- Email: [email protected]
Differential loss of spinal sensory but not motor neurons in the p75NTR knockout mouse
Neuroscience Letters 267 (1999) 45±48
Differential loss of spinal sensory but not motor neurons in the p75 NTR knockout mouse Simon S. Murray a, Perry F. Bartlett b, Surindar S. Cheema a,* a Department of Anatomy, Monash University, Clayton, 3168, Australia The Walter and Eliza Hall Institute of Medical Research, Post Of®ce, Royal Melbourne Hospital, Melbourne 3050, Australia
b
Received 14 December 1998; received in revised form 14 March 1999; accepted 31 March 1999
Abstract Sensory neurons in the dorsal root ganglia (DRG) and motor neurons in the spinal cord express the 75 kDa low-af®nity neurotrophin receptor (p75 NTR) during prenatal development. The p75 NTR gene knockout mouse provides a unique opportunity to assess the role of p75 NTR during this period. Quantitative analysis of the p75 NTR knockout mouse revealed a signi®cant developmental loss of sensory neurons. In the cervico-thoracic ganglia approximately 75% of the neurons are lost, while in the lumbar ganglia the loss is approximately 50%. In contrast, motor neurons were not lost in either the cervical or lumbar spinal cord. These data suggest that p75 NTR is essential for the prenatal survival of a signi®cant number of sensory, but not motor neurons. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: p75 NTR Knockout mouse; Neurotrophin receptor; Neuronal cell death; Differential neuronal loss
The neurotrophin family have been shown to play a crucial role in the development, survival and maintenance of speci®c neuronal populations [1]. These molecules act via speci®c high-af®nity tyrosine kinase (trk) receptors [1], and also bind to the 75 kDa low-af®nity neurotrophin receptor (p75 NTR) [2]. While the interactions between the neurotrophins and the high af®nity trk receptors are well understood [1], the role of p75 NTR remains controversial. There is strong evidence that p75 NTR modulates trk activity. The number of high-af®nity binding sites [19], and the responsiveness [18] of cultured cells to nerve growth factor (NGF) is increased when p75 NTR and trkA are co-expressed. Antibodies directed against p75 NTR reduce the activation of trkA in PC12 cells [3], and sensory and sympathetic neurons in the p75 NTR knockout mouse have a reduced sensitivity to NGF [14]. Growing evidence also indicates that p75 NTR directly signals cell death in several cell types including oligodendrocytes, sympathetic neurons and retinal neurons [7]. Sequence homology studies have shown that p75 NTR belongs to the TNF receptor superfamily and contains a death signaling moiety spanning 90 amino acids [2]. * Corresponding author. Tel.: 161-3-9905-2712; fax: 161-39905-2766. E-mail address: [email protected] (S.S. Cheema)
Sensory neurons deprived of NGF have been shown to survive longer in vitro following a reduction in p75 NTR levels [4], and the rate and extent of apoptosis in PC12 cells is sensitive to the levels of p75 NTR expression [5]. Reducing the levels of p75 NTR using antisense oligonucleotides directed against p75 NTR can prevent the loss of axotomized sensory neurons in the dorsal root ganglion (DRG) of newborn rats [8]. We have investigated whether the sensory DRG and spinal motor neuron populations are affected in the p75 NTR knockout mouse. Since both these populations express p75 NTR during prenatal development [13], the p75 NTR gene knockout model offers a unique opportunity to assess the role of this receptor in the development of sensory and motor neurons. Homozygous transgenic mice lacking the p75 NTR gene were obtained from Jackson Laboratories (Bar Harbor, USA) [15]. BALB/c and 129/svJ mice, which are part of the genetic background of the p75 NTR knockout mouse, were used as the controls. Postnatal mouse pups, 7 days old, were rendered unconscious on ice, the heart was exposed and the animals were perfused transcardially with saline followed by 4% paraformaldehyde in 0.1 M sodium phosphate buffer at pH 7.2. Laminectomies were carried out to expose the cervical and lumbar spinal cords and DRG. Blocks containing the C8 and T1 DRG and spinal segments C5 to T1, and the L4 and L5 DRG and the lumbar spinal cord enlargement
0304-3940/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 9 9) 00 33 0- 4
46
S.S. Murray et al. / Neuroscience Letters 267 (1999) 45±48
were obtained. These tissue blocks were post®xed overnight in the perfusion ®xative at 48C, dehydrated and embedded in paraf®n wax. The cervical and lumbar spinal cords were sectioned in the transverse and horizontal planes respectively. Serial sections 8 mm thick were mounted onto gelatinized slides, de-waxed and stained in 0.1% cresyl violet. Sensory neurons in the DRG and motor neurons in the dorsolateral column of the spinal cord displaying a promi-
Fig. 2. Photomicrographs of cresyl violet stained histological sections illustrating sensory neurons in the DRG of control (A) and p75 NTR knockout (B) mice. The morphology of motor neurons in the spinal cord of control (C) and p75 NTR knockout (D) mice are also shown. Scale bar represents 50 mm and applies to all photomicrographs.
Fig. 1. Histograms illustrating normal sensory and motor neuron numbers in the spinal cords and DRG of p75 NTR knockout and control mouse. The number of sensory neurons in both the C8 and T1 (A) and L4 and L5 (B) DRG in the p75 NTR knockout mouse was signi®cantly less compared with both control groups. Motor neurons were also counted in C7 and C8, and the lumbar enlargement of the spinal cord in control and p75 NTR knockout mouse (C). The number of motor neurons in both the cervical and lumbar spinal cords of the p75 NTR knockout mouse were not signi®cantly different compared with the control groups. Double asterisks denote statistically signi®cant differences at the P , 0:01 level.
nent nucleolus were counted in every ®fth section [9,10]. The cervical motor neuron counts were restricted to spinal segments C7 and C8. The lumbar motor neuron counts were taken from the entire length of the lumbar enlargement. To determine the absolute neuronal numbers, Abercrombie's formula was used [11]. For this calculation, the mean diameters of nucleoli in sensory and motor neurons from wild-type and knockout mice were determined from camera lucida tracings (100 £ ) using a digitizer-linked to a computer image analysis program (SigmaScan Prow) (data not shown). Histograms were constructed using mean and standard error of the mean (SEM) values for each group (Fig. 1). Analysis of variance (ANOVA) statistics with Dunnett's post-hoc tests were used to determine signi®cant differences between groups. The histological appearance of DRG sensory neurons and spinal motor neurons in control (129/svJ) and p75 NTR knockout mice is shown in Fig. 2. The DRG sensory and spinal motor neurons can be clearly identi®ed in both the control and p75 NTR tissues. Fig. 1 is a summary of the numbers of DRG and motor neurons. In the C8 and T1 DRG, the p75 NTR knockout mouse had a 75 and 73% reduction of sensory neurons, respectively (Fig. 1A). In the L4 and L5 DRG, the p75 NTR knockout mouse had a 56 and 48% reduction of sensory neurons, respectively (Fig. 1B). The reduction of sensory neurons observed in the p75 NTR knockout mouse was statistically signi®cant for all DRG counted (**P , 0:01). Analysis of motor neurons counts in both C7 and C8 and lumbar enlargement did not reveal any signi®cant differences between controls and p75 NTR knockout mice (Fig. 1C). In the present study, we found that the numbers of DRG sensory, but not the spinal motor neurons, were signi®cantly depleted in the p75 NTR knockout mouse, suggesting a differ-
S.S. Murray et al. / Neuroscience Letters 267 (1999) 45±48
ential role for this receptor in these two neuronal phenotypes. Up to 75% of cervical and 50% of lumbar DRG neurons are lost during the prenatal period. Several studies have noted that the DRG in the p75 NTR knockout mouse are `macroscopically' smaller, and a reduction in the sensory innervation of the skin suggests that the somatosensory system is compromised in the p75 NTR knockout mouse [6,15]. However, this is the ®rst study to quantify the precise extent of the sensory neuronal loss in the p75 NTR knockout mouse. Our results suggest that there is an absolute requirement of p75 NTR for the survival of a signi®cant number of DRG sensory neurons during prenatal development. The lack of p75 NTR is likely to compromise the action of the high af®nity trk receptors. There is ample in vitro data which indicates that p75 NTR co-expression increases neurotrophin binding to trkA, B and C [2]. Decreased NGF binding has also been noted in DRG sensory and trigeminal neurons obtained from the p75 NTR knockout mouse [12,14]. Down-regulation of p75 NTR, by means of antisense oligonucleotides, is associated with increased death of embryonic (E14) DRG sensory neurons grown in the presence of NGF in the culture medium [4]. Collectively, these studies are consistent with the view that, in DRG sensory neurons, p75 NTR is essential for trk-mediated signaling of survival during prenatal development. Since p75 NTR interacts with trkA, B and C to promote the survival of sensory neurons, it is relevant to compare the degree of sensory neuronal losses in the trk knockout mice. In the trkA knockout mouse, approximately 70±90% of lumbar DRG neurons are lost; in the trkB and trkC knockout mice there is between 20 and 30% loss of lumbar DRG sensory neurons [17]. The reported losses associated with the trk knockout mice are consistent with the 50±75% decrease in the sensory neuron population in the p75 NTR knockout mouse reported in the present study, and further reinforces the view that the interaction between p75 NTR and the trks is essential for survival during prenatal development. It would be of interest to determine at what developmental stage the sensory neurons are lost, as this reduction in neuronal numbers may result from either reduced neuronal proliferation or increased neuronal death around the time of target innervation. A key ®nding in this study was that motor neurons are completely spared in the p75 NTR knockout animal. This occurs despite the expression of p75 NTR by motor neurons from E14 onwards [13], a timeframe that coincides with the period of naturally occurring motor neuron cell death. The observation that motor neuron numbers are not signi®cantly reduced in the p75 NTR knockout mouse is not unexpected, as neither the trkA, B or C knockout mice show a signi®cant loss of motor neurons [16,17]. The precise role of p75 NTR in motor neuron development remains unknown. We thank Mr. Ian Boundy and Ms. Liz Lopes for providing excellent histological assistance. We also thank Ms.
47
Kim Lowry for providing useful comments on the manuscript. S.S.M. is a recipient of the Bethlehem Grif®ths Postgraduate Scholarship. [1] Barbacid, M., The Trk family of neurotrophin receptors, . J. Neurobiol., 25 (1994) 1386±1403. [2] Barker, P., p75NTR: a study in contrasts. Cell Death Differ., 5 (1998) 346±356. [3] Barker, P.A. and Shooter, E.M., Disruption of NGF binding to the low af®nity neurotrophin receptor p75LNTR reduces NGF binding to TrkA on PC12 cells. Neuron, 13 (1994) 203± 215. [4] Barrett, G.L. and Bartlett, P.F., The p75 nerve growth factor receptor mediates survival or death depending on the stage of sensory neuron development. Proc. Natl. Acad. Sci. USA, 91 (1994) 6501±6505. [5] Barrett, G.L. and Georgiou, A., The low-af®nity nerve growth factor receptor p75NGFR mediates death of PC12 cells after nerve growth factor withdrawal. J. Neurosci. Res., 45 (1996) 117±128. [6] Bergmann, I., Priestley, J.V., McMahon, S.B., Brocker, E.B., Toyka, K.V. and Koltzenburg, M., Analysis of cutaneous sensory neurons in transgenic mice lacking the low af®nity neurotrophin receptor P75. Eur. J. Neurosci., 9 (1997) 18± 28. [7] Casaccia-Bonne®l, P., Kong, H. and Chao, M., Neurotrophins: the biological paradox of survival factors eliciting apoptosis. Cell Death Differ., 5 (1998) 357±364. [8] Cheema, S.S., Barrett, G.L. and Bartlett, P.F., Reducing p75 nerve growth factor receptor levels using antisense oligonucleotides prevents the loss of axotomized sensory neurons in the dorsal root ganglia of newborn rats. J. Neurosci. Res., 46 (1996) 239±245. [9] Cheema, S.S., Richards, L., Murphy, M. and Bartlett, P.F., Leukemia inhibitory factor prevents the death of axotomized sensory neurons in the dorsal root ganglia of the neonatal rat. J. Neurosci. Res., 37 (1994) 213±218. [10] Cheema, S.S., Richards, L.J., Murphy, M. and Bartlett, P.F., Leukaemia inhibitory factor rescues motoneurones from axotomy-induced cell death. NeuroReport, 5 (1994) 989± 992. [11] Clarke, P.G., How inaccurate is the Abercrombie correction factor for cell counts? (letter; comment). Trends Neurosci., 15 (1992) 211±212. [12] Davies, A.M., Lee, K.F. and Jaenisch, R., p75-De®cient trigeminal sensory neurons have an altered response to NGF but not to other neurotrophins. Neuron, 11 (1993) 565±574. [13] Ernfors, P., Henschen, A., Olson, L. and Persson, H., Expression of nerve growth factor receptor mRNA is developmentally regulated and increased after axotomy in rat spinal cord motoneurons. Neuron, 2 (1989) 1605±1613. [14] Lee, K.F., Davies, A.M. and Jaenisch, R., p75-De®cient embryonic dorsal root sensory and neonatal sympathetic neurons display a decreased sensitivity to NGF. Development, 120 (1994) 1027±1033. [15] Lee, K.F., Li, E., Huber, L.J., Landis, S.C., Sharpe, A.H., Chao, M.V. and Jaenisch, R., Targeted mutation of the gene encoding the low af®nity NGF receptor p75 leads to de®cits in the peripheral sensory nervous system. Cell, 69 (1992) 737±749. [16] Silos-Santiago, I., Fagan, A.M., Garber, M., Fritzsch, B. and Barbacid, M., Severe sensory de®cits but normal CNS development in newborn mice lacking TrkB and TrkC tyrosine protein kinase receptors. Eur. J. Neurosci., 9 (1997) 2045±2056.
48
S.S. Murray et al. / Neuroscience Letters 267 (1999) 45±48
[17] Snider, W.D., Functions of the neurotrophins during nervous system development: what the knockouts are teaching us. Cell, 77 (1994) 627±638. [18] Twiss, J.L., Wada, H.G., Fok, K.S., Chan, S.D., Verity, A.N., Baxter, G.T., Shooter, E.M. and Sussman, H.H., Duration and magnitude of nerve growth factor signaling depend on the ratio of p75LNTR to TrkA. J. Neurosci. Res., 51 (1998) 442±453.
[19] Verdi, J.M., Birren, S.J., Ibanez, C.F., Persson, H., Kaplan, D.R., Benedetti, M., Chao, M.V. and Anderson, D.J., p75LNGFR regulates Trk signal transduction and NGFinduced neuronal differentiation in MAH cells. Neuron, 12 (1994) 733±745.
Differential loss of spinal sensory but not motor neurons in the p75 NTR knockout mouse Simon S. Murray a, Perry F. Bartlett b, Surindar S. Cheema a,* a Department of Anatomy, Monash University, Clayton, 3168, Australia The Walter and Eliza Hall Institute of Medical Research, Post Of®ce, Royal Melbourne Hospital, Melbourne 3050, Australia
b
Received 14 December 1998; received in revised form 14 March 1999; accepted 31 March 1999
Abstract Sensory neurons in the dorsal root ganglia (DRG) and motor neurons in the spinal cord express the 75 kDa low-af®nity neurotrophin receptor (p75 NTR) during prenatal development. The p75 NTR gene knockout mouse provides a unique opportunity to assess the role of p75 NTR during this period. Quantitative analysis of the p75 NTR knockout mouse revealed a signi®cant developmental loss of sensory neurons. In the cervico-thoracic ganglia approximately 75% of the neurons are lost, while in the lumbar ganglia the loss is approximately 50%. In contrast, motor neurons were not lost in either the cervical or lumbar spinal cord. These data suggest that p75 NTR is essential for the prenatal survival of a signi®cant number of sensory, but not motor neurons. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: p75 NTR Knockout mouse; Neurotrophin receptor; Neuronal cell death; Differential neuronal loss
The neurotrophin family have been shown to play a crucial role in the development, survival and maintenance of speci®c neuronal populations [1]. These molecules act via speci®c high-af®nity tyrosine kinase (trk) receptors [1], and also bind to the 75 kDa low-af®nity neurotrophin receptor (p75 NTR) [2]. While the interactions between the neurotrophins and the high af®nity trk receptors are well understood [1], the role of p75 NTR remains controversial. There is strong evidence that p75 NTR modulates trk activity. The number of high-af®nity binding sites [19], and the responsiveness [18] of cultured cells to nerve growth factor (NGF) is increased when p75 NTR and trkA are co-expressed. Antibodies directed against p75 NTR reduce the activation of trkA in PC12 cells [3], and sensory and sympathetic neurons in the p75 NTR knockout mouse have a reduced sensitivity to NGF [14]. Growing evidence also indicates that p75 NTR directly signals cell death in several cell types including oligodendrocytes, sympathetic neurons and retinal neurons [7]. Sequence homology studies have shown that p75 NTR belongs to the TNF receptor superfamily and contains a death signaling moiety spanning 90 amino acids [2]. * Corresponding author. Tel.: 161-3-9905-2712; fax: 161-39905-2766. E-mail address: [email protected] (S.S. Cheema)
Sensory neurons deprived of NGF have been shown to survive longer in vitro following a reduction in p75 NTR levels [4], and the rate and extent of apoptosis in PC12 cells is sensitive to the levels of p75 NTR expression [5]. Reducing the levels of p75 NTR using antisense oligonucleotides directed against p75 NTR can prevent the loss of axotomized sensory neurons in the dorsal root ganglion (DRG) of newborn rats [8]. We have investigated whether the sensory DRG and spinal motor neuron populations are affected in the p75 NTR knockout mouse. Since both these populations express p75 NTR during prenatal development [13], the p75 NTR gene knockout model offers a unique opportunity to assess the role of this receptor in the development of sensory and motor neurons. Homozygous transgenic mice lacking the p75 NTR gene were obtained from Jackson Laboratories (Bar Harbor, USA) [15]. BALB/c and 129/svJ mice, which are part of the genetic background of the p75 NTR knockout mouse, were used as the controls. Postnatal mouse pups, 7 days old, were rendered unconscious on ice, the heart was exposed and the animals were perfused transcardially with saline followed by 4% paraformaldehyde in 0.1 M sodium phosphate buffer at pH 7.2. Laminectomies were carried out to expose the cervical and lumbar spinal cords and DRG. Blocks containing the C8 and T1 DRG and spinal segments C5 to T1, and the L4 and L5 DRG and the lumbar spinal cord enlargement
0304-3940/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 9 9) 00 33 0- 4
46
S.S. Murray et al. / Neuroscience Letters 267 (1999) 45±48
were obtained. These tissue blocks were post®xed overnight in the perfusion ®xative at 48C, dehydrated and embedded in paraf®n wax. The cervical and lumbar spinal cords were sectioned in the transverse and horizontal planes respectively. Serial sections 8 mm thick were mounted onto gelatinized slides, de-waxed and stained in 0.1% cresyl violet. Sensory neurons in the DRG and motor neurons in the dorsolateral column of the spinal cord displaying a promi-
Fig. 2. Photomicrographs of cresyl violet stained histological sections illustrating sensory neurons in the DRG of control (A) and p75 NTR knockout (B) mice. The morphology of motor neurons in the spinal cord of control (C) and p75 NTR knockout (D) mice are also shown. Scale bar represents 50 mm and applies to all photomicrographs.
Fig. 1. Histograms illustrating normal sensory and motor neuron numbers in the spinal cords and DRG of p75 NTR knockout and control mouse. The number of sensory neurons in both the C8 and T1 (A) and L4 and L5 (B) DRG in the p75 NTR knockout mouse was signi®cantly less compared with both control groups. Motor neurons were also counted in C7 and C8, and the lumbar enlargement of the spinal cord in control and p75 NTR knockout mouse (C). The number of motor neurons in both the cervical and lumbar spinal cords of the p75 NTR knockout mouse were not signi®cantly different compared with the control groups. Double asterisks denote statistically signi®cant differences at the P , 0:01 level.
nent nucleolus were counted in every ®fth section [9,10]. The cervical motor neuron counts were restricted to spinal segments C7 and C8. The lumbar motor neuron counts were taken from the entire length of the lumbar enlargement. To determine the absolute neuronal numbers, Abercrombie's formula was used [11]. For this calculation, the mean diameters of nucleoli in sensory and motor neurons from wild-type and knockout mice were determined from camera lucida tracings (100 £ ) using a digitizer-linked to a computer image analysis program (SigmaScan Prow) (data not shown). Histograms were constructed using mean and standard error of the mean (SEM) values for each group (Fig. 1). Analysis of variance (ANOVA) statistics with Dunnett's post-hoc tests were used to determine signi®cant differences between groups. The histological appearance of DRG sensory neurons and spinal motor neurons in control (129/svJ) and p75 NTR knockout mice is shown in Fig. 2. The DRG sensory and spinal motor neurons can be clearly identi®ed in both the control and p75 NTR tissues. Fig. 1 is a summary of the numbers of DRG and motor neurons. In the C8 and T1 DRG, the p75 NTR knockout mouse had a 75 and 73% reduction of sensory neurons, respectively (Fig. 1A). In the L4 and L5 DRG, the p75 NTR knockout mouse had a 56 and 48% reduction of sensory neurons, respectively (Fig. 1B). The reduction of sensory neurons observed in the p75 NTR knockout mouse was statistically signi®cant for all DRG counted (**P , 0:01). Analysis of motor neurons counts in both C7 and C8 and lumbar enlargement did not reveal any signi®cant differences between controls and p75 NTR knockout mice (Fig. 1C). In the present study, we found that the numbers of DRG sensory, but not the spinal motor neurons, were signi®cantly depleted in the p75 NTR knockout mouse, suggesting a differ-
S.S. Murray et al. / Neuroscience Letters 267 (1999) 45±48
ential role for this receptor in these two neuronal phenotypes. Up to 75% of cervical and 50% of lumbar DRG neurons are lost during the prenatal period. Several studies have noted that the DRG in the p75 NTR knockout mouse are `macroscopically' smaller, and a reduction in the sensory innervation of the skin suggests that the somatosensory system is compromised in the p75 NTR knockout mouse [6,15]. However, this is the ®rst study to quantify the precise extent of the sensory neuronal loss in the p75 NTR knockout mouse. Our results suggest that there is an absolute requirement of p75 NTR for the survival of a signi®cant number of DRG sensory neurons during prenatal development. The lack of p75 NTR is likely to compromise the action of the high af®nity trk receptors. There is ample in vitro data which indicates that p75 NTR co-expression increases neurotrophin binding to trkA, B and C [2]. Decreased NGF binding has also been noted in DRG sensory and trigeminal neurons obtained from the p75 NTR knockout mouse [12,14]. Down-regulation of p75 NTR, by means of antisense oligonucleotides, is associated with increased death of embryonic (E14) DRG sensory neurons grown in the presence of NGF in the culture medium [4]. Collectively, these studies are consistent with the view that, in DRG sensory neurons, p75 NTR is essential for trk-mediated signaling of survival during prenatal development. Since p75 NTR interacts with trkA, B and C to promote the survival of sensory neurons, it is relevant to compare the degree of sensory neuronal losses in the trk knockout mice. In the trkA knockout mouse, approximately 70±90% of lumbar DRG neurons are lost; in the trkB and trkC knockout mice there is between 20 and 30% loss of lumbar DRG sensory neurons [17]. The reported losses associated with the trk knockout mice are consistent with the 50±75% decrease in the sensory neuron population in the p75 NTR knockout mouse reported in the present study, and further reinforces the view that the interaction between p75 NTR and the trks is essential for survival during prenatal development. It would be of interest to determine at what developmental stage the sensory neurons are lost, as this reduction in neuronal numbers may result from either reduced neuronal proliferation or increased neuronal death around the time of target innervation. A key ®nding in this study was that motor neurons are completely spared in the p75 NTR knockout animal. This occurs despite the expression of p75 NTR by motor neurons from E14 onwards [13], a timeframe that coincides with the period of naturally occurring motor neuron cell death. The observation that motor neuron numbers are not signi®cantly reduced in the p75 NTR knockout mouse is not unexpected, as neither the trkA, B or C knockout mice show a signi®cant loss of motor neurons [16,17]. The precise role of p75 NTR in motor neuron development remains unknown. We thank Mr. Ian Boundy and Ms. Liz Lopes for providing excellent histological assistance. We also thank Ms.
47
Kim Lowry for providing useful comments on the manuscript. S.S.M. is a recipient of the Bethlehem Grif®ths Postgraduate Scholarship. [1] Barbacid, M., The Trk family of neurotrophin receptors, . J. Neurobiol., 25 (1994) 1386±1403. [2] Barker, P., p75NTR: a study in contrasts. Cell Death Differ., 5 (1998) 346±356. [3] Barker, P.A. and Shooter, E.M., Disruption of NGF binding to the low af®nity neurotrophin receptor p75LNTR reduces NGF binding to TrkA on PC12 cells. Neuron, 13 (1994) 203± 215. [4] Barrett, G.L. and Bartlett, P.F., The p75 nerve growth factor receptor mediates survival or death depending on the stage of sensory neuron development. Proc. Natl. Acad. Sci. USA, 91 (1994) 6501±6505. [5] Barrett, G.L. and Georgiou, A., The low-af®nity nerve growth factor receptor p75NGFR mediates death of PC12 cells after nerve growth factor withdrawal. J. Neurosci. Res., 45 (1996) 117±128. [6] Bergmann, I., Priestley, J.V., McMahon, S.B., Brocker, E.B., Toyka, K.V. and Koltzenburg, M., Analysis of cutaneous sensory neurons in transgenic mice lacking the low af®nity neurotrophin receptor P75. Eur. J. Neurosci., 9 (1997) 18± 28. [7] Casaccia-Bonne®l, P., Kong, H. and Chao, M., Neurotrophins: the biological paradox of survival factors eliciting apoptosis. Cell Death Differ., 5 (1998) 357±364. [8] Cheema, S.S., Barrett, G.L. and Bartlett, P.F., Reducing p75 nerve growth factor receptor levels using antisense oligonucleotides prevents the loss of axotomized sensory neurons in the dorsal root ganglia of newborn rats. J. Neurosci. Res., 46 (1996) 239±245. [9] Cheema, S.S., Richards, L., Murphy, M. and Bartlett, P.F., Leukemia inhibitory factor prevents the death of axotomized sensory neurons in the dorsal root ganglia of the neonatal rat. J. Neurosci. Res., 37 (1994) 213±218. [10] Cheema, S.S., Richards, L.J., Murphy, M. and Bartlett, P.F., Leukaemia inhibitory factor rescues motoneurones from axotomy-induced cell death. NeuroReport, 5 (1994) 989± 992. [11] Clarke, P.G., How inaccurate is the Abercrombie correction factor for cell counts? (letter; comment). Trends Neurosci., 15 (1992) 211±212. [12] Davies, A.M., Lee, K.F. and Jaenisch, R., p75-De®cient trigeminal sensory neurons have an altered response to NGF but not to other neurotrophins. Neuron, 11 (1993) 565±574. [13] Ernfors, P., Henschen, A., Olson, L. and Persson, H., Expression of nerve growth factor receptor mRNA is developmentally regulated and increased after axotomy in rat spinal cord motoneurons. Neuron, 2 (1989) 1605±1613. [14] Lee, K.F., Davies, A.M. and Jaenisch, R., p75-De®cient embryonic dorsal root sensory and neonatal sympathetic neurons display a decreased sensitivity to NGF. Development, 120 (1994) 1027±1033. [15] Lee, K.F., Li, E., Huber, L.J., Landis, S.C., Sharpe, A.H., Chao, M.V. and Jaenisch, R., Targeted mutation of the gene encoding the low af®nity NGF receptor p75 leads to de®cits in the peripheral sensory nervous system. Cell, 69 (1992) 737±749. [16] Silos-Santiago, I., Fagan, A.M., Garber, M., Fritzsch, B. and Barbacid, M., Severe sensory de®cits but normal CNS development in newborn mice lacking TrkB and TrkC tyrosine protein kinase receptors. Eur. J. Neurosci., 9 (1997) 2045±2056.
48
S.S. Murray et al. / Neuroscience Letters 267 (1999) 45±48
[17] Snider, W.D., Functions of the neurotrophins during nervous system development: what the knockouts are teaching us. Cell, 77 (1994) 627±638. [18] Twiss, J.L., Wada, H.G., Fok, K.S., Chan, S.D., Verity, A.N., Baxter, G.T., Shooter, E.M. and Sussman, H.H., Duration and magnitude of nerve growth factor signaling depend on the ratio of p75LNTR to TrkA. J. Neurosci. Res., 51 (1998) 442±453.
[19] Verdi, J.M., Birren, S.J., Ibanez, C.F., Persson, H., Kaplan, D.R., Benedetti, M., Chao, M.V. and Anderson, D.J., p75LNGFR regulates Trk signal transduction and NGFinduced neuronal differentiation in MAH cells. Neuron, 12 (1994) 733±745.