Extensive functional redundancy in the regulation of Candida albicans drug resistance and morphogenesis by lysine deacetylases Hos2, Hda1, Rpd3 and Rpd31

Current treatment efforts for fungal infections are hampered by the limited availability of antifungal drugs and by the emergence of drug resistance. A powerful strategy to enhance the efficacy of antifungal drugs is to inhibit the molecular chaperone Hsp90. Hsp90 governs drug resistance, morphogenesis and virulence in a leading fungal pathogen of humans, Candida albicans. Our previous work with Saccharomyces cerevisiae established acetylation as a novel mechanism of posttranslational control of Hsp90 function in fungi. We implicated lysine deacetylases (KDACs) as key regulators of resistance to the most widely deployed class of antifungals, the azoles, in both S. cerevisiae and C. albicans. Here, we demonstrate high levels of functional redundancy among the KDACs that are important for regulating Hsp90 function. We identify Hos2, Hda1, Rpd3 and Rpd31 as the KDACs mediating azole resistance and morphogenesis in C. albicans. Furthermore, we identify lysine 30 and 271 as critical acetylation sites on C. albicans Hsp90, and substitutions at these residues compromise Hsp90 function. Finally, we show that pharmacological inhibition of KDACs phenocopies pharmacological inhibition of Hsp90 and abrogates Hsp90-dependent azole resistance in numerous Candida species. This work illuminates new facets to the impact of KDACs on fungal drug resistance and morphogenesis, provides important insights into the divergence of the C. albicans Hsp90 regulatory network and reveals new targets for development of antifungal drugs.

Metal Chelation as a Powerful Strategy to Probe Cellular Circuitry Governing Fungal Drug Resistance and Morphogenesis

Fungal pathogens have evolved diverse strategies to sense host-relevant cues and coordinate cellular responses, which enable virulence and drug resistance. Defining circuitry controlling these traits opens new opportunities for chemical diversity in therapeutics, as the cognate inhibitors are rarely explored by conventional screening approaches. This has great potential to address the pressing need for new therapeutic strategies for invasive fungal infections, which have a staggering impact on human health. To explore this approach, we focused on a leading human fungal pathogen, Candida albicans, and screened 1,280 pharmacologically active compounds to identify those that potentiate the activity of echinocandins, which are front-line therapeutics that target fungal cell wall synthesis. We identified 19 compounds that enhance activity of the echinocandin caspofungin against an echinocandin-resistant clinical isolate, with the broad-spectrum chelator DTPA demonstrating the greatest synergistic activity. We found that DTPA increases susceptibility to echinocandins via chelation of magnesium. Whole genome sequencing of mutants resistant to the combination of DTPA and caspofungin identified mutations in the histidine kinase gene NIK1 that confer resistance to the combination. Functional analyses demonstrated that DTPA activates the mitogen-activated protein kinase Hog1, and that NIK1 mutations block Hog1 activation in response to both caspofungin and DTPA. The combination has therapeutic relevance as DTPA enhanced the efficacy of caspofungin in a mouse model of echinocandin-resistant candidiasis. We found that DTPA not only reduces drug resistance but also modulates morphogenesis, a key virulence trait that is normally regulated by environmental cues. DTPA induced filamentation via depletion of zinc, in a manner that is contingent upon Ras1-PKA signaling, as well as the transcription factors Brg1 and Rob1. Thus, we establish a new mechanism by which metal chelation modulates morphogenetic circuitry and echinocandin resistance, and illuminate a novel facet to metal homeostasis at the host-pathogen interface, with broad therapeutic potential.

Invasive fungal infections pose a serious threat to human health worldwide, with Candida albicans being a leading fungal pathogen. Mortality is in part due to the limited arsenal of effective antifungals, with drug resistance on the rise. The echinocandins, which target the fungal cell wall, are the newest class of antifungal, and echinocandin resistance has already emerged. Here, we screened a library of 1,280 pharmacologically active compounds to identify those that potentiate echinocandin activity against an echinocandin-resistant isolate. The lead compound was a chelator, DTPA, which affects resistance by depleting magnesium. Genome sequencing of mutants resistant to the combination of DTPA and echinocandin revealed mutations in the gene encoding Nik1, which signals upstream of the Hog1 stress response pathway. We established that DTPA acts through Nik1 to modulate Hog1 signaling and enhance echinocandin activity, and that this combination has therapeutic benefits in a murine model of candidiasis. We also discovered that DTPA modulates C. albicans morphogenesis, a key virulence trait. DTPA induced filamentation by chelating zinc, in a manner that is contingent upon core filamentation pathways and specialized circuitry. Thus, we establish novel roles for metal homeostasis in C. albicans pathogenesis, thereby illuminating new therapeutic strategies for life-threatening infectious disease.

Combinatorial strategies for combating invasive fungal infections

Invasive fungal infections are an important cause of human mortality and morbidity, particularly for immunocompromised populations. However, there remains a paucity of antifungal drug treatments available to combat these fungal pathogens. Further, antifungal compounds are plagued with problems such as host toxicity, fungistatic activity, and the emergence of drug resistance in pathogen populations. A promising therapeutic strategy to increase drug effectiveness and mitigate the emergence of drug resistance is through the use of combination drug therapy. In this review we describe the current arsenal of antifungals in medicine and elaborate on the benefits of combination therapy to expand our current antifungal drug repertoire. We examine those antifungal combinations that have shown potential against fungal pathogens and discuss strategies being employed to discover novel combination therapeutics, in particular combining antifungal agents with non-antifungal bioactive compounds. The findings summarized in this review highlight the promise of combinatorial strategies in combatting invasive mycoses.

An Antifungal Combination Matrix Identifies a Rich Pool of Adjuvant Molecules that Enhance Drug Activity against Diverse Fungal Pathogens

There is an urgent need to identify new treatments for fungal infections. By combining sub-lethal concentrations of the known antifungals fluconazole, caspofungin, amphotericin B, terbinafine, benomyl, and cyprodinil with ∼3,600 compounds in diverse fungal species, we generated a deep reservoir of chemical-chemical interactions termed the Antifungal Combinations Matrix (ACM). Follow-up susceptibility testing against a fluconazole-resistant isolate of C. albicans unveiled ACM combinations capable of potentiating fluconazole in this clinical strain. We used chemical genetics to elucidate the mode of action of the antimycobacterial drug clofazimine, a compound with unreported antifungal activity that synergized with several antifungals. Clofazimine induces a cell membrane stress for which the Pkc1 signaling pathway is required for tolerance. Additional tests against additional fungal pathogens, including Aspergillus fumigatus, highlighted that clofazimine exhibits efficacy as a combination agent against multiple fungi. Thus, the ACM is a rich reservoir of chemical combinations with therapeutic potential against diverse fungal pathogens.

Lysine deacetylases Hda1 and Rpd3 regulate Hsp90 function thereby governing fungal drug resistance

The molecular chaperone Hsp90 is a hub of protein homeostasis and regulatory circuitry. Hsp90 function is regulated by posttranslational modifications including acetylation in mammals; however, whether this regulation is conserved remains unknown. In fungi, Hsp90 governs the evolution of drug resistance by stabilizing signal transducers. Here, we establish that pharmacological inhibition of lysine deacetylases (KDACs) blocks the emergence and maintenance of Hsp90-dependent resistance to the most widely deployed antifungals, the azoles, in the human fungal pathogen Candida albicans and the model yeast Saccharomyces cerevisiae. S. cerevisiae Hsp90 is acetylated on lysine 27 and 270, and key KDACs for drug resistance are Hda1 and Rpd3. Compromising KDACs alters stability and function of Hsp90 client proteins, including the drug-resistance regulator calcineurin. Thus, we establish acetylation as a mechanism of posttranslational control of Hsp90 function in fungi, functional redundancy between KDACs Hda1 and Rpd3, as well as a mechanism governing fungal drug resistance with broad therapeutic potential.

Hsp90 governs dispersion and drug resistance of fungal biofilms

Fungal biofilms are a major cause of human mortality and are recalcitrant to most treatments due to intrinsic drug resistance. These complex communities of multiple cell types form on indwelling medical devices and their eradication often requires surgical removal of infected devices. Here we implicate the molecular chaperone Hsp90 as a key regulator of biofilm dispersion and drug resistance. We previously established that in the leading human fungal pathogen, Candida albicans, Hsp90 enables the emergence and maintenance of drug resistance in planktonic conditions by stabilizing the protein phosphatase calcineurin and MAPK Mkc1. Hsp90 also regulates temperature-dependent C. albicans morphogenesis through repression of cAMP-PKA signalling. Here we demonstrate that genetic depletion of Hsp90 reduced C. albicans biofilm growth and maturation in vitro and impaired dispersal of biofilm cells. Further, compromising Hsp90 function in vitro abrogated resistance of C. albicans biofilms to the most widely deployed class of antifungal drugs, the azoles. Depletion of Hsp90 led to reduction of calcineurin and Mkc1 in planktonic but not biofilm conditions, suggesting that Hsp90 regulates drug resistance through different mechanisms in these distinct cellular states. Reduction of Hsp90 levels led to a marked decrease in matrix glucan levels, providing a compelling mechanism through which Hsp90 might regulate biofilm azole resistance. Impairment of Hsp90 function genetically or pharmacologically transformed fluconazole from ineffectual to highly effective in eradicating biofilms in a rat venous catheter infection model. Finally, inhibition of Hsp90 reduced resistance of biofilms of the most lethal mould, Aspergillus fumigatus, to the newest class of antifungals to reach the clinic, the echinocandins. Thus, we establish a novel mechanism regulating biofilm drug resistance and dispersion and that targeting Hsp90 provides a much-needed strategy for improving clinical outcome in the treatment of biofilm infections.

Candida albicans and Aspergillus fumigatus are the most common causative agents of fungal infections worldwide. Both species can form biofilms on host tissues and indwelling medical devices that are highly resistant to antifungal treatment. Here we implicate the molecular chaperone Hsp90 as a key regulator of biofilm dispersion and drug resistance. Compromising Hsp90 function reduced biofilm formation of C. albicans in vitro and impaired dispersal of biofilm cells, potentially blocking their capacity to serve as reservoirs for infection. Further, compromise of Hsp90 function abrogated resistance of C. albicans biofilms to the most widely deployed class of antifungal, the azoles, both in vitro and in a mammalian model of catheter-associated candidiasis. Key drug resistance regulators were depleted upon reduction of Hsp90 levels in planktonic but not biofilm conditions, suggesting that Hsp90 regulates drug resistance through different mechanisms in these distinct cellular states. Reduction of Hsp90 markedly reduced levels of matrix glucan, a carbohydrate important for C. albicans biofilm drug resistance. Inhibition of Hsp90 also reduced resistance of A. fumigatus biofilms to the newest class of antifungal, the echinocandins. Thus, targeting Hsp90 provides a promising strategy for the treatment of biofilm infections caused by diverse fungal species.

Regulatory circuitry governing fungal development, drug resistance, and disease

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.

Intellectual and functional outcome of children 3 years old or younger who have CNS malignancies

PURPOSE

To evaluate the impact of tumor location, clinical parameters, and therapy on neurocognitive, neuroendocrine, and functional outcomes in children < or = 3 years old with intracranial CNS malignancies who survived at least 2 years after diagnosis.

PATIENTS AND METHODS

Records were retrospectively reviewed for 194 children diagnosed from 1985 to 1999 at St Jude Children's Research Hospital (Memphis, TN).

RESULTS

The median age at diagnosis was 1.8 years (range, 0.1 to 3.5 years). Median follow-up was 7.64 years (2.0 to 19.4 years). Tumors were infratentorial (102), diencephalic (53), and hemispheric (39); 47% required ventriculoperitoneal shunts, 36% developed seizure disorders, and 20% developed severe ototoxicity. Therapy included no radiation therapy (RT) in 57 (30%), local RT in 87 (45%), and craniospinal irradiation (CSI) in 49 (25%). Overall survival at 10 years was 78 +/- 4%. In a longitudinal analysis of 126 patients with at least one neurocognitive evaluation (NE), the mean rate of intelligence quotient (IQ) change for patients who received CSI (-1.34 points per year) and local RT (-0.51 points per year) was significantly different from the no RT group (0.91 points per year; P = .005 and P = .036, respectively). Patients with hemispheric tumors had a significantly greater IQ decline (-1.52 points per year) than those with midline tumors (0.59 points per year; P = .038). Among those with NE > or = 5 years after diagnosis, 71.4% of CSI recipients compared with 23% of local RT recipients had IQ less than 70 (P = .021). Patients undergoing CSI were more likely to develop endocrinopathies (P < .0001) and to require special education (P = .0007).

CONCLUSION

In young children with CNS tumors, CSI and hemispheric location are associated with significant declines in IQ scores.

Early neuro-otologic effects of three-dimensional irradiation in children with primary brain tumors

PURPOSE

Central nervous system (CNS) irradiation can cause sensorineural hearing loss. The relationship between the dose to the cochlea and the development of hearing loss is unknown. Conformal radiation therapy (CRT) techniques facilitate accurate cochlear dosimetry. We modeled hearing threshold levels (HTL) after CRT in children with localized primary brain tumors (ependymoma, low- or high-grade astrocytoma, craniopharyngioma, or CNS germinoma) by using cochlear dose and clinical variables.

PATIENTS AND METHODS

We evaluated 72 children (median age, 9.5 years) with audiograms before and every 6 months after CRT (median follow-up, 16.6 months; range, 4.3-42.6 months). We used a mixed-effects model to predict change in hearing for each ear as a function of time, cochlear dose, and clinical variables.

RESULTS

Hearing was affected the greatest in patients with CSF shunts and pre-CRT ototoxic chemotherapy, enhanced by cochlear dose, and was more prominent on the right side. Hearing impairment after CRT alone occurred at low and intermediate frequencies in patients with shunts and supratentorial tumors when the cochlear dose exceeded 32 Gy. Patients with shunts and central supratentorial tumors developed intermediate-frequency hearing loss after CRT alone regardless of dose.

CONCLUSIONS

Hearing loss during the first 4 years after CRT alone is uncommon, although patients with shunts and supratentorial tumors appear to be at increased risk for low- and intermediate-frequency effects. CSF shunting and increased cochlear dose enhance the effect of ototoxic chemotherapy. If possible, the average cochlear dose should be <32 Gy over a 6-week course of treatment until more specific dose data become available.

Lactobacillus endocarditis: a case report of outpatient management

A case of infective endocarditis due to Lactobacillus is presented. The diagnosis was established by positive blood cultures and transesophageal (but not transthoracic) echocardiography. The patient was cured with outpatient ceftriaxone therapy.

Aging differentially alters forms of long-term potentiation in rat hippocampal area CA1

Long-term potentiation (LTP) of the Schaffer collateral/commissural inputs to CA1 in the hippocampus was shown to consist of N-methyl-D-aspartate receptor (NMDAR) and voltage-dependent calcium channel (VDCC) dependent forms. In this study, the relative contributions of these two forms of LTP in in vitro hippocampal slices from young (2 mo) and old (24 mo) Fischer 344 rats were examined. Excitatory postsynaptic potentials (EPSP) were recorded extracellularly from stratum radiatum before and after a tetanic stimulus consisting of four 200-Hz, 0.5-s trains given 5 s apart. Under control conditions, a compound LTP consisting of both forms was induced and was similar, in both time course and magnitude, in young and old animals. NMDAR-dependent LTP (nmdaLTP), isolated by the application of 10 microM nifedipine (a voltage-dependent calcium channel blocker), was significantly reduced in magnitude in aged animals. The VDCC dependent form (vdccLTP), isolated by the application of 50 microM D,L-2-amino-5-phosphonvalerate (APV), was significantly larger in aged animals. Although both LTP forms reached stable values 40-60 min posttetanus in young animals, in aged animals vdccLTP increased and nmdaLTP decreased during this time. In both young and old animals, the sum of the two isolated LTP forms approximated the magnitude of the compound LTP, and application of APV and nifedipine or genestein (a tyrosine kinase inhibitor) together blocked potentiation. These results suggest that aging causes a shift in synaptic plasticity from NMDAR-dependent mechanisms to VDCC-dependent mechanisms. The data are consistent with previous findings of increased L-type calcium current and decreased NMDAR number in aged CA1 cells and may help explain age-related deficits in learning and memory.

Persistent fever in association with infective endocarditis

Fever persisting despite adequate antimicrobial therapy for endocarditis can be an ominous sign. To evaluate the significance of persistent fever in this situation, we reviewed the records of patients at three hospital affiliates of Albert Einstein College of Medicine. Twenty-six patients with 27 episodes of endocarditis and fever lasting for > or = 2 weeks despite appropriate antimicrobial therapy were identified and compared with a matched cohort of 26 patients with endocarditis but without prolonged fever. The median duration of fever in the former group was 35 days. Cardiac infection caused fever in 13 of these patients, seven of whom had myocardial abscesses. Additional causes of infection included drug treatment, nosocomial transmission of pathogens, and pulmonary emboli. Sixteen patients required cardiac surgery (seven on an emergent basis), whereas only two controls underwent such a procedure (P < .001). Twenty-two patients with persistent fever and five controls developed nosocomial complications (P < .001). Six patients with fever died, five from endocarditis-related complications. Thus persistent fever often indicates complicated endocarditis. We present an approach for the evaluation of the patients affected by this condition.

Neural cell adhesion molecule in aged mouse muscle

Expression of the neural cell adhesion molecule was compared in endplate and non-endplate regions of skeletal muscles of mature and old CBF-1 mice, in order to determine whether age-related changes in neuromuscular morphology were correlated with age changes in neural cell adhesion molecule expression. Three muscles were examined: two (soleus and sternomastoid) showed age-related regionalization of nerve terminals as one manifestation of increased synaptic remodelling while the third (diaphragm) did not. Relative neural cell adhesion molecule content in these muscles was measured by densitometry of immunoblots after concentration by affinity chromatography. Expression of the major 140,000 mol. wt form of neural cell adhesion molecule, which was most abundant in the endplate region, was increased in sternomastoid and soleus of old compared to adult mouse, but was unchanged with age in diaphragm. A 70,000-80,000 mol. wt presumably proteolytic polypeptide fragment of neural cell adhesion molecule was increased in immunoblots of all old muscles. Immunocytochemical studies of skeletal muscles showed no difference in neural cell adhesion molecule cellular distribution in mature vs old mice, but in motor nerve of sternomastoid, the number of neural cell adhesion molecule-positive nerve fibers was increased in old mice. Several lines of evidence indicated that partial denervation was rare in old CBF-1 mice, and therefore could not account for the findings above. Selective increase of 140,000 mol. wt neural cell adhesion molecule expression in the junctional regions of those muscles of old mice which show neuromuscular remodelling indicates that this adhesion molecule may play a role in the age-related instability of motor nerve terminals.

Compensatory plasticity of aging at the neuromuscular junction

Several age-related phenomena observed at the neuromuscular junction (NMJ) can be viewed as adaptations to cellular deficits. These compensatory mechanisms, which maintain functional and morphologic integrity, are those present in the adult animal. In the study of compensatory mechanisms with age, the choice of an appropriate animal model is important. Three adaptations are discussed: maintenance or increase of transmitter release despite reduced supply of synaptic vesicles; functional reactive sprouting after partial denervation despite reduced axonal transport; and maintenance of nerve terminal integrity in the face of increased outgrowth and retraction. In all cases, successful adaptation in old animals is obtained at the expense of a more fragile system. Either the compensations themselves or the resulting vulnerability may alter the reactions of the aging nervous system to changes in external and internal milieu.

Plasticity of presynaptic and postsynaptic elements of neuromuscular junctions repeatedly observed in living adult mice

In order to assay the extent of ongoing synaptic remodelling in adult mouse neuromuscular junctions, dynamic structural changes of identified neuromuscular junctions were monitored in vivo over periods up to three months. Nerve terminal outgrowths as small as 1 micron were detectable with a new fluorescent tetanus toxin C-fragment stain combined with fluoresceinated alpha-bungarotoxin to stain postsynaptic acetylcholine receptors. With limited illumination, the new stain did not affect miniature endplate potential frequency, nor morphometric parameters of repeatedly observed neuromuscular junctions. At each observation, areas of presynaptic nerve terminal extending beyond underlying acetylcholine receptor ('preprojections'), and areas of acetylcholine receptor without overlying nerve terminal ('postprojections') were measured. Regions of the neuromuscular junction in which nerve terminal-postsynaptic acetylcholine receptor complexes either 'lengthened' or 'shortened' between observations were also measured. The total area of pre- and postprojections (relative to total junctional area) remained the same over three months but most had been replaced; only 20% of preprojections gave rise to lengthenings, the rest retracted or were unchanged. Lengthening and shortening of branches were about 1-2% of junctional area per month. These more permanent changes occurred against a background of ongoing transient nerve terminal outgrowth and retraction (which constituted 80% of all neuromuscular junction shape changes from one observation to the next, compared with 20% for the postsynaptic component). Breaks in the continuity of the underlying acetylcholine receptor were also observed between observations as were instances where acetylcholine receptor continuity was re-established. A newly observed form of plasticity was a shift in position and angle of pre-existing branches. Establishment of new acetylcholine receptor-positive synaptic regions was mostly preceded by nerve terminal outgrowth on the previous observation. In animals in which spontaneous wheel-running increased locomotor activity approximately tenfold over a period of 35 days, the findings were identical to those in unexercised mice. In summary, in the adult neuromuscular junction, the nerve terminal, not the postsynaptic component, is the dynamic entity, continually changing shape on the scale of micrometers, with relatively small permanent changes. These ongoing exploratory excursions may supply the substrate for synaptic plasticity, which would involve regulation of the dynamics or stability of nerve outgrowth.

Mode of enlargement of young mouse neuromuscular junctions observed repeatedly in vivo with visualization of pre- and postsynaptic borders

The dynamics of structural remodelling during growth of synapses was studied in identified living neuromuscular junctions observed three times at four-day intervals in three- to five-week-old mice. Nerve terminals and acetylcholine receptors in pectineus muscle were stained and visualized with fluorescent ligands of tetanus toxin C-fragment and alpha-bungarotoxin, respectively. In most observations, about 2.5% of nerve terminal area was observed without underlying acetylcholine receptor (termed 'preprojection'), and about 0.4% of acetylcholine receptor without overlying nerve terminal ('post-projection'). Neither overall synaptic growth nor prevalence of pre- and postprojections was affected by repeated observation. In sequential observations, about 80% of the preprojection area at one observation had acquired underlying acetylcholine receptor four days later, while 20% retracted or showed no change. Although approximately one-half of postprojections were also precursors of synaptic regions, their absolute contribution to synaptic growth was small, and some had originated from nerve terminal retraction. Eight per cent of the disparities between pre- and postsynaptic components in second or third observations were the result of nerve terminal outgrowth or retraction. In the four-day intervals, there was about 7.5% lengthening but also about 3% shortening of synaptic area. Preprojectional induction of acetylcholine receptor accounted for at least 25% of lengthening, and apparent concurrent growth at ends or sides of branches accounted for most of the rest (although level of resolution limits this conclusion). Only about 10% of lengthening was attributable to central intercalary growth. In summary, the pectineus neuromuscular junction grows mainly by nerve terminal outgrowth giving rise four days later to underlying acetylcholine receptor and by conjoint lengthening of synaptic complexes but with relatively little contribution by initial acetylcholine receptor extension or intercalary growth. Growth of the neuromuscular junction is not monotonic: nerve terminals retract and synaptic branches shorten as net lengthening proceeds. Compared with non-growing adult neuromuscular junctions, nerve terminal preprojections in growing neuromuscular functions are more prevalent and more likely to give rise to new synaptic regions.

Age differences in morphology of reinnervation of partially denervated mouse muscle

The effect of age on the ability of motor neurons to develop and maintain an enlarged total axonal and synaptic volume was compared in soleus muscles of 5-8-month and 25-30-month mice, 30-120 d after partial denervation. Before and after partial denervation (transection of the L5 root), the total number of muscle fibers was the same in all muscles. However, in young animals, there was only some transient atrophy and hypertrophy mostly receded by 120 d, whereas in old muscle, a more prominent early atrophy was followed by persistent hypertrophy. Ectopic endplates were not found. In zinc-iodide-osmium (ZIO) stained preparations, muscle fibers with small nerve terminals were present at 60 d and were still present in old muscle at 120 d. Fluorescent staining of nerve terminals and acetylcholine receptors revealed that in young muscle, postsynaptic sites were nearly or completely reoccupied by 60 d. In old muscle, about 22% of former junctions were denervated, with the remainder minimally to fully reinnervated. At 60 d and thereafter, collateral sprouts originated from nodes of Ranvier in both young and old muscle and were remyelinated in young but mainly unmyelinated and remarkably tortuous in old animals. These results, confirmed with immunofluorescent strains for myelin basic protein and neurofilaments, account for many of the physiological findings (Jacob and Robbins, 1990). Motor unit size expanded 2.5 times in young and 2 times in old muscle at 60 d after partial denervation. However, the increment in total quantal output and nerve terminal volume per motor neuron was 60-100% greater than control in young but only 20-25% greater in old muscle, with little further recovery. This inability of the motor neuron in old mice to expand the field of innervation may reflect a limitation imposed by reduced axonal transport. The present findings may elucidate the muscle weakness in postpolio syndrome and amyotrophic lateral sclerosis.

Differential effects of age on neuromuscular transmission in partially denervated mouse muscle

The response of the neuromuscular junction to expansion of the motor unit after partial denervation (section of L5 root) was compared in soleus muscles from young (5-8 month) and old (25-30 month) mice. The object was to determine the relative capacity of young and old motor neurons to adapt to an enlarged functional field of innervation, and to delineate physiological parameters that are compromised under these conditions. Neuromuscular function was studied at 30, 60, and 120 d after partial denervation. The initial (18-23) and postoperative number (5-8) of motor units was the same in both age groups. Twitch strength declined in proportion to loss of motor units at 30 d but returned completely (young) or nearly completely (old) by 60 d. In old but not young muscle, the safety factor (assayed by twitch depression in low calcium) was decreased even before functional sprouting had occurred, indicating a reduced safety factor in nondenervated junctions. The proportion of fibers with "long" latencies (delay between stimulation and endplate potential) increased transiently (at 30 d) in young muscle but persisted without recovery at 120 d in regenerated junctions in old muscle. After partial denervation, decline in miniature endplate potential (mepp) amplitude, in mepp frequency, and in estimated quantal content of evoked release was relatively more pronounced in old than in young mice, and in the case of mepp amplitude and frequency, more persistent. Mepp amplitude was also decreased in presumed nondenervated junctions of old muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Hand protection and protection from hands: hand-washing, germicides and gloves

A variety of soaps, detergents, germicides, and protective gloves are available for use by health care workers. Appropriate hand-washing and glove use will reduce the possibility of spread of infectious organisms from patient to staff, from patient to patient, and from staff to patient. Both hand-washing and glove use can have adverse effects. Excessive hand-washing, mechanical irritation from scrubbing, use of germicides, and wearing of gloves can result in irritant and allergic dermatitis. Dermatitis will result in an increased risk of infection to both the worker and the patient.

Motor nerve terminal restoration after focal destruction in young and old mice

Regeneration of soleus motor nerve terminals after focal destruction by black widow spider venom (BWSV) or its active factor alpha-latrotoxin (LTx) was compared in young and old CBF-1 mice. The object was to determine whether previously reported delayed regeneration after nerve injury in old rodents was due to altered removal of debris, or delay or aberrancy in structural or functional restoration of the neuromuscular junction. In addition, the use of a new fluorescent technique permitted for the first time quantitation of the accuracy of early nerve terminal regeneration in mammalian muscle. Immunohistochemical and electron micrographic studies showed no age difference in destruction of terminals and removal of debris 2 days after toxin application. The indirect twitch and structural reinnervation (measured with flourescent techniques) returned to an equal extent in young and old mice beginning at 3 days after LTx treatment. BWSV (as opposed to LTx) delayed regeneration 1 day in young but not in old mice. On the first day of reinnervation, there was perisynaptic outgrowth in both young and old mice, although in the latter, there was a higher incidence of aberrant outgrowth. The relation between return of twitch strength and recovery of nerve terminal area (measured in teased zinc iodide-stained preparations) showed no age dependency. We conclude that factors cited to explain altered reactive sprouting in the aging CNS do not apply to regeneration of peripheral motor nerve terminals. However, it is possible that the aberrant regrowth observed at the neuromuscular junction in old mice will pertain to the aging CNS. Altered axonal rather than nerve terminal regeneration is the likely source of delayed peripheral nerve regeneration in old animals.

Filopodia, lamellipodia and retractions at mouse neuromuscular junctions

In order to determine if mature motor nerve terminals retain structures associated with development such as filopodia and lamellipodia, we studied whole mounts of mature mouse neuromuscular junctions stained with both fluorescent-labelled tetanus toxin C-fragment and alpha-bungarotoxin, and employed electron microscopy in parallel. The rapid fluorescent stain may be of general usefulness. Both filopodia and lamellipodia were found, extending beyond the border of the established postsynaptic receptors. Filopodia often appeared in clusters, were devoid of a synaptic vesicle antigen, and many withdrew in response to cytochalasin D. Control experiments demonstrated that filopodia were not induced by the toxin treatment. The mean number of filopodia per endplate varied from about one in phasic muscle to three in tonic muscle, and was twice as great in immature mouse muscle. Postsynaptic receptor-rich regions without overlying terminals were less numerous than filopodial and lamellipodial projections without underlying receptors. Electron microscopy showed that lamellipodia contained actin-like filaments and immunoreactivity to actin, but no neurofilaments, microtubules, mitochondria or vesicles. Therefore, these structures would not be visualized by in vivo mitochondrial stains. The lamellipodia protruded into the gap between muscle and a closely overlying Schwann cell process. Lamellipodia occupied about 5% of the linear extent of the terminal arbor in whole mounts, but appeared in 16% of random electron micrographic fields. Thus, the lamellipodia and filopodia typical of developing terminals are present in adulthood and represent a distinctive specialization of the nerve terminal, which may interact with the adjacent Schwann and muscle cell. The frequency of filopodia is a function of age and of muscle or motoneuron type. We suggest that some of the factors known to regulate growth of filopodia and lamellipodia in vitro or in development may continue to act at adult presynaptic nerve terminals.

Acetylcholine receptors at mature and aged mouse neuromuscular junctions

The density and distribution of junctional and perijunctional ACh receptors (AChR) were studied in young (8-12 months) and old (24-25 months) C57 mice to determine: (1) if increased amplitude of spontaneous postsynaptic potentials previously reported in old C57 muscle was due to increased junctional AChR; (2) if increased extrajunctional AChR would be found in association with previously reported nerve terminal complexity; and (3) if extrajunctional AChR was present as in disused or denervated muscle. Microdissection of individual muscle fibers combined with I125-alpha-bungarotoxin labeling, gamma counting, measurement of surface area, cholinesterase stains, and autoradiography were used to obtain the results. In both young and old mice there was a sharp gradient in AChR between the end-plate and the perijunctional region. End-plate AChR densities and total AChR per end-plate were the same at old and young end-plates, as were perijunctional values. Thus, neither end-plate nor extrajunctional AChR density changes with age. An increased mepp amplitude reported previously in old CB57 animals must be due to other factors. The perijunctional AChR in old mice show no changes characteristic of disuse or denervation, or those which might give rise to the observed nerve terminal complexity.

Effect of partial denervation and terminal field expansion on neuromuscular transmitter release and nerve terminal structure

The efficacy of evoked ACh release by intact and newly sprouted terminals in response to partial denervation and expansion of the motor neuron terminal field was studied in mouse soleus muscle after section of the L-5 spinal root. From 2 to 4 d after partial denervation until 90 d later, only 3-7 motor units of the normal 21 remained. Regeneration of the dissected nerve was prevented while the remaining motor units were sprouting. The indirect twitch, which was only 20% of direct twitch tension 2-4 d after nerve section, recovered between 28 and 50 d postoperatively. However, the depression of twitch in low Ca/high Mg solution, which was equal to control 2-3 d postoperatively, was 2-3 times more depressed than control by 50 d and remained so up to 90 d. This indicated persistent reduction of the safety factor in sprouted motor units. Intracellular measurement of quantal content in 0.4 mM Ca, 2.75 mM Mg revealed 2 groups of nerve terminals in partially denervated muscle. The quantal content of the first group was greater than contralateral control at earlier times (28-50 d) and only slightly greater than control later (74 and 90 d). This group consisted of the original undenervated terminals, since it was associated with normal miniature endplate potential (MEPP) frequency and end-plate potential (EPP) latency, and presumably with the class of fibers with normal (zinc iodide osmium-stained) nerve terminal morphology and occasional large myelinated preterminal axons.(ABSTRACT TRUNCATED AT 250 WORDS)