Metalloproteases are a group of metal-containing proteolytic activities involved in various physiological functions, the alterations of which often cause major pathologies. This is exemplified by matrix metalloproteinases, the physiological roles of which include the degradation of the extracellular matrix. This is often beneficial to organisms, and matrixins are indeed involved in wound healing and bone resorption. However, matrixin dysfunction can also be extremely deleterious and likely contribute to cancer progression by favoring metastasis and promoting tumor angiogenesis (John and Tuszynski 2001).
Unlike matrixins, which are able to cleave large substrates such as collagen, a ubiquitous constituent of all membranes, another group of enzymes display more restricted substrate specificity. They have been gathered under the generic term of neuropeptidases, because of their avidity for a subset of small peptide substrates. Some of these neuropeptidases also belong to the metalloenzyme family and have been the center of numerous studies aimed at delineating their substrates' spectrum and associated functions. Although the identity of substrates reflecting their in vitro specificity is generally easy to establish once enzymes have been purified, the delineation of their genuine physiological function and the mechanisms underlying their dysfunction in pathological contexts are much more difficult to determine. Those with a well-established function include neprilysin, angiotensin-converting enzyme and aminopeptidase A, which contribute to the control of nociception and blood pressure (Mizutani et al. 2008; Roques et al. 2012). It is worth noting that other peptidases, which have been described decades ago, are still bereft of well-defined functions.
Neurolysin was first described as a novel neurotensin-cleaving enzyme, hence its original name ‘neurotensin-degrading neutral metalloendopeptidase’ (Checler et al. 1986) or endopeptidase 126.96.36.199. Cloning of the enzyme revealed the canonical signature of zinc-containing metalloenzymes (Dauch et al. 1995). As is the case in most peptidases, a close examination of its specificity revealed that the enzyme not only cleaves its canonical substrate, neurotensin, but it also hydrolyses additional peptides such as dynorphin 1-8 and substance P (Rioli et al. 1998). In accordance with its above-cited in vitro substrate specificity, pharmacological studies suggested a potential role of neurolysin in the control of nociception (Vincent et al. 1997). Interestingly, a recent study has identified neurolysin as a non-AT1–non-AT2 angiotensin-binding site, thereby widening its putative function to the renin-angiotensin system (Wangler et al. 2012). Overall, the pharmacological spectrum of neurolysin likely covers distinct physiological functions and could contribute to several pathologies (Fig. 1), but one can reasonably argue that its actual physiological role is still elusive.
In their article, Rashid and Colleagues show, by means of a mouse model of experimental stroke, that upon occlusion/reperfusion (60 min/24 h) of the middle cerebral artery, neurolysin expression monitored by radio-labeling is up-regulated in the ischemic but not in the healthy contralateral hemisphere (Rashid et al. 2013). The augmentation of this expression is correlated with an increased enzymatic activity. The authors demonstrate that neurolysin augmentation is not because of a stroke-associated increase in transcription, as its mRNA remains unaffected. Interestingly, neurolysin has been described as an enzyme that partitions between cytosol and several membrane-associated compartments including plasma membranes and mitochondria (Vincent et al. 1996). In a similar vein, Rashid and Colleagues also documented an apparent enrichment of neurolysin in plasma and mitochondria membrane fractions without a concomitant significant decrease in the cytosolic counterpart. Interestingly, this up-regulation occurred as early as 24 h after reperfusion and lasted for at least 7 days. Overall, the paper by Rashid can be seen as an interesting study unraveling the involvement of neurolysin in post-acute and early phase of the recovery process. In this sense, this study enlarges the putative role of neuropeptidases and, more particularly, neurolysin in the physiopathological context.
The study also raises a series of unresolved issues and important questions concerning the implication of these observations in terms of their translation into putative therapeutic strategies toward facilitation of post-stroke recovery, or even its prevention to some extent. The real mechanisms by which neurolysin is increased during the post-stroke period remain unclear, and the biological significance of neurolysin enrichment in mitochondria remains puzzling given that the nature of its putative substrates in this site is unknown. This raises the question as to whether one should envision neurolysin up-regulation as a deleterious consequence of stroke or as an adaptive mechanism aimed at counteracting cerebral ischemia. In other words, as far as its proteolytic activity is concerned, one should ask whether neurolysin increment in mitochondria is a means of depleting cells from a biologically active enzyme in the cytosol or if it recruits neurolysin at the mitochondria to promote degradation of yet unknown mitochondrial substrates. These questions have obvious implications for any strategy aimed at interfering with neurolysin in a stroke-related context. Should we envision activators or inhibitors of neurolysin?
Another question involves the nature of neurolysin substrate (s) that could participate in the post-acute and recovery phases of stroke. One must admit that it is a difficult issue because neurolysin exhibits the ability to inactivate a series of peptides, many of which have been shown to be modulated during ischemic brain injury. However, whether this end-point observation was consequential or causal remains unclear. It is an important issue nonetheless, particularly in the context of the hypothesis that a cytosolic depletion of neurolysin during the recovery phase would lead to lower degradation rates of its cytosolic substrates. If enhanced levels of cytosolic neurolysin substrates account for part of the recovery process, then one should consider a strategy aimed at designing neurolysin-resistant peptide substrates and/or neurolysin inhibitors (some of which are already available for testing in the experimental stroke model (Dauch et al. 1991; Jiracek et al. 1996) to potentiate their protective function against stroke. Alternatively, if enhanced neurolysin in the mitochondria triggers the post-ischemia recovery process, then mitochondrial substrates would need to be identified to design specific substrate antagonists or, alternatively, synthesize neurolysin activators.
Finally, it is important to note that many peptidases have been previously described as modulators of post-stroke traumatism. Some of these peptidases share common substrates with neurolysin, and the relative contribution of these enzymes to post-stroke recovery remains to be properly understood. In this context, it ought to be pointed out that a neurolysin knockout mouse model, which might prove extremely useful in determining the contribution of endogenous neurolysin to the post-acute and recovery phase of stroke, is still non-existent.
In conclusion, it must be said that this study is a very interesting contribution to the field of both peptidase function and stroke and should be seen as a starting point of investigations into an enzyme that has yet to reveal all its secrets.