On the organization of the thoracic spinal ganglion and nerve in the rat

The Adaptor Protein CD2AP Is a Coordinator of Neurotrophin Signaling-Mediated Axon Arbor Plasticity

Growth of intact axons of noninjured neurons, often termed collateral sprouting, contributes to both adaptive and pathological plasticity in the adult nervous system, but the intracellular factors controlling this growth are largely unknown. An automated functional assay of genes regulated in sensory neurons from the rat in vivo spared dermatome model of collateral sprouting identified the adaptor protein CD2-associated protein (CD2AP; human CMS) as a positive regulator of axon growth. In non-neuronal cells, CD2AP, like other adaptor proteins, functions to selectively control the spatial/temporal assembly of multiprotein complexes that transmit intracellular signals. Although CD2AP polymorphisms are associated with increased risk of late-onset Alzheimer's disease, its role in axon growth is unknown. Assessments of neurite arbor structure in vitro revealed CD2AP overexpression, and siRNA-mediated knockdown, modulated (1) neurite length, (2) neurite complexity, and (3) growth cone filopodia number, in accordance with CD2AP expression levels. We show, for the first time, that CD2AP forms a novel multiprotein complex with the NGF receptor TrkA and the PI3K regulatory subunit p85, with the degree of TrkA:p85 association positively regulated by CD2AP levels. CD2AP also regulates NGF signaling through AKT, but not ERK, and regulates long-range signaling though TrkA(+)/RAB5(+) signaling endosomes. CD2AP mRNA and protein levels were increased in neurons during collateral sprouting but decreased following injury, suggesting that, although typically considered together, these two adult axonal growth processes are fundamentally different. These data position CD2AP as a major intracellular signaling molecule coordinating NGF signaling to regulate collateral sprouting and structural plasticity of intact adult axons.

SIGNIFICANCE STATEMENT

Growth of noninjured axons in the adult nervous system contributes to adaptive and maladaptive plasticity, and dysfunction of this process may contribute to neurologic pathologies. Functional screening of genes regulated during growth of noninjured axons revealed CD2AP as a positive regulator of axon outgrowth. A novel association of CD2AP with TrkA and p85 suggests a distinct intracellular signaling pathway regulating growth of noninjured axons. This may also represent a novel mechanism of generating specificity in multifunctional NGF signaling. Divergent regulation of CD2AP in different axon growth conditions suggests that separate mechanisms exist for different modes of axon growth. CD2AP is the first signaling molecule associated with adult sensory axonal collateral sprouting, and this association may offer new insights for NGF/TrkA-related Alzheimer's disease mechanisms.

Transcriptional changes in sensory ganglia associated with primary afferent axon collateral sprouting in spared dermatome model

Primary afferent collateral sprouting is a process whereby non-injured primary afferent neurons respond to some stimulus and extend new branches from existing axons. Neurons of both the central and peripheral nervous systems undergo this process, which contributes to both adaptive and maladaptive plasticity (e.g., [1], [2], [3], [4], [5], [6], [7], [8], [9]). In the model used here (the “spared dermatome” model), the intact sensory neurons respond to the denervation of adjacent areas of skin by sprouting new axon branches into that adjacent denervated territory. Investigations of gene expression changes associated with collateral sprouting can provide a better understanding of the molecular mechanisms controlling this process. Consequently, it can be used to develop treatments to promote functional recovery for spinal cord injury and other similar conditions. This report includes raw gene expression data files from microarray experiments in order to study the gene regulation in spared sensory ganglia in the initiation (7 days) and maintenance (14 days) phases of the spared dermatome model relative to intact (“naïve”) sensory ganglia. Data has been deposited into GEO (GSE72551).

Sciatic and femoral nerve sensory neurones occupy different regions of the L4 dorsal root ganglion in the adult rat

The topographical distribution of sciatic and femoral nerve sensory neuronal somata in the L4 dorsal root ganglion of the adult rat was mapped after retrograde tracing with one or two of the dyes Fast Blue, Fluoro-Gold, or Diamidino Yellow. The tracers were applied to the proximal transected end of either nerve alone, or from both nerves in the same animal using separate tracers. Three-dimensional reconstructions of the distribution of labelled neurones were made from serial sections of the L4 dorsal root ganglion which is the only ganglion that these two nerves share. The results showed that with little overlap, femoral nerve neurones distribute dorsally and rostrally whereas sciatic nerve neurones distribute medially and ventrally. This finding indicates the existence of a somatotopical organisation for the representation of different peripheral nerves in dorsal root ganglia of adult animals.

Sympathetic and afferent neurones projecting into forelimb and trunk nerves and the anatomical organization of the thoracic sympathetic outflow of the rat

The anatomy of the cervicothoracic sympathetic nerves was studied in the rat. Details of the arrangements of white and grey rami communicantes and superior cervical, middle cervical and stellate ganglia are given. Dorsal root and sympathetic ganglion neurones projecting to skin and muscle of the forelimb and trunk were labelled retrogradely with horseradish peroxidase (HRP) in order to study their number, segmental distribution and location. HRP was applied to forelimb nerves supplying skeletal muscles (Ramus profundus of radial nerve, RP) or hairy skin (N. cutaneus brachii lateralis superior of axillary nerve, CB), to mixed nerves (median nerve, ME; ulnar nerve, UL; radial nerve, RA) and to segmental thoracic nerves supplying hairy skin of the back (dorsal cutaneous nerve, CD) and to mixed internal intercostal nerves (IC). All sensory and sympathetic neurones were located ipsilaterally. In the forelimb nerves sensory somata were commonly restricted to two or three adjacent dorsal root ganglia (usually C6-7 for CB; C7-8 for ME; C7-Th1 for RA and RP; C8-Th1 for UL). Nearly all of the sympathetic somata were located in the middle cervical and stellate ganglia (fusion of C6-Th3). Some 0-0.4% lay in Th4 and Th5, none in the superior cervical ganglia. In the trunk nerves sensory somata were strictly segmentally organized. Sympathetic somata were distributed more widely over 4-5 segments with 50-55% in the segmental ganglion and up to 41% in the next caudal segment. From the data, it is estimated that 400 sympathetic (28%) and 1050 afferent neurones project into CB, 1660 (29%) and 4050 into RA, 540 (42%) and 760 into RP, 1010 (22%) and 3670 into ME, 880 (22%) and 3040 into UL, 350 (25%) and 1040 into IC and 500 (27%) and 1370 into CD.

A rostrocaudal somatotopic organization in the brachial dorsal root ganglia of neonatal rats

A rostrocaudal somatotopic organization in the rat lumbar dorsal root ganglia (DRGs) during development has been demonstrated. The present study shows that such a rostrocaudal somatotopic organization also exists in the brachial DRGs of neonatal rats. WGA-HRP or WGA-HRP/HRP injections into the forepaw of neonatal rats gave rise to completely and partially labeled DRGs. Partial labeling always concerned the caudal or rostral halves of the rostral-most or caudal-most labeled DRGs, respectively. Labeling of forelimb nerves (cutaneous branch of the musculocutaneous nerve, median and ulnar nerve) in neonatal rats also resulted in whole labeled DRGs and labeled rostral or caudal halves of DRGs. Accordingly, just as was found for the hind limb nerves, the position of the somata of the sensory neurons of forelimb nerves can be restricted to the rostral or caudal half of a DRG. Moreover, it was observed that the rostrocaudal organization in the brachial DRGs is maintained in the spinal nerves. Application of tracer to the cut end of intercostal nerves in neonatal rats produced labeling of one whole thoracic DRG. It is therefore possible that the rostrocaudal organization in the brachial and lumbar DRGs is a consequence of the formation of the plexuses. In one instance a subdivision in the labeling pattern of a midthoracic DRG was noticed after a WGA-HRP injection into the thoracic wall of a neonatal rat. The whole thoracic DRG was labeled except the dorsorostral corner. This indicates that during development, some kind of somatotopic organization may exist also in the thoracic DRG.

Sensory neurons of the rat sciatic nerve

Experiments have been undertaken in this laboratory over recent years to accurately determine the numbers and sizes of somatic neurons which contribute to the normal sciatic nerve, at mid-thigh levels, of the adult, albino rat. This article is concerned with the dorsal root ganglion (DRG) neuron population of the sciatic nerve whose cell bodies were identified through retrograde labeling of cut branches of the sciatic with horseradish peroxidase (HRP) and/or its wheat germ conjugate (WGA-HRP). It is essential to understand the neuronal composition of the normal rat sciatic nerve if the consequences of aging, nerve injury, and surgical repair to improve functional regeneration are to be properly evaluated. Neuron counts were determined from camera-lucida paper drawings of all labeled profiles in DRGs L3-L6 at 100 x magnification. The profiles, obtained by labeling individual branches of the sciatic nerve (sural, lateral sural, tibial, peroneal, medial, and lateral gastrocnemius/soleus nerves) were traced from 40-microns-thick, serial, frozen sections. The sizes of the perikarya, areas and diameters, were determined by tracing the perimeters of the drawn profiles on a digitizing tablet. The tablet's output was inputted directly into a specially designed computer spreadsheet which contained a mathematical table for correcting the split-cell error inherent to the sectioning process. Afferents from any given branch of the sciatic normally occupied two to three adjacent ganglia. Sciatic DRG neurons were normally located in lumbar ganglia L3-L6. Nearly 98-99% of all sciatic DRG perikarya resided in the L4 and L5 DRGs. The L6 DRG, traditionally regarded as an important contributor to the rat sciatic, contained merely 0.4% of its afferent neurons while the L3 ganglion, frequently overlooked as a contributor, contained 1.2% of the mid-thigh sciatic afferents. The mean size of rat DRG neurons was about 29 microns (550-600 microns2). The corrected counts revealed that the normal sciatic nerve (at mid-thigh levels), in rats between 2 and 12 months of age, contained a mean, total DRG neuron population of about 10,500 neurons. This is probably an underestimate by 3-5% of the true number due to occasional unreliable labeling of some of the small DRG neurons. It is estimated that the normal, mean number of sciatic DRG neurons of young to middle-aged rats lies somewhere between 10,500 and 11,000 +/- 2000. The data suggest that nearly 20% of all DRG neurons in the sciatic nerve supply muscle afferents. The vast majority of the remaining neurons are involved with innervation of the skin.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for a rostrocaudal organization in dorsal root ganglia during development as demonstrated by intra-uterine WGA-HRP injections into the hindlimb of rat fetuses

The development of the sensory innervation of the rat hindlimb was studied with special attention to the dorsal root ganglia and the lumbar plexus. Injections of wheat germ agglutinin-horseradish peroxidase were made into the hindlimb of 30 rat fetuses of gestational ages ranging from embryonic day 15-18. Additionally wheat germ agglutinin-horseradish peroxidase was applied to the sciatic nerves of 8 neonatal rats and 3 adults. The saphenous nerves of 2 neonatal rats were labeled. Injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the hindlimb of the fetuses result in completely and partially labeled dorsal root ganglia. Partial labeling always concerns the rostral or caudal part of a dorsal root ganglion. The associated dorsal roots of partially labeled dorsal root ganglia are also partially labeled in a corresponding rostrocaudal fashion. Reconstructions of the labeled nerves following injections into the hindlimb suggested that the somata of the sensory neurons of a particular nerve can be restricted to the rostral or caudal half of a dorsal root ganglion. For example: the rostral half of the fourth lumbar dorsal root ganglion belongs to the femoral nerve and its caudal half to the sciatic nerve. The results of application of wheat germ agglutinin-horseradish peroxidase to the central ends of the cut sciatic and saphenous nerves in neonatal rats confirmed these observations. So the rostrocaudal organization in the dorsal root ganglia stems from the distribution pattern of the peripheral nerves.

Morphometrical study of the cat thoracic dorsal root ganglion cells in relation to muscular and cutaneous afferent innervation

The sizes of neuronal somata in cat dorsal root ganglia were determined at the different thoracic segmental levels (T1-T13). The intersegmental variations in the average value and class distribution of diameters were analysed. The maximal and minimal average mean cell diameters were 51.1 and 43.3 microns at the T1 and T2 levels, respectively. Caudally, this value gradually increased from T2 to T8 (47.8 microns) and thereafter decreased progressively to T12 (44.7 microns). At T1, large cells (greater than 50 microns in diameter) were 3.3-fold in excess compared to small ones (less than 35 microns in diameter). The proportion of large to small cells strongly decreased to a 0.9 ratio from T1 to T2, then increased again from T2 (0.9) to T8 (2.3). The size distributions of the overall cell populations were compared to those of neurones supplying muscular targets via the external intercostal nerves or cutaneous targets via the lateral branch of the internal intercostal nerves, identified following the retrograde transport of horseradish peroxidase. The size distribution of cells serving cutaneous nerves was similar to that exhibited by the overall population of ganglion cells. In contrast, the size distributions of cells giving rise to muscle afferents tended towards smaller values. In the thoracic dorsal root ganglia, the cell body sizes of the muscular primary afferents were close to those previously reported for the visceral primary afferents.

Afferent projections from the mammary glands to the spinal cord in the lactating rat--I. A neuroanatomical study using the transganglionic transport of horseradish peroxidase-wheatgerm agglutinin

Horseradish peroxidase-wheatgerm agglutinin was injected subcutaneously into one or more nipples of lactating rats to determine the spinal organization of sensory afferents emanating from the mammary glands. After survival periods of 45-96 h, dorsal root ganglia and segments of the spinal cord and/or medulla oblongata were sectioned and reacted histochemically with tetramethylbenzidine to reveal the transganglionically transported tracer. For each nipple injected, the peroxidase reaction product was found in somata, ranging in diameter from 15 to 60 microns, and fibres in 5-11 contiguous dorsal root ganglia. The number of labelled profiles was highest in the 2-4 central-most ganglia of the series and generally decreased progressively rostrally and caudally. After separate injections into each of the six ipsilateral nipples, labelling occurred in all ipsilateral dorsal root ganglia between the 5th cervical and 6th lumbar spinal segments. Substantial overlap of the spinal projections from adjacent mammary glands was seen, a given dorsal root ganglion innervating 2-3 different glands. Label in the spinal cord was restricted to the medial portion of the superficial dorsal horn. It occurred in what appeared to be terminal fields and fibres essentially in the substantia gelatinosa, but was also seen to extend into the marginal zone and sometimes into deeper regions of the dorsal horn. Label was found in both the gracile and cuneate nuclei of the medulla oblongata, though only occasionally and then only very sparsely. The substantial spread and segmental overlap of labelled mammary afferents, and the fact that most labelled afferents terminated in the dorsal horn, suggest that this spinal region may be an important site for the integration of sensory input from the mammary glands that may play a role in the sensory induction of reflex milk ejection.

Neural relations of cremaster motoneurons, spinal cord systems and the genitofemoral nerve in the rat

The anatomical and biochemical features of primary sensory afferents and the peptidergic innervation of cremaster motoneuron efferents in the genitofemoral (Gf) nerve were analyzed in the rat using immunohistochemical, histochemical, retrograde tracing and lesion methods. Afferent fibers in the Gf nerve were shown to originate from neurons in L1 and L2 dorsal root ganglia (DRG) and to project to L1 to T12.5 in the spinal cord. Some of the DRG neurons giving rise to these fibers contained substance P (SP) or the enzyme fluoride-resistant acid phosphatase but none appeared to contain somatostatin. The dermatome area of the Gf nerve, as determined by plasma extravasation methods, was located in the rostral scrotal and adjacent abdominal region. Identification of cremaster motoneurons by retrograde labelling from the Gf nerve revealed these neurons to be located in the L1 to L2 spinal cord segment, to have prominent rostrocaudally oriented dendritic aborizations and to receive a rich innervation by fibers containing SP, thyrotropin-releasing hormone (TRH) or met-enkephalin (met-Enk). Lesion studies indicated the SP-and met-Enk-containing fibers to be supplied by local intraspinal systems and the TRH-containing fibers by supraspinal systems. In female rats, motoneurons corresponding to the male version of the cremaster motoneuronal pool were less developed and received far fewer peptidergic connections than that observed in males. The multiple neural systems innervating cremaster motoneurons together with sensory afferents in the Gf and other scrotal nerves are suggested to be involved in the contribution of cremaster muscles to thermoregulation of the scrotum.

Preliminary studies on long distance, retrograde transport of horseradish peroxidase in equine peripheral nerves

As a prelude to studies on retrograde axonal transport of neurotoxin (ie, so-called suicide transport) as a means to prevent post neurectomy neuroma formation, preliminary studies were conducted with an innocuous enzymatic marker, horseradish peroxidase (HRP). The proximal stumps of resected medial and lateral palmar digital nerves in six ponies were injected via a tuberculin syringe and needle with 50 micron 1 of a 30 per cent solution of HRP in order to assess long distance retrograde axonal transport. The dorsal root ganglion of the cervical spinal enlargement (ie, C6, C7, C8, T1, T2) were removed at post injection intervals of two, four, six, eight, 10 and 12 days. These were sectioned serially and reacted by the tetramethylbenzidine method to demonstrate transported enzyme in the ganglionic cell bodies which give rise to sensory fibres of the palmar digital nerves. Enzyme, retrogradely transported over axon lengths of 115 cm, was first demonstrated in spinal ganglia four days after injections of the palmar digital nerves. The calculated transport velocity of 287 mm/day, although almost certainly an underestimate, greatly exceeded rates of 72 to 120 mm/day recorded previously with HRP in the peripheral nerves of small laboratory animals. The intensity of the HRP reaction product in ganglionic neurons was strong at four days and it remained unabated in ganglia examined at six, eight, 10 and 12 days post injection. The major sources of the sensory fibres of the palmar digital nerves appeared to be the ganglia of the C8 and T1 spinal segments which contained more than 90 per cent of all labelled neurons.(ABSTRACT TRUNCATED AT 250 WORDS)