Development of neural circuits

Development of neural circuits

1. 1. FORMATION OF THE NERVOUS SYSTEM IN DROSOPHILA

   • a. Life cycle
     • Focus of this lecture: what is occurring internally rather than externally
     • Depending on temperature, it can go from one single cell to a complex organism within 22 hours
     • Development occurs within 10 days
   • b. Specification of the nervous system
     • The neuroectoderm is destined to form the nervous system
       • It elongates to form neural stem cells that forms the whole of the animal brain
         • How do the cells know where they are?
         • How does the cells know how many neurons to create?
         • How do the neurons know where they should connect with one another?
         • Cells must to told to specify into a neuron so they collectively form patterns
     • ii. The segmentation gene cascade
       • Using Drosophila and random mutations, we have identified a series of genes that are important for the development of the organism – they can be divided into different classes… They are reexpressed during neural development
         • MATERNAL GENES
           • Bicoid
           • Caudal
           • Hunchback
         • GAP GENES
           • Kruppel
           • Knirps
           • Tailless
           • Giant
         • PAIR RULE GENES
           • Hairy
           • Fushi-tarazu
           • Even-skipped
           • Odd-skipped
           • Runt
         • SEGMENT POLARITY GENES
           • Wingless
           • Engrailed
       • Genes that provide coordinates along the head to tail axis of the fly
     • iii. AP and DV coordinates
       • The genes within the segmentation cascade are expressed in a pattern along the head to tail axis
       • The dorsal ventral axis have different genes expressed at other positional regions along the axis
         • CALLED COLUMNAR CELLS
     • iv. Neuroblast lineages
       • Taking a segment of neural stem cells and expanded it in a more detailed view using a HYPER NEUROBLAST MAP
         • We would observe a variety of genes expressed by using markers within different neurblasts
           • Each neuroblast/stem cell provides a lineage within the embryo
       • Neuroblast = an embryonic cell from which nerve fibres originate
         • What happens to these cells to form neural and glial cells?
           • Each neuroblast lineage provides a stereotypic progeny of neural/or glial cells
   • c. Early neural differentiation
     • i. Understanding the process in two settings
       • TYPE 1
         • Begins at the ventral ectoderm surface
           • Which starts as epidermis, BUT some cells bud off and sink within the embryo
             • These cells are the neuroblasts
               • The neurblasts then divide into another neuroblast and a ganglion mother cell (GMC)
                 • Which will then go on to form a neuron or a group of neurons/glial cells
       • TYPE 2
         • Neuroblast will form a intermediate neuron projector (INP)
           • This will then go on to mature and form a copy of itself and a ganglion mother cell
             • This ganglion mother cell will then produce glial/or neural cells
         • IMPORTANT DIFFERENCE
           • Type II population of INPs represent a transit amplifying population of INPs that represent a transit amplifying population of type II NBs
             • Which produce more neurons than type I NBs
               • The INP (to some extent) has the same role of a neuroblast
   • d. Asymmetrical division of neuroblasts
     • Daughter cells are different from one another
     • The position of the neuroblast within the embryo leads to orientation within the single cell so that one side (shown in blue) has different stuff to the other side
       • Multiple proteins will determine the cell fate of the ganglion mother cell, including...
         • Numb
           • Localises a the anterior pole
         • Prosper
         • Brat
     • i. Steps:
       • Initial formation of axis polarity (A-B) occurs leading to differential distribution of stuff in the cell
         • Orientation of the mitotic spindle occurs
           • Asymmetrical localization of cell fate is determined within the NB
             • Differential segregation of cell fate determinants between daughter cells occurs
     • ii. Neuroblast clock
       • Neuroblasts have a timer
         • The cell uses a series of transcription factors to regulate neuroblast temporal identity
           • The neuroblast initially has hunchback
             • After division, another factor Kruppel fills the cell
               • Then POU domain protein (Pdm)
                 • Then Castor
                   • Then, depending on the neuroblast, the cell exits the cell cycle and stops dividing
                     • This cascade transcription factors is a system that helps a cell divide
                       • Mutations to the clock causes counting to become distorted and there is no exit from the cell cycle
                         • RESULT = TUMOUR OF NEURAL ORIGIN
   • Mouse neocortex
     • The neural stem cell divisions that occur early in Drosophila development have been observed in the mouse neocortex
       • There is direct neurogenesis through division of the blue cell into a neuron or another neural stem cell
         • Or may be an intermediate precursor cell which will then create neurons
           • This is all orientated in space through the AP axis

2. 2. MAKING NEURAL CIRCUITS UNIQUE

   • a. Regulated expression of Hox genes
     • I. Background
       • A mutation in one single gene within a fly can cause duplication of a whole segment, so that the fly produces an extra set of wings
         • This happens along the head-tail axis
       • Hox genes control the development of different morphologies along the head-to-tail axis
     • ii. Activation of different programmes of spcialisation along the AP axis
       • Patterns of differentiation within the THORAX produces a neural and glial cell
         • The ABDOMEN has a modified cell differentiation programme
           • Instead of producing one neural and glial cell, the same lineage produces two glial cells
             • The controlled expression of Hox genes that cellular programmes that are very stereotyped, are modified so they can form parts of a circuit
   • c. Movement control during development
     • Can we identify the molecular cellular circuits underlying the control of movement?
       • An experiment carried out at Sussex aimed to study what the effects of microRNA mutants were on Hox gene regulation
         • After multiple experiments, no differences were found between WT and mutant e.g. turning behaviour and peristalsis
           • However, after turning the mutant upside down he found the mutant could not turn itself back round
     • Recording and analysing muscle contractions in freely moving Drosophila larvae
       • Using a biochemical procedure that labels the animals and can be studies live as the animals move
         • Researchers have since identified a sequence of movements that concerns the contraction of muscles along the AP axis of the animal
           • Over time, there is a high level of coordinated contraction between the right and left side
             • As a result of that activity of coordination, we can see different behaviours emerge within late embryogenesis
               • - They can be well characterised in this simple organisms
                 • E.g. peristalsis - wave movement of muscles, touch response behavior to see if it retracts its head, self-righting behaviour
   • b. Hox gene regulatorys
     • What regulates the regulators?
     • I. RNA mechanisms
       • Subject to multiple forms of regulation and regulatory RNAs
         • MicroRNA regulation
         • Instead of comparing cell by cell mutant and wt genotypes, it was easier to view how the animal behaved…

3. 3. DROSOPHILA TOOLS TO STUDY NEURAL FUNCTION

   • a. Visualization of the developing nervous system
     • Simple staining can show the nervous system within the embryo
   • b. Ontogenetic control
     • We can quantify the response of individual cells during the motor response
     • Thermogenetic experiment to test escape response movement
       • Using two different fly genotypes, one wt and another genetically modified
         • The modification links to the expression of the dTPRA1 channel in all the cells in the nervous system of the animal
           • This channel is sensitive to changes in temperature
             • The experiment views how each bath reacts to the change in temperatures of stocks
               • Result = the mutant flies change physiology of their nervous system within 20 seconds

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