Authors' response to commentaries

Neurohiolog 3 c~fAging, Vol. t0, pp. 588-590. ': Pergamon Press pie. 1989. Printed hi the U.S.A.

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Authors' Response to Commentaries

LARRY L. BUTCHER AND NANCY J. W O O L F

Universi~' of California, Los Angeles, CA

IN his now classic treatise on scientific method (3), Claude Bernard remarked that " . . . ideas are only intellectual instruments that we use to break into phenomena; we must change them when they have served their p u r p o s e . . . " (p. 41). We embrace this tenet, and, in an area of study as multifaceted and labyrinthine as Alzheimer's disease (AD), fresh approaches and novel ideas, although perhaps the fastest-acting antigens known to science, cannot be considered entirely unwelcome. It was our intent in writing this position paper to stimulate constructive rethinking of an important topic, to generate light as well as heat, and to create a focus for continued dialogue. To varying degrees, all of these goals have been met. Geddes and Cotman propose a pathologic sequence remarkably similar to our own except in the consideration of when proliferative processes in the brain become sufficiently atypical to be indicative of AD. They contend that signs of neuronal growth in brains about to become afflicted with AD are initially normal, reflecting efforts by presumably healthy cells to respond to some unstipulated degeneration of other neurons. At an unspecified time thereafter, this sprouting response becomes abnormal, which heralds the onset of dementing illness. The essential difference between our position and that of Geddes and Cotman is that we take as the starting point for our analysis the atypical morphologic profiles and entities that unequivocally signal AD. Geddes and Cotman have preferred to go back somewhat further in time and speculate on what happens before the pathologic cascade of AD becomes manifest. Although this is conceivably a useful intellectual exercise, it is also the subject for another commentary. A second proposal of Geddes and Cotman is that sprouting results in the replay of select developmental patterns of gene expression [referred to as "retrodifferentiation" in the terminology of other AD theorists (15)], presumably for certain cytoskeletal proteins. Again, and contrary to the contention of Geddes and Cotman, this idea does not conflict with our own. What we do maintain, however, is that, in addition to reexpression in AD of cytoskeletal proteins that transiently appear during development (e.g., vimentin), polypeptide species having no precedent in the organism may also be expressed (21). A strict "replay" of certain fetal patterns of cytoskeletal protein expression need not transpire, therefore, although such developmental reiteration probably occurs to some extent (21). As indicated in our position paper, the 43 kD protein of Wetmur et al. (19) is a possible example of a completely novel polypeptide that is expressed only during AD and at no other time in the life of the person so afflicted. Similarly, the antigen recognized by Alz-50 in the developing brain is apparently associated only with dying cells (1) and not with neurons that persist into adulthood. It is not appropriate, therefore, to consider Alz-50 immunoreactivity in the fetus as evidence that in AD retrogression of mature neurons to an earlier developmental state has occurred. The "outside-in" proposal of Appel and colleagues that the

pathologic cascade of AD is initiated by extra-neuronal accumulation of A4 amyloid peptide deriving from unknown sources and aggregating into entities referred to as diffuse plaques must be viewed with caution. Although such diffuse plaques may appear at an early stage in brains that are about to become afflicted with AD. it is another matter to imply causality from primacy. Indeed, Ihara has noted in his commentary that diffuse plaques are widely distributed throughout the nervous system, including such diverse loci as the basal ganglia and spinal cord, and that their presence is not correlated with dementia. Even if these seemingly omnipresent plaque-like entities created an environment in which the aberrant neuronal growth, senile plaques, and tangle/curly fibers that are presumably directly responsible for dementia could flourish (see Ihara's commentary), why does Atzheimer pathology manifest itself most prominently only in certain regions of the brain and not in all of the areas where diffuse plaques are found? Clearly, other factors are involved, and the possibility exists that diffuse plaques are more epiphenomenotogic than causative. Hammond's cogent commentary enlarges upon our contention that neuronal reorganization in AD is probably a multifactorial process by highlighting the possible roles that the 695 and 751 amino acid amyloid precursor proteins play in neurite outgrowth. Commenting further on this important class of polypeptides. Katzman opines that it would have been more fashionable at the present time to discuss the trophic and toxic actions of [3-amyloid peptide precursor rather than growth factors in general or nerve growth factor (NGF) in particular. It is of no particular consequence to our model, however, if the relevant growth-enabling agent is any of the previously enumerated agents or, more likely, an amalgam of several such substances varying m regional selectivity of action, distribution, and local concentration. What is important is that aberrant neuronal growth is recognized as a significant correlate of AD. The recent finding that NGF increases levels of the mRNA for amyloid precursor protein (11) suggests that both NGF and amyloid precursor protein, depending on their relationship to other aspects of the cellular and neurochemical milieu in which they operate, may play important, possibly interactive, roles in the pathogenesis of AD. Katzman contends that aberrant neuronal growth is a minor feature of the AD brain. Conclusions based on selected data derived from postmortem material and reflecting the end stages of a disorder might not accurately indicate pathologic processes occurring during earlier, more dynamic epochs, however, and it would be premature to conclude thal proliferative processes are not prominent at some stage in the development of AD. Furthermore. in addition to that aberrant growth acknowledged by Katzman to occur in the hippocampus, massive neuronal sprouting in the cerebral cortex of AD patients has been reported recently by Ihara (8), in apparent corroboration of the earlier findings of Scheibel and Tomiyasu (14). Katzman seemingly equates levels of brain trophic agents with

RESPONSE

physiologic saliency and intimates (see also commentary by Williams) that the functional efficacy of central growth factors is little altered in AD, but this conjecture is not supported by the findings of Atterwill and Bowen (2) and the more recent observations of Uchida et al. (18). These latter investigators demonstrated that neurotrophic activity is increased to approximately 160% of normal in AD frontal and parietal cortices (18), perhaps due to the absence of an inhibitory factor(s) (17). Katzman, in concert with Phelps and Williams, also prefers to emphasize the palliative actions of NGF in various animal paradigms of cholinergic dysfunction and in aging, but does not mention that deleterious effects of NGF on radial-arm maze performance have also been reported [e.g., (12)]. While admitting that some of our speculations are "justified by sound scientific evidence," Williams is perplexed by what he perceives to be incomplete attention to semantic detail. Although he apparently favors a more restricted notion of what the term "neurotrophic" means (see also commentary by Phelps), we prefer the broader, more recent conception of Purves and Lichtman (13), namely the "ability of one tissue or cell to support a n o t h e r . . . " (p. 375), usually applied to long-term interactions. Purves and Lichtman (13) further suggest that hormones, neurotransmitters, and trophic agents represent a continuum of extracellular messengers, differing essentially in duration and strategy of action. At least three major effects of trophic factors have been recognized (13), although any given neurotrophic agent may demonstrate one or all of the following actions: promotion of cell survival, influences on the direction of neuronal growth, and control of the extent of terminal arbors. In our opinion, the actions of substances that convey information from one tissue entity to another, such as trophic agents, cannot be divorced from the particular structurofunctional matrix in which they operate (4). In this regard, evidence exists that estrogens, T3, and the other substances we indicated can indeed be called trophic agents within the context of selected neuronal networks [e.g., see (7)]. In a similar vein, Hefti asserts that thyroid hormones are not normally categorized as neurotrophic agents, nor are they viewed as "classic" growth factors as exemplified by insulin (sic). Equally knowledgeable scientists often refer to insulin and insulinlike growth factors as "hormones" (20), however, which underscores the conceptual and terminologic ambiguity that currently exists with respect to transmitters, hormones, and trophic agents (see also commentary of Phelps). Semantic quibbling aside, it is apparent that Williams has misread or misunderstood a number of our proposals, as has Shelanski. We do not suggest anywhere in our manuscript, for example, that epidermal growth factor receptor (sic) is a neurotrophic agent, that " . . . tau and MAP2 epitopes in paired helical filaments correlate with the stabilization (sic) of the cytoskeleton in A D , " or that " . . . neurotrophic factors cause A D . " What we do indicate is that the chemotrophic environment, including balance relationships among multiple agents, is so configured in AD brains as to exacerbate the pathologic cascade of the disorder once it is initiated. If anything, the primary culprit in our model is altered expression of certain categories of cytoskeletal proteins, as well as the possible production of completely novel polypeptide species. Contrary to the apparent perceptions of Appel, Katzman, Phelps and Shelanski, we are highly cognizant of the multiple actions of neurotrophic agents, including their role in cell survival. We disagree, however, that cell mortality attributable to trophic imbalances results only from an absence of those factors and submit that an excess of such agents, or an increase in their saliency due to loss of inhibitory influences, can also lead to neuronal death under certain circumstances, perhaps those attendant with AD. As indicated in our position paper, this laboratory

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has shown recently that prolonged treatment of rat pups with thyroid hormone can produce neuronal death in the cholinergic basal forebrain that is preceded by abnormal neuritic growth similar in some respects to the aberrant proliferative processes seen in the basal nuclear complex of AD patients. It would appear reasonable to suggest, therefore, that the probability of cell death increases when the balance of trophic support for a given neuronal network deviates sufficiently from normal in favor of either greater or lesser trophic efficacy for prolonged periods. In the former case, neuronal degeneration is preceded by a period of aberrant growth; in the latter situation, cells undergo atrophy directly. Because the balance of trophic influence in the AD brain appears to favor increased neuronal growth (17,18), the possibility is raised that cell death in that disorder derives from increased trophic efficacy. If so, then a sound rationale, contrary to Mobley's opinion, would exist for including neurotrophic blocking agents as part of the pharmacologic armamentarium to be considered in the treatment of AD. Indeed, the findings of Mobley himself (11) are not incompatible with such a suggestion. Hefti suggests, without evidence, that clinical findings on patients with thyroid dysfunction are opposite to our predictions. In fact, patients with Graves' disease, a hyperthyroid condition, often report loss of memory (20) and may be misdiagnosed initially as having AD (see also our position paper). Nonetheless, hypothyroidism in aging also can produce a condition mimicking senile dementia, as indicated by Hefti. Both of these findings emphasize our contention that optimal balance among various brain neurochemicals is essential for optimal functioning and that deviations from normal in either direction can have untoward consequences. Appel et al. take us to task for not addressing the nature of the aging-related event(s) that initiates the pathologic cascade in AD, and Shelanski, who offers alternative mechanistic proposals for isolated aspects of our model, inquires why alterations in cytoskeletal protein expression should be considered a primary event in that disorder. Unfortunately, space considerations did not allow us to detail our reasoning on these issues in our position paper, but they have been addressed in considerable detail in another publication (21). Phelps indicates that our model of AD is troubling, but ideas are usually considered troubling only in the context of some reigning hypothesis or theory. That hypothesis is apparently contained in the first paragraph of Phelps' commentary, namely that loss of specific neurons in AD is due to loss of trophic support. As pointed out by Ihara and as detailed in our position paper, however, no currently available experimental evidence supports the underlying premise of that speculation. Phelps further indicates, but not entirely correctly, that there are cases of AD that do not show neurofibrillary tangles at autopsy, and, therefore, neurofibrillary tangles may not relate directly to the etiology of that disorder. The small number of AD patients to which he refers, however, did display neurofibrillary tangles in the hippocampus indistinguishable from those found in "classic" AD (16). In any case, we do not propose that neurofibrillary tangles are the primary deficit in AD. What we do suggest is that abnormal proteins related to the cytoskeleton are produced at an early stage in AD. These may interlink to form paired helical filaments, but it is conceivable that in some, perhaps isolated cases they do not. Various commentators indicated they believe it premature to consider antineurotrophic therapy in AD, especially in view of the ability of certain neurotrophic factors to prevent cell death in some animal preparations. Fundamental differences exist, however, between AD and the animal paradigms mentioned by Phelps, Hefti, and others. In the transected fimbria-fornix preparation, for example, basal forebrain neurons would be prevented from retrogradely transporting NGF from their targets to their somata.

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Accordingly, exogenous NGF administration would be expected to restore trophic levels in cell bodies and presumably rescue those neurons from death. Similarly, the aging rat brain displays decreased amounts of NGF, NGF mRNA, and NGF receptor (9,10), and it is not surprising that NGF injection in those animals would reestablish neurotrophic equilibrium. A rather different profile is found in AD: overall neurotrophic activity is increased (2,18), the m R N A ' s for NGF and its receptor are unchanged (5,6), and an inhibitory factor(s) impacting on trophic support is decreased [(17), see also our position paper]. All of these alterations suggest that in AD, unlike the above-mentioned animal models, a neurochemical imbalance exists in favor of increased trophic efficacy. In our opinion, the administration of additional trophic or related factors to afflicted individuals could conceivably exacerbate this imbalance and increase the probability of cell death by proliferation. It is apparent from the foregoing discourse that restoration of chemotrophic balance, either alone or in combination with other regimens, is attracting increasing attention as a therapeutic strategy in AD (see also commentary by Mobley). In this regard, we agree with Shelanski that timing is critical and reiterate our

position that antineurotrophic therapy might be maximally useful during the period when excessive, abnormal neuronal growth is the predominant pathologic feature of the disorder, possibly earlier stages. Even during this period we do not envision administering antineurotrophic drugs to the extent that an imbalance in the opposite direction occurs and cell death ensues as a function of lack of trophic support. As is the case with any proper pharmacologic strategy, dose-response curves must be constructed with the aim of finding levels of drug usage that restore trophic balance to normalcy. As pointed out in the commentary by Ihara, our suggestion in this regard is more consistent with presently available experimental evidence than proposals based on the conjecture that neurons degenerate in AD due to lack of a specific trophic factor(s). Nonetheless, as indicated in the final paragraph of our position paper and as intimated by Mobley, an optimal overall pharmacologic strategy might include protocols aimed at decreasing, as well as increasing, neurotrophic efficacy. Whether or not one or the other drug regimen is implemented at a given time during the course of AD might depend on the stage of the disorder extant when treatment is instituted.

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