The survival and perpetuation of a species depends on its capacity to cope with stress factors from its environment. One conserved manner by which all living organisms defend themselves at the cellular level when confronted with diverse types of stress is the induction of a defined class of polypeptides termed heat shock proteins (Hsp) [1
]. The small heat shock proteins (sHsp) represent the least conserved subfamily of Hsp as their number and size (ranging from 12 to 40 kDa) vary from species to species. Studies in different experimental systems have revealed a variety of functions for the sHsp under stress conditions. These different roles, including basic chaperoning activity [2
], cytoskeleton protection [4
] and modulation of the apoptotic process [5
] directly represent means of cellular defense against environmental aggression. Contrasting with the classical definition of heat shock proteins as polypeptides induced by stress, cell-specific expression of sHsp in the absence of stress has been reported during the development of a wide range of organisms such as Caenorhabditis elegans
], Drosophila melanogaster
], Xenopus laevis
], Mus musculus
] and man [14
]. Even if functional roles have been demonstrated for certain high molecular weight Hsps in non-stress related processes such as RTK signaling [15
] and spermatogenesis [16
], only preliminary experimental evidence so far support such requirement for sHsp under non-stress conditions [19
]. Their peculiar cell-specific pattern of expression has lead to the hypothesis that sHsp may be implicated in differentiation mechanisms. While recent studies in cultured cells have provided support to this possibility [20
], no such evidence has yet been provided for a multicellular organism.
In Drosophila, sHsps are expressed throughout many stages of the life cycle (reviewed in [21
]). During oogenesis, Hsp27 displays a stage-specific intracellular localization within nurse and follicle cells [23
] while Hsp23, Hsp26 and Hsp27 are respectively expressed in distinct cell types during the spermatogenic process [9
]. During embryogenesis, Hsp27 associates to cells of the brain and of the ventral nerve cord while Hsp26 is found exclusively in the gonads [25
]. Hsp23 also displays a cell-specific pattern of expression during embryonic neurogenesis [26
] and has recently been shown to be strongly downregulated following the targeted expression of the glial "master" gene gcm
]. Despite this increasing knowledge on the developmental expression of sHsps, the precise identity of cells expressing these proteins along with the in vivo
function(s) played by sHsp in these developmental instances remain to be unveiled. The expression of Hsp23 within a highly characterized morphogenetic system (the embryonic nervous system) combined to the isolation of a P-element insertion in the promoter region of its gene, provided the opportunity to precisely define its expression pattern and evaluate its functional implication in a specific developmental process.
This study reports the expression of Hsp23 in neuronal (MP2, VUMs) and glial (midline glia) lineage of the CNS, as well as in a single chordotonal organ per hemisegment and in cells of the amnioserosa. We demonstrate that Hsp23 expression in the neuroectoderm is closely and autonomously linked to the acquisition of MP2 fate as it does not requires the presence of a functional midline and is expanded in a neurogenic mutant where additional MP2s are specified. In vitro transactivation assays support that the Single-minded, Tango and Drifter transcription factors, which are all involved in midline determination and differentiation, may also regulate hsp23 promoter activity. Finally, we evaluate a putative functional contribution of Hsp23 to embryonic neurogenesis through phenotypic analysis of a P-element insertion line resulting in an inhibition of Hsp23 expression in the CNS. The absence of detectable phenotype in the ventral nerve cord of homozygous embryos suggests that the loss of Hsp23 is not detrimental to CNS formation. Furthermore, the failure to observe any differentiation or functional defects following targeted misexpression of Hsp23 indicates that its biological activity is related to non-vital features which are distinct from the normal developmental program.