Let’s Make It Happen: The Role of let-7 MicroRNA in Development - Current Topics in Developmental Biology - page 14

Thus, it was surprising to find SL1-spliced let-7 primary transcripts ( Bracht
et al. , 2004 ). The 3 0 splice site required for trans -splicing is conserved in other
nematodes, and disruption of this signal impairs Drosha processing of the
primary transcripts ( Bracht et al. ,2004 ). Secondary structure predictions by
mfold suggest that splicing alters the context around the precursor hairpin,
perhaps making it a better substrate for subsequent processing. Although
SL-mediated trans -splicing is not found in most vertebrates, the primary
transcripts of several human let-7 genes are part of spliced transcripts. The
mechanistic role of splicing in let-7 biogenesis in worms, and perhaps other
species, is yet to be resolved.
In vertebrates and Drosophila, Exportin-5 delivers miRNA precursors
from the nucleus to the cytoplasm for Dicer processing ( Fig. 1.1 ; Bohnsack
et al. , 2004; Lund et al. , 2004; Yi et al. , 2003 ). The apparent absence of an
Exportin-5 homolog in C. elegans suggests that other cellular transport
factors are involved in miRNA biogenesis in this organism ( Bohnsack
et al. , 2004; Murphy et al. , 2008 ) . The nuclear export receptor XPO-1, as
well as the nuclear cap-binding complex (CBC), have been implicated in
the let-7 biogenesis pathway ( Fig. 1.7 ; Bussing et al. , 2010 ). Depletion of
XPO-1 or either of the two subunits of the CBC results in reduced levels of
precursor and mature let-7 and accumulated levels of pri-let-7. Considering
the role of XPO-1 and the CBC in mediating nuclear export of
m 7 G-capped U snRNAs ( Hutten and Kehlenbach, 2007 ), one possibility
is that capped primary miRNA transcripts are also substrates for transport to
the cytoplasm. Since the cellular location of Drosha is not known in worms,
nuclear export of pri-let-7 transcripts by XPO-1 and CBC could be
important for processing ( Bussing et al. , 2010 ).
The temporal expression of pri-let-7 suggests complex regulation at both
the transcriptional and posttranscriptional levels ( Fig. 1.7 ). Production of
primary let-7 in the L1 and L2 stages is not coupled to accumulation of
precursor and mature miRNA ( Fig. 1.6 ; Van Wynsberghe et al. , 2011 ). The
RNA binding protein LIN-28 mediates this phase of posttranscriptional
regulation ( Lehrbach et al. , 2009; Van Wynsberghe et al. , 2011 ). LIN-28
binds endogenous let-7 pri-miRNAs co-transcriptionally and blocks Drosha
processing ( Fig. 1.7 ; Van Wynsberghe et al. ,2011 ). Since lin-28 is controlled
by lin-4 miRNA, expression of this miRNA at the end of L1 results in a
steady decline of LIN-28 protein and, thus, relief of let-7 processing inhibi-
tion by the third larval stage ( Lehrbach et al. ,2009;Moss et al. , 1997; Van
Wynsberghe et al. , 2011 ). Posttranscriptional regulation of let-7 by LIN-28
was originally discovered in mammalian cells ( Heo et al. ,2008;Newman
et al. , 2008; Viswanathan and Daley, 2010; Viswanathan et al. , 2008; Wulczyn
et al. ,2007 ). In addition to preventing Drosha processing of let-7 pri-miR-
NAs, LIN-28 also recruits TUT4 (PUP-2 in C. elegans )toadda3 0 end U-tail
to let-7 precursors, which blocks Dicer processing and promotes destabiliza-
tion of the RNAs ( Hagan et al. , 2009; Heo et al. , 2008, 2009 ). Comparable to