Download background-for-Flavell-et

The manuscript by Flavell et al. concerns dissection of a neural circuit that underlies
aspects of C. elegans foraging behavior. My goal in picking this paper is to highlight
spatial aspects of gene function, requiring selective inactivation or activation of a
gene in subsets of places that a gene is expressed, and temporal aspects of gene
function/signaling, requiring strategies for rapidly turning pathways on/off. There
is no need to become overly concerned/stuck in details of the neural circuit or
quantitative methods (e.g. hidden Markov models), used to measure the behavioral
states. Here is some quick background:
C. elegans foraging: C. elegans move and turn with different speeds and
frequencies on and off of food. These movement patterns have been interpreted as
foraging strategies. For example, less turns and more rapid movement allows
animals to explore larger areas. The paper explores two movement patterns while
on food: dwelling (defined as lots of turns and relatively little movement away from
a spot), and roaming (less turns and longer distances covered). Animals on food
and without obvious external cues switch back and forth between these states.
C. elegans neurons: All neurons of C. elegans are known by their names! For
example, ASI, ASG, NSM….For the purposes of the discussion, the names don’t matter
other than a way of keeping track of what is expressed and functions where.
Serotonin signaling: 5-hydroxytryptamine, serotonin, is a neuromodulator that is
derived from tryptophan. The rate-limiting enzyme for serotonin production is
tryptophan hydroxylase, tph. The hermaphrodite C. elegans tph-1 is expressed in
only a few neurons: sensory ADF, neurosecretory NSM in the pharyngeal foregut,
and the HSN (involved in egg laying, previous discussion paper). Under certain
conditions, e.g. hypoxia, tph-1 expression also appears in other neurons, ASG. In
turn, serotonin acts through either G-protein coupled serotonergic receptors (ser-1,
-4, -5, -7) or a chloride-gated ion channel (mod-1). Once serotonin is released, it is
taken back up through action of a re-uptake transporter named MOD-5.
Neural secretions: Biogenic amines, such as serotonin, or neuropeptides, such as
PDF, are packaged into vesicles, named dense-core vesicles (DCVs). Secretion of
dense-core vesicles is a beautifully orchestrated set of events that include opening
and closing of ion channels, influx of calcium, and the calcium responsive machinery
that regulates fusion of the DCVs with the plasma membrane (causing release of
DCV contents). Therefore, one can promote or block release of DCVs by depolarizing
or hyperpolarizing (changes in ionic flux across the plasma membrane) of neurons.
Monitoring neural activity: As neural activity often involves a change of calcium
flux in/out of the cells, fluorescent calcium reporters have been used as a proxy for
monitoring neural activity.
mosSCI: This is a transposon-based system for inserting genes into the C. elegans
genome. Until CRISPR came along, there has not been a good way for targeting a
transgene to a specific genomic locus. You need not worry about the details of
mosSCI other than it was a way of inserting a gene somewhere in the genome.
Discussion points:
1. Figure 1: Why did the authors only look at a handful of mutants rather
than doing a screen?
2. What pieces of evidence do the authors use to claim that tph-1 and mod1 are part of the same pathway in these behaviors?
3. Figure 1 (D and E) and Figure 3: Discuss the strategies used to
investigate sites of tph-1 and mod-1 function.
4. Figure 5. What is the point of the optogenetic manipulations?
5. Figure 6. Discuss the strategies used to selectively remove or
reconstitute PDFR-1 function in different neurons.
6. What is the point of the BlaC experiment?
7. At the very big picture level, is there anything in this process that is
reminiscent of the phage lambda (lysis versus lysogeny) decision?