-NEPs, thought to be gliogenic, undergoing adaptive reprograming to replenish the lost GCPs. The first response of the BgL-NEPs is an increase in proliferation, followed by migration to the injured EGL where they initiate expression of Atoh1 and become GCPs (Andreotti et al, 2018;Jaeger and Jessberger, 2017;Wojcinski et al, 2017;Wojcinski et al, 2018). Simultaneously, the WM-NEPs in the lobules reduce their proliferation and differentiation until the EGL is restored, leading to an overall delay in CB development, likely to ensure proper scaling of the different cell types of the cerebellar cortex (Wojcinski et al, 2017).…”