Hawkes, R. | bioRxiv Science

bioRxiv2017

Axons from the trigeminal ganglia are the earliest afferent projections to the mouse cerebellum

The first stage standard model for the development of afferent pathways to the cerebellum is that ingrowing axons target the embryonic Purkinje cells (E13-E16 in mice). Perinatally and early postnatal (E18-P15) the climbing fibers translocate to the Purkinje cell dendrites, and as the granular layer develops the mossy fibers translocate from the Purkinje cell somata and synapse with granule cell dendrites. In this report we describe a novel earlier stage in the development. Immunostaining for a neurofilament-associated antigen (NAA) reveals the early axon distributions with remarkable clarity. Axons from the trigeminal system enter the cerebellar primordium as early as embryo age (E)9. By using a combination of axon tract tracing, analysis of neurogenin1 null mice - which do not develop trigeminal ganglia - and mouse embryos maintained in vitro - we show that the first axons to innervate the cerebellar primordium are direct projections from the trigeminal ganglia. The data show that the early trigeminal projections are in situ before the Purkinje cells are born, and double immunostaining for NAA and markers of the different domains in the cerebellar primordium reveal that they first target the cerebellar nuclear neurons of the nuclear transitory zone (E9-E10), and only later (E10-E11) extend collateral branches to the Purkinje cell plate.

neuroscience↗

bioRxiv2017

Zfp423, A Joubert Syndrome Gene, Is A Domain-Specific Regulator Of Cell Cycle Progression, DNA Damage Response And Purkinje Cell Development In The Cerebellar Primordium

Neurogenesis is a tightly regulated process whose success depends on the ability to balance the expansion/maintenance of an undifferentiated neural progenitor pool with the precisely timed birth of sequential generations of neurons. The Zfp423 gene encodes a 30-Zn-finger transcription factor (TF) that acts as a scaffold in the assembly of complex transcriptional and cellular machineries regulating neural development. While null mutants for Zfp423 feature a severe cerebellar hypoplasia, the underlying mechanism is only partially characterized. Mutations of the human ortholog ZNF423 have been identified in patients carrying cerebellar vermis hypoplasia (CVH) or Joubert Syndrome (JS), associated with other signs of classical ciliopathy outside the central nervous system (CNS). ZNF423 also plays a role in the DNA damage response (DDR). To further characterize the role of ZFP423 in cerebellar neurogenesis, with a focus on Purkinje cells (PC) development, we analyzed two previously undescribed mutant mouse lines carrying allelic in-frame deletions of the corresponding gene, selectively affecting two functionally characterized protein-protein interaction domains, affecting zinc (Zn) fingers 9-20 or 28-30. Some phenotypic defects are allele specific: Zfp423{Delta}9-20/{Delta}9-20 mutants exhibit a depletion of the OLIG2+ PC progenitor pool in the cerebellar ventricular zone (VZ). In these mutants, M-phase progenitors display changes in spindle orientation indicative of a precocious switch from symmetric to asymmetric cell division. Conversely, the Zfp423{Delta}28-30/{Delta}28-30 primordium displays a sharp decrease in the expression of PC differentiation markers, including CORL2, despite an abundance of cycling PC progenitors. Moreover, and importantly, in both mutants VZ progenitor cell cycle progression is remarkably affected, and factors involved in the DDR are substantially upregulated in the VZ and in postmitotic precursors alike. Our in vivo evidence sheds light on the domain-specific roles played by ZFP423 in different aspects of PC progenitor development, and at the same time supports the emerging notion that an impaired DNA damage response may be a key factor in the pathogenesis of JS and other ciliopathies.

neuroscience↗