Alterations of Cell Proliferation and Apoptosis in the Hypoplastic Reeler Cerebellum

Thrombospondin 1 and Reelin act through Vldlr to regulate cardiac growth and repair

Adult mammalian cardiomyocytes have minimal cell cycle capacity, which leads to poor regeneration after cardiac injury such as myocardial infarction. Many positive regulators of cardiomyocyte cell cycle and cardioprotective signals have been identified, but extracellular signals that suppress cardiomyocyte proliferation are poorly understood. We profiled receptors enriched in postnatal cardiomyocytes, and found that very-low-density-lipoprotein receptor (Vldlr) inhibits neonatal cardiomyocyte cell cycle. Paradoxically, Reelin, the well-known Vldlr ligand, expressed in cardiac Schwann cells and lymphatic endothelial cells, promotes neonatal cardiomyocyte proliferation. Thrombospondin1 (TSP-1), another ligand of Vldlr highly expressed in adult heart, was then found to inhibit cardiomyocyte proliferation through Vldlr, and may contribute to Vldlr's overall repression on proliferation. Mechanistically, Rac1 and subsequent Yap phosphorylation and nucleus translocation mediate the regulation of the cardiomyocyte cell cycle by TSP-1/Reelin-Vldlr signaling. Importantly, Reln mutant neonatal mice displayed impaired cardiomyocyte proliferation and cardiac regeneration after apical resection, while cardiac-specific Thbs1 deletion and cardiomyocyte-specific Vldlr deletion promote cardiomyocyte proliferation and are cardioprotective after myocardial infarction. Our results identified a novel role of Vldlr in consolidating extracellular signals to regulate cardiomyocyte cell cycle activity and survival, and the overall suppressive TSP-1-Vldlr signal may contribute to the poor cardiac repair capacity of adult mammals.

Co-cultures of cerebellar slices from mice with different reelin genetic backgrounds as a model to study cortical lamination

Background: Reelin has fundamental functions in the developing and mature brain. Its absence gives rise to the Reeler phenotype in mice, the first described cerebellar mutation. In homozygous mutants missing the Reelin gene ( reln -/-), neurons are incapable of correctly positioning themselves in layered brain areas such as the cerebral and cerebellar cortices. We here demonstrate that by employing ex vivo cultured cerebellar slices one can reduce the number of animals and use a non-recovery procedure to analyze the effects of Reelin on the migration of Purkinje neurons (PNs). Methods: We generated mouse hybrids (L7-GFP relnF1/) with green fluorescent protein (GFP)-tagged PNs, directly visible under fluorescence microscopy. We then cultured the slices obtained from mice with different reln genotypes and demonstrated that when the slices from reln -/- mutants were co-cultured with those from reln +/- mice, the Reelin produced by the latter induced migration of the PNs to partially rescue the normal layered cortical histology. We have confirmed this observation with Voronoi tessellation to analyze PN dispersion. Results: In images of the co-cultured slices from reln -/- mice, Voronoi polygons were larger than in single-cultured slices of the same genetic background but smaller than those generated from slices of reln +/- animals. The mean roundness factor, area disorder, and roundness factor homogeneity were different when slices from reln -/- mice were cultivated singularly or co-cultivated, supporting mathematically the transition from the clustered organization of the PNs in the absence of Reelin to a layered structure when the protein is supplied ex vivo. Conclusions: Neurobiologists are the primary target users of this 3Rs approach. They should adopt it for the possibility to study and manipulate ex vivo the activity of a brain-secreted or genetically engineered protein (scientific perspective), the potential reduction (up to 20%) of the animals used, and the total avoidance of severe surgery (3Rs perspective).

The Phosphofurin Acidic Cluster Sorting Protein 2 (PACS-2) E209K Mutation Responsible for PACS-2 Syndrome Increases Susceptibility to Apoptosis

Phosphofurin acidic cluster sorting protein 2 (PACS-2) is a multifunctional cytosolic membrane trafficking protein with distinct roles in maintaining cellular homeostasis. Recent clinical reports have described 28 individuals possessing a de novo PACS-2 E209K mutation that present with epileptic seizures and cerebellar dysgenesis. As the PACS-2 E209K missense mutation has become a marker for neurodevelopmental disorders, we sought to characterize its biochemical properties. Accordingly, we observed that the PACS-2 E209K protein exhibited a slower turnover rate relative to PACS-2 wild type (WT) upon cycloheximide treatment in 293T cells. The longer half-life of PACS-2 E209K suggests a disruption in its proteostasis, with the potential for altered protein-protein interactions. Indeed, a regulatory protein in neurodevelopment known as 14-3-3ε was identified as having an increased association with PACS-2 E209K. Subsequently, when comparing the effect of PACS-2 WT and E209K expression on the staurosporine-induced apoptosis response, we found that PACS-2 E209K increased susceptibility to staurosporine-induced apoptosis in HCT 116 cells. Overall, our findings suggest PACS-2 E209K alters PACS-2 proteostasis and favors complex formation with 14-3-3ε, leading to increased cell death in the presence of environmental stressors.

Neuroprotective Effect of E3 Ubiquitin Ligase RNF8 Against Ischemic Stroke via HDAC2 Stability Reduction and Reelin-Dependent GSK3β Inhibition

Loss of E3 ubiquitin ligase RING finger protein 8 (RNF8) may lead to neuronal DNA damage and apoptosis. In order to expand on our knowledge on the mechanistic basis underlying neuronal death in ischemic stroke, the present study sought to investigate the potential role of E3 ubiquitin ligase RNF8 on ischemic stroke and explore the underlying downstream mechanism. Middle cerebral artery occlusion (MCAO) in mice and oxygen–glucose deprivation/reoxygenation (OGD/R) in neurons were induced to simulate an ischemic stroke environment. It was found that downregulation of RNF8 and Reelin occurred in MCAO mice and OGD/R-exposed neurons. Silencing of RNF8 enhanced the MCAO-induced neuronal apoptosis and oxidative stress. Mechanistically, RNF8 enhanced the ubiquitination and degradation of HDAC2, thus attenuating OGD/R-induced neuronal apoptosis and oxidative stress. Moreover, HDAC2 inhibited Reelin expression through deacetylation of H3K27me3 in its promoter, causing reduced glycogen synthase kinase-3beta (GSK3β)-Ser9 phosphorylation and the resultant elevated GSK3β activity. By this mechanism, RNF8 alleviated ischemic stroke. Coherently, this study suggests that RNF8 plays a neuroprotective effect against ischemic stroke by downregulating HDAC2 expression and inducing Reelin-induced GSK3β inhibition.

The Reeler Mouse: A Translational Model of Human Neurological Conditions, or Simply a Good Tool for Better Understanding Neurodevelopment?

The first description of the Reeler mutation in mouse dates to more than fifty years ago, and later, its causative gene (reln) was discovered in mouse, and its human orthologue (RELN) was demonstrated to be causative of lissencephaly 2 (LIS2) and about 20% of the cases of autosomal-dominant lateral temporal epilepsy (ADLTE). In both human and mice, the gene encodes for a glycoprotein referred to as reelin (Reln) that plays a primary function in neuronal migration during development and synaptic stabilization in adulthood. Besides LIS2 and ADLTE, RELN and/or other genes coding for the proteins of the Reln intracellular cascade have been associated substantially to other conditions such as spinocerebellar ataxia type 7 and 37, VLDLR-associated cerebellar hypoplasia, PAFAH1B1-associated lissencephaly, autism, and schizophrenia. According to their modalities of inheritances and with significant differences among each other, these neuropsychiatric disorders can be modeled in the homozygous (reln^−/−) or heterozygous (reln^+/−) Reeler mouse. The worth of these mice as translational models is discussed, with focus on their construct and face validity. Description of face validity, i.e., the resemblance of phenotypes between the two species, centers onto the histological, neurochemical, and functional observations in the cerebral cortex, hippocampus, and cerebellum of Reeler mice and their human counterparts.

Caspase-3 Mediated Cell Death in the Normal Development of the Mammalian Cerebellum

Caspase-3, onto which there is a convergence of the intrinsic and extrinsic apoptotic pathways, is the main executioner of apoptosis. We here review the current literature on the intervention of the protease in the execution of naturally occurring neuronal death (NOND) during cerebellar development. We will consider data on the most common altricial species (rat, mouse and rabbit), as well as humans. Among the different types of neurons and glia in cerebellum, there is ample evidence for an intervention of caspase-3 in the regulation of NOND of the post-mitotic cerebellar granule cells (CGCs) and Purkinje neurons, as a consequence of failure to establish proper synaptic contacts with target (secondary cell death). It seems possible that the GABAergic interneurons also undergo a similar type of secondary cell death, but the intervention of caspase-3 in this case still remains to be clarified in full. Remarkably, CGCs also undergo primary cell death at the precursor/pre-migratory stage of differentiation, in this instance without the intervention of caspase-3. Glial cells, as well, undergo a process of regulated cell death, but it seems possible that expression of caspase-3, at least in the Bergmann glia, is related to differentiation rather than death.

Transient vimentin expression during the embryonic development of the chicken cerebellum

Complex morphogenetic events, critical for the development of normal cerebellum foliation and layering, are known to involve type III intermediate filament protein such as vimentin expressed by Bergmann glia. The present study aimed to determine aspects of intermediate and late embryonic pattern of vimentin expression during the corticogenesis of chicken cerebellum at embryonic days 10-19 (E10-E19), using single and double immunohistochemistry/immunofluorescence. Vimentin expression showed partial co-localization with the glial markers GFAP and BLBP. Within cerebellar cortex, vimentin+ fibers were first found within lobules I and X (E10) and gradually extended to all folia (E15-E17), located within the external granule (EGL) the molecular cell layer, showing a radial orientation towards the inner granular layer and the cerebellar white matter oriented longitudinally. Interestingly, within the immature fissures base of most lobules, vimentin+ fibers radiate in a fan shape. Short-term BrdU experiments revealed that EGL cell proliferation was higher in the fissure base compared to folia apex. In addition, following 24-h survival, BrdU+ cells were found in close association to vimentin+ fibers in the EGL pre-migratory zone and within immature molecular layer. Paralleling cerebellum development, vimentin expression gradually extended to all folia sub-regions (base, wall, apex), but, at day E19, it was mainly confined to the folia apex and secondary fissure base. Taken together our data further support the possible role of vimentin+ fibers in the structural events of cerebellum corticogenesis, suggesting the participation of radial/Bergmann glia in chicken cerebellum foliation, similarly to that described for mammalian cerebellum morphogenesis.

The number of Purkinje neurons and their topology in the cerebellar vermis of normal and reln haplodeficient mouse

The Reeler heterozygous mice (reln(+/-)) are haplodeficient in the gene (reln) encoding for the reelin glycoprotein (RELN) and display reductions in brain/peripheral RELN similar to autistic or schizophrenic patients. Cytoarchitectonic alterations of the reln(+/-) brain may be subtle, and are difficult to demonstrate by current histological approaches. We analyzed the number and topological organization of the Purkinje neurons (PNs) in five vermal lobules - central (II-III), culmen (IV-V), tuber (VIIb), uvula (IX), and nodulus (X) - that process different types of afferent functional inputs in reln(+/+) and reln(+/-) adult mice (P60) of both sexes (n=24). Animals were crossed with L7GFP mice so that the GFP-tagged PNs could be directly identified in cryosections. Digital images from these sections were processed with different open source software for quantitative topological and statistical analyses. Diversity indices calculated were: maximum caliper, density, area of soma, dispersion along the XZ axis, and dispersion along the YZ axis. We demonstrate: i. reduction in density of PNs in reln(+/-) males (14.37%) and reln(+/-) females (17.73%) compared to reln(+/+) males; ii. that reln(+/-) males have larger PNs than other genotypes, and females (irrespective of the reln genetic background) have smaller PNs than reln(+/+) males; iii. PNs are more chaotically arranged along the YZ axis in reln(+/-) males than in reln(+/+) males and, except in central lobulus, reln(+/-) females. Therefore, image processing and statistics reveal previously unforeseen gender and genotype-related structural differences in cerebellum that may be clues for the definition of novel biomarkers in human psychiatric disorders.