Insulin signaling in the nematode,

Insulin signaling in the nematode, C. elegans
The well-studied nematode, C. elegans, has provided critical
insights into insulin signaling and its role in regulating animal physiology, longevity and neuronal functions.
About 40 ILP genes (ins-1 to ins-39 and daf-28) have been
identified in the genome (Ritter et al. 2013). Characteristic
of gene duplication events throughout evolution, these ILP
genes are distributed across all six pairs of chromosomes in
the worm as shown in Fig. 2a. The expansive insulin gene
family allows for both divergence and redundancy in the
function of this crucial signaling network. INS-1 is most
similar to the mammalian insulin peptide, but the other
worm ILPs also show conserved structural domains with
human insulin A- and B-chains (Fig. 1a) (Pierce et al.
2001). As described above (Fig. 1b), ILPs bind the conserved
tyrosine kinase receptor DAF-2 (Kimura et al. 1997)
and act via AGE-1 and AKT kinases to phosphorylate
DAF-16 and activate the transcription of target genes
(Paradis et al. 1999; Morris et al. 1996; Lin et al. 1997;
Paradis and Ruvkun 1998). The large number of ILPs
emphasizes the diversity in signaling roles for these ligands
in regulating physiology.
Recent studies analyzing the role of ILPs in the C.
elegans nervous system have identified two flavors of this
signaling pathway: a transcription-dependent and a transcription-
independent component. Recent work shows that
insulin signaling requires a transcription-dependent process
in regulating the development of the neuromuscular junction
(Hung et al. 2013). In this context, multiple insulins
(INS-6 from ASI sensory neurons and INS-4 from both
ASI sensory and motor neurons) act on the DAF-2 receptor
to regulate the effects of a transcription factor, F-box factor
(FSN-1), on neuromuscular junction morphology and
motor neuron–muscle synapse numbers. fsn-1 mutants
have aberrant synapse numbers and morphology, and these
effects are rescued by specifically reducing insulin signaling
in the post-synaptic muscle. These results show that
insulin signals likely antagonize FSN-1 signaling to regulate
neuromuscular synapses. In this example, insulin signaling
interacts with another neuronal signaling pathway to
fine tune a key developmental step using a transcriptiondependent
component.
Interestingly, INS-6 released from the ASI neurons can
also function outside of the neuromuscular junction, for
responding to pathogenic bacteria. C. elegans exposed to
pathogenic bacteria can learn and avoid that pathogen upon a
second exposure (Zhang et al. 2005). In the pathogenavoidance
learning paradigm, ASI sensory neuron-released
INS-6 peptide inhibits the transcription of INS-7 peptide in
the oxygen-sensing URX neurons. In the absence of INS-6,
INS-7 acts through the DAF-2 receptors to regulate the
localization of DAF-16 in downstream RIA interneurons
(Chen et al. 2013). This ILP–ILP loop provides another
example of the complexity and diversity of signaling used to
achieve specific changes in neural networks. Moreover,
these results also show that the same ligand, INS-6 released
from the same ASI neuron, can have multiple distinct roles in the nervous system. Additionally, INS-6 has also been
shown to play a crucial role in coupling environmental
conditions with development by inhibiting dauer entry and
promoting dauer exit (Cornils et al. 2011). In response to
harsh environmental conditions, C. elegans larvae enter a
reversible stage of growth arrest (termed ‘‘dauer’’) wherein
the animal arrests feeding and limits locomotion (Cassada
and Russell 1975). For regulating dauer arrest, the INS-6
signal is also released from ASI sensory neurons. However,
an extra pair of sensory neurons, the ASJs, is also involved in
INS-6 production in the context of dauer arrest (Cornils et al.
2011). Collectively, these results suggests that a high degree
of spatial and temporal control of insulin release enables the
same ligand to perform different roles based on context.
Results from a number of laboratories have shown that
insulins can also modulate neuronal activity in a transcription-
independent manner that occurs on faster timescales.
In a salt chemotaxis learning paradigm, a 10–60-
min exposure to a particular concentration of salt in the
absence of food leads to reduced attraction to that salt
concentration (Tomioka et al. 2006). INS-1, DAF-2 and
AGE-1 all play roles in altering neural activity of the saltsensing
ASE neurons (Oda et al. 2011). Importantly, this
process does not require DAF-16 or transcription. Similarly,
INS-1 released from AIA interneurons has been
shown to suppress AWC sensory neuron responses to food
odors (Chalasani et al. 2010). In both of these examples,
INS-1 acts on short timescales: less than a second in the
case of AWC responses and a few minutes in ASE-associated
behavior. These results show that insulin signaling
can also function independently of transcription in influencing
neuronal function for fast neural modulation.
Recent work from our laboratory demonstrated that the
source of the insulin ligand is also crucial in determining its
function. We showed that in contrast to ASI-released INS-6,
ASE neurons use a proprotein convertase BLI-4 to process
INS-6 and recruit AWC sensory neurons into the salt neural
circuit (Leinwand and Chalasani 2013). In this example, INS-
6 signaling functions on a short timescale of less than one
second and likely do not require transcription. These results
argue that the same ligand (INS-6) can be processed by different
machinery in different neurons and function in a transcription-
dependent and -independent manner based on
context. In summary,modulation of neural function by insulin
signaling is regulated at many levels: at the source and target,
spatially and temporally and through transcription-dependent
and -independent mechanisms.