Are prohormone activating enzymes part of an enzyme cascade?

T I N S - J a n u a r y 1985 of experiments required grows combmatorially. Past e x p e n e n c e has shown us that if a system does not run close to real time, there is not a quick enough feedback to allow researchers to effloently evaluate their solutions Most of the improvements on the speed wtll come from the computer systems that technology is making possible, especially through the use of Very Large Scale Integration (VLSI) electromc c~rcuits_ The importance of VLSI stems from two facts first, VLSI technology has made available fast signal processmg devices that have satisfied most of the requirements for stgnal processing Second, the posslbdlty of designing and building a new archttecture down to the logic circuit level (instead of down to the off-the-shelf

11 microprocessor level) has given us the possibility to make real-time speech recogmtton tn the context of complex tasks a reality In conclusion, we wall see substantial improvements in speech recognition technology m the next few years Nevertheless, these improvements will be rather more evolutionary than revolutionary and will produce systems that are still far from the performance of human to human communication

Acknowledgements This research was sponsored by the Defense Advanced Research Projects Agency (DOD), ARPA Order No 3597, morntoted by the Air Force Avionics Laboratory Under Contract F33615-81-K-1539 The views and conclusions contained m this document are those of the author and

Letter to the Editor Are prohormone activating enzymes part of an enzyme cascade? SIR_

In their recent article ( T I N S , April 1984), James L. Roberts and Dolan Prltchett discussed the possible role of kalhkrem-hke enzymes in the proteolyric processing of p r o h o r m o n e s to their end-product peptldes ~ The kalhkrems are serine proteases with a relatwely high degree of substrate specificity2 Further evidence of the involvement of a serme protease in propolypeptide conversion has come from a recently reported case of human proalbummaemia 3. This particular proalbuminaemia was attributable, not to a mutant proalbumin 4,5 but to a mutant 0qanU-trypsm lnlubitor The mutation converted the inhibitor from an antielastase to an antl-thrombm, suggestmg that the proalbumin converting enzyme was Itself closely related to thrombm 3 Thrombln, hke the kalhkrems, is an argmylesteropeptidase6 This evidence therefore strongly supports the suggestion that p r o h o r m o n e converting enzymes are s e n n e proteases However, there is an interesting corollary to such a view Both kalhkreins and t h r o m b m , as well as all other serme proteases, are themselves d e n v e d from propolypeptide precursors or zymogens Moreover, zymogens are normally without substantml actiwty of their own 7 and

should not be interpreted as representing the official polioes, either expressed or implied, of the Defense Advanced Research Projects Agency or the US Government Selected references There are no recent tutorial books on this topic The interested reader should consult the most recent Proceedings of the Institute o] Electrical and Electromcs Engineers ([EEE) Conference on Acoustics, Speech and Signal Processmg and the IEEE Transacnons on Acousucs, Speech and Signal Pro~essmg The article 'Recogmzlng cootlnuous speech remains an elusive goal', (1983) Spectrum (IEEE) Vol 20, No 11, pp 84-87 contains an account of ongoing projects and applicationsof speech recogmtlon Roberto Btstant t~ at the Computer Scwnce Department, Carnegw-Melhm Umversttv, Ptti~burgh, PA 15232, USA

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are activated only by proteolytlc removal of the pro-sequence If prohormone converting enzymes are serine proteases, t t t s hkely that they too are zymogens. Indeed, zymogen sequences have been identified in the large kalhkrem family uncovered recently 2 This raises the question of how converting enzymes might themselves be activated?' (The enzymic version of 'Sed quts custodtet ipsos Custodes "~s) If they are zymogens, then it would be by proteolytlc cleavage. This would involve another enzyme (although it could also be by autoactivation). Followed through to its logical conclusion then, the evidence suggests the existence of a cascade (Fig 1) The fact that a propolypeptlde convertlng-enzyme cascade might exist has obvious practical consequences for any experimental strategy designed to isolate the prohormone converting enzyme, for there is every posslblhty that the bulk of the enzyme Is held m an inactive, and therefore undetectable, form There is, however, another potentially interesting lmphcahon The existence of a cascade suggests that propolypeptide conversion to end-product peptldes needs to take place rapidly - yet there seems to be no obvious reason why it would need to be so rapid ff this conversion were mtracellular In many cases, the site of conversion of propolypeptlde to active end-product peptide remains unknown There ts evidence that pro-

albumin is converted to albumin close to the cell boundary 9, although equally, there ts evidence that proinsuhn is converted to insulin mtracellularly 1° It is difficult to know at present whether there are two different sites of conversion or not What ts clear is that, if p r o h o r m o n e activation takes place close to, or during secrehon ti, a cascade process has an obvious role to play in ensuring that conversion ts fully complete before enzyme and substrate are diluted by diffusion Immediately after exocytosts of the secretory granule contents

Achvatmg protease

J propolypeptlde --, end-product peptlde Propolypepnde ~s converted to acnve endproduct pepnde through the acnon o f a converttng enzyme Evidence suggests that the convertmg enzyme may be a serme protease which itself requires proteolync cleavage for acnvtty This cleavage either requires the acnon of a second actlvanng protease, or possibly the tmttatlon of autoacnvanon (shown as ~) F i g . 1.

1985, Elsevier Science Ptlbhshers B V , Amsterdam

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References 1 Roberts, J

L

and Prltchett, D (1984) Frends NeuroSc: 7, 105-I07 2 Mason, A J , Evans, B A , Cox, D R , Shine, J and Richards, R I (1983) Nature (London) 303, 300-307 30wen, M C,Brennan, S O,Lewls, J H and Carrell, R W (1983) N Engl J Med 309, 694--698 4 Brennan S O and Can'ell, R W (1978)

Nature (London) 274,908--909 5 Abdo, Y , Rousseaux, J and Dautrevaux M (1981) FEBS Lett 131,286--288 6 Magnusson, S (1971) m The Enzymes, Vol Ill Hydrolysis, Pepnde bonds (Boyer, P D , ed ), 3rd e d n , pp 277-321, Academic Press, New York and London 7 Kassell, B and Kay, J (1973) Science 180, 1(122-1027 8 Juvenal Satires ~ 347

9 Geller. D M , Judah, t D and NIcholl. M R (1072)Btochem J 127.86S-874 10 Dochert), K and Sterner D I ti9,~21 Annu Rev Phystol 44, 625-638 l l Green, D P L (1984) Med tIvpothe~e~ I~ 47-611 l) P L G R E F N

Department of Pharmacology, Umverstty of Otago, PO Box 913, Dunedtn, New Zealand

The uniclue origin of rod photoreceptors in the teleost retina Pamela A. Raymond A populauon of m,tonc cells that produce new neurons has recently been discovered m the layer of photoreceptors in retinas of several species of larval and adult fish These cells generate new rods which are inserted into the photoreceptor mosaic at locations scattered across the entire expanse of the differentiated retina. Subsequently these new rods establish synapttc connecttons with retinal neurons already presenl. The continued production of rods m the fish is apparently needed to maintain visual sensitivity during postembryomc growth of the eye. The extent of postembryonlc growth and neurogenesis in the retina and brain of many species of adult teleost fish has only recently b e e n widely appreciated I This is somewhat surprising since the first thorough documentation of retinal cell addition in adult fish was provided over thirty years ago by Mfiller 2, who counted cells and calculated the total n u m b e r of neurons in the retinas of guppies (Lebtstes renculatus) from hatching to adult stages As the fish grew from 7 m m to 28 m m long he found a threefold increase in the n u m b e r of ganglion cells, cones and inner nuclear layer cells, and nearly a sevenfold increase in numbers of rods More recent studies in juvenile and adult goldfish, as well as other related species (Carasstus spp.), have used cell counts 3,4, optic fiber counts s and autoradiographic techmques 6,7 s to confirm that new neurons are added to the retina as the fish grows Most postembryonic neuronal cell addition in the retina takes place in a narrow carcumferential germinal zone that is located at the peripheral margin2.6, s. Neurogenesis is not restrietod to the germinal zone However, It has been discovered recently that not all neurogenesls in the teleost retina is confined to the circumferential germinal zone Sandy and Blaxter 9 found that when [3H]thymldine was injected into metamorphos-

mg h e r n n g and sole, scattered cells in the outer nuclear layer, among the nuclei of photoreceptors, incorporated the label. Similar dividing cells were found in larval and juvenile goldfish and also in a clchhd fish (Haplochromrs burtoni), In fish allowed to survive for a few weeks or months after the thymidine injection, labeled nuclei of rods, but no other neurons, were seen in the central retina 7,1°. Thus, whereas production of most neurons in the postembryonlc teleost retina is confined to the circumferential edge, new rods continue to be produced by a dispersed population of rod precursors in central regions of the retina which have already differentiated and become functional (Fig 1) The demonstration that new rods are produced by mitotic division within the differentiated teleost retina provided an explanation for some puzzling observations that had been made repeatedly by several investigators but had been difficult to interpret Pnncipal among these was the observation that in many species of teleost fish the larval forms lmtially have no rods, but only cones It. Rods begin to appear either at metamorphosis or sometime during larval development, depending on the species, they then start to accumulate so that their proportions slowly but persistently increase during larval and/or postlarval stages of growth 23-7 An especially dramatic example of this persistent addition of

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rods in the adult teleost retina was described by Locke02 in a deep-sea fish, Chauliodus sloam. The retinas of mature fish of this species contain no cones but only rods, wtuch are arranged in tiers, or banks, in small specimens there is one tier, but in larger animals, there are up to five tiers (Fig 2) Discovery of the rod precursor It is now clear that dividing rod precursors are responsible for the delayed and prolonged production of rods in the teleost retina 7-9 Prevaous attempts to explain the sudden appearance of rods in the differentiated larval retina and the persistent accumulation of new rods in the postlarval retina were unsatisfactory and, in retrospect, somewhat contrived. It was suggested, for example, that cones were transformed into rods 13 or that cells migrated out of the inner nuclear layer into the outer nuclear layer where they differentiated into rods 1], or that rods were produced in the circumferential germinal zone but were then displaced laterally into more central regions of the retina 2,6 T h e possibility that mitotic division of stem cells within the differentiated retina might account for the addition of new rods did not seem to be an acceptable alternative because at that time there was little evidence for rmtotic activity anywhere except at the germinal zone The finding that mitotically active cells give rise to new neurons within fully differentiated regions of retina was totally unanticipated It contradicted all previous conceptions of how retinal histogenesis might be organlzed. To date most studies have observed that m the developing retina cytogenesls and differentlatton take