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Floor Plate
Issues
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Diversity
– Non-neuronal cells
– Neurons
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Developmental origins
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Midbrain dopaminergic cells
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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
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Shh Induction of Spinal Cord Development
– Patterns ventral spinal cord
– v3, MN, v2, v1, v0
Floor Plate: Control of Axon Crossing
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Control of axon crossing
– Netrins
– Shh
Zebrafish Floor Plate Description
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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
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Two cell populations (Odenthal 2000)
– Medial FP (MLP): shh+, netrin1+, fkd4+, fkd7+
– Lateral FP (LFP): foxa2 (fkh)+, fkd4+, netrin-1+
Zebrafish MFP Genetics
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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
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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
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syu (sonic you: shh)
– Affects LFP but not MFP
– Inject syu RNA  increase LFP but not MFP
Zebrafish Summary
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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
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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
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Label with pHH3
– Tal2+ cells negative
– Foxa2 + tal2– cells positive
Tal2 + cells are postmitotic
Multiple Floor Plate Cell types
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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
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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
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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
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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
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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
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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
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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
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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
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Kittappa 2007
Foxa2–/–
– No TH+ cells
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Foxa2+/–
– Motor defects
– Loss of DA neurons
– Like PD
En
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En (Simon 2004)
En is gradually expressed in TH + cells
En –/–
– Early TH+ cells present but lack fibers
– Late TH+ cells absent
Summary
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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?