Question 1

Indicate true (T) and false (F) statements below regarding the formation of orderly neural maps in the mammalian brain. Your answer would be a four-letter string composed of letters T and F only, e.g. FFFF.
( )	As a general rule, neurons that fire together avoid each other.
( )	In the tonotopic map of the auditory system, brain neurons are arranged in the auditory cortex according to the pitch of the sound they respond to, like the keys of a piano.
( )	In the optic tectum in the midbrain, brain neurons are arranged along the anterior-posterior axis according to the frequency of light they respond to, like the colors of a rainbow.
( )	If the optic nerve of a frog is cut and the eye is rotated 180° in its socket, the retinal ganglion cells reconnect to their original targets in the optic tectum, therefore creating an inverted map.

Accepted Answer

As a general rule, "neurons that fire together wire together." Unlike the tonotopic map in the auditory cortex, in which the neurons are arranged according to the frequency of sound to which they respond, the neurons in the retinotopic map in the optic tectum are arranged according to the position of the cell bodies of the retinal ganglion cells to which they are connected. The retinotopic map can be inverted, as shown by the classic optic nerve regeneration experiments in frogs.

Question 2

After reaching sexual maturity, the nematode Caenorhabditis elegans normally doubles in size within about two weeks. This doubling is mainly due to …

Accepted Answer

After cell division in the somatic tissue is halted in the adult worm, endoreplication makes these cells polyploid, which is accompanied by a larger cell size and a larger overall body size.

Question 3

You grow neurons in culture in the presence or absence of nerve growth factor (NGF). You perform this experiment using either wild-type neurons or neurons lacking caspase-3, an effector protein required for programmed cell death by apoptosis in these cells. After a day, you examine cell survival in the culture using a microscope. The survival scores (percentage of cells surviving the treatment) are presented in the following table. Which column (a or b) corresponds to the experiment in the presence of NGF? Which row (1 or 2) corresponds to the experiment using wild-type cells?

Accepted Answer

NGF withdrawal (a) results in apoptosis, but not when caspase-3 is deleted (1).

Question 4

Robo3.1 is an alternative splice form of Robo3 that blocks the reception of Slit signals by the Robo1 and Robo2 receptors. Considering the guidance of commissural neurons crossing the ventral midline in the embryonic spinal cord of a vertebrate, would you expect Robo3.1 activity to increase (I) or decrease (D) as the growth cone crosses the midline? Write down I or D as your answer.

Accepted Answer

Robo3.1 is lost as neurites cross the midline. As a result, the growth cones now respond to Slit and are repelled from the midline.

Question 5

Consider the embryonic cerebral cortex of a mammal. Near which side of the cortex are the cell bodies of radial glial cells located? To which side of the cortex do the first-born neurons eventually migrate?

Accepted Answer

The radial glial cells form a scaffold in the developing cortex to guide the migrating neurons to their destined layer. The first-born neurons migrate and settle closest to the lumenal (inner) surface of the cortex, whereas last-born neurons crawl past them and settle in outer layers. The radial glial cells divide near the lumenal surface (where their cell bodies are located) to give rise to neurons and glia.

Question 6

If explants that can produce axons from either the anterior or posterior retina are placed on a culture substratum that is coated by either anterior or posterior tectal membranes, …

Accepted Answer

In this experiment, posterior explants would only grow on anterior membranes; anterior explants, however, can grow on both membranes.

Question 7

In the following graph, which line (1 to 3) better represents the changes in the level of EphA receptors on retinal axons along the anterior-posterior axis of the mouse retina? Which line better represents the changes in the abundance of EphrinA along the anterior-posterior axis of the mouse optical tectum? Write down your answer as a two-digit number, e.g. 22.

Accepted Answer

Neurons in the posterior retina express EphA. Their axons are repelled by high levels of EphrinA protein in the posterior tectum and thus project preferentially to the anterior tectum.

Question 8

You have grown neurons in a culture dish containing laminin and have induced axon outgrowth. You now incubate the cells in the presence of various concentrations of two secreted proteins (1 and 2). You then fix the cells, stain them with rhodamine-phalloidin (a fluorescent dye that selectively labels actin filaments), and finally use a fluorescence microscope to examine the axons. Some of the axon tips have lost their spread morphology (with lamellipodia shrunk and filopodia retracted) and lack significant fluorescence. You call these "collapsed cones." In the following graph, the percentage of cones that have collapsed is plotted as a function of concentration of proteins 1 and 2. Do you expect these proteins to be chemoattractants or chemorepellents in guiding axonal growth cones? Which one is more potent?

Accepted Answer

Both of these proteins would be expected to be chemorepellents in guiding the migration of growth cones, as they both increase the rate of collapse. Protein 1 is a more potent repellent.

Question 9

An axonal growth cone has reached a muscle fiber and is forming a neuromuscular junction. Which of the following events does NOT normally occur in this process?

Accepted Answer

Signaling occurs in both directions for the formation of a neuromuscular junction. Agrin is released from the growth cone and binds to its receptor, LRP4, on the surface of the muscle cell. In addition to its role as a co-receptor, LRP4 also serves as a signaling molecule to stimulate the formation of presynaptic structures in the neuron.

Question 10

Indicate whether each of the following conditions favors a larger (L) or smaller (S) tissue or body size. Your answer would be a four-letter string composed of letters L and S only, e.g. SSSS.
( )	Hippo overexpression
( )	Insulin-like growth factor overexpression
( )	Myostatin deletion
( )	Growth hormone deficiency

Accepted Answer

Growth hormone stimulates growth, and its deficiency leads to a smaller body size. It acts by inducing the expression of insulin-like growth factor, overexpression of which may lead to a larger body size in an animal. Myostatin specifically inhibits the growth and proliferation of myoblasts; consequently, loss of myostatin results in greatly enlarged muscle tissue. The Hippo pathway inhibits organ and body growth in general; overexpression of Hippo, an upstream component of this pathway, inhibits growth.

Question 11

You have obtained leaf samples from three strawberry varieties A, B, and
A) Variety A
B) Variety B
C) Variety C
C) You isolate the cells, extract their nuclei, and stain them with propidium iodide, a fluorescent dye that binds quantitatively to DNA. You then use a fluorescence-activated cell sorter (FACS) machine to sort the nuclei based on their fluorescence. Guessing from the results, presented in the following histograms, which variety would you expect to yield larger strawberries?
D) They should all be the same

Accepted Answer

In plants, as in animals, cell size correlates with ploidy. Additionally, body and organ size in plants correlates well with ploidy. The fluorescent signal in this experiment corresponds to the nuclear DNA content. Varieties A, B, and C appear to be mostly diploid, octaploid, and tetraploid, respectively.

Question 12

Consider Agrin signaling in the formation of neuromuscular junctions in vertebrates. Shp2 is a protein tyrosine phosphatase that counteracts the tyrosine kinase activity of MuSK. Rapsyn interacts with the acetylcholine receptors and induces their agrin-dependent clustering. Indicate whether each of the following is expected to enhance (E) or suppress (S) acetylcholine receptor clustering in this process. Your answer would be a four-letter string composed of letters E and S only, e.g. EEEE.
( )	Specific inhibition of Shp2
( )	Loss of Rapsyn
( )	Loss of LRP4
( )	Constitutive activation of MuSK

Accepted Answer

Agrin binding to LRP4 activates MuSK, leading to acetylcholine receptor clustering mediated by Rapsyn. As expected, loss of LRP4 or Rapsyn negatively affects receptor clustering and synapse formation. On the other hand, abnormal activation of MuSK or inhibition of its counteracting phosphatases leads to unnecessary clustering.

Question 13

Which of the following is correct regarding neuronal self-avoidance in both invertebrates and vertebrates?

Accepted Answer

Despite employing similar strategies for self-avoidance, neurons in vertebrates and invertebrates use different cell-surface molecules to distinguish self from non-self. DSCAM proteins in Drosophila can be produced from an extraordinary large number of different isoforms generated by alternative RNA splicing. The protocadherins in vertebrates are expressed in a large number of different combinations. In both examples, homophilic recognition results in self-avoidance. This is distinct from the phenomenon of tiling, where each neuron occupies a solitary territory, repelling its neighbors.

Question 14

Which cell behavior depicted below is involved in the tube formation that underlies the development of lungs and trachea?

Accepted Answer

Budding (B) generates the tubes of developing lungs and trachea.

Question 15

The axons and dendrites of neurons that lack DSCAM1 altogether …

Accepted Answer

Drosophila neurons that do not express any DSCAM1 isoform avoid neither themselves nor other neurons, including wild-type neurons.

Question 16

CFSE (carboxyfluorescein succinimidyl ester) is a cell-tracing molecule that, once inside a cell, is modified to yield a highly fluorescent molecule that is retained within the cell. Due to its stability, it can be used to trace cells and follow their divisions: each division dilutes the fluorescent dye twofold. You label T cells from a healthy mouse with CFSE and inject them into either a wild-type strain of mouse or a strain with T cell deficiency, both of which are immunologically compatible with the donor mouse. You later collect and analyze peripheral T cells from the recipient mice and draw the following histograms according to the fluorescent intensity per cell. Based on these results, do you think the total number of T cells in the mouse body is (1) controlled by intracellular programs (as in the thymus, for example) or is (2) regulated as a whole (as in the spleen)? Write down 1 or 2 as your answer.

Accepted Answer

The injected T cells have divided (the CFSE dye is diluted) in the T-cell-deficient mouse but not in the wild-type mouse, supporting the idea that the total population of these cells is regulated as a whole.

Question 17

A cross section of a developing spinal cord in a vertebrate embryo is shown in the schematic drawing below. Indicate which feature in the drawing (labeled A to E) better matches each of the following descriptions. Your answer would be a four-letter string composed of letters A to E only, e.g. AAEB.
  
( )	It contains motor neurons.
( )	It is the floor plate.
( )	It secretes BMP and Wnt signals.
( )	It contains sensory neurons.

Accepted Answer

A represents the roof plate which secretes BMP and Wnt signals; D represents the floor plate which secretes Sonic hedgehog protein. B and C represent the dorsal and ventral regions of the tube, containing sensory and motor neurons, respectively.

Question 18

Mutations in certain components of the cell-cycle machinery in Drosophila melanogaster can be used to slow down the rate of progression through the cell cycle in the wing imaginal discs of the fly larvae, without a major effect on the rate of cell growth. As a result, compared to wild-type flies, the wing in the mutant flies would …

Accepted Answer

The size of the disc is not set to contain a certain number of cells; instead, it is regulated by the total disc size.

Question 19

Sort the following phases in the order that they take place in neural development in vertebrates. Your answer would be a four-letter string composed of letters A to D only, e.g. DACB.
(A)	Neurons are assigned specific characters according to the place and time of their birth.
(B)	Neurons form synapses with other neurons or with muscle cells.
(C)	Synaptic connections are refined and adjusted.
(D)	Neurons extend axons and dendrites toward their target cells.

Accepted Answer

First, the neurons are assigned specific characters according to the place and time of their birth. Next, newborn neurons extend projections along specific routes toward their target cells. In the third phase, neurons form synapses (with other neurons or muscle cells) that are then refined and adjusted in the fourth phase.

Question 20

In the following schematic drawing, a growing axon (in red) is extended to a target neuron to form three synapses. In its journey, it is guided by a variety of mechanisms. Sort the following mechanisms to reflect the order in which they guide the navigation of the growth cone toward the target neuron in this example. Ignore the final chemoattraction step toward the target neuron itself. Your answer would be a six-letter string composed of letters A to F only, e.g. FACBDE.
  
(A)	Chemoattraction
(B)	Chemorepulsion
(C)	Cell surface adhesion
(D)	Contact inhibition
(E)	Extracellular matrix adhesion
(F)	Guidance by pioneer neuron

Accepted Answer

Growth cones use various cues to navigate toward their targets. Please refer to Figure 21-73.

Quiz 21: Development of Multicellular Organisms