The construction of cerebral cortex begins with the formation of radial glia. Once formed, polarized radial glial cells divide either symmetrically or asymmetrically to balance appropriate production of progenitor cells and neurons. Upon birth, neurons use the processes of radial glia as scaffolding for oriented migration. Radial glia thus provide an instructive structural matrix to coordinate the generation and placement of distinct groups of cortical neurons in the developing cerebral cortex. Here we show that Arl13b, a cilia-specific small GTPase mutated in Joubert syndrome patients, is critical for the initial formation of the polarized radial progenitor scaffold. Through developmental stage-specific deletion of Arl13b in mouse cortical progenitors, we found that early neuroepithelial deletion of ciliary Arl13b leads to a reversal in the apical-basal polarity of radial progenitors and aberrant neuronal placement. Arl13b modulates ciliary signaling necessary for radial glial polarity. Our findings demonstrate that Arl13b signaling in primary cilia is important for the initial formation of a polarized radial glial scaffold and suggest that disruption of this process may contribute to aberrant neurodevelopment and brain abnormalities in Joubert syndrome-related ciliopathies.

Primary cilia are essential conveyors of signals underlying major cell functions. Cerebral cortical progenitors and neurons have a primary cilium. The significance of cilia function for brain development and function is evident in the plethora of developmental brain disorders associated with human ciliopathies. Nevertheless, the role of primary cilia function in corticogenesis remains largely unknown. Here we delineate the functions of primary cilia in the construction of cerebral cortex and their relevance to ciliopathies, using an shRNA library targeting ciliopathy genes known to cause brain disorders, but whose roles in brain development are unclear. We used the library to query how ciliopathy genes affect distinct stages of mouse cortical development, in particular neural progenitor development, neuronal migration, neuronal differentiation and early neuronal connectivity. Our results define the developmental functions of ciliopathy genes and delineate disrupted developmental events that are integrally related to the emergence of brain abnormalities in ciliopathies.

Coordination of membrane deformation and cytoskeletal dynamics lies at the heart of many biological processes critical for cell polarity, motility and morphogenesis. We recently showed that slit-robo GTPase-activating protein 2 (srGAP2) regulates neuronal morphogenesis through the ability of its F-BAR domain to regulate membrane deformation and induce filopodia formation. Here we demonstrate that the F-BAR domains of two closely related family members, srGAP1 and srGAP3 (F-BAR(1) and F-BAR(3), respectively) display significantly different membrane deformation properties in non-neuronal COS7 cells and in cortical neurons. F-BAR(3) induces filopodia in both cell types, though less potently than F-BAR(2), whereas F-BAR(1) prevents filopodia formation in cortical neurons and reduces plasma membrane dynamics. These three F-BAR domains can heterodimerize and act synergistically towards filopodia induction in COS7 cells. At the molecular level, F-BAR(2) displays faster molecular dynamics than F-BAR(3) and F-BAR(1) respectively at the plasma membrane which correlates well with its increased potency to induce filopodia. We also show that the molecular dynamic properties of F-BAR(2) at the membrane are partially dependent on F-Actin. Interestingly, acute phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) depletion in cells does not interfere with plasma membrane localization of F-BAR(2), which is compatible with our result showing that F-BAR(2) binds to a broad range of negatively-charged phospholipids present at the plasma membrane, including phosphatidylserine (PS). Overall, our results provide novel insights into the functional diversity of the membrane deformation properties of this subclass of F-BAR-domains required for cell morphogenesis.

Formulation of oxidative phosphorylation and its first observation by means of fluorescence spectroscopy in the 1960s led to the acceptance of bioenergetics as a new field of studies. The new discipline grew fast with the increasing number of papers, related to the energy generation in mitochondria, advancement of the instrumentation, and improvement of observation techniques. As such, fluorescence lifetime imaging microscopy (FLIM) has gained popularity as a sensitive technique to monitor the functional/conformational states of nicotinamide adenine dinucleotide reduced (NADH)one of the main compounds of oxidative phosphorylation. We hereby review the development and current application of cellular metabolism observation via NADH FLIM, illustrating it with the examples of both physiological (cell density, apoptosis, necrosis) and pathological states (inhibition of the electron transfer chain).

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