The unfolding story of two lissencephaly genes and brain development

Orchestrating human neocortex development across the scales; from micro to macro

How our brains have developed to perform the many complex functions that make us human has long remained a question of great interest. Over the last few decades, many scientists from a wide range of fields have tried to answer this question by aiming to uncover the mechanisms that regulate the development of the human neocortex. They have approached this on different scales, focusing microscopically on individual cells all the way up to macroscopically imaging entire brains within living patients. In this review we will summarise these key findings and how they fit together.

How the extracellular matrix shapes neural development

During development, both cells and tissues must acquire the correct shape to allow their proper function. This is especially relevant in the nervous system, where the shape of individual cell processes, such as the axons and dendrites, and the shape of entire tissues, such as the folding of the neocortex, are highly specialized. While many aspects of neural development have been uncovered, there are still several open questions concerning the mechanisms governing cell and tissue shape. In this review, we discuss the role of the extracellular matrix (ECM) in these processes. In particular, we consider how the ECM regulates cell shape, proliferation, differentiation and migration, and more recent work highlighting a key role of ECM in the morphogenesis of neural tissues.

Cortical Morphogenesis during Embryonic Development Is Regulated by miR-34c and miR-204

The porcine brain closely resembles the human brain in aspects such as development and morphology. Temporal miRNA profiling in the developing embryonic porcine cortex revealed a distinct set of miRNAs, including miR-34c and miR-204, which exhibited a highly specific expression profile across the time of cortical folding. These miRNAs were found to target Doublecortin (DCX), known to be involved in neuron migration during cortical folding of gyrencephalic brains. In vivo modulation of miRNA expression in mouse embryos confirmed that miR-34c and miR-204 can control neuronal migration and cortical morphogenesis, presumably by posttranscriptional regulation of DCX.

Doublecortin-Like Kinases Promote Neuronal Survival and Induce Growth Cone Reformation via Distinct Mechanisms

After axotomy, neuronal survival and growth cone re-formation are required for axon regeneration. We discovered that doublecortin-like kinases (DCLKs), members of the doublecortin (DCX) family expressed in adult retinal ganglion cells (RGCs), play critical roles in both processes, through distinct mechanisms. Overexpression of DCLK2 accelerated growth cone re-formation in vitro and enhanced the initiation and elongation of axon re-growth after optic nerve injury. These effects depended on both the microtubule (MT)-binding domain and the serine-proline-rich (S/P-rich) region of DCXs in-cis in the same molecules. While the MT-binding domain is known to stabilize MT structures, we show that the S/P-rich region prevents F-actin destabilization in injured axon stumps. Additionally, while DCXs synergize with mTOR to stimulate axon regeneration, alone they can promote neuronal survival possibly by regulating the retrograde propagation of injury signals. Multifunctional DCXs thus represent potential targets for promoting both survival and regeneration of injured neurons.

Developmental dynamics of PAFAH1B subunits during mouse brain development

Platelet-activating factor (PAF) mediates an array of biological processes in the mammalian central nervous system as a bioactive lipid messenger in synaptic function and dysfunction (plasticity, memory, and neurodegeneration). The intracellular enzyme that deacetylates the PAF (PAFAH1B) is composed of a tetramer of two catalytic subunits, ALPHA1 (PAFAH1B3) and ALPHA2 (PAFAH1B2), and a regulatory dimer of LIS1 (PAFAH1B1). We have investigated the mouse PAFAH1B subunit genes during brain development in normal mice and in mice with a hypomorphic allele for Lis1 (Lis1/sLis1; Cahana et al. [2001] Proc Natl Acad Sci U S A 98:6429-6434). We have analyzed quantitatively (by means of real-time polymerase chain reaction) and qualitatively (by in situ hybridization techniques) the amounts and expression patterns of their transcription in developing and postnatal brain, focusing mainly on differences in two laminated encephalic regions, the forebrain (telencephalon) and hindbrain (cerebellum) separately. The results revealed significant differences in cDNA content between these two brain subdivisions but, more importantly, between the LIS1 complex subunits. In addition, we found significant spatial differences in gene expression patterns. Comparison of results obtained with Lis1/sLis1 analysis also revealed significant temporal and spatial differences in Alpha1 and Lis1 expression levels. Thus, small changes in the amount of the Lis1 gene may differentially regulate expression of Alpha1 and Alpha2, depending on the brain region, which suggests different roles for each LIS1 complex subunit during neural differentiation and neural migration.

LIS1 and DCX: Implications for Brain Development and Human Disease in Relation to Microtubules

Proper lamination of the cerebral cortex requires the orchestrated motility of neurons from their place of birth to their final destination. Improper neuronal migration may result in a wide range of diseases, including brain malformations, such as lissencephaly, mental retardation, schizophrenia, and autism. Ours and other studies have implicated that microtubules and microtubule-associated proteins play an important role in the regulation of neuronal polarization and neuronal migration. Here, we will review normal processes of brain development and neuronal migration, describe neuronal migration diseases, and will focus on the microtubule-associated functions of LIS1 and DCX, which participate in the regulation of neuronal migration and are involved in the human developmental brain disease, lissencephaly.

Polarity regulation in migrating neurons in the cortex

The formation of the cerebral cortex requires migration of billions of cells from their birth position to their final destination. A motile cell must have internal polarity in order to move in a specified direction. Locomotory polarity requires the coordinated polymerization of cytoskeletal elements such as microtubules and actin combined with regulated activities of the associated molecular motors. This review is focused on migrating neurons in the developing cerebral cortex, which need to attain internal polarity in order to reach their proper target. The position and dynamics of the centrosome plays an important function in this directed motility. We highlight recent interesting findings connecting polarity proteins with neuronal migration events regulated by the microtubule-associated molecular motor, cytoplasmic dynein.

A new method of embryonic culture for assessing global changes in brain organization

While dissociated, reaggregated cells and organotypic slice cultures are useful models for understanding brain development, they only partially mimic the processes and organization that exist in vivo. Towards bridging the gap between in vitro and in vivo paradigms, a method for culturing intact brain tissue was developed using whole cerebral cortical hemispheres in which the anatomical and cellular organization of nervous system tissue is preserved. Single, free-floating telencephalic hemispheres were dissected from embryonic mice and placed into defined culture medium on an orbital shaker. Orbital shaking was necessary for optimal growth, and cortices grown under these conditions closely approximated in vivo parameters of cell division, differentiation, migration and cell death for up to 24 h. In addition to wild-type cultures, the method was compatible with genetically altered tissues. One particular advantage of this method is its ability to reveal global anatomical alterations in the embryonic brain following exposure to soluble growth factors. This method should thus be helpful for assessing a wide range of soluble molecules for their systemic effects on the embryonic brain.

Expression of insulin-like growth factor 1 and receptor in ischemic rats treated with human marrow stromal cells

Human bone marrow stromal cells (hMSCs) enhance neurological recovery after stroke in rodents, possibly via induction of growth factors. We therefore elected to test the effects of hMSC treatment on insulin-like growth factor 1 (IGF-1), which plays an important role in growth, development, neuroprotection and repair in the adult. Rats (n=57) were subjected to permanent middle cerebral artery occlusion (MCAo) and injected intravenously with 3 x 10(6) hMSCs or phosphate-buffered saline (PBS) at 1 day after MCAo. Functional outcome was measured after MCAo using a modified Neurological Severity Score (mNSS). Gene expression of IGF-1 and IGF-1 receptor (IGF-1R) in the ischemic brain tissue were measured at 2 and 7 days after MCAo using reverse transcription-polymerase chain reaction (RT-PCR). Immunohistochemistry was performed to measure the expression of bromodeoxyuridine (BrdU), doublecortin (DCX), IGF-1 and IGF-1R at 7, 14 and 30 days after MCAo. Treatment of MCAo with hMSCs significantly improved functional recovery from 14 to 30 days. MAB1281-labeled hMSCs entered the ischemic brain and increased time-dependently. hMSC treatment significantly increased IGF-1 mRNA and BrdU(+), DCX(+), IGF-1(+) and IGF-1R(+) cells compared to PBS-treated rats (p<0.05). The percentage of BrdU(+) or DCX(+) cells colocalized with IGF-1 increased in the hMSC-treated rats compared to the PBS-treated rats (p<0.05). IGF-1 and IGF-1R may contribute to improved functional recovery and increased neurogenesis after treatment of stroke with hMSCs.

Human neocortical development: the importance of embryonic and early fetal events

The identification of numerous genes involved in the development of the cerebral cortex has led to an increased interest in the early stages of corticogenesis, when the first postmitotic neurons migrate into the cortical plate to form the foundation of the adult cortex. However, the cellular substrate of gene expression in early human cortical development is widely unknown. This article analyzes the complex sequence of events in the differentiation of the preplate, the predecessor of the neocortex, and discusses the possible origin and migratory routes of the neuronal populations involved in the transition from preplate to cortical plate. The neuronal classes present in embryonic and early fetal stages are redefined in terms of their relationship with the Reelin-Dab1 signaling pathway whose integrity is essential for successful migration into the cortex. A timetable of developmental steps is provided, and the peculiarities of the preplate derivatives in the human brain, marginal zone, and subplate are discussed. The results presented here may contribute to a deeper understanding of the pathogenesis of migration disorders.

Targeted mutagenesis of Lis1 disrupts cortical development and LIS1 homodimerization

Lissencephaly is a severe brain malformation in humans. To study the function of the gene mutated in lissencephaly (LIS1), we deleted the first coding exon from the mouse Lis1 gene. The deletion resulted in a shorter protein (sLIS1) that initiates from the second methionine, a unique situation because most LIS1 mutations result in a null allele. This mutation mimics a mutation described in one lissencephaly patient with a milder phenotype. Homozygotes are early lethal, although heterozygotes are viable and fertile. Most strikingly, the morphology of cortical neurons and radial glia is aberrant in the developing cortex, and the neurons migrate more slowly. This is the first demonstration, to our knowledge, of a cellular abnormality in the migrating neurons after Lis1 mutation. Moreover, cortical plate splitting and thalomocortical innervation are also abnormal. Biochemically, the mutant protein is not capable of dimerization, and enzymatic activity is elevated in the embryos, thus a demonstration of the in vivo role of LIS1 as a subunit of PAF-AH. This mutation allows us to determine a hierarchy of functions that are sensitive to LIS1 dosage, thus promoting our understanding of the role of LIS1 in the developing cortex.