Primordial germ cells (PGCs) are the embryonic precursors of
gametes. In most model systems, PGCs are migratory and navigate through
or around diverse tissues in order to find the site of the developing
gonads. PGC migration shares conserved features in many species
indicating the process arose in a common ancestor. In vertebrates,
however, the majority of factors implicated in PGC guidance are either
secreted or membrane bound protein growth factors (e.g. stromal derived
factor 1 and Kit ligand); whereas, evidence in Drosophila points to a
lipid-based guidance system .
A recent study bridged the gap by demonstrating that zebrafish PGCs,
like Drosophila PGCs, require 3-hydroxy-3-methylglutaryl-coenzyme A
reductase (HMGCR) for normal migration .
HMGCR is the rate limiting enzyme in isoprenoid and cholesterol
biosynthesis. In flies, HMGCR was shown to act within the somatic
gonadal precursors to control release of a secreted PGC attractant .
Drosophila lack enzymes required downstream of HMGCR for cholesterol
synthesis indicating that isoprenoids are the relevant downstream
effectors . In support of this, Santos and Lehmann demonstrated that mutations in the geranylgeranyl transferase 1 β
subunit cause PGC migration defects. It has been proposed that
geranylgeranylation of small GTPase in the Ras, Rac, or Rho families
regulate secretion of hedgehog  or other putative Drosophila PGC attractants . In zebrafish, evidence also points to a role for HMGCR and isoprenoids in PGC migration .
Inhibition of HMGCR or geranylgeranyl transferase I cause PGC migration
defects. However, in this system it remains unclear whether HMGCR is
required in PGCs themselves or within the soma. Additionally, zebrafish
unlike flies are capable of de novo cholesterol synthesis and this
branch of the pathway was not carefully evaluated.
Cholesterol plays a vital role during vertebrate development.
Mutations in genes required for cholesterol biosynthesis cause severe
developmental defects. Loss of Hmgcr  or squalene synthase  result in early embryonic lethality in mouse models. Mutations in 3b-hydroxysterol-Δ7 reductase (Dhcr7) or lathosterol 5-desaturase (Sc5d) cause skeletal, neural, and in some cases urogenital defects in humans  and in mice [9,10].
Additionally, mutations in genes required for cholesterol transport are
also associated with embryonic lethality or patterning defects .
Several models have been evoked in order to explain the role of
cholesterol during organogenesis. First, cholesterol is the precursor
of steroid hormones, glucocorticoids, and oxysterols, all compounds
known to mediate cell-cell signaling via activation of nuclear hormone
receptors. Second, cholesterol directly regulates cell-cell signalling
by controlling the diffusion  or reception 
of members of the hedgehog growth factor family. Finally, cholesterol
is a key structural component of the plasma membrane. Cholesterol
controls membrane fluidity and modulates membrane protein interactions.
Membrane cholesterol has been shown to influence the activity of growth
factor receptors and cell-adhesion molecules by clustering these cell
surface proteins into lipid rafts . Of particular note, lipid rafts have been shown to affect epidermal growth factor-induced chemotaxis  and migration on fibronectin  in cell culture. This suggests that cholesterol levels might also alter cell migration in vivo.
Considering the roles of cholesterol in cell-cell signalling and
cell migration, we thought it imperative to test whether this branch of
the HMGCR pathway is required for germ cell development in a vertebrate
model. In mice, PGCs migrate from the gut to the genital ridges between
embryonic day 9.5 (E9.5) and E10.5. Steroid hormones and hedgehog
growth factors are unlikely to play a role in this process. Enzymes
required to convert cholesterol into steroid hormones are not expressed
in the gonads until E11.5 .
Likewise, the three vertebrate members of the hedgehog growth factor
family are not expressed in the right place or time to play a role in
PGC guidance in this system [18,19].
However, changes in cholesterol could very well impact the ability of
PGCs to respond to proposed chemoattractants such as SDF1  or KITL .
To examine the role of cholesterol in PGC migration, we first
measured cholesterol in living tissue dissected from E9.5 embryos and
were surprised to find that cholesterol was enriched in the genital
ridges relative to the surrounding tissue. This asymmetric distribution
appears to be maintained via selective uptake of cholesterol by cells
within the genital ridges. We further demonstrate that inhibition of
HMGCR reduced total cholesterol and impaired germ cell survival and
migration in culture. These defects were rescued by co-addition of
geranylgeraniol and cholesterol indicating that both compounds are
required. To test if cholesterol biosynthesis is necessary for PGC
migration in vivo, we examine the number and distribution of PGCs in
embryos lacking Dhcr7 or Sc5d (see Figure 1). PGCs were normally distributed in both lines, but cholesterol levels are only modestly affected in these embryos [9,10].
We conclude that cholesterol is required for PGC migration but that
this requirement can be met by uptake of cholesterol from maternal
sources. We propose that the asymmetric accumulation of cholesterol
within the genital ridges controls signaling interactions required for
PGCs to colonize the gonads.