Molecular Control of Axon Growth: Insights from Comparative Gene Profiling and High-Throughput Screening - International Review of Neurobiology - page 52

is upregulated in DRG neurons in response to peripheral injury and is in-
duced by the cytokine IL6, an activator of axon growth in DRG neurons
( Cao et al., 2006; Miao et al., 2006 ). Despite its positive correlation with
PNS regeneration, however, Socs3 appears to function as a growth
inhibitor, likely by providing negative feedback to dampen growth-
activating signaling by cytokines ( Miao et al., 2006 ). For instance,
overexpression of Socs3 reduces the speed of neurite growth from DRGs
and inhibits RGC regeneration into peripheral nerve graft ( Hellstrom
et al., 2011; Miao et al., 2006 ). Knockout of Socs3 promotes axon
regeneration by RGCs, particularly in conjunction with activation of
mTOR signaling ( Smith et al., 2009; Sun et al., 2011 ). Socs3 therefore
illustrates the ability of work in the PNS to identify important gene
targets to foster CNS regeneration, but also warns against simple
conclusions from gene expression analysis. A priori it is difficult to predict
whether genes upregulated during PNS regeneration are functionally
pro-regenerative, irrelevant, or even, like Socs3, feedback inhibitors of
regeneration. Genome-wide profiling studies, which can aid in
predictions of gene function by identifying potentially interacting genes,
and HTS studies, which directly test gene function, are promising
approaches to address this complexity.
4.2. Expression profiling and Bioinformatic analysis of PNS
injury-induced genes
The PNS injury response has been profiled in multiple studies using micro-
array analysis or molecular techniques such as differential display, SAGE, and
subtractive library construction ( Table 3.1 ). As seen in Table 3.1 , since their
inception in the early 1990s, these studies have rapidly expanded their cov-
erage of the genome and have become increasingly detailed in tracking the
injury response through time. Depending on the study, several hundred to
several thousand gene changes are detected as PNS neurons respond to
axotomy and mount a regenerative response. As with the developmental
datasets discussed earlier, the scale of these changes poses a significant chal-
lenge to identifying genes that function in axon growth.
To manage this complexity, some groups have searched these datasets for
underlying “linchpin” transcription factors that orchestrate PNS regenera-
tion. Gene transcription is required as peripheral neurons transition to a
regenerative state ( Smith & Skene, 1997 ), making such transcription factors
attractive targets to understand and manipulate the regenerative response.
Stam et al. (2007) used microarrays to identify more than 400 genes that