Live discussion and panel of participants scheduled for 7 September 2001.
The live chat and panel of participants scheduled for 7 September 2001 has been postponed until further notice.
By Ming Chen
Much of the research on Alzheimer's disease currently is devoted to the issue of what plaques and tangles do in the brain. However, I would argue that it may be more important to ask,"where do they come from?" This should be the central issue in AD research.
Conceptually, there are two groups of substances in our body which are associated with diseases. The first group includes cancer growth, HIV proliferation, etc, which are caused by pathogens. The second group includes "aging markers" such as hair graying, skin wrinkling, tooth loss, cholesterol deposits, or gallstone.
If plaques and tangles are like cancer or virus, then we would need to look for pathogens that are independent from normal metabolic processes. But if plaques and tangles are analogous to cholesterol, gallstone or cataract deposition, then the direction of our quest would be different, because these aging markers perhaps all originate from aging-associated inefficiencies of normal metabolism. For example, cholesterol accumulation is perhaps the result of inefficiency of its normal degradation, and gallstone is due to slowdown of the mineral clearance, and so on. In these cases, age-related changes in the normal metabolic pathways, rather than a "pathogen," would be key to understanding these disorders.
For a long time, we, like many others, had taken the former scenario as granted because it is so widely believed. Recently, however, based on our experimental findings and a comprehensive review of literature, we were finally led, to our surprise, to the conclusion that the latter scenario is the case. This change did not occur in one day, but step by step through a series of publications over the past several years:
1. We first found that alpha-secretase, the protease responsible for APP normal processing, is a Ca2+-dependent protease (1), possibly calpain (5, 9), a well-known Ca2+-dependent protease. This finding was made several years ago, but its publication proved difficult because it contradicts other reports. This finding has not established the identity of alpha-secretase, but suggested a clue for the regulatory mechanism of this enzyme.
2. Following this clue, we reasoned that if alpha-secretase is regulated by Ca2+, then many agents that have been found to enhance soluble APP (APPs) secretion would also activate Ca2+. Conversely, agents that reduce APPs would have an opposite effect on Ca2+. We immediately checked this possibility by sorting through many reports and found that it is essentially the case (2). Indeed, this finding should not be a surprise because it is well known that protein secretion, an essential part of cell metabolism, is generally under the control of Ca2+ (3, 6).
3. Thus, the concept that APP alpha-processing is a Ca2+-dependent process is supported not only by our finding, but also by an unusually high consistency of many other stuides. Therefore, we reasoned that if alpha-secretase reduces its activity during aging, then APP will accumulate. This, in turn, will provide excessive substrate for beta- and gamma-secretases to overproduce A-beta (2, 7). The two pathways compete for the same APP pool, thus decrease in one must increase the other, in much the same way as the increased deposition of cholesterol must come from the reduced normally-degraded molecules. When a protein becomes deposited in normal aging, the first suspect should be the possible failure in its normal degradation pathway (protein turn-over slowdown).
4. Why will alpha-secretase reduce its activity? Because aging is a process between dynamic life (young) and death (full-stop of metabolisms). So, during this process, most metabolic activities should diminish (6). Ca2+ signaling, like any other essential biochemical pathways (such as cAMP signaling, energy metabolism, protein secretion, and cholestrol catabolism) must be down-regulated during aging.
5. Surprisingly, this model may also explain tangle formation. It is well-known that tau is normally degraded by calpain (indeed, many cytoskeleton proteins are also normally degraded by calpain)(3). Tau is also normally dephosphorylated by phosphatases including calcineurin (Ca2+-dependent phosphase) (2, 6, 7). So, when Ca2+ signal is reduced, tau will accumulate and at the same time become hyperphosphorylated.
6. More importantly, aging and AD are characterized not only by plaques and tangles, but also by a progressive neurotransmission decline. What factor controls neurotransmission? Textbooks tells us that this factor is Ca2+ alone. So, a Ca2+ signaling deficit can explain all three hallmarks, together with the question of why they appear in everybody and at about the same time in life.
7. However, this view collides immediately with a current theory, i.e., "calcium overload" hypothesis, which claims that intracellular Ca2+ levels are increasing throughout aging, leading to cell death. But is is known today that Ca2+ exerts its effects not through "steady-state level" changes (like water levels in swimming pool, as conventionally conceived), but rather through rapid changes in its spike frequency and amplitude (like radio waves) (8). Ca2+ spikes are highly energy-dependent (6) and energy levels must decrease during aging. So, Ca2+ spike frequency will naturally reduce during aging. Intriguingly, the reduced frequency means a "prolonged Ca2+ stay" in the cytosol. Because the spikes in vivo occurs within sub-millisecond, if Ca2+ is measured in second or minute time intervals (as most reports do), it can appear as a slightly "increased level". But such an apparent increase actually means a decreased signaling potency (8).
Although the proposed roles of Ca2+ in plaque and tangle formation is debatable, our model emphasizes one basic point: plaques, tangles and mild memory deficits in most people (except for inherited cases) occur as a result of inefficient NORMAL metabolic pathways and thus are NATURAL events during aging like many other aging markers, but not caused by any pathogens or metabolic "mistakes" like in cancer growth.
This hypothesis predicts:
1. Ca2+ activators (estrogens, nerve growth factors, etc.) together with physical exercise will have neuroprotective effects in the elderly, because they all promote physiological metabolisms. Hence, they will not only increase the production of soluble APP (APPs), but also reduce A-beta (2, 3). This view is similar to the concept of promoting normal metabolism in order to reduce cholesterol deposits, gallstones, osteoporosis, muscle atrophy and other conditions associated with aging.
2. Conversely, animal models for AD can be generated by prolonged use of Ca2+ antagonists or energy metabolism inhibitors (4, 6), or by reducing the intrinsic Ca2+ activators (such as NGF).
3. Presenilins most likely act as Ca2+ channels in vivo (their structures are highly channel-like) and their mutations will REDUCE the Ca2+ channeling ability (2, 3). This will decrease Ca2+ spike FREQUENCY and amplitude, and thus increase A-beta by inactivating alpha-secretase, and also reduce normal degradation of tau and normal neurotranmission altogether. These predictions can be experimentally verified.
1. Chen M. Alzheimer's alpha-secretase may be a calcium-dependent protease. FEBS Lett. 1997 Nov 10;417(2):163-7. Abstract.
2. Chen M. The Alzheimer's plaques, tangles and memory deficits may have a common origin. Part I: a calcium deficit hypothesis. Front Biosci. 1998 May 11;3:a27-31. Abstract.
3. Chen M. The Alzheimer's plaques, tangles and memory deficits may have a common origin. Part II: therapeutic rationale. Front Biosci. 1998 Jun 8;3:A32-7. Abstract.
4. Chen M. The Alzheimer's plaques, tangles and memory deficits may have a common origin. Part III: animal model. Front Biosci. 1998 Jun 17;3:A47-51. Abstract.
5. Chen M, Fernandez HL. The Alzheimer's plaques, tangles and memory deficits may have a common origin. Part IV: can calpain act as alpha-secretase? Front Biosci. 1998 Dec 15;3:A66-75. Abstract.
6. Chen M, Fernandez HL. The Alzheimer's plaques, tangles and memory deficits may have a common origin. Part V: why is Ca2+ signal lower in the disease? Front Biosci. 1999 Apr 1;4:A9-15. Abstract.
7. Chen M. Do the intracellular calcium states in Alzheimer disease need to be revisited? J Neuropathol Exp Neurol. 1999 Mar;58(3):310-1. Abstract.
8. Chen M, Fernandez HL. Ca2+ signaling down-regulation in ageing and Alzheimer's disease: why is Ca2+ so difficult to measure? Cell Calcium. 1999 Sep-Oct;26(3-4):149-54. Abstract.
9. Chen M, Durr J, Fernandez HL. Possible role of calpain in normal processing of beta-amyloid precursor protein in human platelets.Biochem Biophys Res Commun. 2000 Jun 24;273(1):170-5. Abstract.
The Following Three Figures Summarize our "Ca2+ deficit" Hypotheses
Fig. 1. Two different "Ca2+ deficit" hypotheses.
A. The current "calcium overload" hypothesis is based on the traditional concept of steady-state changes of Ca2+ measured at second or minute time intervals. It suggests that the average Ca2+ "levels" are steadily increasing during aging leading to cell death, therefore inhibiting Ca2+ entry will prevent AD.
B. Based on a Ca2+ oscillation concept, we propose that Ca2+ pulse frequency will progressively reduce during aging but this can be measured only at sub-millisecond time scale. If the frequency reduction surpasses a certain limit, Ca2+ gradient will collapse and cell will die. Thus, boosting the pulse frequency by Ca2+ agonists will slow down AD process. Note that the reduced pulse frequency means a "Ca2+ overstay", or "increased average Ca2+ levels", in the cytosol.
Fig. 2. Intermittent and alternate actions of the channels and pumps
To elicit a net ascending slope, Ca2+ channels in a cell must be "on" at the same time while pumps must be "off". This is then followed by an immediate reversal to complete a spike within a fraction of a millisecond. Thus, channels and pumps must operate intermittently as an "integral unit" like an alternate generator. During aging, the turnover speed of the unit will slow down as a result of reduced energy input.
Fig. 3. Distinctive effects of Ca2+ agonists in the brain versus cultured cells.
Why do Ca2+ agonists increase Ca2+ "levels" in the culture cells, but decrease them in the brain? In the living brain, Ca2+ is fully "potentiated" by action potentials and physiological Ca2+ agonists. But in the resting cultured cells, Ca2+ is pulsating at basal-line or surviving levels because the natural action potentials are absent. Therefore, addition of Ca2+ agonists (Stimulated) in the brain will reduce the time of Ca2+ stay in the cytosol (thus decrease Ca2+ "levels"), but increase the average Ca2+ "levels" in cultured cells if measured at second or minute time scale.
The Following Three Figures and a Table Summarize our "Natural Origin" Hypothesis for Plaques and Tangles
Updated -- 10 October 2001
Fig. 4. What will happen during aging?
During aging, many biochemical pathways will naturally slow down, and Ca2+ signaling is no exception. Therefore, numerous aging markers, including plaques and tangles, will appear in essentially everyone and at about the same time in life.
Fig. 5. How have plaques and tangles formed during aging?
Proteins and other large molecules in our body will eventually end up in one of the two mutually exclusive outcomes: either degraded or deposited. If the deposition of cholesterol in aging is due to its insufficient normal degradation/clearance rather than "overactivation" of some abnormal factors, then a similar mechanism will underlie the depositions of APP and tau (and other calpain substrates).
Fig. 6. Subcellular localization of α-, β- and γ-secretases.
α-Secretase is most likely a Ca2+-dependent protease, perhaps calpain, because calpain is known to be active both in the cytosol and at cell surface. Such a "double membrane anchorage" of both a-secretase and APP would allow APP to be normally cleaved only at the lys-16 site. When Ca2+/calpain activity declines during aging, APP will accumulate and be progressively attacked by other proteases including β- and γ-secretases, until they reach the Ab core. (Note that calpain activity is regulated by Ca2+ spike frequency)
Table 1. Why is α-secretase most likely a Ca2+-dependent protease?
Because assuming a-secretase to be a Ca2+-dependent protease can explain the reported actions of these 21 agents altogether, and also because of the simplicity and explanatory power of the assumption for other AD features. Note that none of the other currently proposed candidates for α-secretase has been supported by comparable extensiveness and consistency of the data. Sources are listed in ref. 1. APPs, normally secreted APP.
1. Chen M. Alzheimer's a-secretase may be a calcium-dependent protease. FEBS Lett 1997; 417, 163-7. Abstract.