Progressive Decrease of Mitochondrial Motility during Maturation of Cortical Axons In Vitro and In Vivo

The importance of mitochondria for neuronal function is evident by the large number of neurodegenerative diseases that have been associated with a disruption of mitochondrial function or transport. Mitochondria are essential for proper biological function as a result of their ability to produce ATP through oxidative phosphorylation, buffer cytoplasmic calcium, regulate lipid biosynthesis, and trigger apoptosis. Efficient transport of mitochondria is thought to be particularly important in neurons in light of their compartmentalization, length of axonal processes, and high-energy requirements). However, the majority of these results were obtained using short-term, in vitro neuronal culture models, and very little is currently known about mitochondrial dynamics in mature axons of the mammalian CNS in vitro or in vivo. Furthermore, recent evidence has demonstrated that mitochondrial immobilization at specific points along the axon, such as presynaptic boutons, play critical roles in axon morphogenesis [4, 5]. We report that as cortical axons mature, motility of mitochondria (but not other cargoes) is dramatically reduced and this coincides with increased localization to presynaptic sites. We also demonstrate using photo-conversion that in vitro mature axons display surprisingly limited long-range mitochondrial transport. Finally, using in vivo two-photon microscopy in anesthetized or awake-behaving mice, we document for the first time that mitochondrial motility is also remarkably low in distal cortical axons in vivo. These results argue that mitochondrial immobilization and presynaptic localization are important hallmarks of mature CNS axons both in vitro and in vivo

N-cadherin: A new player in neuronal polarity

Comment on: Gärtner A, et al. EMBO J 2012; <span class="b">31</span>:1893-90

AMP-activated protein kinase (AMPK) activity is not required for neuronal development but regulates axogenesis during metabolic stress

Mammalian brain connectivity requires the coordinated production and migration of billions of neurons and the formation of axons and dendrites. The LKB1/Par4 kinase is required for axon formation during cortical development in vivo partially through its ability to activate SAD-A/B kinases. LKB1 is a master kinase phosphorylating and activating at least 11 other serine/threonine kinases including the metabolic sensor AMP-activated protein kinase (AMPK), which defines this branch of the kinome. A recent study using a gene-trap allele of the β1 regulatory subunit of AMPK suggested that AMPK catalytic activity is required for proper brain development including neurogenesis and neuronal survival. We used a genetic loss-of-function approach producing AMPKα1/α2-null cortical neurons to demonstrate that AMPK catalytic activity is not required for cortical neurogenesis, neuronal migration, polarization, or survival. However, we found that application of metformin or AICAR, potent AMPK activators, inhibit axogenesis and axon growth in an AMPK-dependent manner. We show that inhibition of axon growth mediated by AMPK overactivation requires TSC1/2-mediated inhibition of the mammalian target of rapamycin (mTOR) signaling pathway. Our results demonstrate that AMPK catalytic activity is not required for early neural development in vivo but its overactivation during metabolic stress impairs neuronal polarization in a mTOR-dependent manner

Molecular Specification and Patterning of Progenitor Cells in the Lateral and Medial Ganglionic Eminences

We characterized intrinsic and extrinsic specification of progenitors in the lateral and medial ganglionic eminences (LGE and MGE). We identified seven genes whose expression is enriched or restricted in either the LGE: Boc, Fzd8, Ankrd43 and Ikzf1, or MGE: Mbip, Zswim5, and Adamts5. Boc, Fzd8, Mbip and Zswim5 are apparently expressed in LGE or MGE progenitors, while the remaining three are seen in the post-mitotic mantle zone. Relative expression levels are altered and regional distinctions are lost for each gene in LGE or MGE cells propagated as neurospheres; indicating that these newly identified molecular characteristics of LGE or MGE progenitors depend upon forebrain signals not available in the neurosphere assay. Analyses of Pax6Sey/Sey, Shh−/−, and Gli3XtJ/XtJ mutants suggests that LGE and MGE progenitor identity does not rely exclusively upon previously established forebrain-intrinsic patterning mechanisms. Among a limited number of additional potential patterning mechanisms, we found that extrinsic signals from the frontonasal mesenchyme are essential for Shh and Fgf8-dependent regulation of LGE and MGE genes. Thus, extrinsic and intrinsic forebrain patterning mechanisms cooperate to establish LGE and MGE progenitor identity, and presumably their capacities to generate distinct classes of neuronal progeny

Netrin-1- 3 1 integrin interactions regulate the migration of interneurons through the cortical marginal zone

Cortical GABAergic interneurons, most of which originate in the ganglionic eminences, take distinct tangential migratory trajectories into the developing cerebral cortex. However, the ligand–receptor systems that modulate the tangential migration of distinct groups of interneurons into the emerging cerebral wall remain unclear. Here, we show that netrin-1, a diffusible guidance cue expressed along the migratory routes traversed by GABAergic interneurons, interacts with α3β1 integrin to promote interneuronal migration. In vivo analysis of interneuron-specific α3β1 integrin, netrin-1–deficient mice (α3lox/−Dlx5/6-CIE, netrin-1−/−) reveals specific deficits in the patterns of interneuronal migration along the top of the developing cortical plate, resulting in aberrant interneuronal positioning throughout the cerebral cortex and hippocampus of conditional α3lox/−Dlx5/6-CIE, netrin-1−/− mice. These results indicate that specific guidance mechanisms, such as netrin-1–α3β1 integrin interactions, modulate distinct routes of interneuronal migration and the consequent positioning of groups of cortical interneurons in the developing cerebral cortex

Close Homolog of L1 and Neuropilin 1 Mediate Guidance of Thalamocortical Axons at the Ventral Telencephalon

We report a cooperation between the neural adhesion molecule close homolog of L1 (CHL1) and the semaphorin 3A (Sema3A) receptor, neuropilin 1 (Npn1), important for establishment of area-specific thalamocortical projections. CHL1 deletion in mice selectively disrupted the projection of somatosensory thalamic axons from the ventrobasal (VB) nuclei, causing them to shift caudally and target the visual cortex. At the ventral telencephalon, an intermediate target with graded Sema3A expression, VB axons were caudally shifted in CHL

Organotypic Brain Cultures for Metastasis Research

We thank members of Brain Metastasis Group for critical discussion. Research in the Brain Metastasis Group is supported by MINECO-Retos SAF2017-89643-R (M.V.), Cancer Research Institute CLIP Award 2018 (M.V.), AECC (GCTRA16015SEOA) (M.V.), Bristol-Myers Squibb Melanoma Research Alliance Young Investigator Award 2017 (M.V.), Beug Foundation’s Prize for Metastasis Research 2017 (M.V.), Worldwide Cancer Research (19-0177) (M.V.), H2020-FETOPEN (828972) (M.V.), Fundación Ramón Areces (CIVP19S8163), and La Caixa-Severo Ochoa International PhD Program Fellowship (L.Z.). M.V. is a Ramón y Cajal Investigator (RYC-2013-13365) and an EMBO YIP investigator.N

A PKC-Dependent Recruitment of MMP-2 Controls Semaphorin-3A Growth-Promoting Effect in Cortical Dendrites

There is increasing evidence for a crucial role of proteases and metalloproteinases during axon growth and guidance. In this context, we recently described a functional link between the chemoattractive Sema3C and Matrix metalloproteinase 3 (MMP3). Here, we provide data demonstrating the involvement of MMP-2 to trigger the growth-promoting effect of Sema3A in cortical dendrites. The in situ analysis of MMP-2 expression and activity is consistent with a functional growth assay demonstrating in vitro that the pharmacological inhibition of MMP-2 reduces the growth of cortical dendrites in response to Sema3A. Hence, our results suggest that the selective recruitment and activation of MMP-2 in response to Sema3A requires a PKC alpha dependent mechanism. Altogether, we provide a second set of data supporting MMPs as effectors of the growth-promoting effects of semaphorins, and we identify the potential signalling pathway involved

A Novel Role for Dbx1-Derived Cajal-Retzius Cells in Early Regionalization of the Cerebral Cortical Neuroepithelium

Patterning of the cerebral cortex during embryogenesis depends not only on passive diffusion of morphogens but also on signal delivery by Cajal-Retzius neurons that migrate over long distances