Alix is required during development for normal growth of the mouse brain

Abstract

Alix (ALG-2 interacting protein X) drives deformation and fission of endosomal and cell surface membranes and thereby intervenes in diverse biological processes including cell proliferation and apoptosis. Using embryonic fibroblasts of Alix knock-out mice, we recently demonstrated that Alix is required for clathrin-independent endocytosis. Here we show that mice lacking Alix suffer from severe reduction in the volume of the brain which affects equally all regions examined. The cerebral cortex of adult animals shows normal layering but is reduced in both medio-lateral length and thickness. Alix controls brain size by regulating its expansion during two distinct developmental stages. Indeed, embryonic surface expansion of the Alix ko cortex is reduced because of the loss of neural progenitors during a transient phase of apoptosis occurring between E11.5 and E12.5. Subsequent development of the Alix ko cortex occurs normally until birth, when Alix is again required for the post-natal radial expansion of the cortex through its capacity to allow proper neurite outgrowth. The need of Alix for both survival of neural progenitor cells and neurite outgrowth is correlated with its role in clathrin-independent endocytosis in neural progenitors and at growth cones. Thus Alix-dependent, clathrin independent endocytosis is essential for controlling brain size.

Endosomal sorting complexes required for ESCRTing cells toward death during neurogenesis, neurodevelopment and neurodegeneration

“…A severe reduction is seen in the number of neurites per neuron, also affected are neurite outgrowths along with a significant delay in the maturation of axons. All these correlated with a significant reduction in growth cone area . Furthermore, Alix knockout mice brain show microcephalies which might correlate with defects in cell division during early embryonic cortical development of radial glial cells …”

“…At the onset of neurogenesis around E11.5, RGCs start to give birth to deep‐layer neurons or intermediate progenitors, which further divide to produce more neurons . In ALIX knockout mice brains, microcephaly is partly explained by the abnormal apoptosis of RGCs and neurons detected in E11.5 and E12.5 embryos at the onset of neurogenesis . It may be postulated that ALIX is required for the survival of neural precursors during a limited period in course of development …”

“…Loss of this ESCRT accessory protein decreases the F‐actin content and inhibits F‐actin assembly into higher order structures . In neuronal cultures, depletion of ALIX activity affects the early steps of neuritogenesis . A severe reduction is seen in the number of neurites per neuron, also affected are neurite outgrowths along with a significant delay in the maturation of axons.…”

“…70,71 In ALIX knockout mice brains, microcephaly is partly explained by the abnormal apoptosis of RGCs and neurons detected in E11.5 and E12.5 embryos at the onset of neurogenesis. 48,72 It may be postulated that ALIX is required for the survival of neural precursors during a limited period in course of development. 48 In the nervous system, ALIX and other ESCRT accessory proteins initially control the survival of embryonic neural progenitors, and later affect growth and pruning of axons and dendrites-all steps necessary to form a functional brain.…”

“…78,80 In neuronal cultures, depletion of ALIX activity affects the early steps of neuritogenesis. 72 A severe reduction is seen in the number of neurites per neuron, also affected are neurite outgrowths along with a significant delay in the maturation of axons. All these correlated with a significant reduction in growth cone area.…”