Hedgehog plays a major role during embryogenesis by acting directly on neural development and the dorsoventral pattern. During adulthood, it exerts a role in neuron generation, phenotype determination of neurons, cell cycle, stem-cell maintenance, and apoptosis. These processes include canonical and non-canonical mechanisms [
29]. represents the hedgehog signaling pathway, the impairment of which is lethal during embryogenesis, causing serious birth defects like cyclopia and holoencephaly. In the adult human brain, the hedgehog-signaling pathway is executed in a paracrine manner through sonic hedgehog ligand-secreting neurons in the ventral forebrain, substantia nigra, and septum as well as by ligand-activated glial cells in the sub-ventricular, ventricular, sub-granular zone and cortex [
30]. Some research studies supported evidence that the sonic hedgehog-signaling pathways perform an important role in cortical neuron fate modulation, circuit establishment, and astrocyte-arbitrated synaptic plasticity [
31]. Changes in the cellular response of hedgehog production and pattern can be observed in brain-related disorders like neoplasia, brain-cell injury, and psychiatric disorders [
32]. Some studies reported the correlation of components and hedgehog signaling-pathway activation (e.g., Patched activation, ligand concentration, cellular localization) leads smoothly to an etiology of ASD. Research in this field investigated the associated link between hedgehog components in the autistic brain and their synergism with several confounding factors, including congenital mutations in the gene component of the hedgehog pathway, impairment of the oxidative stress defense system, and inborn cholesterol metabolic errors. High serum levels of sonic hedgehog proteins were detected in autistic subjects when compared with an age-matched control group. The severity of the condition was positively correlated with the serum level of sonic hedgehog proteins. In addition to this, it was also observed that, in autistic subjects, blood levels of hydrogen peroxide (H
2O
2), superoxide anion (O
2−), and hydroxyl radicals (OH
−) were significantly higher. Thus it was suggested that increased oxidative stress induces the activation of a sonic hedgehog-dependent neuroprotection mechanism [32,33]. Other reports also contributed to the hypothesis that additional anti-oxidative pathway components like BCL2 (B-cell CLL/lymphoma 2 apoptosis regulator) apoptosis regulator, glutathione peroxidase, superoxide dismutase, and the BDNF (brain-derived neurotrophic factor) might cause some changes to the sonic hedgehog protein concentration [
34]. Other studies determined an interface between autism phenotypes and the Indian hedgehog and Desert hedgehog proteins. Investigations have shown that the serum concentration of desert hedgehog is lowered in autistic subjects with no exact correlation with disease severity [
35]. Indian hedgehog proteins in serum were elevated in autistic subjects more significantly if there was a positive correlation with the severity of the disease [
36]. Hence it is suggested that hedgehog proteins along with oxidative stress components may be significant biomarkers for ASD.
Figure 2. The hedgehog (hh) signaling pathway showing hh ligands (Shh, Dhh, Ihh) and their receptor Ptch 1. Upon binding with Ptch1, the pathway causes internalization, and Smo inhibition is released. After this, Smo is phosphorylated causing a cascade activation through downstream regulation, and Gli1/2 is processed into the activator forms GliA and GliB. After translocation of GliA into the nucleus, it stimulates target gene expression. The transcriptional repressor precursor Gli3 remains inactive.
During brain development, the hedgehog pathway is interrelated with several developmental and cell-survival mechanisms. Pathway activation can be implemented by both canonical (patched 1 mediated) and non-canonical processes. The non-canonical mechanism is mediated by several kinases like PKA (phosphokinase), GSK3-3β (Glycogen synthase kinase 3β), S6K (Ribosomal protein S6 kinase), DYRK1B (Dual-specificity tyrosine phosphorylation regulated kinase 1B) that can affect the condition of Gli1/2 phosphorylase, thus regulating its activity [37-39]. DYRK1B and S6K are also thought to be associated with the mTOR (mammalian target of rapamycin)/Akt (Protein kinase B) signaling processes, where DYRK1B regulates mTOR/Akt while mTORC1 phosphorylates S6K. Therefore it appears to be evident that there is a connection between Gli1/2 and Phosphoinositide 3-kinase (P13K) mTOR/Akt. Gli plays an important role in dual regulation. Hence, if there is an over-activation of mTOR signaling because of the influence of upstream regulation causing greater S6K activity, hedgehog signaling in the non-canonical pathway becomes upregulated. The biochemical and genetic biomarkers along with oxidative stress and the BDNF biomarker measured in serum, cerebrospinal fluid (CSF), and urine, have been suggested as potential biomarkers for ASD [
40]. Some more research studies are required to check the accuracy and reliability of the hedgehog pathway-related tests in identifying ASD phenotypes. Some other studies investigated the role of hedgehog pathways in the phenotypic acquisition and T-cell differentiation. Considering this as a reference, the T helper and CD4+ cells, along with high levels of Gli2A, were further differentiated into Th2 cells and secreted six-fold more interleukin 4 cells (IL-4) with normal levels of Gli2A after stimulation, suggesting that IL-4, like Gli, acts as a transcriptional target [
41]. It has been reported that the blood serum profile of mothers of autistic offspring show elevated levels of interleukins (IL-4) and dysregulation of T-helper cells and the lymphocytes that regulate them [
42] and that autistic patients [
43] have increased levels of Sonic hedgehog (Shh) and Indian hedgehog (Ihh) ligands [
36]. Conclusively, Gli factors play an important role in cell growth, differentiation, and survival in both the brain and immune system. Hence, more studies are required to prove the accuracy and reliability of the hedgehog-related pathway involvement in the detection of ASD phenotypes as well as modulators for designing novel therapeutic drug targets to treat ASD.