Lecture 2 - Patterning of the Nervous System Flashcards
What patterns the forebrain, midbrain, and hindbrain?
signals from the neural tube
What patterns the early posterior anterior part of the brain?
wnt signaling
-in the anterior portion there are high wnt inhibitors meaning low wnt
-in the posterior portion there is high wnt signaling
What does wnt signaling differences in the anterior and posterior part of the brain do?
establish expression of distinct TF domains and define the mid/hindbrain boundary or MHB
What are Wnt inhbitors which act on the anterior portion of the brain secreted by?
endoderm
What are Wnt signals which act on the posterior portion of the brain secreted by?
paraxial mesoderm
What transcription factor does the forebrain and midbrain express due to wnt inhibitors secreted from the endoderm?
Otx2
What transcription factor does the hindbrain express due to wnt signals secreted from the paraxial mesoderm?
Gbx2
What does the MHB or midbrain/hindbrain boundary secrete?
signals to pattern the MHB
-MHBs secrete FGFs or lateral signals
What does the ZLI or zona limitans intrathalamica secrete?
Shh to divide the forebrain and midbrain
What sends FGF signals to work with Shh gradients from the ventral midline to specify serotonergic and dopaminergic cells in the hindbrain and midbrain respectively?
the isthmic organizer
How come cells in the midbrain and hindbrain respond to FGFs in a different way causing the midbrain to make dopaminergic neurons and the hindbrain to make serotonergic neurons?
midbrain - through wnt inhibition from endoderm expresses Otx2
hindbrian - through wnt signaling expresses Gbx2
-different Tfs allow for the cells to respond to the same signal diffrently
What enables combinatorial coding?
FGF and Shh
What does FGF signaling define?
anterior/posterior
What does Shh signaling define?
dorsal and ventral
How many secreted factors are used in different regions and times to program a vast array of neural diversity?
relatively small number - FGFs, BMPs, Shh, Wnt, and Retinoic Acid
What is the key to understanding neural development?
-initial signals program TF expression which limits future cell fates
-the same signal experienced later is interpreted differently to program subsequent developmental events
From what cells is the neural plate generated?
ectodermal cells that overlie the notochord and the future somites and it is flanked by the epidermal ectoderm
Where does the neural plate fold to form the neural fold?
dorsally at its midline
Where do floor plate cells differentiate?
at the ventral midline of the neural tube
How does the neural tube form?
through fusion of the dorsal tips of the neural folds
Where do roof plate cells form?
at the dorsal midline of the neural tube
From where do neural crest cells migrate?
from the neural tube into and past the somites before populating the sensory and sympathetic ganglia
How are distinct classes of neurons generated at different positions in the embryonic spinal cord?
via different dorsoventral positions
-Shh induces each neural class in the ventral spinal cord
What is Shh released by?
the notochord aka the ventral side
What regions of the spinal cord does Shh encode?
V0, V1, V2, MN, V3
What controls neural patterning in the ventral spinal cord?
Shh signaling gradient
What is Shh?
a morphogen which means that it is released in a concentration gradient
-a ventral to dorsal gradient of Shh signaling established dorsoventral domains of homeodomain protein expression in the progenitor cells within the ventral half of the neural tube
What is a homeodomain protein?
a structural DNA binding protein
What do graded Shh gradients generate?
a corresponding gradient of Gli transcription factor activity
-different concentrations of extracellular Shh lead to proportional intracellular Gli gradients to specify neuronal classes
What occurs with no Shh signaling i.e. on the dorsal side of the spinal cord?
all five transcription factors are expressed
What occurs with more Shh signaling i.e. on the ventralside of the spinal cord?
more and more of these TFs are turned off
-If get 20% of Shh you turn off one TF and show only 4 TF encoding a new cell
-if get 50% of Shh you turn off two TF and show only 3 Tf encoding a new cell
-if get 80% of Shh - turn off four TF
-if get 100% of Shh - turn off all five TF - on ventral side
How are tight all or none fate decisions made at boundaries with similar Shh concentration gradients?
transcription factors act in a cell autonomous manner to repress each others expression conferring unambiguous identity
-small imbalances in initial activity are amplified to repress other factors leading to sharp boundaries
-postmitotic neurons emerge to give die to the 5 major classes of ventral neurons such as the interneurons from v0-v3 and motor neurons
How is the spread of Shh protein controlled with such precision?
most Shh is tethered to the surface of the notochord/floor plate through a lipid modification
-only a small fraction is released and transferred from cell to cell which enable and maintains a stable gradient and transfer of Shh from the source
What is the exact modification the Shh protein undergoes?
the N terminus is cleaved and a palmitate and cholesterol molecule are added on either side
-they then clump into a signaling molecule where the inside is hydrophilic outside is hydrophobic where it will want to bind to cell membrane and as it moves from cell to cell less and less move along creating a concentration gradient
How are small differences in Shh signaling activity converted to all or none decisions about identity?
Shh binds to the receptor patch which is homogeneously arranged in this axis and Shh bound to it leads to translocation of the transcription factor Gli3
-ratio of gli repressor: gli activator determines which targets are expressed
-an extracellular gradient of Shh is converted into a nuclear gradient of Shh target expression which equates to an increase in the silencing of TF expression
What does no Shh signaling result in?
Gli3 repression no TFs turned off all five are expressed
What does 100% Shh signaling result in?
complete Gli3 activation and all 5 Tfs are turned off so none are expressed
How is the dorsal neural tube patterned?
by BMPs also in a morphogenic manner; BMPs come from epidermal cells on top of the neural tube and there is a gradient of BMPs where the ventral side has no BMP while the dorsal side has alot of BMP
How are diverse cells patterned and specified within individual domains?
-the anterior posterior profile of Hox genes expression determines motor neuron subtype
What are hox genes?
major transcription factor family specifying MN subtype; defined by a homeodomain
Where are different hox genes expressed?
discrete but partially overlapping rostrocaudal domains of the hindbrain and spinal cord
How are hox genes encoded?
in a hierarchical fashion where posterioir hox genes supercede anterior hox genes
Are hox genes conserved across species?
yes they are cause they were first found on flies and invertebrates and have hox genes and vertebrates and mammals all have hox genes
On what end of DNA are head hox genes?
3’
On what end of DNA are tail hox genes?
5’
What does the mouse HoxB1 gene control?
the identity and projection of hindbrain MNs
Where is hoxb1 normally expressed?
in r4
Where do r2 axons extend out laterally and become?
trigeminal MNs
Where do R4 cell bodies migrate caudally and become?
facial MNs
If you delete the hox gene in r4 what will it acquire?
if you delete the hox genes in r4 it will they r4 will express the r2 and innervate the trigeminal MNs
How do hox proteins control the identity of neurons in motor columns and pools?
-cross repressive interactions of hox genes refine transcriptional programs in each MN subtype
-this leads to the spectrum of axonal surface receptors that guide axons through local cues to find final muscle target during axon guidance
What is an example of how hox proteins control the identity of neurons in motor columns and pools?
Hox6, Hox9, and Hox10 proteins are expresses in MNs and direct MN identity and peripheral target connectivity
-a more complex hox transcriptional network further downstream controls the MN pool identity where hoxc8 represses hox 5 and actuvates downstream hoxc4 and hox6 which cause Pea3 to be expressed and Scip to be inhibited
If you change the hox code in MN pools you change the pattern of muscle connectivity, if you knock down hoxc6 you get greater scip expression and the muscles only innervate what whole if you overexpress Hoxc6 you get the muscle to only innervate?
the FCU
the pec muscle
How are local axon targeting steps determined?
MNs in the lateral motor column LMC project axons into the ventral and dorsal halves of the limb mesenchyme
-expression of lim homeodomain TFs determine levels of expression of Ephb receptors expressed in the axon surface
How are LMC axons guided to the limb by ephrin expression?
lim homedomain protein Isl1 directs high level of ephrin B expression
-homotypic interactions of ephA/B lead to axon repulsion and collapse
-heterotypic interactions between ephA and ephB lead to attraction and stabilization leading to synpatogenesis
Conserved signaling molecules are used repeatedly within and between diverse organisms at different times and places?
turning on and off hox genes at different places and times can get different organisms
If you mis-express fundamental signaling programs can lead to the development of entire organs?
-Anteopedia change then you can replace the antenna with a leg; posterior hox gene being imposed on an anterior part
-haltere is a gyroscope on the back if a fly that helps is balance; duplicated an abdominal fly so instead os a haltere have two sets of wings and the fly now looks like a dragonfly
What establishes discrete functional areas along AP axis of forebrain?
AP transcription factor gradients
What gradients establish rostral-caudal patterning of the forebrain and define Pax6 vs. Emx2 expression?
FGF8 gradients
cause greater expression of Pax6 over Emx2 causing the rostral end to have more Pax6 and the caudal end to have more Emx2
If you genetically eliminate Emx2 what happens?
the rostral part of brain grows
If you genetically eliminate Pax6 what happens?
the caudal part of the brain expands
Beyond morphogen gradients and transcription factor programs what else influences neural organization?
sensory input
How does sensory input regulate the barrel cortex organization in the developing somatosensory cortex in rodents?
-barrel area of the rodent somatosensory cortex forms a somatotopic representation of the rows of the whiskers on the animals snout
-when you remove a whisker it will be reduced and the neighboring whiskers will take more space in your brain
Summary
