Sonic hedgehog: the super(hero) gene of developmental origin
Summary
Embryogenesis is intimately dependent on genetic processes to develop a single cell into an entire organism. Sonic hedgehog (Shh) is one of the hedgehog genes responsible for body plan segmentation along the antero-posterior axis and the organogenesis of the eyes and central nervous system. Shh gene mutation is associated with abnormal development of the eyes, limbs and brain. Such roles are pivotal not only for embryogenesis, but repressed Shh gene expression can cause several developmental disorders. Regulating Shh expression in the developing embryo is thus critical to the successful development into an organism of complex body plan.
A Molecular Introduction
While inspired by the game character Sonic the Hedgehog, the Sonic hedgehog (Shh) gene belongs to the hedgehog gene family (named after mutant Drosophila embryos grew spiky hairs); encoding the Sonic hedgehog (SHH) protein for vertebrate embryogenesis (Murphy, 2018). In humans, the Shh gene is located on chromosome 7 and can be into processed into 6 different mRNAs (1 unspliced and 5 spliced variants) (“Gene: SHH,” 2020). Once translated, the 6 splicing isoforms are post-translationally modified by autocleavage into amino-terminal (SHH-N) and carboxy-terminal (SHH-C) domains; the former being responsible for morphogenic signaling (Placzek, 1995). In the hedgehog (hh) signaling pathway, SHH is then secreted from the cell and can bind to the membrane-bound Patched-1 (Ptch1) receptor on either the Shh-producing cell (by autocrine signaling) or nearby target cells (paracrine signaling) (Ribes & Briscoe, 2009; Roelink et al., 1995). Receptor binding prevents Ptch1 from inhibiting another signaling protein Smoothened (Smo); activating transcriptional targets of the hedgehog signaling pathway (Chuang et al., 2003). This in turn increases the longevity of active Smo, activating Gli transcription factors downstream that produce Ptch1 and Hedgehog interacting protein 1 (HHIP1) (Ribes & Briscoe, 2009). As HHIP1 binds to Shh and is membrane-bound, this reduces the amount of free Shh able to bind to Ptch1, negatively regulating the signaling pathway (ibid.).
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Roles for Embryo and Adult Development
While negative feedback makes sense biochemically to regulate intracellular signal transduction, this pathway is further critical to pattern formation in the developing embryo. With paracrine signaling of SHH-N from the notochord, SHH-N induces the ventral region of the neural tube to form the floor plate (Yamada et al., 1993 as cited in Placzek, 1995 & Roelink et al., 1995). Consequently, the floor plate becomes the second source of SHH-N production, which with the notochord, can induce other ventral cell types (like motor neurons and ventral interneurons) to differentiate in the neural tube (Yamada et al., 1993 as cited in Placzek, 1995 & Roelink et al., 1995; Placzek, 2000).
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In terms of pattern formation, the threshold concentrations of SHH-N inducing floor plate cells are 5-fold higher than for motor neuron differentiation, indicating the intercellular cascading effect of the hh signaling pathway as SHH-N concentrations decrease from the source cells (notochord) to form ventral cell types along the dorsoventral axis (Roelink et al., 1995). This differential and coordinated signal strategy, as Ribes and Briscoe (2009) term “temporal adaptation,” is further demonstrated temporally as explants exposed to 4nM SHH for 25 hours were able to produce floor plate cells and motor neurons, whereas explants not administered SHH after 12 hrs did not produce these neural plate cells (Ericson et al., 1996). Additionally, ectopic expression of Shh generates floor plate and motor neuron cells in explants whereas Shh-null mice fail to develop these neural plate cells and the notochord degenerates (Ericson et al., 1996; Chiang et al., 1996). Such findings are not only pivotal for the developing embryo, but as well for the motor function during growth as their regulation and control are directly associated with movement disorders like Parkinson’s disease (Placzek, 2000).
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Moreover, given the ventral patterning is so key to notochord maintenance, it comes as no surprise that SHH expression plays a major role in brain, eye and limb development (Chiang et al., 1996). Specifically, Shh-/- mice embryos have shown reduced brain size and abnormal forebrain morphology (ibid.). This is because the midline (that would normally separate the right and left sides of the forebrain) becomes fused with ventral brain lobes, and thus ventral forebrain structures are lost during development (Chiang et al., 1996; MedlinePlus, 2020a). This is significant for the eye development as the optic vesicle invaginates from the forebrain (Macdonald & Wilson, 1996). Thus, comes as no surprise that Shh mutant embryos do not form two, but rather one cyclopic eye (Chiang et al., 1996). This relationship is further established as optic vesicles also fuse with the midline in Shh mutant embryos, preventing the development of the neural and pigment epithelial layers of the retina as well as preventing the eye field’s subdivision into two eyes (Chiang et al., 1996; Macdonald & Wilson, 1996; MedlinePlus, 2020a). In terms of the limbs, they also involve the temporal expression of SHH to form digits (Harfe et al., 2004). In fact, this discovery conceived the concept of morphogens as transplanting of the posterior of one chick limb bud onto the anterior side of the other chick bud led to mirror duplications of the posterior digits on both ends of the limb (Saunder and Gasseling, 1968 as cited in Harfe et al., 2004). These cells responsible for the spatially differentiated signal -the zone of polarizing activity (ZPA)- were also identified to encode Shh for posterior digit formation (Riddle et al., 1993 as cited in Harfe et al., 2004). But while SHH-producing cells produce the posterior digits 3-5, the differences between digits formed lie on the concentration and exposure of SHH during development (Harfe et al., 2004). Consequently, the temporal and spatial patterning of Shh expression across the notochord, optic vesicles and limbs are key not just for the development of the embryo, but also the functioning of the adult.
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Morphogenetic significance of the ancestral super(hero) gene
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It is evident that Shh expression is not only pivotal for embryogenesis but also adult development and function. This perhaps explains their ancestral importance as conserved gene for the development of the brain, eyes and limbs for all vertebrates. Moreover, the genetic-developmental relationship from notochord Shh signaling (to induce motor neuron differentiation and eye field subdivision whilst develop into the brain) highlights their intimate roles during adult functioning (such as seeing a predator, alerting the sympathetic nervous system to increase limb activity for ‘fight-or-flight’). Thus, when Shh expression is depressed during organogenesis, the consequences extend to the adult health. Specifically, over 100 mutations in the Shh gene have found to cause nonsyndromic holoprosencephaly (MedlinePlus, 2020a). This condition occurs when the brain fails to divide into two separate hemispheres and the abnormal development of the eyes and/or face (ibid.). Additionally, at E9.5, the microphthalmia gene is observed in the optical vesicle of Shh mutant embryos, a gene that leads to the condition of abnormally small eyeballs (Chiang et al., 1996; MedlinePlus, 2020b).
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All in all, the cascading effects of the hh signaling pathway during organogenesis depict spatial and temporal patterning of SHH to form the brain, eyes and limbs of the adult body. The morphogen’s intimate effects on notochord development not only influences cell identity, but importantly, body functionality. Unsurprisingly, its super(hero) importance is thus critical and well suited to its name.
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References:
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