BIG1 is required for the survival of deep layer neurons, neuronal polarity, and the formation of axonal tracts between the thalamus and neocortex in developing brain

Abstract

BIG1, an activator protein of the small GTPase, Arf, and encoded by the Arfgef1 gene, is one of candidate genes for epileptic encephalopathy. To know the involvement of BIG1 in epileptic encephalopathy, we analyzed BIG1-deficient mice and found that BIG1 regulates neurite outgrowth and brain development in vitro and in vivo. The loss of BIG1 decreased the size of the neocortex and hippocampus. In BIG1-deficient mice, the neuronal progenitor cells (NPCs) and the interneurons were unaffected. However, Tbr1+ and Ctip2+ deep layer (DL) neurons showed spatial-temporal dependent apoptosis. This apoptosis gradually progressed from the piriform cortex (PIR), peaked in the neocortex, and then progressed into the hippocampus from embryonic day 13.5 (E13.5) to E17.5. The upper layer (UL) and DL order in the neocortex was maintained in BIG1-deficient mice, but the excitatory neurons tended to accumulate before their destination layers. Further pulse-chase migration assay showed that the migration defect was non-cell autonomous and secondary to the progression of apoptosis into the BIG1-deficient neocortex after E15.5. In BIG1-deficient mice, we observed an ectopic projection of corticothalamic axons from the primary somatosensory cortex (S1) into the dorsal lateral geniculate nucleus (dLGN). The thalamocortical axons were unable to cross the diencephalon–telencephalon boundary (DTB). In vitro, BIG1-deficient neurons showed a delay in neuronal polarization. BIG1-deficient neurons were also hypersensitive to low dose glutamate (5 μM), and died via apoptosis. This study showed the role of BIG1 in the survival of DL neurons in developing embryonic brain and in the generation of neuronal polarity.

Haploinsufficiency of ARFGEF1 is associated with developmental delay, intellectual disability, and epilepsy with variable expressivity

“…Regarding the findings on brain MRI, the most common abnormality was a delayed myelination shown by subcortical white matter signal hyperintensity. This is consistent with what was observed in mice which displayed altered neurodevelopment with delayed neural polarization and increased neural apoptosis 15,16 . One individual -patient 3 with the focal motor seizuresdisplayed periventricular nodular heterotopies (PH), a feature that echoes the brain abnormalities observed in a Turkish family with ARFGEF2 (Periventricular heterotopia with microcephaly -OMIM 608097 -Autosomal Recessive) pathogenic variants who displayed severe developmental delay, microcephaly, early-onset refractory epilepsy, bilateral nodular periventricular heterotopia and frequent infections 33 , and a phenotype that partially overlaps the one observed in our cohort.…”

“…The mice displayed developmental delay, altered cerebral and neuronal morphology and had a high susceptibility to seizures 15 . These findings were congruent with previous investigations in which BIG1-deficient mice showed altered axonal projection, delayed neural polarization, and had a smaller neocortex and hippocampus due to increased neuronal apoptosis 16 .…”

Involvement of BIG1 and BIG2 in regulating VEGF expression and angiogenesis

“…Further studies will be required to understand the roles of different Arf family member proteins in the regulation of vessel formation. Early postnatal death of arfgef1-deficient mice (70) and embryonic lethality in arfgef2-deficient mice (71) indicate that murine BIG1 or BIG2 are necessary for early development. Arfgef1 and Arf1 may indirectly mediate expression of the transcription factor in early mouse development (72).…”

Golgipathies reveal the critical role of the sorting machinery in brain and skeletal development

“…Again, the occurrence of both neonatal and progressive manifestations during childhood demonstrates the importance of the ARF pathway throughout development. Interestingly, although BIG1 and BIG2 may play redundant roles in trafficking between the TGN and endosomes [28][29][30] , the brain phenotype revealed by human mutations or deficient mouse models suggests that BIG1 controls maturation steps nature communications (2022) 13:7397 | like peripheral myelination or synaptic transmission [30][31][32] while BIG2 would rather play on the expansion and migration of neural progenitors 33 . Thus, one could speculate that ARF1/3 are differentially regulated at different stages of development, in progenitors or in postmitotic cells, providing an explanation for the diversity and overlap of phenotypes observed.…”