Transcript shh
Floor Plate Issues • Diversity – Non-neuronal cells – Neurons • Developmental origins • Midbrain dopaminergic cells • Comparison to insect midline cells Spinal Cord Development Notochord Induction of Floor Plate Formation in Mammals and Chicks Shh Floor Plate: Shh Induction of Spinal Cord Development • Shh Induction of Spinal Cord Development – Patterns ventral spinal cord – v3, MN, v2, v1, v0 Floor Plate: Control of Axon Crossing • Control of axon crossing – Netrins – Shh Zebrafish Floor Plate Description • Early description (Kuwada 1990) – Consists of 3 rows of cells – Defined by cuboidal shape, and most extend no processes (non-neuronal) – Among earliest identifiable cells in spinal cord and appear before first growth cones – Crossing growth cones are between FP and basal lamina next to notochord Zebrafish Floor Plate Description • Two cell populations (Odenthal 2000) – Medial FP (MLP): shh+, netrin1+, fkd4+, fkd7+ – Lateral FP (LFP): foxa2 (fkh)+, fkd4+, netrin-1+ Zebrafish MFP Genetics • MFP – cyclops (cyc; nodal-TGF-b), one-eyed pinhead (oep; EGF-CFC), schmalspur (sur; foxh1) result in absence of MFP but not LFP • LFP does not require MFP for development Zebrafish LFP Genetics • LFP – no-tail (ntl; fly brachyenteron), floating head (flh), momo (mom) have an absence of notochord • flh or mom: MFP present with associated LFP; LFP absent when MFP absent • In ntl: get expansion of MFP and LFP • LFP can develop without notochord (as can MFP), but LFP only in presence of MFP Shh Control of LFP • syu (sonic you: shh) – Affects LFP but not MFP – Inject syu RNA increase LFP but not MFP Zebrafish Summary • • • • Early cells decide to be either notochord or FP MFP: notochord and shh-independent; requires nodal-related pathway (TGF-b) LFP: dependent on shh from either Notochord or MFP Species differences: – Murine and chicken shh from notochord is required for induction of all FP cells – Murine Shh and netrin-1 are only in MFP and foxa2 (fly fkh) broadly expressed – Chicken Foxa2 in MFP and shh and netrin-1 in LFP Multiple Floor Plate Cell types • • Zebrafish (Schäfer 2007) – foxa2: all MFP; subset LFP – tal2 bHLH gene (fly ~HLH4C) in subset of foxa2– LFP Insect comparison – Both insect midline and vertebrate FP consist of neurons and non-neuronal (glial) cell types Tal2+ LFP cells are Postmitotic • • Label with pHH3 – Tal2+ cells negative – Foxa2 + tal2– cells positive Tal2 + cells are postmitotic Multiple Floor Plate Cell types • Late stage embryonic FP – Sim1+ cells are LFP subset and postmitotic – Sim1+ cells express neuronal marker – Sim1+ cells are V3 interneurons (glutamatergic in other contexts) – Also observed GABAergic neurons Summary • • • Tal2 + cells are neuronal progenitor cells – Subset sim1 + V3 interneurons – May also give rise to other interneurons Tal2 – foxa2+ cells are non-neuronal Insect comparison – Both insect ML and vertebrate have Sim + interneurons Notch Signaling • • Mindbomb (Neur) mutant – Nkx2.2b (vnd), foxa2 (LFP non-neuronal cells), tal2 (LFP neurons) reduced or absent – Foxa2 in MFP (non-neuronal) normal Notch signaling required for LFP and p3 neurons wt mib wt mib Shh Signaling • • • In shh mutants, p3 LFP cells absent In dtr/+ and yot/+ foxa2 LFP was absent and tal2 and sim1 normal Thus, non-neuronal LFP cells require high Shh levels compared to p3 cells Summary • • • • Early – foxa2 + cells proliferate – Tal2 + cells are postmitotic and differentiate into neurons Later – Subset of tal2+ cells become sim1 + V3 interneurons – Also GABAergic KA neurons (sim1 +?) Notch signaling required for LFP and p3 cells Shh signaling: LFP non-neuronal and neuronal cells required different levels of Shh Midbrain FP Cells • • Midbrain dopaminergic neurons – Motor control, reward, addiction, attention, cognition – Loss results in Parkinson disease Origins: mouse (Kittappa 2007) – TH+ cells are in foxa2+ FP cells in midbrain Midbrain FP Cells • • • Shh is expressed in FP Shh-cre X R26R lacZ reporter: lineage marker TH+ cells overlap with b-Gal Development of Midbrain FP Cells Dorsal fate 5-HT1a fate Midbrain FP Cells • • Progenitor cell development – Shh: induces FP – Otx2: required for generation of mesDA precursors – Lmx1a (CG32105): required for mesDA cell fate – Ngn2: proneural function Postmitotic development – Nurr1: controls subset of mesDA gene expression (TH) – Lmx1b (CG4328): survival; affects Pitx3 but not other genes (Nurr1, early TH) – Pitx3: lose TH expression, affects migration, survival – En: required for late maturation and survival Foxa2 • • Kittappa 2007 Foxa2–/– – No TH+ cells • Foxa2+/– – Motor defects – Loss of DA neurons – Like PD En • • • En (Simon 2004) En is gradually expressed in TH + cells En –/– – Early TH+ cells present but lack fibers – Late TH+ cells absent Summary • Vertebrate FP:insect ML – Diversity of neuronal and non-neuronal cell types – MG = non-neuronal FP; both important signaling centers – Sim+ cells present – DA+ cells present – Genes?