Morphogenetic and Histogenetic Roles of the Temporal-Spatial Organization of Cell Proliferation in the Vertebrate Corticogenesis as Revealed by Inter-specific Analyses of the Optic Tectum Cortex Development

The developing optic tectum: An asymmetrically organized system and the need for a redefinition of the notion of sensitive period

The present article summarizes the main events involved in the isthmic organizer and optic tectum determination and analyses how optic tectum patterning is translated, by the organized operation of several specific cell behaviors, into the terminally differentiated optic tectum. The paper proposes that this assembling of temporally/spatially organized cell behaviors could be incorporated into a wider notion of patterning and that, given the asymmetric organization of the developing optic tectum, the notion of "sensitive period" does not capture the whole complexity of midbrain development and the pathogenesis of congenital disorders. The cell behaviors involved in the optic tectum development are organized in time and space by the isthmic organizer. A comprehensive description of the normal optic tectum development, and also its alterations, should consider both domains. Significantly, the identity of each neuronal cohort depends critically on its "time and place of birth". Both parameters must be considered at once to explain how the structural and functional organization of the optic tectum is elaborated. The notion of "patterning" applies only to the early events of the optic tectum development. Besides, the notion of "sensitive period" considers only a temporal domain and disregards the asymmetric organization of the developing optic tectum. The present paper proposes that these notions might be re-defined: (a) a wider meaning of the term patterning and (b) a replacement of the term "sensitive period" by a more precise concept of "sensitive temporal/spatial window".

NGF, TrkA-P and neuroprotection after a hypoxic event in the developing central nervous system

A decrease in the concentration of oxygen in the blood and tissues (hypoxia) produces important, sometimes irreversible, damages in the central nervous system (CNS) both during development and also postnatally. The present work aims at analyzing the expression of nerve growth factor (NGF) and p75 and the activation of TrkA in response to an acute normobaric hypoxic event and to evaluate the possible protective role of exogenous NGF. The developing chick optic tectum (OT), a recognized model of corticogenesis, was used as experimental system by means of in vivo and in vitro studies. Based on identification of the period of highest sensitivity of developmental programmed cell death (ED15) we show that hypoxia has a mild but reproducible effect that consist of a temporal increase of cell death 6 h after the end of a hypoxic treatment. Cell death was preceded by a significant early increase in the expression of Nerve Growth Factor (NGF) and its membrane receptor p75. In addition, we found a biphasic response of TrkA activation: a decrease during hypoxia followed by an increase -4 h later- that temporally coincide with the interval of NGF overexpression. To test the NGF - NGF receptors role in hypoxic cell death, we quantified, in primary neuronal cultures derived from ED15 OT, the levels of TrkA activation after an acute hypoxic treatment. A significant decline in the level of TrkA activation was observed during hypoxia followed, 24 h later, by significant cell death. Interestingly, this cell death can be reverted if TrkA inactivation during hypoxia is suppressed by the addition of NGF. Our results suggest that TrkA activation may play an important role in the survival of OT neurons subjected to acute hypoxia. The role of TrkA in neuronal survival after injury may be advantageously used for the generation of neuroprotective strategies to improve prenatal insult outcomes.

Role of miRNA-9 in Brain Development

MicroRNAs (miRNAs) are a class of small regulatory RNAs involved in gene regulation. The regulation is effected by either translational inhibition or transcriptional silencing. In vertebrates, the importance of miRNA in development was discovered from mice and zebrafish dicer knockouts. The miRNA-9 (miR-9) is one of the most highly expressed miRNAs in the early and adult vertebrate brain. It has diverse functions within the developing vertebrate brain. In this article, the role of miR-9 in the developing forebrain (telencephalon and diencephalon), midbrain, hindbrain, and spinal cord of vertebrate species is highlighted. In the forebrain, miR-9 is necessary for the proper development of dorsoventral telencephalon by targeting marker genes expressed in the telencephalon. It regulates proliferation in telencephalon by regulating Foxg1, Pax6, Gsh2, and Meis2 genes. The feedback loop regulation between miR-9 and Nr2e1/Tlx helps in neuronal migration and differentiation. Targeting Foxp1 and Foxp2, and Map1b by miR-9 regulates the radial migration of neurons and axonal development. In the organizers, miR-9 is inversely regulated by hairy1 and Fgf8 to maintain zona limitans interthalamica and midbrain–hindbrain boundary (MHB). It maintains the MHB by inhibiting Fgf signaling genes and is involved in the neurogenesis of the midbrain–hindbrain by regulating Her genes. In the hindbrain, miR-9 modulates progenitor proliferation and differentiation by regulating Her genes and Elav3. In the spinal cord, miR-9 modulates the regulation of Foxp1 and Onecut1 for motor neuron development. In the forebrain, midbrain, and hindbrain, miR-9 is necessary for proper neuronal progenitor maintenance, neurogenesis, and differentiation. In vertebrate brain development, miR-9 is involved in regulating several region-specific genes in a spatiotemporal pattern.