No abstract

Heparan sulfate proteoglycans are cell surface and secretory proteins that modulate intercellular signaling pathways including Slit/Robo and FGF/FGFR. The heparan sulfate sugar moieties on HSPGs are subject to extensive postsynthetic modification, generating enormous molecular complexity that has been postulated to provide increased diversity in the ability of individual cells to respond to specific signaling molecules. This diversity could help explain how a relatively small number of axon guidance molecules are able to instruct the extremely complex connectivity of the mammalian brain. Consistent with this hypothesis, we previously showed that mutant mice lacking the heparan sulfotransferases (Hsts) Hs2st or Hs6st1 display major axon guidance defects at the developing optic chiasm. Here we further explore the role of these Hsts at the optic chiasm and investigate their function in corpus callosum development. Each Hst is expressed in a distinct pattern and each mutant displays a specific spectrum of axon guidance defects. Particular Hs2st ؊/؊ and Hs6st1phenotypes closely match those of Slit1 ؊/؊ and Slit2 ؊/؊ embryos respectively, suggesting possible functional relationships. To test functional interactions between Hs2st or Hs6st1 and Slits we examined optic chiasm and corpus callosum phenotypes in a panel of genotypes where Hs2st or Hs6st1 and Slit1 or Slit2 function were simultaneously reduced or absent. We find examples of Hs2st and Hs6st1 having epistatic, synergistic, and antagonistic genetic relationships with Slit1 and/or Slit2 depending on the context. At the corpus callosum we find that Hs6st1 has Slit-independent functions and our data indicate additional roles in FGF signaling.

The corpus callosum (CC) connects the left and right cerebral hemispheres in mammals and its development requires intercellular communication at the telencephalic midline mediated by signaling proteins. Heparan sulfate (HS) is a sulfated polysaccharide that decorates cell surface and extracellular matrix proteins and regulates the biological activity of numerous signaling proteins via sugarprotein interactions. HS is subject to regulated enzymatic sulfation and desulfation and an attractive, although not proven, hypothesis is that the biological activity of HS is regulated by a sugar sulfate code. Mutant mouse embryos lacking the heparan sulfotransferases Hs2st or Hs6st1 have severe CC phenotypes and form Probst bundles of noncrossing axons flanking large tangles of midline glial processes. Here, we identify a precocious accumulation of Sox9-expressing glial cells in the indusium griseum region and a corresponding depletion at the glial wedge associated with the formation of Probst bundles along the rostrocaudal axis in both mutants. Molecularly, we found a surprising hyperactivation of Erk signaling in Hs2st Ϫ / Ϫ (2-fold) and Hs6st1 Ϫ / Ϫ (6-fold) embryonic telencephalon that was most striking at the midline, where Erk signaling is lowest in wild-types, and a 2-fold increase in Fgf8 protein levels in Hs6st1 Ϫ / Ϫ embryos that could underpin Erk hyperactivation and excessive glial movement to the indusium griseum. The tightly linked Hs6st1 Ϫ / Ϫ CC glial and axonal phenotypes can be rescued by genetic or pharmacological suppression of Fgf8/Erk axis components. Overall, our data fit a model in which Hs2st and Hs6st1 normally generate conditions conducive to CC development by generating an HS-containing environment that keeps Erk signaling in check.

During embryonic development of the visual system, retinal ganglion cells (RGCs) project their axons towards the brain, passing through the optic chiasm. Axons are guided on this journey by molecular cues in the environment. The heparan sulphate sulphotransferase (Hst) enzymes Hs2st and Hs6st1 are each known to be required for specific aspects of axon guidance in the developing visual system, as revealed by studies of Hs2st ) ⁄ ) and Hs6st1) ⁄ ) mutant embryos. However, it remained possible that these two enzymes have additional, overlapping, functions in RGC axon guidance; but that no effect is manifest in single mutant embryos, because the other enzyme is sufficient to fulfil the shared function. To investigate this possibility, we generated a set of Hs2st;Hs6st1 double mutant embryos that had reduced gene dosage of each of these Hsts, reasoning that any additional phenotypes in these animals would indicate the presence of functional overlap. We first characterised the structure of the mutant Hs6st1 locus, identifying the insertion site of the gene trap vector, to allow us to genotype compound mutants reliably. We found that Hs2st;Hs6st1 ) ⁄ ) mutants that lack both enzymes died prior to E15.5. As the optic chiasm has not formed by this stage, we were unable to determine the effect of complete loss of Hs2st and Hs6st1 on chiasm formation. However, compound mutant embryos lacking one Hst and heterozygous for the other were viable. We found that RGC axon guidance defects in such compound mutants were no more severe than those found in the single mutant embryos. We also found that expression of the Hs6st1 isoform Hs6st3 overlaps with that of Hs6st1 in the developing visual system, suggesting that some Hs6st activity remains present in this region of Hs6st1 ) ⁄ ) mutant embryos.

In the mid-1970s, the Royal Commission on Electrical Power Planning (RCEPP) was ordered by the government of Ontario to review Ontario Hydro’s ambitious expansion plans. Historians have often considered the RCEPP an interesting but ineffective commission as changing economic factors, rather than the Commissions’ recommendations for slower growth, eventually slowed Hydro’s momentum in the early 1980s. This paper explores the Commission as an important venue for energy debate and as a means of facilitating research from public interest groups, including Energy Probe, in the late 1970s. From this debate the Commission negotiated ideas of “soft energy paths”, global resource limits, and cybernetic system thinking into a set of policy recommendations for democratic, systems-based electrical power planning. I argue that the tension between centralized control and local action found in the Commission’s systems approach to planning illustrates the difficulty of collective, long-term, and expert mediated, globalist planning in a period once thought of as a “dawning age of energy conservation.”