Additionally, we find that NVP-BGJ398 the Shh receptor Boc is expressed exclusively in a complementary nonoverlapping population of callosal and local projection neurons in the cortex that are known to preferentially form connections onto deep-layer subcortical projection neurons. This pattern of expression where Shh is expressed by layer V corticofugal “target” neurons, and Boc is expressed by layer II/III callosal inputs is consistent with the model of known connection preferences in cortical microcircuitry (Figure 9). While the peak expression of both Boc and Shh appears to coincide with peak periods of cortical synaptogenesis, both genes continue to be expressed in the cortex

through adulthood. It remains possible that in addition to its role in the initial formation of cortical circuits, Shh function may continue to play an important role in the adult brain Linsitinib concentration in regulating synaptic plasticity of these circuits. Previous studies of Shh function have largely focused on its regulation through the canonical Hh pathway in which Shh binds to Patched and disinhibits Smoothened to promote activation of Gli family

transcription factors. Many studies use Gli activation as a measure of Shh activity within target tissues. However, recent work has shown that Shh function during axon guidance is mediated through a noncanonical pathway that requires the Boc-dependent activation of Src family kinase members, and may not require Gli family transcription (Yam et al., 2009). Considering that Gli1 activation is not found in cortical neurons (Garcia et al., 2010), a similar pathway involving Boc receptor mediated activation of Src family kinases could be responsible for Shh function during cortical circuit development. While Gli activity is not found in postnatal cortical neurons, recent work has shown that Gli1 activation is found in cortical astrocytes. In light of our finding of a population of Shh

expressing glial cells in the cortex, this raises an additional intriguing possibility that Shh could and be signaling to two different cell populations through two distinct signaling pathways. Astrocytes appear to have numerous roles in maintaining normal brain function, including roles regulating synapse formation and even synaptic plasticity (Eroglu and Barres, 2010). Thus Shh expression could provide a mechanism for coordinating the formation of specific circuits by differentially regulating the activities of both neurons and astrocytes. Neurons could be regulated through the noncanonical Src family kinase-dependent Shh pathway, and astrocytes through the canonical Gli-dependent pathway. Shh is most well known for its role in the patterning of the nervous system, and mutations in the human Shh gene are known to cause holoprosencephaly.