Pum2 Shapes the Transcriptome in Developing Axons through Retention of Target mRNAs in the Cell Body

José C. Martínez, MD, PhD	Ulrich Hengst, PhD

Neurons are among the most morphologically complex cells in the body, containing extensive arborizations for receiving inputs from other neurons, and sending signals through long axons projecting up to one meter. A key mechanism underlying the development and maintenance of this complex structure is the localized production of a particular subset of proteins in distal projections. In order to accomplish this, neurons send mRNA transcripts serving as templates for these proteins into projections. These mRNAs can then be translated into protein on demand. The current model for how neurons accomplish this involves the active transport of mRNAs by RNA-binding proteins into projections, primarily through the recognition of specific sequences in mRNAs. While several such sequences have been identified in axonally localized mRNAs, the elements responsible for the localization of most axonal mRNAs remain unknown.

In a recent article published in Neuron, Dr. Ulrich Hengst and colleagues set out to discover novel mechanisms of axonal mRNA localization that might be shared among many mRNAs. The project, led by first author and former graduate student in the Hengst lab, Dr. José Martínez, revealed that a short sequence of eight nucleotides known as the Pumilio binding element (PBE) is enriched in mRNAs that are localized to the soma of developing neurons, but depleted in mRNAs that are localized to axons. The PBE is recognized and bound by the RNA-binding proteins Pumilio (Pum) 1 and 2. Pum2 was found to be specifically localized to the soma, where it acts to prevent the axonal localization of its targets. Reduction of Pum2 or blockage of the PBE resulted in an increase in axonal localization of target mRNAs, while adding the PBE to axonal mRNAs was sufficient to decrease their axonal localization. Furthermore, Pum2 reduction resulted in a decrease in axonal growth and branching in the cortex, as well as impaired regeneration after injury in sensory neurons.

Together, Martínez et al. report a novel mechanism controlling the localization of mRNAs in developing neurons. In addition to previously reported active transport mechanisms, they find that mRNAs can also be localized by exclusion. Additionally, this regulatory relationship may play a role in axonal growth and branching, suggesting that it could be an important component of neural development.

Ulrich Hengst, PhD
Associate Professor of Pathology and Cell Biology (in the Taub Institute for Research on Alzheimer's Disease and the Aging Brain)
uh2112@cumc.columbia.edu

Promotion of Axon Growth by the Secreted End of a Transcription Factor

Ethan McCurdy, PhD	Ulrich Hengst, PhD

Developing neurons face the challenge of spanning immense distances to form synapses with their targets. During neural development, axons respond to a scant number of extracellular signaling molecules that help control aspects of axon growth. However, since the number of these molecules is limited, other types of control exerted at a local level are thought to regulate the extending axon. However, the extent to which axons can actively control their own development is unknown.

Recent work published in Cell Reports by Taub faculty member Ulrich Hengst and lead author Ethan McCurdy has identified a cell-intrinsic mechanism by which axons control their own growth through the generation of the secreted portion of a transcription factor. The authors found that the intramembranous protease S2P cleaves the ER stress-related transcription factor CREB3L2 in developing axons, leading to the secretion of its luminal domain. Upon secretion, this portion of CREB3L2 binds the chemoattractive morphogen Shh and promotes its association with the receptor Ptch1, promoting axon growth.

The significance of this project is the identification of a neuron-intrinsic mechanism for regulating axon growth, placing the axon in a position where it actively controls its own development. Several reports have come out over the last few years that highlight the unconventional activation of stress pathways during normal neuronal physiology. In McCurdy et al. 2019, the authors also note that since S2P and CREB3L2 are activated during the ER stress response, this observation suggests the importance of baseline levels of cellular stress for the developing nervous system.

Ulrich Hengst, PhD
Associate Professor of Pathology and Cell Biology (in the Taub Institute for Research on Alzheimer's Disease and the Aging Brain)
uh2112@columbia.edu