Assembly of sensory-motor connectivity in the spinal cord of mice

Many features critical for proper functioning of an organism are under the control of neuronal circuits. The building blocks for these circuits are formed early during embryogenesis and are generated by the specification of distinct neuronal types. This process of neuronal specification is spatio-temporally regulated by extrinsic and intrinsic factors, defining the progenitor and subsequently postmitotic identity of neuronal subclasses. Postmitotic neurons migrate to specific locations in the central nervous system (CNS), send out axons to and innervate their targets, develop highly specific dendritic trees and form synaptic connections with a variety of inputs. All these processes are likely to be regulated by intriguing interactions of cell autonomous intrinsic and extrinsic factors. During the process of axonal pathfinding and dendrite patterning, activity-independent mechanisms, such as a variety of axon guidance molecules as well as neurotrophic factors are involved in the assembly of neuronal circuits. Although dendritic structures are of crucial importance for the function of neuronal circuits, not much is known about the molecular and cellular mechanisms underlying the formation of neuronal type-specific dendritic morphologies and this topic represented the main focus of my PhD thesis. Specifically, my thesis was centered on the monosynaptic stretch reflex circuit in the spinal cord, which offers a fairly accessible system for studying various aspects of neuronal circuit formation. The main part of my thesis focused on 1) analyzing the correlation between motor neuron (MN) pool specific dendrite patterns and Ia proprioceptive afferent (IaPA)-MN connectivity; 2) determining cellular and molecular factors required for the formation of highly selective IaPA-MN connections. Our study (Vrieseling and Arber, Cell, in press) has demonstrated that there is a strong correlation between MN pool dendrite pattern and monosynaptic IaPA-MN connectivity. MN pools at cervical levels of the spinal cord projecting to Triceps (Tri) and Pectoralis major (Pecmaj) forelimb muscles occupy a dorso-medial cell body position in the ventral horn of the spinal cord and show radial dendrite patterns, extensively projecting into the central grey matter of the spinal cord. In contrast, two other MN pools found at the same rostro-caudal level of the cervical spinal
cord, projecting to the Cutaneous maximus (CM) and Latissimus dorsi (LD) forelimb
muscles, occupy an extreme ventro-lateral position in the ventral spinal cord and project
their dendrites along the grey-white matter border, almost completely avoiding the central
grey matter. Using intracellular recording techniques, I showed that the MN pool
selective dendrite patterns correlated strongly with monosynaptic IaPA-MN connectivity.
Tri and Pectmaj MNs both received monosynaptic inputs from homonymous IaPAs.
However, almost none of the CM and LD MNs received monosynaptic IaPA input. To
elucidate the cellular and molecular factors required for the formation of MN pool
specific dendrite patterns and selective IaPA connectivity, we analyzed a mouse line
mutant in the target-induced ETS transcription factor Pea3. In a previous study it was
shown that Pea3 is specifically expressed in CM and LD MN pools at cervical levels and
that Pea3 regulates MN cell body positioning (Livet et al., 2002). During my thesis, I
showed that in the absence of the ETS transcription factor Pea3, CM and LD MN pools
dramatically change their dendrite morphologies to a radial pattern resembling Tri and
Pectmaj dendrite patterns. Moreover, Pea3 mutant CM and LD MNs received strong
monosynaptic IaPA input from the Tri muscle nerve. Tri MNs do not express Pea3,
hence Pea3 mutation does not genetically change these MNs. Nevertheless, in Pea3
mutants, Tri MNs occupy a different MN pool position due to the altered pool position of
CM MNs. However, the change in Tri MN pool position did not dramatically change its
dendrite pattern nor its monosynaptic IaPA connectivity. From these findings, we
concluded that the target-induced transcription factor Pea3 cell autonomously regulates
MN pool position, dendrite pattern and Ia-MN connectivity.
The minor part of my thesis focused on the role of ETS signaling during late
embryonic dorsal root ganglia (DRG) sensory neuron development (Hippenmeyer et al.,
2005). We found that precocious ETS signaling in DRG sensory neurons dramatically
changed the fate of these neurons. Precocious ETS signaling prevented normal
development of DRG sensory neurons and instead led to abnormal axonal pathfinding,
perturbation of expression of terminal differentiation markers and independence of
neurotrophic support. These findings emphasize the importance for temporal regulation
of factors during development for proper specification of neuronal identity.