Somatic muscle specification during embryonic and post-embryonic development in the nematode C. elegans

Abstract

Myogenesis has proved to be a powerful paradigm for understanding cell fate specification and differentiation in many model organisms. Studies of somatic bodywall muscle (BWM) development in Caenorhabditis elegans allow us to define, with single cell resolution, the distinct hierarchies of transcriptional regulators needed for myogenesis throughout development. Although all 95 BWM cells appear uniform after differentiation, there are several different regulatory cascades employed embryonically and post-embryonically. These, in turn, are integrated into multiple extrinsic cell signaling events. The convergence of these different pathways on the key nodal point, that is the activation of the core muscle module, commits individual cells to myogenesis. Comparisons of myogenesis between C. elegans and other model systems provide insights into the evolution of contractile cell types, demonstrating the conservation of regulatory schemes for muscles throughout the animal kingdom.

FIGURE 1

The major muscle groups of Caenorhabditis elegans hermaphrodites. The major muscle cell types in the hermaphrodite are shown schematically in this lateral view of the animal with anterior to the left and dorsal to the top. At top, are the 95 bodywall muscles (BWMs), the somatic muscle providing locomotive force for the animal. Dark red cells reflect the 81 embryonically derived cells with lighter red shading used to indicate the approximate location of the 14 post-embryonically derived cells. The BWM cells run in four quadrants along the length of the animal; only one side is schematized. At the bottom are several additional muscle cell groups. The pharynx (orange) pumps food into the gut while the enteric muscles (blue) control defecation. The egg-laying muscles are comprised of those controlling the opening of the vulva (vms, in green) and extrusion of zygotes by contraction of uterine muscles (purple). The only muscles of the hermaphrodite not depicted are the gonadal sheath cells.

FIGURE 2

Origin and position of the embryonic somatic muscles of Caenorhabditis elegans. (a) Lineage diagram of the early cell divisions leading to the establishment of the five somatic founder blastomeres (AB, MS, E, C, and D) and single germline precursor (P4). (Reprinted with permission from Ref . Copyright 1983 Society of Developmental Biology) Bodywall muscle (BWM) cells are derived from all somatic founder blastomeres (colored font) except E that gives rise clonally to the gut. The number of BWM cells from each founder lineage is indicated parenthetically. (b) BWM cells from different lineages (colored as in (a)) are schematically superimposed on a Nomarski image of an embryo at the 1.5-fold stage of development. In this lateral view, only the left two BWM quadrants are represented with anterior to the left and dorsal to the top.

FIGURE 3

A comparison of bodywall muscle (BWM) cells in a wild-type and muscle module mutant embryo. (a) Schematic diagram of the left lateral view of an embryo at the 1.5-fold stage of development with the BWM cells (blue lines) and nuclei (filled circles) indicated. (Reprinted with permission from Ref . Copyright 1983 Society of Developmental Biology) (b) Wild-type embryonic BWM differentiation at the 1.5-fold stage of development. Antibodies recognizing myosin heavy chain protein highlight the cytoplasm of BWM beginning to undergo terminal differentiation with nuclei positions visible as unmarked areas near the center of each cell. (c) Absence of detectable BWM in embryos lacking the transcriptional muscle module factors. Myosin heavy chain staining of an embryo at the 1.5-fold stage of development in which the transcription factors HLH-1/myogenic regulator factor (MRF) and UNC-120/serum response factor (SRF) have been eliminated by mutation and the factor HND-1/HAND has been knocked down by hnd-1 RNAi. Loss of all three transcription factors completely abolishes BWM specification and differentiation.

FIGURE 4

Transcriptional hierarchy for embryonic bodywall muscle (BWM) specification. The known zygotic transcriptional cascade is shown for all embryonically derived BWM with the exception of the single BWM cell from the AB lineage. There are at least three distinct pathways resulting in the activation of the core muscle module factors HLH-1/myogenic regulator factor (MRF) and UNC-120/serum response factor (SRF) (red box), which once activated, ensures BWM specification and differentiation. Note the variations within the MS lineage that employs two different SIX-type factors in different sub-lineages. In addition, HND-1/HAND is present in one sub-lineage, but absent from the other.

FIGURE 5

The Caenorhabditis elegans hermaphrodite post-embryonic M lineage. (a) The M lineage showing all differentiated cell types that arise from M. (Reprinted with permission from Ref . Copyright 1977 Society of Developmental Biology) (b) A schematic lateral view of the M lineage through larval development. Times are indicated post-hatching at 25°C. A, anterior; D, dorsal; L, left; P, posterior; R, right; V, ventral.

FIGURE 6

Gene regulatory networks involved in bodywall muscle (BWM) specification in the post-embryonic M lineage. Thick blue lines represent likely direct regulatory relationships, whereas black lines represent relationships based solely on genetic data, and do not distinguish direct versus indirect. Gray lines and text indicate lack of expression in indicated cell. (a) The dorsal M lineage at the 16-M stage showing a left/right pair of cells that will adopt either a coelomocyte (CC) or BWM fate. LET-381/FoxF/C directly regulates the expression of the CC specification factor CEH-34/SIX2, and both genes function in a feed-forward manner to regulate a number of CC differentiation genes and suppress the BWM fate. LET-381/FoxF also inhibits the expression of the sex myoblast (SM) specification factor SEM-2/SoxC to prevent SM production on the dorsal side. Expression of let-381 is regulated by POP-1/SYS-1 asymmetry, dorsoventral (D–V) signaling mechanisms (LIN-12/Notch and SMA-9-SHN) and mesoderm intrinsic factors [HLH-1/myogenic regulator factor (MRF), FOZI-1, and MAB-5/Antp], which may also regulate ceh-34 expression independently of LET-381/FoxF. (b) The ventral M lineage at the 16- and 18-M stages showing a left/right pair of cells that will adopt either a SM or BWM fate. SEM-2/SoxC is likely directly activated in the M lineage by the HOX/PBC complex, and becomes restricted to the SM mother cells due to a combination of POP-1/SYS-1 asymmetry and the lack of LET-381/FoxF on the ventral M lineage. SEM-2/SoxC is then retained in the SM cells and the SM lineage, while the SM sister cells express HLH-1/MRF and FOZI-1 and become BWMs. The initiation of the asymmetric expression of SEM-2/SoxC versus HLH-1/MRF and FOZI-1 may be due to POP-/SYS-1 asymmetry. Once initiated, SEM-2/SoxC and HLH-1/MRF and FOZI-1 appear to mutually repress each other to maintain their proper expression.