Title: The Secreted End of a Transcription Factor Promotes Sensory Axon Growth
Description: During neural development, axons rely on extracellular cues to reach their target regions. Although extracellular signaling is one of the principal determinants for the growth of developing axons, only a small handful of known signaling cues has been identified. The existence of some 86 billion neurons of different subtypes, which ultimately form numerous functional circuits in the human nervous system, means an enormous number of extracellular cues would be required during development. Current views hold that even if more extracellular cues were to be discovered, they would never number large enough to account for the complexity of the human nervous system. Rather, intracellular signaling pathways and other cell-intrinsic mechanisms expand the ways in which a neuron can respond to extracellular cues by tuning the degree of responsiveness to them. Cell-intrinsic signaling pathways also give axons the ability to actively control their own development. These pathways can operate independently of the extracellular environment or even independently of the cell body, where the majority of protein synthesis takes place.

For example, the local translation of proteins in the axon gives it autonomous control to immediately respond to changing demands in the environment. Local translation also occurs in other cell types, but the compartmentalized control over growth is especially important for neurons since the axon can extend up to a meter away from the cell body. In addition to local translation, axonally derived transcription factors, which can be locally synthesized in or localized to the axon, provide another means to control axon development. Axonally derived transcription factors act as physiological sensors and relay information about events happening in the periphery back to the cell body in order to effectuate a global response. It has recently been shown that transcription factors belonging to the OASIS family are activated by proteolysis in axons. Following their activation by proteolytic cleavage, the transcriptionally active N-terminus of these factors is transported to the cell body to activate global transcriptional pathways.

For at least one OASIS family member, CREB3L2, this cleavage event simultaneously produces the C-terminus, which is capable of undergoing secretion. The secreted C-terminus of CREB3L2 acts as an accessory ligand for the activation of Hh pathways in chondrocytes. The generation of two bioactive proteins from one transcription factor, a transcriptionally active portion and a secreted portion, raised the question of whether there was a local function for OASIS transcription factors in axons. Through my research, I identified a mechanism in which DRG axons secrete the C-terminus of CREB3L2, which promotes axon growth in a paracrine manner. CREB3L2 is a transcription factor whose translation is induced by physiological ER stress. For CREB3L2 to be active, it must be cleaved by S2P, which I found is expressed in developing axons. Following proteolysis of CREB3L2 by S2P, the secreted C-terminus of CREB3L2 promotes the formation of Shh and Ptch1 complexes along axons. I found that upon depletion of the secreted CREB3L2 C-terminus, binding of Shh to the Ptch1 receptor is diminished.

Returning the CREB3L2 C-terminus to the cultures exogenously was sufficient to rescue the formation of these complexes. These results highlight an intrinsic role for Shh signaling in developing DRG axons. Moreover, these results demonstrate how ER stress machinery is recruited to axons and promotes axon outgrowth. Finally, these results illustrate a novel, neuron-intrinsic mechanism by which developing axons actively regulate their own growth.