Cannabinoids

The Biosynthesis, Fate and Pharmacological Properties of Endocannabinoids 153

Stella et al. 1997). Twosn-1 DAG lipase isozymes (DAGLαand DAGLβ)havebeen

cloned and enzymatically characterized (Bisogno et al. 2003). They are located in
the plasma membrane, are stimulated by Ca2+, appear to possess a catalytic triad
typical of serine hydrolases, and, like NAPE-PLD, do not appear to be particularly
selective for 2-arachidonate-containing DAGs. Nevertheless, several lines of evi-
dence (Bisogno et al. 2003) suggest that they are responsible for the formation of
the endocannabinoid 2-AG in intact cells:

– Over-expression of DAGLαand DAGLβin COS cells results in significantly

higher levels of 2-AG produced following stimulation with ionomycin, but not

in higher 2-AG basal levels.

– The expression of DAGLαand DAGLβin several cell lines correlates with their

ability to produce 2-AG following stimulation with ionomycin.

– Inhibition of DAGLαand DAGLβactivity with tetrahydrolipstatin in COS cells

and cell lines stimulated with ionomycin results in the impaired production of

2-AG.

– The distribution of the mRNAs encoding for DAGLαcorrelates with the relative

abundance of 2-AG in rodent tissues and organs (Kondo et al. 1998).

• Finally, the two enzymes exhibit a pattern of subcellular expression in nervous
  tissuesthatfitswiththeproposedroleof2-AGeitherasamediatorofneurite
  growth, during brain development (Williams et al. 2003) or as a retrograde sig-
  nal mediating depolarization-induced suppression of neurotransmission and
  heterosynaptic plasticity in the adult brain (Chevaleyre and Castillo 2003; Wil-
  son and Nicoll 2002, for review). In fact, the enzymes are located on axons during
  development and post-synaptically in adult neurons (Bisogno et al. 2003).
  However, DAG-independent biosynthetic pathways for 2-AG have also been
  proposed(Sugiuraetal.2002,forreview),althoughtheirrelevancetotheregulation
  of the endocannabinoid signal has not yet been investigated. Noteworthy is the
  enzymatichydrolysisofaparticulartypeoflysophosphatidicacid,2-arachidonoyl-
  sn-glycero-3-phosphate (Nakane et al. 2002).

2.3

Biosynthesis of Other Putative Endocannabinoids

Very little is known about the biosynthesis of the three most recently proposed
endocannabinoids, 2-AGE, virodhamine and NADA. Regarding 2-AGE (noladin
ether), this compound was previously identified in pig brain (Hanus et al. 2001) and
in some rat tissues and brain areas (Fezza et al. 2002) by using mass-spectrometric
(MS) methods coupled to chromatographic separations. However, a recent study
cast some doubt on the actual existence of 2-AGE in mammalian brain tissue (Oka
et al. 2003). At the time of this study it was already known that (1) the only acyl
ethers to have been detected in animals before the discovery of 2-AGE were 2-acyl
ethers (e.g. alkenyl ethers such as platelet activating factor and plasmalogens); (2)
there was no evidence for the existence of any enzyme catalysing the formation