Migration of dI5 Reelin-Lmx1b-Zfhx3 and Disabled-1-Lmx1b-Zfhx3 neurons contribute to the superficial dorsal horn and lamina V

This study demonstrates that the superficial dorsal horn and lamina V are populated by two distinct neuronal subsets derived from embryonic dI5 progenitors (large Reln+/Dab1+/Lmx1b+/Zfhx3+ projection neurons) and late-born dILB progenitors (smaller interneurons), with their specific migration patterns and final positioning critically dependent on Reelin signaling.

The Gist

Imagine the developing spinal cord as a bustling, high-rise construction site. Its job is to build a complex communication network that allows your body to feel pain, touch, and temperature. To do this, it needs to hire specific workers (neurons) and assign them to specific floors (layers of the spinal cord) where they will live and work for the rest of their lives.

This paper is like a detective story about two specific types of construction workers: Reelin (Reln) and Disabled-1 (Dab1).

The Cast of Characters

1. The Workers (Reln and Dab1 neurons): These are the cells the scientists are tracking. In a healthy adult, most of these workers end up on the "top floors" (the superficial dorsal horn) of the spinal cord, where they help process pain signals.
2. The Foreman (Lmx1b): This is a master blueprint that tells these workers, "You are part of the glutamatergic team." The study found that almost all the Reln and Dab1 workers carry this specific ID badge.
3. The GPS System (Reelin Signaling): Normally, the Reelin protein acts like a GPS or a traffic cop. It tells the workers, "Stop here! This is your floor. Don't go further."
4. The "Group Z" Badge (Zfhx3): This is a special stamp that identifies the "long-distance travelers"—the workers who are meant to send messages far away to the brain (projection neurons), rather than just chatting with neighbors on the same floor (interneurons).

The Journey: A Road Trip with Detours

The researchers watched these workers from the very beginning of the construction project (embryonic days 11.5 to 15.5) to see how they found their way.

The Normal Route (The "Happy Path"):
In a healthy embryo, the workers start at the bottom of the construction site.

• The Early Birds (dI5): These are the big, early-born workers. They have the "Group Z" badge (Zfhx3), meaning they are long-distance travelers.
  • The Coastal Drive: Some of these workers take a scenic route along the very outer edge (the rim) of the spinal cord, circling around like a car driving along a coastal highway. They eventually park in the "Lateral Spinal Nucleus" (a specific parking lot on the side).
  • The Midline Detour: Other early workers gather in the middle (the midline), then turn and drive sideways into "Lamina V" (a specific deep floor).
• The Late Arrivals (dILB): These are smaller, late-born workers. They don't take the long scenic drive. Instead, they fill up the "top floors" (Lamina II) and act as local neighbors, helping to process immediate sensory info.

The Broken GPS (The Mutant Problem):
The scientists then looked at what happens when the "GPS" is broken (in mice that lack the Reelin or Dab1 proteins).

• The Lost Drivers: Without the GPS to say "Stop here," the workers get confused.
• The Coastal Drift: The workers who were supposed to park on the side keep driving too far inward, ending up in the wrong neighborhoods.
• The Midline Traffic Jam: The workers who were supposed to gather in the middle and then move sideways get stuck in a chaotic pile in the center. They can't organize into a neat line to move into their assigned floor (Lamina V).
• The Deep Intruders: Some workers who should have stayed on the top floors end up wandering deep into the lower levels of the building, where they don't belong.

The Big Discovery

The paper solves a mystery: Are these confused workers the "locals" or the "travelers"?

By checking for the "Group Z" badge (Zfhx3), the scientists found that:

1. The large, confused workers found in the wrong places are actually the long-distance travelers (dI5 projection neurons). They are the ones who got lost because the GPS failed.
2. The small workers that fill the top floors are the locals (dILB interneurons). They seem to find their way okay, even if the GPS is broken.

Why Does This Matter?

Think of the spinal cord as a city's emergency response system. If the "pain processing" workers are parked in the wrong buildings or on the wrong floors, the city's alarm system gets glitched.

• In the mutants: Because the workers are mispositioned, the mice feel too much heat (heat hypersensitivity) but can't feel pressure (mechanical insensitivity).
• The Takeaway: The study shows that the Reelin/Dab1 signaling pathway is the essential GPS that guides these specific "long-distance" neurons to their correct parking spots. When this GPS breaks, the workers get lost, and the body's pain system goes haywire.

In short: This paper maps out the road trip of spinal cord neurons, showing that without a functioning GPS, the "long-distance travelers" get lost, leading to a broken pain system.

Technical Summary

1. Problem Statement

The Reelin signaling pathway (involving Reelin, its receptors Apoer2/Vldlr, and the intracellular adaptor Disabled-1/Dab1) is well-established for its role in cortical neuronal layering. However, its specific function in the development of the dorsal spinal cord remains unclear.

• Context: In adult mice, Reelin (Reln+) and Dab1+ neurons are concentrated in pain-processing regions (superficial dorsal horn laminae I-II, lateral lamina V, and the lateral spinal nucleus/LSN).
• The Gap: While it is known that ~90% of adult Reln+ and 70% of Dab1+ neurons in the superficial dorsal horn co-express the transcription factor Lmx1b (marking glutamatergic neurons), their embryonic origins are unknown. Specifically, it is unclear if these neurons derive from the early-born dI5 population or the late-born dILB population.
• The Question: Do Reln+Lmx1b+ and Dab1+Lmx1b+ neurons represent distinct subsets of dI5 projection neurons or dILB interneurons, and how does the absence of Reelin (in Reln-/- mutants) or Dab1 (in dab1-/- mutants) alter their migration and final positioning?

2. Methodology

The study employed a combination of embryonic mouse models, immunohistochemistry, and quantitative spatial analysis.

• Animal Models:
  • Wild-type (WT): Reln+/+ and dab1+/+ mice.
  • Mutants: Reln-/- (reeler) and dab1-/- (using dab1lacZ reporter mice where Dab1 protein is replaced by $\beta$-gal).
  • Stages: Embryonic days (E) 11.5 through E15.5, covering the critical window of dorsal interneuron generation and migration.
• Immunohistochemistry (IHC):
  • Markers: Antibodies against Reln, Dab1, Lmx1b (glutamatergic marker), and Zfhx3 (a specific marker for dI5 projection neurons).
  • Technique: Triple-labeling immunofluorescence combined with Tyramide Signal Amplification (TSA) to detect co-expression of three markers simultaneously.
  • Imaging: Confocal microscopy (Zeiss LSM800) with single optical slices and z-series to ensure precise cellular localization.
• Quantitative Analysis:
  • Dorsal horns were divided into a 12-grid system (mediolateral and dorsoventral axes).
  • The percentage of specific neuronal populations (e.g., Dab1+Lmx1b+) in each grid was quantified across genotypes and ages.
  • Statistical analysis used Two-way ANOVA followed by Holm-Sidak post hoc tests.

3. Key Contributions

• Lineage Tracing: The study definitively links embryonic Reln+ and Dab1+ dorsal horn neurons to the Lmx1b+ dI5 and dILB populations.
• Subtype Differentiation: It distinguishes between two distinct migratory pathways within the Lmx1b+ population:
  1. Large, early-born dI5 projection neurons (expressing Zfhx3).
  2. Small, late-born dILB interneurons (lacking Zfhx3).
• Mechanism of Migration: It elucidates the specific "circumferential" migration path of dI5 neurons along the dorsal horn rim and a separate midline-to-lamina V path, both dependent on Reelin signaling.
• Pathology of Mutants: It provides a cellular mechanism for the nociceptive (pain) abnormalities observed in adult Reln-/- and dab1-/- mice by mapping specific embryonic migration errors.

4. Results

A. Embryonic Origins and Migration Patterns (Wild-Type)

• E11.5–E12.5: Reln+ and Dab1+ neurons appear early, co-localizing with Lmx1b+ dI5 neurons in the lateral dorsal horn. They are distinct from the later-emerging dILB population.
• E13.5–E15.5 (Circumferential Migration): A subset of large, early-born Reln+Lmx1b+Zfhx3+ and Dab1+Lmx1b+Zfhx3+ neurons migrates circumferentially along the outer rim of the developing dorsal horn. These cells settle in the Lateral Spinal Nucleus (LSN) and the lateral reticulated area of lamina V.
• Midline Migration: Another subset of large dI5 neurons migrates medially to the dorsal midline (dorsal to the ventricular zone) before turning laterally to populate Lamina V.
• Interneuron Population: Small, late-born Reln+Lmx1b+ and Dab1+Lmx1b+ neurons (which do not express Zfhx3) fill the core of the superficial dorsal horn (Laminae I-II), identifying them as dILB interneurons.

*B. Migratory Errors in Mutants (Reln-/-)*

In the absence of Reelin, significant positioning errors occur, particularly in the large dI5 projection neurons:

1. Superficial Dorsal Horn: Dab1+Lmx1b+ neurons fail to stop at the lateral rim. Instead, they migrate further medially, resulting in a thickened Lamina I and mispositioned cells at the Lamina I/II border.
2. Deep Dorsal Horn: In WT mice, the deep dorsal horn is largely devoid of Dab1+ neurons. In Reln-/- mice, Dab1+Lmx1b+ neurons migrate tangentially across the deep dorsal horn, failing to reach Lamina V properly.
3. Midline Defects: The midline cluster of Dab1+Lmx1b+ neurons in Reln-/- mice is disorganized and fails to migrate laterally into Lamina V efficiently.
4. Quantification: Statistical analysis confirmed a significant increase in Dab1+Lmx1b+ neurons in medial grids (grid 5 and 8) of Reln-/- mice compared to WT at E14.5 and E15.5.

C. Co-expression Analysis

• Distinct Populations: Reln+Lmx1b+ and Dab1+Lmx1b+ neurons are largely separate subsets, though they overlap in the LSN.
• Zfhx3 Confirmation: The large, migrating neurons along the rim and midline express Zfhx3, confirming their identity as dI5 projection neurons. The small neurons in Lamina II lack Zfhx3, confirming they are dILB interneurons.

5. Significance

• Developmental Mechanism: The study establishes that Reelin signaling is critical for arresting the migration of specific dI5 projection neurons at their target destinations (Lamina V and LSN). Without Reelin, these neurons overshoot or migrate tangentially into inappropriate layers.
• Pain Circuitry: The mispositioning of these glutamatergic neurons (specifically those expressing Dab1 and Lmx1b) in the superficial dorsal horn and Lamina V provides a structural basis for the heat hypersensitivity and mechanical insensitivity observed in adult Reln-/- and dab1-/- mice.
• Clinical Relevance: Understanding how Reelin signaling guides the formation of spinal pain circuits offers potential insights into congenital pain disorders and the developmental origins of chronic pain states.
• Paradigm Shift: It challenges the view that Reelin is solely a cortical organizer, demonstrating its essential role in the precise layering of the spinal cord's sensory processing centers.

In summary, this paper maps the embryonic journey of Reelin/Dab1-expressing neurons, identifying them as a mix of dI5 projection neurons and dILB interneurons, and demonstrates that Reelin signaling is the "stop signal" required for these neurons to correctly populate the spinal cord's pain-processing layers.