Assessing the mechanisms directing cell fate in the dorsal spinal cord

Project Summary
The somatosensory system permits us to perceive and react to the environment through modalities that
include touch, nociception, thermosensation and proprioception. Somatosensory information is received
peripherally and then relayed centrally by different populations of dorsal interneurons (dIs; dI1-dI6) in the spinal
cord. Our research objectives are to understand the mechanisms that establish dIs during development and
then apply these principles towards designing differentiation protocols to direct the formation of specific
populations of dIs from pluripotent stem cells. These cells have the potential to repair damaged sensory
circuits and act as substrates for drug screening platforms. We have focused on dissecting the role of the bone
morphogenetic protein (BMP) family in dI fate specification. BMPs were widely assumed to act as morphogens,
patterning the dorsal spinal cord in a concentration-dependent mechanism similar to the manner in which sonic
hedgehog (Shh) patterns the ventral spinal cord. However, our recent studies using mouse, chicken, and
mouse embryonic stem cell (mESC) models have found that no evidence that BMPs act as morphogens.
Rather, BMPs have signal-specific activities, with differential abilities to direct dorsal progenitor (dP) patterning
and/or differentiation through specific type I BMP receptors. Our recent in vivo and in vitro studies have also
suggested that dI fates are established in a series of nested choice points. In our model, spinal progenitors are
dorsalized by retinoic acid (RA), subdivided into multipotential dP subgroups by BMP signaling, and then
resolve into specific dI fates. Since little is known about this patterning process, we will assess in Aim 1 how
multipotential dP fates are first established by RA and BMP signaling and identify the mechanisms directing
multipotential dPs into specific dI identities. In Aim 2, we will determine the nature of the intracellular response
that permits specific BMPs to drive dPs towards different dI identities, addressing two unresolved questions: [1]
are canonical receptor regulated (R)-Smads activated in a BMP-specific manner to result in distinct patterning
activities? And [2] what factors do the R-Smads in turn regulate to promote dI fates? Together, these studies
will investigate a long-standing problem in developmental biology, i.e., understanding how specific outcomes
arise from a common signal, and shed light on the specification of dIs, cell types needed to permit paralyzed
patients to again interpret their sensory environment. Our specific aims are as follows:
Aim 1: Identify the mechanism(s) that establish multipotential dPs and assign them into individual dI fates.
Hypothesis: RA±BMP4 direct the formation of multipotential dP subgroups, which resolve into specific dI fates
by the asymmetric activation of additional endogenous signaling pathways, such as the Wnt pathway.
Aim 2: Assess the role of the Smad and Id families in BMP-induced dI fate specification.
Hypothesis: BMPs differentially activate Smad1 and/or Smad5, to regulate the activity of Id family members.

Grant Number: 5R01NS123187-04
NIH Institute/Center: NIH
Principal Investigator: SAMANTHA BUTLER