Page 91 - Annual report 2021-22

Annual Report 2021-22 |

Vivek T. Natarajan

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Vivek T. Natarajan’s lab works in the area of skin biology, focusing on cellular and molecular
mechanisms of pigmentation.

Cell Migration and pattern formation in zebrafish

Melanocytes arise from the neural crest and migrate across the body to pattern the embryonic
pigment pattern. The path taken by melanocytes is distinct and well known. Knowledge of various
positive cues and negative contact dependent signals explain the migratory path to a large extent.
However, migration as distinct organized streams from specific dorsal locations remained perplexing.
Observations from T. N. Vivek’s laboratory on zebrafish melanocyte migration indicate the essential
role of PlxnD1 and Sema 3E signalling in establishing the identity of melanocyte progenitor pool. This
population establishes the adult melanocyte pool and replenishes melanocytes upon their loss. This
signalling pathway was known to be involved in the neuronal pathfinding and in endothelial branching.
Here, for the first time, its role as a guidance cue for pigmenting cells was discovered. The
establishment of the melanocyte progenitor pool is abrogated upon depletion of either PlxnD1 or its
cognate ligand Sema 3e. While there are other ligands that activate PlxnD1 in this system Sema3E
appears to be the cognate ligand partner for melanocyte migration.

Based on understanding of the role for PLXND1-SEMA3E deciphered here, and earlier studies on the
expression pattern of Sema3e and kitlg (SCF), T. N. Vivek proposed that melanocyte migration path is
governed by the zone of expression of these two guidance factors. In zebrafish, the earliest expression
of SCF is observed at tail bud around 19hpf, coinciding with induction of mitf expression and initiation
of melanoblast migrations. Between 22-30 hpf Kitla expression is visible in groups of cells at the
horizontal myoseptum in the middle of each somite and in dorsal posterior somites. At later stages
around 4 dpf, after embryonic pigment pattern is established Kitla mRNA is found throughout the skin
and in the dorsal myotome. On the other hand, restrictive gradients of Sema3e originate from within
single motor neurons decorated one per somite between 18 somite stage to 27hpf. At 27 hpf sema3e
is expressed in endodermal tissue ventral to pronephric ducts but progressively decreases until it is
observable only in neural tissue of optic chiasma and otic vessels. It is now evident that Sema3e is
critical for restricting melanocytes into orchestrated streams targeting melanocytes to dermal regions
towards SCF expression zones.

While the role of PLXND1 and SEMA3E signalling seems to be controlling early migration and stem cell
establishment, the CRISPR mutant does demonstrate an adult pattern defect which is distinct from
other well-studied mutants like Picasso and Sparse, clearly indicating a role of this pathway in pigment
pattern maintenance. Once the initial developmental pattern is established, the cellular contacts
between melanophores and other chromatophores remain steadily intact. However, during
regeneration such contacts are interrupted, and cells must migrate out of the stem cell niche to re-
establish the pattern. Sema3E-Plexin D1 axis provides a morphogen-based signalling that can generate
a stable soluble gradient to pattern melanophores. Thereby, this study provides mechanistic
understanding of a candidate pathway that patterns melanophores. The understanding of melanocyte
responses to SEMA3E-PLXND1 guidance signalling would pave the path to predictably influence