Neural Development Review Notes PDF

❖​ Early neural development: subset of ectodermal cells of developing embryo are instructed to from neural tissue→ later on cell identity is specified further along the anterior-posterior and dorsal ventral axes→ finally specific neural cell types are identified ❖​ During gastrulation: arra...

❖​ Early neural development: subset of ectodermal cells of developing embryo are instructed to from neural tissue→ later on cell identity is specified further along the anterior-posterior and dorsal ventral axes→ finally specific neural cell types are identified ❖​ During gastrulation: arrangements of cells of the early embryo into 3 germ layers & ectoderm is told to become neural tissue and skin ​ 3 layers give rise to different things ​ All nervous system comes from ectoderm- neural tube → gives rise to CNS and PNS from ectoderm ❖​ Induction of part of the ectoderm into becoming neural tissue happens in gastrulation ❖​ As dorsal or axial mesoderm go inwards→ releases factors that will tell the ectoderm right above it to become neural tissue ​ We know this happens during gastrulation because if we take the animal cap from a frog embryo (future ectoderm) & culture it by itself before gastrulation (pre gastrula) → gives rise to skin ​ If we take the same part a little later (once dorsal mesoderm started to go in) now the future ectoderm will give rise primarily to neural tissue ​ Suggesting that the timing when induction of neural fates happens is during gastrulation ❖​ What is telling part of the ectoderm to become neural tissue?: the Spemann-Mangold organizer ​ Region of dorsal mesoderm that will give rise to axial mesoderm releases factors that tell surrounding ectoderm to become neural tissue ​ We know this though experiment: transplanted dorsal lip of blastopore (future axial mesoderm) to another embryo = now 2 dorsal lips = 2 axes for host embryo → 2 brains and 2 spinal cords ​ Dorsal lip of blastopore releases factors that can tell the tissue around it to become neural tissue ❖​ In chicken, role of the blastopore is taken by what is called Hensen’s node ​ Hensen’s node: point where the axial mesoderm cells are going in (same thing as dorsal lip of blastopore) & as they go in they release factors that promote induction of neural tissue right above them ​ As Hensen’s node moves more posteriorly its inducing above them the neural tissue and future neural tube ​ Hensen's node is an organizer in chicken/mammals/birds, whereas dorsal lip of blastopore is organizer in frogs→ role is the same ❖​ Now that it is known that the blastopore has inducing power, they tried to purify factors that were release by dorsal lip ​ Experiment: able to show through expression cloning, certain factors were able to rescue embryos that were treated with UV light and would otherwise have no neural tube ​ MORE DETAIL FROM NOTES  ​ Inject different gene products into ventralized embryo→ show that some combinations of gene products of mRNAs are able to induce neural tissue in an embryo that would otherwise not have one, that was treated with UV to artificially not have neural tissue ❖​ This rescue assay they were able to purify a gene product, one mRNA- NOGGIN: was able to rescue ventralized embryos without injection ​ When start injecting diff concentrations of noggin, you are able to generate neural tissue in embryos that otherwise would not ​ Noggin has inducing power ​ Chordin and follistatin have similar activity ❖​ Noggin, chordin and follistatin block BMP signalling ​ BMP signalling in early development is pushing these ectodermal cells to become epidermis→ want to block their activity so part of the ectoderm can turn on neural genes that will turn that part of the ectoderm into neural tissue ❖​ In higher vertebrates, blocking BMP signalling by noggin, chordin and follistatin and activating FGF signalling promotes induction of neural genes that will result in future neural tissue ​ Neural tissues is induced then during gastrulation ❖​ Lateral inhibition: only select cells within prospective neurogenic region will become neural progenitor s or neural stem cells ​ Through stochastic process, few cells within that neurogenic region will be selected to become neuroblastoma (neural stem cell in Drosophila) ​ In future neural ventral cord (neurogenic region) there are preneural clusters that express TF called Asc  ​ Only 1 cell within each cluster (there is multiple clusters) will keep expression of Asc while others lose it (can be any cell in the cluster, whichever has slightly more Asc) ​ Asc is important bc the cells that express it in the neurogenic region are the ones that will turn into neuroblasts or neural progenitors, that cell actively inhibits its neighbors ​ Lateral inhibition happens via notch signaling pathway→ expression of Asc will drive expression of transmembrane protein delta which activates on surrounding cells, notch receptor ​ More Asc = more delta = can inhibit neighbors more than they inhibit you→ that cells stays and expresses Asc ❖​ Once induction and lateral inhibition happens, then cell identity is specified further along the anterior-posterior and dorsal-ventral axes ❖​ From early formation of neural tube, can distinguish different regions of future brain and spinal cord ​ Forebrain (most anterior) Prosencephalon: is divided into telencephalon (cerebral hemispheres) and diencephalon (thalamus, hypothalamus, retina) ​ Midbrain (mesencephalon): single vesicle within neural tube; tectum (superior colliculus) and inferior colliculus  ​ Hindbrain (posterior): divided into metencephalon (pons, cerebellum) and myelencephalon (medulla) ​ Also called rhombencephalon because whole hindbrain is divided into 8 smaller segments called rhombomeres ​ Spinal cord is posterior to hindbrain ❖​ Anterior-posterior specification: anterior and posterior identity and polarity in invertebrates is already decided, there are maternal factors that tell what is going to be posterior vs anterior ​ (invertebrates) Through cascade, TFs and translational regulators subdivide embryos to smaller regions→ triggers expression of segment polarity genes and homeotic genes (Hox genes) ​ Segment polarity genes: subdivide embryo into segments by defining where do they end ​ Hox genes: tells each of those segments what identity it will have and what type of organs, neurons, etc it will develop ​ Ex: more anterior segments express homeotic genes that promote formation of antenna or mouth of the fly embryo ​ Ex: abdominal segments will express a gene called abdominal promoting formation of abdominal structures ❖​ If Hox genes are removed those structures go missing ​ In drosophila: one series of Hox genes divided into 2 clusters → come together in vertebrates and duplicates→ 4 full complements of ancestral hox genes  ​ In humans hox genes are important for identities of neurons in neural tube, hindbrain and spinal cord ​ Rhombencephalon (hindbrain) is segmented into 8 rhombomeres; R1-R8 anterior to posterior & each rhombomere has distinct characteristics ​ Rhombomeres originate different neurons and cranial nerves ❖​ Hox genes important to specify identity of each rhombomere & diff combinations of the hox genes are in each rhombomere ​ Only rhombomere 1 has no expression of hox genes- default ​ Ex: Hoxa-1; when removed (usually promotes specification of R4 and R5) so those rhombomeres go missing and now have RX (has some features of 3) ❖​ How is the expression of anterior and posterior TFs defined in vertebrates?: activator-transformer hypothesis ​ Activators (neural inducers): include BMP inhibitors, FGF signaling, induce neural tissue with anterior characteristics by default, usually becomes forebrain and midbrain→ SOOO need to transform part of the neural tissue to have hindbrain and spinal cord ​ Transformers: include RA, Wnt, FGF) required to transform a portion of neural tissue to more posterior structures like spinal cord and hindbrain (second wave of signals for posterior) ❖​ Retinoic Acid (RA)- can cause embryological defects, MORE IMPORTANTLY it can naturally promote posterior fates at the expense of anterior fates ​ More RA to developing frog embryos→forebrain and eye disappear, at expense we get bigger and thicker spinal cords ​ Block RA→ bigger forbrain and eyes (anterior) and smaller spinal cord (posterior) ​ These transformers do this by turning on TFs that convey posterior identity and inhibit anterior TFs  ❖​ Ex of TFs that promote anterior vs posterior identities: Otx2 and Gbx2 ​ Otx2- expressed in more anterior parts of neural tube & promotes anterior fates→ if removed = forebrain and midbrain go missing ​ Otx2 and Gbx2 cross inhibit each other = sharp boundary between anterior and posterior regions = midbrain-hindbrain boundary MHB (isthmic organizer) ​ Isthmic organizer: important because can induce on more anterior sides midbrain structures and more posterior side of the boundary, inducing cerebellum ​ Secondary organizer: has inducing power→ taking small piece of MHB and moving it to telencephalon can create ectopic or additional cerebellum or midbrain ​ MHB has organizer activity & lineages are restricted between mid and hindbrain ​ The tiny piece of tissue can release factors organizing tissue around it to become cerebellum & midbrain ❖​ Formation and maintenance of the isthmic organizer (MHB) ​ Expression of Otx2 on anterior part of boundary and Gbx2 on posterior part of boundary (again these Tfs cross inhibit each other) and set up expression of Fgf8 ​ Gbx2 promotes expression of FGf8 while Otx2 restricts it ​ Fgf8 trigger expression of other TFs like En1 and Wnt1  ​ These 3 factors regulate formation of midbrain and cerebellum and where the boundary will form Just like there are signals to promote posterior fates (transformers) there are also: ❖​ There are signals that promote polarity/ different fates along the DV axis: Shh ❖​ Shh: acts as morphogen to promote VENTRAL fates ​ Released by notochord and floor plate will act in concentration dependent manner to promote different fates along VENTRAL side of the spinal cord ​ On DORSAL side, we have BMPs that are released by future epidermis covering dorsal part of the neural tube, but also from the roof plate ❖​ BMPs: promote expression of different set of TFs that promote specification of dorsal fates ​ BMPs and Shh act antagonistically- in opposite ways to promote ventral & dorsal fates respectively ​ Both are morphogens: induce ventral and dorsal determinants in concentration dependent manner ❖​ Shh is sufficient to promote ventral fates bc if we over express Shh = more ventral fates, more floorplate, more motor neurons ❖​ Shh is REQUIRED for ventral fate specification Progenitors start generating neurons when beginning to acquire identity along post/ant and DV axis: look at notes ❖​ INITIALLY Progenitors expand to make more progenitor cells or neural stem cells bc start with limited amount of progenitors ❖​ NEXT switch to divide asymmetrically to generate 1 PROGENITOR and 1 NEURON ❖​ FINALLY a lot of progenitors end up quitting (q fraction) ​ QUIT BY: generating 2 glial cells or 2 neurons (quit when produce 2 of the same  cell- has reached final dividios and will no longer self renew) ​ Progenitors can generate glial cells and neurons ​ Neurons→ glia happen later- start to generate fewer neurons as neurogenesis goes on ​ When they quit/how long their cell cycle is + when they start generating glia and stop generating neurons defines how many neurons you end up having ❖​ Progenitors make progenitors→ makes progenitors and neurons→ progenitors and glial cells (neurogenesis→gliogenesis happens at end of embryonic development) ❖​ Choice between becoming neuron or glia is affected by multiple signaling pathways: ​ Involve lateral inhibition- notch and delta signaling participate in pushing progenitor cells from generating neurons to generating glial cells ​ Involve proneural genes Neurons born at ventricle have to migrate to final location ❖​ Organization of mammalian cortex: layered- glutamatergic pyramidal neurons & inhibitory GABAergic interneurons ❖​ 6 layers formed by glutamatergic neurons (majority pyramidal neurons except layer 4, has excitatory neurons) ❖​ Inhibitory neurons from different source will mix with excitatory neurons to keep balance between excitation and inhibition ​ Excitatory neurons: migrater from ventricle and migrate RADIALLY along radial glial cells→ radial glial cells (close to ventricle) act as tracts for those neurons to go to final location but also are the stem cells that generate neurons ​ Radial glial cells (close to ventricle) produce neurons and once born crawl on top of radial glial cells, migrate outward/radially and make their way from ventricle to the surface; INSIDE FIRST→ OUTSIDE LAST ❖​ Inhibitory neurons come from different place; arise from ventral forebrain from Medial Ganglionic Eminence (MGE)- MIGRATE TANGENTIALLY following the surface of the cortex to reach final destination in neocortex (do some radial but mainly tangential) Intrinsic limits in the number of cell divisions. For example, a single neuroblast in Drosophila gives rise to 10 neurons or glia through a series of 5 asymmetric cell divisions. Extracellular signaling factors that promote or inhibit cell division ❖​ Examples where progenitors and neurons change a few times along their path to their final destination: ​ Granule cells of cerebellum- progenitors that produce these cells are born in rhombic lip (structure that surrounds 4th ventricle)--> initially migrate tangentially→ proliferate, generate neurons→ neurons migrate tangentially, then migrate radially on Bergmann glia cells ​ Bergmann glia: specialized radial glia types that serve structural features the cells  will use to migrate radially towards center of cerebellum Signals that contribute to regulation of migration events: ❖​ Reelin: identified through spontaneous mutation in mouse colony; has motor phenotype (ataxia and tremors), saw widespread disorganization of the brain especially in cerebellum and cortex ​ Mutation results in clustering of Purkinje cells which causes the disorganization ​ Layers of the cortex are inverted so neurons that are normally deep layer neurons (5 and 6), in reeler mouse they are on the surface & upper layer neurons are more in the inside ​ No Reelin so there is nothing to tell the neurons to detach from radial glial cell, so they pile up inverting the cortex Neural cell fates are specified ❖​ Cell fates are determined by intrinsic and extrinsic factors ❖​ In C. elegans a cascade of TF (starting with unc-86) regulates specification of touch neurons ❖​ In Drosophila, sequential expression of TFs (starting with HB) regulates specification of different types of ventral cord neurons ❖​ Neuroblastoma progenitor cells switch genetic program that is defined by one of these TFs starting with hunchback (HB) →KR→PDM→CAS ❖​ Ganglion mother cell born from neuroblast will keep program that was present at the time of cell division, but NB itself will switch to next program→ so each ganglion mother cell (GMC)born from NB will have different genetic program to generate diff kinds of neurons from same NB Neuroblasts are precursor of neurons + glia: neuroblasts dividing: ❖​ NB divides asymmetrically→ NB & GMC1→ GMC divides into 2 neurons & NB divides to NB & GMC2 again and that GMC2 → 2 neurons and etc… ❖​ Usually divides 5 times→ 10 neurons  ❖​ So which is going to stay as a Nb and which will become GMC ? ​ Numb involved: the side of the cell that keeps numb after cytokinesis will become GMC and the other side with lost Numb will be Nb→ continues: NB→ Numb is on one side of the cell→ cytokinesis→ etc. etc. NB does it over and over ❖​ In Drosophila: identity of NB is defined by latitude/longitude (anterior/posterior) and timing of birth ​ Hox gene expression also- each segments has different Hox genes being expressed that tell different progenitors or neuroblasts to have different identities along AP axis ​ For DV axis - discussed DPP and diff TFs that are expressed at diff rates in diff ways in diff DV coordinates ​ Msh only expressed by dorsal most Nb ​ Ind expressed by intermediate one ​ Vnd expressed by ventral one ​ 1 Nb gives rise to 4-5 diff GMC ​ Each neuron from 1 Nb have 3 dimensions that are diff from each other: time, AP and DV identity Vertebrate Retinogenesis: ❖​ Parallels Drosophila in ventral cord- timing ❖​ Different types of neurons are born at different times in the retina ​ First fate: RGC ​ Last: muller glia ❖​ What we see in retina is similar to what is seen in cortex with progenitors being flexible ​ Grow early progenitors with other early progenitors→ will still behave as early progenitors: generating early fates of retina ❖​ Heterochronic experiment: culture early progenitors with a lot of late progenitors, the early progenitors start to act like late progenitors generate later fates like rods and muller glia ❖​ Cortical progenitors also lose competence overtime = loss of potential ​ If we get progenitors that give rise to the first layer of cortex (layer 6 i.e.) & move to an older animal that is already generating the latest layers (2 and 3), ear;y will be converted and start to act like late progenitors and generate layer 2 and 3 neurons ​ If do the opposite, old (generating superficial layers)→ young animal, the old progenitors will still behave as old progenitors and generate layer 2 and 3 neurons ​ So older progenitors have lost potential/competence How are fates specifying in the spinal cord? ❖​ Shh promotes expression of Class II Tfs and represses expression of Class I progenitors ​ TFs on dorsal side: Class I ​ TFs on ventral side: class II ​ Can form pairs and cross inhibit each other: pax6+Nkx2.2 and Dbx2+Nkx6.1→ allows progenitor cells to define sharp boundary between them, defining different population of progenitors that would express different combinations of Class I and II Tfs ​ This combination defines characteristics and the type of neuron that will be born from progenitor ❖​ Extrinsic factors (environment) are important for fate specification of neural crest cells ❖​ Neural Crest cells: migratory cells derived from neuron of dorsal neural tube; migrate long distances to form different cell types (sensory, melanocytes, smooth muscle) ​ What defines what neural crest progenitor cells become will be through signals they encounter along migratory path ​ BMPs: factors that tell neural crest progenitor cells to become sympathetic adrenergic cells ​ BMP released by dorsal aorta→ BMP acts on neural crest cells that get close to dorsal aorta→ tells those cells to become sympathetic ganglion neurons Axons ❖​ Axons grow on adhesive substrates ❖​ Repulsive and attractive cues guide axons to their targets ❖​ Guidance cues or signals act on receptors on growth cone ❖​ They act by promoting assembly and stabilization of actin or destabilization of actin ​ Promote civilization & growth of actin filaments = attractive  ​ Promote destabilization = repulsive cues ​ Guidance receptors triggers changes in the actin cytoskeleton to steer the axon ❖​ Class 3 semaphorins repel sensory axons at long range (sema3A)- secreted and can travel and can form gradient to repel axons ❖​ Topographic maps in the retina are initially established by contact-mediated repulsion (ephrin signaling) ❖​ Netrin can act as attractant or repellent: ​ Depends on the receptor expressed by axons ​ For signals to act on neurons there NEEDS to be receptor in order to be able to “see” the signal ​ Chemo-attractant for early spinal cord commissural axons ​ Its receptor: DCC; makes netrin feel like attractant ​ Repellent for trochlear axons ​ Receptor: Unc-5; if they express this it will make neuron feel like netrin is repulsive cue so axons will run away from netrin ​ ❖​ Slits are long range repellents that push commissural axons out of the midline; similar to Sema3A ​ Receptors: Robo ​ Commissural axons initially attracted to midline by netrin bc has receptor for netrin but midline also expresses slit → receptor for slit has to be turned off to allow axons to grow towards the midline attracted by netrin→ once axons hit midline, Robo can be turned back on ​ It gets to the growth cone and in the cone → can now send slit, push axons out of midline and keep them from returning back to midline ​ Commissureless: turns Robo on and off ​ If on: Robo will not reach growth cone ​ If off: Robo is allowed to get to growth cone→ axons now repelled by slit→ pushed out of midline and can’t come back ​ Essentially: For vertebrates it is not called Commissureless, called something else but same thing- proteins that regulate sorting of Robo, intially promoting degredation of Robo so growth cones are insensitive to slit, can enter and grow towards the midline→ then factors are turned off allowing Robo to get to growth cone & can be pushed out of midline by slit